Joan Batchelder

Immunity to blood-stage murine malarial parasites is MHC class II dependent

To determine whether MHC class II antigen presentation is essential for the induction of protective immunity against blood-stage malarial parasites, we used gene-targeted knockout (KO) mice to follow the time-course of nonlethal Plasmodium yoelii and Plasmodium chabaudi infections in two models of MHC class II deficiency. Infection of MHC class II KO (A(-/-)) mice with either parasite species resulted in an unremitting hyperparasitemia, whereas MHC-intact control mice resolved their parasitemia. In contrast, invariant chain KO (Ii(-/-)) mice, which present antigen via recycled but not nascent MHC class II molecules, eventually cured their infections when infected with P. yoelii. P. chabaudi parasitemia declined to subpatent levels in most Ii(-/-) mice but then recrudesced. Immunity to blood-stage malaria may be achieved by cell-mediated and antibody-mediated mechanisms of immunity, as such, the findings in A(-/-) mice indicate an essential role for MHC class II presentation of malarial antigens. Moreover, they suggest that protective immune responses to malarial antigens capable of eliminating blood-stage parasites are T cell dependent and can be induced with antigens processed in early and late endosomes.

Suppression of Plasmodium chabaudi Parasitemia Is Independent of the Action of Reactive Oxygen Intermediates and/or Nitric Oxide

ABSTRACT The killing of blood-stage malaria parasites in vivo has been attributed to reactive intermediates of oxygen (ROI) and of nitrogen (RNI). However, in the case of the latter, this contention is challenged by recent observations that parasitemia was not exacerbated in nitric oxide synthase (NOS) knockout (KO) (NOS2−/− or NOS3−/−) mice or in mice treated with NOS inhibitors. We now report that the time course shows that Plasmodium chabaudi parasitemia in NADPH oxidase KO (p47phox−/−) mice also was not exacerbated, suggesting a minimal role for ROI-mediated killing of blood-stage parasites. It is possible that the production of protective antibodies during malaria may mask the function of ROI and/or RNI. However, parasitemia in B-cell-deficient JH−/− × NOS2−/− or JH−/− × p47phox−/− mice was not exacerbated. In contrast, the magnitude of peak parasitemia was significantly enhanced in p47phox−/− mice treated with the xanthine oxidase inhibitor allopurinol, but the duration of patent parasitemia was not prolonged. Whereas the time course of parasitemia in NOS2−/− × p47phox−/− mice was nearly identical to that seen in normal control mice, allopurinol treatment of these double-KO mice also enhanced the magnitude of peak parasitemia. Thus, ROI generated via the xanthine oxidase pathway contribute to the control of ascending P. chabaudi parasitemia during acute malaria but alone are insufficient to suppress parasitemia to subpatent levels. Together, these results indicate that ROI or RNI can contribute to, but are not essential for, the suppression of parasitemia during blood-stage malaria.

Expansion of the gammadelta T cell subset in vivo during bloodstage malaria in B cell-deficient mice.

Mice rendered B cell‐deficient either by chronic anti‐μ treatment initiated at birth or by gene knockout (JHD and μ‐MT mice) suppressed acute Plasmodium chabaudi infections with a time course similar to intact control mice. Moreover, both kinds of B cell‐deficient mice showed a 50‐ to 100‐fold increase in splenic γδ T cell number after suppression of parasitemia compared with uninfected B cell‐deficient controls; the magnitude of this increase resulted in significantly (P < 0.05) greater numbers of splenic γδ T cells in the B cell‐deficient mice than in infected B cell‐intact controls (about 10‐fold). In contrast, the number of splenic CD4+ αβ T cells was only slightly elevated (< 2‐fold) in both kinds of B cell‐deficient mice compared with their intact controls. The number of splenic γδ T cells following suppression of P. vinckei parasitemia was approximately ninefold greater in JHD mice than in C57BL/6 controls, whereas similar numbers of splenic CD4+ αβ T cells were detected. Maximal numbers of γδ T cells were in cell‐cycle in both JHD and C57BL/6 mice during descending P. chabaudi parasitemia, but the number of γδ T cells in cell‐cycle was greater in B cell‐deficient mice than in intact controls. Interleukin‐10 (IL‐10), a potent TH1 cell‐suppressive molecule, does not appear to down‐regulate the γδ T cell response during malaria in B cell‐intact mice because the magnitude of the γδ T cell response was not significantly greater in IL‐10 knockout mice compared with heterozygote controls. These findings collectively indicate that a markedly enhanced expansion of the γδ T cell population occurs in the absence of B cells, and this expansion occurs predominantly during acute malaria when parasite burdens are similar in B cell‐deficient animals and intact controls. J. Leukoc. Biol. 60: 221–229; 1996.

Splenic γδ T Cells Regulated by CD4+ T Cells Are Required To Control Chronic Plasmodium chabaudi Malaria in the B-Cell-Deficient Mouse

ABSTRACT Little is known about the function and regulation of splenic γδ T cells during chronic Plasmodium chabaudi malaria. The splenic γδ T-cell population continues to expand, reaching levels equal to 4 times the number of splenocytes in an uninfected mouse. Splenic γδ T cells from JH−/− mice with chronic malaria expressed Vγ1+ or Vδ4+ in the same ratio as uninfected controls with Vγ1 cells dominating, but the Vγ2 ratio declined about twofold. γδ T cells from G8 mice specific for the TL antigen increased only 2-fold in number, compared with 10-fold in BALB/c controls, but G8 γδ T cells failed to express the B220 activation marker. Elimination of the parasite by drug treatment caused a slow depletion in the number of splenic γδ, CD4+, and CD8+ T cells. Following challenge, drug-cured JH−/− mice exhibited nearly identical parasitemia time courses as naïve controls. Depletion of either CD4+ T cells or γδ T cells from chronically infected JH−/− mice by monoclonal antibody treatment resulted in an immediate and significant (P < 0.05) exacerbation of parasitemia coupled with a marked decrease in splenic γδ T-cell numbers. The number of CD4+ T cells, in contrast, did not decrease in mice after anti-T-cell receptor γδ treatment. The results indicate that cell-mediated immunity against blood-stage malarial parasites during chronic malaria (i) requires the continued presence of blood-stage parasites to remain functional, (ii) is dependent upon both γδ T cells and CD4+ T cells, and (iii) lacks immunological memory.

CD28 costimulation is required for the expression of T-cell-dependent cell-mediated immunity against blood-stage Plasmodium chabaudi malaria parasites.

ABSTRACT Mice suppress the parasitemia of acute blood-stage Plasmodium chabaudi malaria by an antibody- or T-cell-dependent cell-mediated mechanism of immunity (AMI and CMI, respectively) or by both mechanisms. To determine whether CD28 costimulation is required for expression of these polar immune responses, we first compared the time courses of P. chabaudi malaria in CD28-deficient (CD28−/−) and CD28-intact (CD28+/+) mice. Acute infections in both knockout (KO) and control mice followed similar time courses, with the period of descending parasitemia being prolonged ∼2 weeks in KO mice followed by intermittent low-grade chronic parasitemia. Infected CD28−/− mice produced primarily the immunoglobulin M antibody, which upon passive transfer provided partial protection against P. chabaudi challenge, suggesting that the elimination of blood-stage parasites by CD28−/− mice was achieved by AMI. To determine whether CD28−/− costimulation is required for the expression of CMI against the parasite, we compared the time courses of parasitemia in B-cell-deficient double-KO (JH−/− × CD28−/−) mice and control (JH−/− × CD28+/+) mice. Whereas control mice suppressed parasitemia to subpatent levels within ∼2 weeks postinoculation, double-KO mice developed high levels of parasitemia of long-lasting duration. Although not required for the suppression of acute P. chabaudi parasitemia by AMI, CD28 costimulation is essential for the elimination of blood-stage parasites by CMI.