E1-deleted recombinant adenoviruses have been developed for liver-directed gene therapy because efficient gene transfer to hepatocytes can be achieved in vivo. However, these viruses also express viral proteins in hepatocytes, leading to the development of destructive immune responses. Our previous studies indicated that MHC class I-restricted C D8+CTLs are major effectors in eliminating virus-infected cells, and CD4+ cells are also necessary in developing a fully competent CTL response by secretion of IFN-gamma, which sensitizes the virus-infected hepatocytes to CTLs through up-regulation of MHC class I expression. In this study, we have used adoptive transfer techniques in combination with mice deficient in immune functions to further define the role of CD4+ cells in the primary response to adenovirus-mediated gene transfer to the liver. Our studies indicate that CD4+ cells alone are capable of destroying virus-infected hepatocytes. Adoptive transfer experiments with beta 2m- mice along with in vitro CTL assays suggest that these CD4+ can act as CTL effectors, which are MHC class I-restricted. Depletion of these CD4+ effectors in vivo leads to prolongation of adenovirus-mediated transgene expression in hepatocytes. These results suggest that class I-restricted CD4+ CTLs contribute to elimination of adenovirus-transduced hepatocytes and extend our understanding of functional importance of CD4+ cells in viral pathogenesis.