Alcohol, aging, and innate immunity

Short abstract

Review of aging and alcohol: how innate immunity is modulated in the aged and after alcohol consumption.

Abstract

The global population is aging: in 2010, 8% of the population was older than 65 y, and that is expected to double to 16% by 2050. With advanced age comes a heightened prevalence of chronic diseases. Moreover, elderly humans fair worse after acute diseases, namely infection, leading to higher rates of infection‐mediated mortality. Advanced age alters many aspects of both the innate and adaptive immune systems, leading to impaired responses to primary infection and poor development of immunologic memory. An often overlooked, yet increasingly common, behavior in older individuals is alcohol consumption. In fact, it has been estimated that >40% of older adults consume alcohol, and evidence reveals that >10% of this group is drinking more than the recommended limit by the National Institute on Alcohol Abuse and Alcoholism. Alcohol consumption, at any level, alters host immune responses, including changes in the number, phenotype, and function of innate and adaptive immune cells. Thus, understanding the effect of alcohol ingestion on the immune system of older individuals, who are already less capable of combating infection, merits further study. However, there is currently almost nothing known about how drinking alters innate immunity in older subjects, despite innate immune cells being critical for host defense, resolution of inflammation, and maintenance of immune homeostasis. Here, we review the effects of aging and alcohol consumption on innate immune cells independently and highlight the few studies that have examined the effects of alcohol ingestion in aged individuals.

Abbreviations

cDC

conventional dendritic cell

DC

dendritic cell

NET

neutrophil extracellular trap

NLR

nuclear oligomerization domain‐like receptor

PAMP

pathogen‐associated molecular pattern

pDC

plasmacytoid dendritic cell

pMHC

peptide MHC

PRR

pattern‐recognition receptor

RLR

retinoic acid inducible gene‐I like receptors

ROS

reactive oxygen species

Introduction

Innate immunity is critical for successful host defense, and controlling and resolving the inflammatory and innate immune response is equally important to preventing immune‐mediated tissue damage. Advanced age and alcohol consumption both have immunomodulatory effects, which result in changes in morbidity and mortality rates associated with acute and chronic diseases, including infection [1, 2–3]. Innate immune cell function is independently affected by both increased biologic age [4, 5–6] and alcohol ingestion, in a drinking pattern– and dose‐dependent manner [3, 7]. However, the combined insult of alcohol consumption in older populations on aspects of host defense, innate immunity, and inflammation has not been thoroughly evaluated. Most people older than 65 y drink alcohol, and it has been estimated that >10% of this population drinks at levels considered hazardous to their health [8]. Unhealthy, also termed hazardous, alcohol consumption is defined as 3 drinks/d or 7 drinks/wk for women and 4 drinks/d or 14 drinks/wk for men [9]. Given that both aging and alcohol change innate immune cell function, studies examining the effect of alcohol consumption in older individuals are necessary to understand how host defense and immunoregulation may be altered in this population. Herein, we will review what is currently known about alcohol consumption in older individuals, the independent effects of aging and alcohol misuse on innate immunity, and a synopsis of the few studies that have examined immunity in the context of alcohol consumption and aging.

INFLAMM‐AGING

Underlying many diseases associated with aging is low‐grade, heightened, systemic inflammation, termed inflamm‐aging, characterized by greater levels of proinflammatory mediators, including C‐reactive protein, TNF‐α, IL‐1β, and IL‐6 [10, 11–12]. Inflamm‐aging also occurs in other animals, including mice; aged mice (older than 18 mo) have elevated basal circulating levels of proinflammatory mediators compared with younger (2–6‐mo‐old) mice [10, 13]. A greater incidence of chronic diseases in the elderly, such as type 2 diabetes, osteoporosis, and heart disease, is associated with inflamm‐aging [14]. In addition, inflamm‐aging contributes to worsened outcomes after infection [15, 16–17]. Specifically, the elderly, defined as those older than 65 y are more susceptible to infections and have more complications after infection, leading to longer hospital stays and increased mortality [18, 19]. The underlying dysfunction in innate immune cells associated with aging is at least partly due to inflamm‐aging, and these age‐related changes in innate immunity contribute to poorer outcomes after infection in older populations [20].

PREVALENCE OF ALCOHOL CONSUMPTION IN OLDER INDIVIDUALS

Reports of the frequency of alcohol consumption in the elderly vary based on the population studied. However, it is clear that, globally, many older individuals regularly consume alcohol, and several drink enough to be considered unhealthy or hazardous [9]. Specifically, studies examining older individuals in a variety of countries have estimated that >40% of older humans drink alcohol, 10–20% of the elderly drink at hazardous levels, and more men than women were reported to consume alcohol at dangerous levels [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31–32]. Moreover, the Center for Disease Control and Prevention reported that the largest population of binge drinkers is composed of those older than 65 y [33]. The association between alcohol consumption, or the level of alcohol consumed, with self‐medication to treat symptoms of age‐mediated health deterioration, including pain, has been shown in some, but not all, studies [25, 34, 35]. Alcohol consumption in younger adults has been reported to have both beneficial and detrimental health effects, including effects on the immune system, related to the quantity and pattern of its consumption [7, 36]. Furthermore, the immunologic effects of alcohol consumption and advanced age are surprisingly similar, as, like advanced age, binge (3 or 4 drinks/d for women and men, respectively) and chronic (7 or 14 drinks/wk for women and men, respectively) alcohol consumption alters the function of innate immune cells [7, 9] and worsens mortality after infection in humans and in animal models [3]. Therefore, the circumstances leading to unhealthy alcohol consumption in the elderly and the health consequences of varying patterns of alcohol consumption need to be evaluated in older populations.

INNATE DETECTION OF PATHOGENS WITH ADVANCED AGE OR AFTER ALCOHOL CONSUMPTION

Multiple types of receptors are involved in the detection of pathogen and pathogen‐derived moieties, leading to the activation of similar signaling pathways in many types of innate immune cells. PAMPs are detected by PRRs, which include TLRs, RLRs, and NLRs [37]. These receptors are present on innate immune cells [38] and nonimmune cells, including epithelial and endothelial cells [39, 40]. Expression of PRRs on innate immune cells has been shown to vary with both advanced age and after alcohol consumption. Although not all studies report the same direction of change, most studies examining TLR expression in aging mice suggest advanced age leads to a reduction in TLR expression [41, 42, 43–44]. This finding has also been shown in humans because multiple studies have reported diminished TLR expression in aged, compared with younger, subjects [45, 46, 47–48]. Importantly, it has been consistently demonstrated that signaling downstream of TLRs is reduced with advanced age, leading to lower proinflammatory cytokine production from innate immune cells in older mice [41, 44, 49, 50] and humans [51]. The exception to these findings seems to be triggering of TLR5, which leads to enhanced signaling after cells from aged humans and mice are treated with TLR5 agonists [52, 53]. The effect of alcohol consumption on TLR expression seems to be dependent on the amount and duration of consumption, such that chronic alcohol ingestion in rats and mice has been shown to up‐regulate TLR expression [54, 55–56], whereas binge alcohol consumption was shown to decrease [57] or have no effect [58] on TLR expression. Moreover, binge alcohol consumption inhibits signaling pathways downstream of TLRs, including the MAPK and NF‐κB pathways, in rodent and human cells [59, 60, 61, 62, 63–64], whereas chronic alcohol consumption results in amplified signaling in those same pathways [65, 66, 67, 68–69]. Thus, binge alcohol consumption results in a diminution in the production of proinflammatory cytokines in response to TLR stimulation, namely TNF‐α and IL‐6 [59, 62, 63, 70, 71, 72, 73–74], even in studies in which TLR expression is not altered [58]. Conversely, chronic alcohol consumption results in heightened production of those same cytokines [66, 67, 68–69, 71, 75, 76–77]. However, another study suggests the dichotomy between binge and chronic alcohol ingestion and cytokine production is not that simple; it showed that TNF‐α production by human blood cells was increased after acute alcohol exposure after simultaneous treatment with TLR2 and TLR4 agonists in vitro, which likely mirrors the multiple stimuli that an innate immune cell incurs during a natural infection in vivo [78].

Non‐TLR PRRs can be modulated with advanced age and after alcohol consumption. Aging negatively affects the function and expression of NLRs, RLRs, and their associated signaling complexes in animal models and in human immune cells [42, 79, 80–81]. The effect of alcohol consumption on the stimulation of other PRRs is less well established, although there is evidence that signaling downstream of NLR stimulation is reduced after alcohol exposure in mice and isolated human immune cells [82, 83–84]. In summary, advanced age and alcohol consumption modulate PRR‐mediated signals, resulting in changes in cytokine production, the direction of which is dependent on the amount and frequency of alcohol consumed and pathogen‐derived signals encountered. Thus, the ability of innate immune cells to detect and respond to pathogens and PAMPs is diminished with advanced age and after alcohol consumption, and ingestion of alcohol in the aged will likely have combinatorial or synergistic effects on PRR expression and signaling.

THE EFFECTS OF ADVANCED AGE AND ALCOHOL MISUSE ON INNATE IMMUNECELL SUBSETS

Neutrophils

Neutrophils are quickly recruited to sites of infection or injury. Once present, neutrophils can phagocytose pathogens and extracellular debris, secrete cytokines, and release neutrophil NETs, leading to pathogen clearance. Enhanced or prolonged recruitment and delayed clearance of neutrophils can cause tissue damage [85]. Therefore, a fine balance must be maintained, which requires coordinated activity and timing and depends on the magnitude of inflammatory initiation, maintenance, and resolution [85]. The number of circulating neutrophils present in aged animals and humans is similar to that of younger hosts; however, the proliferation of neutrophil precursors in the bone marrow is lessened with advance age [86, 87, 88–89]. In humans, the life span of neutrophils may differ in older and younger individuals because studies have shown that the normal extension of the neutrophil life span imparted by stimuli (such as PAMPs) no longer occurs in aged populations [89, 90, 91, 92–93]. In mice, binge alcohol consumption was shown to block the differentiation of granulocytes from progenitor cells, including neutrophils, during infection [94, 95, 96, 97–98], which may be due, in part, to alcohol‐mediated changes in G‐CSF or hormone levels [97, 99, 100–101].

In addition to changes in their life span, neutrophil recruitment is altered with advanced age and after alcohol consumption. Some groups have reported that, with advanced age, neutrophil recruitment to the site of inflammation or infection was reduced or delayed, which was not due to lessened neutrophil chemoattractant levels [102, 103, 104, 105, 106–107]; yet, other studies showed heightened neutrophil recruitment in aged mice compared with young mice, possibly because of higher levels of cellular‐adhesion molecules present on the endothelial cells that neutrophils transverse to migrate from the circulation into tissues [108, 109, 110–111]. The directed migration of neutrophils is compromised with advanced age both in vitro and in vivo, likely because of perturbations in signal transduction [104, 112]. Conversely, some studies suggest aging does not affect neutrophil trafficking [113, 114], expression of integrins important for neutrophil entrance into tissues [115], or neutrophil adherence capacity ex vivo [116, 117–118]. However, those are the minority of studies, and the discrepancies likely reflect the difference in pathogen‐ and injury‐mediated signals driving neutrophil migration. Binge alcohol consumption impairs neutrophil recruitment to sites of infection in mice and humans; nevertheless, in some cases, the neutrophils remain at, and/or continue to be recruited to, inflammatory sites longer in rodents given alcohol compared with controls, leading to prolonged inflammation and tissue damage [119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129–130]. Reduction in surface expression of critical integrins (e.g., CD11b/c and CD18) or their ligands on endothelial cells (e.g., ICAM‐1) might contribute to the altered ability of neutrophils to enter tissues after alcohol ingestion [131, 132, 133, 134, 135–136] because of the importance of these receptors for neutrophil migration [137]. Thus, the age‐ and alcohol‐mediated changes in neutrophil trafficking are likely context specific and may reflect changes in the kinetics of the overall immune response in older or intoxicated hosts. Additionally, the multiple pathways and molecules involved in neutrophil recruitment (e.g., expression and activation of integrins, cytoskeletal alterations, and signal transduction capacity) are altered independently with advanced age and after alcohol consumption; therefore, it would be logical to hypothesize that alcohol ingestion in the aged would cause more drastic defects in neutrophil recruitment, resulting in poorer neutrophil‐driven, antipathogen defenses.

Beyond their ability to be recruited to sites of inflammation and infection, other functions of neutrophils are diminished with advanced age and after alcohol consumption. Human and mouse neutrophil production of inflammatory mediators (including myeloperoxidase, elastase, IL‐6, and IL‐8) is lessened with advanced age, but those cells also make more anti‐inflammatory cytokines (e.g., IL‐1 receptor antagonist, IL‐10) [47, 88, 89, 92, 138], suggesting that their ability to promote a normal inflammatory response is reduced with advanced age. Treatment of human neutrophils with alcohol in vitro blunted their production of cytokines, such as IL‐8 and TNF‐α, after stimulation with bacterial products [139]. Other critical functions of neutrophils associated with pathogen clearance, including their ability to phagocytose, form NETs, and produce ROS (critical for intracellular killing of phagocytosed pathogens and for NET formation), are perturbed with advanced age and after alcohol consumption. Neutrophils from older mice and humans are less able to phagocytose and kill pathogens intracellularly compared with those from younger hosts [89, 115, 116, 140, 141, 142, 143, 144–145], which may be due to factors such as altered actin polymerization, changes in bioenergetics, or reductions in the expression of critical phagocytosis receptors [47, 115, 146]. Interestingly, unstimulated neutrophils from older human subjects exhibit higher ROS levels than do those from younger subjects, and the enhanced production of ROS might be responsible for age‐associated damage to the vascular endothelium [147]. Furthermore, another study suggests the observed basal, hyperactivated state of neutrophils in aged mice (i.e., increased ROS production) might be due to microbiome‐derived signals that change with age [148]. Therefore, some differences in the findings examining age‐mediated alterations in neutrophil function may be due to variations in the conditions and/or stimuli studied; unstimulated neutrophils from aged hosts seem to be hyperactivated, yet stimulation of neutrophils from aged hosts results in a reduction in their function.

Most studies report a diminished phagocytic capacity in neutrophils from rodents after chronic or binge alcohol ingestion [121, 149, 150, 151–152], despite two other studies suggesting binge alcohol ingestion has no effect on phagocytosis [153, 154]. A few reports propose that the impaired phagocytosis by neutrophils in alcohol‐treated animals may be due to lower expression levels of important receptors involved in phagocytosis [135, 136, 155]. Interestingly, reports show that both chronic and binge alcohol exposure boosts the production of ROS by neutrophils [121, 149, 150, 156]; yet, the ability of neutrophils to kill pathogens is lessened after both binge and chronic alcohol exposure, which may partially be due to their aforementioned compromised phagocytic capacity [150, 157, 158]. There are no reports to date regarding the effect of alcohol consumption on NET formation, but NET formation is blunted with advanced age, possibly because of poorer ROS production in neutrophils from aged humans and mice [88, 89]. Alcohol consumption and advanced age have negative consequences for multiple neutrophil functions, contributing to the worsened immunity in individuals that have consumed unsafe amounts of alcohol and in the elderly. Thus, neutrophils from aged animals or humans who have consumed unhealthy amounts of alcohol will likely be further weakened in their ability to respond to an injury or infection. Figure 1 summarizes the altered functions of neutrophils associated with alcohol misuse and advanced age.

Figure 1.

The effect of advanced age and unhealthy alcohol consumption on neutrophils. The effects of advanced age and hazardous alcohol consumption are depicted. A downward arrow (↓) denotes a decrease in the illustrated function.

Macrophages

Mϕs perform several critically important tasks, both in the setting of infection/injury and in healthy tissue homeostasis. In the context of an immune response, Mϕs contribute to host defense via detection of microbes, initiation and promotion of inflammatory responses, stimulation of adaptive immune responses, and importantly, resolution of inflammation [159, 160]. Fewer Mϕs, and their weakened functions, have been associated with enhanced susceptibility to infection in aged hosts [161], and both binge and chronic alcohol‐mediated changes in Mϕs lead to worsened outcomes after infection and injury, including greater mortality [162, 163, 164–165]. Age‐mediated changes in Mϕ functions were recently, comprehensively reviewed in Albright et al. [166], but the major findings will be reviewed in this section. Mϕs from aged mice and nonhuman primates are skewed to a more‐proinflammatory state before exogenous stimulation, and they contribute to worsened inflammation in older animals [167, 168–169]. Despite the heightened inflammatory state of Mϕs with advanced age, the signaling cascade after treatment with PAMPs is impaired in Mϕs from aged humans and animals compared with that of younger controls [166]. Specifically, downstream of PRR engagement, the production of proinflammatory cytokines (i.e., TNF‐α, IL‐1β, and IL‐6) by Mϕs from aged mice and humans is decreased after in vitro and in vivo stimulation, and in some cases, a concomitant rise in the levels of the anti‐inflammatory cytokine IL‐10 was reported [43, 45, 48, 49, 166, 170, 171, 172, 173, 174, 175, 176, 177, 178–179]. However, other reports have shown that the levels of proinflammatory cytokines produced by Mϕs from the aged are greater or equivalent to levels produced by Mϕs from younger mice [13, 108, 180, 181–182]. These discrepancies reflect different stimulation conditions and anatomic locations from which the Mϕs were isolated, which suggests specific stimulation conditions highlight different age‐mediated changes in Mϕ function (e.g., in vivo infection or injury vs. TLR mimetics used ex vivo) are changed with advanced age, and is a reminder that Mϕs from various locations (e.g., spleen, peritoneal cavity, lungs) are not functionally identical [183].

Binge and chronic alcohol ingestion‐mediated changes in the ability of Mϕs to detect pathogens through PRRs, the most well‐studied being responses downstream of TLRs, is altered. In particular, there is a reduction in Mϕ TNF‐α production after LPS stimulation of TLR4 [64, 72, 78, 120, 184, 185–186]. Chronic alcohol consumption has the opposite effect on PRR signaling; chronic alcohol consumption results in enhanced TLR and NLR signaling in human and murine Mϕs, resulting in more TNF‐α (and other proinflammatory cytokines, like IL‐1β) produced by these cells [186, 187, 188–189]. Beyond differences in proinflammatory cytokines, alcohol consumption also alters the production of the anti‐inflammatory cytokines IL‐10 and TGF‐β, depending on the duration of the alcohol consumption. Binge alcohol exposure results in heightened production of IL‐10 and TGF‐β by human and rodent Mϕs, but chronic alcohol ingestion leads to lower levels produced by those cells [71, 190, 191, 192–193]. Thus, binge alcohol consumption results in Mϕs that produce more anti‐inflammatory, compared with proinflammatory, cytokines after stimulation, whereas chronic alcohol ingestion shifts that ratio toward more proinflammatory, rather than anti‐inflammatory, cytokines. Therefore, in the context of an infection, Mϕs from aged individuals or those who consume unhealthy amounts of alcohol are impaired in their ability to promote a proper, balanced inflammatory response. Consequently, aged, alcohol‐consuming hosts will likely have Mϕs that are hyperinflammatory without stimuli, but upon infection or injury, produce abnormal levels of cytokines, reducing overall morbidity and survival.

Another key function of Mϕs is phagocytosis, both of foreign pathogens and of dying cells (termed efferocytosis). Most studies show that the phagocytic ability of Mϕs from rodents in the context of infection is reduced with advanced age [194, 195, 196, 197, 198, 199, 200, 201–202]; yet, two studies show that it is augmented [203] or unchanged [204]. However, it has been shown in several models that the blunted phagocytic ability by Mϕs from aged subjects is thought to contribute to poor resolution of inflammation [205] and recovery from injury [206, 207, 208, 209–210], suggesting that impaired phagocytosis by Mϕs in aged hosts has serious consequences for recovery from injury or infection. Some studies have shown that defects in Mϕ phagocytosis are indirect, meaning that the aged microenvironment influences the phagocytic capacity of Mϕs [199], and others have demonstrated that the shortcoming in phagocytic capacities from aged individuals is intrinsic to Mϕs or their progenitors in the bone marrow [200, 211].

As thoroughly reviewed in Karavitis and Kovacs [212], most studies show a decline in the ability of human and rodent Mϕs to phagocytose after both binge and chronic alcohol consumption, although two studies in mice demonstrated an enhancement in phagocytosis by liver Mϕs (Kupffer cells) and splenic Mϕs after binge and chronic alcohol exposure, respectively [212]. The variation in findings across studies likely reflects the heterogeneity of Mϕ populations, the physiologic context in which these cells were examined (i.e., normal homeostasis vs. infection or injury), and whether that function was examined in vitro or in vivo. To conclude, multiple aspects of Mϕ functions are altered with advanced age and after alcohol ingestion, resulting in worsened outcomes after infection or injury. A better understanding of the age‐ and alcohol‐mediated influences on Mϕ functions will shed light onto whether or not drinking in aged individuals will further impair their ability to detect and clear pathogens. However, current evidence suggests that alcohol misuse in the elderly will lead to even greater negative consequences for Mϕ function and, subsequently, host survival. Figure 2 summarizes the hypothesized effects of advanced age and alcohol consumption on Mϕ function.

Figure 2.

Changes in Mϕ function associated with binge alcohol consumption and advanced age. Alterations associated with advanced age and alcohol intoxication are summarized before and after immunologic challenge. An upward arrow (↑) denotes an increase in the specified function, and a downward arrow (↓) signifies a decrease in the denoted function.

DCs

DCs are another very important, innate immunecell population, and evidence suggests they are functionally impaired with advanced age and after alcohol consumption. DCs can be divided into two major subtypes: cDCs and pDCs. cDCs are considered professional APCs; they phagocytose pathogens or pathogen‐derived Ags at the site of infection and traffic to the local, draining lymph node, where they present Ags to cognate T cells [213]. pDCs can also act as APCs, but they are best known for their ability to produce high amounts of type I IFN, particularly in response to viral infections. pDCs can also act as regulatory DCs because they can induce the expansion and promote the function of other regulatory cell subsets, such as T_regs [213]. Multiple studies have examined the frequency of circulating DCs in aged human populations, and they suggest that there are either equivalent or lower cDC and pDC numbers in older vs. younger humans, depending on the study [214, 215, 216, 217, 218, 219, 220–221]. In animal studies, there has also been variation in the findings examining the frequency of DC populations in aged hosts, citing strain‐specific differences in DC frequency in distinct anatomic locations with advanced age in mice [222, 223–224]. Animal studies examining alcohol misuse show differences in DC frequency too, as chronic alcohol consumption causes a reduction in DC number in the spleen and skin, but it does not alter the frequency of DC precursors or DC turnover rate [225, 226–227]. Moreover, DC migration is impaired after chronic alcohol consumption in mice [228, 229]. Thus, similar to Mϕs, advanced age and alcohol consumption affect DCs in distinct compartments differently.

A critical function of DCs is to serve as APCs, which involves Ag acquisition, the up‐regulation of costimulatory molecules, and secretion of cytokines [213]. DCs from older humans have a reduced ability to phagocytose compared with those from younger humans, potentially because of changes in signaling pathways important for cytoskeletal rearrangement [214, 216]. The expression of costimulatory molecules on DCs, such as CD40, CD80, and CD86, is lessened with advanced years, whereas PD‐L1, a molecule that inhibits T cell responses, is enhanced [230, 231]. One study showed that in vitro differentiated DCs from the elderly express similar levels of these molecules as DCs do from younger people [217], but that discrepancy between studies may be due to differentiation of the DCs in vivo [230, 231] vs. in vitro [217]. Chronic alcohol consumption in animals blunts the ability of DCs to up‐regulate costimulatory molecules [227, 232, 233, 234–235]. In aged animals and humans, the production of proinflammatory cytokines by DCs, including type I IFNs, IL‐6, and TNF‐α, are decreased, but the level of the anti‐inflammatory cytokine IL‐10 is greater than in younger hosts [44, 222, 236, 237, 238, 239, 240–241]. However, not all studies support the conclusion that DCs from older hosts are unable to promote inflammation; some studies have reported no difference in the production of those cytokines with age, or an even an increase in IL‐6 and TNF‐α by DCs from older, compared with younger, humans and rodents [46, 214, 217, 242, 243–244]. Interestingly, some of these studies used self‐antigens to promote DC cytokine production, which has implications for the rise in the incidence of autoimmune disease associated with aging [245] and potentially points to a dichotomy in DC function after interacting with self‐ vs. pathogen‐derived moieties. Similar findings have been shown for alcohol consumption; most studies have shown that both binge and chronic alcohol ingestion by rodents reduces the ability of DCs to produce proinflammatory cytokines, including IL‐1β, TNF‐α, IL‐6, and IL‐12 [58, 233, 234–235], but one study examining circulating human DCs from alcoholics showed a heightened ability to produce those same proinflammatory cytokines when compared with control subjects [246]. The differences in the reported findings examining cytokine production by DCs with advanced age or with unhealthy alcohol use reflect the various conditions used (stimulation type, anatomic location that DCs were isolated from, type of DCs, among others). However, any alterations in cytokine production (enhanced or diminished production) by DCs will have negative downstream effects on host immunity because of the role of DCs in bridging the innate and adaptive immune response, leading to poorer pathogen clearance and/or amplified tissue damage.

Any changes in DC function (e.g., costimulatory molecule expression and cytokine production) have the potential to affect T cell responses. In mice, the ability of DCs to prime T cells is lessened with advanced age, both in vitro [237, 247, 248] and in vivo [222, 231, 249]. Conversely, some studies have demonstrated that DCs from older and younger animals and humans have the same capacity to serve as APCs or that DCs from aged hosts have slightly augmented APC abilities [242, 244, 250, 251, 252–253]. The inconsistencies in findings examining the ability of DCs to prime T cells are probably due to differences in the ages, locations, and population purities (ratio of cDCs to pDCs) used in each study, suggesting further work is necessary to elucidate exactly how aging negatively influences DC function. Both chronic and binge alcohol consumption results in a decline in the capacity of DCs to stimulate T cells [232, 233, 234–235]. Therefore, the combination of advanced age and alcohol misuse will likely lead to decreased T cell activation by DCs because of fewer DCs available to stimulate the T cells, reduced costimulatory molecule expression by the DCs, and aberrant cytokine production by those critical APCs, as shown in Fig. 3 . A reduction in DC function, and a subsequent blunted T cell response, weakens host responses to infection and immunologic memory, which will ultimately cause enhanced susceptibility to infection and infection‐mediated complications.

Figure 3.

DC function is impaired with advanced age and alcohol misuse. DC functions altered by aging and alcohol misuse are shown. A downward arrow (↓) signifies a decrease in the function or outcome.

NK cells

NK cells are critical in detecting and killing infected and transformed cells. They respond based on the ratio of signals from stimulatory and inhibitory receptors: a higher proportion of activated stimulatory receptors vs. inhibitory receptors elicits proinflammatory responses, and a higher proportion of activated inhibitory receptors compared with stimulatory receptors elicits anti‐inflammatory responses [254]. In humans, it has been shown that the number of circulating NK cells is greater with advanced age, and the NK cell population shifts such that there are fewer NK cells with an immature phenotype (CD56^bright) and more mature NK cells (CD56^dim), which may contribute to aging‐associated immunosenescence because there are fewer immature NK cells available to respond to new insults [221, 255, 256, 257, 258, 259, 260, 261, 262, 263, 264–265]. Mouse models do not reproduce this age‐related increase in mature NK cells observed in humans, despite using appropriately aged animals (i.e., mice older than 18 mo). In fact, studies have shown that the frequency of NK cells in mice declines with age, including a decrease in mature, compared with immature, NK cells in circulation in old, compared with young, mice [266, 267]. The reduction in circulating NK cells in mice is thought to be due to impaired hematopoiesis and bone marrow egress, but how that relates to humans is not well understood [266, 267–268]. The differences between mice and humans have not been well studied, but one theory to explain the difference is that the specific pathogen‐free mice used in immunologic studies do not experience infections across their life span, like humans do [269]. Studies examining the frequency of NK cells in humans and rodents after alcohol misuse report either no change in NK cell numbers and percentages in the blood and lymphoid organs [270, 271], or they report fewer NK cells [272, 273, 274–275]. The discrepancies among these studies likely lie in the duration of alcohol ingestion and the time after exposure in which NK cells were examined. For example, 1 study demonstrated that 2 wk of alcohol ingestion had no effect on NK cell frequency, but 8 wk of alcohol consumption resulted in a reduction in NK cells [270]. One potential explanation for the reduction in NK cell numbers after alcohol consumption is a reduced proliferative capacity of NK cells after ingestion of alcohol, which has been shown in mice [276].

NK cell function is altered with both advanced age and alcohol misuse. The ratio of expression of inhibitory to stimulatory receptors on NK cells is heightened with advanced age [255, 261]. Some reports have shown that cytokine production by NK cells, such as type I and II IFNs, decreases with advanced age, but others have shown no difference between NK cells from younger and older people [255, 261, 277, 278]. These differences may be due to the aforementioned anatomic and stimulation conditions used by each group interrogating NK cell function in the context of advanced age. A critical role of NK cells is their cytotoxicity in that the diminution of NK cell cytotoxicity in mice leads to decreased cancer immunosurveillance [279, 280]. Some laboratories have documented a diminished cytolytic capacity from NK cells in older individuals [260, 261, 281]. Interestingly, one study extended these findings, showing that impaired immunologic synapse formation, and subsequent declined release of cytolytic granules, occurred with advanced age in human NK cells [260]. Human NK cell production of cytokines and chemokines is also blunted after alcohol consumption [282, 283], and several studies in humans and rodents have shown that NK cell cytotoxicity is lower after alcohol consumption [272, 273–274, 284, 285, 286, 287, 288, 289, 290, 291, 292, 293, 294, 295–296]. Studies examining alcohol misuse that have shown weakened NK cell cytotoxicity, along with a reduction in NK cell frequency, suggest the two likely occur simultaneously, rather than the lessened cytotoxicity measured is because of fewer NK cells to kill target cells [272, 274, 284]. Granzymes (A and B) and perforin are the major effector molecules involved in the ability of NK cells to kill other cells, and both binge and chronic alcohol ingestion in mice leads to a decrease in their expression [297, 298, 299, 300–301]. Given that NK cells treated with alcohol in vitro lose cytotoxic abilities in a dose‐dependent manner, alcohol likely has direct effects on those cells [291, 293, 295, 298]. Additionally, in vivo and ex vivo studies in mice have also suggested direct suppressive effects of alcohol on murine NK cells [292]. However, factors extrinsic to NK cells, including alcohol‐mediated changes in the neuroendocrine interactions, also contribute to the reduction in their cytotoxicity [287, 288–289, 302, 303, 304, 305–306]. In summary, the consensus is that advanced age and alcohol misuse impair multiple functions of NK cells independently, which could reduce host immunity and tumor‐surveillance mechanisms. Therefore, the combination of alcohol consumption and advanced age likely converge to modulate NK cell function, leading to poorer immunosurveillance of cancer cells, as depicted in Fig. 4 . These findings can also be extended to infection, where the killing of infected cells is critical for host clearance of many viruses and intracellular bacteria [307].

Figure 4.

NK cell cytotoxicity is diminished with advanced age and after hazardous alcohol consumption. NK cell killing of a tumor cell is depicted, and the associated defects associated with advanced age or alcohol intoxication are shown with a downward arrow (↓) to demonstrate a decrease in the described function or outcome.

CURRENT EVIDENCE REGARDING ALCOHOL CONSUMPTION, ADVANCED AGE, AND IMMUNITY

Alcohol consumption and disease in older populations

A thorough assessment of the health effects of alcohol use and abuse in older individuals is necessary. A few studies have linked higher incidence, prevalence, earlier onset, and/or worsened disease in older individuals to alcohol consumption. Regardless of age, in adults, the health effects of alcohol depend on the amount of alcohol consumed: low to moderate drinking has been associated with positive health outcomes, whereas hazardous drinking habits have been linked to worsened outcomes [7]. An important consideration in understanding the effects of alcohol consumption in the elderly is that alcohol metabolism is slower in older adults, and consequently, its effects are likely prolonged and potentially worsened [308, 309–310]. Moreover, the elderly are more likely to be taking prescription and over‐the‐counter medications, which can have adverse interactions with alcohol [311, 312]. Additionally, poorer gut‐barrier integrity contributes to disease in the elderly and in alcohol abusers [313, 314]. Therefore, hazardous drinking in the elderly may result in “intestinal intoxication” after low amounts of alcohol consumed, exacerbating inflammation and contributing to worsened disease outcomes. Along those lines, it is known that both advanced age and alcohol consumption alter the microbiome, which can contribute to disease [313, 315, 316]. Thus, the current guidelines regarding healthy alcohol consumption may need to be adapted for older populations.

The evidence to date suggests that hazardous alcohol consumption exacerbates disease and worsens mortality rates in the elderly. There is a decline in overall survival in older humans who consume high amounts of alcohol compared with low to moderate drinkers [317], and, similarly, older alcohol abusers have a greater rate of hospital remittance compared with those that do not consume alcohol at hazardous levels [318]. Hazardous alcohol consumption by older individuals has been associated with a larger incidence of prediabetes, type 2 diabetes, and death from cardiovascular disease, whereas low to moderate levels were linked to reduced incidence rates of all three [319, 320, 321–322]. Another major cause of death in the elderly is infection [2, 323], and alcohol consumption can affect infection outcomes in the elderly. The incidence of respiratory infections is increased in elderly individuals that drink hazardous levels of alcohol [324], and alcohol consumption predisposes older individuals to develop pneumonia [325]. In summary, the mechanism by which alcohol consumption patterns can have beneficial vs. harmful effects on disease, specifically in older populations, is not well understood but clearly merits consideration.

Animal studies examining the effect of advanced age and alcohol ingestion

Despite the heightened infection and pneumonia rates in the elderly associated with unhealthy alcohol consumption, very few studies have examined the effect of alcohol ingestion on immunity in animal models of aging, and none, to our knowledge, have looked at how alcohol alters immune cell function in older humans. Moreover, most of these very few studies have focused on adaptive immune parameters. The only study to date that has examined innate immunity/early inflammation did so indirectly: gene expression patterns of proinflammatory cytokines in the brains of older, alcohol‐treated mice showed that alcohol ingestion enhanced brain region–specific MCP‐1 (also called CCL2 expression) but not IL‐6 or TNF‐α expression [326]. In studies examining aspects of the adaptive immune system, the proliferation of T cells from spleens of older mice fed an alcohol‐containing diet was lower than that of older mice fed a normal diet and compared with younger mice fed either diet [327, 328]. The reduction in T cell proliferation was not due to decreased IL‐2 production (critical for T cell proliferation) by the T cells [327]. Interestingly, those results were recapitulated when T cells from younger or older mice were cultured with alcohol in vitro, suggesting that the suppressive effect of alcohol has a direct effect on the function of immune cells [329]. In addition, the response to sheep red blood cells, a T cell–dependent response, was suppressed after alcohol consumption, and that suppression was even greater when the alcohol‐treated mice were older [327]. More recently, it was shown that T cell cytokine production may be amplified after low‐dose alcohol ingestion in older animals, leading to an enhanced, delayed‐type hypersensitivity response [330]. Together, these studies suggest that the dose of alcohol administered dictates the direction of changes in immune cells of older hosts. T cell responses cannot be independently initiated. They require the involvement of innate immune cells [213]; consequently, changes in innate immunity may contribute to the additive effects of alcohol consumption and advanced age, and those effects may be dependent on the amount and the duration of alcohol ingestion. Although few studies have examined the effect of alcohol administration on immune function in older animals, current evidence suggests that alcohol consumption in older hosts worsens the ability of the immune system to properly respond to challenges.

Conclusions

Advanced age and alcohol consumption have profound effects on innate immune cells. In fact, many of the pathways and functions of innate immune cells are common between aging and alcohol ingestion, as shown in 1, 4. Therefore, the effects of alcohol intoxication in the elderly are likely combinatorial, if not synergistic, resulting in poorer outcomes after infection for this population. The alcohol‐mediated effects on innate immune (and other) cells are dose dependent, and older individuals metabolize alcohol more slowly; thus, alcohol lingers in the body of older people longer than it does in those who are younger, exerting sustained effects on multiple organ systems [310]. Little is known about how alcohol consumption modifies already impaired host‐defense mechanisms in older humans, but the evidence that exists to date suggests that alcohol ingestion may translate to weakened initiation of immunity and immunologic memory in the elderly. Thus, more work examining the effect of a range of doses of alcohol on factors intrinsic and extrinsic to innate immune cells in older humans is critical to set healthy drinking guidelines for this population and to design therapeutics to target immune alterations in those older individuals with unhealthy drinking behaviors.

AUTHORSHIP

L.A.B. composed and E.J.K. edited the manuscript.

DISCLOSURES

The authors declare no conflicts of interest.