Unit: Early responses to parasites and Immunopathology
Director: Jacques A. Louis
The major goal of our Unit is to study in vivo, in various genetically defined mice, the early immunological events leading to resistance or susceptibility to intracellular parasites such as Lesihmania and Toxoplasma. One of the group is studying the mechanisms that would lead either to the initiation of a Th1 like immune response in C57BL/6 resistant mice or to a Th2 like pattern in susceptible BALB/c mice after infection with Leishmania major (L. major). The main focus of the second group is to understand the overwhelming Th1 like response that results in the development of a lethal ileitis in B6 mice after oral infection with Toxoplasma gondii (T. gondii). The third group analyses how the immune response against malaria is influenced by the early mast cell activation mediated by Anopheles saliva.
The crucial role of innate immune response in the development of the adaptive immune response is now well documented. Therefore, we (Noëlle Doyen et Nicolas Rachinel) study the role of Toll like receptors in initiating the early innate immune response. It has been reported that some members of the TLR family are involved in the immune response against L. major. Thus, B6 mice deficient for the expression of the MyD88 molecule, a protein involved in the intracellular signalling pathway after stimulation of the TLR, are susceptible to L. major infection. Our preliminary data indicate that the control of the infection by L.major is delayed in TLR9 deficient mice on a B6 genetic background. In these mice the shift of the immune response towards either a protective Th1 or a non protective Th2 profile is determined by the analysis of the cytokine pattern produced such as IL-12, IFNgIL-4 during the early phase of the infection. We are currently investigating in the lymphoid tissue the expression of different TLRs linked or not to the protective mechanisms against L. major infection. Identification of the different cellular subpopulations (macrophages, dendritic cells, lymphocytes) implicated in TLR expression and activation has been undertaken.
Within the first hours after infection with L.major, a similar expression of IFN-g has been measured in the draining lymph nodes of both susceptible and resistant mice. However, the cellular origin of IFN-g seems to be different according to the mouse strain. NK cells are the source of the early IFN- g produced in resistant mice. We addressed the questions of the direct implication of these cells in the induction of a Th1 like immune response in the resistant strain and whether T cells from the resistant mice were more sensitive to IFN- g than those from susceptible mice.
Our previous work demonstrated that LACK specific Vb4-Va8 CD4 T cells from L. major infected susceptible BALB/c mice exhibit a rapid IL-4 expression. This early IL-4 expression participates in the aberrant Th2 immune response leading to the lack of disease control in susceptible mice. In collaboration with the"Centre OMS de Recherche et de Formation en Immunologie , Lausanne" we recently demonstrated that, this early IL-4 expression by Vb4-Va8 CD4 T production was dependent upon IL-2 production by these cells. In contrast IL-2 produced by some other cell types in response to L. major infection remains inefficient. In the resistant B6 mice, the defect in IL-2 production by the Vb4-Va8 CD4 T might result in their defect in IL-4 production.
The group of D. Buzoni-Gatel investigates the intestinal Th1 and Th2 immune responses after oral infection with T. gondii. In B6 mice, the oral inoculation with the parasite induces a lethal ileitis that shares striking histological and immuno-pathological homologies with those observed in human inflammatory bowel diseases (IBD) such as Crohn or coeliac diseases. Despite the increasing frequency of IBD in developed countries, there is yet no relevant animal model to mimic these diseases, the aetiology of which are still uncertain. However , the T. gondii driven ileitis model is quiet useful to observe 1) the immunological events that would lead to the complete damage of the epithelial barrier and 2) the lack of efficiency of the regulatory mechanisms. Therefore we use this model to determine in vivo the immunological disorders responsible for these pathologies. Thus we recently found that the NKT cell traffic to the infected intestine and that their early activation and subsequent production of IFN-g fully contribute to damage the tissues. Using both genetically deficient mice and blocking antibodies for the NKT cells, we provide the evidence that NKT cells play a crucial role in the development of the immuno-pathological lesions after oral infection with T. gondii. In collaboration with Dr Nadine Cerf-Bensussan (Hôpital Necker), the role of IL-15, an inflammatory cytokine produced by the enterocytes, is currently investigated as well as the significance of IL-15 secretion upon the signalling pathways of anti-inflammatory molecules such as TGF-b.
Together with the group studying Leishmania, the role of TLR9 in the initiation of the inflammatory process following oral infection with T. gondii is investigated. Mice deficient for TLR9 expression fail to develop the inflammation after infection, despite a normal parasite replication. The lack of histological lesions in TLR9-/- is associated with a very significant decrease in INF-g production by the CD4 T lymphocytes from the lamina propria. In collaboration with the Dartmouth Medical School (Hanover, USA), parasite antigens initiating the inflammatory process are currently characterized.
The group of S. Mécheri investigates the role of mast cells in the elicitation of local (skin tissue) and distal (lymph nodes) immune responses caused by mosquito bites. The skin is one of the preferred sites of entry for various pathogens including Plasmodium parasites. We made the hypothesis that mosquito saliva, which contains a myriad of pharmacologically active compounds as well as IgE-inducing allergens, may alter the immune system in favor of the Plasmodium transmission. Among resident immune cells in the skin, mast cells are found in relatively large numbers adjacent to blood and lymphatic vessels. In the dermis, mast cells were found to be intimately connected to dendritic cells and together represent the first line of defense against pathogens. Using in vitro as well as the well-established in vivo model of mast cell deficient W/Wv mice, we have shown that mosquito bites induce dermal mast cell degranulation leading to fluid extravasation and neutrophil influx. This inflammatory response does not occur in mast cell-deficient W/Wv mice, unless these are reconstituted specifically with mast cells. Mast cell activation caused by Anopheles stephensi mosquito bites is followed by the hypertrophy of the draining lymph nodes due to the accumulation of CD3+, B220+, CD11b+, and CD11c+ leukocytes. The leukocyte enrichment of the draining lymph nodes resulted from their sequestration from the circulation rather than local proliferation. With regard to the modulation of antigen-specific immune response by mosquito saliva, we found that mosquito bites inhibit, in a mast cell-dependent manner, the induction of the DTH response to ovalbumin. In addition, a predominant IL-10 response was elicited by mosquito bites in the draining lymph nodes. Based on these results, we speculate that mosquito saliva induces the release of a particular set of inflammatory mediators by activated mast cells that may affect the maturation of adjacent dendritic cells which fail to ultimately elicit fully activated effector T cells. Our goal is to further analyze cytokines and chemokines elicited within the skin and in the draining lymph nodes and to demonstrate the relevance of mast cells and their mediators in malaria pathogenesis.
Keywords: parasites, innate immune response, Th1/Th2 balance, mastocytes