|Year : 1983 | Volume
| Issue : 5 | Page : 486-492
Ocular cells as phagocytes-(A brief review)
Department of Ophthalmology, University of Toronto, Canada
P K Basu
Department of Ophthalmology, University, of Toronto, 1 Spadina Crescent, Toronto, Ontario, M5S, 2 J 5,
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Basu P K. Ocular cells as phagocytes-(A brief review). Indian J Ophthalmol 1983;31:486-92
| Introduction|| |
The phagocytes are cells that have the capacity to take up extracellular matters in bulk into their interior. To achieve this, the cells extend portions of their plasma membrane which then surround the mass to be engulfed [Figure - 1]. These cytoplasmic projections meet and fuse forming an endocytic vacuole containing the foreign matters. The vacuole then merge with the lysosomes and their contents mix. Lysosomes are sac-like bodies containing digestive and catabolic enzymes,  (Kavet and Brain, 1980). The polymorphonuclear leucocytes (PMN) and macrophages are specially adapted for defence against microorganisms and other exogenous materials. For this reason they have been called the "professional phagocytes" (Rabinovitch, 1970). Recent studies have shown that other types of cells located in some tissues can also under certain conditions internalize particulate matters by a process resembling phagocytosis. The cells which have been designated as the "non-professional" phagocytes, have been found in different parts of the body 
In the mamalian eye, the non-professional phagocytes have been located in the retinal pigment epithelium ,, the trabecular meshwork and corneal endothelium  the vitreous humour  and conjunctiva  subconjunctival tissue  corneal epithelium  and corneal stromal cells , The reasons for phagocytes of the rod outer segments by the RPE has been understood to some extent  but the reasons for why other ocular cells have the capability of engulfing foreign particles are largely in the realm of speculations.
In this article, I shall briefly review the ocular non-professional phagocytes with special reference to the work currently in progress in my laboratory.
Retinal Pigment Epithelium (RPE)
It has been shown that in the renewal process of the photoreceptors, the ability to phagocytose the shed rod outer segment (ROS) by the retinal pigment epithelial cells in vivo is of considerable importance  That the RPE can engulf other kinds of substances such as the polystyrene microspheres in vivo and in vitro has also been demonstrated ,, Certain forms of retinal degeneration in animals (e.g. the Royal College of Surgeons (RCS) rats) have been related to the impairment of the phagocytosis of the shed ROS by the retinal pigment epithelium. In view of this, it has been postulated that the retinal degeneration associated with retinitis pigmentosa in humans may be due to a similar defect in the RPE of the patients. Rosenstock and coworkers  have reviewed the different types of models that have been devised to study the mechanisms of phagocytosis by the RPE in as attempt to identify factors which control this function. Recently we have developed a simple organ culture model in which the whole tissue in bulk is incubated in a nutrient medium at 37° C [Figure - 2]. This system can be readily and reliably used to assay the RPE phagocytosis quantitatively, and to identify agents that can stimulate or depress phagocytosis.  We have also developed another in vitro model using RPE tissue cultures in which cells outgrown from the target tissue are cultivated in a nutrient medium at 37°C.  Both models have demonstrated that the mamalian RPE can engulf particulate matters and ROS in vitro [Figure - 3]. Our studies ,, showed that certain conditions known to inhibit the phagocytosis by the professional phagocytes can also inihibit the engulfment of particulate matters by the RPE. These include incubation of the RPE in the presence of cold, iodoacetate, colchicine, cytochalasin B and certain antibiotics such as penicillin. In these studies, we also found that certain antibiotic such as garamycin can stimulate the uptake of foreign particles by the RPE cells.  Besides the antibiotics, we have also tested the effect of ethanol and some other agents such as hydrocortisone, indomethacin, sodium salicylate, azathioprine and found that most of these agents inhibit the phagocytosis of latex particles by RPE (unpublished data). From these experiments, it appears that for the internalization of particulate matters, the RPE cells are very similar to the professional phagocytes.
The trabecular meshwork is covered by endothelial cells. The debris (pigment granules, RBC etc.) that accumulate in the anterior chamber, are carried away with the aqueous humour which flows over these cells. It has been shown recently by several workers , by using both in vivo and in vitro models that these endothelial cells have a phagocytic property. It has been speculated that possibly by removing the foreign particles they help to keep the aqueous channels open. Whether or not some forms of glaucoma are related to a defective phagocytic property of these endothelial cells is not known at present.
The greatest concentration of the vitreous cells (hyalocytes) is at the base of the vitreous body and in the vicinity of the optic nerve. Balazs and Delinger  showed that under certain conditions these cells can engulf foreign matters. However, the role of the hyalocytes as phagocytic cells is not yet known. Profession phagocytes have been shown to be able to take up the degenerated collagenous products after their degeneration by the lysosomal enzyme elaborated by the cells.  The role of the hyalocytes in the degradation of the vitreous collagen in health and disease is not yet known. We have been able to cultivate hyalocytes in tissue culture.  We hope it will be possible in the future to study the phagocytic mechanism of these cells by using in vitro models. A human hyalocyte recovered from a vitrectomy sample which has a striking similarity to a macrophage has been shown in [Figure - 4].
Many kinds of bacteria have been seen within the conjunctival cells .As living bacteria can actively penetrate the plasma membrane, the intracellular presence of bacteria is not necessarily an evidence of phagocytosis. Using metabolically inert particles e.g. latex beads, Latkovic and Nilsson  have demonstrated that these particles are indeed phagocytosed by the conjunctival cells following their instillation in the conjunctival sac of living guinea pigs. Recently we have confirmed their in vivo findings [Figure - 5]. We have also seen that under certain conditions even the subconjunctival cells can engulf particles in vivo  Further, using organ culture and tissue culture models  we have found that the rabbit and bovine conjunctival cells can engulf synthetic particles. Control experiments showed that the engulfment of the particles is dependent on an active metabolic process as well as on a physiological temperature. The phagocytosis was inhibited by treating the cells with sodium iodoacetate and by incubating them at 4° C.
It is very difficult to perform an in vivo experiment to test the phagocytic capacity of the corneal epithelial cells by instilling foreign particles on the cornea. This is because the lid movements constantly dislodge the particles from their contact with the cell surface. Consequently, we used the organ culture model previously distributed by Wine and Basu  and also a tissue culture model to study the phagocytic capability of the rabbit and bovine corneal epithelial cells [Figure - 6][Figure 7] Both models showed that corneal cells can avidly take up foreign particles at 37° C. As seen in the case of the conjunctival and RPE cells, the engulfment of the particles by the corneal cells was inhibited when the cells were incubated in the presence of a metabolic poison or cold temperature. We have also found that in comparison to the unstimulated corneal cells, the regenerating corneal cells have a much greater capacity for phagocytosis of foreign matters. In this respect the cells of the conjunctival epithelium and the corneal epithelium behave in an identical way. We speculate that the cells of the cornea and conjunctiva (being exposed to the environment and thereby being always liable to be injured by trauma and other factors) may actively remove debris from the wounds of these tissues. This probably helps in the remoulding of the injured sites during the healing process. It is possible that because of their exposed nature, these cells may have been endowed with a phagocytic property so that they can contribute to the local defence mechanism against invasion by microbes and other particles floating in the atmosphere.
Laude et al  have shown that human keratocytes in vitro can take up polystyrene latex beads as well as tanned red blood cells. They think that the phagocytosis of these keratocytes is involved in the turnover of the corneal stromal matrix as well as in the initial response of this avascular tissue to injury or bacteria infection. Our studies  using the rabbit and bovine keratocytes confirmed their observations.
In conclusion, I have tried to outline the present status of knowledge about the phagocytic capability of the different ocular cells. The phagocytic role of certain cells (e.g. retinal pigment epithelium in health) are more or less understood but that of the other cells are largely unknown. The in vitro experiments do not necessarily reflect the in vivo condition but they are valuable aids for the study of phagocytosis and its control. I believe that these in vitro systems will be increasingly used and that with time we would have a better understanding of the significance of phagocytosis by the different ocular cells in health and disease.
| Acknowledgement|| |
Mr. F. Carve, Mr. S.M. Hasany and Miss R. Basu gave valuable technical assistance.
| Summary|| |
This article reviews briefly the present status of knowledge about the phagocytic capability of cells of various ocular tissues viz. the retinal pigment epithelium, trabecular meshwork, vitreous humour, conjunctiva and cornea. It describes in short some of the in vivo and in vitro models which have been used to study the phagocytic behaviour of the ocular cells. It also outlines some of the factors that are inhibitory or stimulatory to phagocytosis of foreign particulate matters by these cells. The significance of the phagocytic property of some of the ocular cells (e.g. retinal pigment epithelial cells) is known to some extent. However, the significance of phagocytosis by the other kinds of ocular cells is still largely in the realm of speculation. Some of the speculations have been briefly discussed.
| References|| |
Kavet, R.I and Brain, J.D. : Methods to quantitate endocytosis: a review. J. Reticulo endothelial Soc. 201-221., 1980.
Rabinovitch, M. : Phagocytic recognition. Ind Mononuclear Phagocytes. Ed. Van Furth, R., Blackewell Sci. Pub. Oxford, p. 299, 1977.
Young, R.W. and Bok, D. : Participation of the retinal pigment epithelium in the rod outer segment renewal process. J. Cell Biol., 42: 392403, 1969.
Rosenstock, T., Basu, R., Basu, P.K. and Ranadive, N.S., Quantitative assay of phagocytosis by retinal pigment epithelium. An organ culture model. Expt. Eye Res., 30: 710-729, 1980.
Basu, P.K., Carre, F., Hasany, S.M. and Basu Ranjana: Phagocytic capability of the corneal and conjunctival epithelial and stromal cells in vivo and in vitro. Submitted for publication.
Grierson, I. and Chishol, I.A. : Clearance of debris from the iris through the drainage angle of the rabbit,s eye. Brit. J. Ophthalmol., 62: 694-704, 1978.
Blazes E.A. and Denlinger, J.L. : The pharmacology of the vitreous, In: Drugs and ocular at tissues. Ed. S. Dikstein, S. Karger, Basel, pp 524-538, 1977.
Latkovic, S. and Niltson, S.E.G. : Phagocytosis of latex microspheres by the epithelial cells of: the guinea pig conjunctiva. Acta Ophthabnol., 57: 582-590, 1979.
Basu, P.K., Sarkar, P., Carre, F. and Persad, S. Bovine retinal pigment epithelial cells cultured in vitro: growth characteristes, morphology, chromosomes, phagocytosis ability, tyrosinase activity and effect of freezing. Submitted for publication.
Laude M.A., Birk, D.E., Nagpal, M.L. and Rader, R.L. : Phagocytic properties of human keratocyte cultures, Invest. Ophthalmol. Vis. Sci. 20: 481-489, 1981.
Hollyfield, J.G. : Phagocytic capabilities of the, pigment epithelium. Expt. Eye Res., 22: 457-468, 1976.
Markowitz, S., Goldhar, S., Basu, P.K. Ranadive, N.S. Carre, F. and Basu, R. : Enhancement and inhibition of phagocytic activity in retinal epithelium. Can. J. Ophthalmol., In press.
Alvarado, J.A. and Polanski, J.R. : Biological activity of cultured human trabecular cells. Invest. Ophthalmol. Vis. Sci., 18: 241, (Supplement), 1977.
Rowsey, J.J., Nisbet, R.M., Swedo, J.L. and Katono, L. : Corneal collagenolytic activity in rabbit polymorphonuclear leucocytes, J. Ultrastructure Res., 57: 10-21, 1976.
Basu, P.K. and Carref F. : Long term tissue culture of rabbit vitreous cells. Exp. Eye Res., 3: 1-4. 1964.
Zimianski, M.C.; Dawson, C. R. and Togini, B. Epithelial cell phagocytosis of Listeria monocytogenes in the conjunctiva. Invest. Ophthalmol., 13: 623-626; 1974.
Wine, N.A. and Basu, P.K. ; Studies on corneal wound healing: An in vitro approach. Amer. J. Ophthalmol., 58: 253-258, 1964.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6]