Hepatitis C virus (HCV) is endemic in many countries due to its high propensity for establishing persistence. The presence of HCV-specific T cells in subjects repeatedly exposed to HCV who test negative for HCV RNA and antibodies and who do not have any history of HCV infection has been interpreted as T cell-mediated protection2-5. Here, we show in nonhuman primates that repeated exposure to human plasma with trace amounts of HCV induced HCV-specific T cells without seroconversion and systemic viremia but did not protect upon subsequent HCV challenge. Rather, HCV-specific recall and de novo T cell responses, as well as intrahepatic T cell recruitment and interferon-γ (IFN-γ) production, were suppressed upon HCV challenge, concomitant with quantitative and qualitative changes in regulatory T cells (Treg cells) that occurred after subinfectious HCV exposure and increased after HCV challenge. In vitro Treg cell depletion restored HCV-specific T cell responses. Thus, T cells primed by trace amounts of HCV do not generate effective recall responses upon subsequent HCV infection. Subinfectious HCV exposure predisposes to Treg cell expansion, which suppresses effector T cells during subsequent infection. Strategies to reverse this exposure-induced immune suppression should be examined to aid in the development of T cell-based vaccines against HCV and other endemic pathogens. © 2013 Nature America, Inc.