Influence of the polyclonal activation induced by Plasmodium chabaudi on ongoing OVA-specific B- and T-cell responses

Abstract

Infection by Plasmodium chabaudi results in polyclonal activation, massive proliferation and differentiation of lymphocytes with parasite-unrelated specificities. To verify if polyclonal activation includes experienced B and T lymphocytes and if it modifies pre-established cytokine and Ig-isotype patterns, mice were immunized with ovalbumin (OVA) in alum, a condition that favours T helper 2/immunoglobulin G1 (Th2/IgG1) responses, and infected with P. chabaudi 7 or 80 days later. Polyclonal activation markedly increased the number of anti-OVA Ig-secreting cells in the spleen, an effect more patent in mice infected 7 days after OVA immunization, but also evident in mice infected after 80 days. The Ig-isotype profile predefined by immunization was not qualitatively modified by polyclonal activation. Thus, although P. chabaudi infection preferentially induces IgG2a, the expanded anti-OVA response is dominated by IgG1. Polyclonal expansion of the anti-OVA response did not yield an enlarged memory B-cell pool that could be recalled months later by OVA boosting. Moreover, polyclonal activation of anti-OVA IgGl-secreting cells did not increase this antibody in serum, a probable consequence of the high Ig turnover observed during infection. When OVA-specific T-cell cytokines were evaluated, we observed an increase of both interleukin-4 (IL-4) and interferon-γ (IFN-γ) in mice infected 7 days after immunization, whereas in those infected after 80 days, only IL-4 was augmented. These results suggest that polyclonal activation expands experienced B- and T-cell compartments, preserving their antibody and cytokine patterns.