Suppressive mechanisms in alloantigen-induced T cell responses

Modulation of a human immunosuppressive lymphokine by monosaccharides

Soluble suppressor factor (SSF) is a recently purified human lymphokine produced by peripheral blood lymphocytes (PBL) in serum-free medium as a likely consequence of an autologous mixed lymphocyte reaction. Immunoregulatory actions of SSF include suppression of: polyclonal B cell activation, proliferative responses of normal PBL, and natural killer (NK) and antibody-dependent cellular cytotoxicity. We examined the ability of the monosaccharides fucose (Fuc), galactose (Gal), glucose (Glc), and mannose (Man) to reverse SSF-mediated suppression of NK activity. Fuc and Gal can partially or completely reverse SSF-mediated suppression at four effector:target cell ratios. Man and Glc were unable to significantly reverse SSF-mediated suppression. Fuc or Gal was added to PBL at various times after addition of SSF. SSF-mediated suppression of NK cytotoxicity becomes irreversible with respect to these monosaccharides during the first 24 hr of PBL exposure to SSF. To explore the mechanism behind this block of SSF-mediated suppression. Fuc or Gal (50 mM) was cultured with PBL for 24 hr before addition of SSF, or with SSF for 24 hr before addition to PBL. Our experiments indicate that SSF is directly interacting with these monosaccharides, and may function by recognizing specific sugar moieties on the surface of effector cells.

Cloning of a cDNA for a T cell produced molecule with a putative immune regulatory role

An expression cDNA library was constructed from the helper T cell hybridoma, A.1.1, which has been shown to produce constitutively proteins involved in the down regulation of the immune response. From this library we identified and characterized a cDNA clone, J6B7, by screening with a polyclonal antibody specific for secreted immune regulatory proteins. The mRNA for J6B7 is expressed specifically in some T cells, but not in the thymoma BW5147 or liver cells. J6B7 is 2937 nucleotides in length and contains one open reading frame encoding for a peptide of predicted Mr of 98,042. The nucleotide and deduced amino acid sequences of J6B7 did not reveal significant homology to any published sequences. Hybridization and translation experiments reveal that the J6B7 can hybrid select mRNA from total RNA isolated from either A.1.1 cells or thymic tissue which can be translated in vitro to a peptide which is bound by a monoclonal antibody (mAb) specific for antigenic determinant(s) shared by immune regulatory proteins. Furthermore, the in vitro translated proteins obtained from A.1.1 cells and thymus showed significant suppression of a mixed lymphocyte reaction (MLR) in a dose dependent manner, reaching maximum suppression of 71% and 89%, respectively. These results suggest that the cDNA, J6B7, codes for an immune regulatory protein.

Ultraviolet-induced suppressor T cells and factor(s) in murine contact photosensitivity. III. Mode of action of T-cell-suppressor factor(s) and interaction with cytokines

The mode of action of T-cell-suppressor factor (TsF) induced by ultraviolet B (UVB) preirradiation in terms of interaction with several cytokines was studied. Suppression of murine contact photosensitivity (CPS) to 3,3',4',5-tetrachlorosalicylanilide (TCSA) by preirradiation of the sensitizing site to low doses of UVB was caused by antigen-specific suppressor T cells (Ts) and was not associated with the generation of efferent limb-acting suppressor cells. TsF released by Ts inhibited the proliferation of immune lymph node (LN) cells in vitro and reduced interleukin (IL)-2 production of these cells in an antigen-specific fashion without affecting the IL-2 receptor (IL-2R) expression. Both rIL-2 and rGM-CSF have the ability to restore CPS responses in the UVB-preirradiated mice when administered after but not before photosensitization. However, rIL-2 but not rGM-CSF counteracted the in vivo inhibitory effect of TsF. rGM-CSF did not affect the density of I-A+ epidermal Langerhans cells (LCs). It was suggested that TsF inhibited IL-2-mediated immune T-cell proliferation, while rGM-CSF reconstituted the CPS by enhancing the function of photodamaged LCs. These results indicate multiple steps of the UVB-induced immunosuppression circuit, each of which seems to be controlled by different immunomodulators.

A role for L3T4+ T cells and their lymphokines in the generation of suppressor effector (TS3) cells

The cellular requirements for the in vitro induction of antigen-specific suppressor T cells were examined. Previous reports indicated that Ia-bearing macrophages and anti-idiotypic B cells are required as accessory cells to facilitate the generation of suppressor effector (TS3) cells which regulate the response to the 4-hydroxy-3-nitrophenyl acetyl (NP) hapten. The present study describes two distinct T cell populations which interact to generate antigen-specific TS3. Fractionation of the T cell populations with monoclonal antibody to the L3T4 determinant led to the identification of an NP-specific L3T4- TS3 progenitor population and an L3T4+ helper/inducer subset. In the presence of NP-coupled antigen, the L3T4+ subset could induce progenitor TS3 to differentiate into mature TS3 cells. The activity of the L3T4+ inducer population could be replaced with specifically activated cloned helper cells which were not NP-reactive since an I-Ab-restricted, insulin-reactive, L3T4+ clone was capable of supporting the generation of NP-specific TS3. Inducer activity appeared to be confined to the Th1 but not the Th2 subset. In addition, 18-hr supernatants from antigen-activated clones were capable of substituting for L3T4+ cells or T cell clones in TS3 induction cultures. The TS maturation/differentiation factor(s) active in these supernatants does not appear to be IL-1, IL-2, IL-3, or interferon-gamma alone since purified sources of these lymphokines failed to induce TS3 activity.

Functional analysis of antigen-nonspecific T-cell suppression. I. Effect of mitogen-induced T suppressor cells on helper-T-cell clones

The effect of mitogen-induced nonspecific suppressor T cells (Ts)2 on T-helper-cell activity was investigated using isolated clones of murine T-helper cells as targets. TNP-self-reactive Thy1+, Ly1+ T-cell clones were isolated after continuous culture of T cells derived from picryl chloride-sensitized mice and were characterized by their ability to proliferate in an antigen-specific and MHC-restricted manner. In addition, selected T-cell clones were found to produce both interleukin-2 (Il-2) and T-cell replacing factor (TRF), lymphokines characteristic of helper T cells. Concanavalin A (Con A)-induced Ts cells inhibited the antigen-specific proliferation of these helper-T cell clones in a noncytotoxic manner even in the presence of exogenous Il-2. This implied that failure to proliferate was not merely due to an inability of these clones to produce Il-2. The kinetics of suppression also suggested that early T-cell activation signals were not affected. Furthermore, coculture experiments indicated that while proliferation could be severely inhibited, the actual secretion of lymphokines such as Il-2 and TRF by the T-helper clones was not. Our data suggest that nonspecific Ts modulation of proliferation versus helper factor production are under separate control in cloned T-cell populations, with lymphokine secretion remaining intact in the presence of Con A-induced Ts cells.

Resistance to suppression and acquisition of memory by human T cells

Human memory cells acquire resistance to several types of suppressor cells, including MLR generated suppressor cells. These data suggest one possible mechanism of that resistance, namely retention of IL-2 receptors in the resting state. Cells from a 7-day MLR were separated on a single step percoll gradient. All proliferating cells were found in the interface. Pellet cells were nondividing. Interface and pellet cells had equivalent memory function in a secondary MLR. Thus, there appear to be at least two subpopulations of memory cells, including one that is primed without undergoing division. These subpopulations are functionally distinct. Interface memory cells were 30-50% more resistant than pellet memory cells to MLR generated suppressor cells. On culture day 10, neither pellet nor interface cells displayed significant spontaneous proliferation but exogenous Interleukin 2 (IL-2) produced up to five times as much proliferation in interface cells as in pellet cells. Further, FACS analysis with an anti-TAC equivalent antibody also showed that significantly more interface cells have surface receptors for IL-2. Thus, cells that had previously divided continue to have more and/or higher affinity receptors for IL-2 even after return to the resting state. If a mechanism of suppression in the mixed lymphocyte reaction is to reduce the synthesis/release of Il-2, memory cells may acquire their relative resistance to this suppression by virtue of the increased IL-2 sensitivity of this discrete subpopulation.

Suppression of cytolytic T-cell activity by staphylococcal enterotoxin B-induced suppressor cells: role of interleukin 2

We have previously shown that staphylococcal enterotoxin B (SEB) is a potent inducer of suppressor T cells which function to inhibit antibody production in vitro. In the present paper we extend these studies and show that the SEB-induced suppressor cells also inhibit the development of cytotoxic lymphocytes in mixed-lymphocyte reaction (MLR) cultures. Since further analysis also showed that the level of interleukin 2 (IL-2) in cultures of SEB-primed cells was significantly reduced, experiments were carried out to determine the role of IL-2 in the inhibition of cytotoxic cell activity. Attempts to neutralize the suppression by supplementing MLR cocultures with delectinated supernatants from concanavalin A (Con A)-stimulated rat splenocytes were not successful. In addition, MLR cocultures supplemented on Day 0 with 50 units of affinity-purified IL-2 were also suppressed. Further analysis showed that the IL-2 activity in the supplemented MLR cocultures containing suppressor cells were significantly reduced by Day 3. However, repeated supplementation of the MLR cocultures with exogenous IL-2 was successful in achieving (and maintaining) "normal" levels of IL-2. The cytotoxic cell activity in these MLR cocultures remained suppressed. These results suggest that the inhibition of cytotoxic cell activity by SEB-induced suppressor cells is independent of IL-2 levels in the MLR coculture.

Monoclonal antibodies against a rat leucocyte antigen block antigen-induced T-cell responses via an effect on accessory cells

The MRC OX-45 and OX-46 mouse monoclonal antibodies recognize a rat cell surface glycoprotein of 45,000 MW that is present on a wide variety of haematopoietic cells and on endothelial cells. MRC OX-45 IgG or F(ab')2 blocked the primary mixed lymphocyte response (MLR) and the secondary response of T lymphocytes to the soluble antigen DNP-BGG. In contrast, the antibodies had no effect on the cytotoxic activity of specific (CTL) or non-specific (NK) killer cells or on proliferative responses stimulated by lectins or oxidative mitogenesis. The inhibitory effect was at the level of stimulator cells rather than responders since mouse anti-rat xenogeneic MLRs were inhibited but rat anti-mouse responses were unaffected. However, the effect was not a direct one because inhibition was seen when irradiated spleen cells were used as stimulators but not when cell populations highly enriched for dendritic cells were used. In the latter case, inhibition potentiated by antibody could be restored if a peritoneal cell population enriched for macrophages was added back to the cultures. The inhibitory effects of these monoclonal antibodies seem most likely to be due to potentiation of nonspecific suppression by macrophages.

Inhibition of in vitro lymphocyte function by a soluble AKR leukemic suppressor factor

A soluble suppressor factor(s) found in the serum of AKR mice bearing lymphocytic leukemia (AKR-LSF) has previously been shown to inhibit PHA-induced spleen cell proliferation. In this study we have further characterized the biological activity of the leukemic mouse serum (LMS) in vitro. The LMS inhibits spleen cell proliferation in an MLC as well as generation of functional CTL. The inhibition of CTL generation was not specific since CTL activity resulting from responder and stimulator combinations of various H-2 haplotypes was inhibited. LMS did not inhibit CTL- or NK-mediated cytolytic activity. These results suggested that LMS inhibits lymphocyte proliferative responses but has no effect on cytolytic function. Furthermore, responder cells which had been inhibited by LMS in a primary MLC, washed, and restimulated were able to express CTL activity indicating that the suppression by LMS is reversible. The suppressive effect of LMS is occurring at an early stage of CTL generation since LMS was inhibitory when added on Day 0 or Day 1 but not on Days 2, 3, or 4 of the MLC. Addition of IL-2 did not remove the inhibition by LMS as measured in a PHA proliferative assay. These results suggested that the suppression is not due to a lack of IL-2 but to an inability of the cells to either bind or utilize IL-2.

Class II positive human dermal fibroblasts restimulate cloned allospecific T cells but fail to stimulate primary allogeneic lymphoproliferation

Recently, a number of laboratories have shown that gamma interferon (IFN-gamma) is a potent modulator of HLA class II antigen expression in a variety of cell types ranging from classical antigen presenting cells to those not expected to participate in physiological antigen presentation such as fibroblasts. In order to examine the role of HLA class II expressing fibroblasts in antigen presentation, we established dermal fibroblast (FIB) strains from five HLA typed donors. After optimal preculture with IFN-gamma, class II positive FIB were fully competent to restimulate proliferative responses of two DR specific T cell clones and one DP specific T cell line. However, they failed to elicit strong primary allogeneic proliferation from fully DR mismatched fresh PBMC. This failure was not due to a direct suppressive effect of FIB and could not be corrected by exogenous IL1 or by factors contained in conventional mixed leukocyte culture supernatants.

Immunoregulation by mouse T cell clones. III. Cloned H-Y-specific cytotoxic T cells secrete a soluble mediator(s) that inhibits cytotoxic responses by acting on both Lyt-2- and L3T4- lymphocytes

In this study we report that cloned Thy-1+, L3T4-, Lyt-1-, Lyt-2+, H-Y-specific and H-2Db-restricted cytotoxic T cell lines (CTLL) when induced by lectin or antigen secrete a soluble mediator(s) (SF) that inhibits proliferation and generation of cytotoxic lymphocytes (CTL) in mixed lymphocyte cultures (MLC). The biological activity was separable by gel filtration and appeared as a broad peak in the molecular mass range between 10 000 and 50 000 kDa. It was found that the suppressive activity released by CTLL neither strictly correlates with their cytotoxic potential nor with their ability to produce immune interferon or lymphotoxin. SF was shown to elicit its activity in an antigen-nonspecific manner in that it suppressed the maturation of T lymphocytes responding to both, the appropriate H-Y antigen as well as to unrelated H-2d alloantigens or to the hapten 2,4,6-trinitrophenyl (TNP). The effect of SF on CTL responses was most pronounced in early phases of primary or secondary MLC. When analyzed for its inhibitory activity on precursor cells in populations selected for either Lyt-2- or L3T4- lymphocytes, it was found that SF interfered with the maturation of both subsets. The inhibition of CTL responses elicited by SF could not be reversed by the addition of exogenous interleukin 2. The finding that SF also inhibited the proliferation of some but not all antigen-dependent cloned T cells with helper or cytotoxic potential provides evidence that the factor also may regulate effector lymphocytes. In addition, the results support the assumption that SF exerts its effect directly on the responder rather than the stimulator population, and demonstrate that the development of CTL from their precursor cells is controlled at least in part by the cytotoxic effector cells themselves via a soluble factor(s) that interferes with distinct stages of T cell maturation. These findings again emphasize the expression of multiple functions by CTL and indicate their possible role during the course of an immune response by their capability to eliminate target cells and to secrete a soluble product(s) that mediates feedback control.

Suppressor T cell growth and differentiation. Identification of a cofactor required for suppressor T cell function and distinct from interleukin 2

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.