f PARV4 is a small DNA human virus that is strongly associated with hepatitis C virus (HCV) and HIV infections. The immunologic control of acute PARV4 infection has not been previously described. We define the acute onset of PARV4 infection and the characteristics of the acute-phase and memory immune responses to PARV4 in a group of HCV-and HIV-negative, active intravenous drug users. Ninety-eight individuals at risk of blood-borne infections were tested for PARV4 IgG. Gamma interferon enzyme-linked immunosorbent spot assays, intracellular cytokine staining, and a tetrameric HLA-A2-peptide complex were used to define the T cell populations responding to PARV4 peptides in those individuals who acquired infection during the study. Thirty-five individuals were found to be PARV4 seropositive at the end of the study, eight of whose baseline samples were found to be seronegative. Persistent and functional T cell responses were detected in the acute infection phase. These responses had an active, mature, and cytotoxic phenotype and were maintained several years after infection. Thus, PARV4 infection is common in individuals exposed to blood-borne infections, independent of their HCV or HIV status. Since PARV4 elicits strong, broad, and persistent T cell responses, understanding of the processes responsible may prove useful for future vaccine design.
PARV4 is a small, nonenveloped single-stranded DNA virus of the Parvoviridae family that has been commonly associated with parenteral transmission (1-4). The PARV4 genome contains two open reading frames that encode a nonstructural (NS) and a capsid (VP) protein. Though PARV4 is generally absent from healthy individuals in western countries, 8 to 30% of hepatitis C virus (HCV)-infected individuals have been found to be PARV4 DNA or IgG positive (2,(4)(5)(6)(7)(8). This level can reach up to 95% among HIV-and HCV-coinfected individuals (9). Despite the growing body of evidence emerging on the prevalence of PARV4 exposure in remotely infected cohorts, relatively little is known about the features that accompany acute acquisition of PARV4 in such at-risk cohorts (3,(10)(11)(12).We previously analyzed the immune responses to PARV4 and described a striking T cell response to the NS protein in HCV ϩ and HIV ϩ individuals (13). However, this analysis was cross-sectional and the time point of infection was not known in these cases. Additionally, we were interested in studying PARV4 infection independently of other coinfections. Therefore, we subsequently sought a cohort of individuals who were HCV and HIV-1 negative but had a risk of acquiring PARV4 so that we could study acute acquisition of the virus and the evolution of immune responses in relation to viremia and seroconversion.We describe here a rare cohort of active intravenous drug users (IDUs), both HIV and HCV negative, who acquired PARV4 during the period they were under study. Because of the detailed nature of the study, with monthly follow-up over several years, it was possible to precisely identify the time of PARV4 ser...