Ms4a4a loss preferentially remodels microglia with limited, compartment-specific effects across central and peripheral immune compartments in 5xFAD mice

Background: Genome-wide association studies have identified the MS4A locus as a major modifier of Alzheimers disease (AD) risk and resilience. MS4A4A is enriched in microglia, and we previously demonstrated that loss of Ms4a4a reduces amyloid plaque pathology and attenuates amyloid-associated tau spreading. However, MS4A4A is also expressed in peripheral myeloid populations, raising the possibility that its effects extend beyond brain-resident microglia. Here, we sought to define how constitutive Ms4a4a loss alters immune populations across central nervous system (CNS), CNS-border, and peripheral compartments in the context of amyloid pathology. Methods: We performed multicompartment immune profiling by flow cytometry in 5xFAD Ms4a4a wild-type and 5xFAD Ms4a4a-deficient mice. Myeloid and lymphoid populations were characterized in the brain, dural meninges, skull bone marrow, tibial bone marrow, and spleen. Results: Ms4a4a loss produced its strongest immune effects in brain microglia. 5xFAD Ms4a4a-deficient mice exhibited increased microglial abundance, increased frequency and expression of CD86, increased MHC class II expression intensity, and enrichment of CXCR4-high microglia, whereas P2RY12 and TREM2 were unchanged. Brain macrophages showed increased MHC class II expression, whereas conventional dendritic cells (cDCs) in the brain showed reduced MHC class II expression, with limited changes in other infiltrating immune populations. Immune composition in the dura and skull bone marrow was largely preserved, although MHC class II expression was increased in skull bone marrow cDCs. Peripheral immune composition in tibial bone marrow and spleen was similarly preserved, with selective increases in M2-like macrophages in tibial bone marrow and splenic CD8 T-cell frequency. Conclusions: These findings identify microglia as the immune population most strongly remodeled by Ms4a4a loss in the setting of amyloid pathology. Constitutive Ms4a4a deficiency does not produce broad systemic immune remodeling but instead preferentially alters microglial state, accompanied by more limited, cell type- and tissue-specific effects across other central and peripheral immune compartments. These findings establish MS4A4A as a context-dependent regulator of immune responses to AD pathology with particularly prominent effects on microglia.