Trypanosoma cruzi: effects of anti-thymocyte serum in mice and neonatal thymectomy in rats

CF₁ mice were given eight injections of normal rabbit serum (NRS), Hanks' balanced salt solution (HBSS), or rabbit anti-mouse thymocyte serum (ATS) beginning 3 days prior to and at 3-day intervals subsequent to intraperitoneal (ip) inoculation with 5 × 10⁴ trypomastigotes of a Brazil strain of Trypanosoma cruzi. Markedly enhanced parasitemia, increased numbers of tissue stages (amastigotes), and higher mortality occurred in ATS-treated mice as compared to NRS- or HBSS-treated controls. Administration of three injections of ATS at 3-day intervals during the latter stages of acute Chagas' disease, i.e., when numbers of parasites were declining, resulted in a transitory relapse (increase in numbers) of blood and tissue parasites. No relapse occurred in mice when ATS was administered at 3-day intervals over a period of 15 days during the subacute stage of the disease, i.e., after parasites had disappeared from the blood.Parasitemia and mortality were enhanced in neonatally thymectomized rats when compared to that observed in sham-operated and unoperated control rats following ip injection of 2 × 10⁵ trypomastigotes of T. cruzi. Serum obtained from thymectomized and control rats 5 weeks after inoculation with T. cruzi at a time when the blood of all animals had become microscopically negative for parasites were equally protective in passive transfer experiments, while serum from uninfected controls gave no protection.Gamma globulin levels significantly increased in thymectomized as well as intact rats by the third to fourth week of infection with T. cruzi, reached maximum concentrations in 5–6 wk, and remained elevated significantly at the twelfth week post infection as compared with uninfected controls. No significant changes occurred in total serum proteins or α and β fractions of any group, infected or uninfected.Total circulating leukocytes, especially lymphocytes, were diminished in mice and rats subjected to treatment with ATS or neonatal thymectomy.These data clearly indicate that neonatal thymectomy of rats and ATS treatment of mice suppress the acquired immune response to T. cruzi. Further, passive transfer experiments in rats confirm the protective role of circulating antibody in acquired immunity to Chagas' disease.