T lymphocytes responding to Mls-locus antigens are Lyt-1+, 2- and I-A restricted

Potential alteration of tumor microenvironments by β-mercaptoethanol

The therapeutic effectiveness of immune checkpoint inhibitors in cancer patients is quite profound. However, it is generally accepted that further progress is curtailed by accompanying adverse events and by low cure rates linked to the tumor microenvironment. The multitudes of immune processes altered by low-molecular-weight thiols published over the past decades suggest they have potential to alter tumor microenvironment processes which could result in an increase in immune checkpoint inhibitor survival rates. Based on one of the most studied and most potent low-molecular-weight thiols, β-mercaptoethanol (BME), it is proposed that clinical assessment be undertaken to identify any BME benefits with relevance for proliferation/differentiation of immune cells, lymphocyte exhaustion, immunogenicity of tumor antigens and inactivation of suppressor cells/factors. The BME alterations projected to be most effective are: maintenance/replacement of glutathione in lymphocytes via facilitation of cysteine uptake, inhibition of suppressor cells/soluble factors and inactivation of free-radical, reactive oxygen species.

Mouse endogenous superantigens: Ms and Mls-like determinants encoded by mouse retroviruses

Commonly used inbred mouse strains express different combinations of integrated mouse mammary tumor proviruses (MMTV). This appendix summarizes the proviruses that have been detected. The reported functional properties of those MMTV proviral products which have been identified as superantigens are also summarized, including the ability to elicit primary or secondary T cell responses and to induce Vb-specific clonal deletion during T cell differentiation. In addition, the amino acid sequences of putative ORF gene products of different MMTV are compared.

Superantigens: mechanisms by which they may induce, exacerbate and control autoimmune diseases

Superantigens are polypeptide molecules produced by a broad range of infectious microorganisms which elicit excessive and toxic T-cell responses in mammalian hosts. In light of this property and the fact that autoimmune diseases are frequently the sequelae of microbial infections, it has been suggested that superantigens may be etiologic agents of autoreactive immunological responses resulting in initiation, exacerbation or relapse of autoimmune diseases. This article relates the biology of superantigens to possible mechanisms by which they may exert these activities and reviews the evidence for their roles in various human and animal models of autoimmune disease. Finally, a mechanism of active suppression by superantigen-activated CD4+ T-cells that could be exploited for therapy as well as prophylaxis of human autoimmune diseases is proposed.

Lectins and superantigens: membrane interactions of these compounds with T lymphocytes affect immune responses

1. Lectins and superantigens belong to two different families of macromolecules which are able to interact with cells of the immune system. 2. The principal mechanisms by which they modulate immune responses are presented in this review. 3. Possible similarities shared by these proteins and their common mechanisms of action upon immunocytes will be presented along with a brief discussion regarding the role of these molecules in physiological immune responses and human diseases.

Selective activation of murine V beta 8.2 bearing T cells by Pseudomonas exotoxin A

We have determined that Pseudomonas aeruginosa exotoxin A (PE) can selectively stimulate the proliferation of V beta bearing T lymphocytes. Murine thymocytes were fractionated by selective agglutination with peanut agglutinin (PNA) and the PNA- thymocytes, which represent mature thymocytes, were shown to be responsive to PE stimulation. In addition, mature peripheral T lymphocytes (nylon wool nonadherent splenocytes) were also observed to respond to PE stimulation. Both CD4+ and CD8+ splenic T lymphocyte populations proliferated in response to PE. Flow microfluorimetry analysis of PNA- thymocytes stimulated with PE indicated that V beta 8.2 bearing T cells were preferentially expanded. Thus, our data indicate that PE represents a microbial super antigen which stimulates murine thymocytes which bear the V beta 8.2 element of the T cell receptor.

Selection of murine T cell receptor alpha beta and gamma delta cells in organ cultures established from 14-day embryos

The expression of minor lymphocyte stimulatory locus (Mls) determinants in combination with murine major histocompatibility complex (MHC) class II molecules, leads to the destruction of lymphocytes bearing specific V region-encoded T cell receptor (TcR) products. A much studied example is the elimination of V beta 6+ cells in IE+/Mls-1a mice, in which deletion can be detected 7-10 days after birth but is not fully operational earlier in embryonic life. Here we investigate this transitional period in development and show that selective deletion of V beta 6 occurs in vitro, approximately 1 week after organ cultures are established from 14 day embryos. These unmanipulated organ cultures receive no additional cell immigrants after day 14, suggesting that the cellular elements mediating negative selection (or their direct precursors), are already resident in the fetal thymus by day 14 of gestation. Hence, the developmental timing of the outset of rigorous negative selection of V beta 6 is not dictated by the postnatal entry of deleting elements into the thymus, but perhaps by the maturation of the pre-existing environment. Using a parallel organ-culture approach we have looked at the development of V delta 4 and V gamma 3, TcR gamma delta+ cells in a variety of mouse strains. These receptors have recently been reported to be subject of MHC and non-MHC linked selection, respectively. We find that after an initial period of expansion, the number of V gamma 3-expressing cells dramatically declines. However, this selective loss of V gamma 3 cells is not contingent on the C57BL/6 mouse strain (in contrast to a previous report). These findings are discussed in the context of current models of ontogeny and repertoire selection.

Superantigens

Over the past three years superantigens have come to the forefront of immunological research. Studies in a number of laboratories have indicated that superantigens play a central role in shaping the T-cell repertoire in the development of tolerance, autoimmunity and immunodeficiency. This is in addition to their classic role in the pathogenesis of a number of infectious diseases.

Evidence for quantitative and qualitative differences in functional activation of Mls-reactive T cell clones and hybridomas by antigen or TcR/CD3 antibodies

In this study, we demonstrated that some V beta 6+, CD4+, Mls-1a-specific T cell clones had cytolytic activity when stimulated with anti-T cell receptor(TcR)/CD3 monoclonal antibodies (mAb), but not with targets expressing Mls-1a, although they produced lymphokines (interleukin 2 and interferon-gamma) in response to both types of stimuli. To examine the possibility that lack of cytolysis resulted from expression of the Mls-1a antigen on merely a fraction of splenic B blasts, we (a) used the B cell lymphoma LBB.3.4.16 and (b) measured esterase secretion which is generally concurrent with cytotoxic T lymphocyte (CTL) activity. The B cell lymphoma maximally stimulated the T cell clone for interferon-gamma production when responding and stimulating cells were incubated at a 1:1 ratio, but it was never killed by the Mls-1a-specific T cell clone unless TcR/CD3-specific mAb were added. Furthermore, a fivefold excess of the Mls-1a B cell lymphoma did not induce any secretion of esterase, which was observed only in the presence of the TcR/CD3-specific mAb. Comparison of the reactivity of two Mls-1a-specific T cell hybridomas expressing the same TcR at similar surface density, revealed both quantitative and qualitative differences between CD3-specific mAb and Mls stimulation of the hybridomas. A small quantitative difference in the sensitivity of hybridoma FJ22.5 to stimulation with V beta 6 or CD3-specific mAb resulted in a marked decrease in efficiency of stimulation by Mls-1a for interleukin 2 production and to inability to detect growth inhibition by Mls-expressing cells. A qualitative difference was observed when analyses of inositol phosphate production were performed under optimal conditions of stimulation of the highly responsive T cell hybridoma (FJ8.1): only stimulation with CD3-specific mAb, but not Mls-expressing cells, could induce detectable inositol phosphate production. Lack of cytolysis of Mls-1a class II-expressing B cells may have evolutionary significance in view of the recent mapping of Mls to mouse mammary tumor virus genes.

The effect of peripheral immunization with Mls-1a on the emigration of antigen-specific cells from the thymus

Mature T cells found in the lymph nodes and spleen have the capacity to become activated and to proliferate in response to foreign antigens. The response of the thymus to such immunization is less well understood. We have examined one aspect of the thymic response by determining the effect of peripheral immunization upon cell emigration from the thymus. BALB/c (Mls-1b) mice were injected with spleen cells from DBA/2 (Mls-1a) mice, and V beta 6+ (Mls-1a-reactive) thymic emigrants were identified 3-30 days after immunization. Neither the rate of total cell migration from the thymus nor the proportion of V beta 6+ cells was altered, even though the immunizing spleen cells elicited an immune response in the draining (parathymic) lymph nodes. The same immunogen caused deletion of V beta 6+ cells in both the thymus and lymph nodes after intraperitoneal injection into the neonate. The inability of DBA/2 splenocytes to modify the development of adult thymocytes after intrathymic injection of the cells precluded the lack of entry into the thymus as the reason for the lack of any observed effect in the adult. Our results, therefore, indicate that the development of adult thymocytes is not modified by immunization, and suggest that the differing thymic response of mice injected as adults or neonates is related to changes in the intrathymic antigen presentation capacity associated with age.

Peripheral clonal elimination of functional T cells

A major mechanism for generating tolerance in developing T cells is the intrathymic clonal deletion of T cells that have receptors for those self antigens that are presented on hematopoietic cells. The mechanisms of tolerance induction to antigens not expressed in the thymus remain unclear. Tolerance to self antigens can be generated extrathymically through the induction of clonal nonresponsiveness in T cells with self-reactive receptors. A second mechanism of extrathymic tolerance was identified: clonal elimination of mature T cells with self-reactive receptors that had previously displayed functional reactivity.

Extrathymic tolerance of mature T cells: clonal elimination as a consequence of immunity

The mechanism by which T lymphocytes are tolerized to self or foreign antigens is still controversial. Clonal deletion is the major mechanism of tolerance for immature thymocytes; for mature T cells, tolerance is considered to reflect anergy rather than deletion, and to be a consequence of defective presentation of antigen. This paper documents a novel form of tolerance resulting when mature T cells encounter antigen in immunogenic form. Evidence is presented that exposure of mature T cells to Mlsa antigens in vivo leads to specific tolerance and disappearance of Mlsa-reactive V beta 6+ T cells. Surprisingly, the clonal elimination of V beta 6+ cells is preceded by marked expansion of these cells. Thus, tolerance induction can be the end result of a powerful immune response. These data raise important questions concerning the relationship of tolerance and memory.

Expression of CD4 can confer major histocompatibility complex class II-associated superantigen reactivity upon a T cell receptor derived from a CD8-dependent cytotoxic T lymphocyte clone

It has been shown that reactivity against major histocompatibility complex (MHC) class II-associated Mlsa determinants is mainly mediated by CD4+ V beta 6+ T cells. 3F9 is a CD8+ CTL clone which is specific for the alloantigen H-2Db. While 3F9 is V beta 6+, it is not Mlsa reactive, presumably because it does not express CD4. 3F9 utilizes the same T cell receptor (TcR) V alpha V beta combination as LB2, a CD4+ T helper clone specific for chicken red blood cells (cRBC)/I-Ab and yet differs from LB2 in the junctional sequences in both TcR chains. CD4+ CD8- and CD4-CD8- hybridomas expressing the 3F9 TcR were tested for reactivity against Mlsa and cRBC/I-Ab. Only the CD4+CD8- hybridomas were Mlsa reactive, and antibody inhibition studies revealed that this reactivity was both CD4 and MHC class II dependent. Therefore the expression of the CD4 molecule can make an MHC class I-restricted TcR Mlsa reactive. Neither type of hybridoma reacted against cRBC, thus the main difference in the antigen reactivity between 3F9 and LB2 lies in the TcR junctional regions.

Activation of polyphosphoinositide hydrolysis in T cells by H-2 alloantigen but not MLS determinants

Murine minor lymphocyte-stimulating (Mls) determinants are cell surface antigens that stimulate strong primary T cell responses; the responding T cells display restricted T cell receptor (TCR) V beta gene usage. Interaction of T cells with mitogens or major histocompatibility complex (MHC) antigens activated the polyphosphoinositide (PI) signaling pathway, but this pathway was not triggered by Mls recognition. However, interleukin-2 (IL-2) secretion and proliferation to all three stimuli were comparable. Thus, although recognition of both allo-H-2 and Mls determinants is thought to be mediated by the TCR, these antigens appear to elicit biochemically distinct signal transduction pathways.

On the peptide model of allorecognition: cytotoxic T lymphocytes recognize an alloantigen encoded by two HLA-linked genes

The peptide model of allorecognition hypothesizes that alloreactive cytotoxic T lymphocytes recognize peptides associated with major histocompatibility complex (MHC) molecules. This study characterizes an unusual alloantigen that is recognized by two human cytotoxic T-cell lines and may represent a complex formed by the association of a naturally selected peptide with an MHC molecule. Family studies demonstrate that both components of the alloantigen are products of MHC-linked genes. One component is a class I molecule most likely encoded by the HLA-B locus. The second component is encoded by an MHC-linked gene that shows a limited polymorphism; whether it represents the product of a second class I gene or of some other MHC gene remains to be determined. These data provide experimental evidence supporting the peptide model of allorecognition in human beings and indicate that some naturally selected peptides involved in allorecognition may be derived from MHC-encoded proteins.

Thymic requirement for clonal deletion during T cell development

During T cell differentiation, self tolerance is established in part by the deletion of self-reactive T cells within the thymus (negative selection). The presence of T cell receptor (TCR)-alpha beta + T cells in older athymic (nu/nu) mice indicates that some T cells can also mature without thymic influence. Therefore, to determine whether the thymus is required for negative selection, TCR V beta expression was compared in athymic nu/nu mice and their congenic normal littermates. T cells expressing V beta 3 proteins are specific for minor lymphocyte stimulatory (Mlsc) determinants and are deleted intrathymically due to self tolerance in Mlsc+ mouse strains. Here it is shown that V beta 3+ T cells are deleted in Mlsc+ BALB/c nu/+ mice, but not in their BALB/c nu/nu littermates. Thus, the thymus is required for clonal deletion during T cell development.

TCGF III/P40 is produced by naive murine CD4+ T cells but is not a general T cell growth factor

Several antigen-specific T cell lines were found to secrete a lymphokine upon activation by antigen or lectin that was provisionally termed T cell growth factor III (TCGF III) because it induced the proliferation of a CD4+ T cell clone independently from IL2 and IL4. Amino acid sequence analysis (and the functional properties of TCGF III) revealed that TCGF III was identical with a recently identified lymphokine termed P40. TCGF III/P40 was not only produced by long-term cultured T cell lines but also upon stimulation of freshly isolated Mlsa-reactive T cells. In addition, naive CD4+ T cells secreted TCGF III/P40 upon activation by lectin or allo-major histocompatibility complex structures. However, in spite of its growth-promoting activity for a CD4+ T cell clone this lymphokine does not appear to function as a general growth factor for T cells.

Immune recognition by cytotoxic T lymphocytes of minor histocompatibility antigens expressed on a murine colon carcinoma line

We have characterized in vivo and in vitro responses of mice to the BALB/c-derived carcinoma, C26. BALB/c mice were highly susceptible, in a dose-dependent fashion, to local tumor development following subcutaneous injection of C26. Other strains of mice, including allogeneic strains and major histocompatibility complex compatible strains of different minor histocompatibility (H) backgrounds, were resistant to C26-induced tumors. The basis for resistance of mice to C26 was studied using an in vitro-derived C26 line as target cells in microcytotoxicity assays, and as a source of antigen for in vivo priming. An H-2d-specific alloreactive cytotoxic T lymphocyte (CTL) line was isolated from C57BL/6 mice primed with C26, demonstrating the expression, and immune recognition, of MHC class I antigens on C26. C26 also expressed minor H antigens of BALB background as demonstrated by the ability of CTL specific for BALB minor H antigens to selectively lyse C26. Conversely, minor H antigens on C26 were immunogenic across a minor H barrier as demonstrated by the ability to raise anti-minor H CTL to C26 from minor H disparate strains. Collectively, those experiments indicate that C26 may be useful for immunologic and biochemical studies of murine minor H antigens, and for in vivo and in vitro studies of local immunity.

The T cell receptor V beta 6 domain imparts reactivity to the Mls-1a antigen

A monoclonal antibody secreting hybridoma was established by fusing spleen cells from a rat immunized with a murine T cell clone, OI11, which has I-Ab restricted specificity for the male H-Y antigen and unrestricted specificity for the minor lymphocyte stimulating antigen, Mls-1a, to the mouse myeloma P3X63AG8.653 and screening for the capacity of the hybridoma supernatants to stimulate the OI11 T cell clone. An antibody (RR4-7) was found to be specific not only for the immunizing T cell clone but virtually for all T cells using the V beta 6 TCR gene product as part of their surface antigen receptor. When the expression of the V beta 6 gene in various strains of mice was analyzed, it was found that strains expressing the Mls-1a antigen contained few T cells expressing V beta 6-encoded TCRs. The majority of T cell hybridomas which expressed V beta 6-encoded TCRs were found to be reactive to the Mls-1a antigen. These data confirm the finding of H. R. MacDonald et al. (Nature (London) 332, 40, 1988) that most TCRs encoded by the V beta 6 gene have a biased specificity for the Mls-1a antigen.

Multi-gene/allele control of Mlsb of CBA/H

Festenstein originally described the Mls locus as a single dominant autosomal gene with four alleles which mapped in the 13th linkage group of chromosome 1. We subsequently presented evidence indicating that the mixed leukocyte reaction (MLR) stimulatory products of DBA/2 and CBA/J were controlled by two independently segregating Mls loci and that Mls of C3H was in fact a composite of three independently segregating loci. Recently, Mlsd of CBA/J was shown to be composed of Mlsa of AKR and a product on C3H, which was presumed to be Mlsc. Based on strain distributions, this product cannot be encoded by the Mlsc originally defined by Festenstein. In the present report, three Mls specificities of CBA/H (Mlsb) are defined. Based on the strain distribution, we postulate that these specificities are controlled by three loci, three alleles/locus, or by some combination of the preceding two possibilities.

Multigene control of Mlsc

Festenstein originally described the Mls locus as a single dominant autosomal gene with four alleles which mapped in the 13th linkage group of chromosome 1. We subsequently presented evidence which indicated that the mixed leukocyte reaction stimulatory products of DBA/2 and CBA/J were controlled by two independently segregating Mls loci. Recently, Mlsd of CBA/J was shown to be composed of Mlsa of AKR and Mlsc of C3H. In the present report, classic segregation data is presented which indicates that Mlsc of C3H is controlled by three independently segregating loci. As defined by stimulatory patterns of numerous cell lines, we postulate the following: either one of the loci is shared with BALB.K, CE, C58, and partially with MA/MyJ, one is shared with CBA/H and CBA/J, and one is shared with BALB.K, CBA/J, and partially with CE; or the groups of shared determinants are controlled by different alleles of unique loci (or locus). In any event, Mlsc appears to be composed of at least three independently segregating loci; the number of alleles/locus is being investigated. In addition, C3H was stimulated by BALB.K (both were recently postulated to be Mlsc); this epitope was shared with CBA/J, CBA/H, AKR/Cum, Ma/MyJ, and C58/J.

Neonatal tolerance induction to Mlsa. I. Tolerance to Mlsa is restricted by shared MHC determinants

In this paper we have examined the influence of MHC (major histocompatibility complex) on neonatal tolerance to Mlsa (minor lymphocyte stimulating). By employing a novel approach we have shown that tolerance to Mlsa is restricted by shared MHC determinants. Thus, neonatal Mlsb mice, injected at birth with spleen cells from Mlsa mice, were tested as adults for Mlsa specific responses by interleukin-2 limiting dilution analysis, a technique which allows us to discriminate between responses to MHC + Mlsa and to MHC alone. Tolerance to Mlsa was in the context of any MHC type examined--donor, host, and third-party MHC products. These results show that tolerance to Mlsa is restricted by shared MHC determinants and extend previous studies indicating that activation of Mlsa responses is similarly restricted.

The expression of Mlsc determinants on Mlsa, Mlsb, and Mlsx prototypic strains

In the mouse system, specific determinants other than major histocompatibility complex (MHC) gene products are capable of inducing strong primary proliferative responses in naive T cells. These determinants are encoded by at least two gene loci designated as minor lymphocyte stimulatory (Mls) loci. In order to elucidate the biological role of the Mls system, an effort has been initiated to clarify the fundamental immunogenetic characteristics of the Mls system. In this report, we describe the unexpected finding that Mlsc determinants are expressed on splenocytes of strains including those which have been used as prototypic examples of three other Mls types: Mlsa (DBA/2, DBA/1), Mlsb (BALB/c), and Mlsx (PL/J). The expression of Mlsc by these strains was demonstrated both by the response patterns of unprimed T cells from MHC-identical inbred or F1 hybrid strains and by the responses of a panel of Mls-specific T-cell clones. The experimental results reported here also suggest that the expression of Mls determinants may be influenced by multiple other genes, including MHC-linked genes.

Unidirectional responses to Mls determinants in vivo. Polyclonal T-cell responses to a single common determinant of Mls in different efficiencies?

Polyclonal anti-Mls responses of peripheral (mature) and thymic (immature) lymphocytes were studied in vivo in terms of local host-versus-graft and graft-versus-host reactions. The level of stimulatory activity differed between the various types of Mls antigens, with Mlsa and Mlsd having the highest levels. Mlsc a lower level, and Mlsb the lowest level. The immunogenicity of Mlsa and Mlsd may not be identical, since Mlsd mice responded to the Mlsa determinant, while Mlsa mice did not react with the Mlsd determinants. This suggests that the anti-Mls response is unidirectional. The fact that Mlsb mice made tolerant at birth to Mlsd (Mlsa) antigens behave like Mlsd (Mlsa) mice when responding to Mls antigens supports this suggestion. Furthermore, thymus cells from the Mlsa-tolerized BALB/c (Mlsb) mice were unresponsive to both Mlsa and Mlsd antigens, while those made tolerant to Mlsd were responsive to Mlsa. These results indicate that the polyclonal response to the strongly immunogenic Mlsa,d antigens is unidirectional, and that the immunogenicity of Mlsa and Mlsd is not identical. Based on the results of tolerance experiments and data on T-cell clones, we suggest that the difference in immunogenicity between Mlsa,b,d antigens is due to different efficiencies of the responding T-cell populations, probably because of a quantitative difference in a single common antigenic determinant expressed in each Mls haplotype, which results in different levels of stimulation of T cells according to the avidity for the determinant.

T-cell receptor V beta use predicts reactivity and tolerance to Mlsa-encoded antigens

T lymphocytes reactive with the product of the Mlsa-allele of the minor lymphocyte stimulating (Mls) locus use a predominant T-cell receptor beta-chain variable gene segment (V beta 6). Such V beta 6-bearing T cells are selectively eliminated in the thymus of Mlsa-bearing mice, consistent with a model in which tolerance to self antigens is achieved by clonal deletion.

Self-tolerance eliminates T cells specific for Mls-modified products of the major histocompatibility complex

In mice the product of the Mlsa locus is an unusual antigen capable of interaction with certain products of the major histocompatibility locus (MHC) to form a ligand for a large portion of the T-cell alpha/beta receptor repertoire, including nearly all receptors that use V beta 8.1. The presence of Mlsa/MHC during T-cell development results in the deletion of T cells that express V beta 8.1, documenting the importance of clonal deletion in establishing tolerance to self antigens.

Mls allo-determinants are recognized in an MHC class II antigen-dependent but unrestricted fashion by a discrete set of T cells

Blocking studies carried out with anti-H-2 class II antigen antibodies show that H-2 class II molecules are intimately involved in the recognition of Mlsa determinants by unprimed, specifically responsive T cells. The blocking of the anti-Mlsa response by these antibodies were not due to inhibition of IL-1 production by H-2 class II antigen positive macrophages. A strain analysis indicates that the response to Mlsa is regulated by H-2-linked genes and that this effect is exerted at the level of the stimulator cells; however, the response to Mls epitopes does not appear to be H-2 restricted. Finally, the ability of spleen cells from Mls-incompatible mice to induce a state of Mls-specific, clonal deletion type tolerance by neonatal injection, shows that Mls determinants exist in qualitatively allelic forms and suggests that these determinants are recognized by specific clones of T cells.

Augmented in vitro presentation of Mls determinants after anti-immunoglobulin-induced B cell activation: ontogeny and role of purified B cells

Murine splenocytes which contained B cells activated by in vivo exposure to affinity-purified goat anti-mouse IgD (GaMD) antibody were utilized to present major histocompatibility complex (MHC) and non-MHC minor lymphocyte-stimulating (Mlsa) determinants in a primary mixed lymphocyte reaction (MLR). As the time in hours after in vivo exposure to GaMD increased, splenocytes from adult mice showed a co-ordinate increase in cell size, expression of public and private MHC class II antigenic determinants and MHC and Mlsa antigen-presenting capacity. This augmented alloantigen-presenting capacity was demonstrable with either irradiated or mitomycin C-treated adult splenocytes. In contrast, GaMD-treated neonatal splenocytes from 10-day-old mice demonstrated no significantly increased class II expression or enhanced MHC stimulatory capacity, but nevertheless triggered augmented responder cell proliferation across an Mlsa barrier. Thus, increased class II expression or presenting capacity may not be required for an augmentation in splenocyte Mls-stimulating ability to occur. In vitro exposure of T cell-depleted splenocytes or highly purified small resting B cells to GaMD or lipopolysaccharide (LPS) induced a substantially increased ability in those populations to present MHC and Mlsa antigens in a primary MLR. Hence in vivo or in vitro activation of B lymphocytes in a stimulator cell population may yield more effective presentation of MHC and non-MHC determinants.

Mls determinants and anti-Mls receptors

We review evidence from this laboratory that T cell recognition of Mlsa determinants is not controlled solely by the alpha-beta T cell receptor (TcR) molecule. We propose a model in which Mlsa recognition reflects a receptor-ligand interaction between two sets of complementary accessory molecules, one molecule (Mlsa) being expressed on B cells and the other (the anti-Mlsa receptor) on T cells; this interaction augments recognition of self class II molecules by the TcR. The biological role of Mls molecules might be to facilitate physiological T-B interaction.

Genetic complexity of Mls

For almost 20 years, little new has been described for Mls or the products it encodes. In the present report, data is presented which indicate that either the number of loci or the number of alleles per locus that control Mls-like products is much larger than the two-locus model or five allele-model presently envisaged.

An external stimulus that mimics Mls locus responses

The response to a novel set of T cell mitogens has been analysed and compared to the response to Mls locus differences. These polyclonal T cell activators, staphylococcal enterotoxins A and B, stimulate T cells in a way that requires an antigen-presenting cell bearing class II MHC products and involves the CD4:T cell receptor complex. However, the specificity of MHC recognition by the T cell receptor is lost in this response. Thus, these mitogens produce a response with characteristics similar to that induced by Mls locus differences. These mitogens can be used to analyse the immunobiology of this response, and may help in understanding and identifying the Mls locus product as well.

Mls determinants recognized by T cells

The relationship among different minor lymphocyte stimulatory locus (Mls) determinants, Mlsa, Mlsb, Mlsc and Mlsd, remains unclear. Because of the high degree of cross-reactivity between Mlsa and Mlsd determinants, the weak stimulatory capacity of Mlsc, and the generally non-stimulatory nature of Mlsb, some investigators have recently suggested that Mls is composed of only a single expressed allele originally defined as the a and d alleles. In order to clarify the nature of Mls determinants, T cell clones positively selected for reactivity to the three stimulatory Mls determinants, Mlsa, Mlsc and Mlsd, were generated and their specificities defined by extensive genetic studies. The response pattern of these Mls-specific clones as well as the results of primary mixed lymphocyte responses indicated that: (i) Mlsa and Mlsc determinants recognized by T cells are distinct, demonstrating that polymorphism does exist within the Mls system; and (ii) Mlsd is not an independent Mls type since Mlsd determinants are composed of Mlsa and Mlsc determinants and anti-Mlsd T cell responses are, in fact, the sum of anti-Mlsa and anti-Mlsc responses.

High frequency of Mlsa-reactivity among Ab-restricted, H-Y-specific T cell clones

A high frequency (80%) of Ab-restricted, H-Y-specific T cell clones, but not of Ab-restricted LDH-B-specific T cell clones, was found to be Mlsa-reactive. This reactivity could be inhibited by class II (A and E)-specific antibodies, and required an E-positive stimulator cell. The proliferation was also blocked by T cell receptor-specific and by CD4-specific antibodies. Possible interpretations of these findings are discussed.

Activation of abundant precursors associated with the response of Mlsa H-2k spleens to Mlsc antigen

We have determined the frequency of splenic interleukin 2-producing T-cell precursors that respond to H-2-identical, Mls-disparate stimulating cells. Gene products of the Mlsa and Mlsd alleles are highly stimulatory in primary mixed lymphocyte reactions and activate substantial numbers of T-cell precursors in normal spleens. Although the product of the Mlsc allele has been described as weakly stimulatory in primary mixed lymphocyte reactions, we detected abundant T-cell precursors responsive to Mlsc under our limiting dilution conditions; H-2k Mlsa T-cell precursors responding to Mlsc were as frequent (2.605 per 1000) as H-2k Mlsb or Mlsc T-cells responsive to Mlsd (1.975 per 1000). In contrast, using the frequency of self-responding cells as a threshold, we were unable to detect precursors for Mlsa, Mlsb, Mlsc, or Mlsd in H-2b mice, for Mlsb in H-2k mice, or for Mlsc in H-2k Mlsb or Mlsd mice. Thus, we conclude that in select strain combinations and under our experimental conditions Mlsc stimulators can activate T-cell precursors that are as abundant as precursors reactive to strong stimulators Mlsa and Mlsd.