Chronic hepatitis C virus (HCV) infection is characterized by stable lifelong viremia and a substantially increased risk of serious progressive liver disease. HCV persists in part because T cell immunity fails. Studies in adults have shown that cytotoxic CD8+ T cells are functionally exhausted or target viral epitopes that have mutated to escape recognition. Loss of CD4+ T helper cell activity is a hallmark of chronic hepatitis C but mechanisms that silence this critical immune response have not been identified. Chronic HCV infection appears to be modified by pregnancy. Viremia increases during pregnancy and then decreases, often by several logs, after childbirth. Our central hypothesis is that cellular immunit to HCV is fully restored, at least transiently, in the postpartum period. Two Specific Aims are proposed. Specific Aim 1 is to compare HCV-specific cellular immunity and underlying immunoregulatory changes in women with and without a substantial drop in viremia in the postpartum period. Specifically we will test the hypothesis that viral control after childbirth is associated with a shift of CD4+ and CD8+ T-cells from an exhausted to an effector state, with acquisition of multiple effector functions, susceptibility to signals from survival cytokines, and decreased predisposition to apoptosis. Further, we anticipate that declining viremia after delivery will be associated with changes in plasma cytokines and other pregnancy-related immunoregulatory molecules that promote Th1/Tc1 immunity. Specific Aim 2 is to determine the influence of pregnancy and delivery on evolution of HCV genomes and T-cell repertoire. We predict that CD8+ T cell immunity is relaxed during pregnancy, so that escape mutations in some class I epitopes revert to a sequence that is more fit for HCV replication. Postpartum resurgence of CD8+ T-cell immunity is expected to cause emergence (or re-emergence) of escape mutations in class I epitopes and include new T-cell clonotypes. Spontaneous recovery of T cell immunity that restricts HCV replication would represent a significant departure from the typical pattern of chronic infection described in men and non-pregnant women. We believe that understanding the mechanism(s) of spontaneous HCV control after childbirth is relevant to human health. For instance, it would provide insight into strategies to cure infection during a unique window of low virus replication in the mothers. The studies should also provide information on the relationship between replicative fitness and patterns of escape mutation in viruses that emerge late in pregnancy and are potentially transmissible to HLA semi-matched infants. More generally, the studies should provide a substantially new direction for unraveling the molecular basis of T cell dysfunction and approaches to immunomodulation in chronic hepatitis C and other chronic viral infections.