Contribution of microglia as passenger leukocytes to the fate of intraocular neuronal retinal grafts.

PURPOSE

To determine whether donor-derived microglial cells play a role in dictating the immunogenicity of immature neuronal retinal tissue transplanted intraocularly.

METHODS

Neonatal neural retinas (aged <24 hours) from C57BL/6 or BALB/c mice were implanted in the anterior chamber or the subretinal space of adult syngeneic or allogeneic eyes. After 12 and 35 days of engraftment, retinal grafts were harvested and analyzed immunohistochemically with 20 microg/ml Griffonia simplicifolia (GS) isolectin to identify microglia and define their morphology, monoclonal antibodies to study expression of donor and recipient major histocompatibility complex (MHC) class I and II antigens, and anti-CD3 and -CD14 antibodies to distinguish microglia from T cells and macrophages.

RESULTS

Neonatal retinas were found to contain significant numbers of GS+ cells (microglia) at the time of grafting. By day 12 after grafting, markedly increased numbers of microglia were found in syngeneic and allogeneic grafts. Whereas most microglia in syngeneic grafts displayed a ramified (inactive) morphology at this time, most of the microglia in allografts displayed an ameboid (activated) configuration, with retracted processes and enlarged somas. By day 35 after grafting, although the density of microglia was reduced in syngeneic and allogeneic grafts, intensely labeled GS+ cells were localized in the centers of rosettes in syngeneic, but not in allogeneic, grafts. Instead, donor-derived microglia displayed intense expression of MHC class I and II antigens, and these grafts contained small numbers of recipient-derived T cells, but not macrophages.

CONCLUSIONS

Microglia within developing neuronal retinal transplants display morphologic features that are consistent with the ability to function as "passenger leukocytes," and they distribute themselves within rosettes as though performing surrogate support functions usually adopted by retinal pigment epithelial cells. Because this latter property causes activation of the microglia, it may also cause these cells to enhance the immunogenicity of the allograft.