Search

Humans live in a sea of microbes that are essential to health. Occasionally, commensal microbes are replaced by pathogens, leading to disease. Immune systems evolved primarily to defend against such attacks. Human immunity has two primary constituents: the innate and adaptive immune systems. Innate systems are found widely in multicellular species, whereas the adaptive system is a feature of vertebrate evolution. Innate immunity is largely based on recognition of molecules common in microbes, but not present in their hosts. The adaptive immune system is based on distinction between “self” and foreign molecules (antigens). Lymphocytes of adaptive immunity are of two primary types, T cells and B cells, which display specific antigen receptors: T-cell receptors on T cells and immunoglobulins on B cells. T cells are selected during thymic maturation to distinguish self from nonself molecules. B cells secrete immunoglobulins as antibodies, whereas T cells secrete molecules (cytokines) that can amplify or inhibit inflammatory responses. Other T-cell responses involve cell-to-cell contact to kill target cells or to promote antibody production. Immunological memory, a distinguishing feature of adaptive immunity, represents expansion of clones of lymphocytes with particular antigen-binding specificity such that subsequent encounter results in a greater and more rapid response. Immune system diseases include deficiencies, which increase susceptibility to infection, and physiological or dysregulated inflammatory responses, including autoimmunity.

MHC class I molecules assemble within the endoplasmic reticulum (ER) in complexes that include beta2-microglobulin (beta(2)m), the transporter associated with antigen processing (TAP)and several additional chaperones. Release of class I complexes from the ER is thought to require the binding of an appropriate endogenous peptide, predominantly delivered from the cytosol to the ER by TAP. It was recently demonstrated that exogenous synthetic peptide could 'directly' enter the ER of intact cells, independently of TAP function, and bind to the class I molecule H-2K(b).In TAP-deficient cells, we show that nascent K(b) or K(b)-L(d) chimeric molecules have a high trafficking background; 50-80% of these class I molecules are released from the ER independently of TAP function or the addition of exogenous peptide. The addition of exogenous K(b) cognate peptides enhanced the release of these class I molecules only slightly over the high background. The chimeric class I-b molecule, M3-L(d), differs from K(b)-L(d) only in its peptide binding domains, and M3-L(d) preferentially binds N-formylated peptides, which are rare in eukaryotic cells. Release of M3-L(d) from the ER in the absence of exogenous peptide was negligible. Addition of exogenous formylated peptides induced significant trafficking and surface expression of M3-L(d). These observations suggest that peptide binding is necessary for class I release from the ER even in TAP-deficient cells. These results demonstrate that exogenous peptide not only enters the ER of intact cells independently of TAP but also functionally induces class I antigen presentation.

Human gammadelta T cells expressing the Vgamma2Vdelta2 antigen receptors recognize nonpeptide prenyl pyrophosphate and alkylamine antigens. We find that they also recognize staphylococcal enterotoxin A superantigens in a manner distinct from the recognition of nonpeptide antigens. Using chimeric and mutant toxins, SEA amino acid residues 20-27 were shown to be required for gammadelta TCR recognition of SEA. Residues at 200-207 that are critical for specific alphabeta TCR recognition of SEA do not affect gammadelta TCR recognition. SEA residues 20-27 are located in an area contiguous with the binding site of V beta chains. This study defines a superantigen recognition site for a gammadelta T cell receptor and demonstrates the differences between Vgamma2Vdelta2+ T cell recognition of superantigens and nonpeptide antigens.

Mice infected with Listeria monocytogenes (LM) generate H2-M3wt-restricted CD8 effectors which recognize a heat-killed LM-associated antigen (HAA) presented by macrophages. To characterize HAA, we extracted a bioactive component from LM using SDS or NaOH. Extracted HAA aggregated in hydrophilic solvents but dissociated in the presence of SDS into a smaller subunit which migrated in Sephadex G-200 between chymotrypsinogen (25 kDa) and cytochrome c (12.5 kDa). HAA bioactivity and size was unaffected by proteinase K under conditions which degraded virtually all detectable protein. HAA was also unaffected by other proteases, RNase and DNase, but HAA bioactivity was destroyed by periodate, an agent that degrades carbohydrates. These studies demonstrate that H2-M3wt can present a hydrophobic, non-peptide, microbial antigen, probably glycolipid in origin, to CD8 T cells.

Streptococcus pyogenes (group A Streptococcus) has re-emerged in recent years as a cause of severe human disease. Because extracellular products are involved in streptococcal pathogenesis, we explored the possibility that a disease isolate expresses an uncharacterized superantigen. We screened culture supernatants for superantigen activity with a major histocompatibility complex class II-dependent T cell proliferation assay. Initial fractionation with red dye A chromatography indicated production of a class II-dependent T cell mitogen by a toxic shock-like syndrome (TSLS) strain. The amino terminus of the purified streptococcal superantigen was more homologous to the amino termini of staphylococcal enterotoxins B, C1, and C3 (SEB, SEC1, and SEC3), than to those of pyrogenic exotoxins A, B, C or other streptococcal toxins. The molecule, designated SSA, had the same pattern of class II isotype usage as SEB in T cell proliferation assays. However, it differed in its pattern of human T cell activation, as measured by quantitative polymerase chain reaction with V beta-specific primers. SSA activated human T cells that express V beta 1, 3, 15 with a minor increase of V beta 5.2-bearing cells, whereas SEB activated V beta 3, 12, 15, and 17-bearing T cells. Immunoblot analysis of 75 disease isolates from several localities detected SSA production only in group A streptococci, and found that SSA is apparently confined to only three clonal lineages as defined by multilocus enzyme electrophoresis typing. Isolates of one of these lineages, (electrophoretic type 2) are strongly associated with TSLS. The data identify SSA as a novel streptococcal superantigen that appears to be more related structurally to staphylococcal enterotoxins than to streptococcal exotoxins. Because abundant SSA production is apparently confined to only three streptococcal clonal lineages, the data also suggest that the SSA gene has only recently been acquired by S. pyogenes.

The physiological functions of the mouse telomeric major histocompatibility complex (MHC) class I molecules, including Hmt, are unknown. Hmt presents a polymorphic, N-formylated peptide encoded by the mitochondrial gene ND1 forming the cell surface maternally transmitted antigen (Mta). Because the N-formyl moiety is required for Hmt binding, we proposed that Hmt may function generally in presentation of N-formylated antigens. This hypothesis was validated by a competitive binding assay, demonstrating that synthetic N-formyl peptides from other mitochondrial genes also bound Hmt. Bacteria similarly initiate protein synthesis with N-formylmethionine; indeed, we established that Hmt can also present prokaryotic peptides in an N-formyl-dependent manner. These results indicate biochemical specialization of this MHC-peptide interaction and suggest a unique role for Hmt in prokaryotic host defenses.

The staphylococcal enterotoxins (SEs) are homologous proteins related in their capacity for stimulating both T cells and monocytes. To assess the importance of conserved structure and sequence to functional activity, the role of the disulfide loop and adjacent sequence in these toxins was evaluated. Contrary to previous reports, we demonstrate here that the disulfide loop was required for the mitogenic activity of SEA and SEB. While T cell-stimulatory activity was compromised, reduced and alkylated SEs retained major histocompatibility complex class II-binding and monocyte-stimulatory activities, suggesting that their inability to induce T cell proliferation was due to failure to interact with T cell receptor (TCR) rather than with class II molecules. Reduction and alkylation did not affect the far-ultraviolet circular dichroic spectrum of SEA, suggesting that the loss of mitogenic activity was not associated with significant changes in secondary structure. The disulfide linkage imparts considerable stability to these toxins as peptide cleavages within the loop of SEB were not associated with detectable loss of function, although cleavage in the conserved sequence outside the loop of SEA resulted in loss of mitogenic activity. This report thus establishes a functional role for a conserved element in SEs, the disulfide loop, and further indicates that their class II- and TCR-binding activities can be dissociated.

Maternally transmitted factor (Mtf) is a mitochondrial gene that controls the antigenic polymorphism of the MHC class I maternally transmitted antigen (Mta). Synthetic peptides from the NH2 terminus of the mitochondrially encoded NADH dehydrogenase subunit 1 (ND1) mimic Mtf peptide activity in an allele-specific manner. We show that the minimal ND1-alpha peptide length recognized by Mtaa-specific polyclonal CTLs was between 8 and 12 amino acids, while some Mtaa-specific CTL clones recognized a six amino acid peptide. The N-formyl group at the NH2 terminus of ND1 was essential for Mta activity. Competition experiments using N-substituted ND1-alpha peptides showed that an N-formyl peptide receptor on the target cell, which differs from the chemotactic peptide receptor, was required for Mta expression. The specificity of this receptor can account for the distinct immune restriction of Mta in which Mtf peptides are uniquely restricted by Hmt. It is possible that the Hmt gene product is the N-formyl peptide receptor itself and that it represents a class I antigen presentation molecule specialized for binding, transport, and immune presentation of N-formyl-peptide antigens of mitochondrial and prokaryotic origin.

“Classical” MHC class I (I-a) genes are extraordinarily polymorphic, but “nonclassical” MHC class I (I-b) genes are monomorphic or oligomorphic. Although diversifying (positive) Darwinian selection is thought to explain the origin and maintenance of MHC class I-a polymorphisms, genetic mechanisms underlying MHC class I-b evolution are uncertain. In one extreme model, MHC class I-b loci are derived by gene duplication from MHC class I-a alleles but rapidly drift into functional obsolescence and are eventually deleted. In this model, extant MHC class I-b genes are relatively young, tend to be dysfunctional or pseudogenic, and orthologies are restricted to close taxa. An alternative model proposed that the mouse MHC class I-b gene thymus leukemia Ag (TL) arose ∼100 million years ago, near the time of the mammalian radiation. To determine the mode of evolution of TL, we cloned TL from genomic DNA of 11 species of subfamily Murinae. Every sample we tested contained TL, suggesting this molecule has been maintained throughout murine evolution. The sequence similarity of TL orthologs ranged from 85–99% and was inversely proportional to taxonomic distance. The sequences showed high conservation throughout the entire extracellular domains with exceptional conservation in the putative Ag recognition site. Our results strengthen the hypotheses that TL has evolved a specialized function and represents an ancient MHC class I-b gene.

Class I-b genes constitute the majority of MHC class I loci. These monomorphic or oligomorphic molecules have been described in many organisms; they are best characterized in the mouse, which contains a substantial number of potentially intact genes. Two main characteristics differentiate class I-b from class I-a molecules: limited polymorphism and lower cell surface expression. These distinguishing features suggest possible generalizations regarding the evolution and function of this class. Additionally, class I-b proteins tend to have shorter cytoplasmic domains or in some cases may be secreted or may substitute a lipid anchor for the transmembrane domain. Some are also expressed in a limited distribution of cells or tissues. At least six mouse MHC class I-b molecules have been shown to present antigens to alpha beta or gamma delta T cells. Recent advances have provided insight into the physiological function of H-2M3a and have defined the natural peptide-binding motif of Qa-2. In addition, significant progress has been made toward better understanding of other class I-b molecules, including Qa-1, TL, HLA-E, HLA-G, and the MHC-unlinked class I molecule CD1. We begin this review, however, by arguing that the dichotomous categorization of MHC genes as class I-a and I-b is conceptually misleading, despite its historical basis and practical usefulness. With these reservations in mind, we then discuss antigen presentation by MHC class I-b molecules with particular attention to their structure, polymorphism, requirements for peptide antigen binding and tissue expression.

Although nonclassical (class I-b) gene products represent the majority of murine major histocompatibility complex (MHC) genes, the role of these relatively nonpolymorphic molecules remains uncertain. Recently, one such protein, H-2M3 (formerly designated Hmt), was shown to bind and specifically present N-formylated peptides to cytotoxic T lymphocytes. Because N-formylation is characteristic of prokaryotic proteins, this MHC molecule may be especially adapted for a role in the mammalian defense against bacterial attack. The current studies demonstrate that an MHC molecule, indistinguishable from H-2M3, presents antigens derived from the intracellular pathogen Listeria monocytogenes to Listeria-specific CD8+ cells.

We have shown that cytotoxic lymphocytes generated in primary cultures of NZB spleen cells with H-2-identical BALB/c or B10.D2 stimulator cells exhibit specificity for Qa-1b-associated antigenic determinants. This unidirectional cytotoxicity constitutes the initial demonstration of a primary in vitro response to antigens of the Qa-Tla system. Such responses do not require H-2 homology between effector and target cells in the assay system. In fact, when H-2Dd homologous target cells were employed there was little, if any, evidence for development of primary H-2-restricted responses to minor locus histocompatibility antigens or viral antigens. In view of the recently defined role of Qa-1+, Ly-1,2,3+ cells as regulators of antibody responses, and of the deficiency of such cells in NZB mice, the observation of hyperreactivity for determinants of this system may be relevant to the development of autoimmunity in these animals.

Two substrains of New Zealand Black (NZB) mice have been compared with respect to expression of a maternally transmitted cell surface antigen, Mta, defined by cloned cytolytic T cells, and for restriction enzyme polymorphisms of mitochondrial DNA (mtDNA). These independent assays of maternal cytoplasmic inheritance provide strong evidence for genetic contamination of the NZB/BlPt substrain (NZB/Bl mice from Michael Potter's separate colony at the National Institutes of Health), in which the typical NZB immunologic abnormalities are at least partially ameliorated. The decisive data are the restriction enzyme maps of mtDNA for NZB/BlPt, which were identical with those of the common "old inbred" strains and quite different from those of NZB/BlN (NZB/Bl mice from the breeding facility at the National Institutes of Health). It is probable that the contamination of the NZB/BlPt substrain is related to phenotypic changes in their autoimmune state. More interestingly, the data are consistent with, although they do not prove, involvement of the mitochondrial genome in expression of a cell surface molecule.

Cytotoxic T lymphocytes (CTL) activated in H-2 identical, Qa-1 disparate mixed leukocyte cultures recognize H-2-nonrestricted target antigens indistinguishable by strain or tissue distribution from serologically defined Qa-1 antigens. Cloned Qa-l-specific CTL define determinants encoded by four Qa-1 genotypes; we used anti-Qa-1 sera in antibody blocking experiments to determine if these determinants reside on molecules recognized by Qa-1-specific antibodies. Antisera containing Qa-1.1-specific and TL-specific antibodies blocked recognition of two CTL-defined determinants associated with Qa-1 a . Although both Qa-1 and TL molecules are expressed on activated T cells from appropriate strains, our studies indicated that the CTL recognized Qa-1, not TL. In addition, anti-Qa-1.2 serum inhibited CTL recognition of Qa-1b- and Qa-1c-encoded determinants. Qa-1 d target cells are unique in that they express determinants recognized by anti-Qa-1a CTL and by anti-Qa-1b CTL. Killing of Qa-1 d targets by anti-Qa-1a CTL was not inhibited by anti-Qa-1.1 serum, but was partially inhibited by anti-Qa-1.2 serum. Cytotoxicity of Qa-1 d cells by one anti-Qa-1b CTL clone was inhibited by both anti-Qa-1.2 and anti-Qa-1.1 sera, indicating close association of both serological determinants with the determinants recognized by the CTL. Thus, all of the CTL-defined Qa-1 determinants resided on molecules recognized by Qa-1-specific antibodies, but anti-Qa-1a CTL and Qa-1.1-specific antibodies did not have identical specificities.

The HLA complex is a multigene family on the short arm of human chromosome 6 that encodes molecules critical to self-nonself discrimination (figure 1). Cell surface molecules of two types are involved in this process. Those determined by HLA-A, HLA-B, and HLA-C genes, termed class I molecules, constitute a heavy chain of molecular weight -45,000 that is noncovalently associated with a 12,000 molecular weight light chain, (32 microglobulin. The HLA-A, HLA-B, and HLA-C loci constitute the genes for the class I heavy chains; (32 microglobulin is separately encoded on chromosome 15. Class I molecules are involved in targetcell recognition by a subset of T cells that usually display the surface molecule T8; such cells have been commonly associated with functions of suppressor and cytotoxic cells (including the killing of virus-infected target cells). The HLA-D region encodes a complex of gene products (class II molecules) that are also represented on cell surfaces as heterodimers (-34,000 and "-29,000 Mr). The structural genes for both chains of these molecules are located within the HLA-D region. Three distinct class II molecular species have been characterized; these are termed DR, DQ (formerly MB or DC), and DP (formerly SB). Class II HLA molecules on the surface of antigen-presenting cells are essential for recognition of antigen by T cells that usually express the T4 marker and that have been associated with initial recognitive events in a T cell response, particularly of cells with helper or inducer activities.

Qa-1-specific, H-2-unrestricted cytotoxic T lymphocytes (CTLs) generated from reciprocally immunized B10.BR (H-2k, Qa-1a) and CBA (H-2k, Qa-1b) mice, and immunoprecipitation of cell surface Qa-1 were used to examined Qa-1 region determinant expression by H-2r and H-2f mice. The H-2b, Qa/Tla congenic mice B6-Tlaa and C57BL/6J (B6) were used as prototype Qa-1a and Qa-1b strains, respectively. Cells from H-2r strains expressed determinants recognized by B10.BR anti-CBA CTLs. Reciprocal cold target inhibition of cytolysis demonstrated that some, but not all, Qa-1b-associated determinants recognized on B6 were expressed by H-2r, Qa-1c cells. In contrast, cells from H-2f strains expressed Qa-1 determinants recognized by both B10.lBR anti-CBA and by CBA anti-B10.BR CTLs. Cold target inhibitions indicated that the H-2f, Qa-1d cells expressed some, but not all, Qa-1a-associated antigens recognized on B6-Tla and did not express the same Qa-1b-associated antigens as B6. Furthermore, H-2f strains expressed cell surface antigens immunoprecipitable by antisera specific for both Qa-1a and Qa-1b-encoded determinants; these molecules were of the same molecular weight as those immunoprecipitated from B6 and B6-Tlaa. These data suggest that CTLs and antisera define the same, closely related Qa-1 determinants encoded by a polymorphic locus.

This review has emphasized several recent advances in our understanding of human lymphocyte biology made possible by mAbs that recognize specific functional subpopulations of lymphocytes, coupled with multiparameter flow-cytometric analysis. In the last 3 years, both the T4+ and T8+ subsets of T cells have been further divided according to function and appearance of specific cell surface antigens. In addition, subpopulations of NK cells with different maturation states and B cells with distinct functional abilities have been discovered. Multiparameter flow-cytometric analysis of lymphocyte subpopulations can provide clues to the pathogenetic mechanisms of immune disorders, as has been demonstrated in AIDS patients and children with JRA. The use of multiparameter flow-cytometric analysis to study human immune responses in vitro has enabled a clearer understanding of the complex lymphoid interactions and, as we believe, may play an important role in the analysis of immune-associated disease processes.

An in vitro model was developed to study both primary and secondary proliferative responses of human lymphocytes to hapten-conjugated peripheral blood mononuclear cells. Coculture of human lymphocytes with autologous trinitrophenyl (TNP)-conjugated stimulator cells resulted in primary proliferative responses. Subjects segregated into high and low primary responders with mean stimulation indices of 11 and 2.1, respectively. Restimulation of primed cells from high responder subjects 3 wk after initial sensitization generated secondary proliferative responses. To investigate the antigenic requirements for secondary stimulation, autologous TNP-conjugate primed responders were restimulated with both autologous and allogeneic TNP-conjugated stimulators. In all experiments restimulation with autologous conjugated cells yielded substantially greater proliferative responses than with allogeneic conjugates. Experiments were then performed to ascertain whether HLA determinant homology between primed responder and stimulator cells influenced the level of secondary responsiveness. Homology for HLA-A and B locus serologic determinants was not associated with enhanced responsiveness. In contrast, D region determinant homology, detected by B-cell antigen typing, showed a highly significant positive correlation with the magnitude of secondary responses. The data thus strongly suggest that for secondary proliferative responses to TNP, human T cells recognize hapten in association with HLA-D region determinants.

Because of the central role of IL-2 in clonal expansion of T cells, we have postulated that lymphocyte subpopulations with opposing regulatory functions might be independently regulated by differential requirements for expression of cell-surface IL-2-R. Purified CD4+ and CD8+ cells proliferated in an IL-2-dependent manner to crosslinked anti-T cell receptor antibodies (anti-CD3-Seph). Similarly, both CD4+ and CD8+ cells became IL-2 responsive after incubation in T suppressor cell growth factor (TsGF), a newly described approximately 8,000 Mr product of activated CD4+ cells. In support of our hypothesis, however, we observed that subpopulations of CD4+ and CD8+ cells, possessing distinct cell-surface antigens, showed differential responses to these stimuli. Those cells of suppressor-inducer or suppressor-effector phenotype failed to proliferate when cultured in anti-CD3-Seph plus IL-2, but did proliferate in an IL-2-dependent manner to TsGF. Furthermore, the suppressor-effector population was unresponsive to TsGF plus IL-2 when cocultured in anti-CD3-Seph, suggesting that functionally induced Ts may be refractory to growth stimuli. Conversely, cells with helper-inducer or cytolytic phenotype proliferated when incubated in anti-CD3-Seph and IL-2, while remaining essentially unresponsive to TsGF and IL-2. The results could not be explained by differences in the level of CD3 expression by the T cell subsets. Thus, cells within the helper and suppressor lineages appear to have distinct and reciprocal patterns for the induction of IL-2 responsiveness.

Maternally transmitted antigen (Mta) is a murine cell surface class I-like antigen that is defined by specific cytotoxic lymphocyte reactivity. Mta is unique in that its expression requires cooperation between genetic elements both in the Qa/Tla region of chromosome 17 and in the cytoplasm. In view of the known cytoplasmic, and thus maternal, inheritance of mitochondria, we have directly assessed their potential involvement in Mta expression. The mitochondria-specific lethal dye rhodamine 6G (R6G) was used to control the input of mitochondria into cell hybrids. The parental lines, one of BALB/c and one of NZB origin, were known to differ in Mta and mtDNA phenotype. Our data show that most control BALB/c-NZB hybrids expressed the BALB/c Mta phenotype and likewise contained only BALB/c-type mtDNA. The NZB Mta phenotype was not coexpressed in the control hybrids. However, when the mitochondrial contribution from BALB/c was prevented by R6G treatment, the majority of the resultant hybrids expressed only the NZB Mta type and likewise contained only NZB mtDNA. The exceptional R6G-treated hybrids that continued to express the BALB/c Mta phenotype likewise contained only BALB/c mtDNA. Thus, in every case the mtDNA phenotype correlated with the Mta phenotype of the cells. Together, the data support the remarkable conclusion that mitochondria modulate the phenotypic expression of a cell surface molecule.

We have investigated the cellular and antigenic requirements for incubation of secondary proliferative responses by human T lymphocytes. Two distinct properties of antigen-presenting peripheral blood mononuclear cells were studied: (a) the ability for appropriate cell surface constituents to construct an immunogenic moiety, and (b) the ability to present similar antigenic determinants when they are not covalently bound. Only Ia+ hapten-modified cells were effective stimulators. In contrast, both Ia+ and Ia- cell sonicates could stimulate secondary proliferative responses, but only in the presence of an accessory cell. This accessory cell was present in Ia+ macrophage, but not in Ia+ non-T lymphocyte, preparations. In contrast, macrophages or soluble factors produced by macrophages were not required for primed T cells to undergo hapten-specific proliferation in response to hapten-modified Ia+ stimulator cells. Thus, although all Ia+ cells tested can stimulate primed cells to proliferate, not all Ia+ cells can function as accessory cells for responses to sonicates. This may reflect the unique ability of a subpopulation(s) of Ia+ cells to bind or process sonicates or soluble antigens for appropriate recognition by primed T cells.

Previous studies of HLA-restricted antigen recognition by cloned T cells have frequently demonstrated reactivity that did not correlate precisely with the expression of serologically defined HLA specificities. To further explore such discrepancies, we utilized monoclonal antibody (MoAb) blocking, partial NH2-terminal amino acid sequencing, and Southern blot hybridization techniques to analyze the fine specificity of four autologous trinitrophenyl-specific T cell lines restricted to DR2-linked epitopes. MoAb blocking studies demonstrated that two of these lines recognized determinants on DR molecules while the other two recognized determinants on the same molecule that expresses the DQw1 determinant. However, these latter two lines appeared to recognize a DQw1-related determinant found primarily in association with DR2, but not the other DQw1-associated DR alleles, DR1 and DRw6. To ascertain whether these lines were defining a functional split of DQw1, we performed partial NH2-terminal amino acid sequencing of the molecules precipitated with a DQw1-specific MoAb (Genox 3.53) from different stimulator lines. The results showed that these T cell lines recognized a subtype of DQw1 that is in linkage disequilibrium with DR2. Moreover, we identified characteristic restriction fragment length polymorphisms with a DQβ-specific cDNA that correlated with stimulatory capacity for the DQw1-restricted lines. These results demonstrate that: (1) DQ molecules may provide restriction determinants that are incorrectly assigned to DR molecules on stimulator panel analyses; (2) cloned antigen-specific T cell lines recognize polymorphic regions of class II molecules not distinguished by either conventional typing antisera or xenogeneic MoAb; and (3) the DQw1 epitope(s) is located on a heterogeneous group of DQ molecules that differ from each other in the primary sequence of their β chains.

A mixed leukocyte reaction suppressor factor is produced by spleen cells sensitized in vivo and restimulated in vitro across non-H-2 antigenic barriers. Cells capable of producing this factor appear in the spleens of minor locus-immunized animals later than in animals sensitized to major histocompatibility complex-encoded antigens. However, both H-2 and non H-2-induced factors suppress proliferative responses to any alloantigen. Splenocytes from animals immunized with H-2-identical, minor locus-disparate cells produce suppressor factor in vitro only when restimulated with cells sharing both H-2 and non-H-2 antigens with the in vivo stimulators.

To investigate mechanisms by which antigen, macrophages, and interleukin 2 (IL2) participate in the induction of secondary T-cell proliferative responses, trinitrophenyl (TNP) was presented in three distinct modes: (i) TNP-modified peripheral blood mononuclear cells (TNP-PBMC), (ii) TNP-PBMC cell sonicates, and (iii) TNP-ovalbumin (TNP-OVA). Stimulators were depleted of Mac-120+ macrophages using Mac-120 monoclonal antibody plus complement. TNP-Mac-120 macrophages stimulated primed T cells nearly as well as TNP-unfractionated macrophages (which were about 40% Mac-120+). In contrast, although greater than 70% DR+, Mac-120- macrophages plus either TNP-OVA or TNP-PBMC sonicate elicited minimal responses compared to unfractionated macrophages plus antigen. After 21-28 days of in vitro priming, macrophage-depleted T cells were not stimulated to proliferate by either IL2 alone or sonicates alone. IL2 plus TNP-PBMC sonicates, however, stimulated significant proliferation. Furthermore, this response was considerably greater than that to IL2 plus either TNP-T cell sonicates or TNP-mouse spleen sonicates. Thus, the Mac-120+ macrophage population may have an important antigen-presenting and/or accessory function in the stimulation of primed T cells by soluble or particulate antigen, although it is unnecessary for responses to intact TNP-Ia+ PBMC. In addition, the data suggest that Ia+ sonicates alone may suffice for induction of IL2 responsiveness, but not for endogenous IL2 production and subsequent proliferation by primed T cells.

We examined the regulation of cell surface expression of the Qa-1 alloantigens using a panel of monoclonal anti-Qa-1 cytotoxic T-lymphocyte (mCTL) lines. In contrast to previous reports of tissue-specific expression, we found that Qa-1 was widely expressed, resembling the prototypical class I H-2K/D molecules. We further found that an H-2D-linked gene, which we termed Qdm for Qa-1 determinant modifier, controlled expression of certain CTL-defined Qa-1 antigenic determinants. H-2D khomozygous haplotypes expressed a recessive allele of the modifier, Qdm k, whereas all other H-2 haplotypes tested expressed a dominant allele, Qdm +. The Qdm + allele regulated in trans Qa-1 epitope expression from a Qdm kchromosome, modifying expression of particular CTL-defined Qa-1 antigenic determinants rather than affecting levels of cell surface expression. Mechanisms of Qdm function may include either a novel protein modification system or an unprecedented case of antigen recognition restricted by a nonclassical major histocompatibility complex molecule.

Dialyzable transfer factor, obtained from frozen-thawed peripheral blood leukocytes from a single donor, was given to five anergic patients with chronic mucocutaneous candidiasis. Studies of immunological responses including delayed cutaneous hypersensitivity, in vitro antigen-induced thymidine incorporation, and production of macrophage migration inhibition factor (MIF) were conducted both before and after injection of transfer factor. Before transfer factor, none of the patients had delayed skin responses to any of the natural antigens studied. Their lymphocytes did not produce MIF after exposure to antigens in vitro and only one patient showed increased thymidine incorporation when his lymphocytes were cultured with candida and streptokinase-streptodornase (SK-SD). After injection of transfer factor, four patients developed delayed skin responses to antigens to which the donor was sensitive; no recipient reacted to an antigen to which the donor was nonreactive. Lymphocytes from recipients produced MIF when cultured with antigens that evoked positive delayed skin tests. Only one patient developed antigen-induced lymphocyte transformation and this response occurred only intermittently. Attempts to sensitize three of the patients with the contact allergen, chlorodinitrobenzene, both before and after transfer factor, were unsuccessful. The fifth patient, a 9-yr old boy with an immunologic profile similar to the Nezelof syndrome, did not become skin test-reactive or develop positive responses to the in vitro tests. These findings suggest that transfer factor acts on the immunocompetent cells that respond to antigens with lymphokine production, but has little, if any, effect on cells that respond to antigens by blastogenesis. The failure to sensitize the subjects with chlorodinitrobenzene illustrates the specificity of the immunologic effects of transfer factor, and implies that it does not function through nonspecific, adjuvant-like mechanisms. Failure of transfer factor to produce positive skin tests or MIF production in a patient with Nezelof's syndrome may be evidence that lymphokine-producing cells are thymus derived.

Mouse spleen lymphocytes activated by a mitogenic concentration of concanavalin A (Con A) secrete a factor(s), termed soluble immune response suppressor (SIRS), which is a potent, non-cytotoxic inhibitor of primary plaque-forming cell (PFC) responses by mouse spleen cells to heterologous erythrocytes in vitro. SIRS activity was detected in supernatant fluids of spleen cell cultures within 6 hr of stimulation with Con A and was maximal from 12 to 24 hr after stimulation. SIRS profoundly suppressed 5-day PFC responses when added to antigen-stimulated spleen cell cultures at initiation, was less effective when added 24 hr after initiation, and had no significant effect when added 48 or more hours after initiation. SIRS had to be present in antigen-stimulated spleen cell cultures only during the initial 24 hr of incubation to cause suppression of PFC responses which by kinetic analysis was not manifest until 120 hr of incubation. Background and antigen-stimulated IgM and IgG PFC responses were suppressed by SIRS. Exposure to SIRS did not render spleen cells totally incapable of developing PFC responses to heterologous erythrocytes. Instead, antigen-stimulated PFC responses in control cultures and cultures containing SIRS had the same kinetics and were of comparable magnitude during the first 96 hr of incubation. Thereafter, however, PFC responses in cultures containing SIRS abruptly aborted and the SIRS-induced suppression of PFC responses became evident at 120 and 144 hr of incubation. Although the precise mechanism(s) of action of SIRS is unknown, it appears to function during the early, antigen-dependent phase of the immune response in vitro to abort full expression of the PFC response during the later exponential expansion of the clones of antibody-synthesizing cells.

A central role for major histocompatibility complex (MHC) molecules in presentation of non-MHC antigens to T lymphocytes is generally acknowledged. Among the seminal studies demonstrating this phenomenon were those of Shearer and associates demonstrating that recognition of mouse target cells by trinitrophenyl (TNP)-specific cytolytic T cells required genetic homologies between the killer and target cells at H-2 class I loci (rev., Shearer et al., 1976). This system, employing target cells surface modified by hapten conjugation, has proven particularly useful in subsequent studies of MHC restrictions on antigen recognition by human T lymphocytes. As in the mouse, recognition of TNP-modified target cells by hapten-specific cytotoxic T lymphocytes has appeared to involve restrictions for class I HLA homology (Shaw and Shearer, 1978). The data in human systems have been substantially less compelling, however, with evidence in some instances for effective interactions between target and killer cells that did not involve obvious class I HLA homologies (Shaw and Shearer, 1978; Seldin et al., 1979). In contrast to target cell recognition by cytolytic T-cells, recognition of antigen by helper T-cells has generally required genetic homology between antigen-presenting cells and helper T cells (or B cells and helper T cells) for class II loci of the MHC (rev., Thomas et al., 1977). Subsequently, clear exceptions have been documented with respect to correlations between functional activity and MHC restrictions for antigen recognition (rev., Rich et al., 1986). On the other hand, identification of a dichotomy of T lymphocytes, one subset of which expresses the CD4 molecule (L3T4 in mice) and the other of which expresses the CD8 molecule (Ly2 in mice), has led to a more convincing correlation between T-cell subset and MHC recognition requirements in antigen presentation. That is, recognition of non-MHC antigens by CD4+ (L3T4+) cells requires class II MHC homologies between T-cells and antigen presenting cells, whereas recognition by CD8+ (Ly2+) T-cells is MHC class I restricted (rev., Swain, 1983).

T cell proliferation in response to stimulation with Staphylococcus enterotoxin A (SEA) requires accessory cells that express class II major histocompatibility complex (MHC) molecules. Murine fibroblasts transfected with genes encoding the alpha and beta subunits of HLA-DR, DQ, or DP were used to show that the proliferative response of purified human T cells to SEA is dependent on class II molecules but is not restricted by the haplotype of the responder. Binding of fluoresceinated SEA to class II transfectants and precipitation of class II heterodimers with SEA-Sepharose show that the proliferative response is a result of SEA binding to class II molecules. The binding is specific for class II molecules and is independent of class II allotype or isotype. The ability of SEA to bind class II molecules may be a general characteristic of this class of antigens, now called "superantigens".

Clearer definition of the recognitive structures of human T lymphocytes for antigens will be required to elucidate the molecular basis of diseases and immunological responses induced or regulated by normal or abnormal T-cell function. For this purpose we have investigated the cellular requirements for immune responses in vitro to trinitrophenyl-conjugated peripheral blood mononuclear cells. The responding cell was characterized as a T cell on the basis of rosetting with sheep erythrocytes. T-cell recognition of hapten in proliferative responses depended upon presentation of antigen in an appropriate stimulator-cell context. Neither autologous hapten-modified erythrocytes nor T cells restimulated responses of in vitro-primed lymphocytes. Moreover, hapten-conjugated non-T cells were more effective than modified unfractionated cells in restimulating proliferative responses. Both macrophages and non-T lymphocytes effectively restimulated hapten-conjugate responses. Cell-mixing experiments indicated that the failure of haptenated T cells to stimulate proliferative responses was not because of a lack of fresh macrophages; these experiments suggested instead that T cells do not express appropriate structures necessary to present haptenic determinants in an immunogenic form. Hapten-modified T cells, however, were capable of inducing primed lymphocytes to become efficient cytotoxic effector cells, indicating that T-cell recognitive units for stimulation of proliferative and cytotoxic responses are different. These data support the concept that for induction of proliferative responses, human T cells recognize conventional antigens in association with HLA-D-region-encoded Ia-like molecules.

TNP-specific proliferative cloned human T cell lines were investigated for their synthesis and cell surface expression of HLA-DR molecules and for their capacity to function as antigen presenting cells. Utilizing radioactive amino acid precursors for metabolic labeling, these studies demonstrated endogenous synthesis of HLA-DR molecules by cloned T cells, which by two-dimensional gel electrophoresis were similar to HLA-DR molecules expressed by B cells and monocytes. Moreover, when TNP-modified, the irradiated cloned T cells functioned very effectively to stimulate TNP-specific proliferation by cloned responders; when unmodified they were potent stimulators of allogeneic mixed leukocyte responses. Thus, for haptens covalently attached to cell membrane proteins and for allogeneic HLA antigens, Ia+ cloned T cells can function as effectively for antigen presentation and T cell activation as other Ia+ populations to which such properties have been ascribed.

Mtf, a cytoplasmic, probably mitochondrial factor, controls Mta polymorphism. We tested for dominance between two forms of Mtf to determine whether Mta is controlled by positive or negative genetic mechanisms. We fused Mtf-disparate cells containing distinct mtDNA markers and selected for hybrids containing both. Such mtDNA heteroplasmons codominantly and stably express alternative Mta antigens. Stable codominance excludes negative genetic mechanisms as well as a model of induced nuclear compensation, and implies Mtf controls Mta expression through a positive genetic mechanism.

Mta-specific cytotoxic T lymphocyte (CTL) can be generated in primary cultures of (NZB X B10.D2)F1 spleen cells with H-2-compatible BALB/c stimulator cells. The CTL lyse reciprocal Mta+ (B10.D2 X NZB)F1 as well as H-2-disparate targets, such as B10, B6, and B6-Tlaa; they do not lyse targets from NZB or any F1 hybrid of an NZB mother. The lysis of 51Cr-labeled B10 targets is completely inhibited by unlabeled targets from Mta+ (B10.D2 X NZB)F1, but not from the reciprocal Mta- F1, thus demonstrating H-2-unrestricted lysis of Mta.

Publisher Summary This chapter summarizes the evidence regarding the genetic mechanisms of Mta transmission and the class I nature of the surface antigen. By far the most intriguing aspect of maternally transmitted antigen (Mta) is its strict maternal inheritance. It is because of unawareness of any other cell surface antigen in metazoa that is inherited in this manner. Although the conclusive experiments have yet to be completed, it is thought that the evidence for mitochondrial control of Mta phenotype is rather persuasive. Mta is defined by cytotoxic T-lymphocyte (CTL) recognition; to date no antibodies to Mta have been reported. Mta disparity also induces skin graft rejection as well as weak secondary mixed lyinphocyte responses. Restriction enzyme polymorphism analysis and transfer of mitochondria via cell fusion indicate that the genetic element controlling the Mta polymorphism is probably located within mitochondria.

The effects of aspirin on delayed hypersensitivity were assessed in 40 healthy subjects who were randomly assigned to two equal groups. In a double-blind format, the individuals in one group were placed on 4 gm aspirin daily for 5 days, and individuals in the other group were given placebo. Lymphocyte proliferation was measured before and 72 hours after the initiation of drug, by using three mitogens and four antigens. The percentages of T and B lymphocytes were likewise measured before and during therapy. The subjects were skin tested with the same four antigens 72 hours after the initiation of drug, and the skin tests were read 48 hours later. No significant differences between the two groups were detected when skin tests, lymphocyte proliferation, and percentage of T lymphocytes were compared.

We have segregated DR1+ individuals into two categories according to whether or not their class II+ cells stimulated T lymphocyte clones specific for or restricted to DR1. In a majority of cases (87%), failure to stimulate was a property of cells having the B14;DR1 haplotype and/or nonclassical 21-hydroxylase deficiency. Absence of clonal proliferation could not be explained by release of an intercellular suppressor factor or by stimulator cell absorption of interleukin 2. Homozygous cells inheriting both stimulatory (DR1n) and nonstimulatory (DR1x) haplotypes did not successfully mediate clonal expansion, implying that a trans acting factor operates intracellularly to modify both DR1 alleles or their products. Other DR alleles did not appear to be affected as evidence by normal proliferative responses of T lymphocyte clones restricted to DR2 or DR7 and stimulated by DR1x,2 and DR1x,7 cells, respectively. By two-dimensional gel analysis, we have further identified a 50-kD surface glycoprotein contained in anti-DR immunoprecipitates of DR1x, but not DR1n or non-DR1 cellular lysates. This 50-kD structure had antigenic and peptide identity to DR alpha and beta chains but was resistant to dissociation under conditions that normally separate DR alpha and beta (8 M urea plus 5% 2-mercaptoethanol); boiling in sodium dodecyl sulfate was required to segregate the component polypeptides of the 50-kD heterodimer. We postulate that a product of a novel combinatorial association between constitutive chains of DR may interfere with or compete for normal T cell receptor recognition of DR1 as both an alloantigen and a restricting element. We further propose that gene abnormalities within the class III region of a haplotype associated with nonclassical 21-hydroxylase deficiency may extend into the DR subregion of the major histocompatibility complex with consequent aberrations in DR1 presentation.

Cloned human T lymphocyte lines were generated against trinitrophenyl (TNP)-modified autologous cells from four different individuals. By examining the reactivity patterns of 58 of these T cell clones (TLC) on panels of HLA-typed antigen presenting cells and employing monoclonal antibodies (MoAb) for inhibition studies, we have demonstrated that TNP may be recognized in the context of different DR and DP associated determinants. We did not identify any TLC restricted by determinants associated uniquely with DQ. Determinants associated with the expression of DR1,2,4,5, and 7 as well as the supertypic specifities DRw52 (MT2) and DRw53 (MT3) functioned as restriction elements. In addition, at least two antigenically distinct regions of the DP4 molecule appeared to function in the presentation of TNP. One TNP-specific TLC recognized a determinant associated with the expression of DP4 while another recognized TNP in the context of a highly nonpolymorphic determinant on DP which was expressed on all stimulators tested. Monoclonal antibody blocking studies suggest that these two determinants lie on different portions of the DP molecule. These studies demonstrated that both polymorphic and relatively nonpolymorphic restriction determinants on DR and DP may function in the presentation of conventional antigen to autologous lymphocytes.

Maternally transmitted antigen (Mta) is a mouse major histocompatibility antigen recognized by cytotoxic T lymphocytes. A role for mitochondria in expression of this class I-like cell surface antigen has been previously established. We now show that a labile product of mitochondrial protein synthesis is required for Mta expression. Reexpression of Mta determinants after enzymatic removal occurred within 24 h, and the regeneration process was sensitive to chloramphenicol (CAP), a selective inhibitor of mitochondrial protein synthesis. Additionally, target cells treated with CAP for as little as 18 h showed diminished expression of Mta. The estimated half-life for Mtf products ranged from 6 to 15 h, less than the half-lives of known mitochondrial translation products. This suggests that the Mtf product is not generated by the normal turnover of stable mitochondrial respiratory proteins. Instead, these results indicate the existence of either labile unknown mitochondrially encoded peptides or a rapid turnover pathway for known mitochondrial products.

T-cell-mediated suppression of human immune responses involves a complex interaction between distinct lymphocyte subsets with suppressor-inducer and suppressor-effector functions. Recent studies with subset-specific monoclonal antibodies have defined a characteristic phenotype of suppressor-inducer cells (CD4 + Leu8 + 2H4 + 4B4 − ) that can be distinguished from that of helper cells for antibody synthesis (CD4 + Leu8 − 2H4 − 4B4 + ). Similarly, suppressor-effector cells (CD8 + CD11 + Tp44 − can typically be defined as a subset separable from cytotoxic T cells (CD8 + CD11 − Tp44 + ). Both antigen-specific and nonspecific interactions are important in suppressor T-cell activation and function. Soluble signals required for differentiation of CD8 + suppressor cells include an indomethacin-sensitive monocyte product and interferon gamma. In contrast, proliferation of the CD8 + suppressor cell subset depends on stimulations first by a product of CD4 + Leu8 + cells, T suppressor cell growth factor, and second by interleukin 2. Although the molecular basis of antigen-specific interactions between CD4 + and CD8 + cells in suppressor cell generation has not been defined, it may involve both conventional, presumably MHC-restricted, interactions between antigen and antigen receptors, as well as anti-idiotypic interactions of suppressor-effectors with determinants on suppressor-inducer receptors. Progress in elucidating requirements for activation, growth, and differentiation of suppressor cells should facilitate long-term culture of such cells and lead to clearer understanding of mechanism of suppressor-cell mediated immunoregulation.

We have examined the possible rotes of divalent cations and of cytochalasin B-sensitive processes in the production of mixed leukocyte reaction suppressor factor (MLR-TsF). Ca but not Mg was strictly required for MLR-TsF production. A requirement for Mg could be demonstrated only under conditions of Ca depletion, suggesting an intimate linkage in divalent cation requirements. Ca was required for synthesis as well as release of the suppressor factor. Cytochalasin B inhibited MLR-TsF production only when present during the first 4 hr of culture; such inhibition was reversible with incubations of up to 16 hr. This indicated that cytochalasin B inhibited early events of activation, rather than synthesis or release of MLR-TsF. It was found that both Ca and Mg were required either before or during the cytochalasin-sensitive steps in the activation sequence. The relationship of cytochalasin B-sensitive events in early activation to the antigenic stimulus per se was established by limited exposure of suppressor cells to allogeneic stimulators fixed to poly- l -lysine (PLL)-coated plates. Maximal stimulation was observed with a 4-hr exposure, and this antigen-dependent commitment was cytochalasin B sensitive.