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To be added., Immunoglobulin heavy chain (IgH) class-switch recombination (CSR) replaces initially expressed Cμ (IgM) constant regions (CH) exons with downstream CH exons. Stimulation of B cells with anti-CD40 plus interleukin-4 induces CSR from Cμ to Cγ1 (IgG1) and Cε (IgE), the latter of which contributes to the pathogenesis of atopic diseases. Although Cε CSR can occur directly from Cμ, most mature peripheral B cells undergo CSR to Cε indirectly, namely from Cμ to Cγ1, and subsequently to Cε. Physiological mechanisms that influence CSR to Cγ1 versus Cε are incompletely understood. In this study, we report a role for B cell developmental maturity in IgE CSR. Based in part on a novel flow cytometric IgE CSR assay, we show that immature B cells preferentially switch to IgE versus IgG1 through a mechanism involving increased direct CSR from Cμ to Cε. Our findings suggest that IgE dysregulation in certain immunodeficiencies may be related to impaired B cell maturation.

CIN85 transduces B cell receptor signals to IKK-β, and its expression in B cells is essential for T cell–independent type II antibody responses in mice., CIN85, an adaptor protein which binds the C-terminal domain of tyrosine phosphorylated Cbl and Cbl-b, has been thought to be involved in the internalization and subsequent degradation of receptors. However, its physiological function remains unclear. To determine its role in B cells, we used Mb1-cre to generate mice with a B cell–specific deletion of CIN85. These mice had impaired T cell–independent type II antibody responses in vivo and diminished IKK-β activation and cellular responses to B cell receptor (BCR) cross-linking in vitro. Introduction of a constitutively active IKK-β construct corrected the defective antibody responses as well as cellular responses in the mutant mice. Together, our results suggest that CIN85 links the BCR to IKK-β activation, thereby contributing to T cell–independent immune responses.

Resting antigen-experienced memory B cells are thought to be responsible for the more rapid and robust antibody responses after antigen reencounter, which are the hallmark of memory humoral responses. The molecular basis for the development and survival of memory B cells remains largely unknown. We report that phospholipase C (PLC) γ2 is required for efficient formation of germinal center (GC) and memory B cells. Moreover, memory B cell homeostasis is severely hampered by inducible loss of PLC-γ2. Accordingly, mice with a conditional deletion of PLC-γ2 in post-GC B cells had an almost complete abrogation of the secondary antibody response. Collectively, our data suggest that PLC-γ2 conveys a survival signal to GC and memory B cells and that this signal is required for a productive secondary immune response.

Tyrosine and glycine constitute 40% of complementarity determining region 3 of the immunoglobulin heavy chain (CDR-H3), the center of the classic antigen-binding site. To assess the role of DH RF1-encoded tyrosine and glycine in regulating CDR-H3 content and potentially influencing B cell function, we created mice limited to a single DH encoding asparagine, histidine, and arginines in RF1. Tyrosine and glycine content in CDR-H3 was halved. Bone marrow and spleen mature B cell and peritoneal cavity B-1 cell numbers were also halved, whereas marginal zone B cell numbers increased. Serum immunoglobulin G subclass levels and antibody titers to T-dependent and T-independent antigens all declined. Thus, violation of the conserved preference for tyrosine and glycine in DH RF1 alters CDR-H3 content and impairs B cell development and antibody production.

Dicer is an RNaseIII-like enzyme that is required for generating short interfering RNAs and microRNAs. The latter have been implicated in regulating cell fate determination in invertebrates and vertebrates. To test the requirement for Dicer in cell-lineage decisions in a mammalian organism, we have generated a conditional allele of dicer-1 (dcr-1) in the mouse. Specific deletion of dcr-1 in the T cell lineage resulted in impaired T cell development and aberrant T helper cell differentiation and cytokine production. A severe block in peripheral CD8+ T cell development was observed upon dcr-1 deletion in the thymus. However, Dicer-deficient CD4+ T cells, although reduced in numbers, were viable and could be analyzed further. These cells were defective in microRNA processing, and upon stimulation they proliferated poorly and underwent increased apoptosis. Independent of their proliferation defect, Dicer-deficient helper T cells preferentially expressed interferon-γ, the hallmark effector cytokine of the Th1 lineage.

V(D)J recombination at the immunoglobulin heavy chain (IgH) locus follows the 12/23 rule to ensure the correct assembly of the variable region gene segments. Here, we report characterization of an in vivo model that allowed us to study recombination violating the 12/23 rule, namely a mouse strain lacking canonical D elements in its IgH locus. We demonstrate that VH to JH joining can support the generation of all B cell subsets. However, the process is inefficient in that B cells and antibodies derived from the DH-less allele are not detectable if the latter is combined with a wild-type IgH allele. There is no preferential usage of any particular VH gene family or JH element in VHJH junctions, indicating that 23/23-guided recombination is possible, but is a low frequency event at the IgH locus in vivo.

Interleukin (IL)-10 is a regulator of inflammatory responses and is secreted by a variety of different cell types including T cells. T regulatory cells have been shown to suppress immune responses by IL-10–dependent, but also IL-10–independent, mechanisms. Herein, we address the role of T cell–derived IL-10 in mice with an inactivation of the IL-10 gene restricted to T cells generated by Cre/loxP-mediated targeting of the IL-10 gene. Splenocytes from this T cell–specific mutant secrete increased amounts of proinflammatory cytokines after activation in vitro compared with show enhanced contact hypersensitivity reactions, and succumb to severe immunopathology upon infection with Toxoplasma gondii. Despite intact IL-10 genes in other cell types, the dysregulation of T cell responses observed in the T cell–specific IL-10 mutant closely resembles the phenotype in complete IL-10 deficiency. However, in contrast to complete IL-10 deficiency, sensitivity to endotoxic shock and irritant responses of the skin are not enhanced in the T cell–specific IL-10 mutant. Our data highlight the importance of T cell–derived IL-10 in the regulation of T cell responses and demonstrate that endotoxic shock and the irritant response of the skin are controlled by IL-10 from other cell types.

B cell receptor (BCR) signaling is mediated through immunoglobulin (Ig)alpha and Igbeta a membrane-bound heterodimer. Igalpha and Igbeta are redundant in their ability to support early B cell development, but their roles in mature B cells have not been defined. To examine the function of Igalpha-Igbeta in mature B cells in vivo we exchanged the cytoplasmic domain of Igalpha for the cytoplasmic domain of Igbeta by gene targeting (Igbetac--alphac mice). Igbetac--alphac B cells had lower levels of surface IgM and higher levels of BCR internalization than wild-type B cells. The mutant B cells were able to complete all stages of development and were long lived, but failed to differentiate into B1a cells. In addition, Igbetac--alphac B cells showed decreased proliferative and Ca2+ responses to BCR stimulation in vitro, and were anergic to T-independent and -dependent antigens in vivo.

Immunoglobulin light chain (IgL) rearrangements occur more frequently at Ig kappa than at Ig lambda. Previous results suggested that the unrearranged Ig kappa locus negatively regulates Ig lambda transcription and/or rearrangement. Here, we demonstrate that expression of a VJ lambda 1-joint inserted into its physiological position in the Ig lambda locus is independent of Ig kappa rearrangements. Expression of the inserted VJ lambda 1 gene segment is developmentally controlled like that of a VJ kappa-joint inserted into the Ig kappa locus and furthermore coincides developmentally with the occurrence of Ig kappa rearrangements in wild-type mice. We conclude that developmentally controlled transcription of a gene rearrangement in the Ig lambda locus occurs in the presence of an unrearranged Ig kappa locus and is therefore not negatively regulated by the latter. Our data also indicate light chain editing in approximately 30% of lambda 1 expressing B cell progenitors.

To study homeostasis of peripheral B lymphocytes in the absence of B cell influx from the bone marrow, we generated a mouse mutant in which the recombination-activating gene (RAG)-2 can be inducibly deleted. When RAG-2 was deleted at the age of 8-10 wk, splenic naive follicular B cells were gradually lost over a year of observation, with a half-life of approximately 4.5 mo. By contrast, the pool of marginal zone B cells in the spleen and of B-1 cells in the peritoneal cavity were kept at normal level. In lymph nodes, approximately 90% of the B cells were lost within 4 mo, and B cell numbers remained constant thereafter. Mice in which RAG-2 was deleted at birth maintained a small population of activated B cells with an increased proportion of marginal zone B cells. Additionally, an increase of the pool of IgM secreting cells and B-1a cells was observed.

To determine the function of immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation, we generated mice in which Igalpha ITAM tyrosines were replaced by phenylalanines (Igalpha(FF/FF)). Igalpha(FF/FF) mice had a specific reduction of B1 and marginal zone B cells, whereas B2 cell development appeared to be normal, except that lambda1 light chain usage was increased. The mutants responded less efficiently to T cell-dependent antigens, whereas T cell-independent responses were unaffected. Upon B cell receptor ligation, the cells exhibited heightened calcium flux, weaker Lyn and Syk tyrosine phosphorylation, and phosphorylation of Igalpha non-ITAM tyrosines. Strikingly, when the Igalpha ITAM mutation was combined with a truncation of Igbeta, B cell development was completely blocked at the pro-B cell stage, indicating a crucial role of ITAM phosphorylation in B cell development.

The B cell receptor (BCR) regulates B cell development and function through immunoglobulin (Ig)alpha and Ig beta, a pair of membrane-bound Ig superfamily proteins, each of which contains a single cytoplasmic immunoreceptor tyrosine activation motif (ITAM). To determine the function of Ig beta, we produced mice that carry a deletion of the cytoplasmic domain of Ig beta (Ig beta Delta C mice) and compared them to mice that carry a similar mutation in Ig alpha (MB1 Delta C, herein referred to as Ig alpha Delta C mice). Ig beta Delta C mice differ from Ig alpha Delta C mice in that they show little impairment in early B cell development and they produce immature B cells that respond normally to BCR cross-linking as determined by Ca(2+) flux. However, Ig beta Delta C B cells are arrested at the immature stage of B cell development in the bone marrow and die by apoptosis. We conclude that the cytoplasmic domain Ig beta is required for B cell development beyond the immature B cell stage and that Ig alpha and Ig beta have distinct biologic activities in vivo.

Clonal composition and T cell receptor (TCR) repertoire of CD4(+) and CD8(+) T cells infiltrating actively demyelinating multiple sclerosis (MS) lesions were determined with unprecedented resolution at the level of single cells. Individual CD4(+) or CD8(+) T cells were isolated from frozen sections of lesional tissue by micromanipulation and subjected to single target amplification of TCR-beta gene rearrangements. This strategy allows the assignment of a TCR variable region (V region) sequence to the particular T cell from which it was amplified. Sequence analysis revealed that in both cases investigated, the majority of CD8(+) T cells belonged to few clones. One of these clones accounted for 35% of CD8(+) T cells in case 1. V region sequence comparison revealed signs of selection for common peptide specificities for some of the CD8(+) T cells in case 1. In both cases, the CD4(+) T cell population was more heterogeneous. Most CD4(+) and CD8(+) clones were represented in perivascular infiltrates as well as among parenchymal T cells. In case 2, two of the CD8(+) clones identified in brain tissue were also detected in peripheral blood. Investigation of the antigenic specificities of expanded clones may help to elucidate their functional properties.

Recent work identified Hodgkin and Reed-Sternberg (H/RS) cells in classical Hodgkin's disease (cHD) as clonal progeny of mature B cells. Therefore, it is generally assumed that cHD homogenously represents a B cell lymphoma. In a subset of cHD, however, H/RS cells expressing T cell–associated proteins may be candidates for alternative lineage derivation. Single H/RS cells with cytotoxic T cell phenotype were micromanipulated from three cases of cHD and analyzed by single cell polymerase chain reaction for immunoglobulin heavy (IgH) and light chain (IgL) gene rearrangements, T cell receptor (TCR)-β gene rearrangements, and germline configuration of the IgH and TCR-β loci. H/RS cells from two cases of cHD harbored clonal, somatically mutated Ig gene rearrangements, whereas TCR-β loci were in germline configuration. In contrast, H/RS cells from an additional case harbored clonal TCR-β variable/diversity/joining (VDJ) and DJ gene rearrangements, whereas the IgH locus was in germline configuration on both alleles. Thus, in two cases of cHD with H/RS cells expressing cytotoxic T cell molecules, the tumor cells are derived from mature B cells that aberrantly express T cell markers. In a third case, however, H/RS cells were derived from a T cell, demonstrating that cHD can also occur as a T cell lymphoma.

In mouse mutants incapable of expressing mu chains, VkappaJkappa joints are detected in the CD43(+) B cell progenitors. In agreement with these earlier results, we show by a molecular single cell analysis that 4-7% of CD43(+) B cell progenitors in wild-type mice rearrange immunoglobulin (Ig)kappa genes before the assembly of a productive VHDHJH joint. Thus, mu chain expression is not a prerequisite to Igkappa light chain gene rearrangements in normal development. Overall, approximately 15% of the total CD43(+) B cell progenitor population carry Igkappa gene rearrangements in wild-type mice. Together with the results obtained in the mouse mutants, these data fit a model in which CD43(+) progenitors rearrange IgH and Igkappa loci independently, with a seven times higher frequency in the former. In addition, we show that in B cell progenitors VkappaJkappa joining rapidly initiates kappa chain expression, irrespective of the presence of a mu chain.

The pleiotrophic but overlapping functions of the cytokine family that includes interleukin (IL)-6, IL-11, leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and cardiotrophin 1 are mediated by the cytokine receptor subunit gp130 as the common signal transducer. Although mice lacking individual members of this family display only mild phenotypes, animals lacking gp130 are not viable. To assess the collective role of this cytokine family, we inducibly inactivated gp130 via Cre-loxP–mediated recombination in vivo. Such conditional mutant mice exhibited neurological, cardiac, hematopoietic, immunological, hepatic, and pulmonary defects, demonstrating the widespread importance of gp130-dependent cytokines.

Immunoglobulin (Ig) class switching in B cells is regulated by stimuli transduced by cytokines and cell-cell contact. Among these stimuli, interleukin (IL)-4 has been considered an absolute prerequisite for class switching to IgE in the mouse. Here we report that IL-4-deficient (IL-4-/-) and wildtype mice had comparably elevated serum IgE levels during the course of a murine retrovirus-induced immunodeficiency syndrome, MAIDS. IgE switching in IL-4-/- mice was also induced by injection of anti-IgD antibody. Treatment with anti-IgD induced germline epsilon (g epsilon) transcripts with comparable efficiency in IL-4-/- mice and controls, but the levels of productive epsilon transcripts (p epsilon) were lower by a factor of 200 and serum IgE levels were lower by a factor of 300 in IL-4-/- mice as compared with controls. Induction of g epsilon after anti-IgD treatment of IL-4-/- mice was unaffected by simultaneous treatment with monoclonal antibodies to IL-4 and IL-4 receptor alpha chain. Infection of IL-4-/- mice with Nippostrongylus brasiliensis, a potent stimulus for IgE production, resulted in induction of g epsilon transcripts; however, p epsilon transcripts were barely detectable and serum IgE was not detected. These findings establish a novel IL-4-independent pathway for IgE switching in the mouse that is strongly activated in retroviral infection but weakly in nematode infection. This pathway appears to be dependent on distinct factors that separately control induction of g epsilon transcription and switch recombination to p epsilon.

We have examined the role of endogenously produced interleukin (IL) 4 and IL-10 in the regulation of inflammatory and immune reactions in the skin. In these experiments, irritant and contact hypersensitivity (CH) responses were elicited in mice with targeted disruptions of the IL-4 (IL-4T) or IL-10 (IL-10T) gene. Our study showed that IL-4T and wild-type (wt) mice exhibited equivalent responses to the irritant croton oil. In contrast, the response of IL-10T mice challenged with croton oil was abnormally increased. When IL-10T mice were exposed to a higher dose of irritant, irreversible tissue damage occurred. By comparison, any treatment of wt mice with croton oil resulted in far less tissue damage and resolution of inflammation. Neutralizing antibody studies demonstrated that the necrosis that occurred in IL-10T mice was due to the overproduction of tumor necrosis factor. The anti-tumor necrosis factor antibody treatment of IL-10T mice did not significantly reduce the edema or the influx of inflammatory cells, suggesting that these changes were due to the uncontrolled production of other proinflammatory cytokines. T cell-dependent immune responses were also evaluated using the contact sensitizer oxazolone. The response of IL-4T mice did not differ from wt mice. In contrast, IL-10T mice mounted an exaggerated CH response, increased in both magnitude and duration as compared with wt mice. Based on these studies, we have concluded that IL-10, but not IL-4, is a natural suppressant of irritant responses and of CH, and it limits immunopathologic damage in the skin.

The miR-23∼27∼24 clusters control differentiation of effector T cells. In particular, miR-24 targets IL-4 and miR-27 targets GATA3, thus collaborating in the control of Th2 immunity., Coordinated repression of gene expression by evolutionarily conserved microRNA (miRNA) clusters and paralogs ensures that miRNAs efficiently exert their biological impact. Combining both loss- and gain-of-function genetic approaches, we show that the miR-23∼27∼24 clusters regulate multiple aspects of T cell biology, particularly helper T (Th) 2 immunity. Low expression of this miRNA family confers proper effector T cell function at both physiological and pathological settings. Further studies in T cells with exaggerated regulation by individual members of the miR-23∼27∼24 clusters revealed that miR-24 and miR-27 collaboratively limit Th2 responses through targeting IL-4 and GATA3 in both direct and indirect manners. Intriguingly, although overexpression of the entire miR-23 cluster also negatively impacts other Th lineages, enforced expression of miR-24, in contrast to miR-23 and miR-27, actually promotes the differentiation of Th1, Th17, and induced regulatory T cells, implying that under certain conditions, miRNA families can fine tune the biological effects of their regulation by having individual members antagonize rather than cooperate with each other. Together, our results identify a miRNA family with important immunological roles and suggest that tight regulation of miR-23∼27∼24 clusters in T cells is required to maintain optimal effector function and to prevent aberrant immune responses.

Tonsillar germinal center and immunoglobulin M+ (IgM+)IgD+ B cells as well as peripheral blood (PB) CD5+ and CD5- (conventional) B cells from a 4-yr-old child were isolated and nucleotide sequences of expressed Ig heavy chain variable regions encoded by VH4 gene family members were determined from amplified cDNA. Whereas both tonsillar IgM+IgD+ cells and the majority of IgM-expressing CD5+ and CD5- PB B cells showed no or little somatic mutation, tonsillar germinal center (GC) B cells and IgG-expressing PB B cells carried a high load of somatic mutations in their V region genes. This suggests that somatically mutated memory B cells which have switched isotype accumulate in the PB already at young age. Their frequency seems to increase with age. On the other hand, the antibody repertoire of tonsillar IgM+IgD+ B cells and the majority of IgM-expressing PB B cells is determined by germline-encoded specificities and by generation of variability in the complementary determining region III through VH-DH-JH recombination. A fraction of IgM-bearing PB B cells carries somatically mutated V region genes and probably represents GC-derived B cells which have left the GC at an early stage of the GC reaction without undergoing isotype switching. 10 VH4 germline genes were found to be expressed. Three gene segments were overrepresented in the sequence collection (35 of 50 clones): VH4.21 (30%), V71-4 (20%), and 3D279D (20%). It appears that most potentially functional VH4 germline genes are expressed in peripheral B cells. Some members of this VH gene family are clearly overrepresented over others.

Murine mammary tumor viruses (MMTVs) are retroviruses that encode superantigens capable of stimulating T cells via superantigen-reactive T cell receptor V beta chains. MMTVs are transmitted to the suckling offspring through milk. Here we show that B cell-deficient mice foster nursed by virus-secreting mice do not transfer infectious MMTVs to their offspring. No MMTV proviruses could be detected in the spleen and mammary tissue of these mice, and no deletion of MMTV superantigen-reactive T cells occurred. By contrast, T cell deletion and positive selection due to endogenous MMTV superantigens occurred in B cell-deficient mice. We conclude that B cells are essential for the completion of the viral life cycle in vivo, but that endogenous MMTV superantigens can be presented by cell types other than B cells.

To assess the role of immunoglobulin D (IgD) in vivo we generated IgD-deficient mice by gene targeting and studied B cell development and function in the absence of IgD expression. In the mutant animals, conventional and CD5-positive (B1) B cells are present in normal numbers, and the expression of the surface markers CD22 and CD23 in the compartment of conventional B cells indicates acquisition of a mature phenotype. As in wild-type animals, most of the peripheral B cells are resting cells. The IgD-deficient mice respond well to T cell-independent and -dependent antigens. However, in heterozygous mutant animals, B cells expressing the wild type IgH locus are overrepresented in the peripheral B cell pool, and T cell-dependent IgG1 responses are further dominated by B cells expressing the wild-type allele. Similarly, in homozygous mutant (IgD-deficient) animals, affinity maturation is delayed in the early primary response compared to control animals, although the mutants are capable of generating high affinity B cell memory. Thus, rather than being involved in major regulatory processes as had been suggested, IgD seems to function as an antigen receptor optimized for efficient recruitment of B cells into antigen-driven responses. The IgD-mediated acceleration of affinity maturation in the early phase of the T cell-dependent primary response may confer to the animal a critical advantage in the defense against pathogens.

The anti-(4-hydroxy-3-nitro-phenyl)acetyl (NP) response is dominated by lambda 1 chain-bearing antibodies expressing the VH gene V186.2 in combination with the D element DFL16.1. lambda 1-positive B cells were isolated from the spleens of mice immunized with NP-chicken gamma globulin 6 wk earlier. Rearranged V186.2 genes were amplified from the genomic DNA of these cells and sequenced. In cases where the rearrangement was typical for secondary anti-NP antibodies, the VHDJH sequences were generally heavily mutated. The frequency and the nature of the nucleotide exchanges mirrored those of secondary response antibodies. V186.2 genes with other rearrangements and V186.2-related genes isolated concomitantly were essentially unmutated. These results demonstrate: (a) that somatic antibody mutants are largely restricted to a small compartment of peripheral B cells, namely, that of memory cells; (b) that the memory compartment is strongly selected for high affinity precursors and largely purged from antigen-binding loss mutants; and (c) that the repertoire of binding specificities expressed in the secondary response is established in its final form before secondary immunization.

Immunoglobulin (Ig) heavy chain (HC) class switch recombination (CSR) is a late B cell process that involves intrachromosomal DNA rearrangement. Ku70 and Ku80 form a DNA end-binding complex required for DNA double strand break repair and V(D)J recombination. Ku70−/− (K70T) mice, like recombination activating gene (RAG)-1– or RAG-2–deficient (R1T or R2T) mice, have impaired B and T cell development at an early progenitor stage, which is thought to result at least in part from defective V(D)J recombination (Gu, Y., K.J. Seidl, G.A. Rathbun, C. Zhu, J.P. Manis, N. van der Stoep, L. Davidson, H.L. Cheng, J.M. Sekiguchi, K. Frank, et al. 1997. Immunity. 7:653–665; Ouyang, H., A. Nussenzweig, A. Kurimasa, V.C. Soares, X. Li, C. Cordon-Cardo, W. Li, N. Cheong, M. Nussenzweig, G. Iliakis, et al. 1997. J. Exp. Med. 186:921–929). Therefore, to examine the potential role of Ku70 in CSR, we generated K70T mice that carry a germline Ig HC locus in which the JH region was replaced with a functionally rearranged VH(D)JH and Ig λ light chain transgene (referred to as K70T/HL mice). Previously, we have shown that B cells from R1T or R2T mice carrying these rearranged Ig genes (R1T/HL or R2T/HL mice) can undergo CSR to IgG isotypes (Lansford, R., J. Manis, E. Sonoda, K. Rajewsky, and F. Alt. 1998. Int. Immunol. 10:325–332). K70T/HL mice had significant numbers of peripheral surface IgM+ B cells, which generated serum IgM levels similar to those of R2T/HL mice. However, in contrast to R2T/HL mice, K70T/HL mice had no detectable serum IgG isotypes. In vitro culture of K70T/HL B cells with agents that induce CSR in normal or R2T/HL B cells did lead to the induction of germline CH transcripts, indicating that initial signaling pathways for CSR were intact in K70T/HL cells. However, treatment with such agents did not lead to detectable CSR by K70T/HL B cells, and instead, led to cell death within 72 h. We conclude that Ku70 is required for the generation of B cells that have undergone Ig HC class switching. Potential roles for Ku70 in the CSR process are discussed.

Immunization with an IgG1 but not an IgM monoclonal anti-NP (4-hydroxy-3-nitrophenyl acetyl) antibody induced idiotype-recognizing T helper cells, although these two antibodies carry the same variable regions. The T cells appear to react to an idiotype on the IgG1 but not the IgM antibody. They selectively enhance the expression of that idiotype in the IgG1 fraction of an in vitro anti-NP response.

Hapten-binding receptor material was isolated from sensitized rabbit lymphocytes by a method described previously for murine receptor material. The material was separated into a fraction expressing immunoglobulin determinants (anti-Ig+ fraction) and a fraction lacking known class and type-specific determinants of Ig constant domains (anti-Ig- fraction). We present evidence in support of the notion that--in analogy to the mouse system--the anti-Ig+ fraction is B-cell-derived, whereas the anti-Ig- fraction originates from T lymphocytes. Receptors of the anti-Ig- phenotype are found to express a locus allotypic determinants and, thus, appear to carry variable portions of immunoglobulin heavy chains.

When the IgG1 fraction of anti-idiotypic antibodies raised in guinea pigs is injected into mice, sensitization of idiotypic T and B lymphocytes occurs (1-3). In the present study we analyze the genetic requirements for T-helper cell sensitization by anti-idiotypic antibody. This was done by measuring, in a suitable panel of mouse strains, helper cell responsiveness to two anti-idiotypic reagents which recognize distinct, strain-specific idiotypes, namely the A5A and the S117 marker. Whenever helper cell sensitization by anti-idiotypic antibody was successful, helper function could be specifically inhibited by the same and only the same anti-idiotype. This indicates that helper cells induced by anti-idiotypic antibody express idiotypic determinants on their receptors for antigen. Helper cell sensitization by anti-idiotypic antibody was found in all strains expressing the corresponding or a cross-reactive idiotype at the immunoglobulin level. Idiotype-negative strains were always unresponsive to anti-idiotypic stimulation. In addition, responsiveness did not depend on the H-2 haplotype. Since the A5A and the S117 idiotype are markers for V genes in the heavy-chain linkage group, the present results support the view that the same genes in the Ig-1 complex code for variable portions of immunoglobulins and T-helper cell receptors.

Expression vectors coding for membrane-bound IgM antibodies were introduced into myeloma and B lymphoma cells. Only the lymphoma but not the myeloma cells were able to express the antibodies on the cell surface, although in both cases, complete antibodies were assembled intracellularly. In myeloma cells, the Ig molecules did not reach the Golgi compartment. Thus, the intracellular transport of membrane-bound antibodies is controlled in the B cell lineages in a developmentally ordered fashion.

19S and 7S memory is analyzed in a system of cooperating antigenic determinants. Cooperation occurs in the induction of both 19S and 7S secondary antibodies, and for both responses carrier specificity can be entirely accounted for by presensitization of the animal to carrier determinants. The class distribution of secondary anti-hapten antibody depends on the dose of the hapten primary-carrier conjugate used for priming, and on the time interval between priming with the hapten primary-carrier conjugate and secondary injection. The conditions of priming with the secondary carrier influence the extent of the secondary response but not the class distribution of secondary antibody. The data confirm the cooperation hypothesis of antibody induction. Specifically, we interpret them to mean that in hapten-carrier cooperation, the hapten-specific memory cells are predetermined for the class of the emerging antibodies. Together with the hapten-specific memory cells, the carrier-specific helpers are responsible for the extent of the secondary response.