The clinical spectrum of hepatitis C is variable and the factors responsible for these divergent outcomes with chronic hepatitis C infection remain unknown. We propose to study a cohort of hemophilic siblings infected with hepatitis C to define the natural history, immunologic, and genetic factors that influence its clinical outcome. Patients with hemophilia have a prevalence rate of hepatitis C as high as 90 percent. The sex-linked pattern of inheritance of hemophilia allows us to identify a cohort of siblings both of who have been infected with hepatitis C. Hemophilic siblings are an attractive population to study because: 1) They are all males; 2) Siblings will be relatively close in age; 3) The mode of HCV acquisition is identical; 4) The age at acquisition of hepatitis C is similar 5) The date of acquisition can be confidently estimated upon their factor replacement history; 6) Hemophilic sibs share significant amounts of genetic material. Hemophilic siblings with hepatitis C will 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 will be further studied to define antigen recognition patterns of peripheral CD8 plus CTL and CD4 plus cells and determine their functional significance. Using peripheral blood mononuclear cells, CD8 plus cells will be assayed for CTL activity against three overlapping vaccinia/HCV constructs covering the entire HCV genome followed by fine cloning to identify HCV-specific CTL epitopes. Peripheral CD4 plus cells will be tested for their ability to proliferate to HCV antigens. Using stimulation index, we will quantitate the presence and magnitude of this response. We will also try to identify immunodominant regions targeted by cytotoxic T cells using HLA class I matched hemophilic siblings. Finally, we will identify specific host genes that are preferentially expressed or repressed in patients with delayed progression of their HCV disease. We will quantitate 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 will be 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) will be correlated with delayed progression and diminished pathogenesis in paired hemophilic siblings.