The expression pattern of toll-like receptor (TLR) and cytokine production to TLR agonists in human retinal pigment epithelial cells

Retinal pigment epithelium (RPE) constituting the outer blood-retina barrier plays an important role in ocular defense mechanism. Many studies reported that RPE participates in ongoing immune responses in the retina. However, the exact mechanism is still uncertain. Toll-like receptors (TLRs) participate in the recognition of pathogen-associated molecular patterns (PAMP), such as LPS, zymosan, lipoprotein, and dsRNA. The expression and function of TLRs in human RPE have not been established. In this study, we investigated TLRs expression in human fetal RPE and their recognition of PAMP to determine how human RPE participates in ocular defense mechanism against microbial component. RT-PCR and real time PCR revealed that TLR1 through 5 were constitutively expressed in human fetal RPE, and their expressions were slightly increased by LPS. We deteimined the TNF-α, TL-6, and IL-8 expression in human fetal RPE after treatment with LPS, zymosan, petidoglycan, or poly I:C. RT-PCR demonstrated that LPS and poly I:C treatment increased the production of TNF-α, IL-6, and IL-8 in human fetal RPE. LPS showed more potent effects on TNF-α and IL-8 production. Peptidoglycan and zymosan did not induce the production of TNF-α. CD14, the co-receptor of LPS was weakly expressed and functioned in recognizing LPS in human fetal RPE. These results suggest that human RPE may participate in ocular defense mechanism against microbial component through toll-like receptors.