Skin-recirculating NK cells express α4β1 integrin, E-selectin ligand, and produce IFNγ

Abstract

Abstract NK cells are increasingly recognized as important players of skin immunity in health and during skin diseases, including skin cancers and autoimmunity. Despite their importance, little is known about the functional and migratory properties of skin-associated NK cells. Here, we employed afferent lymph vessel cannulation in sheep to assess NK cells re-entering the lymphatic system from the skin. NK cells (CD3−Nkp46+) consistently comprised 2–5% of total lymphocytes traveling in afferent lymph. The majority of skin-recirculating NK cells expressed perforin and readily produced IFNγ upon stimulation, indicating continuous recirculation of pro-inflammatory NK cells through unperturbed skin. Skin lymph NK cells expressed high levels of α4β1 integrin, similar to other circulating NK cells including those in blood and mesenteric lymph. Furthermore, a subset of lymph-borne NK cells expressed E-selectin ligand (CLA), suggesting a preference to return to skin in a manner similar to skin-homing T cells. Consistent with organ-selective as opposed to ubiquitous homing we observed that skin-recirculating NK cells lacked the gut-homing molecule, α4β7 integrin. In chemotaxis assays, skin-draining NK cells responded strongly to CCL21, a ligand for ‘tissue exit receptor’ CCR7, suggesting that NK cells use CCR7 to egress from skin via lymph. Our results support a model in which skin-specific NK cells with proinflammatory properties continuously survey the skin and are readily available for potential NK memory responses. Additional studies are underway to further assess the function and tissue tropism of skin-draining NK cells.