We showed, for the first time, that IPE expresses a functional LPS receptor complex and, independent of other cell types, secretes proinflammatory mediators when exposed to LPS. The cytokines released by IPE (IL-6, CXCL8, CXCL10, CCL2, CCL4, and CCL5) are similar to those present in the aqueous of patients with AAU
1,43 suggesting that IPE is a source of cytokines during active disease. However, a lack of measurable IL-1β and TNF-α from our culture supernatants is unexpected, given that these cytokines are known to be released on NF-κB activation downstream of LPS stimulation. Certainly, in animal models of EIU, others have detected IL-1β and TNF-α from ocular tissues and serum.
44,45 One explanation may be that unlike immune cells, LPS-stimulated ocular pigment epithelium do not produce IL-1β or TNF-α. Supporting this view, Leung et al.
46 also failed to detect IL-1β or TNF-α in culture supernatants of LPS-stimulated primary RPE and ARPE-19. Others have reported that IPE and RPE do not express TNF-α transcript
39 and preferentially produce IL-1α rather than IL-1β.
47 Our observations may also reflect a limitation of the in vitro model, since gene expression of cultured RPE cells differs from that of native (laser-captured) RPE.
48 Finally, insufficient sensitivity of IL-1β and TNF-α assays may be another explanation, since detection of these cytokines has been inconsistent in clinical studies on uveitis. One group reported IL-1β and TNF-α to be below detection limits in the aqueous of patients with active uveitis,
43 whereas another detected both cytokines in the aqueous of patients with idiopathic uveitis, where concentrations did not differ significantly from noninflamed cataract control aqueous.
49 In children with uveitis, elevated aqueous TNF-α was reported, but IL-1 concentrations did not differ between uveitis and control groups.
50 More clinically relevant cytokines would be CXCL8 and CCL2, given their early appearance in the aqueous and correlation with disease severity,
51,52 whereas no correlation was observed for TNF-α.
52 Therefore, although IPE may respond to LPS with a limited set of cytokines, those produced are clinically important in dictating disease severity.