Skin recirculating B cell subsets in a model of lymph cannulation in sheep

Abstract

Abstract The skin is a key barrier organ and cells of the skin immune system are integral to skin immunity and inflammation. While the trafficking of skin T cells is well defined, less is known about B cells that reside in and traffic through skin. Here we studied skin-associated B cells and their homing receptors using lymph vessel cannulation in sheep. Afferent lymph draining the skin was collected and lymph-borne B cells were compared to B cells from skin, blood, skin and mesenteric lymph nodes, mesenteric efferent lymph, and spleen using flow cytometry. The skin-homing molecule α4β1-integrin as well as activated β1-integrin were upregulated in B cells in skin-draining lymph and skin compared with lymph nodes, suggesting that α4β1 integrin is key to B cell recirculation through and/or homing to skin. Of all tissues analyzed, B cells from skin exhibited the highest frequency in skin-homing molecule E-selectin ligand expression. Among afferent lymph B cells, the CD21− B cell subset showed the highest expression of E-selectin ligand. As recent studies showed a functional endocannabinoid system in the skin, we assessed chemotaxis of skin-recirculating B cells toward 2-arachidonoylglycerol (2-AG), a ligand for cannabinoid receptor 2 (CB2). Interestingly, CD21+ B cells from skin draining lymph were highly enriched in responsiveness to 2-AG as well as CCL20, a skin-expressed chemokine. We propose that α4β1 integrin is integral to all B cell homing to skin and that CD21 expression delineates skin-recirculating B cell subsets with distinct trafficking receptor usage, while CD21− B cells use E-selectin ligand, CD21+ B cells employ receptors for 2-AG and CCL20 to recirculate through skin.