Lauren D. Otto‐Dobos

Following diagnosis but before treatment, up to 30% of breast cancer patients report behavioral side effects (e.g., anxiety, depression, memory impairment). Our rodent mammary tumor model recapitulates aspects of these behavioral sequelae, as well as elevated circulating and brain inflammatory mediators. Neuroinflammation is a proposed mechanism underlying the etiology of mood disorders and cognitive deficits, and therefore may be contributing to tumor‐associated behavioral side effects. The cellular mechanisms by which tumor‐induced neuroinflammation occurs remain unknown, making targeted treatment approaches inaccessible. Here, we tested the hypotheses that microglia are the primary cells driving tumor‐induced neuroinflammation and behavioral side effects. Young adult female BALB/c mice were induced with a 67NR mammary tumor; tumor‐free controls underwent a sham surgery. Mammary tumors increased IBA1 ⁺ and GFAP ⁺ staining in the amygdala and hippocampus relative to tumor‐free controls. However, tumors did not alter gene expression of Percoll‐enriched microglia isolated from the whole brain. While cognitive, social, and anhedonia‐like behaviors were not altered in tumor‐bearing mice, tumors increased central tendency in the open‐field test; microglia depletion did not reverse this effect. Brain region RT‐qPCR data indicated that microglia depletion attenuated tumor‐induced elevations of neuroinflammatory gene expression in a region‐ and mediator‐specific manner. These results indicate a causal role of microglia in tumor‐induced neuroinflammation. This research advances our understanding of the cellular mechanisms underlying tumor‐induced neuroinflammation in order to understand how brain responses (e.g., behavior) may be altered with subsequent cancer‐related immune challenges.

It is poorly understood how solid peripheral tumors affect brain neuroimmune responses despite the various brain‐mediated side effects and higher rates of infection reported in cancer patients. We hypothesized that chronic low‐grade peripheral tumor‐induced inflammation conditions microglia to drive suppression of neuroinflammatory responses to a subsequent peripheral immune challenge. Here, Balb/c murine mammary tumors attenuated the microglial inflammatory gene expression responses to lipopolysaccharide (LPS) and live Escherichia coli (E. coli) challenges and the fatigue response to an E. coli infection. In contrast, the inflammatory gene expression in response to LPS or a toll‐like receptor 2 agonist of Percoll‐enriched primary microglia cultures was comparable between tumor‐bearing and ‐free mice, as were the neuroinflammatory and sickness behavioral responses to an intracerebroventricular interleukin (IL)‐1β injection. These data led to the hypothesis that Balb/c mammary tumors blunt the neuroinflammatory responses to an immune challenge via a mechanism involving tumor suppression of the peripheral humoral response. Balb/c mammary tumors modestly attenuated select circulating cytokine responses to LPS and E. coli challenges. Further, a second mammary tumor/mouse strain model (E0771 tumors in C57Bl/6 mice) displayed mildly elevated inflammatory responses to an immune challenge. Taken together, these data indicate that tumor‐induced suppression of neuroinflammation and sickness behaviors may be driven by a blunted microglial phenotype, partly because of an attenuated peripheral signal to the brain, which may contribute to infection responses and behavioral side effects reported in cancer patients. Finally, these neuroimmune effects likely vary based on tumor type and/or host immune phenotype. image