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Mice were infected with Toxocara canis by an oral administration of embryonated eggs of the parasite, then they were killed between 1st and 8th week after infection to obtain sera and splenic lymphocytes. The lymphocytes were kept in culture medium for 3 to 6 days with either concanavalin A or excretory-secretory antigen of T. canis and assayed for proliferative responses. The obtained sera were examined for the presence of specific IgG and IgM antibodies. It was found that cell-mediated immunity was depressed for 3 weeks after infection and the first signs of the developing immunity were detected only at the 4th week of infection. Subsequently, the blastogenic responses increased reaching a level significantly higher when compared with those of the control cultures at 8th week of infection. Specific toxocaral IgG and IgM antibodies were first detected in the samples collected at the 4th week.
The studies were undertaken to investigate the development of some cell-mediated immune responses in experimental toxocarosis in mice and to assess the influence of these responses on the course of infection. Mice were infected orally with 350 eggs of Toxocara canis and reinfected with the same dose of parasites after 8 weeks. Groups of infected animals were killed each week of the experiments to obtain spleens, livers and brains for further studies. Lymphocytes from removed spleens were analysed by flow-cytometry for CD4 and CD8 expression and cultured in vitro to measure their responses to Concanavalin A and excretory-secretory (ES) antigen of T. canis in a lymphocyte transformation test. Pieces of livers were used to prepare paraffin sections to be stained later with haematoxylin and eosin, whereas whole brains of the infected animals were examined for the presence of parasite larvae. The results of the studies showed depression of T-cell responses to ConA in early stages of infection and significant increase in the blastogenic responses to the ES antigen from week 4 following infection. The depression of T-cell responses was accompanied by lowered CD4+/CD8+ ratio resulting from increased percentages of CD8+ T cells. Histopathological examination of liver sections revealed trapping of larvae in T. canis reinfected mice. The intensity of infection as measured by larval recoveries from the brains of mice increased gradually up to the 8th week of infection, but did not show significant changes after reinfection, testifying to the development of long-lasting protective immunity during primary infection.
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