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Mycobacterium tuberculosis (Mtb) is the leading cause of mortality worldwide due to a single infectious agent. Mtb resides in pulmonary macrophages, neutrophils and phagocytes, which are also key effector cells that limit bacterial growth. The role of other granulocytic phagocytes during Mtb infection is largely unknown. Eosinophils have comparable phagocytic function to neutrophils and an overlapping repertoire of granular contents capable of limiting bacterial growth. Here we characterized eosinophil responses to Mtb in vitro and in vivo. Eosinophils from healthy human donors responded to Mtb in vitro by CD69 up-regulation and CD62L down-regulation. Mtb induced the release of the eosinophil-specific granule proteins EDN and ECP. Mtb exposure resulted in secretion of IL-1a, TNFa, MIP-1a, MIP-1b, and IL-8. These results suggest that Mtb directly affects eosinophil degranulation, and that eosinophils are able to contribute to Mtb-driven inflammation. To investigate whether eosinophils actively participate in the cellular immune response to Mtb in vivo we followed the eosinophilic response after pulmonary Mtb infection in non-human primates. Eosinophils were significantly increased in the BAL fluid of Mtb infected rhesus macaques, providing evidence that eosinophils are being actively recruited to the lung in response to Mtb in vivo. Indeed, when we analyzed Mtb infected lung tissue from patients who had undergone lung resection, eosinophils were enriched in fibrotic TB consolidations, cavity wall and necrotic caseum. We are currently investigating the biological relevance of the eosinophilic response during Mtb infection in the mouse model. This work was supported by the intramural research programs of NIAID, NIH.

Anti-retroviral therapy (ART) of HIV-infected individuals usually leads to clinical improvement, but some individuals develop a worsening of symptoms following ART, referred to as Immune Reconstitution Inflammatory Syndrome (IRIS). CD4 T cells are thought to be a primary mediator of this paradoxical disease and recent work in patients has correlated activated monocytes with the development of IRIS. Here we use a mouse model of Mycobacterium avium associated IRIS to examine myeloid cell responses in IRIS. We find that CD4 T cell reconstitution of chronically infected TCRaKO mice leads to large increases in neutrophils and Ly6C+ CCR2+ monocytes. Prior to CD4 T cell transfer, myeloid cells in chronically infected TCRaKO mice expressed less iNOS but higher TNF compared to WT mice. Within ~12 days after CD4 T cell reconstitution, multiple myeloid cell subsets dramatically upregulated iNOS and TNF reaching levels far above what is ever observed in WT mice at any time during infection. Transfer of IFNgKO CD4 T cells into chronically infected TCRaKO mice failed to drive myeloid cell expansion or increased function, and mice displayed greatly reduced signs of illness. These data indicate that progression of a chronic mycobacterial infection in the absence of CD4 T cell help predisposes myeloid cells to over respond to T cell stimulation when CD4 T cells are later introduced, indicating myeloid cell hyper responsiveness may underly the immunopathogenesis of IRIS.

PD-1 is an inhibitory receptor expressed on the surface of activated T cells. PD-1 previously has been shown to play a significant role in regulating effector T cell responses against viral infections and tumors. However, its role in the control of CD4 T cell responses during infection with fungal pathogens remains poorly understood. Here we examine the role of PD-1 in regulating host responses and resistance to the opportunistic fungal pathogen Cryptococcus neoformans. We find that in wild-type mice C. neoformans elicits Th1, Th2, and Th17 responses, and PD-1 is expressed on each of these effector T cell subsets. In PD-1 deficient mice, the frequency of IFN-γ-producing and t-bet expressing CD4+ T cells were increased, while CD4+ T cells making IL-13, IL-5, and IL-4 were decreased and Th17 responses were unchanged. Also, PD-1 deficient mice show a decrease in eosinophils in the lung, and in fungal burden. These data suggest that PD-1 impairs host resistance to C. neoformans by selectively inhibiting Th1 responses. The work was supported by the NIH Intramural Research Program.