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Many species of bacteria use quorum sensing to sense the amount of secreted metabolites and to adapt their growth according to their population density. We asked whether similar mechanisms would operate in lymphocyte homeostasis. We investigated the regulation of the size of interleukin-2 (IL-2)-producing CD4(+) T cell (IL-2p) pool using different IL-2 reporter mice. We found that in the absence of either IL-2 or regulatory CD4(+) T (T reg) cells, the number of IL-2p cells increases. Administration of IL-2 decreases the number of cells of the IL-2p cell subset and, pertinently, abrogates their ability to produce IL-2 upon in vivo cognate stimulation, while increasing T reg cell numbers. We propose that control of the IL-2p cell numbers occurs via a quorum sensing-like feedback loop where the produced IL-2 is sensed by both the activated CD4(+) T cell pool and by T reg cells, which reciprocally regulate cells of the IL-2p cell subset. In conclusion, IL-2 acts as a self-regulatory circuit integrating the homeostasis of activated and T reg cells as CD4(+) T cells restrain their growth by monitoring IL-2 levels, thereby preventing uncontrolled responses and autoimmunity.

Human CD8+ memory- and effector-type T cells are poorly defined. We show here that, next to a naive compartment, two discrete primed subpopulations can be found within the circulating human CD8+ T cell subset. First, CD45RA-CD45R0(+) cells are reminiscent of memory-type T cells in that they express elevated levels of CD95 (Fas) and the integrin family members CD11a, CD18, CD29, CD49d, and CD49e, compared to naive CD8+ T cells, and are able to secrete not only interleukin (IL) 2 but also interferon gamma, tumor necrosis factor alpha, and IL-4. This subset does not exert cytolytic activity without prior in vitro stimulation but does contain virus-specific cytotoxic T lymphocyte (CTL) precursors. A second primed population is characterized by CD45RA expression with concomitant absence of expression of the costimulatory molecules CD27 and CD28. The CD8+CD45RA+CD27- population contains T cells expressing high levels of CD11a, CD11b, CD18, and CD49d, whereas CD62L (L-selectin) is not expressed. These T cells do not secrete IL-2 or -4 but can produce IFN-gamma and TNF-alpha. In accordance with this finding, cells contained within this subpopulation depend for proliferation on exogenous growth factors such as IL-2 and -15. Interestingly, CD8+CD45RA+CD27- cells parallel effector CTLs, as they abundantly express Fas-ligand mRNA, contain perforin and granzyme B, and have high cytolytic activity without in vitro prestimulation. Based on both phenotypic and functional properties, we conclude that memory- and effector-type T cells can be separated as distinct entities within the human CD8+ T cell subset.

A soluble membrane component of type III pneumococcal polysaccharide-coupled spleen cells (S3-SCSM) induces S3-specific suppressor T cells (Ts) in mice. These Ts can be detected only if mice are pretreated with cyclophosphamide (Cy) or if cells adherent to the lectin Vicia villosa are removed from the spleen cell population prior to transfer. The V. villosa-adherent spleen cells from mice injected with S3-SCSM could abrogate suppression mediated by Ts induced by S3-SCSM in Cy-treated mice. The V. villosa-adherent contrasuppressor cells were shown to be T cells that were I-J+ and of the Lyt-1 phenotype. Contrasuppressor T cells (Tcs) were not present in V. villosa-adherent spleen cell fractions obtained from normal mice, from mice injected with polyvinylpyrrolidone-coupled spleen cells, or from Cy-treated mice injected with S3-SCSM, i.e., mice in which Ts activity is dominant. The V. villosa-adherent cells that abrogated the activity of Ts induced by S3-SCSM in Cy-treated mice did not abrogate suppression mediated by a different subset of S3-specific Ts, suggesting that the Tcs described here do not have activity against all Ts subsets. The ability of S3-SCSM to activate Tcs in normal mice provides an explanation for the inability to detect S3-specific Ts in several previous studies.

A set of T cell clones (TCC) isolated from HLA-DR-, Dw-, DQ-matched allogeneic MLCs was found to proliferate autonomously when stimulated with cells carrying a wide range of class I or II specificities. This apparently unrestricted proliferation was relatively weak, and only low levels of IL-2 were present in the supernatants of stimulated cells. Autologous as well as allogeneic PBMC and B lymphoblastoid cell lines (B-LCL) were capable of stimulating such clones, which were also restimulated by suppressive, but not by helper, TCC. Moreover, such clones displayed the unusual property of autostimulation. mAb inhibition experiments suggested that class II- or class II-restricted antigens were involved in stimulation. Thus, certain "broad" mAbs (TU39, SG520) reacting with multiple locus products inhibited activation of these reagents, but none of those reacting more specifically with DR (TU34, TU37, L243, Q2/70, SG157), DQ (TU22, SPV-L3, Leu 10), or DP (B7/21), or mixtures of these mAbs, were able to do so. Evidence from sequential immunoprecipitation experiments suggested that mAb TU39 bound class II-like molecules other than DR, DQ, and DP on TCC and B-LCL, and it is therefore proposed that such putative novel class II-like molecules may carry the stimulating determinants for these autoreactive clones. DY-reactive clones lacked helper activity for B cells but mediated potent suppressive activity on T cell proliferative responses that was not restricted by the HLA type of the responding cells. Suppressive activity was induced in normal PBMC by such clones, as well as by independent suppressive clones, which was also inhibited only by mAb TU39. These findings lead to the proposal that DY-reactive autostimulatory cells may constitute a self-maintaining suppressive circuit, the level of activity of which would be regulated primarily by the availability of IL-2 in the microenvironment.

BN rats develop interstitial nephritis after immunization with rabbit, but not rat renal tubular antigen. Using RT1n rat strains that differentially express tubular antigen, we investigated the unresponsiveness of BN rats to BN tubular antigen (BN-TBM) using delayed-type hypersensitivity (DTH) responses to BN-TBM as a measure of cell-mediated immunity. Our results indicate that rat strains expressing tubular antigen respond to immunization with BN-TBM with the clonal expansion of antigen-specific, cyclophosphamide-sensitive, OX8+, MHC-restricted suppressor T cells. Such suppression appears to be relevant to the maintenance of tolerance to parenchymal self, since chronic cyclophosphamide therapy abrogates suppression and results in significant interstitial nephritis.

Acute, low dose ultraviolet B radiation of murine body wall skin followed by local application of DNFB produces a state of antigen-specific unresponsiveness. This state is maintained at least in part by an Lyt-1+ T cell that effects unresponsiveness by impairing the induction phase of contact hypersensitivity. The absence of suppressor cells capable of acting at the effector stage of immunity suggests that tolerogenic signals derived from the skin establish suppressor networks that are incomplete and perhaps different from networks that are induced by systemic administration of tolerogens. It is proposed that ultraviolet radiation produces its effects by impairing the antigen-presenting potential of resident Langerhans cells in whose absence hapten-derivatized keratinocytes (or their products) are able to deliver a tolerogenic signal.

Suppressor T cells (Ts-1) induced with trinitrophenyl (TNP)-conjugated syngeneic spleen cells (TNP-SC) can be enriched on antigen-coated plates and are afferent suppressors. In addition, these suppressor cells produced soluble suppressor factors (TsF) that were active in vivo. Therefore, the Ts-1 cells in the TNP system are very similar to the Ts-1 cells in other systems we have studied earlier. Further characterization of these TsF-1 revealed that TsF-1 obtained from TNP-SC-induced Ts-1 is major histocompatibility complex restricted in its activity. Injection of TNP-specific TsF-1 into naive mice did not induce Ts-2 unless additional corresponding antigen was provided. Moreover, the Ts-2 cells induced by administration of both TsF-1 and trinitrobenzene sulfonic acid were antigen specific rather than antiidiotypic.

This report describes a Ts costimulator assay and its use to analyze cofactors required for the expression of suppressor T cell function. Activation of primed MLR-Ts (alloantigen-activated suppressor T cells suppressive of mixed leukocyte reaction) to suppressor T cell factor (TsF) production typically fails in the presence of glutaraldehyde-fixed rather than irradiated allogeneic stimulator cells. However, MLR-TsF production was restored by the addition of 48-h primary MLR supernates; MLR-derived Ts costimulator neither activated primed MLR-Ts in the absence of fixed allogeneic stimulators nor directly suppressed assay MLR. Lack of antigen specificity or genetic restriction and failure to activate unprimed MLR-Ts precursors suggested that Ts costimulator activity differed from previously described Ts inducer functions and was more closely aligned with the lymphocyte- or monocyte-derived interleukins (IL). Three findings distinguished Ts costimulator from IL-2. Depletion of IL-2 activity from MLR supernates by HT2 adsorption failed to affect Ts costimulator function. In addition, MLR supernates prepared in the presence of cyclosporin A contained no IL-2 but expressed Ts costimulator activity. Finally, gel chromatography demonstrated Ts costimulator in peaks of 21,000 and 43,000 mol wt that were largely distinct from the IL-2-containing fractions. Ts costimulator activity was also identified in phorbol myristate acetate (PMA)-induced EL4 supernates and was retained in those supernates after IL-2 depletion by HT2 adsorption. In preliminary functional characterization, MLR supernate-derived Ts costimulator triggered MLR-TsF production from irradiated MLR-Ts in the absence of proliferation. Thus a differentiative rather than proliferative stimulus required for primed MLR-Ts function appears to be provided by this Ts costimulator and has been provisionally termed Ts differentiative factor ( TsDF ). This initial characterization may thus identify one of a possibly distinctive family of interleukins required in the alloantigen-driven activation of suppressor T cells to effector function.

The present studies have identified cloned Lyt-1+2- T suppressor (Ts) cells that are both antigen specific and major histocompatibility complex (MHC) restricted in their activation requirements and that function to regulate the MHC-restricted activation of B cells by T helper (Th) cells. ParentA-restricted Ts clones suppressed, in antigen-specific fashion, the responses generated by (A X B)F1 Th cells cooperating with parentA (B plus accessory) cells, but did not suppress responses by the same (A X B)F1 Th cell population cooperating with parentB (B plus accessory) cells. Moreover, responses of (A X B)F1 leads to parentA Th cells and (A X B)F1 (B plus accessory) cells were suppressed by parentA-restricted Ts clones but not by parentB-restricted Ts clones. Thus, these findings suggest that the cloned Ts cells that have been characterized here function by specifically inhibiting the MHC-restricted interaction between Th cells and B and/or accessory cells. It was further demonstrated in experiments using cloned Th and Ts populations that these Lyt-1+2-Ts cells act not simply as inducers of suppressor but rather function in a restricted fashion as effector cells in the suppressor pathway.

The induction of immunoglobulin (Ig) synthesis in the autologous MLR has an absolute requirement for helper/inducer (Leu-3) T cells, whereas an excess of suppressor/cytotoxic (leu-2) cells suppresses the response. The current study was an effort to assess the immunoregulatory potential to T cells activated in the autologous mixed-leukocyte response (MLR). T cells were cultured with autologous non-T cells for 8-9 d, after which the activated T cells were fractionated into subsets with monoclonal antibodies to T cell markers and HLA-DR antigen. Each population was co-cultured in fresh autologous MLR, and on the 8th day of culture, Ig-secreting cells were measured in a reverse hemolytic plaque assay. The results show that activated Leu-2, DR+ T cells, but neither Leu-2, DR- nor Leu-3 T cells, were at least 50 times more potent as suppressors of IgM and IgG synthesis than fresh Leu-2 cells alone. The activation of this Leu-2, DR+ subpopulation required Leu-3 cells in the primary culture. Furthermore, in the absence of Leu-2 cells in the second culture, little or no suppression was observed, suggesting that the Leu-2, DR+ cells act to amplify or induce suppressor effects of fresh Leu-2 cells. This indicates that at least two distinct subpopulations of Leu-2 cells are required for maximal suppression of an immune response, and that immunoregulatory circuits analogous to those described in the mouse exist in man.

The T4 (CD4) subset of T lymphocytes has been subdivided into two major subsets, a suppressor/inducer subset (T4+,2H4+) and a helper subset (T4+,2H4-) on the basis of the differential expression of the L-C/T200 (CD45) antigens. The 2H4 antigen itself comprises at least three distinct polypeptides at 125,200, and 220 X 10(3) Mr, of which the 200 and 220 X 10(3) Mr polypeptides constitute the highest Mr isoforms of a pool of five distinct L-C/T200 antigens. The T4+,2H4+ subset expresses at least four of these isoforms at 180, 190, 200, and 220 X 10(3) on the cell surface, while the T4+,2H4- subset expresses only the 180 and 190 X 10(3) Mr forms. Pulse-chase analysis and endoglycosidase treatment revealed that the 125 X 10(3) Mr chain of the 2H4 antigen is nonglycosylated, while the 200 and 220 X 10(3) polypeptides are structurally related and derived by N- and O-linked glycosylation from two nascent subunits at 150 and 160 X 10(3) Mr. The function of the T4+,2H4+ subset could be blocked only by an antibody reactive with the L-C/T200 isoforms enriched with O-linked oligosaccharides at 200 and 220 X 10(3) Mr.

We find that a fraction of Ly-1+2- inducer T cell clones inhibits differentiation of memory B cells into IgG-secreting plaque-forming cells. Inhibition of secondary antibody responses was not the result of induction of Ly-2+ T suppressors. Instead, inducer cells directly inactivated B cells, requiring an antigen bridge as well as identity at the major histocompatibility complex (I-A) locus. The interaction between the inducer T cell clone and hapten-specific B memory cells results in an early proliferative response and subsequent failure of B cells to secrete antibody in response to T helper cell signals. Possible mechanisms for this novel type of B cell inactivation are explored.

The ability of murine Peyer's patch (PP) T contrasuppressor cells (Tcs) to reverse oral tolerance to the T cell-dependent (TD) antigen SRBC was studied both in vivo and in vitro. C3H/HeJ mice given SRBC orally for 4 wk are not rendered tolerant to this antigen and were used as a source of PP Tcs cells for adoptive transfer to identically treated, orally tolerized C3H/HeN mice. Transfer of 10(4) or 5 X 10(4) V. villosa-adherent PP T cells resulted in splenic IgM, IgG, and mainly IgA responses in C3H/HeN mice challenged systemically with SRBC. The T cell responsible was Lyt-1+, 2-, L3T4-, I-JK+ and V. villosa lectin-adherent, all characteristics of mature effector Tcs cells. This C3H/HeJ PP Tcs cell subset was also effective when added to in vitro cultures of tolerized spleen cells derived from SRBC-fed, C3H/HeN mice. Interestingly, C3H/HeJ PP Tcs cells restored mainly IgA responses when transferred in vivo or when added to suppressed C3H/HeN splenic cultures. Comparison of the functional activity of Tcs cells derived from spleen or PP of orally immunized C3H/HeJ mice revealed that splenic Tcs cells supported responses of all 3 isotypes; however, PP Tcs cells yielded three-fourfold higher IgA responses, when compared with IgM or IgG anti-SRBC responses. Adherence of C3H/HeJ PP Tcs to an Fc alpha R+ T cell line derived from IgA-specific Th cells resulted in a nonadherent cell fraction that potentiated only IgM and IgG responses, while bound Tcs cells preferentially supported IgA responses. These results suggest that murine PP contain IgA-specific Tcs cells that allow IgA response induction in the presence of Ts cells that mediate oral tolerance.