IL-36γ Is a Key Regulator of Neutrophil Infiltration in the Vaginal Microenvironment and Limits Neuroinvasion in Genital HSV-2 Infection

Herpes simplex virus 2 is a sexually transmitted virus that affects an estimated 11% of the global population, with an increased incidence in women (1). The vaginal epithelium is the first line of defense against HSV-2 infection, and the innate immune response triggered by vaginal epithelial cells is crucial in controlling viral replication and spread during the initial stages of infection (2–6). The IL-36 cytokines were first characterized as members of the IL-1 superfamily that are expressed by epithelial cells at mucosal sites (7, 8). Subsequent study of the IL-36 cytokines, including the agonists IL-36α, -β, and -γ, identified these cytokines as key drivers of chronic inflammatory diseases, including psoriasis (9–13). However, the function of the IL-36 cytokines in host–pathogen interactions is not as well understood. Recently, the IL-36 cytokines have been shown to promote neutrophil recruitment and play a role in both bacterial and viral infections in the lungs (14–18). We have demonstrated that IL-36γ is expressed in the female reproductive tract (FRT) and is induced in response to an array of microbial products, suggesting a role in host defense mechanisms (19). Indeed, we found that HSV-2 stimulated IL36G expression and that treatment with IL-36γ induced an antiviral state that limited viral replication in a human three-dimensional cell culture model and protected against disease pathogenesis after lethal challenge in mice (20). We further revealed that IL-36γ transiently induced polymorphonuclear cell recruitment in the FRT that corresponded with decreased vaginal viral titers and increased survival (20).

Neutrophils are one of the predominant immune cells present in the FRT and function as early responders to pathogens (21, 22). Genital HSV-2 infection robustly recruits neutrophils to the vaginal cavity by 24 h postinfection, and these neutrophils aid in virus clearance from the vaginal mucosa (23). We have shown that neutrophil infiltration to the vaginal cavity corresponded with decreased HSV-2 production and protection against disease pathogenesis after intravaginal (i.vag.) challenge (20). In addition to aiding in HSV-2 clearance, neutrophils can function to provide broad immune protection against pathogens at the FRT barrier through production of antimicrobial peptides, cytokines, chemokines, and reactive oxygen species (ROS) (21). Interestingly, neutrophil elastase has recently been shown to cleave and activate IL-36γ from its proform (24, 25), suggesting an important link between IL-36γ and neutrophils in host defense mechanisms at mucosal sites.

When genital HSV-2 infection spreads beyond the vaginal epithelium, the virus infiltrates autonomic ganglion of the pelvis and the dorsal root ganglia, where it establishes latency (26–29). Periodic virus reactivation can cause ulcerative disease in individuals; however, asymptomatic virus shedding can also occur (30, 31). Although herpetic lesions are the most well-known symptom of genital HSV-2 disease, HSV-2 is a neurotropic virus that can spread through the nervous system. Spread of HSV-2 into the pelvic ganglion can cause damage to the enteric nervous system that leads to urinary retention and constipation in some individuals (32–34). Parr and Parr (35) also demonstrated that the spread of HSV-2 to the dorsal root ganglia provides a pathway for the virus to spread to the spinal cord and brain. Symptoms of HSV-2 neuroinvasion following i.vag. infection in mice include urinary retention, toxic megacolon, and hind limb paralysis (34–36).

In this study, we aimed to determine the requirement for IL-36γ in protection against genital HSV-2 disease pathogenesis. We evaluated IL-36γ–mediated immune cell recruitment to the vaginal microenvironment after i.vag. HSV-2 infection and additionally investigated HSV-2 spread and neuroinvasion to better understand the function of IL-36γ in host defense mechanisms locally in the FRT and systemically after viral infection. Collectively, we demonstrate that IL-36γ plays a key role in recruiting mature neutrophils to the vaginal microenvironment after HSV-2 infection and that IL-36γ may function in a manner that protects against neuroinvasion and HSV-2 disease pathogenesis.

HSV-2 186 was used in all studies and generously provided by Dr. R. Pyles (Univerisy of Texas Medical Branch, Galveston, TX). Stocks were prepared from infected Vero cell culture monolayers and frozen at −80°C. Vero cells were grown in DMEM (Corning, Manassas, VA) as described previously (37). Virus titers in HSV-2 stocks and infected samples were determined using standard plaque assay as previously described (3).

C57BL/6 mice were obtained from The Jackson Laboratory (Bar Harbor, ME). IL-36γ^−/− mice were kindly provided by Dr. T. Denning (Georgia State University, Atlanta, GA). All animals were housed in accordance with the American Association for Laboratory Animal Care standards and provided unlimited access to food and water. All procedures and handling were approved by the University of Arizona Institutional Animal Care and Use Committee and performed in accordance with the Animal Welfare Act to minimize pain and suffering. Mice were treated with depot medroxyprogesterone acetate (DMPA; Greenstone, Peapack, NJ) at days −7 and −1 to synchronize cycles and increase mucosal permeability and susceptibility to infection as previously described (3). For IL-36γ treatment, mice were treated with murine rIL-36γ (250 ng; BioLegend, San Diego, CA) or PBS in 10 μl total volume by i.vag. instillation. At the time of challenge in HSV-2 infection experiments, 10³ PFU of HSV-2 186 in 10 μl of DMEM was i.vag. instilled. Mice were visually inspected daily for disease progression and survival over a 16-d period. Disease severity was scored on a scale from 0 to 5 as previously described (20). Briefly, the scale was as follows: no pathological condition, 0; mild vulvar erythema, 1; moderate vulvar erythema, 2; hair loss and erythema, 3; perineal ulceration, 4; and extension of perineal ulceration to surrounding tissue and/or hind limb paralysis, 5. Moribund mice or those scoring a 5 were euthanized. Vaginal swabs were collected at days 2, 4, and 6 postinfection using a DMEM-soaked urethrogenital calcium alginate swab (Puritan, Guilford, ME) and stored in 1 ml of DMEM at −80°C. Vaginal lavages were collected from mice by i.vag. instilling ∼125 μl of sterile PBS and lavaging the vaginal cavity using an oral feeding tube (Fisher Scientific, Hampton, NH) as previously described (20). The bladder, distal colon, lumbar spinal cord, and brain stem were collected from mice during necropsy, weighed, and stored in DMEM at −80°C. For titration, tissues were homogenized, and virus titers were quantified by standard plaque assay.

Mouse vaginal tissue was collected at 24 and 48 h post–HSV-2 infection and stored in DNA/RNA Shield (Zymo Research, Irvine, CA) prior to RNA extraction using the Zymo Quick-RNA Plus Kit following the manufacturer’s instructions (Zymo Research). cDNA was synthesized using iScript cDNA Synthesis Kit (Bio-Rad, Hercules, CA) from 1 μg of RNA and then analyzed by quantitative PCR (qPCR) using iTAQ Universal SYBR Green Supermix (Bio-Rad) with an Applied Biosystems QuantStudio 6 Flex Real-Time PCR System (Life Technologies, Grand Island, NY). Gene expression was normalized relative to GAPDH, and primer sequences are listed in Table I.

Wild-type (WT) C57BL6/J and IL-36γ^−/− mice were injected i.p. with InVivoMAb anti-mouse Ly6G (α-Ly6G) Ab or InVivoMAb rat IgG2a isotype control (Bio X Cell, West Lebanon, NH) to deplete neutrophils using 200 μg of Ab in 200 μl at day −1 prior to infection and then every other day through the duration of the challenge study. Vaginal lavages were analyzed by flow cytometry to confirm the efficiency of neutrophil depletion.

All lavages were passed through 40-μm filters (Corning BD Falcon, Tewksbury, MA), and cell counting and viability were performed by trypan blue exclusion. Murine vaginal tissue was processed into single-cell suspensions for flow cytometry analysis using a modified protocol as described by Jiang and Kelly (38). Briefly, vaginal tissue was cut into fine pieces (<1 mm) with surgical scissors and transferred to a 1.5-ml microfuge tube with 1 ml of digestion media: RPMI medium (Corning), 10% heat-inactivated FBS (Corning), 1% penicillin/streptomycin (Corning), 10 mM HEPES (Fisherbrand, Hampton, NH), and 0.5 mg/ml collagenase type VIII (Sigma-Aldrich, St. Louis, MO). Tissue was incubated at 37°C with shaking for 1 h. After 1 h, tissue was strained through a 40-μm filter (Corning) and washed with fresh RPMI 1640. Remaining tissue underwent a second incubation with fresh digestion media at 37°C with shaking for 1 h. After the second incubation, tissue was lightly dissociated between sterile glass slides and filtered through 40-μm filters. Cell counting and viability were then evaluated by trypan blue exclusion. Nonspecific binding in samples was blocked by staining with TruStain FcX Ab (anti-mouse CD16/32; BioLegend), and cells were then stained with a mixture of Abs: CD11b Alexa Fluor 647 (clone M1/70), F4/80 PE (clone BM8), Ly6C PerCp (clone HK1.4), Ly6G Alexa Fluor 488 (clone 1A8), and GR-1 FITC (clone RB6-8C5; BioLegend). After staining, cells were fixed in 1% paraformaldehyde, and data were acquired on a BD LSRII flow cytometer (BD Biosciences, San Jose, CA). Unstained, single-stained, and fluorescence minus one controls were used to determine positive and negative staining. Data were analyzed with FlowJo software (Tree Star, Ashland, OR). Dead cells and debris were excluded by forward and side scatter, and singlet events were selected based on forward scatter area, height, and width parameters.

Disease severity, scoring, and systemic virus spread were evaluated by area under the curve (AUC) analysis with an unpaired two-tailed Student t test with Welch correction as previously described (20). Disease incidence and survival were analyzed by log-rank analysis as previously described (3). Unpaired two-tailed Student t test with Welch correction and ordinary one-way and two-way ANOVA with Bonferroni multiple comparisons test were performed for comparisons as indicated. A p value <0.05 was considered significant. All statistical analyses were performed using Prism software, version 8 (GraphPad, San Diego, CA).

We previously demonstrated that i.vag. IL-36γ treatment induced the recruitment of polymorphonuclear cells to the FRT barrier. Therefore, we sought to further understand the role of IL-36γ and immune cell recruitment in the vaginal microenvironment. CD11b⁺Ly6G⁺ neutrophils were the predominant cells in vaginal lavages from DMPA-conditioned naive C57BL/6 mice, representing ∼88–94% of all cells in lavages (Fig. 1A). This finding is consistent with a recent report from Cora et al. (39) that describes neutrophils as the most abundant cells during diestrus in mice. Following treatment with IL-36γ, we measured a significant influx of neutrophils into the vaginal microenvironment at 4 (4.9-fold, p < 0.0001) and 8 h (2.6-fold, p < 0.001) posttreatment. Neutrophil infiltration was transient, and neutrophil counts returned to baseline levels by 12 and 24 h posttreatment (Fig. 1C). We identified macrophages (CD11b⁺F4/80⁺) and monocytes (CD11b⁺Ly6C⁺Ly6G⁻) in vaginal lavages at all time points, but these cells were present at low levels and were not significantly altered by IL-36γ treatment at any time point (data not shown). These results further demonstrate that IL-36γ robustly recruits neutrophils to the vaginal epithelium that can potentially contribute to host defense mechanisms.

We next quantified cellular infiltration in vaginal lavages from IL-36γ^−/− and WT mice before lethal HSV-2 challenge and then at 24 and 48 h postinfection to determine if IL-36γ is necessary for cell recruitment in response to viral infection. We found that there was no difference in the total number of cells or neutrophils in the vaginal microenvironment between naive IL-36γ^−/− and WT mice prior to HSV-2 challenge (Fig. 2). Following HSV-2 challenge in WT mice, the number of neutrophils steadily increased in vaginal lavages at 24 (2.4-fold) and 48 h (4.8-fold, p < 0.0001) postinfection (Fig. 2B). In contrast, lavages from IL-36γ^−/− mice exhibited significantly decreased numbers of CD11b⁺Ly6G⁺ neutrophils at 24 (−3.4-fold, p < 0.0001) and 48 h (−2.9-fold, p < 0.0001) after HSV-2 challenge. Neutrophil counts in IL-36γ^−/− mice were decreased −11.15-fold (p < 0.05) and −19.15-fold (p < 0.0001) at 24 and 48 h postinfection, respectively, compared with WT mice. Macrophages (CD11b⁺F4/80⁺) and monocytes (CD11b⁺Ly6C⁺Ly6G⁻) were present at low levels in lavages and were not significantly altered in either IL-36γ^−/− or WT mice at all time points evaluated (Fig. 2C, 2D). Similar to our findings in vaginal lavages, we measured significantly lower neutrophil counts in IL-36γ^−/− mouse vaginal tissue at 24 h post–HSV-2 infection compared with WT mice (Fig. 2E, Supplemental Fig. 1). We additionally detected macrophages and monocytes in vaginal tissue, and numbers of these cells were not significantly altered in IL-36γ^−/− mice relative to WT mice (Supplemental Fig. 1). Collectively, these results reveal an impaired recruitment of neutrophils to the vaginal microenvironment in IL-36γ^−/− mice after HSV-2 challenge that suggests a crucial function for IL-36γ in regulating neutrophil recruitment after viral infection.

Interestingly, we observed two distinct populations of CD11b⁺Ly6G⁺ cells when analyzing the neutrophil population in vaginal lavages (Fig. 3A, 3B). Prior to HSV-2 challenge, mature (CD11b⁺Ly6G^hi) neutrophils were the predominant population in vaginal lavages from both WT and IL-36γ^−/− mice, with a small pool of immature (CD11b⁺Ly6G^int) neutrophils present. After virus challenge, WT mice maintained the predominant population of Ly6G^hi-stained cells and a small pool of Ly6G^int cells (Fig. 3C). The number of mature neutrophils in WT mice dramatically increased at 24 and 48 h (p < 0.05) postinfection (Fig. 3D), whereas the number of immature neutrophils remained relatively constant (Fig. 3E). Unexpectedly, following infection in IL-36γ^−/− mice, we recorded relatively similar proportions of Ly6G^int- and Ly6G^hi-stained cells. The number of mature neutrophils in IL-36γ^−/− mice significantly (p < 0.0001) decreased at 24 and 48 h postinfection, while counts of immature neutrophils did not significantly change, shifting the ratio of mature to immature neutrophils in the vaginal microenvironment. This is shown in Fig. 3C, illustrating a shift in the neutrophil population in vaginal lavages from IL-36γ^−/− mice at both 24 and 48 h after HSV-2 infection. Collectively, these results demonstrate an important role for IL-36γ in mediating mature neutrophil recruitment in response to genital HSV-2 infection.

In an effort to understand mechanisms that may be contributing to IL-36γ–mediated regulation of neutrophil recruitment and the potential impact on other immune cells, we evaluated expression of several chemokines in vaginal tissue from IL-36γ^−/− and WT mice at 24 and 48 h postinfection (Fig. 4, Table I). These chemokines included those with chemotactic activity for macrophages, monocytes, neutrophils, NK cells, and T cells, among others. From this panel of chemokines, we found that IL-36γ^−/− mouse vaginal tissue exhibited a significant reduction in Cxcl1 expression relative to WT mice at both 24 (−6.4-fold, p < 0.05) and 48 h (−5.9-fold, p < 0.05) postinfection. We further detected a −7.3-fold reduction in Csf3 expression in IL-36γ^−/− vaginal tissue relative to WT at 24 h postinfection, although this was not statistically significant. We did not measure any significant changes in expression of any of the other chemokines assayed. Together, these data suggest that IL-36γ may function to regulate neutrophil recruitment in the FRT in part through the neutrophil chemoattractant KC.

To assess the requirement for IL-36γ in the host response to genital HSV-2 infection, we evaluated disease progression and survival after lethal i.vag. HSV-2 challenge in WT and IL-36γ^−/− mice. We observed that IL-36γ^−/− mice exhibited more rapid disease symptoms (p < 0.0001) compared with WT mice (Fig. 5A). IL-36γ^−/− mice developed hair loss and erythema around the introitus 2.5 d earlier than controls (Fig. 5B). Corresponding with the more rapid onset of disease symptoms, IL-36γ^−/− mice exhibited significantly (p < 0.0001) reduced survival time, succumbing to disease 2.6 d sooner than WT mice (Fig. 5C), indicating that IL-36γ signaling is involved in the host response to genital HSV-2 infection.

We next sought to determine if IL-36γ–mediated protection against genital HSV-2 infection was neutrophil-dependent. Using an anti-mouse Ly6G Ab to deplete neutrophils in mice, we evaluated disease progression and survival after lethal i.vag. challenge. We found that there was no significant difference in disease progression, the incidence of hair loss and erythema, or survival time between WT mice and neutrophil-depleted WT mice (Fig. 5). Isotype control–treated mice exhibited similar disease progression and survival as both WT and neutrophil-depleted WT mice (Supplemental Fig. 2). These data suggest that, although IL-36γ regulates neutrophil recruitment in response to HSV-2 infection, IL-36γ–mediated protection against genital HSV-2 infection and disease pathogenesis is not neutrophil-dependent.

Surprisingly, while monitoring disease progression and survival after lethal challenge, we observed an increased incidence of hind limb paralysis in IL-36γ^−/− mice, an indication of HSV-2 spread to the CNS. We found that 16.67% of WT mice developed hind limb paralysis over the course of infection and that the onset of hind limb paralysis was linked to productive HSV-2 infection in the brain stem, as demonstrated by plaque assay (Fig. 6A, 6B). In contrast, 81.5% of IL-36γ^−/− mice (p < 0.0001) developed hind limb paralysis after lethal challenge, and all mice exhibiting hind limb paralysis had detectable viral titers in the brain stem. There was no significant difference in the viral load in WT and IL-36γ^−/− mice with productive infections in the brain stem (data not shown). The increased spread of HSV-2 to the CNS in IL-36γ^−/− mice indicates a potential neuroprotective function for IL-36γ in the antiviral response.

We then sought to determine the kinetics of the local HSV-2 infection and systemic spread in IL-36γ^−/− mice compared with WT mice to better understand the decreased survival time and increased incidence of neurologic disease in IL-36γ^−/− mice. To measure viral titers and clearance of the virus in the vaginal epithelium, vaginal viral titers were quantified at 2, 4, and 6 d after lethal HSV-2 challenge (10³ PFU; LD₁₀₀). Viral titers in IL-36γ^−/− mice were comparable to WT at both 2 and 4 d postinfection (dPI; Fig. 7A). However, at 6 dPI, IL-36γ^−/− mice still exhibited detectable titers with viral loads comparable to 2 and 4 dPI, whereas WT mice had mostly cleared the virus. Vaginal viral titers in neutrophil-depleted WT mice were similar to WT and IL-36γ^−/− mice at 2 and 4 dPI and, similar to IL-36γ^−/− mice, exhibited delayed clearance at 6 dPI (Supplemental Fig. 2). Trend analyses indicated that IL-36γ^−/− mice exhibited a significant (p < 0.05) delay in HSV-2 clearance from the vaginal epithelium relative to WT mice (Fig. 7E). To evaluate systemic spread of HSV-2 after i.vag. infection, viral titers were quantified in the lumbar spinal cord, distal colon, and bladder of IL-36γ^−/− and WT mice at 2, 4, and 6 dPI. We detected significantly higher viral titers at 4 and 6 dPI in the spinal cord (p < 0.0001 and p < 0.01, respectively), distal colon (p < 0.01 and p < 0.05, respectively), and bladder (p < 0.01 and p < 0.05, respectively) in IL-36γ^−/− mice relative to WT mice (Fig. 7B–D). These differences in titers were most profound at 4 dPI, when there was a 2.3-log increase in titers in the lumbar spinal cord, a 1-log increase in titers in the distal colon, and a 3.3-log increase in HSV-2 titers in the bladder of IL-36γ^−/− mice relative to WT mice. Trend analyses indicated that HSV-2 spread more quickly to the lumbar spinal cord (p < 0.05), distal colon (p < 0.05), and bladder (p < 0.01) in IL-36γ^−/− mice relative to WT mice (Fig. 7F–H). Altogether, these data suggest that IL-36γ may function to limit replication and/or systemic spread of HSV-2 through neurons and the nervous system to protect against disease pathogenesis.

Initially, IL-36γ was investigated in the context of a chronic inflammatory disease, such as psoriasis (40). Recently, however, we and others have demonstrated that IL-36γ plays an important role in host defense mechanisms at mucosal sites (19, 20, 41, 42). IL-36γ induces the production of immune mediators and stimulates immune cell recruitment, suggesting a critical role of IL-36γ in immune protection (16, 17, 19, 20). In this report, we demonstrate that IL-36γ is an essential component of the innate immune response to genital HSV-2 infection, influencing both neutrophil recruitment and the spread of HSV-2 through the nervous system.

Previously, we showed that IL-36γ is expressed by vaginal epithelial cells and that treatment with rIL-36γ induced transient polymorphonuclear cell recruitment in addition to the production of cytokines and chemokines, including the neutrophil chemoattractant KC (19, 20). Using flow cytometry, we performed a time course evaluation of IL-36γ–mediated neutrophil recruitment in the cervicovaginal compartment. Similar to our earlier findings (20), we measured a significant increase in levels of CD11b⁺Ly6G⁺ neutrophils in vaginal lavages at 4 (p < 0.0001) and 8 h (p < 0.001) after treatment with rIL-36γ (Fig. 2C). Comparing neutrophil infiltration in WT and IL-36γ^−/− mice, we measured no differences in neutrophil counts between the different strains prior to infection, suggesting that the normal influx of neutrophils under homeostatic conditions in the FRT is not dependent on IL-36γ. However, upon exposure to HSV-2, we observed that the number of neutrophils in the vaginal microenvironment differed dramatically between WT and IL-36γ^−/− mice (Fig. 2). Consistent with a previous report, we measured a sharp increase in neutrophils in WT mice following i.vag. HSV-2 challenge (23). Surprisingly, there was a notable decrease in neutrophil counts in IL-36γ^−/− mice after lethal challenge, and these neutrophil levels remained suppressed through 48 h postinfection. This was also reflected in vaginal tissue, where we measured a significant decrease in neutrophil counts in IL-36γ^−/− mice relative to WT mice. We did not measure any changes in counts of macrophages or monocytes at 24 or 48 h postinfection in IL-36γ^−/− mice relative to WT mice, indicating that IL-36γ signaling may specifically function to regulate neutrophil recruitment in the FRT. To determine potential mechanisms of IL-36γ–mediated regulation of neutrophil recruitment, we evaluated expression of several chemokines in vaginal tissue from IL-36γ^−/− and WT mice at 24 and 48 h postinfection. Although there was little change in expression of several chemokines in vaginal tissue between IL-36γ^−/− and WT mice, we did measure significantly decreased expression of Cxcl1, a robust neutrophil chemoattractant, at both 24 and 48 h postinfection in IL-36γ^−/− mice (Fig. 4). Similarly, Aoyagi et al. (15) reported decreased KC production coupled with reduced neutrophil infiltration to the lungs in IL-36R^−/− mice after influenza virus infection, suggesting that in the FRT, IL-36 signaling may drive neutrophil recruitment in part through KC.

By distinguishing mature and immature neutrophil populations, we unexpectedly found that the mature neutrophil population was depleted in the IL-36γ^−/− mice through 48 h after HSV-2 infection. The drop in mature neutrophil levels created a shift in the vaginal epithelium, whereby immature neutrophils became the dominant neutrophil population, despite no changes in the overall counts of immature neutrophils. We did measure a 7-fold decrease in Csf3, an important factor for neutrophil maturation, in IL-36γ^−/− vaginal tissue at 24 h post–HSV-2 challenge, although this change was NS. IL-36γ signaling may impact neutrophil maturation factors, and future studies are needed to better understand the impact of IL-36γ on neutrophil mobilization and maturation. Although specific anti–HSV-2 functions of neutrophils are not well defined, neutrophils have been shown to phagocytose virions (43, 44), secrete inflammatory mediators (45, 46), and play a role in viral clearance (23, 47, 48). Although immature neutrophils have many of the functions of mature neutrophils, immature neutrophils have been reported to exhibit a reduction in mobility and trafficking behavior (49, 50). Additionally, immature neutrophils are less efficient in the production of ROS that function as an important component of the host response to infection (51). The dramatic drop in counts of mature neutrophils in IL-36γ^−/− mice did not impact viral titers at 2 or 4 dPI, but titers at 6 dPI in the IL-36γ^−/− mice were similar to those at 2 and 4 dPI, whereas WT mice had almost completely cleared the virus. The decreased neutrophil counts and subsequent delayed clearance of the virus in IL-36γ^−/− mice are consistent with our findings using neutrophil-depleted mice and from a previous report that demonstrated that neutrophil-depleted mice exhibit delayed clearance of HSV-2 from the vaginal epithelium (23). Milligan et al. (52) further demonstrated that, although neutrophils function in viral clearance from the vaginal epithelium, they appear to have a limited role in preventing virus spread to the dorsal root ganglia after i.vag. inoculation. The specific mechanisms by which neutrophils participate in HSV-2 clearance from the vaginal epithelium remain unclear, and the production of ROS (46), neutrophil extracellular traps (53), or phagocytosis (43, 44) may potentially contribute to viral clearance (54, 55). Although neutrophils function in virus clearance from the vaginal epithelium, we found that neutrophils were not essential for protection against genital HSV-2 infection, as neutrophil-depleted mice exhibited disease progression and an overall survival time that were comparable to those of WT mice. Neutrophils may exhibit a function in protection against genital disease pathogenesis, but protection afforded by neutrophils appears to be limited. Collectively, our data indicate that IL-36γ–mediated neutrophil recruitment plays a role in HSV-2 clearance from the vaginal epithelium but does not appear to be essential for protection against disease pathogenesis or in limiting virus replication during acute infection.

Interestingly, we observed a significant (p < 0.0001) increase in the incidence of hind limb paralysis in IL-36γ^−/− mice after lethal i.vag. challenge compared with WT mice. Approximately 81% of IL-36γ^−/− mice developed hind limb paralysis and exhibited detectable titers in the brain stem, whereas only 16% of WT mice developed hind limb paralysis and infection in the brain stem after lethal challenge. Additionally, we measured a more rapid spread of HSV-2 to the spinal cord (p < 0.05), and then to peripheral tissues including the distal colon (p < 0.05) and bladder (p < 0.01), that corresponded with the decreased survival time in IL-36γ^−/− mice. Furthermore, IL-36γ^−/− mice exhibited significantly elevated HSV-2 titers in the spinal cord, bladder, and colon at 4 and 6 dPI. As described earlier, there were no significant differences in vaginal titers at 2 or 4 dPI in WT mice, neutrophil-depleted WT mice, and IL-36γ^−/− mice, indicating that the increased spread of HSV-2 and neuroinvasion may not be due to increased and/or uncontrolled replication of the virus in the vaginal epithelium as a result of the decreased neutrophil counts in IL-36γ^−/− mice. Rather, these data suggest that IL-36γ may function to limit viral infection, replication, and/or spread in neurons and the nervous system. Related neuroprotective functions have also been shown for IFN signaling in neurons that protect against HSV replication and pathogenesis (56, 57). Milora et al. (18) suggested that the IL-36 cytokines may have a neurologic role in immune protection against HSV-1 in a flank skin model, and others have shown that IL-36R is expressed in mixed glial cells (58). A recent report has further shown that IL-36γ was upregulated in spinal neurons and astrocytes in a chronic inflammatory model, indicating an inflammatory function for IL-36γ in the CNS that may function to protect against viral infection and spread (59). Together, our findings suggest a dual role for IL-36γ in neutrophil recruitment in the FRT and in protection against HSV-2 neuroinvasion after lethal genital challenge.

In this study, we reveal that IL-36γ is a crucial innate immune mediator in the FRT that ultimately influences genital HSV-2 disease pathogenesis. Our findings demonstrate that IL-36γ is essential for recruiting neutrophils to the vaginal microenvironment following HSV-2 challenge and may function generally as a regulator of neutrophil recruitment in the FRT. Our data additionally indicate that IL-36γ–mediated protection against genital HSV-2 infection is not neutrophil-dependent and suggest that IL-36γ may function in neurons to protect against HSV-2 neuroinvasion. However, future studies are necessary to determine the mechanisms of IL-36γ–mediated neuroprotection. Ultimately, our findings show that IL-36γ is a key component of the antiviral response to genital HSV-2 infection and may function broadly in host defense mechanisms in the FRT and against other sexually transmitted pathogens.

We are thankful to Dr. Timothy Denning for generously providing a breeder pair of IL-36γ^−/− mice and Dr. Mrinalini Kala and the Flow Cytometry Core at the University of Arizona College of Medicine–Phoenix for assistance with flow cytometry experiments. We thank Dr. Theresa Thomas for critical review of the manuscript.

The authors have no financial conflicts of interest.