Solar-simulating irradiation of the skin of human subjects in vivo produces Langerhans cell responses distinct from irradiation ex vivo and in vitro

It has been postulated that Langerhans cells (LC) provide tolerogenic signa ls in the local impairment of cutaneous immune functions and antigen-specif ic tolerance induced by UV radiation. Studies in vitro and ex vivo have ind icated that UV radiation may down-regulate the expression of costimulatory molecules on LC, leading to reduced antigen-presenting function. In contras t, we recently observed an up-regulatory stage in the number of human epide rmal LC with induced expression of B7 costimulatory molecules 12-24 h after solar-simulating UV radiation (SSR) in vivo. To examine the apparent discr epancy between the observed human LC responses in vitro, ex vivo and in vit ro, we compared the three protocols in a parallel fashion. The intact skin as well as skin explants and epidermal cell suspensions from the same indiv iduals were irradiated with a single erythematogenic dose of SSR. The expre ssion of cell surface markers in the epidermal cells was analysed with flow cytometry 24 h later. The number of CD1a(+)/HLA-DR+ LC increased post-SSR in vitro by a factor of 2.8+/-0.4, whereas in irradiated skin explants es v ivo or in cell suspensions in vitro, reduced numbers were seen. HLA-DR expr ession intensities were found to have increased on DR+ and CD1a(+)/DR+ cell s in vivo. Similarly, SSR induced B7-2 (CD86) expression in CD1a(+) cells s ignificantly in vivo (P=0.031) but reduced the expression Ex vivo or in vit ro. We conclude that the early up-regulatory stage of human LC number and m embrane markers, recorded at 24 h after a single exposure to SSR, is exclus ively an in vivo phenomenon.