Macrophage senescence impairs phagocytosis and phagosome function

吞噬作用是一个进化上古老的过程，可能起源于单细胞生物内化和消化食物的需要，并被保存为巨噬细胞识别和清除颗粒、死亡或受损细胞和病原体的主要机制。我们的实验室最近发现，巨噬细胞的非声速吞噬（即不需要靶标被补体、抗体或其他调理素包裹的吞噬）随着年龄的增长而减少，尽管非声速受体的表达没有改变。此外，我们还发现吞噬溶酶体融合率的年龄相关变化不会影响吞噬体的pH值、蛋白水解或氧化能力。吞噬体融合的延迟足以减少细菌的杀伤。我们比较了幼年（20个月）野生型（WT）小鼠和TNF（肿瘤坏死因子α）敲除小鼠巨噬细胞的吞噬溶酶体融合延迟，有趣的是，巨噬细胞吞噬和吞噬溶酶体成熟缺陷似乎不是由于时间年龄，而是炎症细胞因子TNF的增加。随着年龄的增长，血清和组织中炎症介质的水平增加。骨髓细胞对年龄相关的炎症特别敏感。来自年老（20+ mo） TNF缺乏小鼠的巨噬细胞不会丧失吞噬能力或表现出吞噬溶酶体融合减少。通过分析幼年（3个月）和老年（20+个月）WT和TNF - KO小鼠肺泡巨噬细胞的转录谱，我们发现TNF信号的负调节因子以TNF依赖的方式随着年龄的增长而增加，这使我们假设这是对慢性TNF刺激的适应，最终使这些巨噬细胞对急性炎症事件的反应降低。我们还发现，PI3K（磷酸肌肽3激酶）信号在衰老的巨噬细胞中受损。尽管在其他衰老免疫细胞中也有PI3K信号缺陷的报道，但巨噬细胞中PI3K信号随着年龄的增长而受损的机制及其对吞噬酶体功能的影响程度尚不清楚。总的来说，这些数据支持了我们的假设，即TNF通过增加负调节因子的表达和改变PI3K信号通路来抑制吞噬溶酶体融合并损害吞噬体功能。我们认为巨噬细胞并没有经历“衰老”（如传统描述的那样），而是适应终身暴露于炎症微环境。这种适应包括通过增加炎症负调节因子的表达来适应慢性年龄相关的炎症。通过揭示PI3K和吞噬溶酶体融合途径中被衰老微环境改变的特定成分，我们可能会发现这种独特的衰老表型是否可被操纵。

Phagocytosis is an evolutionarily ancient process, which likely originated from the need of single celled organisms to internalize and digest food, and has been preserved as the major mechanism by which macrophages recognize and remove particulates, dying or damaged cells and pathogens. Our lab has recently discovered that non-opsonic phagocytosis (i.e. phagocytosis that does not require the target to be coated by complement, antibodies or other opsonins) by macrophages decreases with age, even though expression of non-opsonic receptors does not change. Furthermore, we have also found age-related changes in the rate of phagolysosomal fusion occur without affecting the pH, proteolytic or oxidative capacity of the phagosome. This delay in phagosome fusion is sufficient to decrease bacterial killing. We have compared the delay of phagolysosomal fusion between macrophages derived from young (20 mo) wildtype (WT) mice and TNF (tumour necrosis factor alpha) knockout mice, and intriguingly, defective macrophage phagocytosis and phagolysosomal maturation does not seem to be due to chronologic age but rather the increase in the inflammatory cytokine TNF. As we age, levels of inflammatory mediators increase in the serum and tissues. Myeloid cells are particularly sensitive to age-associated inflammation. Macrophages derived from old (20+ mo) mice that are deficient in TNF do not lose phagocytic capacity or demonstrate decreased phagolysosomal fusion. By analyzing transcriptional profiles of alveolar macrophages derived from young (3 mo) and old (20+ mo) WT and TNF KO mice, we have found that negative regulators of TNF signalling increase with age in a TNF dependent manner, leading us to hypothesize that this is an adaptation to chronic TNF stimulation that ultimately makes these macrophages less responsive to acute inflammatory events. We have also found that PI3K (phosphoinositide 3-kinase) signalling is compromised in aging macrophages. Although defects in PI3K signalling have been reported in other aging immune cells, the mechanisms by which PI3K signalling is impaired with age in macrophages and the degree to which this impacts phagolysosomal function is not known. Collectively these data support our hypothesis that TNF inhibits phagolysosomal fusion and impairs phagosomal function by increasing expression of negative regulators and altering PI3K signalling pathways. We propose that macrophages do not so much undergo `senescence' (as traditionally described) but rather they adapt to lifelong exposure to the inflammatory microenvironment. This adaptation includes adapting to chronic age-associated inflammation by increasing expression of negative regulators of inflammation. By uncovering specific components of the PI3K and phagolysosomal fusion pathway that are altered by the aging microenvironment, we may uncover whether this unique senescence phenotype is amenable to manipulation.