DC-SIGN is a member of the C-type lectin family. Mainly expressed by myeloid DCs, it is involved in the capture and internalization of pathogens, including human CMV. Several transcripts have been identified, some of which code for putative soluble proteins. However, little is known about the regulation and the functional properties of such putative sDC-SIGN variants. To better understand how sDC-SIGN could be involved in CMV infection, we set out to characterize biochemical and functional properties of rDC-SIGN as well as naturally occurring sDC-SIGN. We first developed a specific, quantitative ELISA and then used it to detect the presence sDC-SIGN in in vitro-generated DC culture supernatants as cell-free secreted tetramers. Next, in correlation with their inflammatory status, we demonstrated the presence of sDC-SIGN in several human body fluids, including serum, joint fluids, and BALs. CMV infection of human tissues was also shown to promote sDC-SIGN release. Based on the analysis of the cytokine/chemokine content of sDC-SIGN culture supernatants, we identified IFN-γ and CXCL8/IL-8 as inducers of sDC-SIGN production by MoDC. Finally, we demonstrated that sDC-SIGN was able to interact with CMV gB under native conditions, leading to a significant increase in MoDC CMV infection. Overall, our results confirm that sDC-SIGN, like its well-known, counterpart mDC-SIGN, may play a pivotal role in CMV-mediated pathogenesis.

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Langerhans cells (LCs) are epithelial APCs that sense danger signals and in turn trigger specific immune responses. In steady-state, they participate in the maintenance of peripheral tolerance to self-antigens whereas under inflammation LCs efficiently trigger immune responses in secondary lymphoid organs. It has been demonstrated in mice that LC-deprived epithelia are rapidly replenished by short half-life langerin-expressing monocyte-derived LCs (MDLCs). These surrogate LCs are thought to be progressively replaced by langerinhigh LCs arising from self-renewing epithelial precursors of hematopoietic origin. How LCs arise from blood monocytes is not fully understood. Hence, we sought to characterize key factors that induce differentiation of langerinhigh-expressing monocyte-derived Langerhans-like cells. We identified GM-CSF and TGF-β1 as key cytokines to generate langerinhigh-expressing cells but only in serum-free conditions. These cells were shown to express the LC-specific TROP-2 and Axl surface markers and contained Birbeck granules. Surprisingly, E-cadherin was not spontaneously expressed by these cells but required a direct contact with keratinocytes to be stably induced. MDLCs induced stronger allogeneic T cell proliferations but released low amounts of inflammatory cytokines upon TLR stimulation compared with donor-paired monocyte-derived dendritic cells. Immature langerinhigh MDLCs were responsive to MIP-3β/CCL20 and CTAC/CCL27 chemokine stimulations. Finally, we demonstrated that those cells behaved as bona fide LCs when inserted in a three-dimensional rebuilt epithelium by becoming activated upon TLR or UV light stimulations. Collectively, these results prompt us to propose these langerinhigh MDLCs as a relevant model to address LC biology–related questions.

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