Enhancing safety of cytomegalovirus-based vaccine vectors by engaging host intrinsic immunity

Rhesus cytomegalovirus (RhCMV)-based vaccines maintain effector memory T cell responses (T-EM) that protect similar to 50% of rhesus monkeys (RMs) challenged with simian immunodeficiency virus (SIV). Because human CMV (HCMV) causes disease in immunodeficient subjects, clinical translation will depend upon attenuation strategies that reduce pathogenic potential without sacrificing CMV's unique immunological properties. We demonstrate that intrinsic immunity can be used to attenuate strain 68-1 RhCMV vectors without impairment of immunogenicity. The tegument proteins pp71 and UL35 encoded by UL82 and UL35 of HCMV counteract cell-intrinsic restriction via degradation of host transcriptional repressors. When the corresponding RhCMV genes, Rh110 and Rh59, were deleted from 68-1 RhCMV (Delta Rh110 and Delta Rh59), we observed only a modest growth defect in vitro, but in vivo, these modified vectors manifested little to no amplification at the injection site and dissemination to distant sites, in contrast to parental 68-1 RhCMV. Delta Rh110 was not shed at any time after infection and was not transmitted to naive hosts either by close contact (mother to infant) or by leukocyte transfusion. In contrast, Delta Rh59 was both shed and transmitted by leukocyte transfusion, indicating less effective attenuation than pp71 deletion. The T cell immunogenicity of Delta Rh110 was essentially identical to 68-1 RhCMV with respect to magnitude, T-EM phenotype, epitope targeting, and durability. Thus, pp71 deletion preserves CMV vector immunogenicity while stringently limiting vector spread, making pp71 deletion an attractive attenuation strategy for HCMV vectors.