Suppressor cells and loss of B-cell potential in mice infected with Trypanosoma brucei.
The functional changes in splenic lymphoid populations from mice infected with T. brucei strain S42 were studied throughout the 3 weeks of infection. Within a week of infection, proliferation of B and T cells profoundly increased as shown by 3H-labelled thymidine incorporation and fluorescent staining of surface Ig; the spleen cells secreted high levels of both IgM and IgG immediately cells were put into culture; but with progressing infection this Ig production declined. The early effect on T cells was reflected by lack of responsiveness to PHA. B-cell potential was studied in low-density cultures treated with lipopolysaccharide (E. coli). Normal spleen cells proliferate extensively in these cultures with subsequent secretion of IgG as well as IgM. The ability to proliferate and produce Ig in response to LPS was severely depressed by day 7 and almost totally absent by day 12 of infection. Removal of T cells from the spleen cells obtained early in infection partly restored the response to LPS but as the infection neared its fatal end, B-cell potential appeared to become exhausted. Macrophages obtained from infected mice even early in infection profoundly depressed the ability of normal spleen cells to proliferate and secrete immunoglobulin in LPS cultures. The general immunodepressing effect of trypanosomes can be attributed to clonal exhaustion of B-cell potential caused by an undefined blastogenic stimulus from the parasites which may operate at least in part by the generation of suppressive T cells and macrophages.