Efferocytosis by Macrophages Attenuates Inflammatory Responses Following Ultraviolet B-Induced Apoptosis in Corneal Stromal Cells

PURPOSE: To investigate the role of efferocytosis in maintaining corneal immune homeostasis after ultraviolet B (UVB)-induced keratocyte apoptosis and its impact on inflammatory responses in both in vitro and in vivo settings. METHODS: Human corneal stromal fibroblasts (HCFs) were exposed to UVB radiation (150 mJ/cm²) to induce apoptosis and co-cultured with M1 macrophages using a transwell system. In this in vitro efferocytosis model, UVB-irradiated HCFs (BHCFs) were evaluated for efferocytosis-related markers, including milk fat globule epidermal growth factor 8 (MFG-E8) and MER proto-oncogene, tyrosine kinase (MERTK), as well as inflammatory cytokines such as interleukin (IL)-1β and IL-6. Paracrine effects on nearby M1 macrophages were assessed by analyzing changes in cytokine profiles and expression of myeloid/macrophage markers. To validate the physiological relevance of these findings, an in vivo mouse model was established by subconjunctival injection of clodronate liposomes in UVB-exposed mice to reduce corneal macrophages. RESULTS: BHCFs showed higher TUNEL positivity and significantly more efferocytosis when co-cultured with M1 macrophages. This was accompanied by upregulation of MFG-E8 and MERTK and downregulation of IL-1β and IL-6. Microenvironmental M1 macrophages exhibited reduced IL-1β, increased transforming growth factor beta 1, and downregulation of CD14, CD68, CD80, and CD11c. In vivo macrophage reduction impaired Mertk activation and failed to suppress IL-1β upregulation in the cornea. CONCLUSIONS: Efferocytosis contributes to corneal immune homeostasis after UVB-induced apoptosis by resolving inflammation and modulating macrophage phenotype. These findings support the existence of an efferocytic mechanism in the corneal stroma.