Role of cellular and humoral immunity and helper cell involvement in permissiveness to infection by Schistosoma mansoni

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Abstract
Cellular and humoral parameters were studied in permissive (mouse) and nonpermissive (rat) animals during the first few weeks following infection by Schistosoma mansoni. Comparison was made between the antigen‐induced proliferation of lymphocytes of the two species. Extract of cercaria, schistosomula and adult worms obtained from infected mice served as the test antigen in 3 and 6 days incubation in vitro thymidine uptake assay. Both spleen and thymus cells of mice and rats differed in their reactivity with the schistosome antigens, but the difference was much more pronounced in the thymocytes. In this case, whereas cells obtained from mice were not induced to proliferate by any of the antigens, thymocytes of the infected rat showed specific increasing proliferation rates as a function of time after infection. In the rat, a subset of T cell population, the T helper cell, appears to be involved in the immune response, as established by the use of anti‐helper monoclonal antibody W3/25. In the presence of these antibodies the immune reactivity of the rat thymocytes with antigens from different stages of the parasite was markedly reduced. The effect of W3/25 antibodies was observed not only in vitro, in the lymphocyte proliferation assay, but also in vivo, by a significant increase in the extent of infection in the W3/25‐treated rats, as compared to untreated controls. Differences between the two hosts were also observed in their humoral immune response against the schistosome antigens, as manifested by complement‐dependent cytotoxicity. In these assays mouse sera were more effective than rat sera. When autologous complement was evaluated, mouse complement had a higher cytotoxic effect than rat complement in killing of the parasite. Both sources of complement were less effective than guinea pig complement in causing antibody‐mediated cytotoxicity.