Skin microbiome regulates SCF level in Keratinocytes and defines mast cell maturation

Abstract

Abstract Background Mast cell (MC) progenitors enter the skin and become mature in the surrounding microenvironment during embryogenesis and they fully mature 8 to 15 days after birth (in mouse). We published that skin commensal bacteria work together with skin resident cells (especially keratinocytes), in order to retain and differentiate mast cells into the skin. In the present study, we aim to demonstrate that this effect is due to TLR2 sensing of microbiome. Methods we used germ free mice (GF), hairless (hr/hr) mice and hr/hr/Tlr2−/− mice. Analyses were performed by immunohistochemistry, qPCR and FACS. IgE cross-linking and 48/80 have been used to test in vivo MCs degranulation. Results we found that GF mice present with less mature skin MCs than pathogen free mice (PFM). MCs immaturity correlated with a low level of stem cell factor (SCF) in keratinocytes, a critical MC differentiation and growth factor. Furthermore, compound 48/80, a known degranulation agent, induced less edema in the paws of GF mice than in those of conventional mice. A smaller population of c-Kit positive MCs in the GF mice normalized after being co-housed with PFM mice for 5 weeks to reconstitute their microbiome. On the other hand, hr/hr mice, have a skin increased number of mature and proliferating MCs that correlates with a more gram positive microbiome than their littermates. When hr/hr were backcrossed into Tlr2−/− (so they cannot sense the gram positive commensals) the number of MCs in skin dramatically decreased. Conclusions While baseline keratinocyte SCF expression is essential to recruit MCs into the skin, TLR2 stimulation by gram positive skin microbiome, promotes SCF keratinocyte production, which then enhances the maturation and proliferation of skin MCs.