The role of suppressor T cells in regulation of immune responses

Suppressor T cells play important roles in the regulation of immune responses and the mediation of dominant immunologic tolerance. Studies of suppressor T-cell function have been hampered until their recent identification as a minor fraction (approximately 10%) of CD4 ( +) T cells that coexpress CD25. CD4(+)CD25(+ ) T cells have been shown to play a critical role in the prevention of organ- specific autoimmunity and allograft rejection. Because tumor antigens are self- antigens, it is not surprising that CD4(+)CD25(+) T cells also inhibit the induction of tumor immunity. The spectrum of activity of CD4(+ ) CD25(+) cells extends to non-self-antigens, including infectious agents. Indeed, T cell-mediated suppression might be responsible for the low level of chronic infection seen with many pathogens. Interestingly, however, this persistent level of infection might be beneficial to the host and needed for maintenance of immunologic memory. Although CD4(+ ) CD25(+) T cells are capable of inhibiting T(H)2 responses, their role in the suppression of allergic responses has not been firmly established. Depending on the desired immune response, enhancement or restraint of suppressor T-cell function might be required. Therefore immunologic or pharmacologic manipulation of regulatory T-cell populations represents an important future approach to immunotherapy of a wide range of immune responses.     

The role of antigen-presenting cells in the regulation of allergen-specific T cell responses

Allergic reactions in atopic patients follow from a generalized enhanced polarization of Th cells, predominantly imposed by factors derived from antigen-presenting cells from a pathogen-stressed tissue; these sample information not only on antigen structures but also on the nature of the stress. Antigen-presenting cells of atopic individuals show aberrant characteristics which, through a highly interactive communication network, play an active role in aberrant Th-cell polarization. This generalized bias may follow from intrinsic abnormalities of antigen-presenting cells and also from a low degree of cross-regulation by micro-organisms.     

Characterization of efferent T suppressor cells induced by Paracoccidioides brasiliensis-specific afferent T suppressor cells.

Previously, we reported that Paracoccidioides brasiliensis culture filtrate antigen (Pb.Ag) when injected i.v. into mice induces antigen-specific suppressor cells which down-regulate the anti-P. brasiliensis delayed-type hypersensitivity (DTH) response. The suppressor cells are present in both spleens and lymph nodes of Pb.Ag-treated animals and suppress the afferent limb but not the efferent limb of the DTH response to P. brasiliensis. The suppressor cells induced by Pb.Ag are L3T4+ Lyt-1+2- I-J+ T cells and are considered to be equivalent to the Ts1 cells described for other antigen-specific suppressor cell pathways. This report provides data which show that Ts1 cells induced by Pb.Ag or a soluble factor derived from Ts1 cells (TsF1) stimulates the production of second-order or efferent suppressor cells. The second-order suppressor cells are detectable in spleens and lymph nodes of mice 7 days after injection of Ts1 cells or TsF1 and are specific in suppressing the paracoccidioidal DTH response. In addition, the second-order suppressor cells are T cells with an L3T4- Lyt-2+ I-J+ phenotype and are effective in suppressing only the efferent limb of the P. brasiliensis DTH response. On the basis of the characteristics defined in this study, the paracoccidioidal second-order suppressor cells are equivalent to the Ts2 cells described for other antigen-specific suppressor-cell pathways. Thus, the suppressive circuit induced by Pb.Ag is similar to the suppressor-cell pathways that regulate the DTH responses to azobenzenearsonate, 4-hydroxy-3-nitrophenyl acetyl, lysozyme, and Cryptococcus neoformans antigen. We propose that such a suppressor-cell circuit as defined here with the murine model could be responsible for the depressed cell-mediated immune responses observed in paracoccidioidomycosis patients who have antigen circulating in their sera.     

NK cell regulation of T cell-mediated responses

NK cells promote adaptive immune responses through their production of type 1 and type 2 cytokines or chemokines. Secretion of these factors by activated NK cells influences the differentiation of B and T lymphocytes. Increasing evidence indicates that NK cells are also directly involved in dendritic cell (DC) maturation. By contrast, a potential role for direct cell-cell interactions between NK and T lymphocytes, in particular CD4(+) T cells, has not been explored. We provide evidence that activated human NK cells are able of promoting TcR-dependent proliferation of resting autologous peripheral blood CD4(+) T cells by a process that involves costimulatory molecules of the immunoglobulin (Ig) and tumor necrosis factor (TNF) superfamilies. These findings suggest a novel link between natural and adaptative immune responses.     

Regulation of T cell-mediated cytotoxic allograft responses. I. Evidence for antigen-specific suppressor T cells

Murine lymph node cells draining a local lymphoid cell allograft (LNd) exhibited weak cytolytic activity when tested immediately after surgical removal, However, upon mere in vitro cultivation, high cytotoxic activity developed. The in vitro generation of cytotoxic activity could be inhibited by treatment of the cells with mitomycin C. Upon transfer into a primary mixed lymphocyte culture, mitomycin C-treated LNd cells effectively suppressed the induction of cytotoxic T lymphocytes (CTL). Using the velocity sedimentation technique at 1 × g the suppressor cells could be confined to the blast cells within LNd cells, the sensitized prekiller T cells being nonsuppressive. Results derived from independent experimental approaches suggested that the in vivo sensitized blast suppressor cells exhibited an immunological specific suppression, i.e. they selectively suppressed the in vitro induction of CTL reactive towards those H-2 transplantation antigens against which the suppressor cells had been immunized. Thus, antigen-specific suppressor cells (which were Thy-1.2 positive) appeared to be specifically activated during the in vivo sensitization procedure.     

Modulation of cellular anti-tumour responses by tumour-specific T suppressor lymphocytes.

Tumour-specific Ts cells, induced by a protocol which simulates early stages of tumourigenesis in BALB/c mice by the syngeneic plasmacytoma ADJ-PC-5, suppress the generation of specific cytotoxic T lymphocytes (CTL) in a primary in vitro mixed lymphocyte tumour culture (MLTC) of BALB/c spleen cells against ADJ-PC-5. The influence of these Ts cells on the generation of non-specific killer cell activity has now been analysed. The data show that non-specific killer cells generated during in vitro MLTC lyse both ADJ-PC-5 and YAC-1 cells. They are different from tumour-specific CTL as shown by cold target inhibition experiments and on the basis of their different phenotype. The generation of non-specific killer activity during MLTC can be completely suppressed by ADJ-PC-5 specific Ts cells. Activation of non-specific killer cells by recombinant interleukin-2 (r IL2) alone is not influenced by these Ts cells. The in vitro data are best explained by assuming that the target for suppression are ADJ-PC-5 specific T helper cells. Their inactivation will result in depletion of IL2 in the culture, which is required to activate non-specific killer cells. Prevention of activation of specific and non-specific killer cells in vitro by activated tumour-specific Ts cells in the presence of the tumour, suggests that similar mechanisms might operate in vivo and that inactivation of tumour-specific T helper cells might be of central relevance.     

The distinctive specificity of antigen-specific suppressor T cells.

Although suppressor T cells have been cloned in only a few instances, the existence of a functional cadre of T cells that acts to downregulate the immune response is well documented. In this review Eli Sercarz and Urszula Krzych describe studies on suppressor T-cell (TS-cell) specificity that provide some support for the conclusion that the TS cell is a distinctive cell type with an expressed repertoire that is different from that expressed by helper T (TH) cells. They go on to explore the interaction between cells recognizing TS-cell-inducing determinants (SDs) and TH-cell-inducing determinants (HDs), and their relationship to immunogenicity and Ir gene effects.     

Myeloid-derived suppressor cells help protective immunity to Leishmania major infection despite suppressed T cell responses

Th1/Th2 cytokines play a key role in immune responses to Leishmania major by controlling macrophage activation for NO production and parasite killing. MDSCs, including myeloid precursors and immature monocytes, produce NO and suppress T cell responses in tumor immunity. We hypothesized that NO-producing MDSCs could help immunity to L. major infection. Gr1(hi)(Ly6C(hi)) CD11b(hi) MDSCs elicited by L. major infection suppressed polyclonal and antigen-specific T cell proliferation. Moreover, L. major-induced MDSCs killed intracellular parasites in a NO-dependent manner and reduced parasite burden in vivo. By contrast, treatment with ATRA, which induces MDSCs to differentiate into macrophages, increased development of lesions, parasite load, and T cell proliferation in draining LNs. Altogether, these results indicate that NO-producing MDSCs help protective immunity to L. major infection, despite suppressed T cell proliferation.     

Functional analysis of the defective T cell regulation of the antigen-specific PFC response in SLE patients: differentiation of suppressor precursor cells to suppressor effector cells.

The investigation described here is concerned with the T cell regulation of the antigen-specific antibody response which has been studied in patients suffering from systemic lupus erythematosus (SLE). Apart from the fact that T helper cell activity was found to be less efficient, it appeared that the peripheral blood leucocytes (PBL) of patients in an active stage of the disease did not contain the suppressor precursor cells, which functions as the target cell for the inductive signal of T mu+ suppressor inducer cells. The absence of the suppressor precursor cells in SLE patients coincided with the absence of T gamma+ suppressor effector cells. Characterization of the (post-thymic) precursor cells (derived from normal donors) with the aid of monoclonal antibodies of the OKT series and several other markers pointed out that this population contains OKT4+ as well as OKT8+ cells. Further experiments demonstrated that the cells are capable of rosetting with autologous erythrocytes, and do not bear Fc receptors for IgM or IgG. Considering the various findings as a whole the conclusion is warranted that the post-thymic suppressor precursor T cell can differentiate into a suppressor effector cell only after interaction with T suppressor inducer cells.     

Loss of suppressor T cells in active multiple sclerosis. Analysis with monoclonal antibodies

To determine whether abnormalities of immunoregulatory T cells are associated with multiple sclerosis (MS), we characterized peripheral lymphocytes in 33 patients with untreated MS and compared them with 42 normal persons and 29 age-matched control subjects who had other neurologic diseases. For this analysis, we used monoclonal antibodies to the surface antigens of helper (T4) and suppressor (T5) T-cell subsets and to a common T-cell antigen (T3). In contract to normal persons and the controls with other neurologic diseases, the patients with MS had a reduced percentage of T3-positive (T3+) cells (P less than 0.05). More importantly, there was a selective decrease in T5-positive (T5+) cells in 11 of 15 patients with active MS, but in only one of 18 patients with inactive MS and in none of the normal persons or controls with neurologic disease (P less than 0.00001). Serial analysis of five patients with MS showed a correlation between the absence of the T5+ subset and disease activity. Thus, there is loss of peripheral suppressor cells in many patients with active MS, suggesting that immunoregulatory abnormalities contribute to the pathogenesis of the disease.     

Fine antigenic specificity and genetic restriction of lysozyme-specific suppressor T cell factor produced by radiation leukemia virus-transformed suppressor T cells

Culture supernatants obtained from a radiation leukemia virus-transformed, hen egg-white lysozyme (HEL)-specific, suppressor T cell line are able, when injected into mice, to specifically suppress the anti-HEL antibody response. Suppression is observed on both primary and secondary anti-HEL antibody responses evaluated by direct and developed hemolytic plaque assays. Culture supernatants from this HEL-specific suppressor T cell line do not suppress the antibody response induced by a structurally related lysozyme, demonstrating the presence in the culture supernatant of a suppressor factor endowed with fine antigenic specificity. The suppressor factor is able to selectively suppress the anti-HEL antibody response induced by the N-terminal C-terminal peptide of the HEL molecule indicating that the fine specificity of this factor is restricted to an antigenic epitope present in this region of the HEL molecule. The suppressive activity is restricted by genes located within the H-2 complex and analysis of the suppression induced in recombinant mice demonstrates that the interaction between HEL-specific suppressor T cell factor and its cellular target requires identity in the I-J region of the H-2 complex.     

Suppression of the cytotoxic T cell response to minor alloantigens in vivo. Linked recognition by suppressor T cells

This report describes the activity of transferable suppressor T cells (Ts) generated in vivo in response to minor alloantigens. These Ts cells are antigen specific in both primary and secondary in vivo cytotoxic T lymphocyte responses to minor alloantigens and are the result of a host response rather than of a graft-vs.-host reaction. The Ts cells are produced soon after immunization and their activity is transient. They act via "linked recognition", since they can suppress the cytotoxic T lymphocyte response to noncross-reactive minor antigens, but only if these are presented on the same antigenic cell. A model for dominant low responsiveness in (high X low responder)F1 animals is proposed, whereby Ts cells, activated via the low responder allele, work by linked recognition to suppress helper cells activated via the high responder allele.     

The regulation of herpes simplex virus-specific CTL induction by suppressor cells

The incubation of herpes simplex virus (HSV) immune murine splenocytes with HSV antigens induced suppressor cells which inhibited HSV-specific cytotoxic T lymphocyte (CTL) induction. The cell mediating the suppression was identified as a Thy 1+ Lyt 2+ I-J+ cell. The induction of this suppressor cell required the participation of at least three leukocyte populations. That is, depleting the cultures of either Lyt 1+ or Lyt 2+ splenocytes resulted in a failure to induce suppressor cell activity. Likewise the removal of macrophage-like antigen-presenting cells (APC), in particular I-A- I-J+ APC, abolished suppressor-cell induction. Though the Lyt 2+ I-J+ cells had to be provided by HSV-immune mice, both the APC and the Lyt 1+ cells could be provided by HSV-naive mice. Though the induction of the suppressor cell was virus specific, its action was nonspecific as evidenced by the suppression of influenza-specific CTL induction. The implication of our results for the understanding and manipulation of herpesvirus disease is briefly discussed.     

Amplification and suppression of human T cell proliferative responses by alloactivated T cells

Human regulator T (TR) lymphocytes presensitized to allogeneic non-T cells in unidirectional mixed leucocyte cultures (MLC) were shown to be capable of either amplifying or suppressing the MLC proliferative responses of autologous T cells when tested in limiting dilution co-culture assays. Although the concentration of alloantigen present during presensitization influenced the TR suppressive activity, considerable suppression was observed over a 10-20-fold range of stimulators. Thus, the concentration of stimulators was not an extremely critical factor in the generation of TR suppression. Much more important, however, was the duration of MLC presensitization because both TR amplification and suppression of T cell-mediated responses were critically time-dependent. In addition, the deletion of rapidly proliferating T cells from MLC at times of marked amplifier activity resulted in the impaired development of TR suppressor populations. These data directly demonstrate that distinct subpopulations of alloactivated TR cells can be distinguished by varying the presensitization interval and that maximal TR suppression requires earlier proliferation of alloreactive T cells.     

The control of T cell responses by dendritic cell subsets

Dendritic cells are known as the most efficient antigen-presenting cell type to activate na?ve T cells; however, they are able to do more than just efficiently present antigen to T cells. They are key modulators of the immune response that can influence Th cell differentiation by preferentially inducing Th type 1 or 2 cell responses, and the differential polarisation of CD4(+) T cells appears to be mediated by discrete dendritic cell subsets.     

T cell suppression in vitro. I. Role in regulation of antibody responses

Suppression of the antibody response by supraoptimal numbers of T helper cells was studied in vitro and found to have both a specific and a nonspecific component. Suppression did not depend on direct cell contract of T and B cells, as supernatants of activated T cells were just as inhibitory. Suppression by excess T cells (or T cell supernatant) was abrogated by the addition of macrophages. By using a tolerance induction protocol, it was shown that T cell supernatants induce partial tolerance in both T and B cells. This tolerance occurred in three different experimental settings – if adherent cells were physically removed by surface adherence, in the presence of anti-macrophage serum, or in the presence of very high concentrations of T cell supernatant. All three conditions stress the importance of the interaction between T cell supernatants and macrophages. The nonspecific form of suppression was not analyzed in detail here, but was also shown to be partly abrogated by macrophages, suggesting that it may be analogous to antigenic competition, one model of which has recently been shown to be abolished by addition of macrophages. The results suggest the presence of homeostatic feedback loops, due to excess T cell function – T cell products suppress the function of T cells, preventing the induction of further helper cells, and also suppress B cells directly. The balance between T cell suppression and cooperation is affected by macrophage function.     

Revisiting and revising suppressor T cells.

A great deal of experimental evidence supports the phenomenon of immunological suppression. The molecular mechanisms to explain the phenomenology have, however, remained controversial. In this review, the data are reinterpreted in light of the recent advances in the understanding of T-cell subsets, the cross-regulatory properties of lymphokines and the differential presentation capacities of different antigen-presenting cell types.     

Inhibition of T suppressor cell function by local administration of an active cyclophosphamide derivative at the sensitization site.

In previous work, we have shown that local administration of active cyclophosphamide (CY) derivatives, or other cytostatic drugs, at the sensitization site induced a similar immunopotentiation to that of systemic CY. Since it is well documented that CY can inhibit suppressor cells, it was proposed that immunoenhancement by locally administered drugs might be based on a similar principle. The objective of the present study was to test this hypothesis, using an experimental model of Ts mediated suppression of delayed type hypersensitivity to sheep erythrocytes. In this model, mice are made tolerant to sheep erythrocytes by i.v. injection of a high dose of sheep erythrocytes. Local treatment of sheep erythrocytes-tolerant mice with the active CY derivative Z7557 at the site of attempted sensitization reversed suppression in a dose-dependent manner. Local treatment with the cytostatic drug etoposide (VP-16) and systemic CY treatment were also effective. In transfer experiments, the function of afferently acting suppressor cells was blocked by local treatment with Z7557 or systemic CY. These data support the concept of anti-suppressor cell activity of locally administered cytostatic drugs. As with CY, the pharmacological basis of this effect remains to be determined.     

Generation of functionally active suppressor cells by haemorrhage and haemorrhagic serum.

Mitogen (PHA)-induced proliferation of peripheral blood mononuclear cells (PBMC) was reduced by more than 70% 2 h after the haemorrhage of 30% of blood volume. Experiments using isolated macrophages and lymphocytes showed that post-haemorrhage macrophages were functionally normal and that lymphocytes were responsible for the observed haemorrhage-induced depression of proliferative response. Surface marker determinations showed that at least some, if not all, of the haemorrhage-induced suppressor cells are of the OX8+ phenotype. Exposure of PBMCs to serum from bled animals also brought about activation of OX8+ suppressor T cells. These results suggest that the depressed proliferative response of PBMCs induced by haemorrhage or by exposing the cells to haemorrhagic serum (serum from bled animals) is due to the activation of OX8+ suppressor T cells.