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PURPOSE: Reliable biomarkers for predicting subsequent sepsis among patients with suspected acute infection are lacking. In patients presenting to emergency departments (EDs) with suspected acute infection, we aimed to evaluate the reliability and discriminant ability of 47 leukocyte biomarkers as predictors of sepsis (Sequential Organ Failure Assessment score ≥ 2 at 24 h and/or 72 h following ED presentation).METHODS: In a multi-centre cohort study in four EDs and intensive care units (ICUs), we standardised flow-cytometric leukocyte biomarker measurement and compared patients with suspected acute infection (cohort-1) with two comparator cohorts: ICU patients with established sepsis (cohort-2), and ED patients without infection or systemic inflammation but requiring hospitalization (cohort-3).RESULTS: Between January 2014 and February 2016, we recruited 272, 59 and 75 patients to cohorts 1, 2, and 3, respectively. Of 47 leukocyte biomarkers, 14 were non-reliable, and 17 did not discriminate between the three cohorts. Discriminant analyses for predicting sepsis within cohort-1 were undertaken for eight neutrophil (cluster of differentiation antigens (CD) CD15; CD24; CD35; CD64; CD312; CD11b; CD274; CD279), seven monocyte (CD35; CD64; CD312; CD11b; HLA-DR; CD274; CD279) and a CD8 T-lymphocyte biomarker (CD279). Individually, only higher neutrophil CD279 [OR 1.78 (95% CI 1.23-2.57); P = 0.002], higher monocyte CD279 [1.32 (1.03-1.70); P = 0.03], and lower monocyte HLA-DR [0.73 (0.55-0.97); P = 0.03] expression were associated with subsequent sepsis. With logistic regression the optimum biomarker combination was increased neutrophil CD24 and neutrophil CD279, and reduced monocyte HLA-DR expression, but no combination had clinically relevant predictive validity.CONCLUSIONS: From a large panel of leukocyte biomarkers, immunosuppression biomarkers were associated with subsequent sepsis in ED patients with suspected acute infection.CLINICAL TRIAL REGISTRATION: NCT02188992.

Purpose Cellular immune dysfunctions, which are common in intensive care patients, predict a number of significant complications. In order to effectively target treatments, clinically applicable measures need to be developed to detect dysfunction. The objective was to confirm the ability of cellular markers associated with immune dysfunction to stratify risk of secondary infection in critically ill patients. Methods Multi-centre, prospective observational cohort study of critically ill patients in four UK intensive care units. Serial blood samples were taken, and three cell surface markers associated with immune cell dysfunction [neutrophil CD88, monocyte human leucocyte antigen-DR (HLA-DR) and percentage of regulatory T cells (Tregs)] were assayed on-site using standardized flow cytometric measures. Patients were followed up for the development of secondary infections. Results A total of 148 patients were recruited, with data available from 138. Reduced neutrophil CD88, reduced monocyte HLA-DR and elevated proportions of Tregs were all associated with subsequent development of infection with odds ratios (95% CI) of 2.18 (1.00–4.74), 3.44 (1.58–7.47) and 2.41 (1.14–5.11), respectively. Burden of immune dysfunction predicted a progressive increase in risk of infection, from 14% for patients with no dysfunction to 59% for patients with dysfunction of all three markers. The tests failed to risk stratify patients shortly after ICU admission but were effective between days 3 and 9. Conclusions This study confirms our previous findings that three cell surface markers can predict risk of subsequent secondary infection, demonstrates the feasibility of standardized multisite flow cytometry and presents a tool which can be used to target future immunomodulatory therapies. Trial registration The study was registered with clinicaltrials.gov (NCT02186522). Electronic supplementary material The online version of this article (10.1007/s00134-018-5247-0) contains supplementary material, which is available to authorized users.

CD4(+) regulatory T (Treg) cells expressing CD25 and the transcription factor forkhead box P3 (FOXP3) are indispensable for immunological self-tolerance and homeostasis. FOXP3(+)CD25(+)CD4(+) T cells in humans, however, are heterogeneous in function and differentiation status, including suppressive or nonsuppressive cells as well as resting or activated Treg cells. We have searched for cell surface markers specific for suppression-competent Treg cells by using a panel of currently available monoclonal antibodies reactive with human T cells. We found that CD15s (sialyl Lewis x) was highly specific for activated, terminally differentiated, and most suppressive FOXP3(high) effector Treg (eTreg) cells and able to differentiate them in various clinical settings from nonsuppressive FOXP3(+) T cells secreting inflammatory cytokines. For example, CD15s(+)FOXP3(+) eTreg cells were increased in sarcoidosis, whereas it was nonsuppressive CD15s(-)FOXP3(+) T cells that were expanded in lupus flares. FOXP3(+) cells induced from conventional CD4(+) T cells by T-cell receptor stimulation hardly expressed CD15s. CD15s(+)CD4(+) T-cell depletion was sufficient to evoke and enhance in vitro immune responses against tumor or viral antigens. Collectively, we have identified CD15s as a biomarker instrumental in both phenotypic and functional analysis of FOXP3(+)CD4(+) T-cell subpopulations in health and disease. It allows specific targeting of eTreg cells, rather than whole FOXP3(+)CD4(+) T cells, in controlling immune responses.

INTRODUCTION: Critically ill patients are at high risk of nosocomial infections, with between 20% and 40% of patients admitted to the intensive care unit (ICU) acquiring infections. These infections result in increased antibiotic use, and are associated with morbidity and mortality. Although critical illness is classically associated with hyperinflammation, the high rates of nosocomial infection argue for an importance of effect of impaired immunity. Our group recently demonstrated that a combination of 3 measures of immune cell function (namely neutrophil CD88, monocyte HLA-DR and % regulatory T cells) identified a patient population with a 2.4-5-fold greater risk for susceptibility to nosocomial infections.METHODS AND ANALYSIS: This is a prospective, observational study to determine whether previously identified markers of susceptibility to nosocomial infection can be validated in a multicentre population, as well as testing several novel markers which may improve the risk of nosocomial infection prediction. Blood samples from critically ill patients (those admitted to the ICU for at least 48 hours and requiring mechanical ventilation alone or support of 2 or more organ systems) are taken and undergo whole blood staining for a range of immune cell surface markers. These samples undergo analysis on a standardised flow cytometry platform. Patients are followed up to determine whether they develop nosocomial infection. Infections need to meet strict prespecified criteria based on international guidelines; where these criteria are not met, an adjudication panel of experienced intensivists is asked to rule on the presence of infection. Secondary outcomes will be death from severe infection (sepsis) and change in organ failure.ETHICS AND DISSEMINATION: Ethical approval including the involvement of adults lacking capacity has been obtained from respective English and Scottish Ethics Committees. Results will be disseminated through presentations at scientific meetings and publications in peer-reviewed journals.TRIAL REGISTRATION NUMBER: NCT02186522; Pre-results.

INTRODUCTION: Sepsis is an acute illness resulting from infection and the host immune response. Early identification of individuals at risk of developing life-threatening severe sepsis could enable early triage and treatment, and improve outcomes. Currently available biomarkers have poor predictive value for predicting subsequent clinical course in patients with suspected infection. Circulating leucocytes provide readily accessible tissues that reflect many aspects of the complex immune responses described in sepsis. We hypothesise that measuring cellular markers of immune responses by flow cytometry will enable early identification of infected patients at risk of adverse outcomes. We aim to characterise leucocyte surface markers (biomarkers) and their abnormalities in a population of patients presenting to the hospital emergency department with suspected sepsis, and explore their ability to predict subsequent clinical course.METHODS AND ANALYSIS: We will conduct a prospective, multicentre, clinical, exploratory, cohort observational study. To answer our study question, 3 patient populations will be studied. First, patients with suspected sepsis from the emergency department (n=300). To assess performance characteristics of potential tests, critically ill patients with established sepsis, and age and gender matched patients without suspicion of infection requiring hospital admission (both n=100) will be recruited as comparator populations. In all 3 groups, we plan to assess circulating biomarker profiles using flow cytometry. We will select candidate biomarkers by cross-cohort comparison, and then explore their predictive value for clinical outcomes within the cohort with suspected sepsis.ETHICS AND DISSEMINATION: The study will be carried out based on the principles in the Declaration of Helsinki and the International Conference on Harmonisation Good Clinical Practice. Ethics approval has been granted from the Scotland A Research Ethics Committee (REC) and Oxford C REC. On conclusion of this study, the results will be disseminated via peer-reviewed journals.TRIAL REGISTRATION NUMBER: NCT02188992; Pre-results.

Natural Regulatory T cells (Tregs) are defined by stable expression of the cell surface proteins CD4 and CD25, low surface expression of CD127 and expression of the transcription factor FOXP3. The contribution of Treg to the prevention of autoimmunity and the maintenance of immune homoestasis is the subject of ongoing interest, as alterations in Treg numbers and function are implicated in a wide range of diseases. The in vitro benchmark for determining Treg function is suppression of proliferation of unmatched effector T cells in a mixed lymphocyte reaction (MLR) over a 3-6-day time period. As an alternative to this assay, we show that a 7-h CD154 expression assay is rapid, simple and provides a reliable readout of suppressor function. Using multiple Treg-like cell types including natural (n)Treg, inducible (i)Treg and Treg cell lines, we show that suppression of CD154 expression is a surrogate for suppression of proliferation. We propose this as a suitable alternative to the MLR assay, as it is rapid and may be more amenable to high-throughput screening, analysing large cohorts of clinical samples or assaying transiently suppressive populations.

Advancements in cell analysis capabilities have significantly expanded our understanding of the complexity of immunological systems. As the need for deeper analysis has increased, many laboratories have employed a set of lineage markers to define a cell population, followed by the addition of unique markers specific to their biological question. Utilizing regulatory T cells (Treg) as a model population, we describe the design of an 8-color backbone panel on a 12-Color BD FACSLyric™ flow cytometer that can be supplemented with 4 drop-in markers (colors) to characterize two critical facets of Treg biology: activation and homing. Intelligent panel design enabled identification of human naïve and activated Treg cells utilizing established Treg markers (CD3, CD4, CD25, CD127, FoxP3, CD45RA). CD15s and CD161 were included in the 8-color backbone panel to identify functionally suppressive effector and/or pro-inflammatory cytokine secreting Tregs. Importantly, addition of a 4-color drop-in activation panel (PI16, CD147, CD39 and HLA-DR) or a 4-color drop-in homing panel (CCR4, CCR6, CXCR3 and CD31) did not impact resolution of the backbone Treg population(s), while providing new and interesting insights into Treg biology. We highlight the utility of a modular panel design approach that provides an efficient, scalable, and standardized solution for complex analysis of essentially any cell population of interest.

The costimulatory requirements required for peripheral blood T regulatory cells (Tregs) are unclear. Using cell-based artificial APCs we found that CD28 but not ICOS, OX40, 4-1BB, CD27, or CD40 ligand costimulation maintained high levels of Foxp3 expression and in vitro suppressive function. Only CD28 costimulation in the presence of rapamycin consistently generated Tregs that consistently suppressed xenogeneic graft-vs-host disease in immunodeficient mice. Restimulation of Tregs after 8–12 days of culture with CD28 costimulation in the presence of rapamycin resulted in >1000-fold expansion of Tregs in

Human regulatory T cells inhibit graft-versus-host disease that can occur after tissue transplantation, in part through expression of programmed death ligand 1 and modulation of antigen-presenting cells., Immunotherapy using regulatory T cells (Treg) has been proposed, yet cellular and molecular mechanisms of human Tregs remain incompletely characterized. Here, we demonstrate that human Tregs promote the generation of myeloid dendritic cells (DC) with reduced capacity to stimulate effector T cell responses. In a model of xenogeneic graft-versus-host disease (GVHD), allogeneic human DC conditioned with Tregs suppressed human T cell activation and completely abrogated posttransplant lethality. Tregs induced programmed death ligand-1 (PD-L1) expression on Treg-conditioned DC; subsequently, Treg-conditioned DC induced PD-L1 expression in vivo on effector T cells. PD-L1 blockade reversed Treg-conditioned DC function in vitro and in vivo, thereby demonstrating that human Tregs can promote immune suppression via DC modulation through PD-L1 up-regulation. This identification of a human Treg downstream cellular effector (DC) and molecular mechanism (PD-L1) will facilitate the rational design of clinical trials to modulate alloreactivity., Author Summary Graft-versus-host disease (GVHD) is the most serious complication of bone marrow transplants between individuals (so-called allogenic transplants). The class of suppressor immune cells called regulatory T cells (Tregs) inhibit GVHD by dampening the effects of donor immune cells in the grafted tissue. The cellular and molecular mechanisms involved in this process have not been fully characterized, particularly for human cells. In this study, we report that human Tregs, which we generated from precursor cells ex vivo, express high levels of a cell surface protein called PD-L1 (programmed death ligand-1) that is known to mediate immune suppression. Coculture of these Tregs with allogeneic antigen-presenting cells (APCs), which are known to initiate GVHD, increased, in turn, the amount of PD-L1 on the APCs. The Treg-conditioned APCs were then less able than unconditioned APCs to provoke GVHD in a mouse model of the condition, preventing the death of the animals after transplantation. We found that an antibody against PD-L1 blocked the immunosuppressive effects of Tregs or Treg-conditioned APCs, indicating that this protein is an important part of the molecular mechanism. These findings are potentially important for attempts to modulate immune responses in disease by transplanting T cells into patients.

A spontaneously arising murine plasmacytoma, HPC-202, derived from a BALB/c.H-2 b congenic mouse that lacks any detectable H-2 determinants on its cell surface is described. However, the expression of H-2 determinants is inducible by interferon-γ. The H-2 negative cell surface phenotype permits the HPC-202 tumor to escape H-2 allospecific cytotoxic cell lysis but not NK cell lysis, as well as to grow, to varying degrees, in some H-2 incompatible hosts. In those strains which exhibit a resistance to HPC-202 growth, resistance does not map to a single gene within the major histocompatibility complex of the mouse. Resistance is also radiosensitive and is therefore presumably due to a rapidly dividing cell population. The utility of this tumor as a model system to study both the non-H-2-restricted natural resistance to tumor growth, and the mechanism by which H-2 genes are regulated by cells is discussed.

Introduction: CD23 is a 45-kDa transmembrane glycoprotein corresponding to the low-affinity receptor for the immunoglobulin E (IgE). There are two isoforms - CD23a expressed on B-cells, and CD23b primarily expressed on monocytes, T-cells and platelets. The CD23 protein can be cleaved and released into the serum as a soluble form of CD23 (sCD23). The sCD23 is a 25kDa fragment that can be found in serum, plasma and urine in patients with chronic lymphocytic leukemia. Objective: To evaluate a novel and robust method of sCD23 detection using CBA detection, and correlate with known markers of prognostication and clinical outcome. Methods: At Barts & The London School of Medicine, historical serum samples have been taken at diagnosis from patients with CLL between 1984 to 2012 with corresponding clinical data. Serum analysis was obtained from 60 healthy controls and assessed for sCD23 in ng/ml (France and Canada). sCD23 levels (ng/ml) were evaluated against known prognostic markers and clinical features, including: ZAP-70 (>20%), CD38 (>20%), B2M, cytogenetics (17p-/11q- vs 12+/13q-/normal), LDH, splenomegaly, hepatomegaly, and lymphadenopathy. 107 samples from 86 patients underwent retrospective assessment of sCD23 from stored serum. The sCD23 cytometric bead array (CBA) flow cytometric assay (BD Biosciences) captures the soluble analyte with beads of known size and fluorescence.1 Capture beads are identified and sCD23 quantification (in ng/ml) is performed based on PE fluorescence excitation. Results were acquired using FACSDiva (BD Biosciences) and analysed using FCAParray. Statistical analysis was performed and significance was set at Results: sCD23 levels from the 86 patients ranged from 1.92 to 2441 ng/ml with a median value of 234.5 ng/ml. The sCD23 levels from the healthy control group ranged from 0.43 to 5.16 ng/ml with a median value of 1.56 ng/ml (figure 1). Figure 1: sCD23 levels between healthy controls and patients Figure 1:. sCD23 levels between healthy controls and patients Within the CLL patient cohort, elevated sCD23 significantly correlated with 6 out of the 8 prognostic & clinical indicators (Table 1). Table 1: Median sCD23 (ng/ml) for CLL patients by prognostic risk stratification. Indicator Median sCD23 (ng/ml) Level of significance (MWU-test) ZAP-70 positive (n=19) 1,096.0 P In addition, there was a strong correlation with response to therapy. Those patients who never required therapy (n=21) had significantly lower levels of sCD23 (mean 93.36ng/ml) over those who achieved a partial remission (n=21, mean 650.2 ng/ml p Figure 2: Mean sCD23 level (ng/ml) at diagnosis against response to therapy when required. Figure 2:. Mean sCD23 level (ng/ml) at diagnosis against response to therapy when required. Two patients had sequential serum samples taken throughout therapy. The patient with elevated sCD23 at diagnosis remained elevated through each treatment cycle to only achieve partial response, whilst the patient with low sCD23 at diagnosis responded well to therapy and achieved a complete remission (p=0.01, Figure 3). Figure 3: sCD23 following therapy. sCD23 non-responder did not respond to FC or FCR therapy . The responder received Chlorambucil+R then FCR. Figure 3:. sCD23 following therapy. sCD23 non-responder did not respond to FC or FCR therapy . The responder received Chlorambucil+R then FCR. Conclusion: We have evaluated in a pilot study the sCD23 CBA assay, which is an easy and reproducible assay. Elevated levels of sCD23 at diagnosis correlated with “physical” prognosis, as well as known biological markers of prognostication. Those patients with elevated levels of sCD23 at diagnosis correlated with response to therapy regardless of therapy given, and there is evidence that patients who do not respond to therapy retain their elevated levels of sCD23 during therapy. This simple assay could provide the basis for individualised risk stratification and prediction of response to therapy based on an individual analyte. Ref: 1 Grelier A et al. Cytometry B. 2014 Mar;86(2):91-7. Disclosures Farren: BD Biosciences: Research Funding. Nauwelaers:BD Biosciences: Employment. Keenan:BD Bioscience: Employment. Warner:BD Biosciences: Employment. Agrawal:BD Biosciences: Research Funding.

Introduction Over the past decade, treatments for patients with chronic lymphocytic leukemia (CLL) have produced complete remissions (CR) without evidence for minimal residual disease (MRD), particularly for younger and/or fitter patients. In this setting, achieving an MRD-negative CR has prognostic implications, yielding longer progression free survival (PFS) and overall survival (OS) than for patients who achieve a CR with persistent MRD. Complicating efforts to incorporate testing for MRD in clinical practice has been the lack of a defined consensus on the methods of MRD detection. In this study, we report on a novel combination of mAbs for MRD detection by flow cytometry based on two antigens: the NK-cell receptor and tumor specific antigen, CD160; and the tumor associated antigen, receptor tyrosine kinase-like orphan receptor 1 (ROR-1). Objective To compare a novel single-tube, tumor-specific (CD160+ROR1) targeted approach to MRD detection against the previously published CD160 flow cytometric assay (CD160FCA) (Farren et al, 2011) and the new, single-tube 8-color ERIC assay. Methods Between October 2012 and July 2013, prospective assessment of MRD was performed on peripheral blood in 56 patients (86 samples). We developed a flow cytometric assay using mAbs specific for CD160 or ROR1 (Fukuda et al, 2008). For this we used the following mAb from BD Biosciences: CD2 FITC, CD5 Pe-Cy7, CD19 PerCP5.5, CD45V500, CD160PE, ROR-1 AF647 (“ROR-160FCA”) and a sequential gating strategy. This was compared with CD160FCA and the 8-color ERIC consortium protocol (unpublished). Light chain analysis (LCR) was performed in all cases and reported where detectable. A proof of concept spiking experiment simulating MRD was prepared by mixing CLL and normal peripheral blood leukocytes in a serial dilution to a level of 10-5(n=3). Statistical analysis was performed using Spearman Rank correlation coefficients, Mann-Whitney t-test, and Bland-Altman method comparison. Significance was set at Results To establish the proof of principle, MRD levels ranging from 0.001% to 100% (Neat CLL) were prepared by serial dilutions, in which MRD levels could be established by the ROR-160FCA to 10-5 (n=3). Assessment of the observed incidence against expected incidence of CLL MRD demonstrated a highly significant correlation (R2=0.96, p=0.01). In the study, the range of detectable disease went from 54 samples had levels of disease Conclusion Monitoring minimal residual disease in CLL is a key focus for clinical trials, as MRD is an important prognostic marker in CLL in terms of PFS and OS. Here we provide a single tube assay, ROR-160FCA, which is unique by targeting two antigens restricted to malignant B-cells, CD160 and ROR1. ROR-160FCA is equivalent in MRD detection compared to both CD160FCA and the current ERIC assay under development. The two tumor-specific antigens give ROR-160FCA the potential for improved sensitivity, particularly where limited sample is available. Furthermore, it only requires a simple sequential gating strategy, is rapid, and appears more cost effective than other Methods. References Farren TW, Giustiniani J, Liu FT et al. Blood. 2011;118 (8):2174-2183. Fukuda T, Chen L, Endo T et al. Proc Natl Acad Sci U S A. 2008;105 (8):3047-3052. Disclosures: Farren: BD Biosciences: Research Funding. Warner:BD Biosciences: Employment, Research Funding. Agrawal:BD Biosciences: Research Funding.

The binding of murine IgG1, IgG2a, IgG2b, IgG3 protein molecules to Staphylococcal A protein is presented. Differential elution by sodium thiocyanate gradients is described. Proteins of the IgG2 class (either a or b subclass) require between 1.5 and 2.0 M for complete elution, whereas, IgG1 proteins are fully eluted with only 0.5 M NaSCN. These differential elution patterns can be utilized to distinguish between, or prepare proteins of different Ig classes. It is proposed that this quantitative rather than qualitative distinction between Ig subclass binding to Staph A, indicates the existence of multiple binding sites per molecule, with different markers of such sites being present on the heavy chain of the different Ig classes.

Normal C3H bone marrow cells were grown 7 days in medium containing L cell-derived colony stimulating factor-1 (CSF-1). During the first 4 days of culture, erythroid and granulocytic cells decreased while macrophages increased exponentially with a doubling time of about 31 hr. Only 0.3% of all cells in the initial bone marrow suspension formed discrete colonies of mononuclear phagocytes, but by day 6 60% of the nonadherent cells were capable of forming macrophage colonies, representing a 200-fold enrichment of the original progenitor population. Using flow cytometry, mononuclear phagocytes obtained after 4 days of culture were separated into two distinct phenotypes based on their autofluorescence. Nonadherent cells were a discrete population of small cells exhibiting low autofluorescence, and the adherent cells were a broad heterogenous population of large cells exhibiting high autofluorescence. A panel of currently available rat monoclonal antibodies (MABs) against murine hematopoietic cells were used to determine whether unique subsets of macrophages could be resolved. The MABs RA 31B6 and H-11 stained virtually all the nonadherent cells but not adherent cells. The MABs E-2 and 11–4.1 (anti-H-2Kk) stained almost all the adherent cells and demonstrated no significant staining of nonadherent cells. Nearly all the nonadherent and adherent cells were stained by the MABs DNL 4.4 and MAC-1. Additionally, the data suggest that the epitopes for MAC-2 and MAC-3 and γ2a Fc receptors develop late in nonadherent progenitor cells as they mature into adherent macrophages.

Simultaneous dual immunofluorescence and flow cytometry was used to study sixteen lymphocyte phenotypes in 209 men including: healthy homosexuals, lymphadenopathy patients (LAN), and AIDS patients. Significant differences between the distribution of lymphocytes in healthy homosexuals and healthy heterosexuals were decreased percentages of helper/inducer T cells (Leu 3), increased cytotoxic/suppressor T cells (Leu 2), and consequently a decreased Leu 3/Leu 2 ratio. The increased Leu 2 cells were identified as functionally cytotoxic subset Leu 2+ 15- phenotype rather than suppressor cells which are Leu 15+. Leu 2 and Leu 3 bearing cells exhibited an excess of membrane-bound immunoglobulins which were easily elutable at 37 degrees C. An increased percentage of an HLA-DR framework determinant bearing T cells were also detected. Within the NK cell family, Leu 7 cells were moderately increased and the functionally unidentified Leu 2+ 7+ population was strikingly elevated. LAN or AIDS patients were compared to healthy homosexual controls. Lower percentages of Leu 3 cells and higher percentages of Leu 2 cells were evident in LAN patients. These subsets were similar in LAN and AIDS patients. The increase in Leu 2+ cells was due to the Leu 2+ 15- cytotoxic subset. Fewer T cells had immunoglobulin in LAN and AIDS. A definite increase in Leu 2+ DR+ cells but not Leu 3+ DR+ cells occurred in AIDS compared to LAN or healthy controls. NK cell changes already present in healthy homosexuals persisted in LAN and AIDS patients. No differences in the distribution of B cells was detected in any intergroup comparisons. Changes in monocytes or pan-T cells were relatively insensitive measures of immunologic alterations among any of the groups. These results indicate many of the changes in lymphocyte subsets seen in AIDS and LAN subjects are already present in a carefully screened population of healthy homosexuals in San Francisco. Many of the changes in Leu 2 and NK family of cells suggest a possible adaptive response to viral or neoplastic challenge. Whether these interesting phenotypic alterations relate to functional changes in response to such challenge of the identified subsets waits further investigation.

Allotypic analysis of IgG 2 anti-red cell autoantibodies in NZB hybrid mice was performed by the Coombs (antiglobulin) test using alloantisera to mouse immunoglobulins. The results in 34 of 35 mice showed that only one parental allele was expressed in the autoantibody population of any given mouse. Either allele could be expressed and no preference for either parental type was evident. These results are held to favor the concept of a monoclonal or restricted origin of the autoantibody producing cell population, and do not indicate a role for allotype linked immune response gene control in autoantibody production.

Monoclonal antibody (mAb) anti-Leu M5 reacts with a two-chain molecule composed of a 150-kDa alpha subunit noncovalently associated with a 95-kDa beta subunit and probably is specific for an epitope on the 150-kDa alpha chain. This p150/95 antigen is the third member of a family of polypeptides sharing a common 95-kDa beta chain, which includes the lymphocyte function-associated antigen LFA-1 (p177/95) and complement receptor CR3 (Mo1/MAC-1/OKM1; p165/95) antigens. Sequential immunoprecipitation with anti-p95 beta chain mAb specifically removed the antigens detected by anti-LFA-1, anti-CR3 and anti-Leu M5 mAb. Certain patients with recurrent bacterial infections are genetically deficient in expression of the LFA-1 and Mo1 antigens, and have impaired granulocyte function. Granulocytes from a patient with this disease also failed to react with anti-Leu M5. Stimulation of normal granulocytes with f-Met-Leu-Phe, C5a-desArg, or calcium ionophore resulted in increased expression of Mo1 and Leu M5 antigens on the cell surface, but did not significantly increase expression of LFA-1 antigen. In functional assays, anti-Leu M5 did not inhibit T cell-mediated or natural killer cell-mediated cytotoxicity. In addition, anti-Leu M5 neither inhibited the binding of complement-coated particles to CR1 or CR3 nor did it affect the binding of EC3dg to neutrophils (CR4). These studies clearly indicate that the p150/95 antigen recognized by the anti-Leu M5 antibody is a structurally distinct member of the LFA-1/CR3 family.

We have previously reported that the addition of monocytes results in enhanced modulation of the T65 antigen when normal or leukemic lymphoid cells were cultured in vitro with the T101 monoclonal antibody. In the present investigation, we extend these findings to demonstrate that monocyte-enhanced modulation is a phenomenon that occurs with a variety of T and B lymphoid antigens identified by murine monoclonal antibodies. Two patterns of monocyte-enhanced modulation were observed: (1) augmentation by monocytes of existing antigen modulation by the T101 and anti-Leu-4 antibodies, and (2) induction by monocytes of previously unrecognized modulation with the anti-Leu-2 and anti-Leu-9 antibodies. Enhancement of modulation by monocytes was also detected with antibodies to surface IgM and HLA-DR antigens. Antigen modulation on lymphoid cell lines appeared to be more variable than on fresh cells, with or without monocytes. Monocyte-enhanced antigen modulation was not demonstrated with two monoclonal antibodies against solid tumors. Monocyte-enhanced modulation was shown to be dependent upon the Fc portion of the antibody, but independent of proteolytic or oxidative compounds released by monocytes. These findings indicate that the results obtained during in vitro studies of antigen modulation may vary with the source of cells and the extent to which monocytic cells are present. In addition, these findings suggest an enhanced role for Fc receptor-bearing cells of monocytic origin in antigen modulation following in vivo administration of monoclonal antibodies.

Since preliminary evidence indicated that a gene locus associated with the chocolate coat color had an inhibitory effect on the spontaneous development of autoimmune hemolytic anemia–which in turn was associated with a dominant gene–we accordingly initiated the development of a congenic line of NZB mice carrying the chocolate coat color locus (and other genes closely linked to this locus). After eight backcross generations to NZB, an inbred line of mice carrying the chocolate coat color gene was established and then studied for incidence of anti-RBC autoantibodies. The congenic line NZB.ch shows a considerably lower incidence of such autoantibodies, particularly in male mice. Although further analysis is still required, we propose that the genetic control of autoimmunity is based on the resultant interaction of several gene loci: 1) a single dominant locus (A) that predisposes to an autoimmune state rather than the maintenance of (self) tolerance, 2) a modifying locus (M) linked to ch., subject to gene dosage, which particularly in the recessive state can inhibit the effect of the A locus, and 3) a cluster of specific immune response genes that are associated with specific autoantibody responses, e.g. antinucleic acid antibodies, and are probably H2 linked. Our current analysis does not suggest that the A or M loci are linked to the MHC. It is also proposed that the wild-type allele of the M locus is carried by normal mouse strains, that NZB mice have a completely inactive allele at this locus, and that NZC mice have a third allele detectable only in the homozygous state.

Many recent studies have demonstrated that cytotoxic T lymphocytes (CL) activated to various antigens other than those of the H-2 complex, will lyse target cells only when H-2 compatibility exists between the CL and target cell. From these observations, it has been inferred that T lymphocytes might only be capable of responding to H-2 antigen or antigens that become associated with H-2 region gene products. Our results suggest that this is not the case, and that in some situations, cytotoxic T lymphocytes can specifically lyse target cells of different H-2 types. Two in vitro systems are described where primary induction of cytotoxic T lymphocytes to oncofetal and plasmacytoma antigens results in CL capable of lysing suitable targets bearing these antigens, of either syngeneic or allogeneic derivation. Thus it is proposed that although interaction antigens involving H-2 components may preferentially activate T lymphocytes, this does not imply a restriction on the recognition potential of T lymphocytes.

We have observed that a subset of E+ cells bind human monomeric IgG (FcR-IgG). In the present study, we have separated the E+, FcR-IgG+ cells by flow cytometry and tested their NK activity against the tumor KS62 using the chromium-release assay. Virtually all the NK activity residing within the E+ subset was mediated by E+, FcR-IgG+ cells. These findings are discussed in relation to the cellular lineage of the human NK cell.

A new antigenic specificity, designated Lym 7.3, defines a third allele of the Ly 7 locus. The monoclonal antibody recognizing this determinant was produced by a hybridoma derived from immunization of B10.D2 Ign mice against a (BALB/c X NZB) F1 murine B lymphoma cell line (WEHI-5). Previously, we reported that another antibody (BD5-334.5) from this fusion reacted with an alloantigen similar or identical to the Ly 7.2 antigen originally described by McKenzie and coworkers. The Lym 7.3 specificity is qualitatively and quantitatively identical to the Lym 7.2 determinant in tissue distribution. Both antibodies react with a majority of splenocytes, peripheral blood lymphocytes, lymph node cells, and mitogen activated blasts from positive strains, but do not react with kidney, liver, or brain tissues by absorption analysis. B lymphocytes expressed significantly higher cell surface density of Lym 7.2 and Lym 7.3 than T lymphocytes. However, these antibodies clearly could be distinguished on the basis of inbred and congenic strain distribution. Three distinct phenotypes were presented: (1) Lym 7.2+ Lym 7.3+; (2) Lym 7.2+ Lym 7.3-; and (3) Lym 7.2- Lym 7.3-. Genetic studies suggested both specificities were encoded by a single locus.

Various murine lymphoid tumor cell lines were examined for their alterations in intracellular levels of cyclic AMP following exposure to several pharmacologic agents. The agents tested in the present study included 3-isobutyl-1-methylxanthine (MIX), isoproterenol (ISO), prostaglandin E2 (PGE), histamine, and cholera toxin (CT). B cell tumors were generally found to be less responsive to beta-adrenergic and PGE-induced increases in cyclic AMP than were T cell tumors. An exception to this general finding was the murine B lymphoma cell line WEHI-231, which demonstrated marked sensitivity to ISO and PGE. Macrophage tumors were generally found to be even less responsive to PGE than were the B cell tumors, again with some notable exceptions (P388.D1). Cholera toxin produced differential effects in B, T, and macrophage tumors, both in terms of the absolute magnitude of the cyclic AMP response and the kinetics of this response. A T lymphoma cell line (EL-4) was identified that seems to be totally unresponsive to cell surface receptor-mediated increases in intracellular levels of cyclic AMP. The results of this study are discussed in terms of the usefulness of murine lymphoid tumors for the study of pharmacologic modulation of the immune response by cyclic AMP-elevating agents. These results demonstrate the high degree of cellular heterogeneity that can exist within normal lymphoid populations of cells, and suggest that cloned cell lines may be useful in the biochemical characterization of subsets of lymphoid cells.

Radiation injury in defined populations of B cells was investigated utilizing an allotype congenic transfer system. The amount of donor immunoglobulin present in the recipient's serum was found to be directly proportional to the number of viable cells injected, and on this basis approximate D37 values for the inoculated B cells were determined in several experiments and found to fall in the 70 to 145 rad range depending upon the specific experimental conditions. Since the sensitivity of T cells to radiation-induced interphase death can be modified by prior exposure to select mitogens and antigens, an attempt was made to demonstrate a similar phenomenon with respect to B cells. The results indicate that the radiosensitivity of B cells can be slightly influenced by prior incubation with mitogens and only equivocally changed by activation with antigen.

Murine B cell lymphomas and myelomas were examined for the expression of a determinant previously found exclusively on normal pluripotent stem cells colony-forming unit-spleen (CFU-s). This determinant(s), which is defined by a rabbit antimouse brain antiserum (R alpha MB), is present on the tumor stem cell population of some but not all B cell neoplasms examined. The determinant is not detected on tumor cells of the macrophage or T cell lineage. Absorption of the activity in R alpha MB with myeloma cells, concomitantly removed reactivity with the normal stem cell, CFU-s, and the myeloma stem cell, plasmacytoma CFU-s. Sorting analysis further showed that the antigen was diminished within a positive tumor population as cells acquired the capacity to secrete immunoglobulin. These studies suggest that this normal stem cell-associated antigen may also be an early differentiation antigen for the B cell lineage, and is expressed on some stem cells of B cell tumors.

A microculture system for the in vitro induction of tumour-specific immunity with syngeneic spleen cells and plasma-cell tumours is described. The system generates cytotoxic lymphocytes assessed by their ability to lyse 51Cr-labelled target cells in a micrototoxicity assay. Both the inductive and effector phases are subject to many variables in materials and methodology and the optimal conditions are defined in this study. Of particular importance are the responder- to stimulator-cell ratios, the presence of 2-mercaptoethanol, the day of harvest, the method of target-cell 51Cr labelling, and preincubation of the cytotoxic lymphocytes prior to assay.

In semisolid agar cultures containing mercaptoethanol, cells from the spleen, lymph nodes, marrow, peritoneal cavity, thoracic duct, and blood of normal mice generated clusters and colonies of up to 3,000 cells. Colony numbers and growth were markedly enhanced by the addition of sheep red cells. The frequency of colony-forming cells in the spleen or lymph nodes was 0.5-2.0%, and cluster forming cells were approximately five times more numerous. The mononuclear cells comprising these colonies had the electronmicroscopic morphology of immature lymphoid and plasma cells. The majority of the cells possessed Fc receptors, 61-69% reacted with anti-mu-serum and 4-11% with anti-gamma2-serum. Analysis of single cells from individual colonies indicated a higher frequency of the cells with membrane immunoglobulin and a clonal pattern of anti-mu or anti-gamma-reactivity. The clonal nature of colonies was supported by an analysis of NIP-binding cells in colonies grown from CBA spleen cells enriched for NIP-binding cells. Mass-harvested colony cells synthesized immunoglobulin in short-term liquid cultures. It is concluded that the colonies are clones of functionally active B-lymphoid cells.

In this report we demonstrate that when the murine macrophage tumor cell line P- 388D1 is incubated for 48-72 h with either concanavalin A-stimulated rat spleen cell supernatant or cloned murine immune interferon (IFN-gamma), the cultured cells release a cell-free factor activity that in turn induces the cell surface expression of Ia antigen on the murine monocyte cell line WEHI-3. This IFN-gamma-stimulated, Ia-inducing activity cannot be blocked with an anti-IFN-gamma heteroantiserum that does block the induction of Ia expression on WEHI-3 by both cloned murine IFN-gamma and rat Con A supernatant. The Ia-inducing factor ( IaIF ) generated from P- 388D1 after stimulation by IFN-gamma does not demonstrate any antiviral activity. The P- 388D1 -derived IaIF is not shed plasma membrane Ia glycoprotein molecules, as demonstrated by the inability of the active component to bind specifically to an anti-I-Ad affinity column or to a protein A column after the active supernatant is first treated with an excess of anti-I-E/Cd,k monoclonal antibody.

The binding of Staph A protein to murine immunoglobulins has previously been thought to be restricted to the IgG2 and IgG3 classes. In this study five IgM proteins were assessed for binding, and of these one showed marked Staph A binding. Pepsin digestion of this IgM molecule produced several different sized fragments, and the binding studies with these fragments indicated that the binding site is in the CH2 domain.

Chicken embryos injected with pooled spleen cells from outbred embryos are tolerant to histocompatability antigens of the donor population. Tolerance can be shown by the failure of peripheral blood cells (PBC) from tolerant birds to produce graft versus host lesions on the chorioallantoic membrane of outbred chicken embryos. It is shown that PBC from tolerant chickens can reduce by up to 85% the GVH reactivity of PBC from normal chickens. These results are interpreted to indicate that the PBC of birds tolerant to allogeneic cells contain active suppressor cells that can reduce the activity of normal cells in their response to allogeneic stimulation. The implications of these results are discussed in relation to the pathogenesis of Marek's disease—a common herpes virus-induced lymphoid neoplasm of chickens. It is suggested that resistance to neoplasia in this disease occurs because of the establishment of a population of active suppressor cells that prevents a neoplastic lymphoid proliferation.

The presence of oncofetal antigens (OFA) on a wide variety of murine tumor cells was demonstrated to a totally in vitro system of cellular immunity. Nonimmune spleen lymphocytes were cocultivated with irradiated syngeneic fetal liver cells and, at various times after initiation of culture, were tested for the presence of cytotoxic lymphocytes (CL) by 51Cr-release assay with labeled tumor target cells. Significant cytotoxic activity was regularly detected after such culture, whereas only minor levels appeared in control cultures of spleen lymphocytes with irradiated syngeneic spleen cells. Specificity of the reaction was assessed by inhibition tests in which nonlabeled cells were admixed to the CL and 51Cr-labeled tumor targets. Fetal liver cells gave significant inhibition; however, no inhibition was found with adult spleen cells. Various tumor types gave inhibition, and fibrosarcomas were more effective than plasmacytomas or lymphomas. The results suggested that all tumor types tested possess such OFA, as well as their unique or virus-associated, tumor-associated transplantation antigens, and that the in vitro system permits a more active response to the tumor-associated OFA than that observed in in vivo studies.

A tumour-associated antigen has been identified which is expressed on both BALB/c plasmacytomas and T lymphomas, but which does not induce a detectable cell-mediated immune response in the strain of origin of the tumours. The (BALB/c x C57BL) F1 hybrid, however, makes a readily detectable response both in vivo and in vitro in vitro experiments with various BALB/c H-2 F1 cogenic mice indicated that the genetic control of the response was not H-2 linked, and was associated with other C57BL genes.

In previous studies with in vitro activated cytotoxic T lymphocytes, we have demonstrated the presence of oncofetal antigens (OFA) on a range of murine tumor cells. The present studies with the same tumor lines attempt to determine whether these antigens are also capable of activating lymphocyte responses in vivo. Several experimental designs were followed, each being performed many times (1). Preimmunization of mice with irradiated fetal liver cells and followed by challenge with viable tumor cells did not consistently produce a state of tumor resistance. However, injection of live fetal cells frequently led to enhanced tumor growth (2). The growth of tumors in multiparous mice can be inhibited, in contrast to controls, but this effect was relatively short lived after parturition (3). Preimmunization with fetal cells in vivo did not result in augmented secondary cytotoxic T cell responses in vitro (4). Immunization of mice with irradiated tumor cells frequently led to a state of resistance to the clonal growth of hemopoietic fetal cells, although again the level of resistance was usually relatively weak. From the overall results, we conclude that OFA are relatively poor immunogens on tumor cells or fetal cells in vivo, and in contrast to in vitro responses, do not act as potent tumor transplantation antigens.

Monoclonal antibodies directed against rat kappa light chains have been generated by immunizing SJL/J mice with soluble rat immunoglobulin, followed by fusion of immune spleen cells with the P3-X63-Ag8 myeloma cell line. Monoclonal antibodies from three of these hybridoma cell lines, MAR 18.5, 80.2, and 103.6, have been extensively characterized. MAR 18.5, 80.2, and 103.6 antibodies are of the gamma 2a kappa isotype, and bind strongly to protein A, allowing easy purification. Monoclonal antibody from clone 18.5 binds equally well to Ig of both RI-1a and RI-1b allotypes, whereas 80.2 and 103.6 antibodies selectively bind to RI-1b. These monoclonal antibodies can be FITC conjugated for use as a second antibody in indirect immunofluorescence assays, or radiolabeled for use in radio immunoassays requiring a specific antirat kappa antibody. The antiallotype specific monoclonal antibodies also may be of use in the study of rat immunoglobulin genetics.

Although the Lyt-1 antigen has previously been considered a T cell-specific marker, recent evidence suggests that a population of Thy-1-, Lyt-1+ cells exists in normal lymphoid tissues. In this study, we have observed that the WEHI-55, WEHI-259, and CH5 B cell lymphomas express high levels of the Lyt-1 antigen, as detected by monoclonal antibodies using the fluorescence activated cell sorter. Three other B cell lymphomas of the 11 examined also gave weak but detectable reactions with the anti Lyt-1 monoclonal antibody. Except for the expression of the Lyt-1 antigen, these lymphomas are typical of cells in the B cell lineage with respect to surface phenotype. The Lyt-1 glycoprotein immunoprecipitated from metabolically labeled WEHI-55 cells is similar in structure to the Lyt-1 glycoprotein on thymocytes. These findings are similar to recent reports that B-type human lymphocytic leukemia cells express the putative human homologue of Lyt-1, the Leu-1 antigen.

Tumor-associated antigens (TAA) were demonstrated on plasmacytomas derived from BALB/c, NZB, and C3H mouse strains, by in vivo and in vitro techniques. By immunizing the appropriate F1 hybrid mice with these tumors, it was possible to show that all the plasmacytomas expressed cross-reactive tumor-associated transplantation antigens. When cytotoxic lymphocytes (CL) were induced in vitro by the coculturing of syngeneic or F1 hybrid spleen cells with irradiated plasmacytoma cells, "shared" and "unique" plasmacytoma TAA were demonstrable. This was accomplished by inducing CL in vitro against one syngeneic plasmacytoma and assaying for lytic activity on a range of 51Cr-labeled BALB/c, NZB and C3H plasmacytoma cells in vitro. In a second in vitro assay, unlabeled plasmacytoma cells were tested for their ability to inhibit the lysis of a particular 51Cr-labeled plasmacytoma, with the use of CL induced in vitro against it. The possibility that these TAA were "self" antigens was excluded by demonstrating in the inhibition assay that they were not present on T lymphomas and spleen cells of the same strain, and that CL "autosensitized" in vitro could not significantly lyse 51Cr-labeled plasmacytoma cells in vitro. From both in vivo and in vitro studies of immunity to these tumors, it was concluded that any one plasmacytoma line possesses multiple TAA of both shared and unique types.

A comparison of the growth of a number of BALB/c tumours in BALB/c, BALB/c.nu/ + (heterozygous nude) and BALB/c.nu (homozygous) nude mice has shown that a majority of BALB/c lymphoid tumours grow at a significantly reduced rate in BALB/c.nu mice. The three non-lymphoid BALB/c tumours (carcinomas and sarcomas) tested and a minority of lymphoid tumours, however, grew as well, or better, in the BALB/c.nu mice. Furthermore a correlation, without exception to date, has emerged from a comparison between the susceptibility of a tumour cell line to in vitro lysis by spleen cells from syngeneic nude mice and its growth in vivo in syngeneic nude mice. Only those lymphoid tumour cell lines lysed in vitro show the reduced growth rate in syngeneic nude as compared to syngeneic normal mice. Tumour cell lines, both lymphoid and non-lymphoid, which are resistant to in vitro lysis do not show the reduced in vivo growth rate in syngeneic nude mice. Both the in vivo and in vitro effect can be partially abolished by irradiating the nude mice prior to use, suggesting that a radiation-sensitive non-T-cell surveillance of lymphoid tumours is operating in nude mice. While the identity of the antigens is still unknown, the specificity of the phenomena suggest that C-type RNA oncogenic viral determined antigens are involved.

Binding of antibody to the surface of B lymphocytes was shown to involve the Fc piece of the immunoglobulin molecule. This property was not shared equally by all immunoglobulin classes as revealed by direct binding and inhibition studies. Total IgG globulin was found to label cells more heavily than IgM, and IgG1-containing fractions more heavily than IgG2 fractions lacking IgG1. The ability of various purified myeloma proteins to inhibit attachment of antibody to B cells was examined. Pretreatment of B cells with excess IgG2a, IgA, or light chain proteins failed to inhibit, whereas IgG1 proteins and to a lesser extent Ig2b and IgM proteins at the same concentrations did so. At lower protein concentrations, IgG1 myeloma protein alone retained the capacity to inhibit binding. The conclusion was reached that the receptor on B cells for antibody has a marked predilection for the IgG1 class. Although IgM and IgG2b antibody may bind, they do so with lower avidity and probably in insignificant amounts if IgG1 antibody is present in excess. No evidence was found to implicate complement in the binding process. For example, heat-inactivated sera at high dilution retained the ability to label B cells, while the use of purified low molecular weight anticomplementary factor, a potent inhibitor of C'3, did not interfere with the formation of the bond between antibody and cell surface. The failure of anti-mouse immunoglobulin F(ab)'2 fragments to prevent access of antibody to B cells implied that the antibody-binding receptor and antigen-binding (immunoglobulin) receptor are discrete entities on the B cell membrane. Despite this, a marked similarity between their surface distribution was observed on electron microscopy. The antibody-binding receptor was shown to be a marker for mature B cells. It did not appear to be present on hematopoietic precursor stem cells and was lost during differentiation to antibody-forming cells.

SummaryThe NZC strain of mouse is known to have a defect involving the transplantation of hematopoietic stem cells. Attempts to test the immune system of this strain show a normal antibody response in vivo. However, in adoptive transfer systems, NZC cells were shown to be minimally responsive on transfer, although (Balb X NZB) F1 cells could respond in the NZC environment. In vitro studies of the response of NZC cells to SRBC, phytohemagglutinin and pokeweed all show reduced responses. Evidence is presented which suggests that there is a defect in the number of progenitor cells for the immune system, possibly involving only one component cell population.We are grateful to Dr. Malcolm Moore for valuable discussions in the course of this work, and to Mrs. D. Barr, and Misses G. Cousins and P. Bell for competent technical assistance.

Nineteen different radiation-induced thymomas originating in BALB/c, NZB and NZW mouse strains were tested for Ig synthesis by short-term culture with 14C-amino acids followed by radioimmunoelectrophoresis. Twelve were negative, six produced light (L) chains and one synthesized a protein similar to IgM. The IgM producer, WEHI-22, and one of the L chain producers, WEHI-105, were established as cloned cultured cell lines and shown to possess the lymphoid cell characteristics of susceptibility to inhibition by low concentrations of thymidine and of cortisol. Both were also shown to bear the θ antigen, a thymus-derived (T) lymphocyte marker. Cells of the WEHI-22 line also possessed surface immunoglobulin readily detectable by binding of labeled anti-Ig antibody and were able to bind mouse antibody-coated sheep erythrocytes forming rosettes, while WEHI-105 did neither. The specificity of the Ig receptor mediating rosette formation appeared from myeloma protein competition experiments to be very similar to that of the receptor described for non-thymus-derived (B) cells. It was considered that WEHI-22 cells either possess a mixture of T and B cell characteristics or are neoplastic variants of T cells possessing surface IgM with antiglobulin activity.

Addition of 125I-labeled anti-immunoglobulin protein to cell suspensions from mouse thymus, spleen and thoracic duct lymph (TDL) resulted in labeling of surface immunoglobulins of lymphoid-like cells. Near-optimal conditions for cell labeling were established with varying anti-immunoglobulin concentrations and times of exposure. On the average 20.1% to 36.3% of TDL and spleen cells labeled under such conditions, whereas thymus cells labeled less heavily (0.3% to 3.7%) with identical materials. Normal rabbit sera (NRS) or anti-immunoglobulin absorbed with purified immunoglobulins produced essentially no labeling. The total heavy chain labeling on TDL and spleen cells exceeded light chain labeling (64.6% vs 31.8% and 79.8% vs 33.0%, respectively). This might suggest that labeled cells had multiple surface heavy chain determinants or that the anti-heavy chain antibodies cross-reacted with an unidentified immunoglobulin-like surface antigen. There was increased labeling with TDL suspensions from neonatally thymectomized mice (65% with anti-light chain). This was consistent with the hypothesis that primarily non-thymus-derived cells were labeled by these techniques.

Rabbit antisera against intact sheep erythrocytes (SRBC) were assayed by hemolysis and PCA. In one serum, antibodies mediating both PCA and hemolysis were present, whereas another serum contained mainly hemolytic antibodies. The PCA reaction could be developed by intravenous challenge with either fresh SRBC or their lysate, but not with stroma or formalin-fixed SRBC. Both stroma and formalin-fixed cells could, however, bind the hemolytic antibodies, which the lysate failed to do. The results indicate that one of the anti-SRBC serum contained antibodies against stromal antigens, which could be detected only by direct red cell binding assays (hemolysis or hemagglutination), and antibodies against a soluble internal antigen, possibly hemoglobin, which could be detected only by PCA reactions.