Leishmania major shapes the early cytokine and chemokine response at the site of infection

Abstract

Thesis (Ph. D.)--University of Rochester. School of Medicine and Dentistry. Dept. of Microbiology and Immunology, 2007.Compartmentalization of the immune response ensures tight regulation of T cell activation in the lymph node (LN) and precise effector T cell delivery and function at sites of inflammation. During differentiation within the LN, functionally discrete effector T cells acquire distinct homing properties that direct them to sites of inflammation. We show that such tissue-specific accumulation of cytokine secreting effector T cells can be subverted by a pathogen at the infected tissue site. On Leishmania major (L. major) infection, the draining LN (dLN) of both susceptible and resistant mice contained both IL-4 and IFNγ-producing T cells early in infection. In contrast, primarily IL-4 producing cells were found in the infected tissue. Despite a striking absence of IFNγ producers in the L. major-infected tissue, L. major-specific IFNγ effectors were readily detectable at an independent inflammatory site. Recruitment of non-Leishmania specific effectors to the infection site was similarly restricted. Importantly, provision of strong inflammatory signals (CpG) failed to disrupt the selective cytokine production at the L. major infection site, suggesting that L. major actively modifies the local milieu. To formally test this idea, we assessed the ability of the L. major infected site to recruit a third-party effector population. Mice were infected with L. major in one ear and immunized with ovalbumin (OVA) in the contralateral ear. Two weeks after infection, mice were rechallenged with soluble OVA directly in the L. major infected ear. Indeed, while both anti-OVA IL-4 and IFNγ producers were found in the OVA-immunized ear and dLN, only IL-4 producing, OVA specific cells were found in the L. major-infected tissue. Mechanistically, IFNγ producers could fail to be recruited, be recruited but not be retained or be recruited and then functionally modified or terminated. Our data suggests a role for L. major in shaping the early chemokine microenvironment. Early upon infection, we observed a striking restriction in chemokine expression at the infection site; expression of CCL7 but not Th1-attracting chemokines. Moreover, L. major potently inhibited CpG-induction of various Th1-attracting chemokines in part through a direct action of the parasite on the infected macrophage. These data implicate a role for the local chemokine milieu in shaping the resulting cytokine repertoire at the site of infection. In addition we have developed techniques that will enable us to track effector cell function and localization within the whole mouse. The use of fluorescent parasites in combination with cytokine reporter mice, whole mount immunohistochemistry and bioluminescence imaging will allow us to monitor the recruitment/retention and functional status of antigen specific effector cells at the site of infection. We suggest that the pathogen-driven restriction to non-pathogenic IL-4 producing cells enables L. major to gain early host residency. This work demonstrates the need to understand the immune response at infected tissue sites in addition to the repertoire generated within the dLN. This will promote the development of therapies, which target either the generation of the appropriate immune response or direct an existing response to the appropriate location in order to combat infection