HLA-DR mediated cell death is associated with, but not induced by TNF-alpha secretion in APC

Tumor necrosis factor alpha (TNFalpha) is a pleiotropic cytokine involved in inflammatory responses which can trigger both cell apoptosis and cell activation. In antigen presenting cells (APC), TNFalpha increased antigen presentation, notably by up-regulation of HLA class II expression. In addition to their role in antigen presentation, HLA-DR molecules transduce intracellular signals which lead to cytokine up-regulation or cell death. We have previously observed that the susceptibility of APC to HLA-DR mediated apoptosis increase throughout their maturation. We therefore investigated the relationship between TNFalpha production and susceptibility to HLA-DR-mediated apoptosis of different APC. The hematopoietic progenitor cell line (KG1), monocytic cell line (THP-1), monocyte-derived dendritic cell (DC), and B-lymphoid cell line (Raji) have been studied. We report that apoptosis susceptibility and spontaneous TNFalpha release are correlated in these different cells. However, while autocrine TNFalpha production was critical for DC maturation, upregulation of TNFalpha release after HLA-DR crosslinking was not observed and neutralization of endogenous TNFalpha did not modify HLA-DR-mediated apoptosis. These data reveal that HLA-DR mediated apoptosis susceptibility and spontaneous TNFalpha release are regulated in a parallel manner and that while TNFalpha may induce maturation of APC to an "apoptosis sensitive" stage, there is no direct role for TNFalpha in HLA-DR-mediated apoptosis of APC.