Intranasal infection by MCMV reveals key roles for viral evasion of MHC-I to enable viral spreading and CD4 T cell help to promote functional CD8 T cell responses

Abstract

Abstract Natural Cytomegalovirus (CMV) infection occurs primarily through the oral/nasal mucosa. Recent evidence from murine (M)CMV infection suggested that MCMV is spread to salivary gland (SG) by dendritic cells after intranasal (i.n.) inoculation. In agreement with this, a recombinant MCMV, which cannot replicate in hematopoietic cells, could not spread to SG after i.n. infection. However, the mechanisms by which infected cells evade immune control while spreading the virus are unclear. In fact, previous work suggested that deleting all three genes blocking MHC-I antigen presentation results in only a subtle defect in total latent viral loads and SG replication in vivo. Surprisingly, we found that viral evasion of MHC-I antigen presentation was critical for MCMV to replicate persistently in nasal mucosa and spread to SG after i.n. inoculation, in contrast to intraperitoneal (i.p.) infection. Remarkably, depletion of CD4 T cells reversed this immune restriction on viral spreading, enabling even MCMV lacking all evasion genes to replicate in nasal mucosa and spread to SG after i.n. infection. This was not due to a direct anti-viral effect of CD4 T cells as delaying CD4 T cell depletion until day 7 post infection restored immune control of MCMV at later times. Instead, priming and expansion of functional CD8 T cells was impaired by lack of CD4 T cells after i.n. infection, in contrast to i.p. infection. Thus, intranasal inoculation revealed a critical role for CD4 T cell help in the promotion of protective CD8 T cell responses. Moreover, our data suggest since MCMV spreads only in infected hematopoietic cells after i.n. infection, viral evasion of MHC-I is necessary to protect CD8 T cells from killing infected cells carrying MCMV.