Complement contributes to protective immunity against reinfection by Plasmodium chabaudi chabaudi parasites

Abstract

We have studied the impact of deficiency of the complement system on the progression and control of the erythrocyte stages of the malarial parasite Plasmodium chabaudi chabaudi. C1q-deficient mice and factor B- and C2-deficient mice, deficient in the classical complement pathway and in both the alternative and classical complement activation pathways, respectively, exhibited only a slight delay in the resolution of the acute phase of parasitemia. Complement-deficient mice showed a transiently elevated level of gamma interferon (IFN-gamma) in the plasma at the time of the acute parasitemia compared with that of wild-type mice. Although there was a trend for increased precursor frequencies in CD4(+) T cells from C1q-deficient mice producing IFN-gamma in response to malarial antigens in vitro, intracellular cytokine staining of spleen cells ex vivo showed no difference in the numbers of IFN-gamma(+) splenic CD4(+) and CD8(+) cells. In contrast, C1q-deficient animals were significantly more susceptible to a second challenge with the same parasite. C1q-deficient animals showed a reduced level of anti-malarial immunoglobulin G2a (IgG2a) antibody 100 days after primary infection. However, following a significantly higher parasitemia, C1q-deficient mice had increased levels of IgM and IgG2a anti-malarial antibodies. In summary, this study indicates that while complement plays only a minor role in the control of the acute phase of parasitemia of a primary infection, it does contribute to parasite control in reinfection.

FIG. 1

Acute-phase parasitemia in complement-deficient and wild-type mice. Although parasitemias were essentially similar during the initial days of infection in C1qa^−/− (open circles) and wild-type (solid circles) mice, the peak of infection was slightly delayed in the C1qa^−/− animals, resulting in a delay in crisis (P < 0.05 [∗] at day 10). H2-Bf/C2^−/− mice (open squares) had parasitemias similar to those in C1qa^−/− animals. The error bars indicate the SEM.

FIG. 2

C1qa^−/− mice suffer from a more severe secondary infection. Six days after secondary infection, all of the wild-type (solid circles; n = 11) and C1qa^−/− (open circles; n = 9) animals had detectable parasitemias (>0.01%). The percentage of blood cells that were parasitized was significantly greater at this point in the C1qa^−/− mice (P < 0.05 [∗]; Student's t test on log-transformed data). The error bars indicate the SEM.

FIG. 3

Anti-malaria specific antibodies in C1qa^−/− and wild-type mice. Throughout the primary infection, antibody responses in C1qa^−/− mice (open bars) were not significantly different from those in the wild-type controls (solid bars). Prior to secondary infection (day 100), the C1qa^−/− mice (n = 5) exhibited a lower level of IgG2a anti-malarial antibodies than the wild-type mice (n = 5) (P < 0.01 [∗∗]; Mann-Whitney test). The mice were reinfected on day 102 (indicated by arrows), and antibody levels were measured on day 120 (the antibody titers for day 120 correspond to the second y axis because of significant increases in serum titers at this time). After the second challenge, the C1qa^−/− animals (n = 9) had significantly greater levels of IgM (P < 0.05 [∗]; Mann-Whitney test) and IgG2a (P < 0.05; Mann-Whitney test) than the wild-type mice (n = 11). Anti-malarial IgG1, IgG2b, and IgG3 titers were similar in C1qa^−/− and wild-type mice throughout the infection (data not shown). The data shown correspond to means + SEM (n = 5 to 11 at each time point).

FIG. 4

(A) Scatter plot showing the circulating quantities of IFN-γ during the acute phase of infection. In the infected wild-type mice (solid circles), IFN-γ was detected only at the peak of parasitemia (days 7 to 9); however, in the C1qa^−/− mice (open circles), IFN-γ was detectable much earlier in the infection and the levels were significantly higher on days 5, 7, and 9 (P < 0.01, [∗∗], P < 0.01, and P < 0.05 [∗], respectively; Mann-Whitney tests). ND, undetectable levels of IFN-γ in the serum. (B) H2-Bf/C2^−/− animals (open squares) exhibited kinetics in the production of IFN-γ similar to those of wild-type mice (solid circles), but the levels of protein in the serum were significantly greater in the H2-Bf/C2^−/− mice on days 6, 8, and 9 (P < 0.01, P < 0.001 [∗∗∗], and P < 0.05, respectively; Mann-Whitney tests).