DNA damage accumulation is expected to occur with aging and accumulates as a result of many endogenous and exogenous factors. Genomic instability in the aging process can be associated with somatic mutations, copy-number alterations, and chromosome abnormalities for nuclear as well as mitochondrial DNA (mtDNA) [2]. Such DNA alterations may affect essential genes and transcriptional pathways, resulting in dysfunctional cells.

In the literature, the causes of genomic instability in Mφ are scarce and one work points out that it can be induced by pathogens such as Mycobacterium tuberculosis [34]. M. tuberculosis is the causative agent of tuberculosis with a pathological outcome associated with the formation of granulomas. The granulomas formed in the development of several chronic diseases (due to persistent inflammatory stimuli) can modulate molecular programs, which are involved in TRM differentiation and relate to clinical outcomes [35] and DNA damage [36].

Since Mφ are the most abundant immune cell type in the skin, and this tissue is directly exposed to several environmental factors, such as UV radiation, genomic instability could be a central hallmark of aging in aged Mφ; after all, numerous DNA injuries can lead to accumulated damage and altered Mφ function in this tissue [37]. This in turn can lead to altered gene expression of molecules such as cytokines, MHC class II, transcription factors, an exacerbated production of reactive oxygen species (ROS) [15], and NF-κB signaling in response to DNA damage [38], rendering Mφ more susceptible to apoptosis, impairing their phagocytosis function [39], and contributing to inflammaging [40].

Telomeres protect the ends of chromosomes from degradation and abnormal recombination. Considered a primary hallmark of aging, telomere attrition causes the loss of chromosome protective structures as they gradually get shorter [2]. This shortening process has also been closely connected with inflammation [41,42].

Comparing young and old mice, Kang and colleagues (2018) observed that the shortening of telomeres in Mφ leads to increased ROS production, the same phenotype observed for genomic instability (see Section 2.1). Furthermore, in their experiment, knockout mice for Telomerase RNA Component (Terc^−/−), a gene that encodes for the RNA that serves as a template for the telomere repeats, showed telomere dysfunction in Mφ, which is associated with hyper inflammation and mitochondrial abnormality, followed by oxidative stress with hyperactivation of the Nod-like receptor protein 3 (NLRP3) inflammasome (see Section 2.6) [43].

The increase in oxidative stress can induce DNA breakdown, which can lead to mutations that may explain most of the changes described in the aged Mφ. The main inflammatory signaling pathway, NF-κB, regulates the maintenance of telomeres and telomerase activity [44], just as the latter regulate NF-κB activity [45]. This relationship leads to a defective autophagic response and overexpression of inflammatory cytokines, such as TNF-α, IL-6, and IFN in circulating Mφ [46].

Epigenetic alterations involve changes in DNA methylation (DNAm) patterns, post-transcriptional modification of histones, and chromatin remodeling [2,47]. These aging-induced epigenetic changes in Mφ are mainly responsible for controlling the inflammatory profile and cell differentiation [48,49].

DNAm undergoes predictable time-dependent modifications across CpG islands and is influenced by both intrinsic and extrinsic processes. Molecular clocks have been developed in order to calculate the “biological age” of biological samples using methylome data [50,51], including a skin-specific Molecular Clock [51]. Age-associated epigenetic remodeling involves highly localized gain/loss of DNAm at the binding sites of transcription factors associated with the monocyte-macrophage differentiation process [52]. Despite a major lack of comprehension regarding the cause or effect role of epigenetic changes and aging phenotypes, recent studies have shed light on at least a few events that connect epigenetic changes in Mφ and age-related phenotypes, such as inflammation and differentiation.

For instance, aging-associated changes in DNAm, particularly the demethylation in the tumor necrosis factor (TNF-α) promoter, a cytokine predominantly produced by Mφ, revealed a possible link between inflammation, Mφ, and chronic age-related diseases. The promoter demethylation has been described to occur in peripheral blood leukocytes and Mφ of aging subjects and is accompanied by a reduction of TNF-α reporter gene activity [53], possibly associated with chronic inflammatory processes.

Other studies have also revealed that protein-3 containing the Jumonji domain (Jmjd-3), a H3K27 demethylase, promotes the induction of Irf-4, and SMYD-3, an H3K4 methyltransferase, of IL-4 and IL-12 [54,55]. These histone-modifying proteins play a central role in aging and their activities increase in the course of this process. In Mφ, they contribute to the positive regulation of regenerative and anti-inflammatory profiles [55,56].

These epigenetic alterations in the skin microenvironment contribute to inflammaging and can be directly linked to clinical outcomes. For instance, the chronic exposure of Mφ to inflammatory triggers and products of dead or senescent cells (see Section 2.7) can impose epigenetic changes that cause Mφ’ altered response capacity during skin aging [52].

Proteostasis is defined as the process of protein homeostasis maintenance and comprises a complex proteostasis network (PN), mainly composed by specialized proteins such as chaperones and cochaperones, translational machinery, the ubiquitin-proteasome system (UPS), and the autophagy machinery [57]. The PN has the role of controlling protein synthesis, modification, secretion, and degradation. It also reduces misfolded proteins by restoring, removing, or degrading them through the unfolded protein response (UPR) activity to prevent their accumulation in cellular compartments [58]. The chaperone-mediated autophagy (CMA) is another player in the proteostasis balance. HSPA8 is a central component of CMA and is an abundant protein in Mφ and other immune cells. Together with a co-chaperone complex, HSPA8 recognizes “CMA-targeting recognition motifs” in the targeted protein sequence, unfolding the substrate and delivering it to a protein called lysosome-associated membrane protein 2A (LAMP2A), which internalizes the targeted proteins for subsequent degradation in the lysosomal lumen [59]. If those mechanisms fail to restore homeostasis, apoptotic pathways may be activated to ensure survival of the organism [58].

Autophagy is a paramount process in the maintenance of skin homeostasis throughout aging, the consequences of age-related autophagy decay affecting different skin types, including LC. An example describing the consequences of the loss of proteostasis on aged skin is the change of elastin, collagen, and melanin levels [60,61,62] found in wrinkled and hypopigmented skin [60,63]. The balance in the composition of those proteins is essential for skin function and health and, interestingly, Mφ play an important role in this context. For instance, Mφ synthesize metalloelastases (e.g., metalloelastase 12) that participate in the elimination of nonfunctional elastin aggregates generated in the skin as a consequence of photoaging [64,65].

If on one side, healthy Mφ are important contributors to the maintenance of proteostasis, aged Mφ exhibit diminished inositol-requiring enzyme 1α (IRE1α) activation (a stress sensor that activates UPR) and increased susceptibility to endoplasmic reticulum (ER) stress-dependent apoptosis [66]. During high levels of ER stress, UPR activates IRE1α, which in turn assists the alternative splicing of X-box binding protein 1 (XBP1) mRNA [67]. After its activation, the transcription factor XBP1 induces the expression of cytokines such as pro–IL-1β [68]. However, it has been shown that Toll-like Receptors (TLR) in mice and human Mφ can directly activate XBP1, without UPR mediation, or even in synergy with ER stress [67], leading to the splicing of XBP1 and activation of a sustained proinflammatory environment by IL-1β, IL-6, and TNF [67,69].

In fact, it has been shown that the inflammasome is activated in the context of excessive misfolded protein accumulation, which is exacerbated in autophagy- or p62 (sequestosome 1)-deficient Mφ [70,71]. In addition, ER stress can also be transferred from neighboring parenchymal cells to TRMs by upregulating the splicing of UPR components, such as Grp78, Gadd34, Chop, and Xbp-1 [72,73]. This phenomenon of a “transmissible” ER stress state is mediated by the production of IL-4, IL-10, and by apoptotic bodies from stressed cells [72]. Therefore, the age-related loss of proteostasis in the skin affects Mφ and seems to contribute to the local inflammaging phenotype.

Aging directly affects the sensors and molecular targets of nutrients and fluid homeostatic regulation [74]. One of the underlying mechanisms by which aging promotes deregulated nutrient sensing is by promoting disturbed insulin sensitivity, which compromises the capacity of some tissues to uptake and metabolize nutrients [75].

Importantly, nutrient sensing components have been directly linked to longevity, including the mammalian targets of rapamycin (mTOR) and AMP-dependent protein kinase (AMPK), the two major components of nutrient sensing and metabolic regulation. AMPK has an inhibitory effect on mTOR signaling, which is activated during nutrient starvation, leading to a rise in the AMP:ATP ratio [76]. In association with nicotinamide adenine dinucleotide (NAD), high AMP levels activate sirtuins, responsible for insulin signaling pathway and longevity regulation. Consequently, the excess of nutrient availability can promote aging-associated diseases [77,78].

Accordingly, the modulation of nutrient sensing signaling influences several immune cell types including Mφ. It has been said that the Mφ immunometabolism influences Mφ polarization and activation, processes which are tightly linked to skin homeostasis and inflammaging. Mφ metabolic signatures have been closely connected with the M1-like and M2-like phenotypes; the M1-like Mφ heavily relying on glycolysis, and the M2-like Mφ being more dependent on oxidative phosphorylation [79]. During the aging process, Mφ function and phenotypes are disturbed due to many factors, including nutrient sensing dysregulation and the installation of a chronic low-grade inflammation environment in the tissue. In this sense, deregulated nutrient sensing can increase Mφ glycolysis and suppress the oxidative phosphorylation (via attenuation of IL-4-induced anti-inflammatory responses), favoring the accumulation of M1-like Mφ in the aged skin [80,81,82,83]. The presence of M2-like profile is therefore reduced, causing skin damage and promoting the progression of age-associated diseases [84,85,86].

Furthermore, FOXO and mTOR are targets of the insulin and insulin-like growth factor 1 (IGF-1) signaling (IIS) pathway and are influenced by nutrient status, altering tissue homeostasis, and inflammation [87,88]. In a very elegant experiment, the skin of mice lacking both the insulin and IGF-1 receptor in myeloid cells was enriched in noninflammatory Mφ phenotype after the induction of dermatitis. When compared to controls, it showed evidence of a proinflammatory IR/IGF-1R-dependent pathway and a connection between cutaneous inflammatory responses and diseases such as insulin-resistant diabetes mellitus type 2 [89]. In addition, SASP has been highly associated with Mφ inflammatory factors in conditions of hyperglycaemia, contributing to the fueling of low-grade inflammation in diabetes [90]. Under nutrient starvation, FOXO1 migrates to the nucleus after phosphorylation and seems to stimulate proinflammatory TLR4 signaling and IL-8β production in Mφ. FOXO1 migration also stimulates the expression of the anti-inflammatory cytokine IL-10 in M2-like cells [91], supporting the phenotypic development of aging Mφ in distinct directions [19].

Bone marrow-derived Mφ have also shown an increasing expression of growth hormone receptor (GH-R) and GH-R-dependent induction of inflammatory components in aged mice. Current evidence suggests that the downregulation of NLRP3 inflammasome in Mφ by GH-R is capable of maintaining immune system homeostasis and extending health- and lifespan [92].

Since nutrient sensing signaling pathways can be pharmacologically modulated and are closely linked to inflammation, interesting observations could be made regarding the manipulation of Mφ phenotypes in the skin. For instance, the modulation of AMPK/mTOR/NLRP3 inflammasome signaling using Metformin revealed that the drug treatment promoted reduced NLRP3 signaling and promoted the regenerative M2-like phenotype in skin Mφ, paving a way to re-establish skin Mφ equilibrium [93]. Several other anti-inflammatory drugs target immunometabolism and may also contribute in this sense, as revised by [94].

Mitochondrial production of ROS is crucial for the skin’s defense against pathogens [95]. However, dysfunctional mitochondria in Mφ can generate excessive production of ROS and cause damage to important intracellular structures, including the mtDNA [22], contributing to defective apoptosis and activation of inflammasomes [96]. Aged Mφ present mitochondrial dysfunction associated with decreased ATP production, reduction of mitochondrial membrane potential (ΔΨm), and increased oxidative stress, as well as depreciated antioxidant defense response that can impair Mφ functions and lead to senescence [97]. For instance, during lung Streptococcus pneumoniae infections, impaired mitochondrial function of aged Mφ increases lung pathology and oxidative stress [98].

The activity of the oxidative phosphorylation system is also altered in aging. During aerobic respiration, oxygen can be reduced prematurely, generating a high amount of ROS, a process that is exacerbated in senescent cells [99]. Besides that, Minhas and colleagues (2019) demonstrated the importance of NAD⁺ levels to maintain mitochondrial respiration and regulate Mφ phagocytosis in an anti-inflammatory homeostatic state, both in vitro and in vivo. NAD⁺ levels can be replenished by de novo synthesis and via the kynurenine pathway. Blockage of de novo NAD⁺ synthesis impaired phagocytosis and resolution of inflammation in aged Mφ [100].

In addition to sensing and cleansing cellular debris, Mφ also detect accumulation of mitochondrial garbage in the cellular microenvironment, leading to a continuous stimulation of these cells and thus their activation [101] and thus sustaining an environment of chronic low-grade inflammation with production of cytokines and ROS [97]. ROS accumulation in intracellular microenvironments (not only in the mitochondria) can cause DNA damage in aged tissues [28]. In the context of the skin, ultraviolet (UV) radiation-induced mtDNA injury also leads to more ROS production, accelerating photoaging [102]. In photodamaged skin, xanthine oxidase-induced ROS is reported to be the cause of alterations in collagen biosynthesis in cultured human dermal fibroblasts [103]. Moreover, other enzymatic and non-enzymatic sources of ROS are observed in the skin. Besides mitochondrial ROS production via electron transport chain and UV-induced ROS, there is production of ROS via peroxisomes, ER, and skin cell membranes [104]. In cultured Mφ, Ives and colleagues (2015) demonstrated that xanthine oxidase (XO) is the major source of ROS [105]. XO expression and activity has also been shown to be increased in old mice, closely associating with oxidative stress and exacerbated ROS formation [21].

As extensively revised by Beek and colleagues (2019), there is a tight link between mitochondrial dysfunction and ER in aged Mφ, that results in impaired calcium and redox homeostasis and leads to oxidative stress and activation of several pathways, including the inflammasome. Consequently, a proinflammatory environment is promoted by enhancing IL-1β secretion and nuclear translocation of NF-kB [19]. Furthermore, it has been proposed that this age-related oxidative-inflammatory stress (“oxi-inflammaging”) occurs in a vicious cycle during aging [106]. Taken together, it can be expected that age-related ER and oxidative stresses can contribute to an enhanced production of pro-inflammatory cytokines in Mφ and thus to systemic inflammaging, favoring the onset of pathologies.

Cellular senescence is a cellular state characterized by cell cycle arrest, even under growth-promoting conditions [2]. Other phenotypes of cellular senescence include apoptosis resistance and SASP [107]. In the human skin, senescent keratinocytes and fibroblasts accumulate with age and support a feedforward system mainly mediated by SASP to accelerate tissue function decay [108]. Senescent cells show increased production of proinflammatory cytokines, chemokines, growth factors and metalloproteinases [109], telomere attrition (see Section 2.2), epigenetic alterations (see Section 2.3), loss of proteostasis (see Section 2.4), and dysfunctional mitochondria (see Section 2.6), underscoring the many facets of the senescent cell phenotype.

The age-related alterations of immune system elements have been defined as immunosenescence and are characterized by changes in anatomical barriers, lymphoid organs and immune cell function, all of which synergize to result in a systemic organismal low-grade inflammation, deemed inflammaging [5].

Intrinsic and extrinsic factors of immunosenescence affect both recruited Mφ and TRMs. In homeostatic conditions, activated Mφ clear cell debris [110], but when senescent cell clearance is not effective, such cells accumulate and intensify SASP, causing several alterations in the local milieu, including Mφ dysfunction [111]. Using a mouse model, Prattichizzo and colleagues (2018) demonstrated that in hyperglycemic conditions, both cellular senescence and SASP can be induced in Mφ [90]. In aged tissues, the activated Mφ produces molecules that drive inflammatory response, such as IL-6, matrix metalloproteinases, chemokines and other mediators [112,113]. Together, these indicate that dysfunctional Mφ are both a result of dysfunctional niches and cellular senescence, in addition to contributing to the maintenance of low-grade tissue inflammation.

Stem cell exhaustion is a consequence of the sum of several hallmarks of aging mentioned above and is likely one of the main culprits for the loss of tissue regenerative capacity, and consequently organismal aging. Examples of that loss have already been related to immunosenescence [114,115].

Skin homeostasis is mainly maintained by two stem cell types: dermal mesenchymal stem cells (dermal MSCs), present in the inner layer of dermis, and epidermal stem cells (ESCs), located in the basal epidermal layer. While ESCs are responsible for epidermal cell renewal, a consequence of the capacity to differentiate into different cell lineages of the skin, such as keratinocytes and melanocytes [116,117], dermal MSCs are capable of differentiating into subcutaneous adipocytes, osteoblasts, and chondrocytes [118,119]. With aging, a loss of ESC and dermal MSC production and differentiation is observed, with consequent deceleration of skin cell renewal and reduction of skin healing capacity [119,120].

Tissue-resident hematopoietic cells, progenitors of Mφ, can self-maintain independently of hematopoietic stem cells (HSC). Mφ derived from yolk-sac are replaced by HSC-derived ones only in a few organs, including epidermis skin layer (LC originating from erythro-myeloid progenitors) [121]. This statement suggests a more important role is played by the cell’s origin than its tissue location in life span [14]. As a consequence, it is not surprising that the aged skin tends to have less LC in its composition, decreasing antigen-specific immunity [122].

Wound repair is a continuous process comprising four phases: hemostasis, inflammatory, proliferative, and remodeling (or resolution) phases [123]. With the advance in aging, there is a delayed lesion reepithelization and decreased tensile strength [124,125,126]. That may also indicate the importance of the presence of Mφ subpopulations in key moments of wound skin repair [123]. The M1-like Mφ profile contributes to the early stage of skin healing by promoting an inflammatory response with the production of high levels of proinflammatory cytokines. On the other hand, M2-like Mφ are responsible for the tissue repair process itself, including the regulation of re-vascularisation processes, fibroblast proliferation, and myofibroblast conversion, in addition to collagen production via inhibition of the AMPK/mTOR/NLRP3 inflammasome signaling axis, as discussed in Section 2.5. [93]. Immune cells such as Mφ are capable of activating epidermal stem cells for re-epithelialization under the establishment of an inflammatory wound microenvironment [127].

Through single-cell transcriptomic data analysis, a study was capable of characterizing a dermal subpopulation of Mφ that contributes to local nerve regeneration and axon sprouting after a mechanical injury [128]. Another study also observed that during skin repair, Mφ are also capable of stimulating the proliferation of adipocyte precursors [129]. Aged Mφ tend to lose their ability to migrate into wounds, with consequent retention of the Mφ at the dermis and increased tissue damaging release of ROS and proinflammatory cytokines [130]. In the epidermis, such an excessive proinflammatory microenvironment depletes epidermal stem cells, further contributing to compromised skin healing capacity [131].

The age-related changes in intercellular communication have been characterized in the autocrine, paracrine, endocrine, and neuroendocrine levels. The neurohormonal signaling (e.g., renin-angiotensin, adrenergic, insulin-IGF1 signaling) tends to be deregulated in aging as inflammatory reactions increase, immunosurveillance against pathogens and premalignant cells declines, and the composition of the peri- and extracellular environment changes, thereby affecting the mechanical and functional properties of all tissues, including the skin [2,132].

Recently, it was observed that mast cells are important in the recruitment of Mφ in aging through the change in the pattern of chemoattractant cytokines [133]. But there have still been few studies on how these intercellular communications involve Mφ in the skin. Still, it is already known that the age-related changes in intercellular communication are associated with inflammation. The accumulation of tissue damage throughout life, the likelihood of cytokines being secreted by senescent cells, the enhanced activation of the NF-κB transcription factor, and the occurrence of a defective autophagy response all seem to foster the immune system failure [96,134].