014 Commensal microbes augment expression of the hair follicle chemokine Ccl20 to facilitate accumulation of Tregs in neonatal skin

013 Heterogeneous CD4 migratory T cells rapidly and constitutively infiltrate skin and co-exist with populations of resident memory T cells (TRM) C Park, Y Pan, X Fu, T Tian, S Divito, C Lin, R Clark and TS Kupper 1 Dermatology, BWH/HMS, Boston, MA and 2 Wellman Laboratories, MGH, Boston, MA Long term resident TRM cells have long been described in skin, but recently populations of migratory T cells have been identified that co-exist in human skin with TRM. While the function of skin CD4 TRM cells is well described in murine models, re-circulating migratory CD4 T cells have not been well characterized in this setting. Thus, to explore the function of putative skin migratory CD4 T cells, we used a model of C. albicans skin infection that we have previously shown generates a TRM population of sessile CD4 Th17 T cells. Months after the infection had resolved, a faster moving population of CD4 T cells could be observed in the reticular dermis by intravital (IV) microscopy; unlike TRM, these cells expressed little CD69 or CD103, produced negligible IL-17, and interacted infrequently with CD11c+ DC. By IV microscopy, these migratory deep dermal cells could actually be observed entering the previously infected skin from blood in real time with rapid kinetics, a process that could be blocked completely by the sphingosine 1 phosphate receptor (S1P1) inhibitor FTY720 (fingilomod). These migratory CD4 cells expressed CCR7, CD27, were heterogeneous for Lselectin, and were negative for KLRG-1. This phenotypic profile suggests that they are a combination of skin homing TCM cells and the recently described “T migratory memory” (TMM) cells, rather than TEM cells (which are uniformly KLRG-1+). In contrast, Th17 CD4 TRM cells in the same field are all negative for these markers. Later in the time course, CD4 FoxP3+ Treg cells also infiltrated the skin and accumulated as a major population of TMM cells coexisting with Th17 CD4 TRM cells. These results suggest that heterogeneous populations of CD4 T cells constitutively migrate into skin and provide immunosurveillance that may modify that provided by TRM. 014 Commensal microbes augment expression of the hair follicle chemokine Ccl20 to facilitate accumulation of Tregs in neonatal skin TC Scharschmidt, KS Vasquez, ML Pauli, H Truong, JL Sonnenburg, SE Millar and MD Rosenblum 1 Dermatology, University of California, San Francisco, San Francisco, CA, 2 Dermatology, University of Pennsylvania, Philadelphia, PA and 3 Microbiology and Immunology, Stanford University, Stanford, CA We have previously reported that a wave of regulatory T cells (Tregs) into neonatal skin is responsible for establishing tolerance to skin commensal microbes. However, mechanisms mediating the abrupt accumulation of this population remain undefined. Tregs localize to hair follicles (HFs) in both mouse and human skin. Migration of Tregs into neonatal skin coincides with initial colonization of the skin by commensal microbes and HF neogenesis. We hypothesized that both commensal microbes and HF development play a role in facilitating Treg migration to neonatal skin. Germ-free neonates as well as K5-Dkk1 mice, which fail to develop HFs, demonstrate reduced numbers of neonatal skin Tregs. Using RNA-seq and flow cytometry we identified Ccr6 as a key receptor preferentially expressed by neonatal skin Tregs. The Ccr6 ligand, Ccl20, is a chemokine known to be produced by HF keratinocytes. We found that expression of Ccl20 increased in neonatal skin during the period of peak Treg influx and was significantly reduced under germ-free conditions. Accordingly, Ccl20 was exquisitely capable of driving migration of neonatal Tregs in vitro. Taken together, these data suggest that HFs and commensal bacteria jointly coordinate migration of Tregs into neonatal skin during this critical developmental window by a Ccl20-dependent mechanism.