Altered immune responses in mice with concomitant Schistosoma mansoni and Plasmodium chabaudi infections

Abstract

Mixed parasitic infections are common in many parts of the world. However, little is known about how concurrent infections affect the immunity to and/or pathogenesis of each other. Protection and elimination of blood-stage Plasmodium chabaudi chabaudi AS in resistant mice are characterized by a sequential activation of CD4(+) Th1 and Th2 cells. The patent egg-laying stage of the murine model of Schistosoma mansoni is associated with a strong Th2 response to both Schistosoma and unrelated antigens. In this study, we investigated how infection of mice with S. mansoni would affect the immune response to and pathogenesis of a P. chabaudi infection. C57BL/6 mice infected with S. mansoni for 8 weeks were infected with blood-stage P. chabaudi. Malaria parasitemias were significantly higher in these mice than in mice infected with P. chabaudi only. In doubly infected mice, both spleen cell proliferative and Th2 responses to S. mansoni soluble egg antigen (SEA) or anti-CD3 were suppressed up to 1 month after the malaria infection. Findings for SEA-specific immunoglobulin M (IgM) and IgG serum antibody levels were similar. No significant effects were seen on P. chabaudi-induced gamma interferon responses. However, tumor necrosis factor alpha (TNF-alpha) production was significantly lower in double-infected mice. Thus, a defect in TNF-alpha production might contribute to the increased malaria parasitemias seen in S. mansoni-P. chabaudi-infected mice. Taken together, our data show that schistosoma and malaria infections profoundly affect each other, findings which might have implications for the development of vaccines.

FIG. 1

Malaria parasitemia (A) and RBC counts (B) in peripheral blood of mice infected with P. chabaudi AS alone (•) or concomitantly with S. mansoni (○). Each value represents the mean ± SEM for 20 to 40 mice. ∗, statistically significant difference (P < 0.05) between mice with P. chabaudi infection only and mice with concurrent S. mansoni-P. chabaudi infection. Solid lines represent mean RBC counts from S. mansoni-only-infected control animals ± SEM (−−−).

FIG. 2

SEA- and P. chabaudi-specific antibodies in serum samples from mice with P. chabaudi infection only (■) or with concurrent S. mansoni-P. chabaudi infection (). Anti-SEA (A to C) and anti-P. chabaudi (D to F) antibodies were measured by ELISA. IgG1 (A and D), IgG2a (B and E), and IgM (C and F) levels were determined and are presented as absorbance (OD) at 405 nm. Results shown were obtained from serum dilutions of 1:1,000 for SEA-specific IgG1 and IgG2a, 1:10,000 for SEA-specific IgM, and 1:1,000 for P. chabaudi-specific IgG1, IgG2a, and IgM antibodies. Bars represent mean OD ± SEM for three to five mice, and solid lines represent pooled data from control animals infected with S. mansoni only assayed in parallel at each time point ± SEM (−−−). #, statistically significant differences (P < 0.05) between mice with S. mansoni infection only and mice with concurrent S. mansoni-P. chabaudi infection; ∗, statistically significant differences (P < 0.05) between mice with P. chabaudi infection only and mice with concurrent S. mansoni-P. chabaudi infection.

FIG. 3

Stimulation indices (SI) in spleen cell cultures from mice with P. chabaudi infection only (■) or from mice with concurrent S. mansoni-P. chabaudi infection (). Cultures were stimulated with pRBC (A), SEA (B), or anti-CD3 (C). Bars represent mean SI for three to five mice ± SEM, and solid lines represent pooled data from S. mansoni-only-infected control animals assayed in parallel at each time point ± SEM (−−−). Note the different scales on the y axes. #, statistically significant differences (P < 0.05) between mice with S. mansoni infection only and mice with concurrent S. mansoni-P. chabaudi infection.

FIG. 4

Th2 cytokine responses in in vitro-stimulated spleen cell cultures from mice with P. chabaudi infection only (■) or from mice with concurrent S. mansoni-P. chabaudi infection (). IL-4 (A and B) and IL-5 (C and D) were measured in the SN from 72-h spleen cell cultures stimulated with SEA (A and C) or anti-CD3 (B and D). Bars represent mean values from three to five mice ± SEM, and solid lines represent pooled data from S. mansoni-only-infected control animals assayed in parallel at each time point ± SEM (−−−). Note the different scales on the y axes of panels C and D. #, statistically significant differences (P < 0.05) between mice with S. mansoni infection only and mice with concurrent S. mansoni-P. chabaudi infection.

FIG. 5

IFN-γ responses in in vitro-stimulated spleen cell cultures from mice with P. chabaudi infection only (■) or from mice with concurrent S. mansoni-P. chabaudi infection (). IFN-γ was measured in the SN from 72-h spleen cell cultures stimulated with pRBC (A) or anti-CD3 (B). Bars represent mean levels for three to five mice ± SEM, and solid lines represent pooled data from S. mansoni-only-infected control animals assayed in parallel at each time point ± SEM (−−−). Note the different scales on the y axes. #, statistically significant differences (P < 0.05) between mice with S. mansoni infection only and mice with concurrent S. mansoni-P. chabaudi infection.

FIG. 6

TNF-α responses in in vitro-stimulated spleen cell cultures from mice with P. chabaudi infection only (■) or from mice with concurrent S. mansoni-P. chabaudi infection (). Cultures were stimulated with pRBC (A) or anti-CD3 (B). Bars represent mean levels for three to five mice ± SEM, and solid lines represent pooled data from S. mansoni-only-infected control animals assayed in parallel at each time point ± SEM (−−−). ∗, represents statistically significant differences (P < 0.05) between mice with P. chabaudi infection only and mice with concurrent S. mansoni-P. chabaudi infection. nd, not determined.