MHC class II and negative regulators of hematopoiesis

HLA-DR antigens a side from their antigen-presenting function have recently been shown to allow for transmembrane signaling. Findings in several models suggest that such a signaling function may occur in hematopoietic tissues. Signals generated via HLA-DR result in upregulation of tumor necrosis factor alpha (TNF-alpha) and nitric oxide as well as an enhanced expression of Fas and its natural ligand (Fas-L), all molecules associated with programmed cell death (apoptosis). While a role of HLA-DR in hematopoietic failure remains hypothetical, the data presented here can be accommodated into a model of hematopoietic failure initiated by HLA-DR mediated signals.