Regulation of Langerhans cell function via blood borne factor(s)

Langerhans cells (LC) are epidermal dendritic cells that are functionally labile. Freshly obtained LC (fLC) readily activate allogeneic T cells, but they are incapable of activating autologous T cells; and even when pulsed with antigen, they fail to activate naive, antigen-specific T cells. When fLC are cultured for 2-3 days in the presence of keratinocytes, LC swiftly up-regulate surface expression of class I and II MHC molecules, and express de novo the co-stimulatory molecules B7 and ICAM-1. In addition to displaying enhanced ability to activate allogeneic T cells, cultured LC acquire the novel functional property of activating autologous T cells. It is believed that keratinocyte-derived GM-CSF is the primary driving force responsible for the conversion of fresh to cultured LC in vitro. However, in vivo administration of GM-CSF, either intracutaneously or systematically, fails to induce LC to undergo functional transformation in situ. Moreover, despite a high level of GM-CSF in the circulation, fLC from mice bearing GM-CSF-producing tumors display no ability to activate syngeneic T cells. These observations suggest that a homeostatic factor that antagonizes the effect of GM-CSF may be present in vivo. To test this possibility, we have examined the functional properties of LC prepared from mouse skin that had been explanted in vitro for 3 days. We found that the functional and phenotypic features of these cells closely resembled those of LC cultured in single cell suspension: the cells strongly expressed B7-1 and B7-2, and displayed enhanced expression of class II MHC molecules; they readily activated naive autologous T cells. Strikingly, when explants or suspensions of fresh epidermal cells were cultured in the presence of 10% mouse serum they failed to acquire syngeneic T cell activating properties; and surface up-regulation of Ia and de novo expression of B7 was inhibited. However, the cultured cells still expressed surface Ia and readily activated allogeneic naive T cells. If mouse serum was added only during the last 24 h of culture, the LC displayed full functional transformation. Human, rabbit and bovine serum showed no inhibitory effect on mouse LC. Our data suggest that mouse serum contains a factor (or factors) that inhibits, in a species-specific manner, epidermal LC from undergoing functional transformation in vitro, and this factor may maintain epidermal LC in the 'fresh' functional program in vivo.