UV-B induced HSV-1 reactivation leads to infectious virus in the corneas of virtually all latently infected mice and requires an intact STING to develop herpetic stromal keratitis

Reactivation of latent herpes simplex type 1 results in virus returning to the cornea leading to recurrent herpetic stromal keratitis (rHSK). We compare two competing models to reactivate viruses from latency, UV-B irradiation and cyclophosphamide. Results revealed that while both result in corneal recrudescence, only UV-B irradiation results in rHSK. To better understand the dynamics of reactivation, we analyzed corneas for both the presence of infectious viruses and the dynamics of exposure to multiple reactivations using UV-B. We noted that multiple reactivations result in progressively worse corneal disease. We also noted that expression of IFNα and STING, surragate markers for the presence of virus, are induced by the presence of reactivated virus. Studies to determine the importance of STING to the development of HSK revealed that in the absence of STING, mice do not develop significant HSK and the magnitude of the infiltrate of CD45 + cells in these corneas is significantly reduced. The resulting paucity of CD45 + CD11b + GR-1 + F4/80-neutrophils, and to a lesser extent CD45 + CD11b + GR-1-F4/80 + macrophages in B6-STING KO mice following reactivation is likely the underlying cause for lack of rHSK as has been noted by ourselves and others. These results underscore the critical importance of STING’s role in developing rHSK.

Although the immune mechanisms are not fully understood, HSK is an immunoin ammatory disease triggered by reactivation of HSV-1 that has traveled from the latently infected trigeminal ganglia to the eye where it produces disease by reactivating an in ammatory response in the eye against the virus [6]. Therefore, it is bene cial to conduct HSK experiments using recurrent models of disease in addition to those involving primary infection models, because recurrent models most resemble HSK disease in humans [7][8][9]. Primary and recurrent HSK have distinct mechanisms, because different cytokines and chemokines play different roles in each disease. For example, interleukin-6 plays a role in primary HSK but not recurrent HSK [10]. Clinically speaking, recurrent HSK mice models result in features different from primarily infected mouse models [7]. Just like HSK disease in humans, recurrent infections in mice result in microdendrites, focal stromal opacities, disciform endotheliitis, and corneal neovascularization [7,8]. Meanwhile, primary infections in mice result in a different clinical picture, unlike human disease, which includes multiple epithelial dendrites and diffuse stromal opaci cation [3]. Mice are not the only animals used to study recurrent HSK disease. Along with mice, dogs [11][12][13] and rabbits [13][14][15][16][17][18][19] have also been used to create HSV-1 reactivation models.
Due to economic and genetic bene ts corresponding to using mice, mice have been the species of choice when studying both primary [3,6,7] and recurrent models of HSK [8,9,[20][21][22][23][24][25][26][27]. One of the early murine models of HSK used a modi cation of what is used in rabbits. In this model iontophoresis along with timolol was used, the addition of timolol being a key factor as this model displayed improved frequency of reactivation when compared to iontophoresis alone [20]. Other reactivation techniques used to reactivate latent infections in mice include using UV irradiation, hyperthermia, transcorneal epi iontophoresis, immunosuppressant treatment, and treatment with sodium butyrate in phosphatebalanced saline [8,9,[21][22][23][24][25][26][27]. Approximately 24 hours post thermal stress, mice treated with hyperthermia resulted in reactivation detected through harvested trigeminal ganglia [26]. The rest of the techniques mentioned above assessed reactivation by measuring the amount of viral shedding collected from eye swabs or eye washes [21][22][23][24][25][26][27]. Immunosuppressant and UV-B radiation resulted in a greater frequency of reactivation compared to other methods, which suggests UV-B irradiation and/or immunosuppressants may be the ideal technique of reactivation when designing a recurrent model to study HSK [34,35].
The UV-B irradiation model has been used in many previous studies that have made important discoveries regarding HSK. Xia,et. al. [28] utilized UV-B irradiation in experiments to demonstrate that elevated levels of interleukin-17 could be involved in the development of recurrent HSK [19]. Furthermore, the importance of LAT expression, as was reported in rabbits [13], was also determined to be important in the UV-irradiation model in mice [9]. There are other studies pursuing the clinical potential of T-cell based immunotherapy or using CRISPR-Cas9 to cure recurrent HSK. These studies, which utilized a UV irradiation murine model, generated data suggesting the potential e cacy of such treatment in treating HSK [29].
The current study was designed to investigate several issues. The rst being a comparison of UV-B inducted and CP-induced reactivation. After demonstrating the superiority of the UV-B reactivation model over CP, we next performed studies to better determine what percentage of mice display reactivation following UV-B treatment of latently infected mice. These studies were followed by an analysis of the consequence of serial reactivations to corneal disease in individual mice. Finally, we discovered that STING expression was increased following reactivation, which led us to determine the role that STING expression plays in HSK.

Mice
All procedures were carried out in accordance with ARRIVE Guidelines and the relevant guidelines and regulations for animal research. These investigations with mice conformed to the Association for Research in Vision and Ophthalmology (ARVO) Statement for the Use of Animals in Ophthalmic and Vision Research. These investigations, including experimental protocols and methods, were also reviewed and approved by the Saint Louis University Animal Studies Committee. C57BL/6 (B6) were purchased from NCI, C57BL/6J-Sting1 gt /J (B6-Sting KO) were purchased from Jackson Labs and bred in our animal facility. NIH inbred strain of mice were originally obtained from Harlan OLAC (Oxford, England) and have been bred in the Saint Louis University animal facility for over 15 years under the supervision of the staff in Comparative Medicine.

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For all experiments, except those shown in Table 1 (which used age matched female mice), we used age matched male and female mice from the aforementioned strains that were 6-10 weeks old. For recurrent disease, mice were typically infected on the scari ed cornea with 10 6 PFU HSV-1 McKrae strain as previously described [10], unless otherwise indicated in the results section. Each mouse received an intraperitoneal (IP) injection of 0.5 ml pooled human serum (Sigma Chemicals, St. Louis MO; ED50 for virus neutralization = 1:1600) concurrent with infection. Administration of human anti-HSV antibodies at the time of ocular infection has been shown to protect mice from death and corneal disease during primary infection, while allowing for the establishment of latency and subsequent reactivation of virus after corneal UV-B exposure [8, 10,21,30,[33][34][35]. These human antibodies are undetectable at the time of UV-B irradiation, 5 weeks after primary infection. To con rm infection, eye swabs taken three days post-infection were evaluated for the presence of infectious virus. Typically, 100% of these infected mice were shedding virus at day 3 [7], should any mouse not demonstrate infectious virus, it would be eliminated from the study. Furthermore, it is rarely observed (less than 10%) that these mice will develop signi cant scarring from primary infection. Should they display such scarring, they are also eliminated from the study. There were no signi cant differences between any of these treatments by students t-test.

UV-B irradiation and virus reactivation.
Mice were reactivated from latency as previously described [10,21]. Brie y, the eyes of all latently infected mice were examined for corneal opacity before irradiation, and only animals with clear corneas were used. At least 5 weeks after primary infection, the eyes of latently infected and control mockinfected mice were exposed to 250 mJ/cm2 of UV-B light using a TM20 Chromato-Vu transilluminator (UVP, Inc., San Gabriel, CA), which emits UV-B at a peak wavelength of 302 nm. Irradiated mice were swabbed with sterile cotton applicators from day 0 to day 7, unless otherwise indicated. The day 0 swab was performed to eliminate any animal with infectious virus in their tear lm prior to reactivation from consideration in that particular study. The swab material was cultured on Vero cells, as described [7], to detect recurrent virus shedding from the cornea. Reactivation was de ned as the nding of any HSVpositive eye swab on any day after UV-B exposure, with day 0 swabs serving as a control [21]. We also determined the result on HSK by subjecting mice to multiple reactivations by UV-B irradiation. This was accomplished by successively reactivating mice every 30 days at which point the clinical disease was resolving. Mice were observed for clinical disease following these reactivations and viral swabs taken for 7 days following each reactivation.

Cyclophosphamide Procedure
Mice that are latently infected with HSV-1 and control mice are injected with 150mg/kg of cyclophosphamide (CP) (Sigma-Aldrich) on days 0, + 2, and + 4 intraperitoneally. CP was diluted in saline based on each individual animal's weight. Mice were swabbed with sterile cotton applicators from day 0 to day 7 following CP treatment, unless otherwise indicated. The swab material was cultured on Vero cells, as described above, to detect recurrent virus shedding from the cornea. Reactivation was de ned as the nding of any HSV-positive eye swab on any day after CP treatment, with day 0 swabs serving as a control [21].
Clinical evaluation.
On the designated days after viral infection or UV-B reactivation, a masked observer, who is unaware of the experimental groups, examined the mouse eyes through a binocular dissecting microscope to score clinical disease. Stromal opaci cation was rated on a scale of from 0 to 4, where 0 indicates clear stroma, 1 indicates mild stromal opaci cation, 2 indicates moderate opacity with discernible iris features, 3 indicates dense opacity with the loss of de ned iris detail except pupil margins, and 4 indicates total opacity with no posterior view. Corneal neovascularization was evaluated as described previously [10,21] using a scale of from 0 to 8, where each of the four quadrants of the eye is evaluated for the presence of vessels that have grown into them. Corneal blink re ex was tested by loosely holding the mouse and touching the cornea with the blunt tips of surgical forceps without touching the eyelashes and whiskers. The cornea was divided into 5 areas (4 quadrants and center area), loss of blink re ex referred to the inability of the mouse to blink when an area was touched and was recorded as 0. Positive blink re ex referred to the ability to blink when an area of the cornea was touched and was recorded as 1. The total score of the 5 areas would be the nal score of corneal blink re ex for a mouse. A score of 0 indicated a complete loss of corneal sensation such that the mouse failed to blink when any area of the cornea was touched. A score of 5 indicated retention of some degree of sensation such that the mouse blinked when any area of the cornea was touched [30,33]. Note: Uninfected, UV-B irradiated control mice were used as a baseline for any effects due to UV-B irradiation.

Plaque Assays for virus in the cornea
Mice were irradiated with UV-B and sacri ced on Day 3 and whole eyes removed to determine the presence of virus within the corneal samples. Eyes were frozen in Eppendorf tubes on dry ice a placed stored at -80C until use. Eyes were thawed by adding 50ul media to each tube and the eye placed on a sterile Petri dish to remove cornea with disinfected scissors and forceps along the limbus under microscope. The corneas were then ground 30-40 times using a plastic pestle, following with addition of 400ul media. At this point the ground material was subjected to sonication using a Branson Soni er 450 (Marshall Scienti c, Hampton, NH) each cornea was subjected to a 30 second pulse followed by 15 second rest and another 30 second pulse on ice to control for any heat generation. The sonicate is then plated on VERO cells or placed at 4C until plated. Titers for each cornea were determined by plaque assay.

RNA isolation
Mice were euthanized and eyes were frozen on dry ice and stored at -80C. Upon thawing, 1 ml of TRIzol reagent (Life Technology) was added to each individual eye. The cornea was removed and ground 30 to 40 times to homogenize it. This was followed by addition of 200 ul chloroform and allowed to sit for 2-3 minutes at which time it was centrifuged at 12,000 x g for 15 minutes at 4C. The top 0.4 ml of the aqueous phase is collected and transferred to a fresh tube to which 400 ul 70% alcohol was added and then applied to a spin cartridge from PuireLink RNA mini kit (Thermo Fisher Scienti c). These cartridges were centrifuged at 12,000 x g for 15 minutes washed with buffer and then buffer with ethanol. Finally, the RNA is eluted with 30 ul RNase-free water by incubating for 1 minute at RT and then centrifuging the cartridge for 2 minutes at 12,000 g to elute RNA. Store at -80C.

Detection of ICP4 lytic gene expression
Individual corneas were removed from female mice and placed in separate tubes, treated with DNase to remove DNA, and RNA was isolated. Once puri ed, the RNA it was reverse-transcribed using a High-Capacity RNA-to-cDNA kit (Applied Biosysytems). We next performed the qPCR using a CFX96 Optics module (Bio-Rad Laboratories), operated by CFX Manager Software version 3.1, using ready-made Power SYBR Green PCR Master Mix (Applied Biosysytems) according to the manufacturer's protocol. For ICP4 mRNA, the forward primer was 5' -GCG TCG AGG TCG T -3' and the reverse primer was 5'-CGC GGA GAC GGA G -3'. Gene expression of each cornea was quanti ed relative to the expression level of GAPDH (Forward primer: 5'-ACTCCACTCACGGCAAATTC-3' and reverse primer: 5'-TCTCCATGGTGGTGAAGACA-3'). We determined that GAPDH message could be reliably detected between 22 and 25 cycles of PCR expansion. Positive expression of ICP4 required between 25 and 28 cycles of PCR expansion. Any product detected above 32 cycles was deemed unreliable and thus considered negative.

Western Blot
Corneas were removed from mice at the indicated time points and stored at -80C until use. For every 2 corneas, add 150 µL of ice-cold lysis buffer (Cell Signaling Technology, Danvers, MA) in addition of 1 mM PMSF (Cell Signaling Technology, Danvers, MA) rapidly to the tube, homogenize with a tissue grinder, then maintain constant agitation for 2h at 4°C (e.g., place on an orbital shaker in the fridge). Centrifuge for 20 min at 12,000 rpm at 4°C in a microcentrifuge. Gently remove the tubes from the centrifuge and place on ice, aspirate the supernatant, and place in a fresh tube kept on ice; discard the pellet. Immuno uorescence for IFN-α FITC conjugated anti-CD4 (clone RM4-5), PE-conjugated anti-CD8α (clone 53-6.7) (from PharMingen) and eFlour 660-conjugated CD11b (clone M1/70) (from eBiosciences, San Diego, CA). The samples were analyzed by the Microbiology ow cytometry core who did the gating and provided us with the data. The strategy we used for analysis was to initially gate on live cells by the and then the CD45 + cells. These CD45 + cells were further evaluated for T cell markers CD4 and CD8, or for macrophage markers F4/80 + CD11b + GR-1 − , or neutrophil markers GR-1 + CD11b + F4/80 − . Cells were then analyzed on a ow cytometer (FACSAria with FACSDIVA data analysis software; BD Biosciences).

Statistics
All statistical analyses were performed with the aid of Sigma Stat for Windows, version 2.0 (Jandel, Corte Madera, CA). The log rank test was used to compare disease scores. Student's unpaired t-test was used to compare virus titer data and ow cytometry numbers.

Results
It has been reported that both CP and exposure to UV-B light leads to HSV-1 reactivation [31]. However, they have never been compared head-to-head for both their ability to reactivate or the resulting corneal disease which occurs following such treatments. Our laboratory compared CP with UV-B in a mouse strain, inbred female NIH mice, which has been shown to display the highest reactivation rates following UV-B irradiation [34,35]. Results indicate that the reactivation rate for CP treatment was very similar to that seen when UV-B irradiation was used (Table 1). When days shedding virus were compared, UV-B had a mean shedding of 4.1 days and CP, 2.3 days which was statistically signi cant (P = 0.014). It is interesting to note that these studies also tested the combination of CP + UV-B the result of which did not result in statistically greater numbers of mice shedding virus into the tear lm (60%) than either UV-B (54%) or CP (50%) alone (Table 1). These data indicate that there is no additive effect of combined CP + UV-B treatment in stimulating virus release into the tear lm.
When these mice were monitored for corneal disease following reactivation, the disease in CP treated mice was almost non-existent while UV-B reactivation resulted in signi cant viral-induced HSK disease (Fig. 1). We also monitored corneal disease in animals that were treated with both CP and UV-B irradiation, and such analysis indicated that these animals, like those treated with CP alone, did not develop signi cant corneal disease (data not shown), further indicating the immune suppression, which has been shown to have signi cant immunosuppressive effects (32), likely does not allow for the development HSK.
As we evaluated these data, we were faced with the critical issue as to how well does shedding of virus into ocular tear lm re ect actual number of mice that have virus in the corneas following UV-B reactivation. We had always believed that detection of viruses in tear lm would be, at best, a re ection of the lower limit to how many mice are reactivating. To begin to better understand what a more accurate rate of reactivation would be we assayed several different parameters to determine the presence of virus in corneas following UV-B reactivation.
We have recently reported that viral antigen was expressed in virtually all corneas following reactivation [33]. While it is formally possible that the results shown in that publication [33] could be due to persistent antigen in the cornea, we did not believe that to be the case. Nonetheless, we decided to remove corneas from latently infected female mice 3 days after UV-B reactivation grind them and then plate this tissue on Vero cells as a means of detecting intact infectious virus in reactivated corneas. The data from these extracts were then compared to latently infected mice that were not subject to UV-B reactivation. As shown in Table 2, less than 10% of mice that did not undergo UV-B treatment possessed an infectious virus in their corneas. While almost all UV-B treated latently infected female mice (91%) demonstrated the presence of infectious virus (Table 2), the percent of male mice displaying infectious virus following UV-B treatment was 70%. While this difference may seem to be signi cant, upon statistical analysis it was not (P > 0.05). However, the difference in mean virus titer between female mice (913 ± 414) and male mice (20.6 ± 12)was signi cant (P < 0.05) ( Table 2). Statistical analysis indicates that there is no difference between female and male UV-reactivated mice for % positive samples. For other comparisons between these two groups, female mice displayed greater levels for mean (P < 0.05) and for Range (P < 0.05). Comparison between UV-reactivated and latently infected only reveal that the UV-reactivated mice were greater for all parameters measured (P < 0.005).
We next performed similar studies in which RNA was isolated from corneas derived from either latently infected mice that were subjected to UV-B treatment or those corneas from latently infected mice that were not exposed to UV-B treatment. We used the expression ICP4 as our target gene as it is critical in the ability of the virus to make infectious progeny [36]. Results from these studies indicated that only mice from latently infected mice that were UV-B reactivated possessed detectable viral RNA (4 of 9 mice), while none of the latently infected and unreactivated mice possessed detectible viral RNA (0 of 9 mice). However, the limitations of isolating viral RNA from corneal tissue indicates that it is very di cult to effectively isolate su cient RNA for a full analysis. Nonetheless, we are con dent that only corneas from reactivated mice have measurable RNA following UV-B reactivation.
We had previously hypothesized that multiple reactivations might lead to successively progressive corneal disease. When we performed such an analysis, we found that our hypothesis was correct. Mice, even those without any corneal disease following their rst exposure to UV-B irradiation, did upon subsequent reactivation stimulation developed signi cant corneal pathology (Table 3A) when compared to naïve mice undergoing UV-B irradiation alone (Table 3B). The number of animals used in 3A was 20 and 10 for 3B.
Data is expressed as mean ± standard error of the mean.
Statistical differences between the rst reactivated group and other groups were only achieved for the third reactivation (P < 0.05) and fourth reactivation group (P < 0.01).
To more fully understand the events that occur in the cornea following UV-B reactivation we evaluated the type I interferon response. Many other investigators have shown that type I IFN production is very important in resistance to primary infection [37,38]. However, little is known about such responses following UV-B reactivation. Consequently, we evaluated corneal sections at Days 1, 2, 3 and 5 following UV-B reactivation for the presence of IFNα. Representative results from Day 3 indicate that following reactivation there are a signi cant number of cells expressing IFNα found in reactivated corneas (Fig. 2B), but not found in corneas of latently infected, but not UV-B reactivated mice ( Fig. 2A). Likewise, mice receiving UV-B treatment did not display many cells positive for IFNα expression (Fig, 2C). IFNα was not detected at Day 1 post-infection and Day 2 only had a few positive cells in the latently infected and reactivated group, which is why we chose to show Day 3 where differences were most apparent. These observations are similar to what we generally observe as following reactivation there is no virus in tear lm swabs detected [7]. As indicated in the gure legend, we did not detect signi cant IFNα expression by Day 5 post-reactivation.
We next evaluated STING expression following either primary infection with HSV-1 or following UV-B induced reactivation. We chose this pathway as this is one of the critical pathways that stimulates the innate immune response following infection (39,40). We initially attempted to determine levels in phosphorylation of STING as a means of determining activation of this sensing molecule. However, we quickly found that such an analysis would be impossible due to the transient presence of phosphorylated STING that is seen in cultured cells [41]. However, when Western blots of primary infection were run, we noted that STING is expressed following infection (Fig. 3A), while STING expression in the uninfected eye could not be detected. Furthermore, STING expression is maximally expressed at Day 1 following infection and almost undetectable by day 3 post-infection. Thus, we decided to measure STING expression in latently infected mice following UV-B reactivation. Figure 3B indicates that there is expression of STING in latently infected mice undergoing UV-B treatment, while the contralateral eye did not demonstrate measurable STING. That said, this gure also demonstrated that UV-B treatment of uninfected corneas does result in STING expression in some, but not all mice. In contrast, latently infected mice undergoing UV-B treatment displayed levels of STING expression that are greater than in uninfected mice treated with UV-B (Fig. 3B).
This increase in STING expression seen in latently infected mice following UV-B treated mice allowed us to hypothesize that STING expression might also be related to corneal disease. It had been previously reported that mice with compromised STING expression displayed signi cantly greater mortality following HSV-1 infection (42,43). Consequently, we decided to compare female B6-STING KO mice to female wild-type B6 mice in our reactivation model. We chose female mice for this analysis as they routinely display greater disease (8). Our initial attempts to latently infect B6-STING KO mice underscored their increased sensitivity to HSV-1 infection [42.43], in that mice infected with our normal dose of 10 6 PFU HSV-1 McKrae displayed a 60% mortality rate even though they had been given anti-HSV-1 antibody at the time of infection. The wild-type B6 mice did not demonstrate any deaths following our standard means of establishing latency ( disease when compared to their wild-type counterparts did (Fig. 4). We next evaluated the in ammatory in ltrate in wild-type and B6-STING KO mice. This analysis demonstrated that the B6-STING KO mice displayed signi cantly fewer CD45 + cells (leukocytes) (Fig. 5). We next evaluated these CD45 + cells for T cell, neutrophil and macrophage subsets. Such an analysis indicated that the B6-STING KO mice had signi cantly fewer cells of all subsets at day 21 post-reactivation and signi cantly fewer CD8 + T cells, GR-1 + (neutrophils) and F4/80 + (macrophages) cells at day 28 post-reactivation (Fig. 5). These data indicate that STING expression is required for mice to display signi cant recurrent HSK following UV-B induced reactivation.

Discussion
Recurrent HSV-1 infection which results in HSK, is a leading cause of infectious blindness involving the cornea in the developed world [1][2][3][4]. Following primary corneal infection by HSV-1, the virus travels to the TG where it has the capacity to establish latency. This established latency can be disrupted by various stressors leading to reactivation of the virus and spread to peripheral tissues [1][2][3][4]. In humans, primary disease is typically without clinical symptoms [2][3][4]. Surveys indicate that most cases demonstrating clinical disease are a result of reactivation of a latent HSV-1 infection [2][3][4]. Disease is initiated by the presence reactivated virus in the cornea, which in turn, initiates an immunopathologic condition that results in corneal in ammation, which when severe, will result in chronic damage that might result in permanent damage to the cornea [2,4]. There are several animal models of this disease, mostly involving rabbits and mice as the animal of choice [8, 9,13,15]. Our laboratory has been studying a mouse model of both acute infection and to a much greater extent, recurrent model of HSK that requires some sort of stimulation that reactivates the virus from latency in the TG [8, 21,[44][45][46]. While we were using the UV-B induced model originally described by Shimeld, et al. [35,36], we decided several years ago to compare that means of inducing reactivation with using CP, which had also been shown to reactivate an active infection from latency [47,48]. This comparison con rmed that UV-B induced reactivation was superior to CP induced reactivation as UV-B treatment not only leads to latently infected cells in the TG releasing infectious virus, but also results in corneal disease, but CP treatment did not lead to corneal disease. This underscores the immunosuppressive nature of CP treatment [49] and at the same time restricts it's use as a method to establish a recurrent model of HSK.
We had posited that tear lm data was an underestimation of the true reactivation rate following UV-B induced reactivation. This hypothesis was strongly supported from our recent publication indicating that most, if not all, corneas possessed HSV-1 antigen following UV-B reactivation [33]. Nonetheless, one could argue that there might have been residual antigen in corneas remaining from the primary infection at the time of reactivation. To convince ourselves that our hypothesis was accurate, we removed corneas from mice 3 days post-reactivation and made extracts that were then tested for infectious virus. We clearly show that the great majority of latently infected mice treated with UV-B contained infectious virus while less than ten percent of latently infected mice who were not subjected to UV-B treatment contained infectious virus. These studies revealed the interesting observation that there was a tendency of female mice to demonstrate a somewhat greater percentage of mice with infectious virus and higher mean titers of virus than seen in similarly treated male corneas from latently infected mice undergoing UV-B induced reactivation. Though, upon statistical analysis, only the mean viral titers for female mice proved to be signi cant. While we can only speculate of this observation, one could interpret these results to be a partial explanation for why we see greater disease in female vs. male mice in our reactivation model [8]. It is also interesting to note that the range of viral titers for female corneas with infectious virus was very broad ranging from 8 pfu to 6800 pfu ( Table 2). This could be viewed as indicating that disease is most severe in those bearing the greatest viral load. However, a recent report by Drayman, et al. [50] indicates that immune activation is exclusively restricted to cells that are undergoing unproductive infection rather than productive infection. This report further suggests that cells that are producing infectious virus are not capable of stimulating a signi cant innate immune response due to the effectiveness of infectious virus to control such responses in these cells. Consequently, the activation of an immune response to the virus may not be driven by the total number of productively infected cells, but rather the number of nonproductively infected cells. This in part may explain the di culty of detecting ICP4 message in reactivated corneas. It is known that ICP4 is required for expression viral genes that produce infectious virus [36]. Interestingly, most UV-B-reactivated mice express some level of disease, while those latently infected and not UV-B-reactivated nor those corneas from uninfected mice undergoing UV-B irradiation alone do not display remarkable disease.
Another concept that we decided to examine was the hypothesis that serial reactivation would result in progressively increased disease in mice. Our data supports that is the case. We believe that this is potentially the result of two, non-mutually exclusive mechanisms. The rst would be that serial reactivation would act as a booster for the pathogenic CD4 + T cells that mediate corneal disease [44,51]. The second would be the result of changes in nerve ber regrowth into the cornea following infection [51] and reactivation [33]. We have recently published that such regrowth leads to increased disease due to the preferential regrowth of sympathetic vs sensory nerves, which predisposes the cornea to increased in ammation and that CD4 + T cells are required for that to occur [33]. Future studies could be performed to evaluate the relative contribution of these factors to increased disease. Furthermore, this observation suggests that controlling reactivation occurrences is very important in preventing serious complications from recurrent HSK.
That concept led us to investigate surrogate markers to determine the extent of immune activation following UV-B reactivation in cells of the cornea. The two markers we chose to investigate were IFNα and STING. Analysis of corneas following reactivation of virus indicated that exposure to UV-B irradiation alone will stimulate a few cells to express IFNα, but that all corneas from latently infected mice that were UV-B-reactivated demonstrated signi cantly more IFNα. This, we believe is an indication that the corneas of latently infected mice are displaying the presence of new virus following reactivation [34]. We next monitored the expression of the known pattern recognition molecule STING that is activated following HSV-1 infection [35][36][37]. The cGAS-cGAMP-STING pathway recognizes cytoplasmic DNA and recruits and activates the kinases TBK1 and IKKβ resulting in the activation of IFF3 and NF-κB, which culminates in the production of pro-in ammatory cytokines and type I IFNs [52][53][54][55][56]. Initially, we attempted to distinguish phosphorylated STING from unphosphorylated STING as a means of detecting activated STING. This proved to be impossible in vivo when using corneas from these mice, probably due to the transient nature of phosphorylated STING and the di culty of detecting phosphorylated STING in the presence of murine corneas. However, we noted that the latently infected mice following UV-B induced reactivation expressed more STING protein as determined by Western blot analysis than either corneas from un-reactivated and latently infected mice or corneas from uninfected mice receiving UV-B treatment alone. It should be noted that we demonstrated that mice undergoing primary infection also expressed signi cantly greater levels of STING than corneas from uninfected mice, which was not detectable in uninfected mice. Thus, our data demonstrates that the presence of HSV-1, whether via primary infection or the result of UV-B induced reactivation, stimulates STING protein expression in these infected corneas.
We were somewhat surprised that infection/reactivation induces STING expression, as the virus produces at least two different proteins that interfere with the STING pathway of innate immune activation. This has been shown by data indicating that viruses lacking VP22 also have a more robust cGAMP activity, which indicates an interaction between VP22 and cGAS that reduces cGAMP production [56]. Thus, VP22 from HSV-1 acts as an antagonist of IFN-β activation by the cGAS-STING pathway by colocalizing and inhibiting the enzymatic activity of cGAS (56). In addition, ICP27 is a multifunctional immediate early tegument protein essential for HSV-1 replication and is conserved among all herpesviruses. ICP27 functions in transporting mRNA transcripts from the nucleus to the cytosol [57]. Nevertheless, ICP27 also can inhibit innate immunity [58] by counteracting the production of type I IFNs. ICP27 was shown to interfere with the production of type I IFN by interacting with STING and TBK1 and reducing the phosphorylation of IRF3 [59]. Christensen et al. [57] suggested a model for this inhibition whereby ICP27 has low a nity for inactivated TBK1 but higher a nity to TBK1 in the TBK1-STING complex leading to an interaction with both factors [59]. Thus HSV-1 invests some of its genetic capital to directly target this pathway. Nonetheless, our data clearly indicates that HSV-1 infection/reactivation results in an induction of STING production.
We nally went on to determine the impact of STING expression on HSK. As described earlier in this report, the STING pathway is a major factor in the cell's ability to recognize the presence of unwanted DNA in the cytoplasm [52][53][54][55][56]. It had also been reported that in the absence of STING, mice were highly susceptible to infections by HSV-1 [39,40]. These reports indicate that this pathway is critical in developing an innate immune response to HSV-1.       Flow cytometric analysis of corneas from wild-type displayed great numbers of in ammatory cells than corneas from B6-STING KO mice. We reactivated B6 (n=15) and B6-STING KO mice (n=15) and harvested their corneas on Day 21 and Day 28 following UV-B irradiation. Cells were subjected to ow cytometric analysis for the indicated markers. Results indicate that wild-type B6 mice had a greater total in ammatory in ltrate (CD45 + cells) and a corresponding increase in cell numbers for neutrophils (CD45 + CD11B + F4/80 -GR-1 + ), CD4 + T cells, CD8 + T cells and macrophages (CD45 + CD11B + GR-1 -F4/80 + ) at Day 21 post-reactivation and that a similar pro le was seen at day 28 as well, though the number of cells were substantially reduced at this time point. Data represents mean + S.E.M. for individually analyzed corneas from these groups. Results were analyzed by student's t-test and signi cance was determined for all cell types at Day 21 and all except CD4 + T cells at Day 28 (p<0.05-0.01) (see asterisks).