biomedicines
Review

Immunosurveillance of Cancer and Viral Infections with
Regard to Alterations of Human NK Cells Originating from
Lifestyle and Aging
Xuewen Deng 1, *, Hiroshi Terunuma 1,2 and Mie Nieda 1
12*

Citation: Deng, X.; Terunuma, H.;

Biotherapy Institute of Japan, Inc., 2-4-8 Edagawa, Koto-ku, Tokyo 135-0051, Japan;
terunuma@bij-net.com (H.T.); nieda.bij@gmail.com (M.N.)
N2 Clinic Yotsuya, 5F 2-6 Samon-cho, Shinjuku-ku, Tokyo 160-0017, Japan
Correspondence: tohxuewen@bij-net.com; Tel.: +81-3-5632-6080; Fax: +81-3-5632-6083

Abstract: Natural killer (NK) cells are cytotoxic immune cells with an innate capacity for eliminating
cancer cells and virus- infected cells. NK cells are critical effector cells in the immunosurveillance ofcancer and viral infections. Patients with low NK cell activity or NK cell deficiencies are predisposed
to increased risks of cancer and severe viral infections. However, functional alterations of human NK
cells are associated with lifestyles and aging. Personal lifestyles, such as cigarette smoking, alcohol
consumption, stress, obesity, and aging are correlated with NK cell dysfunction, whereas adequate
sleep, moderate exercise, forest bathing, and listening to music are associated with functional
healthy NK cells. Therefore, adherence to a healthy lifestyle is essential and will be favorable for
immunosurveillance of cancer and viral infections with healthy NK cells.
Keywords: NK cell; immunosurveillance; lifestyle; aging; cancer; viral infection

Nieda, M. Immunosurveillance ofCancer and Viral Infections with
Regard to Alterations of Human NK
Cells Originating from Lifestyle andAging. Biomedicines 2021, 9, 557.
https://doi.org/10.3390/
biomedicines9050557
Academic Editors: Veronique Baudand Amedeo Amedei
Received: 22 March 2021
Accepted: 14 May 2021
Published: 17 May 2021

Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims inpublished maps and institutional affiliations.

1. Introduction
The average global life expectancy has increased substantially over the past few
decades [1]. The aging of the population has led to a high prevalence of chronic diseases
such as cancer [2]. People with cancer have 5.8–11.2 years shorter life expectancy than theirpeers without cancer [3]. Lifestyles affect the incidence of chronic diseases including cancer
and life expectancy [4]. Lifestyle factors including cigarette smoking, alcohol consumption,
physical exercise, stress, obesity, sleep, forest bathing, and listening to music. Healthy
lifestyles are related to a long life expectancy free from major diseases such as cancer [5]and higher resistance to viral infections. Conversely, unhealthy lifestyles contribute to
the progression of severe diseases of such as COVID-19 [6] and increased risk of cancer
progression [7]. Therefore, the promotion of a healthy lifestyle is critical to the defense
against cancer and viral infections, and in improving life expectancy free of cancer andother associated chronic diseases [5]. In this paper, we review the immunosurveillance ofcancer and viral infections with regard to alterations of natural killer (NK) cells originatingfrom lifestyles and aging.
2. NK Cells

Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
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Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

NK cells were first identified in 1975 from the observation of their natural capacity tokill tumor cells without prior in vivo sensitization [8,9]. NK cells originate from the bonemarrow, and human NK cells comprise 5–20% of all lymphocytes and are defined phenotypically by their expression of CD56 and lack of CD3 expression. Two distinct populationsof human NK cells could be identified, based upon their cell-surface density of CD56. The
majority of human NK cells have low-density expression of CD56 (CD56dim ) and expression
high levels of CD16, which comprise about 90% of peripheral blood NK cells, whereas 10%of NK cells are CD56bright CD16dim or CD56bright CD16negative . The CD56dim subset is more

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naturally cytotoxic, whereas the CD56bright subset, which is localized in lymphoid tissue
has the capacity to produce abundant cytokines [10–12]. However, CD56bright NK cells canalso become cytotoxic upon appropriate activation and play roles in different disease states,
such as cancer and infections [13]. NK cell functions are tightly regulated by a balanceof activating and inhibitory germline-encoded receptors [14–16]. Healthy cells express
major histocompatibility complex class I (MHC-I) molecules, which mark these cells as“self”. MHC-I molecules act as ligands for inhibitory receptors on NK cells and contributeto “self-tolerance”, by preventing the-killing of these healthy cells by NK cells [15,17].
The MHC-I-specific inhibitory receptors include killer cell immunoglobulin-like receptors(KIRs) and lectin-like CD94-NKG2A heterodimers [15,16].
3. NK Cell Immunosurveillance of Cancer and Viral Infections
Cancer cells and virus-infected cells, down regulate MHC-I expression, which impairs
the engagement with inhibitory receptors on NK cells, and lowers the inhibitory signaling
threshold [15,17–19]. In addition, cancer cells and virus- infected cells upregulate NK cell
activity ligands, which in response to NK-cell -activating receptors, such as NKG2D, CD244,
NKp30, and NKp46, induce signaling pathways that trigger NK cell responses [15,16]. The
coactivation of these receptors overcomes the NK regulatory balance to mount an effective
response against cancer cells and virus- infected cells [15]. NK cell activation can resultin direct lysis of target cells, through cytotoxic degranulation by perforin and granzyme
B [15], and the simultaneous production of inflammatory cytokines, such as interferon-γ
and tumor necrosis factor-α, which activate other immune components including adaptive
response components [17,20].
NK cells play key roles in host defense against viral infections [21,22]. In humans, NK
cells are important to the innate immune response against members of the herpesvirus,
poxvirus, and papillomavirus families [23,24]. Patients with identified NK cell deficiencies
are predisposed to particularly severe, recurrent viral infection [24,25]. Mouse models
provide additional evidence that NK cells critically help in the control of several viral
infections, most notably murine cytomegalovirus, poxviruses, and influenza viruses [26,27].
Since their discovery, a numerous studies have demonstrated the NK-cell-mediatedkilling of many types of tumor cell line in vitro in experimental animal models [28–33].
NK cells contribute to the immunosurveillance of human cancer [34]. A prospective
cohort study of 3625 people from the general population with 11-year follow up for cancer
incidence and death indicated that people with medium or high NK cell activity had areduced risk of cancer, whereas those with low NK cell activity manifested an increased
cancer risk [35]. Additionally, patients with cancer, such as colorectal cancer [36], prostatecancer [37], and breast cancer [38] had decreased NK cell activity. Patients with metastatic
cancer had furthermore decreased NK cell activity [38]. Several studies have demonstrated
that a high degree of intratumoral infiltration of NK cells is associated with a favorable
outcome [39–41]. Moreover, NK cell activity predicted the response to chemotherapy andimmunotherapy for patients with cancer [42–44]. Therefore, functional healthy NK cells are
critical for anti-tumor and anti-virus immunities, whereas NK cell dysfunction increases
the vulnerability to viral infections and cancer (Figure 1).

Biomedicines 2021, 9, 557

A

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Healthy NK cell

Cancer cell/virus-infected cell
Clearance

B

Normal cell

Dysfunctional NK cell
Progression of
the diseases

Activating receptors
NKG2D, NCRs, etc.

Inhibitory receptors
KIRs, NKG2A, etc.

Ligands of activating
receptor

HLA class I

Cytokines andCytotoxic granules

Figure 1. Schematic depiction of NK cell immunosurveillance of cancer and viral infections. (A): Healthy NK cells expressing
activating receptors can recognize virus- infected cells and cancer cells, and release sufficient amount of cytotoxic granules
and cytokines to kill and clear virus-infected cells and cancer cells. (B): Dysfunctional NK cells expressing an imbalance ofactivating and inhibitory receptors with high expression of inhibitory receptors, malfunctions in the recognition of cancer
and virus-infected cells and the release of cytotoxic granules and cytokines to kill them, evasion of immunosurveillance ofviral infections and cancer, and the spread of viral infections and progression of cancer.

4. Lifestyles and NK Cells
Owing to the unique capacities of NK cells in host immunoprotection, especiallyanti-tumor and anti-viral immunities, research data on NK cells in relation to lifestyles
have accumulated over the past decades since the 1980s (Table 1). In this paper we provide
an updated overview of the association of human NK cells with personal lifestyles, such as
cigarette smoking, alcohol consumption, stress, obesity, sleep, exercise, forest bathing andlistening to music. In particular, we examine findings of human studies on the alterations ofNK cell count, percentage, subsets, phenotype, activity, cytokine secretion, and functional
granule components in association with lifestyles.

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Table 1. Overview of Lifestyles Associated Alterations of Human NK Cells.

Cigarette Smoke

Alcohol
Consumption

StressStudy

NK Cells CountNK Cell (%)

NK Cell
Subsets

Receptors
Expression on
NK CellNK Activity

Chemokine andCytokine
Production

Functional
GranuleComponents

Takeuchi M et al. [45]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Kusaka Y et al. [46]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Morimoto K et al. [47]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Jung YS et al. [48]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Li Q et al. [49]

↓

N/A

N/A

N/A

N/A

N/A

Perforin↓, GRN↓,
GrA↓

Kusaka Y et al. [46]

N/A

N/A

N/A

N/A

↔

N/A

N/A

Morimoto K et al. [47]

N/A

N/A

N/A

N/A

↔

N/A

N/A

Li Q et al. [49]

↔

N/A

N/A

N/A

N/A

N/A

GrA↓

Ochshorn-Adelson
M et al. [50]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Laso FJ et al. [51]

↑

N/A

N/A

N/A

AWLD↑, ALC↓

N/A

N/A

Perney P et al. [52]

↓

N/A

N/A

N/A

N/A

N/A

Perforin↓

Redwine L, et al. [53]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Irwin M et al. [54]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Morimoto K et al. [47]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Koga C et al. [55]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Andersen BL et al. [56]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Morikawa Y et al. [57]

↓

↓

N/A

N/A

↓

N/A

N/A

Bosclo P et al. [58]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Glaser R et al. [59]

N/A

↓

N/A

N/A

↓

N/A

N/A

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Table 1. Cont.

Study

NK Cells CountNK Cell (%)

NK Cell
Subsets

Receptors
Expression on
NK CellNK Activity

Chemokine andCytokine
Production

Functional
GranuleComponents

Lynch LA et al. [60]

N/A

↓

N/A

NKB1↑, CD158b↑,
CD69↑

N/A

N/A

N/A

Tobin LM et al. [61]

↓

↓

N/A

CD69↑, PD-1↑,
Glut-1↔

↓IFN-γ↔

GrB↓, Perforin↓↔

CD56bright ↑,CD56bright :NKG2D↑,
CD56dim :NKG2D↓, CD69↓

N/A

CD56bright :IFN-γ↑,
CD56dim : IFN-γ↓

N/A

↓

N/A

TRAIL↓

Bahr I et al. [62]N/A

CD56dim ↓Obesity

Sleep deprivation

Naujoks W et al. [63]

N/A

↔

↔

NKp46↓, CD158i↓,
NKG2A↑,
Siglec-7↑, CD62L↓,
CD27↓

Laue T et al. [64]

N/A

↔

↔

TRAIL↓, CD107a↓

N/A

N/A

N/A

Viel S et al. [65]

↔

↔

N/A

NKp46↓, CD94↓,
CD69↑,
CD16↓, CD107a↓

N/A

IFN-γ↔, MIP1-β↓

GrB↑

Irwin M et al. [66]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Irwin M et al. [67]

↓

↓

N/A

N/A

↓

N/A

N/A

Moldofsky H et al. [68]

N/A

N/A

N/A

N/A

↓

N/A

N/A

Dinges DF et al. [69]

↑

N/A

N/A

N/A

↑

N/A

N/A

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Table 1. Cont.

Exercise

Forest bathing

Music

Study

NK Cells CountNK Cell (%)

NK Cell
Subsets

Receptors
Expression on
NK CellNK Activity

Chemokine andCytokine
Production

Functional
GranuleComponents

Li Q et al. [49]

↑

N/A

N/A

N/A

N/A

N/A

Perforin↑, GRN↑,
GrA↑, GrB↑

Nieman DC et al. [70]

↔

↑

N/A

N/A

↑

N/A

N/A

Pedersen BK et al. [71]

N/A

↑

N/A

N/A

↑

N/A

N/A

Moro-Garcia
MA et al. [72]

N/A

↑

N/A

N/A

↑

N/A

N/A

Jung YS et al. [73]

N/A

N/A

N/A

N/A

↑

N/A

N/A

Nieman DC et al. [74]

↔

N/A

N/A

N/A

↑

N/A

N/A

Li Q et al. [75]

N/A

↑

N/A

N/A

↑

N/A

Perforin↑, GRN↑,
GrA↑,GrB↑

Li Q et al. [76]

↑

↑

N/A

N/A

↑

N/A

Perforin↑, GRN↑,
GrA↑, GrB↑

Li Q et al. [77]

↑

↑

N/A

N/A

↑

N/A

Perforin↑, GRN↑,
GrA↑, GrB↑

Li Q et al. [78]

↑

↑

N/A

N/A

↑

N/A

Perforin↑, GRN↑,
GrA↑,GrB↑

Li Q [79]

↑

N/A

N/A

N/A

↑

N/A

Perforin↑, GrA↑,
GrB↑

Wachi M et al. [80]

N/A

↑

N/A

N/A

↑

IFN-γ↑, IL-10↓

N/A

Koyama M et al. [81]

↔

N/A

N/A

N/A

↔

N/A

N/A

Hasegawa Y et al. [82]

N/A

↑

N/A

N/A

↑

N/A

N/A

N/A: Not Available; ↔: No change; AWLD: Chronic Alcolism Without Liver Disease; ACL: Alcohol-Induced Cirrhosis. The up arrows “↑”, increase; down arrows “↓”, decrease.

Biomedicines 2021, 9, 557

7 of 164.1. Cigarette Smoking
Cigarette smoke is a major health risk factor that increases the risk of developing
many types of cancer [83]. In a murine lung metastasis model, cigarette smoking impaired
NK-cell-dependent tumor immunosurveillance, and the resulting altered immunity was
associated with an increased tumor burden [84]. Studies in humans have shown that
NK cell activity is reduced in smokers [45–48]. Recently, a large cross-sectional study has
shown an inverse correlation between NK cell activity and the number cigarettes smoked
daily, and the total number of cigarettes smoked [48]. Cigarette smoke reduces the cytolytic
granules of perforin, granzymes, and granulysin on NK cells [49].
These finding suggest that cigarette smoking compromises NK cell function, enabling
cancer cells to evade immunosurveillance.
4.2. Alcohol Consumption
Chronic heavy consumption of alcohol by humans has been implicated as an etiological factor or cofactor in various types of cancer [85]. Alcohol is known to have a generally
suppressive effect on the immune system in humans [86,87]. However, research data about
the effects of alcohol on NK cell activity are conflicting [50]. There are results indicating
the NK cell count in the peripheral blood is increased [51] or decreased [52] by alcohol.
Some results have shown an alcohol-related decrease in NK cell activity [53,54], whileother studies did not find any significant effect of alcohol on NK cell activity [46–48]. A
study of nonalcoholic male volunteers administered alcohol either intravenously or orally
to has showed that a single dose did not alter NK cell activity even when the blood alcohol
concentrations reached 89 mg/dl. However, when NK cells are exposed to alcohol in vitro
for 4h at concentrations of 80 mg/dl and above, a significant concentration-dependent
decrease in NK cell activity was observed [50]. Another study comparing the effects ofalcohol on NK cells between patients with chronic alcoholism without liver disease (AWLD)
and those with alcohol-induced cirrhosis (ALC) has shown that alcohol by itself induces anincrease in the number and activity of NK cells in peripheral blood. In contrast, the NK
cell activity is constantly depressed in the stage of alcoholic cirrhosis, suggesting that the
behavior of NK cells in peripheral blood in chronic alcoholism differs depending on thepresence or absence of ALC [51]. In addition, the level of perforin expression by NK cells
is decreased in patients with chronic alcoholism [52]. The alterations of NK cells inducedby chronic alcohol consumption, particularly in the stage of alcoholic cirrhosis, causes
decreased immunosurveillance, which may contribute to the higher incidence of cancer.
4.3. Stress
Psychological stress affects various immune functions [88]. The hypothalamic-pituitaryadrenal (HPA) axis has been considered as a pathway along which psychological stress- istransposed into an impaired immune function [89]. NK cells express adrenergic receptors,
which respond to catecholamines triggered by a stress response [90]. Subjects who complained of an unstable mental state have lower NK cell activity than those who reported astable mental state [47]. Anxiety about cancer suppresses NK cell activity [55]. Stress levels
in patients with breast cancer negatively correlated with NK cell activity [56]. Chronic job
stress is associated with decreased NK cell count, proportion, and activity in peripheral
blood [57]. People with job insecurity have decreased NK cell activity [58]. Stress depresses
interferon (IFN) production by leukocytes concomitant with decreased NK cell activity [59].
Therefore, stress increases vulnerability to cancer and viral infections.
4.4. Obesity
Obesity increases the risk of many cancers and severe COVID-19 infection [91,92], andis responsible for up to 40% of cases of cancer [93]. Obesity drives chronic inflammation,
which precedes the development of comorbid diseases including cancer [94]. NK cellanti-tumor responses are negatively regulated in obesity [95]. The number of NK cells isdecreased in obese people [60,61], and is further decreased in obese people with unhealthy

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metabolic profiles [60]. There are increased CD56bright NK subset and decreased CD56dim
subset in obese people, and correlates with body mass index (BMI) [62]. Obesity is associated with various alterations of the NK cell phenotype. Studies demonstrated that theexpression level of the functional marker TRAIL and the activating NK receptor NKp46 are
reduced in obese people [63–65]. In contrast, studies showed a highly activated NK cells inobese people with increased expression levels of CD69, and programmed cell death protein (PD)-1, as well as the inhibitory receptors of NKB1, CD158b, CD158i, NKG2A/CD94
complex, and Siglec-7 [60,61,63,65]. In addition, the expressions of CD16, and the adhesion
molecule CD62L, and the maturation and differentiation marker CD27 on NK cells are
downregulated in obese people [63]. Despite the enhanced activation, NK cells showeda decreased expression level of degranulating marker CD107a and decreased secretionof granzym B and perforin as well as the macrophage inflammatory protein (MIP)-1β in
response to cancer cell lines in obese people [61,64,65]. Furthermore, the cytotoxicity of
NK cells against cancer cells is significantly impaired in obese humans [61,63]. Aside fromthe characterization of total NK cells, the impact of obesity on the CD56dim and CD56brightsubset-specific NK cell phenotype has recently been investigated, which demonstrated
decreases in NKG2D-positive and CD69-positive CD56dim NK cells, while there was anincrease in NKG2D-positive CD56bright NK cells in obese people. In addition, a study ofcytokine production demonstrated a decreased expression level of IFN-γ positive CD56dim
NK cells, while there was an increased expression of IFN-γ positive CD56bright NK cells
in obese people. These alterations are correlated with BMI [62]. Therefore, obesity is
associated with alterations of NK cell frequency and phenotypes and an impaired capacity
to defend against cancer cells and virus-infected cells [61,63], which contribute to a higher
cancer risk and susceptibility to viral infections in obese people. However, weight loss wasfound to reverse the obesity-induced defects in NK cells [62,96,97].
4.5. Sleep Deprivation
Sleep has a critical role in promoting health [98]. Sleep affects two primary effector
systems, the HPA axis and the sympathetic nervous system (SNS), which in turn regulate
immune responses [98,99]. NK cell activity is dependent in part on sleep. NK cell countand activity are minimum during the early part of the night and reach the maximum
in the late morning hours [100]. There is a positive association between sleep time andefficiency with NK cell activity [101]. Loss of sleep at night resulted in decreases in both thenumber of NK cells and NK cell activity. After a night of recovery sleep, NK cell activity
returned to baseline levels [66,67]. In addition, one night of sleep deprivation decreased
NK activity [68]. In contrast, two nights of sleep deprivation increased NK activity [69].
The alterations of NK cell triggered by sleep loss were found to be mediated throughaugmented levels of glucocorticoids and catecholamine [99]. The resulting reduction in
NK cell activity increased the vulnerability to cancer and viral infections.
4.6. Exercise
People who exercise almost daily have a reduced number of days of sickness [70,102].
Research has shown that positive immunological changes occur during moderate exercise [103]. NK cells have attracted the attention of exercise scientists for more than
30 years [104]. It has been reported that acute physical exercise strongly affects the NK cellcount in peripheral blood [105]. The NK cell number increases immediately after cessationof exercise, followed by transient decreases in NK cell count and activity. Depending on
the exercise regime (type, duration, and intensity), the decreases have been observed after
15–30 min and can persist for more than 24 h [105,106]. NK cells are rapidly mobilized
into the circulation in response to acute exercise, most likely by epinephrine-dependentβ–adrenergic signaling [107,108]. This mobilization primarily affects CD56dim NK cells
and is driven by the expression of adhesion molecules of CD11a and CX3CR1 [107,109].
In view of the physical fitness level, studies unanimously revealed an increased NK cell
activity in subjects with good physical constitution [70–74]. NK cell count and activity

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are increased in racing cyclists compared with nonathletes [71]. Consistent with thesefindings, athletes were found to have higher NK cell count and activity [72,74]. People
with exercise habits have higher counts of NK cells and perforin-, granulysin-, granzyme
A/B- expressing NK cells [49]. Therefore, exercise habits strengthen the NK cell function to
defend against cancer and viral infections.
4.7. Forest Bathing
A forest bathing trip is a short, leisurely visit to a forest and is regarded as being
similar to natural aromatherapy. Incorporating forest bathing trips into a good lifestyle was
first proposed in 1982 by the Forest Agency of Japan. The results of studies using the Profile
of Mood States test demonstrated that a forest bathing trip significantly increased thescore for vigor and decreased the scores for anxiety, depression, and anger [75,76]. Forest
bathing stabilizes autonomic nervous activity and significantly decreases the concentrationsof stress hormones such as adrenaline and noradrenaline [75,77,78]. The frequency andcount of NK cells, and the expression levels of granulysin, perforin, and granzymes A/B
on NK cells increased on the forest bathing days, resulting in an increase in NK cell
activity [75–79,110]. The increased NK cell activity lasted for more than 30 days after thetrip [75,77,79]. The mechanisms underlying the increased NK cell activity during forest
bathing may be partially related to an attenuated stress hormone response and also tobreathing in of volatile organic compounds, called phytoncides produced by trees, such
as α–pinene and limonene [75–77,110]. In vitro research data indicate that phytoncides
increase NK cell activity in a dose-dependent manner and prevent dichlovos-induced
inhibition of NK cell activity [79]. Therefore, forest bathing trips have a positive effect on
immunosurveillance against cancer and viral infections.
4.8. Music
Music therapy has attracted the attention of various fields such as psychiatry andgeriatrics. Music therapy improves mood states by decreasing tension/anxiety, depression/dejection, anger/hostility, fatigue/inertia, and confusion/bewilderment and increases
vigor/activity [80,81,111,112]. Music reduces the levels of stress hormones [113] and provides relief from stress [114,115]. The frequency of NK cells and NK cell activity are
increased after music therapy [80,82]. Therefore, music provides a “wellness environment”,
which improves defense against cancer and viral infections.
5. Aging with NK Cells
Aging is accompanied by dysregulated immune function that contributes to an increased susceptibility to diseases, such as cancer and infections [116]. Physiological aging
is associated with changes in the composition, phenotype, and function of circulating NK
cells [117]. Studies to date have shown the effects of aging on human NK cells (Table 2).
Significant increases in the percentage and absolute number of NK cells are the generalobservations reported [118–120]. Regarding to NK cell subsets, studies have shown that the
proportions and number of CD56dim NK cells increases with age [118,120,121], and older
adults possess significantly fewer CD56bright NK cells [118–121], resulting in a marked
age-related increase in the CD56dim :CD56bright ratio [118,120,121]. Regarding the effects
of aging on the NK cell phenotype, there is an age-related decrease in the expression
level of activating receptors, such as NKp30 and/or NKp46 [118,120], and the expression
levels of inhibitory receptors such as CD94 and/or NKG2A also show age-associatedreductions [118,120,121]. Additionally, the proportion of CD57, a marker of NK maturity, ishigher in older adults [120], whereas the expression level of CD69, an activation marker, increases with aging [119]. However, the increased expression level of CD57 on NK cells has
been shown to be affected by infection by viruses, such as human cytomegalovirus [122].
CD57 is not necessarily a marker of aging/immunosenescence [123]. Importantly, aging
is accompanied by decreased NK cell activity, which may be due to an age-associatedimpairment in perforin secretion [120]. Prospective studies have demonstrated that low

Receptors ExNK Ac- Chemokine and Cy- Functional GranNK Cells NK Cell NK Cell Subdy
pression on NK
tivitytokine Production ule Components
Count
(%)
sets
Cell
2021, 9, 557
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NKp46↓,
eira A etBiomedicinesal.
CD56bright↓,
↑
↑
N/A
N/A
N/A
NKp30↓, CD94↓8]
CD56dim↑ernier M, et↑↔
CD56bright↓
CD69↑
N/A
IFN-γ↔
↔
NK cell activity is associated with an increased susceptibility to viral infections [124] and19]cancer[35].
bright↓, NKp46↓, CD94↓,
CD56
↓
↑
N/A
Perforin↓
et al. [120]
N/A
dim
↑
CD56
CD57↑
Table
2. Overview
of Aging
Associated Alterations of Human NK Cells.
CD56bright↓,
al. [121]
N/A
N/A
NKG2A↓
N/A
N/A
N/A
Receptors
Chemokine
Functional
NK Cells CD56dim↑
NK CellStudy
NK Cell (%)
Expression on
NK Activity
and Cytokine
GranuleCount
Subsets
NKdownCell arrows “↓”, decrease.
Production
Components
N/A: Not Available; ↔: No change. The up arrows “↑”, increase;
Almeida-Oliveira
A et al. [118]↑↑CD56bright ↓,CD56dim ↑NKp46↓,
NKp30↓, CD94↓

N/A

N/A

N/A

6. Immunosurveillance of Cancer and Viral Infections by Healthy NK Cells

Le Garff-Tavernier
↑
↔
CD69↑
↔
IFN-γ↔
N/A
CD56bright ↓M, et al. [119] NK cells are lymphocytes of the innate immune system, which can deal promptly
Hazeldine
NKp46↓, CD94↓,
CD56bright ↓, andwith stressedvirus-infected
cells,
whileN/A
also regulating
theN/A cells, such↑ as cancer cells↓
Perforin↓
J et al. [120]CD57↑
CD56dim ↑body’s adaptive immunoresponses[125]. However, alterations in NK cells are associatedCD56bright ↓,N/A
N/A
NKG2A↓
N/A
N/A
N/A
dim
with certain lifestyles [47,49,124,126]
CD56 ↑and aging [117]. Personal lifestyles, such as cigarette
Not Available;↔: No change.
The up obesity,
arrows “↑”, increase;
down arrows“↓”, decrease. with NK cell dyssmoking,N/A:
alcoholconsumption,
stress,and agingare associatedfunction, whereas adequate sleep, exercise, forest bathing, and listening to enjoyable mu6. Immunosurveillance of Cancer and Viral Infections by Healthy NK Cells
sic are associatedwith a healthy NK cell function (Figure 2). Therefore, adherence to a
NK cells are lymphocytes of the innate immune system, which can deal promptly
healthy lifestyle is essential
and will be favorable to the immunosurveillance of cancer andwith stressed cells, such as cancer cells and virus-infected cells, while also regulating theviral infections withhealthy NK cells.
body’sfunctional
adaptive immunoresponses[125]. However, alterations in NK cells are associatedLutz CT et al. [121]with certain lifestyles [47,49,124,126] and aging [117]. Personal lifestyles, such as cigarettesmoking, alcohol consumption, stress, obesity, and aging are associated with NK cell
dysfunction, whereas adequate sleep, exercise, forest bathing, and listening to enjoyable
music are associated with a healthy NK cell function (Figure 2). Therefore, adherence to ahealthy lifestyle is essential and will be favorable to the immunosurveillance of cancer andviral infections with functional healthy NK cells.

Figure 2. Schematic short list of the impacts of lifestyles and aging on NK cells. Cigarette smoking,alcohol consumption, stress, obesity, and aging suppress NK cell function, leading to a dysfunctional
NK cells, whereas sleep, exercise, forest bathing, and listening to music enhance NK cell function
with maintained functional healthy NK cells.

Figure 2. Schematic short list of the impacts of lifestyles and aging on NK cells. Cigarette smoking,alcohol consumption,
stress,obesity, and aging suppress NK cell function, leading to a dysfunc7. Future
Directions
tional NK cells, whereas
sleep,
andfactors
listeningenhance
NK cell
Although exercise,
there are forest
many bathing,confounding
for tothemusic
associations
amonglifestyle,function with maintained
cells. future research is needed to provide scientific
NK cells, functionaland cancerhealthyor viral NK
infections,evidence of immunosurveillance of cancer and viral infections in relation to the alterationsof NK cells caused by lifestyles and aging. Clinical studies are also necessary to clarify the
7. Future Directions
importance of improving lifestyles in disease prevention.
Although thereFor
aremany
confounding
for the associations
among lifestyle,diseasetreatmentusing NK factors
cells, NK-cell-based
adoptive immunotherapy
has been
NK cells, and cancer
or viral
infections,is needed
to provide
scientificintroduced
for cancer
treatmentfutureand hasresearchbeen considered
for viral
infections [19,127,128].
To obtain convincingofevidence
theviralimportanceof NK
for disease
it isevidence of immunosurveillancecancer ofandinfections
incells
relationto thetreatment,
alterationsof NK cells caused by lifestyles and aging. Clinical studies are also necessary to clarify theimportance of improving lifestyles in disease prevention.

Biomedicines 2021, 9, 557

11 of 16

necessary to treat cancer or viral infections using the combination of NK cell therapy andlifestyle improvement. To this end, further studies are necessary.
8. Conclusions
NK cells play key roles in the innate elimination of cancer cells and virus-infected cells.
Alterations of NK cells are associated with lifestyles and aging. Adherence to a healthy
lifestyle is essential and will be favorable to the immunosurveillance of cancer and viral
infections with healthy NK cells.
Author Contributions: Conceptualization, X.D. and H.T.; writing—original draft preparation, X.D.;
writing—review and editing, H.T. and M.N. All authors have read and agreed to the publishedversion of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data sharing not applicable.
Conflicts of Interest: X.D., H.T. and M.N. hold patents for feeder-cell-free NK cell expansion culture
technology and are stock holders of Biotherapy Institution of Japan Inc., a biotech startup company
developing NK-cell-based immunotherapy and Mesenchymal Stromal Cell (MSC)-based products,
such as adipose MSCs, and MSC-derived conditioned medium (CM) concentrates and extracellular
vesicle (EV) isolates (StemSup® ) for clinical research.

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