Adipose tissue is unique in terms of its immune effects on apoptotic cell clearance, as adipocyte apoptosis triggers inflammatory cytokine responses in macrophages, and is a potential inducer of adipose tissue inflammation. Classically, there are two types of adipose tissue in mammals: white adipose tissue (WAT) and brown adipose tissue (BAT). Adipocytes of the WAT accumulate neutral lipids in a large droplet, whereas BAT adipocytes have multilocular lipid droplets and high lipolytic activity, and oxidize fatty acids into ATP, and generate heat.
Apoptosis is an inevitable process in tissue development, and is a key component of the necessary physiological tissue turnover for proper tissue healing (reviewed in ). During this process, however, apoptotic cells pollute their surroundings with damage-associated molecular patterns, potential autoantigens, modified lipids and aged molecules, which can all impair tissue integrity and initiate immune responses. The safe removal of apoptotic debris is, hence, crucial to prevent self-immunity or the development of tissue damaging inflammation. This important task is performed by tissue-resident macrophages, which clear tissues of hazardous materials from the dead or dying cells without evoking inflammation . The “immunologically silent” removal of apoptotic cell contents is particularly relevant when apoptotic cells are cleared from damaged, infected or inflamed tissues. Macrophages exposed to the content of apoptotic cells adopt a pro-resolving, anti-inflammatory or tolerogenic phenotype, which is often termed alternative- or M2-macrophage activation (reviewed in [3,4]). Apoptotic cell clearance induces interleukin-10 (IL-10) and tumor growth factor beta (TGFβ) production in macrophages concomitant with the expression of various M2-activation-associated molecules such as the high affinity scavenger receptor for the hemoglobin-haptoglobin complex CD163 and the mannose receptor CD206 .
Adipose tissue is unique in terms of its immune effects on apoptotic cell clearance, as adipocyte apoptosis triggers inflammatory cytokine responses in macrophages, and is a potential inducer of adipose tissue inflammation . Classically, there are two types of adipose tissue in mammals: white adipose tissue (WAT) and brown adipose tissue (BAT) [7,8]. Adipocytes of the WAT accumulate neutral lipids in a large droplet , whereas BAT adipocytes have multilocular lipid droplets and high lipolytic activity, and oxidize fatty acids into ATP, and generate heat [10,11]. Beyond their role in fat metabolism, WAT and BAT have their own, specific, endocrine and immune functions . Excess development of WAT leads to obesity, which is associated with a decline in BAT mass.
Number of adipocytes, which build up the adipose tissues, is stable over a prolonged period of time, and the mature adipocytes are relatively resistant to apoptosis [13,14,15]. Albeit adipocyte apoptosis appears during development and remodeling of the adipose tissue, and the adipokine C1q /TNF-related protein 9 (CTRP9) increases the uptake of apoptotic cells by macrophages , adipocyte number in the WAT depots is relatively constant during infancy and childhood . The lipid content of the WAT increases along postnatal development due to the expansion of the cell volume of adipocytes , and a notable increase of adipocyte number appears at the onset of puberty . In infancy, adipocytes undergo differentiation ( Figure 1 a) and activate lipolysis in response to insulin and β-adrenergic stimulation [19,20]. Two adipocyte populations can be identified later, in puberty and in adulthood: small, still maturing cells, and larger, lipid-laden mature adipocytes ( Figure 1 a) [18,21]. The number of mature adipocytes increases in severe obesity, and newly generated adipocytes also appear. An increase in adipocyte number is termed adipocyte hyperplasia, whereas an increment in cell volume due to lipid accumulation is termed adipocyte hypertrophy  ( Figure 1 a).
Adipocyte differentiation is associated with an increase in the expression of survival factors, such as the BCL2 apoptosis regulator and the baculoviral inhibitor of apoptosis repeat-containing 2  ( Figure 1 b). WAT adipocytes are more resistant to apoptosis than BAT adipocytes . Adipocyte apoptosis becomes prevalent under pathological conditions—for instance, lipodystrophy is associated with adipocyte apoptosis and necrosis . Furthermore, the antiviral cytokine interferon alpha (IFNα) triggers adipocyte apoptosis both in vitro and in vivo in mice , and patients treated with HIV-1 protease inhibitors develop lipodystrophy due to adipocyte apoptosis . In congenital generalized lipodystrophy, both WAT and BAT mass is lost due to adipocyte apoptosis and necrosis .
How macrophages respond to apoptotic adipocytes may depend on the immunological context in which adipocyte death occurs, the signals provided by the apoptotic cells , and additional immune signals such as complement and cytokines . Under homeostatic conditions, when apoptotic adipocytes are scarce and Th2 cytokines are expressed in the WAT, the patrolling ATMs clear the dying adipocytes and adopt an M2-like activation state . It is plausible that the first wave of apoptosis of adipocytes during the development of obesity is well controlled by the ATMs, and they are able to maintain an M2-like activation state . However, the prevalence of damage-associated molecules and pro-inflammatory lipid species in the dying adipocytes can switch the function of ATMs towards the release of inflammatory cytokines and reactive oxygen species. Moreover, the capacity of ATMs to clear apoptotic cells may be exhausted in obesity , and the apoptotic or necrotic cell debris aggravates inflammation.
Signals that allow apoptotic cells to control macrophage behavior are complex, and involve diverse molecules and metabolites released by the apoptotic cells themselves. Apoptotic cells seem to actively elaborate cell metabolites, which serve as “find-me” signals, and display molecular patterns on the cell surfaces which serve as “eat-me” signals for the macrophages  ( Figure 2 ). After ingestion of the apoptotic cells, cellular contents are lysed and processed in the phagosomes of the macrophages, ultimately producing signal molecules that control macrophage functions  ( Figure 2 ). The study of the metabolites that shape immune response towards apoptotic cells is a new field of immunometabolism, recently termed as “efferotabolism” (efferocytosis-associated metabolism) . Here, I review the possible metabolites of the apoptotic adipocytes that potentially control ATM behavior.
Microparticles released by apoptotic cells, and the apoptotic debris itself, contain membrane lipids and lipid mediators, which are metabolized in the macrophage phagosome to provide ligands for nuclear receptors (NRs) and trigger M2-activation (reviewed in [3,56], Figure 2 ). NRs are transcription factors that regulate gene transcription in a ligand-dependent manner . Macrophages express several NRs, including peroxisome proliferator activator receptors (PPARs), liver X receptor (LXR), retinoid X receptor (RXR), retinoic acid receptor (RAR) and vitamin D receptor (VDR), and are all implicated in phagocytosis, a crucial mechanism for apoptotic cell clearance [59,60,61,62,63]. Apoptotic cells fail to inhibit pro-inflammatory cytokine responses in macrophages lacking PPARs or RXRs [62,63,64]. Furthermore, the absence of the PPARβ/δ ligand-binding domain is sufficient to impair M2-activation stimulated by apoptotic cells . This suggests that PPAR/RXR signaling may be activated by lipid metabolites derived from cell membranes of the internalized apoptotic cells. However, the uptake of apoptotic cell membranes may also induce inflammasome activation in macrophages. Mitochondrial membranes, as well as entire mitochondria of the apoptotic cells, are released by apoptotic cells and trigger inflammasome activation due to the presence of damage-associated molecules in the apoptotic mitochondrial membrane, such as cardiolipin [65,66] ( Figure 2 ).
Vitamin D suppresses macrophage pro-inflammatory polarization in various settings [92,93,94]. For example, vitamin D supplementation attenuates inflammatory cytokine expression and promotes M2-macrophage traits in the epicardial adipose tissue of swine, whereas vitamin D deficiency exacerbates inflammation by increasing the number of pro-inflammatory macrophages . Vitamin D also increases the expression of the antimicrobial peptide cathelicidin  and induces autophagy . In the context of adipose tissue biology, cathelicidin is known to be expressed by the subcutaneous fat depot, and adipocyte autophagy is thought to mitigate obesity-associated diseases [97,98]. The intake of vitamin D from apoptotic adipocytes may affect ATM functions, potentially reducing pro-inflammatory cytokine expression, inducing autophagy, and aiding in the acquisition of antimicrobial traits ( Figure 2 ).
Apoptotic cells release metabolites that initiate cellular responses in neighboring cells and in macrophages that clear apoptotic debris. These signals are often called as “find-me” signals, since they promote the encounter between macrophages and apoptotic cells  and are released actively by dying cells through pannexin channels and possibly through other membrane channels .
Polyamines self-assemble with phosphate ions in the cell nucleus and generate so-called nuclear aggregates of polyamines, which interact with genomic DNA and control DNA conformation, protection and packaging . Polyamines also affect the interactions of NRs with their coactivator complexes . Given that NRs are important in both adipose tissue development and macrophage functions, especially in apoptotic cell removal (reviewed in ), it is plausible that polyamines shape macrophage functions in the process of efferocytosis at a transcriptional level ( Figure 2 ).
Beyond its traditional role as a lipid storage site, adipose tissue is increasingly recognized as being pivotal in metabolic and endocrine physiology . ATMs are positioned at the interface between these functions, and interactions between ATMs and adipocytes potentially affect systemic metabolism and endocrine health ( Figure 3 a). Uptake of apoptotic cells is a core function of macrophages and serves as a communication channel between adipocytes and ATMs. The rich immunometabolism of apoptotic cell contents shapes ATM function, which in turn sustains healthy metabolism by balancing the protective and destructive immune functions of ATMs. Protective immunity ensures the neutralization of apoptotic cells and favors cell metabolism and energy expenditure ( Figure 3 a). When ATMs process materials of the dying adipocytes correctly, they signal to the endocrine organs and other immune cells, which ultimately supports adipose tissue development and metabolism ( Figure 3 a). By contrast, destructive immune traits appear in response to danger signals from dying adipocytes ( Figure 3 b). The resulting ATM phenotype damages metabolism through uncontrolled inflammation or self-immunity , leading to a dysfunctional interplay between ATMs and adipocytes and driving the development of metabolic diseases [155,156]. ( Figure 3 b). As adipocyte death is prevalent in obese adipose tissue, the majority of the literature has focused on the role of ATMs in the setting of obesity. However, the immune functioning of ATMs begins at birth, and is crucial to sustain energy expenditure in infancy [23,52,155,157]. Adipocyte apoptosis may occur in the adipose tissue of the neonate, but we know very little about the impact of ATM-adipocyte interactions in early postnatal development, even though adipose tissue quality in infancy is contingent on ATM function and determines metabolic health in adulthood [23,158]. It is thus important to comprehend how physiological adipocyte death (i.e., during adipose tissue development) shapes ATM functions in postnatal life.
Several signals that potentially define whether ATMs safely recycle adipocyte contents or trigger inflammation are known and have been reviewed here ( Figure 3 b). In the coming decades the treatment of obesity and associated chronic diseases will likely remain a major challenge to health systems worldwide; thus, it is timely and important to understand the impact of innate immune signaling—including apoptotic adipocyte-derived signals—in the adipose tissue.