Macrophages in the Iris Ciliary Body and Choroid

Conventional histological and ultrastructural studies revealed macrophages in the human iris stroma to early microscopists. In particular, their propensity to phagocytose melanin shed from iris pigment epithelium throughout life make them a characteristic feature of histological preparations of human eyes where some were thought to represent a subpopulation of "Clump cells" (61). More recently, studies of normal rat and mouse iris, ciliary body, and choroid revealed rich networks (~600-800 cells/mm2) of RTM (58,62-65) (Figs. 4A,5A,6A). In the rat these cells are CD68+ (ED1+) CD163+ (ED2+) and CD 169+ (ED3+) and in the mouse are F4/80+, CD 169+ (SER4)+, CD11b+58. The success of these studies was due in part to the combination of a wholemount approach to immunostaining (66). The layered structure of the eye particularly lends itself to this approach, which is used extensively in dermatological retinal neurobiological research. The RTMs of the uveal tract display a largely perivascular distribution, suggesting a guardian role at the blood-tissue interface (Fig. 6A). In light of the role of macrophages in mediating innate immune responses by detecting exogenous microbial stimuli via TLRs, mannose receptors, and CD14 (vide supra), this location close to the fenes-trated vascular beds of the ciliary body and choroid (but not the iris) would strategically place them as a first line of detection to blood-borne pathogen invasion within the eye.

Figure 4 (A) Immunoperoxidase staining of frozen sections of human ciliary processes with an anti-macrophage marker revealing numerous macrophages (*) in the stroma. (B) Rat ciliary process wholemount stained with anti-MHC class II mAb revealing the high density of dendriform stained cells (putative DCs), whose ramifications are finer than those in the iris.

Figure 4 (A) Immunoperoxidase staining of frozen sections of human ciliary processes with an anti-macrophage marker revealing numerous macrophages (*) in the stroma. (B) Rat ciliary process wholemount stained with anti-MHC class II mAb revealing the high density of dendriform stained cells (putative DCs), whose ramifications are finer than those in the iris.

Recent data from our laboratory expands our earlier functional analysis of resident macrophages within the iris (59) and provides insight into the roles of macrophages in primary and secondary immune responses. Several groups have shown that activated macrophages associated with body cavities or mucosal surfaces secrete a range of soluble mediators that inhibit T-cell proliferation (67-69). Using in vitro assays of freshly isolated iris macrophages we noted that they exhibited a functional phenotype that lacked lymphocytostatic properties, but possessed the ability to ingest, process, and effectively present soluble Ag to Ag-specific T cells (70). This functional data suggests that iris macrophages, unlike mucosal or "body cavity" macrophages, do not produce nitric oxide in response to T cell-derived signals, such as IFN-y. This may be due to their exposure to high concentrations of TGF-^ and/or calcitonin gene related peptide (CGRP).

Figure 5 (A) Mouse iris stained with anti-CD169 mAb revealing the network of macrophages. (B) Mouse iris stained with anti-MHC class II mAb to reveal the DC network.

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