Donor bone marrow-derived macrophage MHC II drives neuroinflammation and altered behaviour during chronic GVHD in mice.

Graft-versus-host disease (GVHD) remains the leading cause of non-relapse mortality after allogeneic stem cell transplantation for haematological malignancies. Manifestations of GVHD in the central nervous system (CNS) present as neurocognitive dysfunction in up to 60% of patients, however, the mechanisms driving chronic GVHD in the CNS are yet to be elucidated. Our studies of murine chronic GVHD revealed behavioural deficits associated with broad neuroinflammation and persistent Ifng upregulation. By flow cytometry, we observed a proportional shift in the donor-derived T-cell population in the chronic GVHD brain from early CD8 dominance to later CD4 sequestration. RNA sequencing of the hippocampus identified perturbations to structural and functional synapse-related gene expression, together with the upregulation of genes associated with IFN-γ responses and antigen presentation. Neuroinflammation in the cortex of mice and humans during acute GVHD was recently shown to be mediated by resident microglia-derived TNF. In contrast, infiltration of pro-inflammatory MHC class II+ donor bone marrow-derived macrophages (BMDM) was identified as a distinguishing feature of chronic CNS GVHD. Donor BMDM, which comprised up to 50% of the CNS myeloid population, exhibited a transcriptional signature distinct from resident microglia. Recipients of MHC class II knockout bone marrow grafts exhibited attenuated neuroinflammation and behaviour comparable to controls, suggestive of a critical role of donor BMDM MHC class II expression in CNS chronic GVHD. Our identification of disease mediators distinct from those in the acute phase indicates the necessity to pursue alternative therapeutic targets for late-stage neurological manifestations.