Ccl2 But Not Cx3cr1 Is Involved In The Turnover Of Retinal Microglia In Irradiated Mouse

Abstract

Purpose: : We have shown previously that a total turnover of retinal microglia occurs in mice within 6 months after full-body irradiation. The molecular mechanism that controls bone marrow (BM)-derived cells trans-endothelial migration at the blood-retina barrier following irradiation, however, remains unknown. In this study, we investigated the role of the CC chemokine CCL2 and the CX3C chemokine receptor CX3CR1 in the homing of BM-derived cells into the retina in this setting.

Methods: : Three months old ccl2^-/- or wild type (WT) C57BL/6 mice were lethally irradiated with gamma rays (10 Gy). To test the role of CCL2 on the recruitment of BM-derived cells, irradiated mice (CCL2^-/- and WT) were transplanted with 1×10⁷ BM cells from eGFP-actin transgenic mice. To test the role of CX3CR1 on the recruitment of BM-derived cells, irradiated WT mice were transplanted with BM cells from either eGFP-actin transgenic mice or Cx3cr1^gfp/gfp mice. At different days, animals were sacrificed and eyes collected for immunohistochemical investigation. Chemokine gene expression in the retina of irradiated mice was examined by real-time qRT-PCR.

Results: : Deletion of ccl2 or cx3cr1 did not affect the reconstitution of the immune system following BM transplantation. Full-body irradiation resulted in a significant upregulation of chemokine CCL2 in the retina. Six months post-BM transplantation, all Iba-1⁺ cells were eGFP⁺ in the retina from WT mice, suggesting a total turnover of microglia. In ccl2^-/- recipient mice, however, only cells in the juxtapapillary and peripheral marginal retinal areas were Iba-1 and eGFP dual positive. The majority of the eGFP cells in other retinal area (equatorial area) had a round, small or amoeboid shape and were negative for Iba-1 staining, and they were located in the ganglion cell layer. Intravascular eGFP⁺ cells were detected in the inner- and outer-plexiform layers of the retina. In WT recipient mice that received BM cells from either the eGFP-actin transgenic mice, or CX3CR1^gfp/gfp mice, the numbers of GFP⁺ cells migrated into the retina were similar between different donor groups. The cells were distributed in both the inner- and outer-plexiform layers of the retina.

Conclusions: : CCL2 but not CX3CR1 is involved in the recruitment of BM-derived cells into the neuroretina in mice with full-body irradiation.

Keywords: microglia • radiation damage: light/UV • retina