A Humanised TAP Mouse Model to Investigate CD8+ T Cell Immunity

Abstract

Herpes simplex virus type 1 (HSV-1) is exceptionally common, having a prevalence of 67% worldwide. Although the disease commonly manifests as mild cold sores, it can result in severe conditions like HSV-1 keratitis, which is a leading cause of blindness in one eye, HSV-1 encephalitis and neonatal herpes both of which could be fatal. The high frequency of infection and severity of the rare outcomes makes HSV-1 a clinically significant pathogen. CD8+ T cell are primarily responsible for recognising and eliminating virally infected cells. HSV-1 can avoid detection by CD8+ T cells by deploying a range of strategies to evade immune detection. ICP47 is an immunomodulatory protein produced by HSV-1, which is a potent inhibitor of the transporter associated with antigen presentation (TAP). ICP47 binds to TAP and blocks peptide transport for presentation on MHC-I and, ultimately prevents recognition of HSV-1 infected cells by CD8+ T cells. However, the function of ICP47 has been impossible to accurately assess in models in vivo, because the binding of ICP47 to TAP is species-specific. In particular, the affinity of ICP47 binding TAP is 100 times lower for mouse TAP proteins than human TAP proteins. To address this, we have developed mice that express human TAP1 and TAP2 (HuTAP) instead of mouse TAP1 and TAP2. We found that broadly, HuTAP mice have a similar immune profile to wildtype (WT) mice, as well as a similar ability to clear acute HSV-1 infection and establish latency. In contrast, the ability of activated HSV-1-specific CD8+ T cells to protect against HSV-1 replication and pathogenesis is greatly compromised in HuTAP mice. Using a virus lacking ICP47, we confirmed that this reduced protection was dependent on ICP47, suggesting ICP47 more efficiently inhibited TAP and the subsequent antigen presentation in HuTAP mice. Recognition of HSV-1 infected cells by in vitro activated CD8+ T cells, adoptively transferred one day before the infection was seen as early as one day after infection, which was marked by greater numbers of CD8+ T cells in the skin of WT mice at the infection site. This result suggests a role for effective early antigen presentation in the recruitment of immune cells in WT mice. Similarly, HSV-1-specific, tissue-resident, memory CD8+ T cells lodged in the skin were only partially protective in HuTAP, compared with WT mice. Together these demonstrate that in our HuTAP mice model the interaction of ICP47 and TAP as it occurs in humans and will be an invaluable tool for assessing the impact of cellular immunotherapies that rely on harnessing CD8+ T cells to treat HSV-1 and possibly other viral infections. We also used HuTAP mice to investigate whether the species of TAP alters the specificity of CD8+ T cell responses for other viruses, using vaccinia virus (VACV). We saw that the total CD8+ T cell response against VACV was similar in HuTAP and WT mice. However, we found significantly reduced CD8+ T cell responses for 2 out of 19 immunogenic VACV peptides that we tested. We found the difference in the CD8+ T cell response to be due to the ability of HuTAP mice to present these peptides during VACV infection and not changes in the T cell receptor repertoire. Taken together, this new mouse model shows how the species of TAP impacts antigen presentation and thereby the CD8+ T cell mediated responses and therefore is a valuable tool in understanding antigen presentation during viral infection. Show more