Sialoadhesin expression in intact degenerating retinas and following transplantation

Resident microglial cells normally do not express sialoadhesin (Sn; a sialic acid-binding receptor), whereas recruited inflammatory macrophages have been shown to do so. The expression of Sn was examined in the course of photoreceptor cell degeneration and after transplantation.

Sn expression was analyzed in retinas of rd1 and rds mice. For transplantation studies, neonatal (P2) retinal cells derived from GFP mice were injected intraocularly in adult rd1 mice and control mice. Antibodies recognizing different Sn epitopes, CD11b, and MHC-II were used to identify activated microglial cells in intact retinas and 21 days after transplantation.

In rd1 mice, a few CD11b-positive cells were observed in the outer nuclear layer in the central retina at postnatal day (P)11 and in increasing numbers between P12 to P21. In rds mice, CD11b-expressing cells were found from P16 onward. No Sn-expressing cells were observed within the rd1 or rds mouse retinas at any of the ages examined (up to P150). Specific staining was observed only in cells found in the vitreous margin of the retina and in surrounding tissues (sclera, cornea, ciliary body, choroid). After transplantation to normal and rd1 mice, a variable number of Sn-positive cells were detected within the grafts, in the graft-host interface, and in the subretinal space.

The significant activation of microglia/macrophages observed in the various stages of degeneration in rd1 and rds mouse retinas is not accompanied by Sn expression. However, Sn-expressing cells are observed after transplantation. The occurrence of such cells could be of significance for the integration and long-term survival of retinal grafts, as the expression of Sn could facilitate other phagocytic receptors.