The epidermal layer of human skin is thought to contain a single dendritic cell (DC) population known as a Langerhans cell (LC). These cells can be identified by their high surface expression of HLA-DR, CD1a and the C-type lectin receptor (CLR) langerin. However, using flow cytometry we have identified two additional CD1a⁺ epidermal cell populations that appear distinct from LCs in both the epidermis of the skin and the anogenital tract. 

Firstly, a second CD1a⁺/langerin⁺ population was observed which can be discerned from LCs by their lower surface expression levels of Siglec 3, HLA-DR and langerin and their lack of expression of CD4. Secondly, a CD1a⁺/langerin^- population was observed which can be distinguished from both langerin expressing populations by the differential surface expression of several CLRs. They also express higher levels of CCR5 and Siglec 9 and unlike the other two subsets express CD11c.

Cell sorting and flow cytometry allowed us to determine how these cells interact with T cells and we found that the langerin^- DCs induced the highest rate of T cell proliferation. Using reporter T cell lines, we also determined that the langerin^- DC population transferred HIV to T cells much more efficiently. We are currently classifying the cytokine expression profile of the epidermal and dermal CD1a⁺ populations via flow cytometry. The results of these experiments will be presented.