Whyte, M. L. | bioRxiv Science

bioRxiv2022

Characterization of the HHV-6 U20 immunoevasin

Roseoloviruses (HHV-6A, -6B, and -7) infect >90% of the human population during early childhood, and are thought to remain latent or persistent throughout the life of the host. As such, these viruses are among the most pervasive and stealthy of all viruses; they must necessarily excel at escaping immune detection throughout the life of the host, and yet very little is known about how these viruses so successfully escape host defenses. Herein, we characterize the HHV6A and HHV6B U20 gene products, which are encoded within a block of genes unique to the roseoloviruses, and therefore of particular interest. Despite 92% amino acid identity, U20 proteins from HHV6A and 6B have been shown to possess different host evasion functions. Here we characterize expression, trafficking, and post-translational modifications of U20 during HHV6A infection. While U20 localized to lysosomes in HHV-6A-infected cells, HHV-6B U20 trafficked to the cell surface and was rapidly internalized. HHV-6B U20 trafficked slowly through the secretory system, receiving several post translational modifications to its N-linked glycans indicative of surface expressed glycoproteins. Interestingly, U20 is also phosphorylated on at least one Ser, Thr, or Tyr residue. These results provide a framework to understand the role(s) of U20 in evading host defenses. ImportanceHHV6A and HHV6B U20 proteins are virus-encoded integral membrane glycoproteins possessing class I MHC-like folds. As such, it is tempting to speculate that they are involved in host evasion. Indeed, although they share 92% identity, HHV6A U20 has been shown to target NK activating ligands (1) and HHV6B U20 has been shown to inhibit TNF receptor signaling and apoptosis (2). Here, we have performed cell biological and biochemical characterization of the trafficking, glycosylation, and post-translational modifications occurring on HHV6A and -6B U20, and we demonstrate U20 expression in the context of HHV6 infection.

cell biology↗

bioRxiv2020

The roseoloviruses downregulate the protein tyrosine phosphatase PTPRC(CD45)

Like all herpesviruses, the roseoloviruses (HHV6A, -6B, and -7) establish lifelong infection within their host, requiring these viruses to evade host anti-viral responses. One common host-evasion strategy is the downregulation of host-encoded, surface-expressed glycoproteins. Roseoloviruses have been shown to evade host the host immune response by downregulating NK-activating ligands, MHC class I, and the TCR/CD3 complex. To more globally identify glycoproteins that are differentially expressed on the surface of HHV6A-infected cells, we performed cell surface capture of N-linked glycoproteins present on the surface of T cells infected with HHV6A, and compared these to proteins present on the surface of uninfected T cells. We found that the protein tyrosine phosphatase CD45 is downregulated in T cells infected with HHV6A. We also demonstrated that CD45 is similarly downregulated in cells infected with HHV-7. CD45 is essential for signaling through the T cell receptor and as such, is necessary for developing a fully functional immune response. Interestingly, the closely related {beta}-herpesviruses human cytomegalovirus (HCMV) and murine cytomegalovirus (MCMV) have also separately evolved unique mechanisms to target CD45. While HCMV and MCMV target CD45 signaling and trafficking, HHV6A acts to downregulate CD45 transcripts. ImportanceHuman herpesviruses-6 and -7 infect essentially 100% of the worlds population before the age of 5 and then remain latent or persistent in their host throughout life. As such, these viruses are among the most pervasive and stealthy of all viruses. Host immune cells rely on the presence of surface-expressed proteins to identify and target virus-infected cells. Here, we investigated the changes that occur to proteins expressed on the cell surface of T cells after infection with human herpesvirus-6A. We discovered that HHV-6A infection results in a reduction of CD45 on the surface of infected cells. Targeting of CD45 may prevent activation of these virus-infected T cells, possibly lengthening the life of the infected T cell so that it can harbor latent virus.

microbiology↗