Costimulation of T Cell Activation by Integrin-associated Protein (CD47) Is an Adhesion-dependent, CD28-independent Signaling Pathway

The integrin-associated protein (IAP, CD47) is a 50-kD plasma membrane protein with a single extracellular immunoglobulin variable (IgV)-like domain, a multiply membrane-spanning segment, and alternatively spliced short cytoplasmic tails. On neutrophils, IAP has been shown to function in a signaling complex with β₃ integrins. However, the function of IAP on T cells, which express little or no β₃ integrin, is not yet defined. Here, we show that mAbs recognizing IAP can enhance proliferation of primary human T cells in the presence of low levels of antiCD3, but have no effect on T cell proliferation on their own. Together with suboptimal concentrations of anti-CD3, engagement of IAP also enhances IL-2 production in Jurkat cells, an apparently integrin-independent function of IAP. Nonetheless, costimulation by IAP ligation requires cell adhesion. IAP costimulation does not require CD28. Furthermore, anti-IAP, but not anti-CD28, synergizes with suboptimal anti-CD3 to enhance tyrosine phosphorylation of the CD3 ζ chain and the T cell–specific tyrosine kinase Zap70. Ligation of human IAP transfected into the hemoglobin-specific 3.L2 murine T cell hybridoma costimulates activation for IL-2 secretion both with anti-CD3 and with antigenic peptides on antigen-presenting cells (APCs). Moreover, ligation of IAP but not CD28 can convert antagonist peptides into agonists in 3.L2 cells. Using costimulation by IAP ligation as an assay to analyze the structure–function relationships in IAP signaling, we find that both the extracellular and multiply membrane-spanning domains of IAP are necessary for synergy with the antigen receptor, but the alternatively spliced cytoplasmic tails are not. These data demonstrate that IAP ligation initiates an adhesiondependent costimulatory pathway distinct from CD28. We hypothesize that anti-IAP generates the costimulatory signal because it modulates interactions of the IgV domain with other plasma membrane molecules; this in turn activates effector functions of the multiply membrane-spanning domain of IAP. This model may have general significance for how IAP functions in cell activation.     

Regulatory functions of CD8+CD28- T cells in an autoimmune disease model

CD8+ T cell depletion renders CD28-deficient mice susceptible to experimental autoimmune encephalomyelitis (EAE). In addition, CD8-/-CD28-/- double-knockout mice are susceptible to EAE. These findings suggest a role for CD8+ T cells in the resistance of CD28-deficient mice to disease. Adoptive transfer of CD8+CD28- T cells into CD8-/- mice results in significant suppression of disease, while CD8+CD28+ T cells demonstrate no similar effect on the clinical course of EAE in the same recipients. In vitro, CD8+CD28- but not CD8+CD28+ T cells suppress IFN-gamma production of myelin oligodendrocyte glycoprotein-specific CD4+ T cells. This suppression requires cell-to-cell contact and is dependent on the presence of APCs. APCs cocultured with CD8+CD28- T cells become less efficient in inducing a T cell-dependent immune response. Such interaction prevents upregulation of costimulatory molecules by APCs, hence decreasing the delivery of these signals to CD4+ T cells. These are the first data establishing that regulatory CD8+CD28- T cells occur in normal mice and play a critical role in disease resistance in CD28-/- animals.     

Redirected lysis of human melanoma cells by a MCSP/CD3-bispecific BiTE antibody that engages patient-derived T cells

Melanoma-associated chondroitin sulfate proteoglycan (MCSP; also called HMW-MAA, CSPG4, NG2, MSK16, MCSPG, MEL-CSPG, or gp240) is a well characterized melanoma cell-surface antigen. In this study, a new bispecific T-cell engaging (BiTE) antibody that binds to MCSP and human CD3 (MCSP-BiTE) was tested for its cytotoxic activity against human melanoma cell lines. When unstimulated peripheral mononuclear blood cells (PBMCs) derived from healthy donors were cocultured with melanoma cells at effector:target ratios of 1:1, 1:5, or 1:10, and treated with MCSP-BiTE antibody at doses of 10, 100, or 1000 ng/mL, all MCSP-expressing melanoma cell lines (n=23) were lysed in a dose-dependent and effector:target ratio-dependent manner, whereas there was no cytotoxic activity against MCSP-negative melanoma cell lines (n=2). To investigate whether T cells from melanoma patients could act as effector cells, we cocultured unstimulated PBMCs with allogeneic melanoma cells from 13 patients (4 stage I/II, 3 stage III, and 6 stage IV) or with autologous melanoma cells from 2 patients (stage IV). Although cytotoxic activity varied, all 15 PBMC samples mediated significant redirected lysis by the BiTE antibody. When PBMC or CD8 T cells were prestimulated by anti-CD3 antibody OKT-3 and interleukin-2, the MCSP-BiTE concentrations needed for melanoma cell lysis decreased up to 1000-fold. As MCSP is expressed on most human melanomas, immunotherapy with MCSP/CD3-bispecific antibodies merits clinical investigation.     

The modulation of CD40 ligand signaling by transmembrane CD28 splice variant in human T cells

The role of CD40 ligand (CD40L)/CD40 signaling in T cell-dependent B cell differentiation and maturation has been amply documented. The mechanism of CD40 signaling in B cells has been well established, whereas the signaling mechanism of CD40L in T cell costimulation remains unknown. In this study we show that CD28i, a transmembrane splice variant of CD28 costimulatory receptor, complexes with CD40L in human T cells. The cross-linking of CD40L resulted in the coendocytosis of CD28i with CD40L. The tyrosine phosphorylation of CD28i followed the cross-linking of CD40L, and the overexpression of CD28i augmented the c-Jun NH2-terminal kinase, p21-activated kinase 2, and nuclear factor kappaB activation. These data indicate that CD28i, by functioning as a signaling adaptor, transduces CD40L signaling as well as CD28 signaling in human T cells.     

Developmental regulation of Lck targeting to the CD8 coreceptor controls signaling in naive and memory T cells.

The question of whether enhanced memory T cell responses are simply due to an increased frequency of specific cells or also to an improved response at the single cell level is widely debated. In this study, we analyzed T cell receptor (TCR) transgenic memory T cells and bona fide memory T cells isolated from virally infected normal mice using the tetramer technology. We found that memory T cells are qualitatively different from naive T cells due to a developmentally regulated rearrangement of the topology of the signaling machinery. In naive cytotoxic T cells, only a few CD8 molecules are associated with Lck and the kinase is homogeneously distributed inside the cell. However, in vivo priming of naive T cells induces the targeting of Lck to the CD8 coreceptor in the cell membrane and the consequent organization of a more efficient TCR signaling machinery in effector and memory cells.     

Proline residues in CD28 and the Src homology (SH)3 domain of Lck are required for T cell costimulation.

The Src family tyrosine kinases Lck and Fyn are critical for signaling via the T cell receptor. However, the exact mechanism of their activation is unknown. Recent crystal structures of Src kinases suggest that an important mechanism of kinase activation is via engagement of the Src homology (SH)3 domain by proline-containing sequences. To test this hypothesis, we identified several T cell membrane proteins that contain potential SH3 ligands. Here we demonstrate that Lck and Fyn can be activated by proline motifs in the CD28 and CD2 proteins, respectively. Supporting a role for Lck in CD28 signaling, we demonstrate that CD28 signaling in both transformed and primary T cells requires Lck as well as proline residues in CD28. These data suggest that Lck plays an essential role in CD28 costimulation.     

CD4~+CD25~+调节性T细胞在肝硬化发病中的作用及意义

目的检测病毒性肝硬化患者外周血Treg细胞和IL-10细胞因子表达的变化,探讨其相关的临床意义。方法收集2014年6月到2016年6月我院收治的病毒性肝硬化患者84例,另取同期我院健康体检者40例作为正常对照。Treg细胞的表达使用流式细胞仪检测,IL-10的表达使用ELISA法检测。结果对照组Treg细胞比例和IL-10表达分别为5.14±1.82%和21.14±4.22ng/L,肝硬化组为2.56±1.02%和9.53±2.61ng/L,对照组上述指标均显著高于肝硬化组(P0.05,P0.01)。Child-Pugh评分分级A级、B级和C级患者Treg细胞比例和IL-10表达各级之间存在显著性差异(P0.05),其中A级显著高于B级(P0.05),B级又显著高于C级(P0.05)。Treg细胞比例和IL-10表达在乙肝病毒性肝硬化患者和丙肝病毒性肝硬化患者之间无显著性差异(P0.05)。结论病毒性肝硬化患者外周血中Treg细胞比例和IL-10表达均减低,提示其可能参与了肝硬化的发生、发展过程,并可能为肝硬化的治疗提供新思路。     

B7-1 Engagement of Cytotoxic T Lymphocyte Antigen 4 Inhibits T Cell Activation in the Absence of CD28

Ligation of cytotoxic T lymphocyte antigen 4 (CTLA4) appears to inhibit T cell responses. Four mechanisms have been proposed to explain the inhibitory activity of CTLA4: competition for B7-1 and B7-2 binding by CD28; sequestration of signaling molecules away from CD28 via endocytosis; delivery of a signal that antagonizes a CD28 signal; and delivery of a signal that antagonizes a T cell receptor (TCR) signal. As three of these potential mechanisms involve functional antagonism of CD28, an experimental model was designed to determine whether CTLA4 could inhibit T cell function in the absence of CD28. TCR transgenic/recombinase activating gene 2–deficient/CD28–wild-type or CD28-deficient mice were generated and immunized with an antigen-expressing tumor. Primed T cells from both types of mice produced cytokines and proliferated in response to stimulator cells lacking B7 expression. However, whereas the response of CD28^+/+ T cells was augmented by costimulation with B7-1, the response of the CD28^−/− T cells was strongly inhibited. This inhibition was reversed by monoclonal antibody against B7-1 or CTLA4. Thus, CTLA4 can potently inhibit T cell activation in the absence of CD28, indicating that antagonism of a TCR-mediated signal is sufficient to explain the inhibitory effect of CTLA4.     

On the role of CD4+ T cells in the CD8+ T-cell response elicited by recombinant adenovirus vaccines.

We have investigated the role of CD4(+) T cells in the development of the CD8(+) T-cell response after immunization with recombinant adenovirus (rAd). In the absence of CD4(+) T cells, the "unhelped" CD8(+) T-cell population exhibited a reduction in primary expansion and long-term survival that appeared to be due to inadequate priming of na?ve T cells. There were few functional or phenotypic differences between the helped and unhelped CD8(+) T-cell populations with the exception of O-glycosylated CD43, a marker of effector cells, which was augmented on the unhelped CD8(+) T-cell population. In some cases, the unhelped CD8(+) T-cell population exhibited reduced ability to control virus infection; however, this appeared to be a function of the reduced frequency of antigen-specific CD8(+) T cells. Most notably, the unhelped CD8(+) T-cell population exhibited no defect in secondary expansion. These results provide insight into the role of CD4(+) T cells during the primary CD8(+) T-cell response generated by rAd vaccines and identify potential benefits and issues that must be considered when using adenovirus vaccines under conditions where CD4(+) T-cell function may be limiting, such as vaccination of human immunodeficiency virus patients.     

慢性乙肝患者外周血CD4+细胞HLA-DR、CD8+细胞CD28表达的检测及其意义

目的 检测乙肝患者外周血CD4^＋淋巴细胞表面HLA-DR和CD8^＋淋巴细胞表面CD28的表达情况,评价乙肝患者的细胞免疫状态。方法 荧光抗体CD4-PECY5、HLA-DR-FITC和CD8-PE、CD28-FITC标记淋巴细胞,流式细胞仪分别测定患者CD4^＋淋巴细胞表面HLA-DR和CD8^＋淋巴细胞表面CD28表达的百分率,并与HBV-DNA结果比较。结果 ①与正常对照组比较,乙肝患者组外周血CD4^＋淋巴细胞表面HLA-DR的表达显著升高（P〈0.001）,CD8^＋淋巴细胞表面CD28的表达显著降低（P〈0.01）;②乙肝患者DNA阳性（〉4000拷贝/ml）与阴性（〈4000拷贝/ml）组CD4^＋淋巴细胞表面HLA-DR和CD8^＋淋巴细胞表面CD28的表达比较均无显著性差异（P〉0.1）。结论 乙肝患者外周血CD4^＋细胞的免疫活化增强,CD8^＋细胞与抗原递呈细胞结合的作用减弱;淋巴细胞的活化情况与病毒复制情况及含量多少无关。     

CD27 promotes survival of activated T cells and complements CD28 in generation and establishment of the effector T cell pool

CD27, like CD28, acts in concert with the T cell receptor to support T cell expansion. Using CD27(-/-) mice, we have shown earlier that CD27 determines the magnitude of primary and memory T cell responses to influenza virus. Here, we have examined the relative contributions of CD27 and CD28 to generation of the virus-specific effector T cell pool and its establishment at the site of infection (the lung), using CD27(-/-), CD28(-/-), and CD27/CD28(-/-) mice. We find that primary and memory CD8+ T cell responses to influenza virus are dependent on the collective contribution of both receptors. In the primary response, CD27 and CD28 impact to a similar extent on expansion of virus-specific T cells in draining lymph nodes. CD27 is the principle determinant for accumulation of virus-specific T cells in the lung because it can sustain this response in CD28(-/-) mice. Unlike CD28, CD27 does not affect cell cycle activity, but promotes survival of activated T cells throughout successive rounds of division at the site of priming and may do so at the site of infection as well. CD27 was found to rescue CD28(-/-) T cells from death at the onset of division, explaining its capacity to support a T cell response in absence of CD28.     

Activation of CD4+ T cells in the presence of a nondepleting monoclonal antibody to CD4 induces a Th2-type response in vitro

In vitro experiments using purified rat CD4+ T cells in primary and secondary mixed leukocyte cultures (MLC) have been carried out to explore the mechanism of inhibition of cell-mediated autoimmune disease in the rat by a nondepleting monoclonal antibody (mAb) to CD4. Previous work has shown that W3/25, a mouse anti-rat CD4 mAb of immunoglobulin G1 isotype, completely prevents the development of the paralysis associated with experimental allergic encephalomyelitis (EAE) in Lewis rats, but does so without eliminating the encephalitogenic T cells. The in vitro experiments described in this study have shown that when CD4+ T cells were activated in the presence of the anti-CD4 mAb in a primary MLC, the synthesis of interferon (IFN) gamma, but not interleukin (IL) 2, was completely inhibited. After secondary stimulation, now in the absence of the mAb, the synthesis of IL-4 and IL-13 mRNA was greatly enhanced compared with that observed from CD4+ T cells derived from primary cultures in which the mAb was omitted. As IL-4 and IL-13 are known to antagonize cell-mediated immune reactions, and as EAE is cell- mediated disease, the data suggest that the W3/25 mAb controls EAE by modifying the cytokine repertoire of T cells that respond to the encephalitogen. The capacity for the mAb to suppress IFN-gamma synthesis provides, in part, an explanation for this change in cytokine production. These findings are discussed in terms of what is known of the factors that determine which cytokine genes are expressed on T cell activation. Possible implications for the evolution of T cell responses in human immunodeficiency virus infection are also discussed.     

Role of IL-2 secreted by PADRE-specific CD4+ T cells in enhancing E7-specific CD8+ T-cell immune responses

CD4(+) T helper cells are known to play an integral role in the generation of CD8(+) T-cell immune responses. We have previously shown that co-administration of DNA vaccines containing E6 or E7 protein of human papillomavirus 16 (HPV-16) combined with DNA encoding invariant (Ii) chain in which class II-associated Ii peptide (CLIP) region is replaced with the CD4(+) T helper epitope, PADRE (Pan-DR-epitope) (Ii-PADRE DNA) enhanced HPV antigen-specific CD8(+) T-cell immune responses in vaccinated mice. In the current study, we investigated the enhancement of HPV E7-specific CD8(+) T-cell immune responses by PADRE-specific CD4(+) T cells. We showed that intradermal administration of Ii-PADRE DNA at the same location as E7-expressing DNA is necessary to generate strong E7-specific CD8(+) T-cell immune responses. We also showed that PADRE-specific CD4(+) T cells generated by Ii-PADRE DNA vaccination expressed Th1 cytokine profile. Furthermore, our in vitro study demonstrated that PADRE-specific CD4(+) T cells stimulated with PADRE-loaded dendritic cells secrete IL-2 that leads to the proliferation of E7-specific CD8(+) T cells. Thus, our data suggest that activated PADRE-specific CD4(+) T helper cells may be required at the vicinity of E7-specific CD8(+) T cells where they secrete IL-2, which enhances the E7-specific CD8(+) T-cell immune responses generated by DNA vaccination.     

Potency of mature CD40L RNA electroporated dendritic cells correlates with IL-12 secretion by tracking multifunctional CD8(+)/CD28(+) cytotoxic T-cell responses in vitro

Electroporation of mature dendritic cells (DC) with RNA-encoding CD40L greatly enhances the production of interleukin (IL)-12, a proinflammatory cytokine necessary for the induction of T-cell immunity. Results presented herein reveal a correlation between the priming of CD28(+) antigen-reactive effector memory cytotoxic T lymphocytes (CTL) displaying 3 or 4 simultaneous effector functions and the quantity of IL-12 produced by postmaturation electroporation-CD40L DC. By using multiparameter flow cytometry, the quantities of IL-12 needed to prime naive antigen-reactive T cells to simultaneously produce interferon-γ and tumor necrosis factor-α in the presence or absence of IL-2 secretion in conjunction with lytic activity defined by CD107a expression can be used to determine the overall potency of a DC product. In the presence of IL-12, CTL differentiation toward lytic function is not accompanied by a reduction in the secretion of interferon-γ and tumor necrosis factor-α. Therefore, by measuring the availability of IL-12 one can predict the potency of a DC immunotherapeutics in relation to its ability to drive distinct effector memory CTL subsets with multifunctional activities.     

Effective Depletion of Regulatory T Cells Allows the Recruitment of Mesothelin‐Specific CD8+ T Cells to the Antitumor Immune Response Against a Mesothelin‐Expressing Mouse Pancreatic Adenocarcinoma

Vaccine‐induced CD8⁺ T‐cell responses can eradicate developing tumors in vivo in mouse models. Translating these successes into approved treatments for cancer patients has been challenging, since many of these models lack expression of clinically proven/relevant tumor antigens. We have shown that mesothelin is a clinically relevant CD8⁺ T‐cell target in human pancreas cancer, which is also highly conserved among species. Here, we utilize the murine mesothelin‐expressing pancreatic tumor model (Panc02) to identify the immune‐relevant mesothelin‐derived peptides and study interventions that enhance the antitumor response. We first screened overlapping peptides of the entire murine mesothelin protein to identify two new CD8⁺ mesothelin‐restricted epitopes. These peptides were then evaluated for recognition by vaccine‐induced T cells from mice treated with vaccine in sequence with low‐dose cyclophosphamide (CY) and an anti‐CD25 IL‐2Rα monoclonal antibody (PC61). These treatments are both known to deplete subpopulations of T regulatory cells (Tregs). Our findings demonstrate that combined Treg‐depleting therapies synergize to enhance vaccine efficacy. Furthermore, our data supports mesothelin as a relevant antigen in murine and clinical models and the use of Panc02 as a clinically relevant murine model of pancreatic cancer for evaluating antigen‐targeted immunotherapies in immune‐tolerant hosts.     

T Cell Receptor (TCR)-induced Death of Immature CD4+CD8+ Thymocytes by Two Distinct Mechanisms Differing in Their Requirement for CD28 Costimulation: Implications for Negative Selection in the Thymus

Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities. One mechanism of negative selection in developing T cells is the induction of apoptosis in immature CD4⁺CD8⁺ (DP) thymocytes, referred to as clonal deletion. Clonal deletion is necessarily T cell receptor (TCR) specific, but TCR signals alone are not lethal to purified DP thymocytes. Here, we identify two distinct mechanisms by which TCR-specific death of DP thymocytes can be induced. One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28. The other mechanism is initiated by TCR signals in the absence of simultaneous costimulatory signals and is mediated by subsequent interaction with antigen-presenting cells. We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.     

Activation of natural killer T cells by alpha-galactosylceramide rapidly induces the full maturation of dendritic cells in vivo and thereby acts as an adjuvant for combined CD4 and CD8 T cell immunity to a coadministered protein

The maturation of dendritic cells (DCs) allows these antigen-presenting cells to initiate immunity. We pursued this concept in situ by studying the adjuvant action of alpha-galactosylceramide (alphaGalCer) in mice. A single i.v. injection of glycolipid induced the full maturation of splenic DCs, beginning within 4 h. Maturation was manifest by marked increases in costimulator and major histocompatibility complex class II expression, interferon (IFN)-gamma production, and stimulation of the mixed leukocyte reaction. These changes were not induced directly by alphaGalCer but required natural killer T (NKT) cells acting independently of the MyD88 adaptor protein. To establish that DC maturation was responsible for the adjuvant role of alphaGalCer, mice were given alphaGalCer together with soluble or cell-associated ovalbumin antigen. Th1 type CD4+ and CD8+ T cell responses developed, and the mice became resistant to challenge with ovalbumin-expressing tumor. DCs from mice given ovalbumin plus adjuvant, but not the non-DCs, stimulated ovalbumin-specific proliferative responses and importantly, induced antigen-specific, IFN-gamma producing, CD4+ and CD8+ T cells upon transfer into naive animals. In the latter instance, immune priming did not require further exposure to ovalbumin, alphaGalCer, NKT, or NK cells. Therefore a single dose of alphaGalCer i.v. rapidly stimulates the full maturation of DCs in situ, and this accounts for the induction of combined Th1 CD4+ and CD8+ T cell immunity to a coadministered protein.     

Antibody and B7/BB1-mediated ligation of the CD28 receptor induces tyrosine phosphorylation in human T cells

CD28 is an adhesion receptor expressed as a 44-kD dimer on the surface of a major subset of human T cells. The CD28 receptor regulates the production of multiple lymphokines, including interleukin 2 (IL-2), by activation of a signal transduction pathway that is poorly understood. Here we show that ligation of CD28 by a monoclonal antibody (mAb) or by a natural ligand, B7/BB1, induces protein tyrosine phosphorylation that is distinct from T cell receptor (TCR)-induced tyrosine phosphorylation. CD28-induced protein tyrosine phosphorylation was greatly enhanced in cells that had been preactivated by ligation of the TCR, or by pretreatment with phorbol esters. Rapid and prolonged tyrosine phosphorylation of a single substrate, pp100, was induced in T cells after interaction with B7/BB1 presented on transfected Chinese hamster ovary (CHO) cells. Anti-B7 mAb inhibited B7/BB1 receptor-induced tyrosine phosphorylation, indicating that B7-CD28 interaction was required. CD28-induced tyrosine phosphorylation was independent of the TCR because it occurred in a variant of the Jurkat T cell line that does not express the TCR. Herbimycin A, a protein tyrosine kinase inhibitor, could prevent CD28-induced tyrosine phosphorylation and CD28-induced IL-2 production in normal T cells. The simultaneous crosslinking of CD28 and CD45, a tyrosine phosphatase, could prevent tyrosine phosphorylation of pp100. These results suggest that specific tyrosine phosphorylation, particularly of pp100, occurs directly as a result of CD28 ligand binding and is involved in transducing the signal delivered through CD28 by accessory cells that express the B7/BB1 receptor. Thus, this particular form of signal transduction may be relevant to lymphokine production and, potentially may provide a means to study the induction of self-tolerance, given the putative role of the costimulatory signal in the induction of T cell activation or anergy.     

Regulatory roles of T mu and T gamma cells in the collaborative cellular initiation of the extrinsic coagulation pathway by bacterial lipopolysaccharide

The Shwartzman reaction is a classic biologic response in which the coagulation system is activated in vivo. Cellular initiation of the extrinsic coagulation protease cascade can be mediated by one or more limbs of the lymphoid response to diverse biological stimuli. The T cell-instructed monocyte and macrophage responses that have been implicated are mediated by a number of different cellular pathways and are elicited not only by antigens and allogeneic cells but also by other stimuli such as immune complexes and the lipid A moiety of bacterial lipopolysaccharide (LPS). The latter response has been implicated in the pathogenesis of the disseminated intravascular coagulation associated with bacterial infection. In the rapid collaborative cellular pathway response to LPS, we have described a relatively rigorous requirement for T helper cells in induction of the biosynthesis of tissue factor and Factor VII by monocytes. To elucidate potential regulatory aspects of this cellular procoagulant response, we provide the first evidence for the existence of T suppressor cells for the cellular procoagulant response to LPS by the rapid T cell-instructed pathway. Human peripheral blood lymphocytes were separated by cytoaffinity into Fc gamma-positive and Fc mu-positive cells and were characterized for their functional properties in the procoagulant response. T mu cells mediated the monocyte response, consistent with their identity with instructor cells. T gamma cells suppressed the response of monocytes to LPS in the presence of T mu cells, suggesting that they possess suppressor function for this response. The T gamma suppressor cells required stimulation by LPS to express their suppressor function and they exerted their suppressive effect directly on the monocyte. The existence and participation of LPS-responsive T suppressor cells on the cellular procoagulant response in vitro add a new dimension to the complexity of the rapid pathway of the collaborative cellular procoagulant response and may be important in the pathogenesis of disseminated intravascular coagulation.