Ocular surface immune cell diversity in dry eye disease

Dry eye disease (DED) is a multifactorial chronic ocular surface inflammatory condition. Disease severity has been directly related to the immuno-inflammatory status of the ocular surface. Any perturbation in the orchestrated functional harmony between the ocular surface structural cells and immune cells, both resident and trafficking ones, can adversely affect ocular surface health. The diversity and contribution of ocular surface immune cells in DED have been of interest for over a couple of decades. As is true with any mucosal tissue, the ocular surface harbors a variety of immune cells of the innate-adaptive continuum and some of which are altered in DED. The current review curates and organizes the knowledge related to the ocular surface immune cell diversity in DED. Ten different major immune cell types and 21 immune cell subsets have been studied in the context of DED in human subjects and in animal models. The most pertinent observations are increased ocular surface proportions of neutrophils, dendritic cells, macrophages, and T cell subsets (CD4+; CD8+; Th17) along with a decrease in T regulatory cells. Some of these cells have demonstrated disease-causal association with ocular surface health parameters such as OSDI score, Schirmer's test-1, tear break-up time, and corneal staining. The review also summarizes various interventional strategies studied to modulate specific immune cell subsets and reduce DED severity. Further advancements would enable the use of ocular surface immune cell diversity, in patient stratification, i.e. DED-immunotypes, disease monitoring, and selective targeting to resolve the morbidity related to DED.