SPONTANEOUS APOPTOSIS OF DENDRITIC CELLS IS EFFICIENTLY INHIBITED BY TRAP (CD40-LIGAND) AND TNF-ALPHA, BUT STRONGLY ENHANCED BY INTERLEUKIN-10

In the lymphoid tissues, adaptive immune responses are initiated by the interaction of interdigitating dendritic cells (IDC) with naive T cells. To understand this interplay better, we used mature Langerhans cells (mLC), migrating from human epidermis, as the correlate of IDC ex vivo to evaluate the different effects of tumor necrosis factor (TNF)-alpha, TNF-related activation protein (TRAP; CD40-ligand) and interleukin-10 (IL-10) on induction or prevention of apoptotic cell death in these cells. Spontaneous decrease of mLC viability in culture was due to apoptosis, as determined by the appearance of typical morphological changes such as dilatation of the endoplasmic reticulum (ER), chromatin condensation and membrane blebbing. IL-10 strongly reduced mLC viability, whereas TRAP and TNF-alpha facilitated the survival of mLC. Spontaneous DNA fragmentation was detectable after 24 h in culture. IL-10 led to an earlier onset of DNA fragmentation, whereas TRAP and TNF-alpha delayed internucleosomal DNA cleavage. We found that IL-10-treated mLC were readily ingested and removed by macrophages. TNF-alpha and TRAP, in contrast, reduced engulfment of mLC by macrophages. Interestingly, IL-10, even at low concentrations, reverted the effects of TNF-alpha and TRAP in inhibiting mLC apoptosis. Furthermore, IL-10 led to the down-regulation of various surface antigens, especially of CD86 and CD54, whereas TNF-alpha and TRAP enhanced the expression of MHC class I and II antigens and of the accessory molecules CD40, CD54, CD80 and CD86. Taken together, these results show that mLC spontaneously undergo apoptosis in culture and that the progression of mLC to apoptosis is inhibited by TRAP and TNF-alpha, but accelerated by IL-10.