Characterization of two suppressor cells that together prevent in vivo development of cytolytic T cells to hapten-altered self

Down-regulation of cytotoxic T lymphocyte generation by two distinct suppressor-cell systems

Two distinct suppressor systems have been described that are capable of down-regulating in vivo generation of cytotoxic T cells directed toward haptenaltered self-antigens. One system, induced by hapten, involves three T cells that others have shown to function sequentially to suppress DTH. The initiator of this cascade is a T cell that is readily induced in spleens of mice injected intravenously with syngenic membrane-coupled hapten. This Ts, when triggered by the same syngeneic membrane-coupled hapten that induced it, elaborates a factor. The other two Ts arise in lymph nodes and spleens of mice painted epidermally with hapten. One of the two Ts in this set is readily armed by the factor of the first Ts. The factor confers its specificity and genetic restriction upon the accepting Ts. The latter, when properly triggered, makes a factor that is taken up by its companion Ts, which actually suppresses by way of a nonspecific factor. Whereas this Ts cascade is operative at the efferent limb of DTH, it mediates suppression only at the afferent phase of the CTL response. A distinctly different suppressor system is induced by minor locus (Mls) antigen. When Mlsd lymphoid cells are injected intravenously into Mlsc-possessing mice, an Lyt-1+ T-suppressor cell is generated that can be found in the spleen as well as among peritoneal exudate cells. This Ts interacts with macrophages to accomplish nonspecific suppression of the CTL response that is detectable both in vivo as well as in vitro. A Ts soluble product has been found to be effective to suppress CTL generation in vitro only when macrophages are present in culture. The macrophage that accomplishes suppression is I-A-. Although the afferent limb of the CTL response is down-regulated by this suppressor system, our in vitro culturing system is so structured as to make the helper T cell inactive. Thus, the mechanism of suppression must be oriented to the other early participants in the response, namely, precursor CTL, helper and differentiation factors, and/or the antigen-presenting cell.

A syngeneic splenic cell antigen induces a suppressor T cell in lymph nodes that controls cytotoxic T-cell and primary antibody responses

The nonspecific suppression of immunological responses that is generated within host popliteal lymph nodes upon exposure to syngeneic normal spleen cells has been examined. The suppression, which had previously been described as being capable of preventing initiation of cytotoxic T lymphocytes (CTLs) to hapten-altered self antigens, arises within 3 to 7 days after injecting the spleen cells. Suppression was shown to be attributable to an induced T cell that was functional when transferred intravenously. Although the cell surface marker(s) on both splenic B and T cells that stimulates appearance of Ts has not yet been identified, the cells possessing the marker were not required to be viable to cause the induction. We have shown here that the Ts is fully functional when it is put in the antigenic site used for CTL immunization. The induced Ts has been identified as bearing the Lyt 2.1 cell surface marker. Furthermore, it has been shown to be insensitive to cyclophosphamide (CY), thus differentiating it from the naturally occurring Ts cell (TS0) that is known to be CY sensitive. In addition to preventing induction of CTLs toward hapten-altered self antigens, exposing popliteal lymph nodes to syngeneic spleen cells induced Ts capable of suppressing the primary IgM antibody response to sheep red blood cells. The Ts cells that suppressed the primary antibody response possessed the same Lyt cell surface markers and CY insensitivity as the Ts that mediated suppression of the CTL response. Thus, evidence that two dissimilar immunological reactions may be down-regulated by the same suppressor mechanism has been provided. Results of a kinetic study showed that the Ts prevented development of both the humoral and the cell-mediated immune responses by affecting their inductive phases. Possible targets for suppression that more than likely would have to be common to the two widely different immune responses have been indicated.

The suppressor T cell induced by syngeneic splenic cell antigen down-regulates hapten-specific cytotoxic T cells by elaborating a factor inhibitory for IL2-dependent cell replication

An in vitro study has been made of the mechanism by which a suppressor T cell, that is induced in lymph nodes by a syngeneic splenic cell antigen, prevents generation of cytotoxic T cells specific for hapten-altered self antigens. When popliteal lymph node cells exposed in vivo to syngeneic splenic cells were immunized in vitro with heat-treated syngeneic TNP-coupled thymocytes and excess helper factors, the Ts remained inactive. In this condition the exposed popliteal lymph node cells routinely demonstrated approximately twice the CTL response developed by lymph node cells from normal mice. Nevertheless, when triggered in vitro by splenic antigen on either X-irradiated B or T cells, the exposed but not the normal lymph node cells exhibited reduced hapten-altered self-specific CTL responses. Furthermore, T cells within spleen cell-exposed popliteal lymph node cell populations when reexposed to splenic T cells made a factor that was found to be suppressive of CTL generation by normal lymph node cells in vitro. The nondialyzable T-cell suppressor factor (TsF) did not appear to act on lymph node precursor CTLs, nor on helper T cells but instead acted at the level of utilization of helper factors in the development of CTLs. In an examination of the effect of TsF on cellular replication, TsF was found to be nontoxic for CTLL-20, an IL-2-dependent T cell, and it did not hinder the uptake of IL-2 by receptor blockade of this cell. Nevertheless, the replication of CTLL-20 that is IL-2 driven was diminished in the presence of TsF. Similarly, TsF was found to be inhibitory for T-cell proliferation stimulated by mitogen but had no effect on a B myeloma cell proliferative response. Thus, TsF appears to act as an inhibitor of a T cell's capability to replicate despite the presence of the stimulus for replication, namely, IL-2.

Natural cytotoxic T cells (NCTC) that differ from natural killer (NK) and natural cytotoxic (NC) cells are present in Peyer's patches of mice

We have examined noninduced cytotoxicity of mouse gut associated and peripheral lymphoid tissues for a wide variety of syngeneic as well as allogeneic cell lines and lymphoblasts. Lymphoid cells from Peyer's patches were found to lyse these targets in a 3-hr chromium release assay whereas lymphoid cells from intestinal mucosa, mesenteric or peripheral lymph nodes, spleen, and thymus did not. The variety of targets toward which Peyer's patch cells were cytotoxic established the latter as nonspecific and H-2 unrestricted. The cell responsible for the lytic event was identified as possessing Thy 1.2 and Ia surface antigens. This naturally cytotoxic T cell (NCTC) was found to be adherent to nylon-wool but not to plastic plates. Although both natural killer cell (NK) and non-NK targets served as targets for the NCTC, the latter were further differentiable from NK cells by lack of asialo GM1 surface marker, which is present on NK cells. In addition, NCTC remained fully functional in mice given either of the drugs cyclophosphamide or cortisone. Each of these drugs, in the doses used, markedly reduced poly(I:C)-induced NK activity. Thus, NCTC differs from NK on the basis of the spectrum of targets against which it is functional, phenotypic surface markers, insusceptibility to stimulation with poly(I:C), and insensitivity to diminution by the immunosuppressive drugs cyclophosphamide and hydrocortisone. Since NCTC is a Thy 1.2 antigen-bearing cell and is detectable in a 3-hr cytotoxic assay, it also differs from the natural cytotoxic (NC) cell. NC lacks the Thy 1.2 marker and becomes detectable only in an 18-hr cytotoxic assay. Thus, NCTC is neither an NK nor an NC cell. We have discussed the possibility that the three naturally occurring cells may be related by being dedifferentiated descendants of an antigen-specific cytotoxic T lymphocyte (CTL). Alternatively, since NCTC is confined to an anatomical site prone to ample antigenic exposure and is still identifiable as a T cell, it may be in linear transition from the CTL to the NK or NC stages.

Ongoing hapten-specific delayed-type hypersensitivity prevents generation of cytolytic T lymphocytes to the same hapten

Cytolytic T lymphocytes (CTL) specific for trinitrophenyl (TNP)-altered self antigens can be generated in vivo through the simultaneous injection of TNP-modified syngeneic spleen cells and H-2-compatible, minor histocompatibility locus (Mls)-disparate auxiliary spleen cells into the footpads of mice. The latter stimulates host helper cells to produce differentiative and proliferative signals required for the generation of CTL. Advent of this protocol allowed investigation of the initiation of two different cell-mediated immune responses, delayed-type hypersensitivity (DTH) and the generation of CTL, in the same experimental animal. Mice presensitized for CTL were able to develop DTH as well as normal controls. However, when mice were first sensitized for DTH, they were thereafter incapable of generating CTL. This effect was hapten specific, relatively long lasting, and preventable by treating mice with cyclophosphamide before sensitizing for DTH. Adoptive transfer of lymphoid cells from DTH-immune mice conferred DTH reactivity upon naive recipients but not a suppressed CTL response. Therefore, cells mediating DTH were not responsible for suppression of CTL. The mechanism for suppression has been discussed from the viewpoint of the suppressor-T-cell circuits that are known to be generated when animals are sensitized for DTH and which are susceptible to treatment with cyclophosphamide.