Plasmacytoid dendritic cells prime IFN‐γ‐secreting melanoma‐specific CD8 lymphocytes and are found in primary melanoma lesions

Plasmacytoid dendritic cells (PDC) are a small population of leukocytes specialized in the production of type I IFN. It has been shown that PDC have a potent T cell stimulatory capacity in allogeneic mixed lymphocyte reaction, However, their role in initiating primary immune responses remains elusive. We report that blood PDC efficiently prime naive CD8+ lymphocytes specific forthe melan‐A26–35 epitope to become IFN‐γ producing cells in vitro. In addition, we found that CD40L‐stimulated PDC induce expression on primed melan‐A‐specific T cells of cutaneous lymphocyte antigen and L‐selectin (CD62L), homing receptors that allow the migration of effector cells to the inflamed skin. Finally, we show that PDC can be found in the peri‐tumoralarea of most primary cutaneous melanomas in vivo and that type I IFN‐containing supernatants derived from PDC increase melanoma cell surface expression of CD95 and MHC class I and class II molecules in vitro. Our results suggest a new immunomodulatory role for tissue infiltrating PDC, which may prime tumor‐specific T cell responses and affect tumor growth via soluble factors.