The staphylococcal enterotoxins and their relatives

Molecular insights into human immunodeficiency virus type 1 pathogenesis

Infection with human immunodeficiency virus type 1 leads to a persistent but progressive cytopathic process that culminates in the near complete destruction of the CD4+ subset of T cells. The levels of human immunodeficiency virus type 1 replication and virus burden increase throughout the clinical course of disease reflecting a balance between the viral and cellular regulatory influences as well as the ability of the host immune system to eliminate infected T cells. Human immunodeficiency virus type 1 replication is dependent on the state of cellular activation and involves both inducible host cell derived transcription factors and at least three virus-derived gene products. Further study of the mechanism of action of these factors, particularly those encoded by the virus, may facilitate the future development of highly specific and effective therapies for human immunodeficiency virus type 1.

Molecular characterization of Mls-1

Recently a series of endogenous and exogenous superantigens have been described which have one common feature, namely, they lead to in vivo deletion and in vitro stimulation of T cells expressing particular T cell receptor V beta genes. The Mls antigens represent the prototypes of these molecules. We have mapped Mls-1 to the endogenous mammary tumor virus (MMTV) Mtv-7, while other SAG have also been associated with various MMTV. The open reading frame gene of the MMTV encodes the SAG. Thus, the new terminology MMTV sag has been proposed for this gene. Transfection experiments suggest that the expression of MMTV sag is tightly controlled, probably by a negative acting factor encoded within the open reading frame. Furthermore, a pronounced IL-4 effect is seen in the functional detection of the transfected Mtv-7 sag. Since this lymphokine does not influence the mRNA level of the endogenous or transfected MMTV genes, it is likely that it exerts its effect by increasing transcription of MHC class II genes, whose products are required for functional detection of Mls. We have identified one mouse strain, MA/MyJ, which has an Mls-1 phenotype but does not contain Mtv-7. The SAG activity of this strain was mapped to a new mammary tumor provirus, Mtv-43, not seen in other inbred strains. Sequence analyses revealed that the predicted amino acid sequences of the Mtv-7 and the Mtv-43 sag genes are very similar. This is particularly striking in the C-terminus, where all other MMTV sag sequences differ 100%. Thus, this region of the molecule seems to control the V beta specificity of SAG molecules. It is likely that the SAG expression provides an advantage for the infectious MMTV, probably by facilitating its transmission by T cells from the site of primary residence in the gut to its final destination, the mammary glands.

Molecular aspects of autoimmunity: a review

Molecular genetic techniques are being widely applied to the study of autoimmune diseases. Major advances have been made in diabetes, rheumatoid arthritis and coeliac disease. Work on experimental models of autoimmune uveitis suggests that similar advances will follow in this field. The application of molecular genetics to the study of immunology has lead to great advances in our understanding of the anatomy of antigen recognition. This work has lead to the identification of some of the structural determinants of antigen binding by MHC molecules and is helping to explain some MHC-disease associations. More recently, molecular studies of the T cell receptor have characterized patterns of T cell receptor expression in humans and have lead to the identification of regions of the T cell receptor critical for antigen recognition. These techniques will hopefully provide insights into the nature of autoimmunity and permit the identification of targets for disease specific immunotherapies. This review describes attempts to corelate MHC structure and function in the context of autoimmunity and discusses some of the strategies for analyzing T cell receptor usage in autoimmune disease.

Retroviral super-antigens and T cells

For many years immunologists have been intrigued by a series of potent antigens encoded in the murine genome. These antigens, originally termed minor lymphocyte stimulating (Mls) antigens, are capable of inducing extremely strong T cell proliferative responses when presented in the context of MHC class II molecules. Recently, Mls antigens have been shown to stimulate T cells bearing particular T cell receptor V beta elements, leading to the designation of super-antigens. The endogenous expression of these super-antigens in mice results in the clonal elimination of large numbers of T cells in order to maintain self-tolerance. In this review we discuss the recent identification of endogenous super-antigens as retroviral gene products. In addition, we analyze the role of class II MHC molecules in the presentation of endogenous super-antigens to T cells. Finally, we discuss the dramatic effect of retroviral super-antigens on the T cell repertoire.

T cell receptors, immunoregulation, and autoimmunity

T cell receptor (TCR) peptide vaccines have proven useful in the prevention and treatment of autoimmune disease in animal models. Prospects for developing TCR peptide vaccines for human autoimmune disease are only now being explored. Preliminary indications provide cause for optimism that immunization with TCR peptides eventually will be a viable treatment option for autoimmune pathologies in humans. In the long term, development of this technology may permit reliable manipulation of T cell immunity, leading to treatments for autoimmunity, T lymphoproliferative disorders, and, in the broadest interpretation, any pathogenesis mediated by oligoclonal T cell populations.

Factors involved in peripheral T cell tolerance: the extent of clonal deletion or clonal anergy depends on the age of the tolerized lymphocytes

After injection of SEB (staphylococcus enterotoxin B), normal adult mice, or thymectomized irradiated mice (TX irr.) reconstituted with lymphocytes taken from normal adult mice became specifically tolerant of SEB. At the same time the percentage of Vbeta8 positive CD4 lymphocytes known to be responsive to SEB was almost 50% decreased, indicating that a high level of clonal deletion was realized. In contrast, mice with an exclusively old T cell compartment (old thymectomized mice, TX irr. mice reconstituted several months previously) became tolerant of SEB without deleting their Vbeta8 + CD4 + cells, indicating that clonal anergy was the major mechanism in play in the induction of tolerance. Finally, TX irr. mice reconstituted with single positive thymocytes known to become recent thymic emigrants developed tolerance for SEB together with a high level (70%) of clonal deletion. Altogether these results indicated that the mechanism involved in peripheral tolerance depended on the age of the lymphocyte: very young lymphocytes underwent mainly clonal deletion whereas long lived lymphocytes underwent predominantly clonal anergy.

Effects of growth factors, hormones, bacterial lipopolysaccharides, and lipotechoic acids on the clonal growth of normal ureteral epithelial cells in serum-free culture

In vitro tissue culture techniques were employed to study the effects of bacterial endotoxins on the growth of normal epithelial cells from the human ureter (NHU). Primary cultures of NHU cells were initiated from explant outgrowth cultures of human ureteral tissue and cultured on collagen gel in F-12* medium containing 1% fetal calf serum (FCS). Optimal clonal growth of secondary cultures of NHU cells seeded at relatively low seeding cell densities, directly on plastic dishes, was achieved in F-12* medium containing bovine pituitary extract (0.5% BPE) and 0.05% BSA. Results indicated that insulin in the F-12* medium could be replaced by three orders of magnitude less IGF-1. Further clonal growth experiments demonstrated that PGE1 is growth stimulatory and can replace BPE as a growth factor requirement. This finding was in agreement with the fact that BPE growth requirement could be replaced by cholera toxin or dibutyryl cAMP. These results suggested that both BPE and cholera toxin operated by activation of a cAMP-dependent mitogenic pathway. Seven gram-negative bacterial lipopolysaccharides (LPS) and three gram-positive bacterial lipotechoic acids (LT) were tested for their effects on NHU clonal growth. Three out of the five LPS derived from Escherichia coli (strains 055:B5, 0128:B12, and 0127:B8), LPS from Klebsiella pneumoniae, and LPS from Pseudomonas aeruginosa all showed significant growth inhibitory effects at minimally effective doses ranging from 5 to 25 micrograms/ml. LPS derived from E. coli strain (0111:B4) had no growth effects at the highest concentration tested (100 micrograms/ml). In contrast, LT derived from Streptococcus pyogenes, S. faecalis, Staphylococcus aureas, and Bacillus subtilis all markedly enhanced clonal growth at concentrations ranging from 1 microgram/ml less than [LT] less than 50 micrograms/ml. LT from Strep. pyogenes was inhibitory to clonal growth at 100 micrograms/ml. The growth inhibitory effects of LPS were shown to be sensitive to the presence of hydrocortisone in the growth medium, indicating that LPS effects on growth are mediated via the arachidonic acid cascade. We speculate that these results indicate a link between the susceptibility of uroepithelial tissue to the pathogenic microflora seen in urinary tract diseases and the differential sensitivity of proliferation-competent uroepithelial cells to growth inhibition by LPS produced by gram-negative bacteria. However, further studies with uropathogenic serotypes will be necessary to corroborate this possibility. The growth-stimulating activity of LTs produced by gram-positive bacteria may be due to their ability to bind to cell-associated fibronectin and to activate the fibronectin receptor as part of ligand receptor-induced mitogenic transmembrane signalling pathway.

Stimulator cell-dependent requirement for CD2- and LFA-1-mediated adhesions in T lymphocyte activation by superantigenic toxins

The staphylococcal enterotoxins and related microbial T cell mitogens stimulate T cells by cross-linking variable parts of the T cell receptor (TCR) with MHC class II molecules on accessory or target cells. We have used cloned human T cells and defined tumor cells as accessory cells (AC) to study the requirements for T cell activation by these toxins. On AC expressing high levels of CD54 (intercellular adhesion molecule-1, ICAM-1) and CD58 (lymphocyte function-associated antigen-3, LFA-3), mAb to CD2 were relatively ineffective in inhibiting the response to the toxins and antibodies to the lymphocyte function-associated antigen-1 (LFA-1) did not inhibit at all. If added together, however, these mAb inhibited the response completely. Similar results were obtained using antibodies to the target structures of CD2 and LFA-1. In contrast, on cells expressing low levels of LFA-3, mAb to LFA-1 but not to CD2 were strongly inhibitory. The same pattern of inhibition was found when these same cells were used as presenters of specific antigen to the T cells. These data show that adhesions via CD2 or LFA-1 are alternatively required for the stimulation of the T cells by superantigenic toxins and demonstrate another similarity between T cell stimulation by superantigens and by specific antigen recognition.

Streptococcus associated toxic shock

In the past few years, there appears to have been a change in the spectrum of disease caused by group A beta-haemolytic streptococcus (GABHS), and a toxic shock-like syndrome caused by this organism has recently been described in adults. We report four children with an acute illness characterised by rapid progression of shock, erythematous rash, multisystem organ involvement, electrolyte derangements, and desquamation who fulfil the previously established diagnostic criteria for toxic shock syndrome. Three of the children had extensive cutaneous and soft tissue infection and the fourth had peritonitis. All four developed bacteraemia. Treatment included aggressive cardiovascular resuscitation and antibiotic therapy. Although no patient died, they suffered multiple and severe complications requiring prolonged treatment and hospitalisation. Streptococcal toxic shock syndrome is a separate and clearly defined entity occurring in previously healthy children.

Endogenous ligands selecting T cells expressing particular V beta elements

It has recently become clear that the minor lymphocyte stimulatory antigens (Mls) and other endogenous ligands which lead to the partial or total deletion of T cells bearing particular V beta segments are encoded by mouse mammary tumor virus (MMTV). We review here the genetic analyses of multiple V beta 11 and V beta 3 deletion ligands and demonstrate the involvement of MMTV in all examples. Several features of Mls and the V beta 11/V beta 3 deleting ligands identify them as members of the superantigen family. Bacterial superantigens are known to bind both MHC class II and the TCR in regions distinct from conventional peptide antigens. Within the MMTV genome, the 3' LTR has been identified as encoding superantigen function. We present data demonstrating that in vitro translation identifies the major product of the open reading frame (ORF) within the 3' LTR as a type II integral membrane glycoprotein. It is proposed that the type II membrane glycoprotein interacts with MHC and TCR in a manner analogous to the bacterial superantigens and distinct from conventional peptide antigen. Several unanswered questions regarding superantigen action remain; what determines total or partial deletion? How is Mls transferred between cells? These questions are addressed in the discussion.

Immunopotentiation of anti-viral and anti-tumor immune responses using anti-T cell receptor antibodies and mitogens

Although the immunosuppressive properties of anti-CD3 mAbs are now widely recognized, we have accumulated data characterizing the T cell activating properties of these antibodies. While in some situations these activating properties may be viewed as unwanted side-effects (for instance OKT3-mediated T cell activation may be responsible for some of the first dose toxicity seen with patients receiving OKT3 for suppression of allograft rejection), we have shown that anti-CD3 mAb therapy can augment host immune responses and provide protection against some tumors and viral infections. Importantly, this augmented response allows the development of long term, specific immunity. Because the immunosuppressive and activating properties of anti-CD3 mAbs are so closely overlapping, we have sought to identify other agents that are capable of activating T cell subsets selectively. We have found that SEB activates T cell subsets selectively in vivo and that this activation can be exploited to prevent the outgrowth of a malignant murine tumor. Studies currently in progress, including phenotypic and functional analysis of TILs and in vivo T cell subset depletions, should result in a more precise understanding of how SEB-induced T cell activation inhibits tumor growth.

Thymocyte activation and death: a mechanism for molding the T cell repertoire

The programmed death of thymocytes and T cells was studied. Injection of anti-TCR antibodies into adult mice caused the specific deletion of CD4+ CD8+ thymocytes, an effect that was largely reversed by cyclosporin A. Surprisingly, using either anti-TCR antibodies or superantigens, it was found that the susceptibility of these thymocytes to clonal deletion changed during ontogeny. Double positive thymocytes from newborn and young (3 week old) mice were readily depleted, whereas thymocytes from 1 week old mice were relatively refractory. The differences between these groups could not be accounted for by cell surface TCR expression, TCR-mediated early signal transduction pathways such as phosphoinositide hydrolysis or Ca2+ mobilization, or differences in susceptibility to Dex- or ionomycin-induced programmed cell death. These results suggest that there is a relatively synchronous wave of maturing thymocytes that are susceptible to deletional signals during fetal life and shortly after birth, but not 7 days after birth. By 3 weeks of age, the next wave (or waves) of susceptible cells have populated the thymus. These observations closely follow the experimental model known as "neonatal tolerance," and we suggest that the failure to tolerize 1 week old mice in that system reflects an alteration in the cells' susceptibility to clonal deletion. In a separate set of experiments exploring the mechanisms of PCD, it was found that although the activation- and glucocorticoid-induced PCD pathways were distinct (being distinguishable by their sensitivity to CsA and the glucocorticoid antagonist RU-486), they were mutually antagonistic. Attempts to identify the level of the antagonism failed to demonstrate any direct interference between the two stimuli, up to and including the transcription and translation of a GRE-controlled reporter gene. Based upon these observations, we propose the following model of thymocyte development: glucocorticoids eliminate thymocytes with little or no avidity for self; antagonism between glucocorticoids and cellular activation allows thymocytes that recognize self with low or moderate avidity to survive (positive selection); activation of thymocytes that recognize self with high avidity dominates the antagonistic effect of glucocorticoids, leading to PCD (negative selection).

Rheumatoid arthritis

The immunopathogenesis of rheumatoid arthritis is discussed in two ways. First, we consider the major question of whether T cells are likely to drive the disease. Second--and assuming T cells to be important--we discuss available data on the components of the trimolecular complex (major histocompatibility complex class II-antigen-T-cell receptor), which are possibly involved in the disease. Our two main points are that the most important questions concerning the pathogenesis of rheumatoid arthritis require answers from immunointervention in patients, and that animal experiments can be increasingly used in interpreting current experiments in humans.

Stimulation of human T cells by microbial 'superantigens'

The enterotoxins and the TSST of S. aureus, the erythrogenic toxins A and C of S. pyogenes and a still uncharacterized exoprotein of M. arthritidis belong to a family of exotoxins that have in common a potent mitogenic activity for T lymphocytes of several species. These proteins stimulate CD4+ and C8+ T cells, as well as a fraction of gamma delta TCR-bearing T cells by cross-linking variable parts of the T cell antigen receptor with MHC class II molecules on accessory or target cells. They are functionally bivalent molecules having distinct interaction sites for variable parts of the TCR and for nonpolymorphic parts of the MHC class II molecule. For alpha beta TCR-bearing T cells the V beta is the dominant site of interaction with the toxins. However, there is only a preferential but not exclusive stimulation of T cells carrying a certain V beta, because T cell clones carrying e.g. V beta 5 or V beta 8 can respond also to those toxins that do not stimulate V beta 5+ and V beta 8+ T cells in bulk cultures. Therefore, different TCR bind to these toxins with different affinities and the specificity of the TCR-toxin interaction is quantitative rather than qualitative in nature. Murine T cells respond to the mitogen of M. arthritidis that is a natural pathogen for mice and rats much better than to the toxins of the human pathogenic bacteria, whereas the opposite is true for human T cells. This could indicate that the toxins have been adapted to the host's immune system in evolution. The T cell-stimulating activity contributes to the pathogenesis of the respective diseases.

Reduction of lupus nephritis in MRL/lpr mice by a bacterial superantigen treatment

The effects of biweekly intravenous injections of Staphylococcus Enterotoxin B (SEB) into autoimmune MRL-lpr/lpr (MRL/lpr) mice were investigated. Rather than causing the expansion of V beta 8+ T cells, SEB administration resulted in the reduction V beta 8+, CD4-CD8- "double-negative" (DN) T cells. This was shown by FACS analysis as this putative pathogenic population was diminished in both spleen and lymph node. The symptoms of systemic lupus erythematosus (SLE) in MRL/lpr, which include high titers of anti-DNA antibodies and circulating immune complexes and proteinuria, were reduced in SEB-treated mice in a dose-dependent manner. The clinical parameters of SLE in MRL/lpr, which include lymph node hyperplasia and necrotic vasculitis, were suppressed in 50-micrograms SEB-treated mice. T cells bearing V beta 6 T cell receptor, which does not interact with SEB, were not reduced with SEB administration. Thus, disease suppression was associated with a specific reduction in the number of V beta 8+, DN T cells. These results implicate a possible therapeutic role of superantigen-based immunotherapy in V beta-restricted, T cell-dominated clinical syndromes.

Maintenance of multiallelic polymorphism at the MHC region

Models that purport to explain the maintenance of MHC polymorphism must be able to explain a variety of phenomena. (1) The range of MHC allele frequencies at some of the loci is very large, with some alleles quite common and many others rare, while at others the range of allele frequencies is far narrower. (2) MHC alleles and their frequencies often have long persistence times, in some cases tens of millions of years. (3) Random-mating populations appear to be in Hardy-Weinberg equilibrium for MHC. (4) There is no obvious, strong and consistent selection pressure yet detected that acts differentially on different MHC genotypes. (5) Because the allelic composition of the MHC polymorphism does change over evolutionary time, the MHC system must be capable of accommodating new alleles with similar properties without destruction of the equilibria that permit the maintenance of the older alleles. In this review I examined the degree to which a large number of models that have been proposed fit these criteria. These include heterosis, marginal overdominance, conditional heterosis, assortative mating, maternal-fetal incompatibility, molecular mimicry, minority advantage, pathogen adaptation, and optimum allele frequency models. Most of the models do poorly at accounting for a number of the above phenomena. The last class, optimum allele frequency models, have the most satisfactory set of properties. However, optimum allele frequency models require mechanisms that somehow "feed back" from the frequency of an allele in the population to the fitness of an organism carrying that allele. Thus, these models require that MHC polymorphisms be maintained by some type of group selection. Evidence for an against optimum allele frequency selection, and ways in which this type of selection might be detected experimentally, are presented.

Limited T-cell receptor beta-chain heterogeneity among interleukin 2 receptor-positive synovial T cells suggests a role for superantigen in rheumatoid arthritis

Rheumatoid arthritis (RA) is a disease affecting the synovial membranes of articulating joints that is thought to result from T-cell-mediated autoimmune phenomena. T cells responsible for the pathogenesis of RA are likely present in that fraction of synovial T cells that expresses the interleukin 2 receptor (IL-2R), one marker of T-cell activation. We report herein an analysis of T-cell receptor (TCR) beta-chain gene expression by IL-2R-positive synovial T cells. These T cells were isolated from uncultured synovial tissue specimens by using IL-2R-specific monoclonal antibodies and magnetic beads, and TCR beta-chain transcription was analyzed by PCR-catalyzed amplification using a panel of primers specific for the human TCR beta-chain variable region (V beta). Multiple V beta gene families were found to be transcribed in these patients samples; however, three gene families, V beta 3, V beta 14, and V beta 17, were found in a majority of the five synovial samples analyzed, suggesting that T cells bearing these V beta s had been selectively retained in the synovial microenvironment. In many instances, the V beta 3, V beta 14, or V beta 17 repertoires amplified from an individual patient were dominated by a single rearrangement, indicative of clonal expansion in the synovium and supportive of a role for these T cells in RA. Of note is a high sequence similarity between V beta 3, V beta 14, and V beta 17 polypeptides, particularly in the fourth complementarity-determining region (CDR). Given that binding sites for superantigens have been mapped to the CDR4s of TCR beta chains, the synovial localization of T cells bearing V beta s with significant CDR4 homology indicates that V beta-specific T-cell activation by superantigen may play a role in RA.

Identification of the staphylococcal enterotoxin A superantigen binding site in the beta 1 domain of the human histocompatibility antigen HLA-DR

The staphylococcal enterotoxin A (SEA) is a superantigen that must bind to class II molecules of the major histocompatibility complex to be recognized by T cells. In humans, most HLA-DR class II allelic and isotypic forms, such as DR1, bind SEA well. DRw53 is an exception, binding SEA very poorly. We have localized this difference to a single residue (amino acid 81) in the beta 1 domain. A highly conserved histidine at residue 81 allows SEA binding, but a tyrosine does not. Residue 81 is predicted to lie in an alpha-helix on the surface of the molecule, with its side chain pointing up out of the pocket associated with binding of conventional peptide antigens. This finding supports the hypothesis that superantigens and conventional antigens bind to different sites on the class II molecule.

Selective depletion in HIV infection of T cells that bear specific T cell receptor V beta sequences

The mechanism of T cell depletion during infection with the human immunodeficiency virus (HIV) is unclear. Examination of the repertoire of T cell receptor V (variable) regions in persons infected with HIV revealed the absence of a common set of V beta regions, whereas V alpha usage was normal. The lack of these V beta segments did not appear to correlate with opportunistic infections. The selective elimination of T cells that express a defined set of V beta sequences may indicate the presence of an HIV-encoded superantigen, similar to those encoded by the long terminal repeat of the mouse mammary tumor virus.

Crosslinked staphylococcal enterotoxin B stimulates CD8+ T cells only in the presence of unlinked costimulator signals

The superantigen staphylococcal enterotoxin B (SEB) binds to class II MHC expressing cells and subsequently causes selective activation of T cells carrying appropriate T cell receptor (TCR) V beta chains. Apparently SEB acts as a bifunctional molecule by bridging class II MHC structures with the appropriate TCR-V beta chains. This assumption predicts that immobilized SEB ought to stimulate purified, class II MHC negative murine T cells. We show here that immobilized SEB lacks the ability to trigger murine CD8 T cells. Responsiveness obtained at a high T cell concentration is due to contaminating class II MHC-positive lymphocytes. Complementation of the culture system with syngeneic irradiated B cells blasts effectively restores responsiveness. The proliferating cells exhibit SEB specific cytotoxicity and a bias for V beta 8 expression. Since no evidence for leakiness of SEB covalently bound to sephadex beads was obtained, the data imply that immobilized SEB in fact binds to the TCR of T cells expressing the appropriate V beta chains. However, for primary activation additional costimulatory signals are required which can be provided in an unlinked fashion by activated B cells. Resting B cells are activated by immobilized SEB to cells expressing high costimulator activity. As such, the data point out a third function of SEB.

Characterization and clonal distribution of four alleles of the speA gene encoding pyrogenic exotoxin A (scarlet fever toxin) in Streptococcus pyogenes

Streptococcus pyogenes strains producing pyrogenic exotoxin A (scarlet fever toxin) have recently caused episodes of streptococcal toxic-shock-like syndrome (TSLS). We exploited knowledge of genetic diversity and relationships among exotoxin A-producing patient strains provided by multilocus enzyme electrophoresis to select strains for comparative sequencing of toxin genes. Our analysis identified four alleles of speA in natural populations, one of which (speA1) occurs in many distinct clonal lineages and is probably old. Two other alleles (speA2 and speA3), characterized solely by single amino acid substitutions, were each identified in single clones that together have caused the majority of TSLS episodes. It is unlikely that these alleles have had a long association with S. pyogenes clones. A fourth allele (speA4) also is present in a single phylogenetic lineage and is 9% divergent from the other three toxin alleles. An absence of synonomous (silent) nucleotide changes in speA2 and speA3 is unusual and suggests that the allelic variation is not selectively neutral, which implies that the toxins are not functionally equivalent. These results may be important in helping to understand the recent increase in frequency and severity of disease caused by S. pyogenes.

The zeta chain is associated with a tyrosine kinase and upon T-cell antigen receptor stimulation associates with ZAP-70, a 70-kDa tyrosine phosphoprotein

Stimulation of the T-cell antigen receptor (TCR) leads to tyrosine phosphorylation of a number of cellular proteins, including phospholipase C (PLC) gamma 1 and the TCR zeta chain. We describe here a 70-kDa tyrosine phosphoprotein (ZAP-70) that associates with zeta within 15 sec following TCR stimulation. The phosphorylation of ZAP-70 and its association with zeta is independent of the other TCR chains since stimulation of a functional CD8/zeta chimeric receptor in a TCR-negative T cell leads to coprecipitation of ZAP-70 with the chimeric protein. In a Jurkat cell expressing the TCR and the CD8/zeta chimeric protein, tyrosine phosphorylation and association of ZAP-70 occurs exclusively with the stimulated receptor complex. In addition, a tyrosine kinase that does not appear to be fyn associates with the cytoplasmic domain of zeta and phosphorylates zeta and ZAP-70 in vitro.

Monoclonal antibody-targeted superantigens: a different class of anti-tumor agents

The bacterial superantigen staphylococcal enterotoxin (SE) A (SEA) directs cytotoxic T lymphocytes (CTLs) expressing particular sequences of the T-cell receptor (TCR) beta chain to lyse tumor cells expressing major histocompatibility complex (MHC) class II molecules, which serve as receptors for SEs. We now report that chemical conjugates of SEA and the colon carcinoma-reactive monoclonal antibodies (mAbs) C215 or C242 mediate T cell-dependent destruction of colon carcinoma cells lacking MHC class II molecules. SEA was covalently linked to the mAbs C215 and C242 via a PEG-based hydrophilic spacer. The C215-SEA conjugate targeted CD4+ as well as CD8+ CTLs to lyse a panel of colon carcinoma cells lacking MHC class II molecules. T-cell recognition of mAb-SEA conjugates was SEA specific, since SEB-selective T-cell lines with potent cytotoxic activity towards Raji cells coated with SEB did not respond to the C215-SEA conjugate. Unconjugated SEA did not induce T-cell lysis of MHC class II- colon carcinoma cells but efficiently directed CTLs against MHC class II+ Raji cells and certain interferon-treated MHC class II+ colon carcinoma cells. These results suggest that SEA-mAb conjugates retain the SEA-related selectivity for certain TCR beta-chain variable region (V beta) sequences but, in contrast to unconjugated SEA, mediate the TCR interaction in a MHC class II-independent manner. The cytotoxic activity mediated by C215-SEA and C242-SEA conjugates was blocked by excess of C215 mAb and C242 mAb, respectively, showing that the specificity in the targeting of mAb-SEA conjugates is defined by the antigen reactivity of the mAb. These results demonstrate that bacterial superantigens may be successfully conjugated to mAb with preserved T cell-activating capacity. The circumvention of MHC class II binding of SEs by conjugation to mAb suggests that such conjugates may find general application as antitumor agents, taking advantage of the extreme T cell-activating potency of superantigens.

Allelic exclusion at DNA rearrangement level is required to prevent coexpression of two distinct T cell receptor beta genes

In mice double transgenic for functionally rearranged T cell receptor (TCR) V beta 2 and V beta 8.2 genes we found that most T lymphocytes express both TCR beta chains simultaneously. These T cells show no abnormality in thymic selection in vivo and their TCRs are capable of transducing activation signals in vitro. These results indicate that multispecific T cells may appear in the periphery if allelic exclusion of TCR beta genes is not established at the level of gene rearrangement.

Induction of interleukin-1 in human monocytes by the superantigen staphylococcal enterotoxin A requires the participation of T cells

Nanogram quantities of the bacterial superantigen Staphylococcal Enterotoxin A (SEA) induced significant amounts of extracellular IL-1 alpha and IL-1 beta in human peripheral blood mononuclear cells. Induction of maximal IL-1 alpha and IL-1 beta levels by lipopolysaccharide (LPS) required microgram quantities. LPS induced detectable extracellular IL-1 content within 3-6 hr and maximal levels were detected already after 12 hr. Induction of IL-1 production by SEA showed a delayed release with peak values after 24-48 hr. IL-1 beta was the major species of IL-1 seen in both SEA- and LPS-stimulated culture supernatants. SEA was in general a relatively stronger inducer of extracellular IL-1 alpha than LPS. SEA-induced extracellular IL-1 production in human monocytes was entirely dependent on the presence of T cells, whereas addition of T cells to LPS-stimulated purified human monocytes only marginally enhanced the extracellular IL-1 production. The capacity to induce extracellular IL-1 production in monocytes in response to SEA was high in the CD4+ 45RO+ memory T cell subset, whereas CD4+ 45RA+ naive T cells and CD8+ T cells had lower IL-1-inducing capacity. The T cell help for IL-1 production could not be replaced by a panel of T cell-derived recombinant lymphokines added to SEA-stimulated monocytes, including IFN-gamma and TNF, indicating the participation of cell membrane-bound ligands or hitherto unidentified soluble mediators.

Influence of major histocompatibility complex haplotype on the mitogenic response of T cells to staphylococcal enterotoxin B

The abilities of antigen-presenting cells (APC) from nine independent major histocompatibility complex haplotypes and a number of intra-H-2 recombinant congenic strains of mice to present staphylococcal enterotoxin B (SEB) and induce proliferation in murine T-cell receptor V beta 8+ T-cell clones were compared. SEB presented by APC of all haplotypes tested induced significant responses in each of the T-cell clones. The magnitude of response was similar for most haplotypes, but there were limited quantitative differences between certain haplotypes. SEB presented by APC from H-2b mice as well as the intra-H-2 recombinant strains B10.GD and B10.A(4R), which do not express cell surface I-E (designated I-E-), induced the poorest T-cell responses. However, APC from AfE-, AsE-, and AqE- mice were as potent in SEB presentation as APC expressing both I-A and I-E. Antibodies against I-E were more effective than anti-I-A antibodies at inhibiting responses to SEB presented by APC expressing both I-A and I-E, whereas responses induced by APC expressing I-A but not I-E were blocked by antibodies against I-A. Thus, our results show that I-A can present SEB efficiently but that expression of both I-A and I-E on the same APC results in presentation of SEB predominantly by I-E. In addition, experiments using four distinct I-E- strains of mice indicate that I-A alleles differ in their ability to present SEB.

Mls--a retrovirus exploits the immune system

The identity of minor lymphocytes stimulating (Mls) antigens, endogenous superantigens that can activate, or induce the deletion of, large portions of the T-cell repertoire, has recently been revealed: they are encoded by mouse mammary tumor viruses (MMTV) that have integrated into the germ line as DNA proviruses. As Hans Acha-Orbea and Ed Palmer point out, Mls-mediated modulation may be only the tip of the retrovirus iceberg; already murine leukemia virus (MuLV), with similar superantigen properties, has been discovered.

The T-cell-receptor repertoire in the synovial fluid of a patient with rheumatoid arthritis is polyclonal

We have analyzed the T-cell-receptor repertoire expressed in the synovial fluid of a patient with rheumatoid arthritis by using an inverse polymerase chain reaction. Total RNA was isolated from Ficoll-purified mononuclear cells and converted into circularized double-stranded cDNA. Specific amplification of alpha- and beta-chain variable regions (V alpha and V beta) was achieved with inverted alpha- and beta-chain constant region (C alpha and C beta) primer pairs, and the amplification products were cloned into phage vectors. A total of 78 alpha and 76 beta clones were sequenced, and 67 and 72 productively rearranged alpha and beta genes were identified, respectively. Thirty-one V alpha, 33 alpha-chain joining region (J alpha), 29 V beta, and 12 beta-chain joining region (J beta) gene segments were found in the productively rearranged clones, indicating that the T-cell repertoire expressed in the synovial fluid of this RA patient is highly heterogenous and polyclonal. Comparison of peripheral blood and synovial fluid repertoires showed that the most abundant V beta sequences, V beta 2.1 and V beta 3.1, were enriched in the inflamed joint by a factor of 2 to 3. It is possible that T cells expressing these V beta gene segments, which recognize bacterial superantigens, play a role in the disease.

New approaches to the therapy of demyelinating disease

The pace of research on the pathogenesis and treatment of multiple sclerosis, the principal human demyelinating disease of the central nervous system, has intensified in the past 3 years, due in part, to the application of advances in molecular and cellular immunology. Many lessons that have been learned in an animal model of central nervous system demyelinating disease, experimental allergic encephalomyelitis, also apply to multiple sclerosis and certain successful approaches for the treatment of this disease are now being attempted in humans.

Staphylococcus aureus enterotoxin mediated specific non-responsiveness of human T cells

Exotoxins produced by certain strains of Staphylococcus aureus are able to both stimulate and induce non-responsiveness in T cells expressing specific T cell antigen receptor V beta gene elements. The exposure of human CD4+ T cells to the appropriate enterotoxin rendered them anergic to restimulation with their natural ligand, although responsiveness to exogenous IL-2 remained intact. The loss of antigen-dependent proliferation was associated with the down-regulation of the TCR complex that was paralleled by enhanced cell surface CD2 and CD25. Further analysis of the phenotypic changes revealed that membrane levels of CD28 were increased only on activation, suggesting a differential expression of this protein on activated and anergic T cells. During the induction of anergy it was observed that the synthesis of the lymphokines IL-2, IL-4 and IFN-gamma was differentially regulated. IL-4 and IFN-gamma, but not IL-2 were detected in the supernatants of overnight cultures of T cells exposed to tolerising concentrations of toxin. Transcription of IL-4, as determined by polymerase chain reaction at selected intervals, was elevated during the induction of anergy and accounted for the presence of the protein in the supernatants. In contrast, no tight coupling was observed between protein and mRNA levels for IL-2, suggesting post-translational regulation.

Conventional antigen and superantigen may be coupled to distinct and cooperative T-cell activation pathways

CD4+ T cells are equipped to detect two major classes of ligands. Infectious microbial agents, including bacteria and retroviruses, carry a class of proteins termed superantigens that are recognized by the T-cell receptor in association with class II products of the major histocompatibility complex. Proteins expressed by other cells and organisms are processed by macrophages into peptides that are presented to CD4+ T cells by class II molecules. We have examined CD4+ T-cell clones that proliferate vigorously in response both to conventional peptide antigens and to bacterial or retroviral superantigens. The response to peptide antigen is characterized by a rapid and sustained increase in the levels of intracellular free Ca2+ and a vigorous cytokine response. In contrast, the proliferative response of these clones to bacterial or retroviral superantigen is not accompanied by detectable increases in intracellular Ca2+ or by significant cytokine production. Further analysis of T-cell activation indicates that interaction of a single T-cell receptor with the two types of ligand may be coupled to functionally distinct signaling pathways that interact in a synergistic fashion to achieve T-cell activation.

Characterization of human T cells reactive with the Mycoplasma arthritidis-derived superantigen (MAM): generation of a monoclonal antibody against V beta 17, the T cell receptor gene product expressed by a large fraction of MAM-reactive human T cells

While all known microbial superantigens are mitogenic for human peripheral blood lymphocytes (PBL), the functional response induced by Mycoplasma arthritidis-derived superantigen (MAM) is unique in that MAM stimulation of PBL consistently results in T cell-dependent B cell activation characterized by polyclonal IgM and IgG production. These immunostimulatory effects of MAM on the humoral arm of the human immune system warranted a more precise characterization of MAM-reactive human T cells. Using an uncloned MAM reactive human T cell line as immunogen, we have generated a monoclonal antibody (mAb) (termed C1) specific for the T cell receptor V beta gene expressed by the major fraction of MAM-reactive human T cells, V beta 17. In addition, a V beta 17- MAM-reactive T cell population exists, assessed by MAM, induced T cell proliferation and cytotoxic T cell activity. mAb C1 will be useful in characterizing the functional properties of V beta 17+ T cells and their potential role in autoimmune disease.

Analysis of T cell stimulation by superantigen plus major histocompatibility complex class II molecules or by CD3 monoclonal antibody: costimulation by purified adhesion ligands VCAM-1, ICAM-1, but not ELAM-1

Many ligands of adhesion molecules mediate costimulation of T cell activation. The generality of this emerging concept is best determined by using model systems which exploit physiologically relevant ligands. We developed such an "antigen-specific" model system for stimulation of resting CD4+ human T cells using the following purified ligands: (a) major histocompatibility complex class II plus the superantigen Staphylococcus enterotoxin A, to engage the T cell receptor (TCR); (b) adhesion proteins vascular cell adhesion molecule 1 (VCAM-1), intercellular adhesion molecule 1 (ICAM-1), and endothelial leukocyte adhesion molecule 1 (ELAM-1), to provide potential cell surface costimulatory signals; and (c) recombinant interleukin 1 beta (rIL-1 beta)/rIL-6 as costimulatory cytokines. In this biochemically defined system, we find that resting CD4+ T cells require costimulation in order to respond to TCR engagement. This costimulation can be provided by VCAM-1 or ICAM-1; however adhesion alone is not sufficient since ELAM-1 mediates adhesion but not costimulation. The cytokines IL-1 beta and IL-6 by themselves cannot mediate costimulation, but augment the adhesion ligand-mediated costimulation. Direct comparison with the model of TCR/CD3 engagement by CD3 monoclonal antibody demonstrated comparable costimulatory requirements in both systems, thereby authenticating the commonly used CD3 model. The costimulation mediated by the activation-dependent interaction of the VLA-4 and LFA-1 integrins with their respective ligands VCAM-1 and ICAM-1 leads to increased IL-2R alpha (CD25) expression and proliferation in both CD45RA+ CD4+ and CD45RO+ CD4+ T cells. The integrins also regulate the secretion of IL-2, IL-4, and granulocyte/macrophage colony-stimulating factor. In contrast the activation-independent adhesion of CD4+ T cell to ELAM-1 molecules does not lead to T cell stimulation as measured by proliferation, IL-2R alpha expression, or cytokine release. These findings imply that adhesion per se is not sufficient for costimulation, but rather that the costimulation conferred by the VLA-4/VCAM-1 and LFA-1/ICAM-1 interactions reflects specialized accessory functions of these integrin pathways. The new finding that VLA-4/VCAM-1 mediates costimulation adds significance to observations that VCAM-1 is expressed on a unique set of potential antigen-presenting cells in vivo.

The contribution of non-MHC genes to susceptibility to autoimmune diseases

Genetic studies of experimental models of autoimmune diseases, including systemic lupus-like syndromes and organ-specific autoimmunity, provide major information on genetic control of autoimmune diseases. In addition to genes known to be linked to the major histocompatibility complex (MHC), these studies point to multiple genes located outside the MHC that influence the onset and the progression of autoimmune diseases. Identification of these genes and of their interrelationships is now a major task that will be facilitated by recent progress in molecular biology and gene mapping. Among candidate genes, antigen-receptor genes (i.e., immunoglobulin- and T-cell receptor genes) most likely contribute an important part of the autoimmune susceptibility in several of these animal models. Available linkage data suggest a similar involvement of these antigen-receptor genes in several human autoimmune diseases. In addition to a better understanding of pathogenic mechanisms associated with autoimmunity, the knowledge of these disease-predisposing genes is expected to permit a better classification of often complex syndromes as well as the design of new treatments.

Biodegradable and biocompatible poly(DL-lactide-co-glycolide) microspheres as an adjuvant for staphylococcal enterotoxin B toxoid which enhances the level of toxin-neutralizing antibodies

Microspheres composed of biocompatible, biodegradable poly(DL-lactide-co-glycolide) (DL-PLG) and staphylococcal enterotoxin B (SEB) toxoid were evaluated as a vaccine delivery system when subcutaneously injected into mice. As measured by circulating immunoglobulin G (IgG) antitoxin titers, the delivery of SEB toxoid via DL-PLG microspheres, 1 to 10 microns in diameter, induced an immune response which was approximately 500 times that seen with nonencapsulated toxoid. The kinetics, magnitude, and duration of the antitoxin response induced with microencapsulated toxoid were similar to those obtained when an equal toxoid dose was administered as an emulsion with complete Freund adjuvant. However, the microspheres did not induce the inflammation and granulomata formation seen with complete Freund adjuvant. The adjuvant activity of the microspheres was not dependent on the superantigenicity of SEB toxin and was equally effective at potentiating circulating IgG antitrinitrophenyl levels in response to microencapsulated trinitrophenyl-keyhole limpet hemocyanin. Empty DL-PLG microspheres were not mitogenic, and SEB toxoid injected as a mixture with empty DL-PLG microspheres was no more effective as an immunogen than toxoid alone. Antigen-containing microspheres 1 to 10 microns in diameter exhibited stronger adjuvant activity than those greater than 10 microns, which correlated with the delivery of the 1- to 10-microns, but not the greater than 10-microns, microspheres into the draining lymph nodes within macrophages. The antibody response induced through immunization with microencapsulated SEB toxoid was protective against the weight loss and splenic V beta 8+ T-cell expansion induced by intravenous toxin administration. These results show that DL-PLG microsphere vaccine delivery systems, which are composed of pharmaceutically acceptable components, possess a strong adjuvant activity for their encapsulated antigens.

Staphylococcal exotoxins stimulate nitric oxide-dependent murine macrophage tumoricidal activity

The staphylococcal exotoxins toxic shock syndrome toxin 1 (TSST-1) and enterotoxin B were tested for their ability to stimulate murine peritoneal macrophages (PM) for tumoricidal activity. Both toxins were found to stimulate oil-elicited, gamma interferon-primed PM monolayers to kill nonadherent P815 tumor targets. The mechanism of killing of toxin-stimulated tumoricidal activity involved the production of nitric oxide, as nitrite could be demonstrated in culture fluids, and NG-monomethyl-L-arginine, an inhibitor of nitric oxide production, abrogated toxin-stimulated tumoricidal activity. TSST-1 stimulated the secretion of tumor necrosis factor by PM monolayers in the presence and absence of gamma interferon. The mechanism of toxin-stimulated tumoricidal activity was also determined to be independent of the production of reactive oxygen intermediates in that TSST-1 failed to stimulate H2O2 production by PM. These results demonstrate that the staphylococcal exotoxins are capable of stimulating macrophage production of nitric oxide for tumor cytotoxicity and suggest that the nitric oxide thus produced may subsequently play a role in the pathogenesis of the diseases caused by these toxins.

Stimulation of CD2-negative T cells by staphylococcal enterotoxin superantigens

A minor fraction of CD3+ T cells lacks expression of the CD2 antigen, which is the target for an "alternative" T cell activation pathway. CD2-CD3+ T cells can be stimulated by anti-CD3 or anti-T cell receptor (TCR) antibodies, indicating that the CD3/TCR signal transduction pathway functions in the absence of cell surface CD2. In the present study we have analyzed whether CD2-CD3+ T cells also respond to antigen stimulation. We show here that cloned CD2-negative T cells expressing the alpha/beta TCR are activated by one or several staphylococcal enterotoxin "superantigens". Activation of CD2-CD3+ T cell clones by staphylococcal enterotoxins resulted in IL-2 production and/or proliferative activity, and was dependent on the presence of HLA class II-bearing feeder cells. These data demonstrate that T cells can recognize (and respond to) antigen in the absence of a functional CD2 molecule.

Altered antigen receptor signaling in anergic T cells from self-tolerant T-cell receptor beta-chain transgenic mice

T-cell tolerance to the minor lymphocyte-stimulating antigen Mls-1a in a T-cell receptor (TcR) V beta 8.1 transgenic line of mice is maintained by both clonal deletion and clonal anergy. Approximately 20-50% of peripheral CD4+ (but not CD8+) T cells isolated from these mice are anergic and fail to proliferate following TcR ligation. We have examined key events in T-cell signaling in peripheral T cells isolated from these mice. In this report, we show that the anergic CD4+ T cells did not mobilize calcium or express receptors for interleukin 2 (IL-2) following TcR ligation. However, the cells retained viability and functional potential because stimulation with phorbol 12-myristate 13-acetate and ionomycin bypassed the block in receptor-mediated signaling and induced IL-2 receptor expression and proliferation of the anergic cells.

Relative ability of distinct isotypes of human major histocompatibility complex class II molecules in binding staphylococcal enterotoxin A

Relative ability of distinct isotypes of human major histocompatibility complex class II molecules to bind staphylococcal enterotoxin A (SEA) was investigated. SEA-binding was observed in L cells transfected with DR2 and DQw1 genes. By contrast, it was not detected in L cells transfected with DPw4 and DP (Cp63) genes. All the transfectants supported SEA-induced IL-2 production by human T cells. Levels of the accessory activity were low in the DPw4 and DP (Cp63) transfectants compared with the DR2 and DQw1 transfectants. In view of the observation that all the transfectants express well the transfected gene products on their surface, the results indicate that DR and DQ molecules bind SEA with high affinity, while DP molecules bind it with fairly low affinity.

The Mycoplasma arthritidis T-cell mitogen, MAM: a model superantigen

The superantigens are receiving a great deal of attention as a new group of potent immunomodulatory molecules. They are produced by diverse microbial agents including staphylococci, streptococci and mycoplasmas and are also encoded by murine tumor viruses (the Mls antigens). Superantigens activate T cells by a unique pathway which can lead to modification of the T-cell repertoire and induction of autoimmunity. Here, Barry Cole and Curtis Atkin review their observations on the Mycoplasma arthritidis superantigen, MAM, and discuss how MAM might contribute to the acute and chronic inflammatory disease mediated by this organism.

Evidence for the effects of a superantigen in rheumatoid arthritis

While studying the alpha beta T cell receptor repertoire in rheumatoid arthritis (RA) patients, we found that the frequency of V beta 14+ T cells was significantly higher in the synovial fluid of affected joints than in the peripheral blood. In fact, V beta 14+ T cells were virtually undetectable in the peripheral blood of a majority of these RA patients. beta-chain sequences indicated that one or a few clones dominated the V beta 14+ population in the synovial fluid of individual RA patients, whereas oligoclonality was less marked for other V beta's and for V beta 14 in other types of inflammatory arthritis. These results implicate V beta 14-bearing T cells in the pathology of RA. They also suggest that the etiology of RA may involve initial activation of V beta 14+ T cells by a V beta 14-specific superantigen with subsequent recruitment of a few activated autoreactive v beta 14+ T cell clones to the joints while the majority of other V beta 14+ T cells disappear.

Characterization of rabbit ileal receptors for Clostridium difficile toxin A. Evidence for a receptor-coupled G protein

The purpose of this study was to characterize the surface receptor for toxin A, the enterotoxin from Clostridium difficile, on rabbit intestinal brush borders (BB) and on rat basophilic leukemia (RBL) cells. Purified toxin A was radiolabeled using a modified Bolton-Hunter method to sp act 2 microCi/micrograms, with retention of full biologic activity. 3H-Toxin A bound specifically to a single class of receptors on rabbit BB and on RBL cells with dissociation constants of 5.4 x 10(-8) and 3.5 x 10(-8) M, respectively. RBL cells were highly sensitive to toxin A (cell rounding) and had 180,000 specific binding sites per cell, whereas IMR-90 fibroblasts were far less sensitive to toxin A and lacked detectable specific binding sites. Exposure of BB to trypsin or chymotrypsin significantly reduced 3H-toxin A specific binding. Preincubation of BB with Bandeirea simplicifolia (BS-1) lectin also reduced specific binding, and CHAPS-solubilized receptors could be immobilized with WGA-agarose. The addition of 100 nM toxin A accelerated the association of 35S-GTP gamma S with rabbit ileal BB, and preincubation of BB with the GTP analogues GTP gamma S or Gpp(NH)p, significantly reduced 3H-toxin A specific binding. Our data indicate that the membrane receptor for toxin A is a galactose and N-acetyl-glucosamine-containing glycoprotein which appears to be coupled to a G protein.

Protective effect of cyclosporin A on immune abnormalities observed in the murine acquired immunodeficiency syndrome

The murine leukemia viruses (MuLV) designated LP-BM5 induce an immunodeficiency disease in susceptible strains of mice with many features in common to human acquired immunodeficiency syndrome (AIDS), including lymphadenopathy and profound immunodeficiency associated with enhanced susceptibility to infection and terminal B cell lymphomas. The disease, termed murine AIDS (MAIDS), crucially depends on the presence of B cells and CD4+ T cells, suggesting that mutual activation of these two cell types is central in the pathogenesis of the immunodeficiency syndrome. Cyclosporin A (CsA), whose immunosuppressive effect is attributed mainly to inhibition of interleukin 2 and interferon-gamma expression, interferes in T-B cell interactions. Here we show that chronic treatment with CsA (40 or 60 mg/kg/day) before and after infection with LP-BM5 MuLV protects against the development of immunodeficiency disease as assessed by functional, serological and histopathological criteria. The protection was not complete, suggesting both CsA-sensitive and CsA-resistant components to the pathogenesis of this syndrome, and was found to be independent of ecotropic MuLV expression. These results underline immunopathological mechanisms in the progression of immune abnormalities in MAIDS that are susceptible to inhibition of CsA and may serve as an experimental basis for developing a treatment of the human disorder with immunomodulators.

Characterization of T-cell subsets and T-cell receptor subgroups in pigtailed macaques using two- and three-color flow cytometry

In order to characterize macaque T-lymphocyte subsets, we used a chromophore from a dinoflagellate, peridinin chlorophyll A protein (PerCP), which, like fluorescein isothiocyanate (FITC) and R-phycoerythrin (PE), can be excited by a 488-nm laser and emits light at 670 nm without spectral overlap with FITC and PE. Mouse monoclonal antibodies were conjugated with FITC, PE, and PerCP to detect CD4+ and CD8+ cells in macaque peripheral blood lymphocytes (PBL) subsets before and after activation and in nonactivated thymocytes. Resting and activated macaque blood CD4+ T-cells could be clearly delineated into discrete subsets with either CD28, CD45RA, or CD45RO as a second marker and CD26, CD29, CD44, or CD69 as a third marker. CD8+ cells were further subdivided by expression of similar combinations of markers. A subset of CD8+ CD28- T-cells in blood expressed the activation marker CD69, suggesting that they were already activated. Virtually all CD4+CD8+, CD4+CD8-, and CD4-CD8+ macaque thymocytes expressed CD2, CD3, and CD18 and not CD25, CD44, or CD45O, but macaque thymocyte subpopulations did differ in their expression of CD28 and CD29. The expression of T-cell receptor (TCR) subgroups on macaque PBL and thymocytes was analyzed before and after activation with staphylococcal enterotoxins (superantigens). The pattern of T-cell variable-region expression in macaques was similar to that seen in humans, with a high frequency of T cells expressing V beta 8. After superantigen stimulation, only minor changes in TCR V beta expression were detectable in PBL. A dramatic increase in V beta 8 expression was seen after stimulation of macaque thymus with staphylococcal enterotoxin D (SE-D), a minor increase after toxic shock syndrome toxin 1 (TSST-1) stimulation, and a simultaneous decrease in V beta 6 levels.