Early Induction of CCL7 Downstream of TLR9 Signaling Promotes the Development of Robust Immunity to Cryptococcal Infection

Abstract

We investigated mechanisms by which TLR9 signaling promoted the development of the protective response to Cryptococcus neoformans in mice with cryptococcal pneumonia. The afferent (week 1) and efferent (week 3) phase immune parameters were analyzed in the infected wild-type (TLR9+/+) and TLR-deficient (TLR9−/−) mice. TLR9 deletion diminished 1) accumulation and activation of CD11b+ dendritic cells (DCs), 2) the induction of IFN-γ and CCR2 chemokines CCL7, CCL12, but not CCL2, at week 1, and 3) pulmonary accumulation and activation of the major effector cells CD4+ and CD8+ T cells, CD11b+ lung DCs, and exudate macrophages at week 3. The significance of CCL7 induction downstream of TLR9 signaling was investigated by determining whether CCL7 reconstitution would improve immunological parameters in C. neoformans-infected TLR9−/− mice. Early reconstitution with CCL7 1) improved accumulation and activation of CD11b+ DCs at week 1, 2) restored early IFN-γ production in the lungs, and 3) restored the accumulation of major effector cell subsets. CCL7 administration abolished the difference in lung fungal burdens between TLR9+/+ and TLR9−/− mice at week 3; however, significant reduction of fungal burdens between PBS- and CCL7-treated mice has not been observed, suggesting that additional mechanism(s) apart from early CCL7 induction contribute to optimal fungal clearance in TLR9+/+ mice. Collectively, we show that TLR9 signaling during the afferent phase contributes to the development of protective immunity by promoting the early induction of CCL7 and IFN-γ and the subsequent early recruitment and activation of DCs and additional effector cells in mice with cryptococcal pneumonia.

Altered Immune Response Differentially Enhances Susceptibility to Cryptococcus neoformans and Cryptococcus gattii Infection in Mice Expressing the HIV-1 Transgene

“…Among these, MCP-1, MIP-1␣, TNF-␣, and IFN-␥ all play a role in leukocyte recruitment to the lungs in response to C. neoformans infection (42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52). Accordingly, reduced production of these four cytokines may explain, at least in part, the strikingly sparse inflammatory cell response to C. gattii compared to that to C. neoformans in the non-Tg C3H mice.…”

“…During pulmonary C. neoformans infection, upregulation of MCP-1 and MCP-3 (CCL7) production is required for CCR2-mediated recruitment of T cells, dendritic cells, and macrophages, formation of bronchovascular cell infiltrates, and development of protective Th1 immunity (42)(43)(44)(45)(46)(47)(48). Furthermore, SJL/J mice, which are resistant to C. neoformans infection, show enhanced MCP-1 mRNA expression compared to susceptible C57BL/6 mice (62).…”

Interleukin-17A Enhances Host Defense against Cryptococcal Lung Infection through Effects Mediated by Leukocyte Recruitment, Activation, and Gamma Interferon Production

“…Our final objective was to investigate potential cellular and molecular mechanisms through which IL-17A enhanced fungal clearance and intracellular sequestration in this model system. We specifically questioned whether IL-17A altered the known relationships between fungal containment, IFN-␥ production, and the activation status of lung dendritic cells and macrophages (56,57,60,61). Using intracellular cytokine staining in conjunction with flow cytometric analysis, we first evaluated the expression of IFN-␥ by CD4 and CD8 T cells at 1, 2, 4, and 8 weeks postinfection.…”

“…We evaluated the expression of MHC-II and the costimulatory molecules CD80 and CD86, as their increased expression by lung dendritic cells is positively associated with Th1 polarization and increased expression on lung macrophages correlates with enhanced fungicidal activity (39,61,63). Expression of these markers was evaluated on both the CD11c ϩ…”

DAP12 Inhibits Pulmonary Immune Responses to Cryptococcus neoformans

“…Studies on the intracellular signaling molecules CARD9 (3) and MyD88 (4-6), which can mediate events downstream of CLRs and TLRs or interleukin-1 receptor superfamily members, respectively, suggest that these pathways play a role in mounting an effective immune response to C. neoformans. However, so far, only TLR9 and mannose receptor, which does not contain intracellular signaling motifs, have been identified as potential PRRs for C. neoformans in vivo (6)(7)(8)(9)(10). There are conflicting reports regarding the importance of TLR2 (4,5,11), and other PRRs for fungal cell wall components, including Dectin-1, Dectin-2, and TLR4, were found to have limited roles, if any, in disease outcomes after C. neoformans challenge (4,5,(11)(12)(13)(14).…”