The role of myeloid cell heterogeneity during spontaneous choroidal neovascularization in Vldlr knockout mice.

Background: Myeloid cells are heterogeneous cells that are critical for spontaneous choroidal neovascularization (CNV) in the Vldlr^-/- mouse model. However, the specific myeloid cell subtype necessary for CNV remains unknown.

Methods and results: To investigate the role of monocytes, we bred Ccr2^-/- and Nr4a1^-/- mice into the Vldlr^-/- background. We found that Ccr2 and Nr4a1 deficiency had no effect upon macrophage counts, CNV lesion number, or total CNV area. Next, we investigated the role of microglia by generating Vldlr^-/-Tmem119^CreER/+Rosa26^DTR/+ mice. Diphtheria toxin (DT) treatment reduced macrophage counts at CNV lesions and CNV lesion number, but did not affect total CNV lesion area. To target microglia via a second strategy, we generated Vldlr^-/-Cx3cr1^CreERCsf1r^iDTR mice and treated them with a single low dose of tamoxifen to target microglia without affecting choroidal macrophages. DT treatment in Vldlr^-/-Cx3cr1^CreERCsf1r^iDTR mice decreased macrophage counts at CNV lesions and CNV lesion number but again had no effect upon total CNV lesion area. To target choroidal macrophages and microglia, we treated Vldlr^-/-Cx3cr1^CreERCsf1r^iDTR mice with 9 tamoxifen treatments. DT-treated mice showed dramatic reductions in macrophage counts, CNV number, and total lesion area.

Conclusions: These data suggest that monocytes and monocyte-derived macrophages are dispensable, microglia are likely initiators for CNV development, and choroidal macrophages are potential key contributors to CNV growth and/or maintenance in the Vldlr^-/- model.