Granulocytes in Ocular HSV-1 Infection: Opposing Roles of Mast Cells and Neutrophils.

Herpes simplex virus type-1 (HSV-1) is a neurotropic double-stranded DNA virus with significant clinical interest due to its endemic nature, life-long persistence, and status as the leading cause of infectious corneal blindness.1,2 The global burden of corneal HSV-1 infection is estimated to be 1.5 million annual occurrences including 40,000 new cases of severe monocular visual impairment or blindness.3 More than 60% of the human population is infected with and harbors latent HSV-1 in neuronal ganglia.2,4,5 Periodic reactivation of HSV-1 facilitates shedding of infectious progeny in external mucosae innervated by infected neurons, often resulting in herpes labialis (i.e., cold sores) or ocular herpes keratitis.2,6 Farooq et al.7 have reviewed the mechanisms of HSV-1 cell entry and spread in the eye. Intermittent but recurrent HSV-1 reactivation in the cornea contributes to the development of herpes stromal keratitis (HSK), a blinding immunopathology characterized by inflammation, scarring, and neovascularization.2,3 Investigating processes of immune surveillance and innate countermeasures during the earliest stages of infection is important to better understand HSV-1 pathogenesis, focus development of novel therapeutics, and ultimately prevent HSK. The innate immune response to HSV-1 infection in the cornea is multifactorial. It is abundantly clear that type-1 IFN-α/β signaling in both resident cells and infiltrating leukocytes is required for control of HSV-1.8 Due to the avascular nature of the normal cornea, circulating leukocytes must extravasate from blood vessels surrounding the cornea in the limbus and migrate through the tissue in accordance with the local milieu of inflammatory mediators (i.e., cytokines, chemokines, eicosanoids, etc.) following injury or pathogenic insult.9–12 Acute corneal HSV-1 infection provokes a potent influx of innate leukocytes including inflammatory monocytes, macrophages, natural killer (NK) cells, and neutrophils (PMNs). Of these responders, inflammatory monocytes contribute to virus clearance during the first 48 hours post infection (pi).8 Conversely, current evidence suggests that PMNs are not responsible for HSV-1 clearance.13,14 Resident and perhaps infiltrating dendritic cells also respond to HSV-1 infection or regulate the activity of other leukocytes in the cornea.15 In addition to resident dendritic cells, an often overlooked bone marrow–derived cell residing in the mucosa of the external eye is the mast cell (MC).16 Mast cells are long-lived radioresistant granulocytes strategically localized in tissues exposed to the external environment to facilitate pathogen surveillance.17,18 Mast cells express an armament of innate sensors to accommodate this protective role.18 Upon stimulation, MCs undergo rapid degranulation of cytoplasmic granules containing preformed proteases and potent vasoactive mediators including histamine and TNFα . Following degranulation, MCs produce many soluble factors de novo to fine-tune subsequent innate and adaptive responses.17,18 The scope and functional diversity of immunomodulatory mast cell granule products have recently been reviewed by Wernersson and Pejler.19 Though the activities attributed to MCs in general are broad, it is recognized that the MC phenotype and products are regulated in a niche- and stimulus-specific manner.19,20 Mast cells have been implicated in the maintenance of immune privilege and contribute to pathogen surveillance and defense in various anatomic sites including the skin, lung, bladder, and gut.18,19,21,22 In the human host, MCs reside in an organized fashion within the eye-associated lymphoid tissue including the lacrimal gland, limbus, and conjunctiva.23–25 Whereas the contributions of ocular MCs are appreciated in IgE-mediated allergic conjunctivitis, they have not been investigated with respect to pathogen defense in the eye. In the present article, we hypothesized MCs are involved in the innate immune response to ocular HSV-1 infection due to their proximity to the cornea and immunomodulatory potential. Mice deficient in MCs were found to be more sensitive to HSV-1 infection, but counterintuitively exhibited greater corneal edema. Collectively, our results underscore a novel role for MCs in the eye as an innate sentinel against acute HSV-1 infection by promoting virus clearance and limiting inflammation to preserve the visual axis. We also found that MCs serve as a type of gatekeeper in the corneal vasculature. Imbalances in eicosanoid mediators are implicated in increased corneal PMN infiltration and inflammation in KitW-sh mice. Neutrophils were identified as surrogates for HSV-1 replication using in vivo, in vitro, and adoptive cell transfer approaches. Lastly, we establish that myeloid-derived suppressor cells (MDSC) do not compromise the use of KitW-sh mice in our model investigating the impact of MCs on the innate and early adaptive immune response to acute HSV-1 infection.