Modulation of the MR1-MAIT cell axis by human herpes simplex viruses

Abstract

The ubiquitously expressed antigen-presenting molecule major histocompatibility complex (MHC) class I-related gene protein (MR1) presents microbial metabolites derived from bacteria and fungi biosynthesis of vitamin B. Recognition of surface MR1-antigen complexes by mucosal-associated invariant T (MAIT) cells, in conjunction with other signals of infection, determine the nature and strength of MAIT cell effector responses. While there is no evidence that viruses synthesise MR1 ligands, here we identify multiple points of modulation of the MR1 antigen presentation pathway by human herpes simplex viruses (HSV) type 1 and type 2. HSV-1 and HSV-2 are significant human pathogens that establish primary infection principally in the orofacial and genital mucosa; locations monitored by large populations of mature MAIT cells. Techniques such as infection with mutant viruses, ectopic viral and cellular gene expression, RT-qPCR, flow cytometry, immunoblotting and high throughput fluorescence microscopy are used to characterise the modulation of MR1 during HSV infection. Three viral products, namely virion host shutoff protein (vhs) viral RNase, infected cell protein (ICP) 22 and US3 viral serine threonine kinase, all contribute to the rapid and profound downregulation of MR1 protein. In addition, initial loss of MR1 transcripts results from vhs RNase activity. ICP22 expression downregulates newly synthesised MR1 in the endoplasmic reticulum. US3, however, primarily targets loss of mature surface molecules. By contrast, limiting ligand availability to promote MR1 maturation and surface expression prior to HSV infection establishes that pre-existing surface MR1 is upregulated due to impaired endocytosis. In conclusion, HSV encodes multiple immune-modulatory mechanisms to downregulate surface MR1, presumably in part to offset an increased retention of mature molecules on the plasma membrane. This is the first examination of viral modulation of the MR1 biosynthesis pathway.