THE DISTRIBUTION OF IMMUNE CELLS IN THE UVEAL TRACT OF THE NORMAL EYE PAUL G. McMENAMIN Perth, Western Australia SUMMARY function of these cells in the normal iris, ciliary body Inflammatory and immune-mediated diseases of the and choroid. The role of such cell types in ocular eye are not purely the consequence of infiltrating inflammation, which will be discussed by other inflammatory cells but may be initiated or propagated authors in this issue, is not the major focus of this by immune cells which are resident or trafficking review; however, a few issues will be briefly through the normal eye. The uveal tract in particular considered where appropriate. is the major site of many such cells, including resident tissue macro phages, dendritic cells and mast cells. This MACRO PHAGES review considers the distribution and location of these and other cells in the iris, ciliary body and choroid in Mononuclear phagocytes arise from bone marrow the normal eye. The uveal tract contains rich networks precursors and after a brief journey in the blood as of both resident macrophages and MHe class 11+ monocytes immigrate into tissues to become macro­ dendritic cells. The latter appear strategically located to phages. In their mature form they are widely act as sentinels for capturing and sampling blood-borne distributed throughout the body. Macrophages are and intraocular antigens. Large numbers of mast cells professional phagocytes and play a pivotal role as are present in the choroid of most species but are effector cells in cell-mediated immunity and inflam­ virtually absent from the anterior uvea in many mation.1 In addition, due to their active secretion of a laboratory animals; however, the human iris does range of important biologically active molecules such contain mast cells. Small numbers of what are as cytokines, reactive oxygen intermediates and nitric presumed to be trafficking lymphocytes are present in oxide, they play a role in other processes including the uveal tract of normal eyes. There is little data immune regulation, tissue reorganisation and angio­ available on the presence or absence of eosinophils. genesis.I The heterogeneous nature, function, mor­ The role of these various cell types in immune phology and immunophenotype of the monocyte/ homeostasis and ocular inflammation is briefly con­ macrophage series in different tissue microenviron­ sidered. ments is well established? Cells recognised as belonging to the 'mononuclear-phagocyte system' Local and systemic inflammatory and immune­ include Kupffer cells (liver), mesangial cells (kidney), mediated disorders affecting the eye are propagated alveolar macrophages (lung), microglia (brain par­ not only by newly infiltrating inflammatory cells but enchyma and retina) and peritoneal macrophages. also by the complement of immune cells normally Macrophages undergo a complex process of activa­ present in non-inflamed ocular tissues. In addition, tion driven by molecular inductive signals, e.g. immune cells in the normal eye almost certainly play interferon-gamma (IFN'Y), usually at sites of inflam­ an important role in local immunological homeo­ mation. The activation process particularly involves stasis. Several types of immune cells, such as 'young' or newly recruited macrophages (responsive macrophages, dendritic cells, mast cells, lymphocytes macrophages). Resident tissue macrophages are and eosinophils, normally reside for various periods poorly responsive to such signals.1 Fully activated during their life cycle in peripheral tissues. The major macrophages are capable of effecting a range of aimof this review will be to consider the current state destructive powers specifically targeted towards of knowledge on the distribution, phenotype and certain foreign organisms or tumour cells. An important, although less well understood aspect of Correspondence to: P. O. McMenamin. Department of Anatomy and Human Biology, The University of Western macrophage biology is the nature of the signals Australia. Nedlands (Perth) 6907 Western Australia. responsible for suppressing activation. Activated Eye (1997) 11, 183-193 © 1997 Royal College of Ophthalmologists 184 P. G. McMENAMIN , 2 pm IMMUNE CELLS IN THE UVEA 185 macrophages may express low/moderate amounts of Macrophages in the Ciliary Body major histocompatibility complex (MHC) class II Immunohistochemically stained wholemounts have molecule and thus may play a role in antigen revealed that macrophages are distributed in the presentation in secondary immune responses? connective tissue stroma of the ciliary body and ciliary processes in the rat and mouse eye (Fig. 3b). Macrophages in the Iris Laser scanning confocal microscopy, which provides Classical histological and clinical studies described 'optical sections' of the thick ciliary processes, is a accumulations of large, heavily pigmented cells in the valuable adjunct to conventional light microscopy in iris stroma close to the pupil margin, and near the iris these studies. The relationship between macrophages base (Fig. l), which were assumed to be of and nerve fibres in dual immunofluorescent stained neuroepithelial origin.4 Electron microscopic studies5 preparations is currently being investigated in our revealed that there were two types of cells: type II laboratory (Fig. 5a-c). Specific investigations of the clump cells, considered to display the morphological distribution of macrophages in the normal human features of neuroepithelial cells, were rare; and type I ciliary body are rare; however, studies by the author cells, the predominant type, were considered to be support the observations in rodents and suggest the melanin-containing macrophages (Fig. 2). More human ciliary body is richly endowed with resident recently the use of specific anti-macrophage mono­ tissue macrophages (Fig. 6). clonal antibodies (mAbs: EDl, ED2 and ED3) has revealed that the normal rat iris contains a rich Macrophages in the Choroid network of resident tissue macrophages whose Like the anterior uveal tract the connective tissue density and distribution is best appreciated in stroma of the normal rodent choroid possesses a rich immunostained lflS wholemount preparations population of resident tissue macrophagesy,12 The (Fig. 3).6,7 Prior to the use of wholemounts there choroid in the rat and mouse eye, which is only had been only a limited amount of data on the 20-30 f..lm thick in places (Fig. 7), can be processed presence of macrophages in the iris stroma of the rat8 for immunostaining as wholemounts either with the and mouse eye.9 Resident tissue macrophages in the sclera attached (A. Kijlstra, personal communica­ normal iris display mixed morphological appearances tion) or as a separate layer dissected free from the ranging from pleiomorphic, bipolar to mildly dendri­ sclera (Fig. 8). Such methods have revealed rich form (Fig. 3b) and are distributed in a largely networks (approximately 600 cells/mni) of largely perivascular and perineural manner similar to many perivascular resident tissue macrophages throughout other populations of resident interstitial macro­ the choroid which are EDl +, ED2+ and ED3+ in the phages. Quantitative studies in rodents6,7,lO indicate ratll,12 and SER4+ and F4/80+ in the mouse (Fig. 8). they are present at densities of 700-800 cells/min2. The remarkably rich networks of resident tissue To the author's knowledge there are no studies of macrophages in the anterior and posterior uveal tract macrophages in human eyes using comparable are likely to perform a variety of functions. In investigative approaches. Immunostaining studies in common with other populations of interstitial macro­ humans are complicated by the presence of melano­ phages it is likely these cells are involved in cytes; however, some preliminary immunofluores­ phagocytosis of tissue debris, tumour cells and cence data (Fig. 4a) and conventional micro-organisms (see review!3). The pattern of immunohistological studies (Fig. 4b) confirm the distribution of the resident tissue macrophages presence of a similar network of dendriform and close to vascular beds suggests a sentinel or guardian pleiomorphic resident macrophages located through­ role at the blood-tissue interface. Whether uveal out the iris stroma. tract macrophages possess CDl4 and thus the Fig. I. Light micrograph of a human iris wholemount revealing the 'clump cells' (arrowheads) near the pupil margin (P). Note the posterior iris pigment epithelium has been brushed of! Fig. 2. Transmission electron micrograph of a large melanin-containing macrophage characterised by large complex phagolysosomes, ruffled cell border and eccentric nucleus. A melanin granule in the process of being phagocytosed is identifiable (arrowhead). Fig. 3. (a) Upper panel shows a frozen section of the rat anterior segment stained with ED2 mAb (anti-resident-tissue tnacrophages). Note that only rare cells can be identified in the iris and ciliary body using this method. The diagram in the lowerpanel illustrates the view of the iris and ciliary body obtained by preparing wholemounts. (b) Iris wholemount from a normal rat eye stained with ED2. Compare the numbers of cells in (b) with those in the upper panel in (a) CP, ciliary processes. Fig. 4. (a) Immunofluorescence (PM2K, anti-human macrophage antibody) of human iris wholemount revealing pleiomorphicand dendriform immunopositive cells (arrowheads). (b) Conventional frozen section of the human iris stained with PM2K and visualised with a red chromogen, to display the resident tissue macrophages in