Macrophage senescence impairs phagocytosis and phagosome function

吞噬作用在进化上是一个古老的过程，它可能起源于单细胞生物内化和消化食物的需要，并被保存下来，作为巨噬细胞识别和清除颗粒、死亡或受损的细胞和病原体的主要机制。我们的实验室最近发现，巨噬细胞的非调理吞噬作用(即不需要被补体、抗体或其他调理素包被的吞噬作用)随着年龄的增长而减少，即使非调理受体的表达没有改变。此外，我们还发现与年龄相关的吞噬小体融合率的变化不会影响吞噬小体的pH、蛋白分解或氧化能力。吞噬小体融合的这种延迟足以减少细菌的杀灭。我们比较了来自年轻(20mo)野生型(WT)小鼠和肿瘤坏死因子α基因敲除小鼠的巨噬细胞的吞噬融合延迟，有趣的是，有缺陷的巨噬细胞吞噬和吞噬小体成熟似乎不是由于年龄原因，而是炎性细胞因子TNF的增加。随着年龄的增长，血清和组织中的炎性介质水平会增加。髓系细胞对年龄相关的炎症特别敏感。缺乏肿瘤坏死因子的老年(20+mo)小鼠的巨噬细胞不会丧失吞噬能力或表现出吞噬小体融合减少。通过分析来自年轻(3mo)和老年(20+mo)WT和TNFKO小鼠的肺泡巨噬细胞的转录谱，我们发现，随着年龄的增长，TNF信号的负调控因子以一种依赖于TNF的方式增加，这导致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较差。我们还发现，在老化的巨噬细胞中，PI3K(磷脂酰肌醇3-激酶)信号受到损害。虽然PI3K信号在其他老化免疫细胞中的缺陷也有报道，但随着年龄的增长，巨噬细胞中PI3K信号受损的机制以及这对吞噬酶体功能的影响程度尚不清楚。总之，这些数据支持我们的假设，即肿瘤坏死因子通过增加负调控因子的表达和改变PI3K信号通路来抑制吞噬小体融合并损害吞噬小体功能。我们认为，巨噬细胞并不像传统描述的那样经历“衰老”，而是适应终生暴露在炎性微环境中。这种适应包括通过增加炎症负性调节因子的表达来适应与年龄相关的慢性炎症。通过揭示由衰老微环境改变的PI3K和吞噬溶酶体融合途径的特定成分，我们可能揭示这种独特的衰老表型是否易于操纵。

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.