Hemophilic siblings with hepatitis C undergo a detailed clinical evaluation to stage their liver disease and to identify sibling pairs with clinically and/or histologically discordant levels of disease activity. These siblings pairs are further studied to define antigen recognition patterns of lymphocyte cells including peripheral CD8 plus cytotoxic T lymphocytes (CTL) and CD4 plus cells and determine their functional significance. Using peripheral blood mononuclear cells, CD8 plus cells are assayed for CTL activity against three overlapping vaccinia/hepatitis C virus (HCV) constructs covering the entire HCV genome followed by fine cloning to identify HCV-specific CTL epitopes. Peripheral CD4 plus cells are tested for their ability to proliferate to HCV antigens. Using stimulation index, Drs. Fried and colleagues are quantitating the presence and magnitude of this response. They are also trying to identify immunodominant regions targeted by cytotoxic T cells using HLA class I matched hemophilic siblings. Finally, they are identifying specific host genes that are preferentially expressed or repressed in patients with delayed progression of their HCV disease. They are quantitating the expression of mRNAs encoding host antiviral defense and immunoregulatory elements in peripheral blood mononuclear cells (PBMCs) and liver tissue from sibling pairs that have discordant chronic hepatitis C using mRNA libraries that are screened by high density oligonucleotide arrays. The expression levels of these genes (including, but not limited to, interferon alpha, beta, and gamma; IRF-1 and IRF-2; interferon induced protein kinase; the cellular protein activator of PKR (PACT) RNase L; interferon-inducible RNA-specific adenosine deaminase; a ribonuclease specific for inosine- containing RNA; chemokine receptors CCR1, CCR3, CCR5, and their signal transduction elements; 2'-5'-oligoadenylate synthetase; tumor necrosis factor; FAS receptor; signal transduction components of these antiviral pathways, and both type 1 and 2 cytokines) are correlated with delayed progression and diminished pathogenesis in paired hemophilic siblings.