Influence of Plasmodium chabaudi adami on the isotypic distribution of the antibody response of mice to sheep erythrocytes

Suppression of adaptive immunity to heterologous antigens during Plasmodium infection through hemozoin-induced failure of dendritic cell function

BACKGROUND

Dendritic cells (DCs) are central to the initiation and regulation of the adaptive immune response during infection. Modulation of DC function may therefore allow evasion of the immune system by pathogens. Significant depression of the host's systemic immune response to both concurrent infections and heterologous vaccines has been observed during malaria infection, but the mechanisms underlying this immune hyporesponsiveness are controversial.

RESULTS

Here, we demonstrate that the blood stages of malaria infection induce a failure of DC function in vitro and in vivo, causing suboptimal activation of T cells involved in heterologous immune responses. This effect on T-cell activation can be transferred to uninfected recipients by DCs isolated from infected mice. Significantly, T cells activated by these DCs subsequently lack effector function, as demonstrated by a failure to migrate to lymphoid-organ follicles, resulting in an absence of B-cell responses to heterologous antigens. Fractionation studies show that hemozoin, rather than infected erythrocyte (red blood cell) membranes, reproduces the effect of intact infected red blood cells on DCs. Furthermore, hemozoin-containing DCs could be identified in T-cell areas of the spleen in vivo.

CONCLUSION

Plasmodium infection inhibits the induction of adaptive immunity to heterologous antigens by modulating DC function, providing a potential explanation for epidemiological studies linking endemic malaria with secondary infections and reduced vaccine efficacy.

The response of CD4+ T cells to Plasmodium chabaudi chabaudi

We have studied the role of CD4+ T cells in the immune response to Plasmodium chabaudi chabaudi. From in vivo experiments in which the different subsets of T cells were depleted, it is clear that CD4+ T cells are essential for the generation of protective immunity. Our limiting dilution analysis show that the CD4 T-cell response to P. chabaudi antigens is heterogeneous, in that distinct functions can be performed by different responding T cells, and these responses change during infection. During the first phase of the infection the predominant response is that of a TH1-type cell, producing IL-2 and IFN-gamma. This correlates with the appearance of IFN-gamma in the serum of infected animals. After the clearance of the acute parasitemia, i.e. in the second phase of the infection, the specific response is characterised by TH2 cells, which are effective helper cells for antibody production and presumably are necessary for the switch of IgM to IgG. CD4+ T cells are effector cells are not necessary in the second phase of the infection; mice which have been depleted of CD4+ T cells at this time are able to control their infection in a manner similar to untreated mice. This ability to control parasitemia coincides with the production of specific IgG but not IgM antibodies and the predominance of TH2 type helper cells. Therefore, our data suggest that malaria-specific IgG antibodies are important effectors in the second phase of an infection with P. chabaudi chabaudi.

Expression of Fc gamma receptors on splenic T cells of mice infected with Plasmodium chabaudi adami

In previous studies, it was shown that mice infected with Plasmodium chabaudi adami have a deficiency in their production of IgG1 immunoglobulin, suggesting isotype-specific immunoregulation. In order to examine this phenomenon in further detail the expression of Fc gamma receptors (Fc gamma R) on T cells obtained from mice infected with P. chabaudi was studied by flow cytometry. There was an increase in the number of splenic T cells which expressed Fc gamma R during infection. At the peak of the acute stage of infection (10-15 days) up to 40% of T cells were positive for Fc gamma R expression. These Fc gamma R were present on about 40% of both Lyt-2+ and L3T4+T cells. The isotype preference of these receptors on control Thy-1+ T cells is IgG1 greater than IgG2b greater than IgG2a as determined by an inhibition assay and fluorescence-activated cell sorter (FACS) analysis. However, 2 to 3 weeks after infection this pattern was altered such that IgG2b and IgG2a represented the major isotypes binding to the Fc gamma R of the L3T4+ T cell. At this stage of infection Fc gamma R on L3T4+ cells fail to bind IgG1. In the Lyt-2% T cells IgG1 and IgG2b remained the best inhibitors. These data suggest that there may be changes in Fc gamma R expression on T cells during infection reflected particularly in a decreased ability of IgG1 to bind to the Fc gamma R of L3T4+ cells.