Sex-specific neurological consequences of neurotropic influenza A virus infections

Influenza A virus (IAV) infection can increase the risk of neuroinflammation, potentially increasing the risk for neurodegenerative diseases. Some IAV strains, such as avian H7N7 subtype, are neurotropic and capable of directly invading and infecting neurons and glia cells. Sex is a factor influencing the severity of IAV infections. Numerous studies indicate that females of the reproductive age mount stronger innate and adaptive immune responses to IAVs compared to males, leading not only to a higher morbidity and mortality but also potentially increasing the risk for neuronal damage. A better understanding of IAV neurotropism and the mechanisms, leading to deleterious neurological manifestations in a sex-specific manner, is crucial. To explore sex-specific responses, primary hippocampal triple co-cultures derived from male and female mice were utilized and infected with the recombinant neurotropic H7N7 strain rSC35M. The results show a stronger interferon-β and pro-inflammatory chemokine secretion in female-derived cultures after neurotropic IAV infection. Sex-specific differences in microglia activation upon infection were observed which resulted in a transition into a hyper-ramified state in both sexes and an increase in amoeboid-shaped microglia only in male-derived cultures. These differences were reflected in alterations observed in neurons. Only females-derived neurons showed an increase in spine density 24 h after IAV infection. Interestingly, this is correlated to sex-specific differences in synaptic pruning mediated via microglia cells. Amoeboid-shaped microglia seem to exhibit a preference for postsynaptic terminals, thereby potentially mitigating neuronal hyperexcitability. In contrast, hyper-ramified microglia might fulfill different tasks via targeting presynaptic terminals and by this means limiting viral spread. Infection triggered strong secretion of CCL3 and CCL5 in both sexes, reflecting widespread neuroimmune activation. Notably, the cognate chemokine receptor CCR5 was selectively upregulated in neurons, with earlier induction in males and a delayed response in females. Mechanistically, these data implicate a CCL3/CCL5–CCR5 cascade engaging mTOR, which suppresses autophagy and promotes neurodegeneration. Phosphorylated tau (pTau) accumulated in a sex- and region-dependent pattern: female neurons showed rapid nuclear pTau elevation by 6 h, whereas male neurons exhibited delayed accumulation by 72 h. Total tau levels declined in both sexes, possibly indicating a compensatory reduction in tau burden. These findings suggest that neurotropic IAV infection activates sex-specific neuroimmune pathways converging on the CCL5-CCR5-mTOR axis, leading to autophagic dysfunction and tau pathology. The accelerated tau phosphorylation in female neurons mirrors clinical observations of greater tau accumulation in women with Alzheimer’s disease. These insights underscore the importance of considering sex as a biological variable in understanding virus-driven neurodegenerative processes and may offer a mechanistic link between viral invasion of the central nervous system and Alzheimer’s-like disease progression.