Vaccination against coccidiosis: host strain-dependent evocation of protective and suppressive subsets of murine lymphocytes.

BALB/c mice are normally more resistant than C57BL/6 (B6) mice to infection with Eimeria vermiformis, but these phenotypes can be reversed by oral or parenteral vaccination with a crude antigen prepared from the parasite. Treatment of mice with antibodies specific for CD4+ or CD8+ T cells showed that the increased susceptibility of vaccinated BALB/c mice was associated with the presence of CD4+ T cells. This finding was confirmed when the recipients of CD4+ T cells selected from the mesenteric lymph nodes (MLN) of vaccinated BALB/c mice produced more oocysts after challenge than the recipients of a similar population of cells from sham-vaccinated mice. The residual population of cells (presumably enriched for CD8+ T cells, 'CD8+'), on the other hand, conferred some protection and, in B6 mice, the findings were reversed. Thus, vaccination induced suppressive or protective CD4+ cells and protective or suppressive 'CD8+' cells, depending upon the normal resistance/susceptibility phenotype of the host. Examinations of the isotypes (IgG1, IgG2a) of specific serum antibodies, and of the levels of IFN-gamma and IL-5 cytokines released by MLN cells stimulated ex vivo, did not allow any further characterization of the mechanisms involved.