Analysis of B cell and T cell proliferative responses induced by monomorphic and pleomorphic Trypanosoma brucei parasites in mice.

Cells collected from C57B1/6 mice infected with monomorphic and pleomorphic clones of Trypanosoma brucei parasites (ILTat 1.4 and GUTat 3.1) were analysed for the incorporation of 125I-Iododeoxyuridine into DNA of total splenic lymphocytes and of B and T lymphocytes isolated on a fluorescence activated cell sorter. The monomorphic T. brucei ILTat 1.4 parasites triggered delayed and low splenic DNA synthetic responses in comparison to those arising in mice infected with the pleomorphic T. brucei GUTat 3.1 organisms. Mice infected with both parasite clones mounted splenic DNA synthetic responses similar to those arising in animals infected with the pleomorphic organisms alone and similar responses were induced by lethally irradiated T. bruceiGUTat 3.1 and T. brucei ILTat 1.4 parasites. In mice infected with the pleomorphic parasites, DNA synthesis was first detected in the T cell population and B cell DNA synthetic responses were detected between 1 and 2 days later. In contrast only T cell DNA synthetic responses were detected after infection with the monomorphic T. brucei ILTat 1.4 parasites. It is suggested that the previously reported failure of monomorphic T. brucei parasites to trigger antibody production in infected mice is a result of their inability to stimulate B lymphocytes.

Regulation of parasite-specific antibody responses in resistant (C57BL/6) and susceptible (C3H/HE) mice infected with Trypanosoma (trypanozoon) brucei brucei.

After infection with 10(3) T. brucei GUTat 3.1, C57BL/6 mice produced antibody responses and controlled the first parasitaemic wave whereas C3H/He mice did not. The inability of C3H/He mice to control parasitaemia resulted from an impaired ability of parasite-induced antibody-containing cells to secrete immunoglobulin. Antibody-containing cells in infected C3H/He mice regained the ability to secrete antibody within 24 h after trypanosome elimination by treatment with Berenil, suggesting that the block in antibody secretion was maintained by living parasites or short-lived components of degenerating parasites. Infected C3H/He mice also had an impaired ability to produce a rabbit erythrocyte-specific antibody response on challenge with rabbit erythrocytes and this response recovered when parasites were eliminated from the blood 24 h before analysis. It was not possible to inhibit secretion of antibody by rabbit erythrocyte-induced plasma cells either by incubating them with serum from infected C3H/He mice or by injecting large numbers of living trypanosomes into C3H/He mice already responding to rabbit erythrocytes. The process leading to failure of parasite and rabbit erythrocyte-induced antibody-containing cells to become high rate antibody-secreting cells was not identified but did not appear to correlate with any obvious change in the intra-cellular morphology of the antibody-containing cells.

Regulation of the growth and differentiation of Trypanosoma (Trypanozoon) brucei brucei in resistant (C57Bl/6) and susceptible (C3H/He) mice.

While Trypanosoma brucei brucei GUTat 3 were equally infective for C3H/He and for C57Bl/6 mice at doses ranging from 5 to 5 x 10(3) organisms and had similar prepatent periods in both strains of mice, infected C57Bl/6 mice displayed lower parasitaemia, shorter times to parasite wave remission and survived for a longer time than infected C3H/He mice. Parasite growth and differentiation rates and host immune responses were similar for the first 5 days in both strains of mice after infection with 10(3) T.b.brucei GUTat 3 but, thereafter, parasite differentiation proceeded more rapidly and specific antibodies reached higher titres in C57Bl/6 than in C3H/He mice. In contrast, parasite growth and differentiation rates were similar in irradiated mice of both strains. Furthermore, following inoculation of intact mice with irradiated T.b.brucei GUTat 3, C3H/He mice actually mounted higher titred antibody responses than C57Bl/6 mice showing that they were not intrinsically defective in their capacity to respond to GUTat 3 antigens. Parasite differentiation occurred at the same rate in irradiated (650r) C57Bl/6 mice and in irradiated C57Bl/6 mice reconstituted with syngeneic spleen cells although T.b.brucei GUTat 3 specific antibody was detected in the latter mice prior to peak parasitaemia. Furthermore, it was shown directly in C57Bl/6 mice that there was no selective destruction of slender form T.b.brucei GUTat 3 parasites during the phase of accumulation of stumpy form parasites. These studies indicate that the more rapid differentiation of T.b.brucei GUTat 3 parasites in infected C57Bl/6 mice as compared to infected C3H/He mice was unlikely to be directly related to the more efficient antibody response in the infected C57Bl/6 mice. The observations suggest that there might be an association between host mechanisms which regulate differentiation of T.b.brucei parasites and those which regulate antibody responses.

Trypanosoma brucei variable surface antigen is released by degenerating parasites but not by actively dividing parasites.

Surface antigen biosynthesis and fate in monomorphic and pleomorphic Trypanosoma brucei was examined to assess how slender and stumpy form T. brucei parasites present their variant specific glycoprotein (VSG) to the host immune system. Monomorphic and pleomorphic T. brucei did not release recently synthesized VSG in vitro. Slender form T. brucei, either from monomorphic or pleomorphic populations, did not release VSG in vivo. Detection of free VSG in plasma from irradiated mice infected with pleomorphic parasites correlated with the appearance of stumpy form parasites and possibly arose as a result of degeneration of those parasites. The in vivo released VSG was found to react well with some but not all antibodies directed against VSG determinants. Monoclonal and monospecific antibodies which react with VSG on living trypanosomes did not react with the released VSG whereas VSG-specific monoclonal antibodies which do not react with the surface of living T. brucei did react with the released VSG. It was unclear whether released VSG had lost a conformational determinant expressed on trypanosome-attached VSG or whether antibodies which react strongly with VSG on living trypanosomes are of such low avidity that they fail to bind released VSG. The results suggest that trypanosome-attached VSG is more important for stimulation of protective humoral responses than released VSG. The requirements for stimulation of protective anti-VSG responses are reported elsewhere (Sendashonga & Black 1982).

Humoral responses against Trypanosoma brucei variable surface antigen are induced by degenerating parasites.

An analysis was made of the inductive stimuli for anti-T. brucei variant surface glycoprotein (VSG) responses and the role played by humoral immunity in trypanosome wave control. The first wave parasitaemia was influenced by the rate of parasite differentiation from rapidly dividing slender forms to short lived stumpy forms. Remission of first wave parasitaemia was caused by a humoral immune response against external determinants of surface expressed VSG. Anti-VSG responses were accompanied by anti-trypanosome plasma membrane responses and were followed by non-specific responses. Responses appeared to be initiated by fragments of parasites on which VSG external determinants and plasma membrane antigens were accessible and were possibly accelerated and amplified by a trypanosome mitogen which was not VSG. The parasite fragments may have arisen as a result of degeneration of stumpy form but not slender form parasites.