Turnover of Dermal Dendritic Cells.
Authors: Bogunovic Milena , Ginhoux Florent , Loubeau Martine , Merad Miriam 
Published: October 2019

Abstract
We have previously shown that in steady state conditions, epidermal Langerhans cells (LCs) derive from a radio-resistant precursor that self-renew in the skin throughout life (Merad et al. 2002). In the current study we addressed the mechanisms that regulate the turnover of other subsets of skin antigen presenting cells (APCs). Based on cell surface marker expression, we characterized two populations of APCs in the mice dermis including CD11c+ CD11b+ cells and CD11b+ CD11c- cells (likely corresponding to dermal dendritic cells (DDCs) and macrophages respectively). To explore the turnover of DDCs we reconstituted lethally irradiated CD45.2+ C57BL/6 mice with bone marrow (BM) cells isolated from CD45.1+ C57BL/6 congenic mice and followed the replacement of host DDCs by donor DDCs. Interestingly, we found that despite 95% blood and BM chimerism, 20% of CD11c+ DDCs remained of host (CD45.2+) origin for at least 1 year after transplant, while 80% of DDCs and dermal macrophages appeared to be of donor (CD45.1+) origin as early as at 4 weeks after transplant. The radio-resistant CD11c+ cells homogenously expressed CD11b, F4/80, high-level MHC II and the co-stimulatory molecules CD40 and CD86, while only 10% of them expressed the LC specific marker (langerin) suggesting that these cells represent migrating LCs. To explore whether radio-resistant DDCs are able to proliferate locally, we administered bromodeoxyuridine (BrdU) to (CD45.1+ BM > CD45.2+ recipient) chimeric mice, 8 weeks after transplant. Because at the time of BrdU administration, blood cells in chimeric mice were of donor origin, incorporation of BrdU in host DDCs should reflect local proliferation. Indeed, we found that 30% of host DDCs incorporate BrdU after two weeks of BrdU labeling suggesting that radio-resistant DDCs proliferate in situ. In contrast to steady state conditions, exposure of chimeric mice to ultraviolet (UV) light led to complete replacement of the remaining host DDCs by circulating donor DDCs. To explore the nature of the chemokines playing a role in the recruitment of circulating DDCs to inflamed skin, we reconstituted lethally irradiated CD45.1+ mice with a 1:1 mixture of CD45.1+ autologous BM cells and CD45.2+ BM cells deficient in the chemokine receptors CCR2 or CCR6. Two weeks after transplant, we exposed the mice to UV light and followed the recruitment of mutant and wild type DDCs in the skin. Importantly, we found that 3 weeks after UV exposure, the % of CD45.2+ CCR2−/− DDCs was 8 fold lower than CD45.1+ CCR2+/+ DDCs, while there was no difference between the % of CCR6−/− and CCR6+/+ DDCs. These results suggest that the recruitment of donor DDCs is dependent on the chemokine receptor CCR2 but not CCR6, while CCR2 and CCR6 were required for the recruitment of circulating LC precursors. Taken together, our results suggest that, similar to LCs, a small population of DDCs is radio-resistant and proliferate in quiescent skin. Whether this subset of DDCs represents a local precursor for epidermal LCs will be the subject of further studies.