Eastern and Venezuelan Equine Encephalitis Viruses Differ in Their Ability To Infect Dendritic Cells and Macrophages: Impact of Altered Cell Tropism on Pathogenesis

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Abstract
Eastern and Venezuelan equine encephalitis viruses (EEEV and VEEV, respectively) cause severe morbidity and mortality in equines and humans. Like other mosquito-borne viruses, VEEV infects dendritic cells (DCs) and macrophages in lymphoid tissues, fueling a serum viremia and facilitating neuroinvasion. In contrast, EEEV replicates poorly in lymphoid tissues, preferentially infecting osteoblasts. Here, we demonstrate that infectivity of EEEV for myeloid lineage cells including DCs and macrophages was dramatically reduced compared to that of VEEV, whereas both viruses replicated efficiently in mesenchymal lineage cells such as osteoblasts and fibroblasts. We determined that EEEV infection of myeloid lineage cells was restricted after attachment, entry, and uncoating of the genome. Using replicon particles and translation reporter RNAs, we found that translation of incoming EEEV genomes was almost completely inhibited in myeloid, but not mesenchymal, lineage cells. Alpha/beta interferon (IFN-alpha/beta) responses did not mediate the restriction, as infectivity was not restored in the absence of double-stranded RNA-dependent protein kinase, RNase L, or IFN-alpha/beta receptor-mediated signaling. We confirmed these observations in vivo, demonstrating that EEEV is compromised in its ability to replicate within lymphoid tissues, whereas VEEV does so efficiently. The altered tropism of EEEV correlated with an almost complete avoidance of serum IFN-alpha/beta induction in vivo, which may allow EEEV to evade the host's innate immune responses and thereby enhance neurovirulence. Taken together, our data indicate that inhibition of genome translation restricts EEEV infectivity for myeloid but not mesenchymal lineage cells in vitro and in vivo. In this regard, the tropisms of EEEV and VEEV differ dramatically, likely contributing to observed differences in disease etiology.

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