It is estimated that an average adult human turns over roughly 330 ± 20 billion cells every day as part of healthy living.^{1, 2} This translates to 0.4% of our body mass. Such a large number for cell turnover then begs the question—what are these cells and why? The reasons for this are multi-factorial. First, there are cells in the body that have a finite life span, such as neutrophils (~1 day) and erythrocytes (~120 days), and there are also other cell types such as many hematopoietic cells that have a life span of a few days to few weeks; these need to be removed after their useful life span and replaced by new cells. Second, there are many aspects of development where we generate excess cells, of which only a few are deemed fit to progress to full maturation, and the rest undergo death and need to be removed; examples of this include development of T cells in the thymus, B cells in the bone marrow, and also adult neurogenesis in the brain.¹ Third, there are also “damaged” cells that emerge daily in the body, such as due to light/UV damage, for example skin and photoreceptors of the eye.³ Thus, all these turnover events result in a large number of cells undergoing death essentially in all organs and tissues, albeit at different magnitude.² Although there are many different forms of death processes, the cells that are destined die via homeostatic turnover do so primarily via the process of caspase-dependent apoptosis.⁴

What happens to these dying cells? Despite the billions of dying cells per day, when one looks at tissues, it is hard to recognize dying cells, even in those with high cellular turnover. This is because the recognition and clearance of dying cells is remarkably efficient.^{5, 6} Just like there is a dedicated set of molecules and mechanisms to induce programmed cell death, we also possess a dedicated machinery to recognize and remove these dying cells.⁷ Such clearance under homeostasis conditions occurs quickly, efficiently, and from an immunological perspective, quietly.⁸ It is worth noting that just like the apoptotic cell death machinery, the clearance processes are also highly conserved evolutionarily, and studies from the nematode, flies, zebrafish, mice, and humans have established the conserved components of the clearance process.^{9, 10} This volume of Immunological Reviews focuses on different aspects of the cell clearance process and its implications to homeostasis and disease.

While there are different forms of phagocytosis, the recognition and clearance of apoptotic cells by phagocytes has been termed “efferocytosis,” a term originally coined by Dr. Peter Henson. (where “effero” means “carry to the grave”).¹¹ This should be distinguished from Fc receptor mediated phagocytosis or complement receptor-mediated phagocytosis, which involves opsonization of target cells by specific ligands and uptake via the respective receptors. A key distinction from the other forms of phagocytosis is that the apoptotic cell clearance does not induce an immune response; further efferocytosis is also actively anti-inflammatory.⁸ This makes sense when one considers the fact that if we were to induce an inflammatory response to billions of cells that we clear every day, we may all end up as walking bags of inflammation. However, the failure to clear the apoptotic cells promptly can induce secondary necrosis that can lead to inflammatory sequelae, as detailed by some of the chapters in this volume.

Works from a number of laboratories have now established that there are different steps to recognition and removal of dying cells.^{1, 12} The first step is the recognition of the dying cells. When phagocytes such as macrophages, dendritic cells, or healthy neighbors are in close proximity to the dying cell, specific molecules on the apoptotic cells are recognized by specific receptors on the engulfing cell leading to subsequent intracellular signaling, cytoskeletal reorganization, and corpse internalization.¹³ While there are many ligands exposed on cells undergoing apoptosis, the best detailed is the exposure of the phospholipid phosphatidylserine (PtdSer).^{14, 15} PtdSer is normally kept via an active and energy dependent process on the inner leaflet but gets exposed on the outer leaflet as part of the apoptotic process.^{15, 16} This PtdSer in turn, gets recognized by multiple PtdSer recognition receptors—either directly, or indirectly, via intermediary bridging molecules.^{16, 17} As part of the first chapter, Dr. Tal Brustyn-Cohen details one of the best described receptor families linked to clearance of apoptotic cells, namely the TAM receptors.¹⁸ These TAM receptors recognize PtdSer indirectly via Gas6 or Protein S that bind PtdSer, and this chapter details the role of TAM receptors in different contexts and highlights their role in other physiology.

One of the challenges in studying cell clearance in the mammals is that multiple homologues for engulfment receptors and signaling molecules, and the complexities that arise when individual knockouts do not often have a clear phenotype. Thus, defining the function of individual molecules, as well as visualizing cell death and clearance in vivo have been a challenge for the past couple of decades. Will Wood, Andrew Davidson, and colleagues detail the beautiful model systems for cell death and efferocytosis in the fruit fly Drosophila that have provided many new insights.¹⁹ They also detail new approaches that have been developed to visualize apoptosis and efferocytosis in vivo, plus the different molecular mechanisms of clearance employed by Drosophila macrophages to clear dying cells.

After a phagocyte engulfs an apoptotic cell, a second challenge ensues—that is, digesting the corpse. This is no small feat, as this involves digesting another cell that is often nearly the same size as the phagocyte itself. Further, many phagocytes engulf multiple apoptotic cells.²⁰ Mylvaganam and Freeman take a comprehensive approach to how a phagocyte resolves a phagolysosome as well as aspects of the membrane traffic, and the role of solute carriers in managing some of the contents of the corpse. They also put this in disease contexts with lysosomal storage disorders.²¹

Another inherent challenge that the phagocyte faces is how to handle all the excess biomass. To put it another way, when a phagocyte engulfs an apoptotic cell, it basically doubles its lipids, carbohydrates, and proteins, to name a few of the corpse contents. Further, phagocytes such as macrophages ingest multiple corpses in succession, leading to even greater challenge of dealing with all this excess metabolic overload.^{20, 22} Shilperoort, Tabas, and colleagues beautifully detail the many aspects of this macrophage immunometabolism.²³ They detail how amino acids such as arginine and methionine and their subsequent conversion within phagocytes impact continued uptake of additional corpses, macrophage responses, and in turn, disease processes. The authors also detail lactate regulation in macrophage responses, relevance to human disease such as atherosclerosis, and limitations to the current studies.

While phagocytes such as macrophages get a lot of attention as “professional phagocytes” capable of engulfing many corpses, there are also nonprofessional phagocytes. Although these phagocytes may do cell clearance with slower kinetics than macrophages, they play an important role in routine clearance of the many cells in the body. The retinal pigmented epithelial cells (RPE) of the retina provide a beautiful example, as they clear on a daily basis the “used” photoreceptors that are damaged from light during the day and they need to be removed to allow new photoreceptors to take their place.^{24, 25} Another beautiful aspect of the RPE is that they are postmitotic, and we are born and die with the same number of RPE. This means that the RPE cells do the clearance throughout the lifetime, in addition to their many nurse cell functions for the photoreceptors to maintain a healthy retina. Silvia Finnemann and colleagues detail the background on clearance by RPE cells, the receptors, and mechanisms of RPE-mediated clearance, as well as diseases that arise when this clearance is disturbed and lead to retinal inflammation.²⁶

Just like we do not fully appreciate the importance of garbage workers until they go on strike, the importance of the “cellular clearance crew” has gotten much better appreciation in the past two decades when failures in clearance, or complexities associated with cell death and cell clearance contribute to inflammatory diseases or links to cancer.⁸ This is detailed in four of the final chapters of this volume. First, Christopher Gregory details beautifully the complexity of cell death in the cancer context.²⁷ He details how apoptotic cells and their products (including extracellular vesicles and other factors released by the dying cells) regulate the tumor microenvironment; this includes how responses of the macrophages within solid tumors, either due to direct contact with the apoptotic cells or their released products, lead to reshaping the tumor microenvironment for tumor growth. This is followed by a detailed description by Wagoner, Michael Elliott, and colleagues on the antibody-mediated phagocytosis of tumor cells—which occurs when an antibody-bound tumor cell is recognized via Fc receptors, primarily by macrophages.²⁸ The authors also present challenges and modifications that occurs to the phagocytes as part of the FcR-mediated phagocytosis, and some approaches to overcome them.

It has become increasingly clear that many auto-inflammatory diseases such as atherosclerosis, arthritis, and certain forms of colitis have a component of defective or minimally the release of certain components from the late stage dying cells, that promote a pro-inflammatory milieu, and in turn, chronic inflammation.¹ Further, if some of the self-antigens are presented in this pro-inflammatory environment, this can evolve to autoimmunity. Schneider and Arandjelovic detail the inflammatory components of arthritis. Interestingly, some of the components of the engulfment machinery has additional roles, such as in neutrophil migration to the arthritic joints that in turn can also contribute to arthritis.²⁹ Lastly, Gabrielle Fredman and Sayeed Khan discuss the role of specialized pro-resolving mediators (SPMs) in clearance of dead cells.³⁰ They highlight the role of SPMs in facilitating clearance of not only apoptotic cells but also necroptotic cells, and further link these to non-resolving diseases such as atherosclerosis.

In sum, in the past couple of decades of investigations on how cells die, how they are removed, and the consequences of such regulated and efficient phagocytosis to homeostasis have exploded. This has led to a remarkable increase in knowledge on the molecules and mechanisms, and how defects in clearance contribute to disease states in specific tissue contexts. In this collection of reviews, the authors not only highlight contributions from their own laboratories, but they also put these discoveries in the larger context of what is known, the challenges, and how to go about addressing the next set of questions in the field. The research on cellular turnover, with a role in essentially every single tissue, is bound to continue, with modulating phagocytosis providing opportunities for treating multiple diseases.³¹

据估计，作为健康生活的一部分，一个成年人每天平均更换大约 330 ± 200 亿个细胞。^{1, 2}这相当于我们体重的 0.4%。如此大量的细胞更新引出了一个问题：这些细胞是什么？为什么？造成这种情况的原因是多方面的。首先，体内有些细胞的寿命是有限的，例如中性粒细胞（约 1 天）和红细胞（约 120 天），还有其他细胞类型，例如许多造血细胞，其寿命为几天到几周；这些电池在其使用寿命结束后需要被移除并被新电池取代。其次，在发育的许多方面，我们都会产生多余的细胞，其中只有少数被认为适合完全成熟，其余的则会死亡并需要去除；这方面的例子包括胸腺中 T 细胞的发育、骨髓中 B 细胞的发育以及大脑中成人神经发生的发育。¹第三，体内每天都会出现“受损”细胞，例如由于光/紫外线损伤，例如皮肤和眼睛的光感受器。³因此，所有这些周转事件都会导致所有器官和组织中的大量细胞死亡，尽管程度不同。²虽然死亡过程有许多不同的形式，但注定要通过稳态更新而死亡的细胞主要是通过半胱天冬酶依赖性细胞凋亡过程。⁴

这些垂死的细胞会发生什么？尽管每天有数十亿个垂死细胞，但当人们观察组织时，很难识别垂死细胞，即使是那些细胞更新率高的细胞。这是因为对垂死细胞的识别和清除非常有效。^{5, 6}就像有一套专门的分子和机制来诱导程序性细胞死亡一样，我们也拥有一套专门的机制来识别和清除这些垂死的细胞。⁷体内平衡条件下的这种清除过程快速、高效，而且从免疫学的角度来看，是安静的。⁸值得注意的是，就像细胞凋亡机制一样，清除过程在进化上也是高度保守的，对线虫、果蝇、斑马鱼、小鼠和人类的研究已经确定了清除过程的保守组成部分。^{9, 10}本期《免疫学评论》重点关注细胞清除过程的不同方面及其对体内平衡和疾病的影响。

虽然吞噬作用有不同的形式，但吞噬细胞对凋亡细胞的识别和清除被称为“胞吞作用”，该术语最初由 Peter Henson 博士创造。（其中“effero”的意思是“带到坟墓”）。¹¹这应与 Fc 受体介导的吞噬作用或补体受体介导的吞噬作用区分开来，后者涉及特定配体对靶细胞的调理作用并通过各自的受体进行摄取。与其他形式的吞噬作用的一个关键区别是，凋亡细胞清除不会诱导免疫反应。进一步的胞吞作用也具有积极的抗炎作用。⁸考虑到这一事实是有道理的：如果我们对每天清除的数十亿个细胞引发炎症反应，我们最终可能都会成为行走的炎症袋。然而，未能及时清除凋亡细胞可能会诱发继发性坏死，从而导致炎症后遗症，如本卷某些章节所详述。

许多实验室的工作现已证实，识别和去除垂死细胞有不同的步骤。^{1, 12}第一步是识别垂死细胞。当巨噬细胞、树突状细胞或健康邻居等吞噬细胞靠近垂死细胞时，凋亡细胞上的特定分子会被吞噬细胞上的特定受体识别，从而导致随后的细胞内信号传导、细胞骨架重组和尸体内化。¹³虽然正在凋亡的细胞上暴露了许多配体，但最详细的是磷脂磷脂酰丝氨酸 (PtdSer) 的暴露。^{14, 15} PtdSer 通常通过活性和能量依赖过程保留在内部小叶上，但作为细胞凋亡过程的一部分暴露在外部小叶上。^{15, 16}该 PtdSer 依次被多个 PtdSer 识别受体识别——直接或通过中间桥接分子间接识别。^{16, 17}作为第一章的一部分，Tal Brustyn-Cohen 博士详细介绍了与凋亡细胞清除相关的最受描述的受体家族之一，即 TAM 受体。¹⁸这些 TAM 受体通过结合 PtdSer 的 Gas6 或 Protein S 间接识别 PtdSer，本章详细介绍了 TAM 受体在不同情况下的作用，并强调了它们在其他生理学中的作用。

研究哺乳动物细胞清除的挑战之一是吞噬受体和信号分子的多个同源物，以及个体敲除通常不具有明确表型时出现的复杂性。因此，定义单个分子的功能以及体内细胞死亡和清除的可视化一直是过去几十年的挑战。Will Wood、Andrew Davidson 及其同事详细介绍了果蝇细胞死亡和胞吞作用的美丽模型系统，该系统提供了许多新的见解。¹⁹他们还详细介绍了已开发的可视化体内细胞凋亡和胞吞作用的新方法，以及果蝇巨噬细胞清除垂死细胞所采用的不同分子清除机制。

吞噬细胞吞噬凋亡细胞后，第二个挑战随之而来——即消化尸体。这是一个不小的壮举，因为这涉及消化另一个通常与吞噬细胞本身大小几乎相同的细胞。此外，许多吞噬细胞吞噬多个凋亡细胞。²⁰ Mylvaganam 和 Freeman 采用综合方法来研究吞噬细胞如何解析吞噬溶酶体以及膜运输的各个方面，以及溶质载体在管理尸体某些内容物中的作用。他们还将其置于溶酶体贮积症的疾病背景中。²¹

吞噬细胞面临的另一个固有挑战是如何处理所有多余的生物量。换句话说，当吞噬细胞吞噬凋亡细胞时，它的脂质、碳水化合物和蛋白质基本上都会增加一倍，仅举几例尸体内容物。此外，巨噬细胞等吞噬细胞连续吞噬多具尸体，导致处理所有这些过量的代谢超载面临更大的挑战。^20、22 Shilperoort、Tabas 及其同事精美地详细介绍了这种巨噬细胞免疫代谢的许多方面。²³他们详细介绍了精氨酸和蛋氨酸等氨基酸及其随后在吞噬细胞内的转化如何影响额外尸体的持续摄取、巨噬细胞反应，进而影响疾病过程。作者还详细介绍了巨噬细胞反应中的乳酸调节、与动脉粥样硬化等人类疾病的相关性以及当前研究的局限性。

虽然巨噬细胞等吞噬细胞作为能够吞噬大量尸体的“专业吞噬细胞”而受到广泛关注，但也存在非专业吞噬细胞。尽管这些吞噬细胞可能以比巨噬细胞慢的动力学进行细胞清除，但它们在体内许多细胞的常规清除中发挥着重要作用。视网膜的视网膜色素上皮细胞（RPE）提供了一个美丽的例子，因为它们每天都会清除“使用过的”光感受器，这些光感受器在白天因光而受损，需要将其移除以让新的光感受器取代它们。 。^{24, 25} RPE 的另一个优点是它们是有丝分裂后的，我们出生和死亡时的 RPE 数量相同。这意味着 RPE 细胞除了为感光器提供许多护理细胞功能以维持健康的视网膜外，在整个生命周期中都会进行清除工作。Silvia Finnemann 及其同事详细介绍了 RPE 细胞清除的背景、受体和 RPE 介导的清除机制，以及当这种清除受到干扰并导致视网膜炎症时出现的疾病。²⁶

就像我们在垃圾工人罢工之前无法充分认识到他们的重要性一样，“细胞清理人员”的重要性在过去二十年中得到了更好的认识，当清理失败或与细胞死亡和细胞相关的复杂性时，“细胞清理人员”的重要性得到了更好的认识。清除会导致炎症性疾病或与癌症的联系。⁸本书最后四章详细介绍了这一点。首先，克里斯托弗·格雷戈里（Christopher Gregory）精美地详细介绍了癌症背景下细胞死亡的复杂性。²⁷他详细介绍了凋亡细胞及其产物（包括细胞外囊泡和垂死细胞释放的其他因子）如何调节肿瘤微环境；这包括实体瘤内巨噬细胞由于与凋亡细胞或其释放的直接接触而产生的反应如何导致重塑肿瘤生长的肿瘤微环境。随后，Wagoner、Michael Elliott 及其同事详细描述了抗体介导的肿瘤细胞吞噬作用，这种吞噬作用发生在抗体结合的肿瘤细胞通过 Fc 受体（主要是巨噬细胞）识别时。²⁸作者还提出了作为 FcR 介导的吞噬作用一部分的吞噬细胞所面临的挑战和修饰，以及克服这些挑战和修饰的一些方法。

越来越清楚的是，许多自身炎症性疾病，如动脉粥样硬化、关节炎和某些形式的结肠炎，都与晚期死亡细胞释放某些成分有缺陷或最低限度有关，这些成分会促进促炎环境，并且反过来，慢性炎症。¹此外，如果某些自身抗原出现在这种促炎症环境中，则可能会演变成自身免疫。施奈德和阿兰杰洛维奇详细介绍了关节炎的炎症成分。有趣的是，吞噬机制的一些组件具有额外的作用，例如中性粒细胞迁移到关节炎关节，这反过来也可能导致关节炎。²⁹最后，Gabrielle Fredman 和 Sayeed Khan 讨论了专门的促溶解介质 (SPM) 在清除死细胞中的作用。³⁰他们强调了 SPM 在促进清除凋亡细胞和坏死性细胞方面的作用，并进一步将这些细胞与动脉粥样硬化等无法解决的疾病联系起来。

总之，在过去的几十年里，关于细胞如何死亡、如何被清除以及这种受调节和有效的吞噬作用对体内平衡的影响的研究已经爆发。这使得人们对分子和机制以及清除缺陷如何导致特定组织环境中的疾病状态的了解显着增加。在这本综述中，作者不仅强调了他们自己实验室的贡献，而且还将这些发现置于已知的知识、挑战以及如何解决该领域的下一组问题的更大背景下。细胞更新的研究基本上在每个组织中都发挥着作用，必然会继续下去，调节吞噬作用为治疗多种疾病提供了机会。³¹