Pay more attentions to the complexity of the resident immune cells in ocular surface

Ocular surface is a special interface between the inner ocular tissue and the external environment.It provides effective physical and biological barriers to prevent environmentally harmful substances and pathogenic microbes invading into the eye.This protection is afforded by unique local anatomy and cellular components, especially the resident immune cells such as innate lymphoid cells(ILCs), macrophages, mast cells, Langerhans cells(LCs) and γδ T cells.Recent studies reinforce the importance of LCs and mast cells as inducers of immune tolerance.Most adult tissue macrophage populations are seeded before birth and maintained in the steady state independently of monocytes from blood.However, under inflammatory condition, some transient monocytes enter into tissue and become macrophage compartments.Monocytes are subdivided into two main phenotypically and functionally distinct subsets.The first main subset is dedicated to the surveillance of endothelial integrity.The second main subset compasses classical monocyte functions such as rapid migration to the sites of injury and the replenishment of peripheral dendritic cells and macrophage compartments.In addition, newly identified ILCs are founded in the ocular surface tissues.These cell groups defens the different stimulus from external environment by producing different cytokines and growth factors.Accumulating data highlight some key roles of tissue-resident immune cells in ocular surface homeostasis and pathology.