Herpes simplex virus (HSV) type 2 infection occurs primarily at the genital mucosal surfaces and is a leading cause of ulcerative lesions. Despite the availability of animal models for HSV-2 infection, little is known regarding the mechanism of immune induction within the vaginal mucosa. Here, we examined the cell types responsible for the initiation of protective Th1 immunity to HSV-2. Intravaginal inoculation of HSV-2 led to a rapid recruitment of submucosal dendritic cells (DCs) to the infected epithelium. Subsequently, CD11c⁺ DCs harboring viral peptides in the context of MHC class II molecules emerged in the draining lymph nodes and were found to be responsible for the stimulation of IFNγ secretion from HSV-specific CD4⁺ T cells. Other antigen-presenting cells including B cells and macrophages did not present viral peptides to T cells in the draining lymph nodes. Next, we assessed the relative contribution to immune generation by the Langerhans cells in the vaginal epithelium, the submucosal CD11b⁺ DCs, and the CD8α⁺ lymph node DCs. Analysis of these DC populations from the draining lymph nodes revealed that only the CD11b⁺ submucosal DCs, but not Langerhans cell–derived or CD8α⁺ DCs, presented viral antigens to CD4⁺ T cells and induced IFNγ secretion. These results demonstrate a previously unanticipated role for submucosal DCs in the generation of protective Th1 immune responses to HSV-2 in the vaginal mucosa, and suggest their importance in immunity to other sexually transmitted diseases.

Cytotoxic CD8+ T cells play a critical role in controlling herpes simplex virus (HSV) infection and reactivation. However, little is known about the spatiotemporal dynamics of CD8+ T cells during HSV lesion evolution or about their involvement in immune surveillance after lesion resolution. Using quantum dot–conjugated peptide–major histocompatibility complex multimers, we investigated the in vivo localization of HSV-2–specific CD8+ T cells in sequential biopsies of human genital skin during acute, resolving, and healed stages of HSV-2 reactivation. Our studies revealed that functionally active CD8+ T cells selectively infiltrated to the site of viral reactivation. After lesion healing in concert with complete reepithelialization and loss of HSV DNA from skin biopsies, HSV-2–specific CD8+ T cells persisted for more than two months at the dermal–epidermal junction, adjacent to peripheral nerve endings. In two out of the six sequentially studied individuals, HSV-2 DNA reappeared in clinically and histologically normal–appearing skin. Detection of viral DNA was accompanied by increased numbers of both HSV-specific and total CD8+ T cells in the dermis. These findings indicate that the frequency and clinical course of HSV-2 reactivation in humans is influenced by virus-specific CD8+ T cells that persist in peripheral mucosa and genital skin after resolution of herpes lesions.

Infection with HSV-2 increases the likelihood of HIV acquisition, but suppression of HSV-2 reactivation with antiviral drugs does not seem to reduce the acquisition of HIV. Laurence Corey and colleagues provide a potential mechanism underlying this observation, showing that even after acyclovir treatment for the HSV-2 infection, many inflammatory and immune cells possessing the receptors required for HIV infection persist in the mucosa, making the initial 'spark' of infection more likely. To explore the mechanism by which herpes simplex virus (HSV)-2 infection is related to HIV-1 acquisition, we conducted in situ analysis of the cellular infiltrate from sequential biopsies of HSV-2 lesions from patients on and off antiviral therapy. CD4+ and CD8+ T cells and a mixed population of plasmacytoid and myeloid dendritic cells (DCs), including cells expressing the C-type lectin receptor DC-SIGN, persisted at sites of HSV-2 reactivation for months after healing, even with daily antiviral therapy. The CD4+ T cells that persisted reacted to HSV-2 antigen, were enriched for expression of the chemokine receptor CCR5, and were contiguous to DCs expressing the interleukin-3 receptor CD123 or DC-SIGN. Ex vivo infection with a CCR5-tropic strain of HIV-1 revealed greater concentrations of integrated HIV-1 DNA in cells derived from healed genital lesion biopsies than in cells from control skin biopsies. The persistence and enrichment of HIV receptor–positive inflammatory cells in the genitalia help explain the inability of anti–HSV-2 therapy to reduce HIV acquisition.

Herpes simplex virus 2-infected individuals are shown to have skin-tissue-resident CD8αα+ T cells that persist at the portal of virus release in genital skin and mucosa, possess oligoclonal T-cell-receptor-β CDR3 sequences that are stable over time, and express gene signatures of antiviral and cytolytic function indicative of viral-specific immune surveillance. Using herpes simplex virus 2 (HSV-2) as a model pathogen, Jia Zhu et al. demonstrate that a unique subset of tissue-resident CD8+ T cells persists in the genital mucosa. In HSV-2-infected individuals, virus-specific and clonally related CD8αα+ T cells — rather than conventional CD8αβ+ T cells — are the predominant tissue-resident memory cells localized to the source of HSV-2 release, where they contribute to immune surveillance. These findings suggest that vaccines or immunotherapeutic approaches that increase the number and density of tissue-resident CD8αα+ cells may enhance control of not only HSV-2 but of other genital tract infections, such as HIV. Most herpes simplex virus 2 (HSV-2) reactivations in humans are subclinical and associated with rapid expansion and containment of virus. Previous studies have shown that CD8+ T cells persist in genital skin and mucosa at the dermal–epidermal junction (DEJ)—the portal of neuronal release of reactivating virus—for prolonged time periods after herpes lesions are cleared1,2. The phenotype and function of this persistent CD8+ T-cell population remain unknown. Here, using cell-type-specific laser capture microdissection, transcriptional profiling and T-cell antigen receptor β-chain (TCRβ) genotyping on sequential genital skin biopsies, we show that CD8αα+ T cells are the dominant resident population of DEJ CD8+ T cells that persist at the site of previous HSV-2 reactivation. CD8αα+ T cells located at the DEJ lack chemokine-receptor expression required for lymphocyte egress and recirculation, express gene signatures of T-cell activation and antiviral activity, and produce cytolytic granules during clinical and virological quiescent time periods. Sequencing of the TCR β-chain repertoire reveals that the DEJ CD8αα+ T cells are oligoclonal with diverse usage of TCR variable-β genes, which differ from those commonly described for mucosa-associated invariant T cells and natural killer T cells. Dominant clonotypes are shown to overlap among multiple recurrences over a period of two-and-a-half years. Episodes of rapid asymptomatic HSV-2 containment were also associated with a high CD8 effector-to-target ratio and focal enrichment of CD8αα+ T cells. These studies indicate that DEJ CD8αα+ T cells are tissue-resident cells that seem to have a fundamental role in immune surveillance and in initial containment of HSV-2 reactivation in human peripheral tissue. Elicitation of CD8αα+ T cells may be a critical component for developing effective vaccines against skin and mucosal infections.

Herpes simplex virus-2 (HSV-2) shedding episodes in humans vary markedly in duration and virologic titer within an infected person over time, an observation that is unexplained. To evaluate whether host or virological factors more closely accounted for this variability, we combined measures of viral replication and CD8⁺lymphocyte density in genital biopsies, with a stochastic mathematical model of HSV-2 infection. Model simulations reproduced quantities of virus and duration of shedding detected in 1,003 episodes among 386 persons. In the simulations, local CD8⁺lymphocyte density in the mucosa at episode onset predicted peak HSV DNA copy number and whether genital lesions or subclinical shedding occurred. High density of CD8⁺T cells in the mucosa correlated with decreased infected cell lifespan and fewer infected epithelial cells before episode clearance. If infected cell lifespan increased by 15 min because of CD8⁺lymphocyte decay, then there was potential for a thousandfold increase in the number of infected cells. The model suggests that the rate of containment of infected cells by the peripheral mucosal immune system is the major driver of duration and severity of HSV-2 reactivation in the immunocompetent host.

A stochastic mathematical model suggests that very low numbers of herpes simplex virus 2 particles are released frequently into the genital tract in infected individuals.