Murine norovirus mutants adapted to replicate in human cells reveal a post-entry restriction.

RNA viruses lack proofreading in their RNA polymerases and therefore exist as genetically diverse populations. By exposing these diverse viral populations to selective pressures, viruses with mutations that confer fitness advantages can be enriched. To examine factors important for viral tropism and host restriction, we passaged murine norovirus (MNV) in a human cell line, HeLa cells, to select mutant viruses with increased fitness in non-murine cells. A major determinant of host range is expression of the MNV receptor CD300lf on mouse cells, but additional host factors may limit MNV replication in human cells. We found that viruses passaged six times in HeLa cells had enhanced replication compared with the parental virus. The passaged viruses had several mutations throughout the viral genome, which were primarily located in the viral non-structural coding regions. Although viral attachment was not altered for the passaged viruses, their replication was higher than the parental virus when the entry was bypassed, suggesting that the mutant viruses overcame a post-entry block in human cells. Three mutations in the viral NS1 protein were sufficient for enhanced post-entry replication in human cells. We found that the human cell-adapted MNV variants had reduced fitness in murine BV2 cells and infected mice, with reduced viral titers. These results suggest a fitness tradeoff, where increased fitness in a non-native host cell reduces fitness in a natural host environment. Overall, this work suggests that MNV tropism is determined by the presence of not only the viral receptor but also post-entry factors. IMPORTANCE: Viruses infect specific species and cell types, which is dictated by the expression of host factors required for viral entry as well as downstream replication steps. Murine norovirus (MNV) infects mouse cells, but not human cells. However, human cells expressing the murine CD300lf receptor support MNV replication, suggesting that receptor expression is a major determinant of MNV tropism. To determine whether other factors influence MNV tropism, we selected for variants with enhanced replication in human cells. We identified mutations that enhance MNV replication in human cells and demonstrated that these mutations enhance infection at a post-entry replication step. Therefore, MNV infection of human cells is restricted at both entry and post-entry stages. These results shed new light on factors that influence viral tropism and host range.

Murine norovirus mutants adapted to replicate in human cells reveal a post-entry restriction.

RNA viruses lack proofreading in their RNA polymerases and therefore exist as genetically diverse populations. By exposing these diverse viral populations to selective pressures, viruses with mutations that confer fitness advantages can be enriched. To examine factors important for viral tropism and host restriction, we passaged murine norovirus (MNV) in a human cell line, HeLa cells, to select for mutant viruses with increased fitness in non-murine cells. A major determinant of host range is expression of the MNV receptor CD300lf on mouse cells, but additional host factors may limit MNV replication in human cells. We found that viruses passaged six times in HeLa cells had enhanced replication compared with the parental virus. The passaged viruses had several mutations throughout the viral genome, which were primarily located in the viral non-structural coding regions. While viral attachment was not altered for the passaged viruses, their replication was higher than the parental virus when entry was bypassed, suggesting the mutant viruses overcame a post-entry block in human cells. Three mutations in the viral NS1 protein were sufficient for enhanced post-entry replication in human cells. We found that the human cell-adapted MNV variants had reduced fitness in mouse BV2 cells. Although the mutant viruses had increased fitness in HeLa cells, they did not have increased fitness in mice. Overall, this work suggests that MNV tropism is not only determined by the presence of the viral receptor but also post-entry factors.

Adrenergic signaling controls early transcriptional programs during CD8+ T cell responses to viral infection.

Norepinephrine is a key sympathetic neurotransmitter, which acts to suppress CD8 + T cell cytokine secretion and lytic activity by signaling through the ß2-adrenergic receptor (ADRB2). Although ADRB2 signaling is considered generally immunosuppressive, its role in regulating the differentiation of effector T cells in response to infection has not been investigated. Using an adoptive transfer approach, we compared the expansion and differentiation of wild type (WT) to Adrb2-/- CD8 + T cells throughout the primary response to vesicular stomatitis virus (VSV) infection in vivo. We measured the dynamic changes in transcriptome profiles of antigen-specific CD8 + T cells as they responded to VSV. Within the first 7 days of infection, WT cells out-paced the expansion of Adrb2-/- cells, which correlated with reduced expression of IL-2 and the IL-2Rα in the absence of ADRB2. RNASeq analysis identified over 300 differentially expressed genes that were both temporally regulated following infection and selectively regulated in WT vs Adrb2-/- cells. These genes contributed to major transcriptional pathways including cytokine receptor activation, signaling in cancer, immune deficiency, and neurotransmitter pathways. By parsing genes within groups that were either induced or repressed over time in response to infection, we identified three main branches of genes that were differentially regulated by the ADRB2. These gene sets were predicted to be regulated by specific transcription factors involved in effector T cell development, such as Tbx21 and Eomes. Collectively, these data demonstrate a significant role for ADRB2 signaling in regulating key transcriptional pathways during CD8 + T cells responses to infection that may dramatically impact their functional capabilities and downstream memory cell development.

Tradeoffs for a viral mutant with enhanced replication speed.

RNA viruses exist as genetically heterogeneous populations due to high mutation rates, and many of these mutations reduce fitness and/or replication speed. However, it is unknown whether mutations can increase replication speed of a virus already well adapted to replication in cultured cells. By sequentially passaging coxsackievirus B3 in cultured cells and collecting the very earliest progeny, we selected for increased replication speed. We found that a single mutation in a viral capsid protein, VP1-F106L, was sufficient for the fast-replication phenotype. Characterization of this mutant revealed quicker genome release during entry compared to wild-type virus, highlighting a previously unappreciated infection barrier. However, this mutation also reduced capsid stability in vitro and reduced replication and pathogenesis in mice. These results reveal a tradeoff between overall replication speed and fitness. Importantly, this approach-selecting for the earliest viral progeny-could be applied to a variety of viral systems and has the potential to reveal unanticipated inefficiencies in viral replication cycles.