Sex differences in hippocampal cytokine networks after systemic immune challenge

Increased production of cytokines in the in the brain during illness or injury modulates physiological processes, behavior, and cognitive function. It is likely that the pattern of cytokines, rather than the activation of any individual cytokine, determines the functional outcome of neuroimmune signaling. Cytokine networks may thus be particularly useful for understanding sex differences in immune and neuroimmune activation and outcomes. In this project, we aimed to determine the activation and resolution of hippocampal cytokine networks in both male and female mice. We measured 32 cytokines in the hippocampus and periphery of male and female mice at rest, 2, 6, 24, 48, and 168 hours after an acute systemic injection of lipopolysaccharide (LPS; 250g/kg). We hypothesized that males and females would exhibit both differences in individual cytokine levels and differences in network dynamics of hippocampal cytokines. Cytokines with sex-specific activation by LPS included male-specific elevations of IFN{gamma}, CSF1, CSF2, and IL-10; and female-specific activation of the IL-2 family and IL-4. We also observed differences in time course, where females showed more rapid elevations, and faster resolution of cytokine activity compared with males. Network analysis using ARACNE and Cytoscape demonstrated markedly different hippocampal cytokine networks across sex even at baseline, and sex differences in cytokine network activation states in response to LPS. Analysis of global shifts in cytokine concentrations further identified a period of cytokine and chemokine downregulation at 48 hours that was more pronounced in females compared with males. Together, these findings demonstrate that sex differences in neuroimmune responses include both differences in intensity of the cytokine response, and importantly differences in cytokine networks activated. Such sex differences in cytokine networks in the brain are likely critical for short and long-term functional outcomes associated with neuroimmune activation.