FcγR- and CD9-dependent synapse-engulfing microglia in the thalamus drive cognitive impairment following cortical brain damage in mice.

Chronic neuroinflammation gives rise to diverse microglial states across the brain, yet how region-specific microglial remodeling contributes to cognitive dysfunction remains unclear. Here we report that synapse-engulfing microglia in the thalamus drive cognitive impairment after cortical brain damage in mice, primarily studied in females. Region-specific manipulations of microglia during the chronic phase show that reactive microglial changes in the thalamus, but not in the hippocampus, impair recognition memory. Single-cell RNA sequencing reveals an enrichment of synapse-engulfing CD9hi microglia in the thalamus. Antibody-based CD9 blockade in the thalamus, as well as microglia-selective CD9 disruption, rescues thalamic synaptic loss, restores neuronal activity, and improves recognition memory. Further analysis shows that the blood-brain barrier disruption and subsequent γ-immunoglobulin (IgG) extravasation facilitate the generation of CD9hi microglia in an Fcγ receptor III-dependent manner. These findings demonstrate that the induction of synapse-engulfing CD9hi microglia in the thalamus by IgG/FcγRIII signaling drives recognition memory deficits following cortical damage.