Although commensal bacteria are crucial in maintaining immune homeostasis of the intestine, the role of commensal bacteria in immune responses at other mucosal surfaces remains less clear. Here, we show that commensal microbiota composition critically regulates the generation of virus-specific CD4 and CD8 T cells and antibody responses following respiratory influenza virus infection. By using various antibiotic treatments, we found that neomycin-sensitive bacteria are associated with the induction of productive immune responses in the lung. Local or distal injection of Toll-like receptor (TLR) ligands could rescue the immune impairment in the antibiotic-treated mice. Intact microbiota provided signals leading to the expression of mRNA for pro–IL-1β and pro–IL-18 at steady state. Following influenza virus infection, inflammasome activation led to migration of dendritic cells (DCs) from the lung to the draining lymph node and T-cell priming. Our results reveal the importance of commensal microbiota in regulating immunity in the respiratory mucosa through the proper activation of inflammasomes.

Summary Unlike other types of T helper (Th) responses, whether the development of Th2 cells requires instruction from particular subset of dendritic cells (DCs) remains unclear. By using an in vivo depletion approach, we have shown that DCs expressing CD301b were required for the generation of Th2 cells after subcutaneous immunization with ovalbumin (OVA) along with papain or alum. CD301b+ DCs are distinct from epidermal or CD207+ dermal DCs (DDCs) and were responsible for transporting antigen injected subcutaenoulsy with Th2-type adjuvants. Transient depletion of CD301b+ DCs resulted in less effective accumulation and decreased expression of CD69 by polyclonal CD4+ T cells in the lymph node. Moreover, despite intact cell division and interferon-γ production, CD301b+ DC depletion led to blunted interleukin-4 production by OVA-specific OT-II transgenic CD4+ T cells and significantly impaired Th2 cell development upon infection with Nippostrongylus brasiliensis. These results reveal CD301b+ DDCs as the key mediatorsof Th2 immunity.

Summary Candida albicans is a dimorphic fungus responsible for chronic mucocutaneous and systemic infections. Mucocutaneous immunity to C. albicans requires T helper-17 (Th17) cell differentiation that is thought to depend on recognition of filamentous C. albicans. Systemic immunity is considered T cell independent. Using a murine skin infection model, we compared T helper cell responses to yeast and filamentous C. albicans, We found that only yeast induced Th17 cell responses through a mechanism that required Dectin-1 mediated expression of interleukin-6 (IL-6) by Langerhans cells. Filamentous forms induced Th1 without Th17 cell responses due to the absence of Dectin-1 ligation. Notably, Th17 cell responses provided protection against cutaneous infection while Th1 cell responses provided protection against systemic infection. Thus, C. albicans morphology drives distinct T helper cell responses that provide tissue specific protection. These findings provide insight into compartmentalization of Th responses, C. albicans pathogenesis and have critical implications for vaccine strategies.

Regeneration of skin’s barrier function after injury requires temporally coordinated cellular interactions between multiple cell types. Macrophages are essential inflammatory cells in skin wound regeneration. These cells switch their phenotype from inflammatory in the early regenerative stages to anti-inflammatory in the midstages of healing to coordinate skin repair. However, little is known about how different subsets of anti-inflammatory macrophages contribute to skin wound healing. Here, we characterize midstage macrophages (CD45+/CD11b+/F4-80+) and identify two major populations: CD206+/CD301b+ and CD206+/CD301b−. The numbers of CD206+/CD301b+ macrophages increased concomitantly with repair, when the anti-inflammatory phenotype switch occurs in midstage healing. Using diphtheria toxin–mediated depletion models in mice, we show that selective depletion of midstage CD301b-expressing macrophages phenocopied wound healing defects observed in mice where multiple myeloid lineages are depleted. Additionally, when FACS-isolated subpopulations of myeloid cells were transplanted into 3-day wounds of syngeneic mice, only CD206+/CD301b+ macrophages significantly increased proliferation and fibroblast repopulation. These data show that the CD301b-expressing subpopulation of macrophages is critical for activation of reparative processes during the midstage of cutaneous repair.

Rapid induction of CD8+ cytotoxic T lymphocyte (CTL) responses is critical to combat acute infection with intracellular pathogens. CD4 + T cells help prime antigen-specific CTLs in secondary lymphoid organs after infection in the periphery. Although the frequency of naïve precursors is very low, the immune system is able to efficiently screen for cognate CTLs through mechanisms that are not well understood. Here we examine the role of CD4 + T cells in early phases of the immune response. We show that CD4 + T cells help optimal CTL expansion by facilitating entry of naïve polyclonal CD8 + T cells into the draining lymph node (dLN) early after infection or immunization. CD4 + T cells also facilitate input of naïve B cells into reactive LNs. Such "help" involves expansion of the arteriole feeding the dLN and enlargement of the dLN through activation of dendritic cells. In an antigen-and CD40-dependent manner, CD4 + T cells activate dendritic cells to support naïve lymphocyte recruitment to the dLN. Our results reveal a previously unappreciated mode of CD4 + T-cell help, whereby they increase the input of naïve lymphocytes to the relevant LN for efficient screening of cognate CD8 + T cells.cell trafficking | herpes simplex virus | innate immunity | vascular biology | antiviral immunity