Sex-specific immune-brain coupling in hippocampal circuits and Alzheimer's disease vulnerability.

Women face twice the lifetime risk of Alzheimer's disease (AD) compared with men. Inflammatory burden is widely implicated in AD, but whether sex-differentiated vulnerability is determined by cytokine levels or by the brain's sensitivity to those signals remains unclear. Here, we examined sex-specific associations between peripheral inflammatory markers, medial temporal lobe (MTL) white matter microstructure using NODDI diffusion MRI, and hippocampal-dependent memory in 121 cognitively unimpaired older adults. Women exhibited widespread alterations in MTL white matter microstructure relative to men. In women, peripheral cytokines (TNFα, IL-10) were associated with microstructural variation in hippocampal-prefrontal tracts, whereas these associations were absent in men. An indirect association between TNFα and memory through hippocampal cingulum neurite density was observed only in women. Despite these microstructural and coupling differences, cytokine levels were comparable between sexes, indicating that risk may be better captured by the coupling between peripheral immune signals and circuit integrity than by cytokine levels alone. These findings identify a sex-specific pattern of immune-brain coupling that reframes inflammatory risk in AD as a property of circuit-level sensitivity rather than overall inflammatory burden.