Search

We previously showed that CD8+ T cells are required for optimal primary immunity to low dose Leishmania major infection. However, it is not known whether immunity induced by low dose infection is durable and whether CD8+ T cells contribute to secondary immunity following recovery from low dose infection. Here, we compared primary and secondary immunity to low and high dose L. major infections and assessed the influence of infectious dose on the quality and magnitude of secondary anti-Leishmania immunity. In addition, we investigated the contribution of CD8+ T cells in secondary anti-Leishmania immunity following recovery from low and high dose infections. We found that the early immune response to low and high dose infections were strikingly different: while low dose infection preferentially induced proliferation and effector cytokine production by CD8+ T cells, high dose infection predominantly induced proliferation and cytokine production by CD4+ T cells. This differential activation of CD4+ and CD8+ T cells by high and low dose infections respectively, was imprinted during in vitro and in vivo recall responses in healed mice. Both low and high dose-infected mice displayed strong infection-induced immunity and were protected against secondary L. major challenge. While depletion of CD4+ cells in mice that healed low and high dose infections abolished resistance to secondary challenge, depletion of CD8+ cells had no effect. Collectively, our results show that although CD8+ T cells are preferentially activated and may contribute to optimal primary anti-Leishmania immunity following low dose infection, they are completely dispensable during secondary immunity., Author Summary It is known that CD8+ T cells are important for primary immunity to low dose L. major infection, but whether low dose-induced immunity is long lasting and whether CD8+ T cells are also important for memory immune response to low dose L. major is unknown. We studied whether infectious dose affects primary anti-Leishmania immunity and the contribution of CD8+ T cells in immunity following recovery from low and high dose infections. We found that low and high dose infections preferentially induced proliferation and cytokine production by CD8+ and CD4+ T cells, respectively, during early and late stages of infections. Also, both low and high dose-infected mice were solidly protected against secondary L. major challenge. Depletion of CD4+ cells in mice that healed low and high dose infections abolished resistance to secondary challenge, but depletion of CD8+ cells had no effect. Together, our results show that although CD8+ T cells are selectively activated and contribute to optimal primary immunity after low dose infection, they are not required for secondary immunity. This research further enhances our understanding of the immunobiology of cutaneous leishmaniasis.

Background BALB/c mice are highly susceptible while C57BL/6 are relatively resistant to experimental Trypanosoma congolense infection. Although regulatory T cells (Tregs) have been shown to regulate the pathogenesis of experimental T. congolense infection, their exact role remains controversial. We wished to determine whether Tregs contribute to distinct phenotypic outcomes in BALB/c and C57BL/6 mice and if so how they operate with respect to control of parasitemia and production of disease-exacerbating proinflammatory cytokines. Methodology/Findings BALB/c and C57BL/6 mice were infected intraperitoneally (i.p) with 103 T. congolense clone TC13 and both the kinetics of Tregs expansion and intracellular cytokine profiles in the spleens and livers were monitored directly ex vivo by flow cytometry. In some experiments, mice were injected with anti-CD25 mAb prior or post T. congolense infection or adoptively (by intravenous route) given highly enriched naïve CD25+ T lymphocytes prior to T. congolense infection and the inflammatory cytokine/chemokine levels and survival were monitored. In contrast to a transient and non significant increase in the percentages and absolute numbers of CD4+CD25+Foxp3+ T cells (Tregs) in C57BL/6 mouse spleens and livers, a significant increase in the percentage and absolute numbers of Tregs was observed in spleens of infected BALB/c mice. Ablation or increasing the number of CD25+ cells in the relatively resistant C57BL/6 mice by anti-CD25 mAb treatment or by adoptive transfer of CD25+ T cells, respectively, ameliorates or exacerbates parasitemia and production of proinflammatory cytokines. Conclusion Collectively, our results show that regulatory T cells contribute to susceptibility in experimental murine trypanosomiasis in both the highly susceptible BALB/c and relatively resistant C57BL/6 mice., Author Summary BALB/c mice are highly susceptible while C57BL/6 is relatively resistant to experimental Trypanosoma congolense infection. Acute death observed in infected BALB/c mice is usually associated with the excessive production of pro-inflammatory cytokines. Regulatory T cells (Tregs) have been shown to play a significant role in the pathogenesis of many diseases including those caused by parasites. However, the role of Tregs in the pathogenesis of T. congolense infection remains unclear. We were interested in addressing the following questions: Do Tregs contribute to the distinct phenotypic outcomes observed in T. congolense-infected BALB/c and C57BL/6 mice? If so, where and how do they operate with respect to parasitemia and cytokine response? By selectively altering the numbers of these cells either by targeted depletion with monoclonal antibody or adoptive transfer of highly enriched naïve CD25+ cells prior to infection, we show that Tregs impairs efficient parasite control and impacts on production of disease-exacerbating proinflammatory cytokines. Collectively, our findings suggest that Tregs contribute to enhanced susceptibility to experimental T. congolense infection in mice.

Background Visceral leishmaniasis is the most clinically relevant and dangerous form of human leishmaniasis. Most traditional drugs for treatment of leishmaniasis are toxic, possess many adverse reactions and drug resistance is emerging. Therefore, there is urgent need for identification of new therapeutic targets. Recently, we found that mice with an inactivating knock-in mutation in the p110δ isoform of pi3k, (p110δd910a) are hyper resistant to L. major, develop minimal cutaneous lesion and rapidly clear their parasite. Here, we investigated whether pi3k signaling also regulates resistance to L. donovani, one of the causative agents of visceral leishmaniasis. Methodology/Principal Findings WT and p110δD910A mice (on a BALB/c background) were infected with L. donovani. At different time points, parasite burden and granuloma formation were assessed. T and B cell responses in the liver and spleen were determined. In addition, Tregs were expanded in vivo and its impact on resistance was assessed. We found that p110δD910A mice had significantly reduced splenomegaly and hepatomegaly and these organs harbored significantly fewer parasites than those of WT mice. Interestingly, infected p110δD910A mice liver contains fewer and less organized granulomas than their infected WT counterparts. Cells from p110δD910A mice were significantly impaired in their ability to produce cytokines compared to WT mice. The percentage and absolute numbers of Tregs in infected p110δD910A mice were lower than those in WT mice throughout the course of infection. In vivo expansion of Tregs in infected p110δD910A mice abolished their enhanced resistance to L. donovani infection. Conclusions/Significance Our results indicate that the enhanced resistance of p110δD910A mice to L. donovani infection is due to impaired activities of Tregs. They further show that resistance to Leishmania in the absence of p110δ signaling is independent of parasite species, suggesting that targeting the PI3K signaling pathway may be useful for treatment of both visceral and cutaneous leishmaniasis., Author Summary Visceral leishmaniasis (VL) is the most dangerous form of human leishmaniasis in terms of mortality and morbidity and is spreading to several non-endemic areas because of global traveling and military conflicts. The emergence of Leishmania-HIV coinfection and increased prevalence of drug resistant strains have compounded an already bad situation. In addition, the drugs available are toxic, expensive and have several side effects. Therefore, a detailed understanding of protective immune response is extremely important in order to identify new therapeutic targets. The phosphoinositide 3 kinase (PI3K) family of enzymes mediate several important immunologic and physiologic cellular process including proliferation, differentiation, growth and host defense. We previously showed that genetic inactivation of the p110δ isoform of PI3K results in resistant to L. major (the causative agent of cutaneous leishmaniasis (CL)). Here, we investigate the role of PI3K in immunity to VL and the mechanisms underlying its protective effect. Collectively, our results demonstrate that signaling via the p110δ also regulates immunity to L. donovani, an effect that is dependent on the impact of p110δ signaling on expansion and function of regulatory T cells in vivo. Thus, our studies suggest that targeting the p110δ pathway may be a novel therapeutic strategy for controlling VL and CL.