Chagas' disease results from the infection of the protozoan parasite Trypanosoma cruzi and affects several million people in South America. Several alterations of the immune response have been described in this disease, such as severe immunosuppression of both cellular and humoral responses and massive polyclonal stimulation with the generation of autoantibodies crossreacting with host cells and tissues. We have obtained monoclonal antibodies (mAbs) from T. cruzi-infected mice that recognized a 50/55-kD antigen (GP50/55) on the T. cruzi membrane, but not in other parasites of the family Trypanosomatidae. One of these GP50/55-specific mAbs (C10) crossreacts with a 28-kD antigen (p28) expressed on the membrane of greater than 85% of activated mouse T and B lymphocytes, after in vitro activation with concanavalin A, Salmonella typhosa lipopolysaccharide, phorbol dibutyrate ester, or antigen, and on several murine T and B lymphocyte cell lines. Human T and B lymphocytes also express upon activation with phytohemagglutinin or Staphylococcus aureus Cowan I (SAC) a similar antigen recognized by mAb C10, although in a lower proportion of cells (30-40%). Furthermore, this mAb was able to suppress mouse and human T and B cell proliferation to any of those stimuli. In addition, sera from chagasic patients and T. cruzi-infected mice, but not from control patients or littermates, contain antibodies that recognize a similar p28 antigen on B lymphocytes. Furthermore, the immunoglobulin fractions of some chagasic sera also suppress the proliferation of human T lymphocytes. These results suggest a possible pathological role of autoantibodies as an alternative mechanism for T. cruzi-associated immunosuppression.

Purified molecules (70,000 mol wt) from a T-suppressor (Ts) clone bind to sheep erythrocyte glycophorin and specifically suppress the response to this antigen. Papain splits purified 70,000-mol wt Ts molecules into two peptides: mol wt 45,000 and 24,000. The 45,000-mol wt peptide nonspecifically suppresses antibody response to several antigens and lacks antigen-binding activity. The 24,000-mol wt peptide does not suppress but retains antigen-binding activity. The results indicate that papain splits the Ts molecule into a "constant" region responsible for function and a "variable" region responsible for antigen-binding. Since binding of the 70,000-mol wt molecule to antigen also results in release of the 45,000 mol wt subunit, this cleavage may allow Ts molecules specific for one determinant to suppress immunity to complex foreign proteins.

We have generated an antigen-specific T suppressor clone that synthesizes 70,000-mol wt peptides that have antigen-specific-binding activity. Although these data also indicated that antigen-binding peptides completely inhibited the in vitro primary response to a complex antigen, suppression might reflect the combined biologic activities of many different 70-mol wt polypeptides or polypeptides associated with the 70,000-mol wt material by noncovalent interactions. The protein responsible for antigen-specific suppression was therefore purified to virtual homogeneity after sequential separation of internally labeled supernate peptides on Sephacryl S-200 and DEAE-cellulose columns followed by isoeleetrofocusing. The resulting protein is greater than 95 percent homogeneous according to sodium dodeeyl sulfate-polyacrylamide electrophoresis and represents two peptides having two very close but distinguishable isoelectric point values of approximately 5.0. The purified molecules are retained by columns coated with lentil lectin or antigen but not by columns coated with antisera specific for immunoglobulins, the I region of the major histocompatibility complex or Ly-1 or Ly-2 antigens. Less than 50 pg of the purified glycoprotein specifically and completely suppresses production of anti-sheep erythrocyte plaque-forming cell by mixtures of 10(6) Ly-1 cells and B cells and this is a result of inactivation of Ly-l-mediated helper function. Specific inactivation of T (Th) cells by the 70,000-mol wt molecule is rapid, specific, and requires the presence of antigen. The mechanism of specific suppression of Th function may depend upon two functionally distinct regions of the 70,000-mol wt molecule: one that binds antigen and a second that mediates suppression.

We have generated continuously propagatable T lymphocyte clones to study antigen-specific T cell functions. All Ly-2+ clones mediate suppressive activity and secrete a characteristic pattern of polypeptides that differs from Ly-2- T cell clones. Cells of one clone, Cl.Ly23/4, specifically bind glycophorin from sheep erythrocytes (SRBC). After incubation with [35S]methionine, supernate material from this clone also contains biosynthetically labeled 70,000-mol wt proteins that specifically bind to SRBC and this binding is inhibited by glycophorin from sheep but not other erythrocytes. These antigen-binding 70,000-mol wt peptides specifically and completely suppress primary anti-SRBC responses generated by mixtures of primed Ly-1+2- cells and B cells. Suppression by these antigen-binding peptides reflects direct inhibition of T-helper activity.