CNS cGVHD is associated with resident microglia proliferation and activation and donor-derived macrophage infiltration. (A) Allogeneic transplantation model for the delivery of 5 × 10⁶ TCD BM from C57Bl/6 (H2^b) donors expressing eGFP under the Csf1r promoter (B6.Csf1r-eGFP) with no T cells or with 0.5 × 10⁶ CD3⁺ T cells into lethally irradiated B6D2F1 (H2^b/d) recipients. (B-F) Representative confocal images of parenchymal brain regions of interest in GVHD and TCD mice, showing Iba1⁺ microglia/macrophages, GFP⁺ donor BMDMs, and merged images counterstained with 4’,6-diamidino-2-phenylindole (DAPI; cell nuclei). Green arrows identify donor-derived (GFP⁺) macrophages, and white arrows indicate resident (GFP⁻) microglia. Stereological quantification of each region shows the total Iba1⁺ cell population alongside the number of GFP⁺ donor BMDMs. (B) Meninges at days 35 and 100 after transplant. Original magnification ×40; scale bar, 30 μm. (C) Choroid plexus at days 35 and 100 posttransplantation. Original magnification ×40; scale bar, 30 μm. Dashed lines outline choroid plexus in the lateral ventricle adjacent to surrounding tissue. (D) Cortex at days 35 and 100 after transplant. Original magnification ×20; scale bar, 50 μm. (E) Hippocampal dentate gyrus at days 35 and 100 after transplant. Original magnification ×20; scale bar, 50 μm. (F) Habenula at days 35 and 100 after transplant. Original magnification ×40; scale bar, 30 μm. Data presented as mean ± standard error of the mean (n = 4-5 mice per group). Statistical differences calculated with unpaired Student t test. *P < .05, **P < .01, ***P < .001, ****P < .0001.