A comparative analysis of humoral immune responses raised by suppressed and impaired mouse cytomegalovirus mutants

Human cytomegalovirus (HCMV) is a broadly distributed, species-specific betaherpesvirus that frequently causes morbidity and mortality in individuals with impaired, senescent, and immature immunity. If HCMV replicates in pregnant women, virus transmission and congenital infections frequently cause mental defects, deafness, microcephaly, or even death of infants. Despite an urgent need, no approved HCMV vaccine is available. Mice infected with mouse cytomegalovirus (MCMV) serve as a model to investigate general principles of cytomegalovirus (CMV)-specific immune responses. In principle, viruses can be attenuated through the deletion of genes that are either required for efficient virus production or for immune evasion, and we refer to these two types of alteration as impairing and suppressing attenuation, respectively. A prototype for a suppressed virus mutant is an MCMV mutant lacking the M27 gene (ΔM27-MCMV) that causes an exaggerated interferon (IFN) susceptibility. Accordingly, the replication of ΔM27-MCMV proceeds wt-like in the absence of IFN signalling in vitro, but is drastically suppressed by IFNs in vitro and in vivo. A mutant lacking M34 (ΔM34-MCMV), which replicates approximately 100-fold impaired in cell culture irrespective of the presence or absence of IFNs and was highly attenuated in vivo, is our prototype for an impaired mutant. A recent publication reported a comparable impairment of ΔUL34-HCMV (Turner et al., 2022). ΔM34-MCMV infection induced MCMV-specific ELISA-reactive IgG responses. Furthermore, the immune serum of ΔM34-MCMV-infected mice activated the fragment crystallizable (Fc) receptors CD16, CD32, CD64, and FcγRIV in antibody-dependent cellular cytotoxicity (ADCC) surrogate assays. Ultimately, infection with ΔM34-MCMV protects mice against subsequent challenge infections. In this project we compare the immune responses of mice infected with ΔM27-MCMV & ΔM34-MCMV and show that despite differing in the type of attenuation and susceptibility to innate immunity between these two MCMV mutants, both raise similar Fcγ receptor and MCMV-specific IgG responses. Moreover, we determine that ΔM34-MCMV is a viable vaccine candidate with the implication that investigation into a ΔUL34-HCMV live-attenuated vaccine may yield promising results.