Parasitic infection by Leishmania is a well-known example of a chronic inflammatory disease. Upon infection/injury, leukocytes are recruited to the affected site, and further polarized. The activation of Wnt signaling is possibly one of the initial molecular responses to maintain tissue homeostasis and tissue repair. It is known that platelets are important players in these processes. Although chronic inflammation in parasitic infection is a consequence of constant interaction between the host immune system and parasites, how chronic immune responses are elicited and modulated by parasitic infection remains elusive. The important insight of this proposal is that platelets release significant amounts of the Wnt antagonist Dickkopf-1 (Dkk-1) following parasite recognition and such elevated levels of Dkk-1 regulate multiple levels of the immune response to support chronic inflammation. Consequently, this sequence of events favors parasite survival and constant immunopathology in the host. This systemic increase in Dkk-1 has potent effects on the immune system leading to the development of chronic TH2 immune responses and a potentiation of IL-10. Importantly, CD4 T cells developing under TH1 conditions were driven towards a TH2 (IL-4, IL-13, IL-10) phenotype in the presence of Dkk-1. We propose two aims to examine the interplay between platelets/Dkk-1 at several pivotal points of Leishmania-host innate and adaptive immune responses. In the first aim, the mechanisms by which Dkk-1 is released from platelets will be addressed. As activation is TLR2 dependent, Leishmania mutants that lack characterized surface virulence factors will be utilized. Given the known importance of PMN leukocytes (neutrophils) in Leishmania infection, we will also address the mechanism by which Dkk-1 increases leukocyte-platelet aggregates (LPA) and recruits leukocytes to the infection site. The second aim will specifically probe the consequences of conditional deletion in CD4 T cells of LRP6, the receptor for Dkk-1, and c-Maf and in neutrophils deletion of the receptor for IL-10 and LRP6. The goal is to investigate the contribution of these factors in platelets, CD4 T cells and neutrophils in parasitic infection leading to disease susceptibility or a failure to elicit sterile immunity that results in persistent infection. Taken together, this proposal will provide novel biological insight towards understanding parasitic recognition and evasion during infection and consequent pathogenesis. Our proposal has a primary focus on investigating the mechanism by which overexuberant immune responses are elicited and maintained by host-pathogen interaction. Since we propose multifaceted role(s) of platelet-derived Dkk-1 in modulating immune responses, our study will identify important roles of platelets in leishmaniasis, and contribute to developing Dkk-1 and related biological pathways as effective therapeutic targets in leishmaniasis and potentially other infectious diseases.

PROJECT NARRATIVE This project will characterize the initial interactions of the Leishmania major parasite interactions with the immune system that lead to activation of type 2 immunity. It emphasizes the central role of the Wnt ligands derived from platelets in regulating lymphocyte differentiation and proliferation. Important parasite structures leading to the cellular responses and the role of wnt ligands in amplifying the response are identified.