Anti-IL-2 treatment impairs the expansion of T(reg) cell population during acute malaria and enhances the Th1 cell response at the chronic disease.

Plasmodium chabaudi infection induces a rapid and intense splenic CD4(+) T cell response that contributes to both disease pathogenesis and the control of acute parasitemia. The subsequent development of clinical immunity to disease occurs concomitantly with the persistence of low levels of chronic parasitemia. The suppressive activity of regulatory T (T(reg)) cells has been implicated in both development of clinical immunity and parasite persistence. To evaluate whether IL-2 is required to induce and to sustain the suppressive activity of T(reg) cells in malaria, we examined in detail the effects of anti-IL-2 treatment with JES6-1 monoclonal antibody (mAb) on the splenic CD4(+) T cell response during acute and chronic P. chabaudi AS infection in C57BL/6 mice. JES6-1 treatment on days 0, 2 and 4 of infection partially inhibits the expansion of the CD4(+)CD25(+)Foxp3(+) cell population during acute malaria. Despite the concomitant secretion of IL-2 and expression of high affinity IL-2 receptor by large CD4(+) T cells, JES6-1 treatment does not impair effector CD4(+) T cell activation and IFN-γ production. However, at the chronic phase of the disease, an enhancement of cellular and humoral responses occurs in JES6-1-treated mice, with increased production of TNF-α and parasite-specific IgG2a antibodies. Furthermore, JES6-1 mAb completely blocked the in vitro proliferation of CD4(+) T cells from non-treated chronic mice, while it further increased the response of CD4(+) T cells from JES6-1-treated chronic mice. We conclude that JES6-1 treatment impairs the expansion of T(reg) cell population during early P. chabaudi malaria and enhances the Th1 cell response in the late phase of the disease.

Analysis of the activation profile of dendritic cells derived from the bone marrow of interleukin-12/interleukin-23-deficient mice.

We have previously shown that macrophages from interleukin (IL)-12p40 gene knockout (IL-12/IL-23-/-) mice have a bias towards the M2 activation profile, spontaneously secreting large quantities of transforming growth factor-beta1 (TGF-beta1) and producing low levels of nitric oxide (NO) in response to lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma). To verify whether the activation profile of dendritic cells (DCs) is also influenced by the absence of IL-12/IL-23, bone marrow-derived DCs from IL-12/IL-23-/- and C57BL/6 mice were evaluated. At first we noticed that approximately 50% of the C57BL/6 DCs were dead after LPS-induced maturation, whereas the mortality of IL-12/IL-23-/- DCs was < 10%, a protective effect that diminished when recombinant IL-12 (rIL-12) was added during maturation. Similarly to macrophages, mature IL-12/IL-23-/- DCs (mDCs) produced higher levels of TGF-beta1 and lower levels of NO than C57BL/6 mDCs. NO release was IFN-gamma-dependent, as evidenced by the poor response of IFN-gamma-/- and IL-12/IL-23-/-IFN-gamma-/- mDCs. Nevertheless, IFN-gamma deficiency was not the sole reason for the weak NO response observed in the absence of IL-12/IL-23. The high level of TGF-beta1 secretion by IL-12/IL-23-/- mDCs could explain why exogenous IFN-gamma partially restored the NO production of IFN-gamma-/- mDCs, while IL-12/IL-23-/- IFN-gamma-/- mDCs remained unresponsive. We also showed that CD4+ T-cell proliferation was inhibited by C57BL/6 mDCs, but not by IL-12/IL-23-/- mDCs. IFN-gamma and NO appear to mediate this antiproliferative effect because this effect was not observed in the presence of mDCs from IFN-gamma-/- or IL-12/IL-23-/- IFN-gamma-/- mice and it was attenuated by aminoguanidine. We conclude that the presence of IL-12/IL-23 during LPS-induced maturation influences the activation profile of DCs by a mechanism that is, only in part, IFN-gamma dependent.

Role of CD28 in polyclonal and specific T and B cell responses required for protection against blood stage malaria.

The role of B7/CD28 costimulatory pathway in the polyclonal and specific lymphocyte activation induced by blood stages of Plasmodium chabaudi AS was investigated in CD28 gene knockout (CD28(-/-)) and C57BL/6 (CD28(+/+)) mice. Analysis of the spleen during the acute infection revealed a similar increase in T and B cell populations in both groups of mice. Moreover, CD28(-/-) mice were able to develop a polyclonal IgM response to P. chabaudi. On the contrary, the polyclonal IgG2a response was markedly reduced in the absence of CD28. Production of IFN-gamma; up-regulation of CD69, CD40L, CD95 (Fas), and CD95L (Fas ligand); and induction of apoptosis were also affected by the lack of CD28. Interestingly, the ability to control the first parasitemia peak was not compromised in acutely infected CD28(-/-) mice, but CD28(-/-) mice failed to eradicate the parasites that persisted in the blood for >3 mo after infection. In addition, drug-cured CD28(-/-) mice were unable to generate memory T cells, develop an anamnesic IgG response, or eliminate the parasites from a secondary challenge. The incapacity of CD28(-/-) mice to acquire a full protective immunity to P. chabaudi correlated with an impaired production of specific IgG2a. Moreover, reinfected CD28(-/-) mice were protected by the adoptive transfer of serum from reinfected CD28(+/+) mice containing specific IgG2a. Our results demonstrate that the polyclonal lymphocyte response is only partially affected by the absence of CD28, but this coreceptor is essential to generate specific T and B cell responses required for complete protection against P. chabaudi malaria.