Non-Canonical Activation of HSF1 Stimulates mTORC1-Mediated Translation in HCMV-Infected Monocytes

Human cytomegalovirus (HCMV) is a major cause of organ disease among immunonaïve and immunocompromised individuals. HCMV infection stimulates the survival of normally short-lived circulating monocytes, allowing these blood cells to mediate the dissemination of the virus from the initial point of infection to distant organ sites. We previously showed that HCMV induces a non-canonical phosphorylation of Akt within infected monocytes that activates the stress response transcription factor Heat Shock Factor 1 (HSF1). In this study, we demonstrate that HSF1 is necessary for the survival of HCMV-infected monocytes using both pharmacological and genetic approaches. In contrast, HSF1 inhibition had minimal effect on the viability of uninfected cells, indicating the specific involvement of HSF1 on the survival of infected monocytes. Surprisingly, the aberrant activation of HSF1 by HCMV did not trigger nuclear relocalization, suggesting that HSF1's regulation of monocyte viability occurs within the cytoplasm. Indeed, we found that HCMV-activated, cytoplasmic HSF1 directly binds to mTOR, a critical component of the mTORC1 complex involved in the regulation of mRNA translation. SUnSET (Surface Sensing of Translation) assays revealed HCMV-activated HSF1 increases mRNA translation through mTORC1. Ribosomal profiling identified the increased translation of a selected subset of pro-survival transcripts, including cIAP2, which we validated to selectively stimulate the survival of HCMV-infected monocytes. Taken together, these data demonstrate that the non-canonical activation of HSF1 in infected monocytes drives mTORC1-dependent translation of antiapoptotic transcripts, ensuring the survival and dissemination of infected monocytes.

HCMV is a primary driver of morbidity and mortality in individuals with compromised or immature immune systems. Spread of HCMV throughout the body relies on the infection of peripheral blood monocytes, which spread the virus to end-organ tissues. However, the naturally short lifespan of monocytes must be overcome to allow for viral spread to occur. Here, we demonstrate that HCMV uniquely regulates the cellular stress response to promote the long-term survival of infected monocytes. Specifically, HCMV activates the cellular stress response transcription factor HSF1 to block the progression of apoptosis. In contrast to traditional heat shock stress where HSF1 translocates into the nucleus to mediate transcription, HCMV infection retains activated HSF1 in the cytoplasm where it binds to mTOR to promote protein synthesis of prosurvival factors necessary for the survival of infected monocytes. Overall, our study provides insight into the complex regulator mechanisms through which HCMV usurps host stress responses to promote viral dissemination.