Acute graft versus host disease (GVHD) is initiated by donor T cells which recognize host antigens in association with MHC class II (Ia) molecules. Recent studies strongly suggest that cytokines, particularly TNF-alpha, IL-2 and interferon-gamma play an important role in acute GVHD. Attention has been recently focused on the role of TNF-alpha in acute GVHD. Serum TNF-alpha levels are elevated in human GVHD, blood and skin cells of patients with acute GVHD express TNF-alpha, antibodies to TNF-alpha ameliorate murine and human GVHD and infusion of TNF-alpha in mice induces changes that mimic GVHD. We hypothesize that engagement of la molecules on host cells by alloreactive donor T cells is an important inducer of TNF-alpha release in acute GVHD. Microbial superantigens which bind to la molecules may further contribute to TNF-alpha release from la+ monocytes, and from activated la+ T cells and keratinocytes present in acute GVHD. To define the mechanisms of induction of TNF-alpha gene expression via la molecules, we will: 1. Examine the induction of TNF-alpha in acute GVHD. We will: a) assess TNF-alpha expression in patients with acute GVHD, b) examine the induction of TNF-alpha in monocytes and T cells during alloreaction, c) examine the induction of TNF-alpha by superantigen. 2. Analyze the function and structure of la-coupled signal transducing pathways. We will: a) examine the activation of PKC by la ligands, b) examine the phospholipases and second messengers involved in PKC activation, c) examine the activation of tyrosine phosphorylation pathways by la ligands, d) identify la-associated molecules. 3. Identify the DNA response elements and regulatory proteins involved in transcriptional activation of TNF-alpha via la. We will examine: a) transcriptional activation of the TNF-alpha gene by la ligands using CAT assays and b) induction of specific nuclear protein/DNA complexes by la ligands. The studies proposed should provide a better understanding of the pathogenesis of acute GVHD and have important implications for la mediated signal transduction in immune cells as well as in non-immune cells which display aberrant la expression in inflammation cancer and autoimmunity.