Kinetics of immunoglobulin and specific antibody responses of CBA mice infected with Trypanosoma rhodesiense

Interleukin-4-dependent immunoglobulin G1 isotype switch in the presence of a polarized antigen-specific Th1-cell response to the trypanosome variant surface glycoprotein

This study examines B-cell immunoglobulin (Ig) class-switching events in the context of parasite antigen-specific Th-cell responses in experimental African trypanosomiasis. Inbred mice were infected with Trypanosoma brucei rhodesiense, and the coordinate stimulation of Th-cell cytokine responses and B-cell responses to the trypanosome variant surface glycoprotein (VSG) was measured. The cytokines produced by T cells in response to VSG, at both the transcript and protein levels, were gamma interferon and interleukin-2 (IL-2) but not IL-4 or IL-5. Isotype profiles of antibodies specific for VSG showed that IgG1, IgG2a, and IgG3 switch responses predominated; no VSG-specific IgE responses were detected. To determine whether cryptic IL-4 responses played a role in promoting the unexpected IgG1 switch response, IL-4 knockout mice were infected; the cytokine responses and Ig isotype profiles of IL-4 knockout mice were identical to those of the wild-type control mice except for dramatically reduced IgG1 levels in response to VSG. Thus, these results revealed an IL-4-dependent component of the VSG-driven B-cell Cmu-to-Cgamma1 switch. We speculate that an IL-4 response is mediated primarily by cells other than T lymphocytes since IL-4-secreting but parasite antigen-unresponsive, "background" cells were detected in all infected mice and since infected nude mice also displayed a detectable IgG1 switch response. Overall, our results suggest that B-cell clonal stimulation, maturation, and Ig class switching in African trypanosomiasis may be partially regulated by unusual mechanisms that do not include antigen-specific Th1 or Th2 cells.

Defective immunoglobulin M responses to vaccination or infection with Schistosoma mansoni in xid mice

Mice vaccinated with irradiated Schistosoma mansoni cercariae develop a persistent immunoglobulin M (IgM) antischistosomulum antibody response. To investigate the possible role of antilarval IgM antibodies in the effector mechanism of vaccine-induced immunity, CBA/N mice, which have an X-linked genetic defect resulting in impaired IgM antibody responses to certain antigens, were analyzed for their resistance to a challenge infection. When either infected with unattenuated parasites or vaccinated with irradiated cercariae, mice of this inbred strain failed to produce detectable IgM antibodies to schistosomulum surface membrane and soluble worm antigens. To analyze the effect of this IgM deficiency on immunity, F1 hybrids were constructed between CBA/N females and nondefective C57BL/6J males. As expected, vaccinated (CBA/N X C57BL/6J)F1 females, as well as (CBA/J X C57BL/6J)F1 males and females, produced normal IgM antibodies to both surface antigens and worm antigen extracts. However, such antibodies were not produced by (CBA/N X C57BL/6J)F1 males (hemizygous for xid). Nevertheless, (CBA/N + C57BL/6J)F1 males displayed the same high levels of immunity to challenge infection as (CBA/N X C57BL/6J)F1 females and (CBA/J X C57BL/6J)F1 males and females. These results indicate that vaccine-induced immunity is not dependent on an IgM response to schistosome antigens.