Targeting immune-mediated, age-related, biology has the potential to be a transformative therapeutic strategy. However, the redundant nature of the multiple cytokines that change with aging requires identification of a master downstream regulator to successfully exert therapeutic efficacy. Here, we discovered CCR3 as a prime candidate, and inhibition of CCR3 has pro-cognitive benefits in mice, but these benefits are not driven by an obvious direct action on central nervous system (CNS)-resident cells. Instead, CCR3-expressing T cells in the periphery that are modulated in aging inhibit infiltration of these T cells across the blood-brain barrier and reduce neuroinflammation. The axis of CCR3-expressing T cells influencing crosstalk from periphery to brain provides a therapeutically tractable link. These findings indicate the broad therapeutic potential of CCR3 inhibition in a spectrum of neuroinflammatory diseases of aging.

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Recent research into the proteomic composition of plasma has identified specific circulating factors that increase with age and are directly associated with cognitive decline and neurodegeneration. CCL11 (eotaxin) is one such factor elevated with aging that has been shown to reduce neurogenesis in the dentate gyrus as well as impair hippocampal learning and memory when administered to young mice (Villeda, et al. 2011). Given the increase in CCL11 levels observed in plasma from Alzheimer's disease patients, it is possible that CCL11 plays a role in mechanisms underlying cognitive function in neurodegenerative disease, including neuroinflammation. A CCR3 (CCL11 receptor) antagonist was administered to 23-month-old aged C57Bl/6 mice for 3 weeks to measure its effect on age-dependent neuroinflammation. Immunohistochemical assessment of astrocytes and microglia was performed on brain tissues from mice, and circulating cytokines were measured in the plasma. We then used acute and chronic models of LPS administration in young C57Bl/6 mice to determine the effects of CCR3 inhibition in models of induced neuroinflammation, and again performed immunohistochemical assessment of neuroinflammatory markers. We demonstrate that treatment with a CCR3 antagonist resulted in reduced levels of systemic inflammatory cytokines and decreased neuroinflammation in the hippocampus of aged mice. Additionally, treatment with the antagonist diminished LPS-induced neuroinflammation, significantly reducing activated microglia in the hippocampus. CCL11 plays a significant role in the neuroinflammation associated with aging and neurodegenerative disease. Our findings demonstrate that targeting the CCR3 receptor with an antagonist has therapeutic potential to alleviate associated disease phenotypes.

As the population ages, dementia and aging-related diseases are becoming increasingly prevalent. Mounting evidence suggests that there are factors present in young blood that can counteract the effects of brain aging. Similarly, we have identified factors in aged blood whose increase with aging is associated with negative effects on cognition. Among these factors is eotaxin, a chemokine known to induce cognitive deficits in young wild-type mice and is implicated in aging-associated diseases such as Alzheimer's disease and Parkinson's disease. To test whether targeting the primary eotaxin receptor, CCR3, could prevent eotaxin-induced cognitive deficits, we treated young C57Bl/6 mice with recombinant eotaxin and co-administered a small molecule targeting the CCR3 receptor, twice a day for 18 days. In a second experiment, 23-month-old aged C57Bl/6 mice were treated with the CCR3 antagonist for 3 weeks to assess effects on aging-induced cognitive dysfunction driven in part by endogenous increases in eotaxin. We also treated young C57Bl/6 mice with LPS to induce brain inflammation, treated with the CCR3 antagonist, and assessed markers of peripheral and brain inflammation. CCR3 antagonist treatment significantly improved performance on the Y maze and Barnes maze tests for cognition in eotaxin-treated young mice. CCR3 antagonist treatment was also effective in reversing age-associated cognitive decline in aged mice with similar improvements in performance in the Y maze and Barnes maze tests. In addition, we found that the locomotor dysfunction observed in aged mice was improved with CCR3 antagonist treatment. We then investigated effects on neuroinflammation, and found that treatment with the CCR3 antagonist prevented the LPS-induced increase in microglial activation in the brain. These results implicate the potential therapeutic utility of CCR3 antagonism for improving neuroinflammation and cognition in aging-associated neurodegenerative diseases such as Alzheimer's disease.

The concept that young plasma can have beneficial effects on cognitive processes in mice has led to a new consideration of it as a therapeutic source. Plasma is comprised of a large number of proteins with differing functional roles; therefore it is likely that multiple mechanisms are relevant for this efficacy. We have sought to firstly determine whether the concept is translatable to human plasma and thus has direct therapeutic potential and secondly to explore the mechanistic underpinnings of young plasma's beneficial effects on cognition. Plasma from 18 year old healthy donors was used to test efficacy in two strains of immunocompromised mice. NOD scid and NSG animals were aged prior to dosing to a point where their cognitive and motor functions are significantly diminished compared to young animals. Repeated dosing, at levels scaling to approximately 1 unit of plasma per infusion in man, was performed with varying frequency parameters. The efficacy of plasma was tested in a behavioral test battery incorporating measures of motor and cognitive function. On completion of behavioral assays, brains were collected and immunohistological assessments performed to assess multiple cellular and biochemical endpoints. Young human plasma improved both motor (distance and velocity in the open field) and cognitive (Barnes maze, Y-maze) function in aged, immunocompromised, mice. The behavioral effects correlated with histological changes including doublecortin staining and BrdU labeling, indicative of neurogenesis. Furthermore, the effects of young plasma were long-lived, with histological and behavioral changes evident weeks after dosing. The plasma proteome provides a novel source of therapeutics that may be of benefit in disorders involving cognition, including Alzheimer's disease. The ability of human plasma to have long term effects involving mechanisms such as neurogenesis indicate the disease modifying potential of such therapeutics.