AIM: To explore whether efferocytosis impacts ocular surface inflammation in high-iron environment by regulating macrophage polarization.

METHODS: A total of 50 healthy C57BL/6 male mice aged 6-8wk were randomly divided into normal control group, iron group, inhibitor group, enhancer group and solvent control group, with 10 mice in each group. The normal control group was injected intraperitoneally with 0.2mL of normal saline, and the other groups were injected intraperitoneally with 50mg/mL iron dextran of 0.2mL, once every 3d. From the 14d, the inhibitor group, the enhancer group and the solvent control group were injected intraperitoneally with the same volume(0.2mL)50mg/kg XMD8-92, 10mg/kg simvastatin and 50% DMSO solvent once a day, respectively. The anterior segment of the eyes was observed under slit lamp microscope on the 7, 14, 28d after intraperitoneal injection, and the ocular surface inflammation index and corneal fluorescein staining score were evaluated. The cornea, conjunctiva and lacrimal gland tissues were taken at 28d for the HE staining and immunofluorescence staining, and RT-PCR were used to detect the expression of macrophage polarization related indexes(CD86, CD206, iNOS, Arg-1); Western blot were used to detect the expression of efferocytosis related signal factors(Gas6, MerTK); ELISA was used to detect the expression of inflammatory factors(IL-1β, TNF-α, MMP-9).

RESULTS: After injection for 28d, compared with the normal control group, the ocular surface inflammatory index and corneal fluorescein staining score were increased in the iron group and the solvent control group. HE staining showed incomplete corneal epithelium, reduced conjunctival goblet cells, unclear lacrimal gland structure and relatively disordered arrangement of cells. In all tissues, the expressions of polarization related indexes of M1 macrophages such as CD86 and iNOS were up-regulated, while those of M2 macrophages such as CD206 and Arg-1 were down-regulated, and the expressions of inflammatory factors such as IL-1β, TNF-α and MMP-9 were up-regulated(all P<0.05). Compared with the iron group and the solvent control group, the ocular surface inflammation index and corneal fluorescein staining score of the inhibitor group were further increased. HE staining showed obvious exfoliation of corneal epithelium, further decrease or even disappearance of conjunctival goblet cells, disorder of lacrimal gland structure and irregular arrangement of cells. In all tissues, the expression of signal factors related to efferocytosis such as Gas6 and MerTK was down-regulated(all P<0.05), the expression of polarization related indexes of M1 macrophages such as CD86 and iNOS and the expression of inflammatory factors such as IL-1β, TNF-α and MMP-9 were further up-regulated(all P<0.05). But the ocular surface inflammation index and corneal fluorescein staining score decreased in the enhancer group. HE staining showed the integrity of corneal epithelial, the increase of conjunctival goblet cells and the improvement of lacrimal gland structure and morphology. In all tissues, the expression of signal factors related to efferocytosis such as Gas6 and MerTK was up-regulated(all P<0.05), and the expression of polarization related indexes of M2 macrophages such as CD206 and Arg-1 was up-regulated, while the expression of inflammatory factors such as IL-1β, TNF-α and MMP-9 was down-regulated(all P<0.05).

CONCLUSION: High-iron environment induces macrophages polarize to M1, which aggravates ocular surface inflammation and tissue damage. Efferocytosis by regulating the polarization of macrophages impact the occurrence of ocular surface inflammation in high-iron environment.