The Role of Tissue Resident Memory CD4 T Cells in Herpes Simplex Viral and HIV Infection.

Tissue-resident memory T cells (TRM) were first described in 2009. While initially the major focus was on CD8 TRM, there has recently been increased interest in defining the phenotype and the role of CD4 TRM in diseases. Circulating CD4 T cells seed CD4 TRM, but there also appears to be an equilibrium between CD4 TRM and blood CD4 T cells. CD4 TRM are more mobile than CD8 TRM, usually localized deeper within the dermis/lamina propria and yet may exhibit synergy with CD8 TRM in disease control. This has been demonstrated in herpes simplex infections in mice. In human recurrent herpes infections, both CD4 and CD8 TRM persisting between lesions may control asymptomatic shedding through interferon-gamma secretion, although this has been more clearly shown for CD8 T cells. The exact role of the CD4/CD8 TRM axis in the trigeminal ganglia and/or cornea in controlling recurrent herpetic keratitis is unknown. In HIV, CD4 TRM have now been shown to be a major target for productive and latent infection in the cervix. In HSV and HIV co-infections, CD4 TRM persisting in the dermis support HIV replication. Further understanding of the role of CD4 TRM and their induction by vaccines may help control sexual transmission by both viruses.