Developmentally programmed early-age skin localization of iNKT cells supports local tissue development and homeostasis.

Skin is exposed to various environmental assaults and undergoes morphological changes immediately after birth. Proper localization and function of immune cells in the skin is crucial for protection and establishment of skin tissue homeostasis. Here we report the discovery of a developmentally programmed process that directs preferential localization of invariant natural killer T (iNKT) cells to the skin for early local homeostatic regulation. We show that iNKT cells are programmed predominantly with a CCR10+ skin-homing phenotype during thymic development in infant and young mice. Early skin localization of iNKT cells is critical for proper commensal bacterial colonization and tissue development. Mechanistically, skin iNKT cells provide a local source of transferrin that regulates iron metabolism in hair follicle progenitor cells and helps hair follicle development. These findings provide molecular insights into the establishment and physiological functions of iNKT cells in the skin during early life.

Differential regulation of CD8+ CD86+ Vγ1.1+ γδT cell responses in skin barrier tissue protection and homeostatic maintenance.

Compared to αßT cells, γδT cells are more innate-like and preferentially function as the first line of defense in barrier tissues. Certain populations of γδT cells possess adaptive immune cell properties but their regulation is not well understood. We herein report that while innate-like γδT17 cells dominated in the skin of WT mice, Vγ1.1+ γδT cells with adaptive T cell-like properties predominantly expanded in the skin of TCRß-/- and B2m-/- mice. Commensal bacteria drove expansion of Vγ1.1+ skin γδT cells, functional properties of which correlated with local immune requirements. That is, Vγ1.1+ skin γδT cells in TCRß-/- mice were a heterogeneous population; while Vγ1.1+ skin γδT cells in B2m-/- mice were mostly CD8+ CD86+ cells that had a similar function of CD8+ CD86+ skin αßT cells in supporting local Treg cells. We also found that intrinsic TGF-ß receptor 2-derived signals in skin CD8+ αßT and γδT cells are required for their expression of CD86, a molecule important in supporting skin Treg cells. Our findings reveal broad functional potentials of γδT cells that are coordinately regulated with αßT cells to help maintain local tissue homeostasis.

Preferential Perinatal Development of Skin-Homing NK1.1+ Innate Lymphoid Cells for Regulation of Cutaneous Microbiota Colonization.

Proper immune cell development at early ontogenic stages is critical for life-long health. How resident immune cells are established in barrier tissues at neonatal stages to provide early protection is an important but still poorly understood question. We herein report that a developmentally programmed preferential generation of skin-homing group 1 innate lymphoid cells (ILC1s) at perinatal stages helps regulate early skin microbiota colonization. We found that a population of skin-homing NK1.1+ ILC1s was preferentially generated in the perinatal thymi of mice. Unique thymic environments and progenitor cells are responsible for the preferential generation of skin-homing NK1.1+ ILC1s at perinatal stages. In the skin, NK1.1+ ILC1s regulate proper microbiota colonization and control the opportunistic pathogen Pseudomonas aeruginosa in neonatal mice. These findings provide insight into the development and function of tissue-specific immune cells at neonatal stages, a critical temporal window for establishment of local tissue immune homeostasis.

Dysregulated Microbial Fermentation of Soluble Fiber Induces Cholestatic Liver Cancer.

Dietary soluble fibers are fermented by gut bacteria into short-chain fatty acids (SCFA), which are considered broadly health-promoting. Accordingly, consumption of such fibers ameliorates metabolic syndrome. However, incorporating soluble fiber inulin, but not insoluble fiber, into a compositionally defined diet, induced icteric hepatocellular carcinoma (HCC). Such HCC was microbiota-dependent and observed in multiple strains of dysbiotic mice but not in germ-free nor antibiotics-treated mice. Furthermore, consumption of an inulin-enriched high-fat diet induced both dysbiosis and HCC in wild-type (WT) mice. Inulin-induced HCC progressed via early onset of cholestasis, hepatocyte death, followed by neutrophilic inflammation in liver. Pharmacologic inhibition of fermentation or depletion of fermenting bacteria markedly reduced intestinal SCFA and prevented HCC. Intervening with cholestyramine to prevent reabsorption of bile acids also conferred protection against such HCC. Thus, its benefits notwithstanding, enrichment of foods with fermentable fiber should be approached with great caution as it may increase risk of HCC.

Interleukin-25 Mediates Transcriptional Control of PD-L1 via STAT3 in Multipotent Human Mesenchymal Stromal Cells (hMSCs) to Suppress Th17 Responses.

Multipotent human mesenchymal stromal cells (hMSCs) harbor immunomodulatory properties that are therapeutically relevant. One of the most clinically important populations of leukocytes is the interleukin-17A (IL-17A)-secreting T (Th17) lymphocytes. However, mechanisms of hMSC and Th17 cell interactions are incompletely resolved. We found that, along with Th1 responses, hMSCs strongly suppressed Th17 responses and this required both IL-25--also known as IL--17E-as well as programmed death ligand-1 (PD-L1), a potent cell surface ligand for tolerance induction. Knockdown of IL-25 expression in hMSCs abrogated Th17 suppression in vitro and in vivo. However, IL-25 alone was insufficient to significantly suppress Th17 responses, which also required surface PD-L1 expression. Critically, IL-25 upregulated PD-L1 surface expression through the signaling pathways of JNK and STAT3, with STAT3 found to constitutively occupy the proximal region of the PD-L1 promoter. Our findings demonstrate the complexities of hMSC-mediated Th17 suppression, and highlight the IL-25/STAT3/PD-L1 axis as a candidate therapeutic target.

STAT3 negatively regulates type I IFN-mediated antiviral response.

Type I IFNs are crucial cytokines of innate immunity for combating viral infections. Signaling through type I IFN receptors triggers the activation of STAT proteins, including STAT1, STAT2, and STAT3. Although an essential role of STAT1 and STAT2 for type I IFN-induced antiviral response has been well established by studies of gene-targeted mice and human mutations, the role of STAT3 for this response remains unclear. Using gain-of-function and loss-of-function approaches, we demonstrated that STAT3 negatively regulates type I IFN-mediated response. STAT3 knockdown or knockout cells displayed enhanced gene expression and antiviral activity in response to IFN-α/ß. Restoration of STAT3 to STAT3KO cells resulted in attenuation of the response. Upon viral infection, increased type I IFN production in STAT3KO cells resulted in enhanced STAT activation and ISG expression. One mechanism for the enhanced IFN production and response in the absence of STAT3 might operate through an MDA5-dependent manner. STAT3 also appeared to suppress IFN response directly in a manner dependent on its N-terminal domain and independent of its function as a transcriptional factor. Taken together, these results define STAT3 as a negative regulator of type I IFN response and provide a therapeutic target for viral infections.