Cytotoxic T-Lymphocyte Target Proteins and Their Major Histocompatibility Complex Class I Restriction in Response to Adenovirus Vectors Delivered to Mouse Liver

A prerequisite for successful human gene therapy is the development of efficient and safe transfer technologies. Recombinant adenovirus vectors have several features which make them attractive vehicles for gene delivery. They transduce a wide variety of cell types, do not require host cell proliferation for gene transfer, and are able to transduce cells in vivo (6, 17, 19, 20, 25). The adenovirus genome is comprised of early and late genes; expression of the former leads to the activation of a cascade which culminates in the formation of new virions (9). First-generation recombinant adenoviruses used in gene therapy have been rendered replication defective by deletion of the E1A and E1B genes. Enthusiasm for the use of recombinant adenoviruses with deletions of the E1 genes in gene therapy has been diminished by the observation that deletion of E1 sequences is insufficient to completely ablate expression of other early and late viral genes. Previous studies revealed that the block in replication achieved by deleting E1 can be overcome in vitro with high multiplicities of infection (MOIs) or through cellular factors with E1-like function (10, 24). Expression of viral genes in infected-host cells leads to direct toxicity or to the stimulation of adenovirus-specific, major histocompatibility complex (MHC) class I-restricted cytotoxic T lymphocytes (CTLs) which can contribute to the loss of transgene expression (31, 33, 34). The antigenic potential of reporter proteins as well as therapeutic proteins in models of gene replacement therapy is another potential problem. A few recent reports have highlighted the role of the transgene product in inducing destructive cellular immune responses (28, 32). The capsid proteins of adenovirus vectors stimulate CD4+ T helper cells which recognize antigenic peptides in association with MHC class II determinants. Both TH1 and TH2 subsets are activated, the former contributing to the CTL response by augmenting the stimulation of CD8+ T cells as well as by increasing expression of MHC class I on the target cell via gamma interferon (36). The antigenic targets recognized by MHC class I-restricted CTLs have been extensively studied for a variety of viruses including influenza virus, vesicular stomatitis virus, and lymphocytic choriomeningitis virus. Epitopes within internal regulatory proteins, as well as integral membrane proteins, have been identified as targets for CTL-mediated destruction of virus-infected cells (1, 2, 29, 37, 38). For adenovirus, however, little is known about the antigen specificity of the CTL response. Studies with replication-competent, wild-type adenovirus revealed E1A encoded within the E1 locus as a strong immunodominant antigen (15, 18). These data, however, are not relevant to the use of adenoviruses in gene therapy when E1A and E1B had been deleted. In this study, a library of recombinant vaccinia viruses expressing viral regulatory and structural genes as well as two reporter genes was used to precisely identify the targets within E1-deleted recombinant adenoviruses which elicit CD8+ T-cell responses. Experiments were performed in four strains of inbred mice, three of which had different mouse H-2 haplotypes. This study revealed that the level of CTL response to adenovirus antigens or the transgene product is dependent on the MHC haplotype of the host.