Microglial activation without peripheral immune cell infiltration characterises mouse and human cerebral small vessel disease

Aims Cerebral small vessel diseases (SVDs) involve diverse pathologies of the brain's small blood vessels, leading to cognitive deficits. Cerebral magnetic resonance imaging (MRI) reveals white matter hyperintensities (WMHs), lacunes, microbleeds and enlarged perivascular spaces in SVD patients. Although correlations of MRI and histopathology help to understand the pathogenesis of SVD, they do not explain disease progression. Mouse models, both genetic and sporadic, are valuable for studying SVD, but their resemblance to clinical SVD is unclear. The study examined similarities and differences between mouse models of SVDs and human nonamyloid SVD specimens. Methods We analysed four mouse models of SVD (hypertensive BPH mice, Col4a1 mutants, Notch3 mutants and Htra1−/− mice) at different stages for changes in myelin, blood‐brain barrier (BBB) markers, immune cell populations and immune activation. The observations from mouse models were compared with human SVD specimens from different regions, including the periventricular, frontal, central and occipital white matter. Postmortem MRI followed by MBP immunostaining was used to identify white matter lesions (WMLs). Results Only Notch3 mutant and hypertensive BPH mice showed significant changes in myelin basic protein (MBP) immunostaining, correlating with MRI patterns. These changes were linked to altered microglial morphology and focal plasma protein staining around blood vessels, without peripheral immune cell infiltration. In human specimens, both normal‐appearing white matter (NAWM) and WMLs lacked peripheral cell infiltration. However, WMLs displayed altered microglial morphology, reduced myelin staining and occasional fibrinogen staining around arterioles and venules. Conclusions Our data show that Notch3 mutants and hypertensive BPH/2J mice recapitulate several features of human SVD, including microglial activation, focal sites of demyelination and perivascular plasma protein leakage without peripheral immune cell infiltration. Human and mouse cerebral small vessel diseases present microglial activation without peripheral immune cell infiltration. The areas of microglial activation coincide with myelin lesions rather than focal extravascular serum proteins. Notch3 mutants and hypertensive BPH/2J mice recapitulate several features of human SVD.