Background
With the development of medical and sanitary conditions over the past 50 years, the average human life expectancy in most countries has significantly increased [
1]. In China, owing to the long life expectancy and low birth rate, the fraction of the population aged > 60 years will outnumber children aged < 5 years by 2020, and between 2015 and 2050, Chinese population will rapidly double from 12 to 22% of the world’s population, reaching approximately 2 billion by 2050. The rate of ageing in Chinese population is fast, and the proportion of elderly people will reach 28% of the population by 2040 [
2].
Ageing is always accompanied by poor health and declining physiological functions, elevating incidences of infectious diseases and a number of pathologies that have a common inflammatory origin, such as atherosclerosis and cardiovascular disease, type II diabetes, arthritis, dementia, Alzheimer’s disease, osteoporosis or cancer [
3]. This poses great challenges and burdens pertaining to social health and well-being, healthcare and economic growth; therefore, attention to healthy ageing prospects is crucial.
Immunosenescence is a likely mechanism underlying these ageing-related diseases. It is defined as the state of deregulated immune function contributing to increased susceptibility of the elderly to infection and possibly to autoimmune diseases and cancer [
4]. When immunosenescence occurs, the functional capacity of the immune system of the host gradually declines with age. The most marked changes in the immune system that occur with age involve the T cell compartment, the arm of the immune system that protects against pathogens and tumours [
4,
5]. Therefore, immunosenescence can reflect ageing-related decrease in immune function at the cellular and serological levels [
6] as well as can concomitantly occur with the hyper stimulation of both innate and adaptive immune systems that may result in a low-grade chronic state of inflammation known as inflammaging [
7,
8]. One of the key underlying mechanisms of this phenomenon is thymus involution, which is almost complete by the age of 60 years.
The gut microbiota, including a wide variety of microorganisms in large numbers inhabiting in the gastrointestinal (GI) tract, plays an important role in maintaining GI homeostasis and regulating host metabolism as an integral part of the host system. Studies have shown that each individual has at least 160 taxonomic species belonging to 1000–1150 prevalent bacterial species; their collective genome (‘microbial group’) comprises at least 100 times as many genes as the human genome [
9]. The composition of the microbiota develops during the first few years of life and is relatively stable during adult life [
9,
10]. Gut microbiota could be influenced by host genetics, health, diet, ageing and probiotics [
10‐
13]. Ageing-related changes in gut microbial composition may be associated with many diseases and disorders in the elderly [
11,
14,
15]. Some recent studies have also indicated that gut microbes, particularly certain selected strains, might enhance cell-mediated immunity in host animals, thereby altering ageing-related immunosenescence [
16]. However, mechanisms and timing of changes in gut microbial composition with age remain unclear, and only a few previous studies have focussed on the potential association between human immunosenescence and gut microbiota during the ageing process.
The present study was conducted to characterize ageing-related changes in properties of gut microbiota from adult to elderly stages and to investigate whether these changes are associated with host immunity.
Discussion
Several studies have observed differences in gut microbial composition between the elderly and younger people. Such ageing causes changes in gut microbial community and may therefore influence host physiological functions, including immunity, and could be involved with potent pathogens of various ageing-related diseases and disorders, particularly of noncommunicable diseases [
19]. For example, immunosenescence could be caused by an abnormally activated immune response to gut microbiota, which might be due to diminished mucosal tolerance, ageing-related changes in gut microbiota or both [
20]. Evidence has been obtained from studies in which the administration of dietary probiotics improved systemic immune responsiveness [
21]. Therefore, immunosenescence and changes in gut microbiota might be associated with each other and might concurrently affect health. However, the association between these two changes has not been studied in great depth. In the present study, the associations between age, gut microbiota composition and immunity in both adults and the elderly were analysed with participants from southwest China, are relatively isolated region with its own characterized local culture and diet. They represent the general population of a restricted geographic area and can be considered as a relatively homogeneous cohort in terms of their lifestyle and dietary habits. In China, most people retire at the age of 60 years, with their lifestyle greatly changing after this age. Based on this, the study participants were divided into two groups as follows: middle-aged group (50–59 years) and elderly group (≥ 60 years).
Human gut microbiota comprises approximately 500–1000 species that belong to only a few known bacterial phyla. The most abundant phyla are
Firmicutes,
Bacteriodetes,
Proteobacteria,
Verrumicrobia and
Actinobacteria [
22]. The
Bacteroidetes/
Firmicutes ratio varies from 2.5 to 0.09 between infants and adults, with 1.6 in the elderly [
23]. Low
Bacteroidetes/Firmicutes ratio has been suggested as one of the hallmarks of human gut microbiota because this ratio has been observed to be lower in obese individuals in some studies in humans [
24]. In the present study,
Firmicutes (58.80%) and
Bacteroidetes (21.06%) were the most abundant, followed by
Proteobacteria (10.56%),
Actinobacteria (5.51%) and
Verrucomicrobia (2.70%), which were detected in all of the tested participants. These results are in agreement with those reported by the previous studies [
22]. However,
Bacteroidetes/Firmicutes ratio was almost 0.35, which was relatively lower than those reported in other elderly people [
23]. Overall, the results obtained in the present study indicate that although all the tested participants are clinically healthy, they are at a potential risk of metabolic disorders in the context of their intestinal microbial composition. Conversely, these results might raise a question regarding the reported association between low
Bacteroidetes/Firmicutes ratios and metabolic disorders considering that the tested participants are from the region in southwest China with a less obese and overweight population, whose diets are well known for being healthy.
The present study also indicated that some genus compositions, including
Alistipes,
Desulfovibrio,
Faecalibacterium,
Lachnobacterium and
Oxalobacter, markedly decreased with age in the adults aged ≥ 50 years, whereas the percent of genus
Sutterella significantly increased in the elderly group. Earlier studies have demonstrated a significant decrease in beneficial bacteria, such as
Bacteroides,
Lactobacillus and
Bifidobacterium, with age [
15]. Elderly individuals had a higher proportion of species belonging to phylum
Bacteroidetes, suggesting that
Bacteroidetes are beneficial to the elderly [
3]; However, in the present study,
Alistipes (in the phylum
Bacteroidetes) decreased with age, both in middle-aged and elderly groups.
Faecalibacterium and
Lachnobacterium from the phylum
Firmicutes decreased with age in middle-aged and elderly groups, respectively.
Faecalibacterium, particularly
F. prausnitzii is one of the important commensal microbes of human gut microbiota. They can synthesize butyrate and other short-chain fatty acids through the fermentation of dietary fibre to promote host health. In healthy adults,
F. prausnitzii represents more than 5% of the intestinal bacteria. Lower than normal levels of
F. prausnitzii are have been known to be associated with the development of Crohn’s Disease, obesity, asthma and major depressive disorder [
25‐
27].
Lachnobacterium belongs to the family
Lachnospiraceae; members of this family may provide protection against colon cancer in humans by producing butyric acid [
28]. In addition,
Desulfovibrio and
Oxalobacter (in the phylum
Proteobacteria) decreased in the middle-aged and elderly groups, respectively.
O. formigenes, a member of the genus
Oxalobacter, is an obligate anaerobe and digests oxalate as a carbon and energy source for cells. A lack of
O. formigenes colonizing the colon is a risk factor for the development of hyperoxaluria and calcium oxalate stones. Further, levels of the genus
Coprococcus were significantly higher in the elderly group than those in the middle-aged group.
Coprococcusis a genus of anaerobic cocci; compared with younger adults,
Enterococcilevels are higher in the elderly, as reported in previous Asian studies [
19]. Based on these previous studies, the ageing-related decreases in
Alistipes,
Desulfovibrio,
Faecalibacterium,
Lachnobacterium and
Oxalobacter among the tested participants could negatively affect their health. To the best of our knowledge, this is the first study to demonstrate that levels of
Alistipes,
Desulfovibrio,
Faecalibacterium,
Lachnobacterium, Oxalobacter and
Coprococcus could be decreased during the process of ageing although the tested subjects were specific in terms of their geographic origin and life style. Our findings indicate that ageing could negatively affect the quality and quantity of human gut microbiota though different mechanisms across different populations and that its effects are not limited to
Bifidobacteria and
Lactobacillus.
In addition to gut microbial composition, several studies have shown lower richness and diversity of gut microbiota colonized in nonagenarians or centenarians than those in young adults [
29]. In this study, the microbial richness (Chao index and observed species) of the tested participants were observed to decrease with age in the elderly group. These results were consistent with those reported in the previous studies although ageing-related changes in gut microbiota have been observed to occur at a much earlier age. Recent studies have suggested that the loss of richness and diversity in gut microbiota, or called dysbiosis is associated with increased frailty, which might be caused by the accumulation of disorders and an imbalance in the intestinal ecosystem that leads to decreased immunity and increased susceptibility to pathogens [
30].
Immunosenescence is most likely a combination of autoimmunity, immunodeficiency and immune dysregulation, which accompanies a chronic state of low-grade inflammation associated with physiological ageing [
31]. Evidence has been presented to support the hypothesis that the inflammatory status of the elderly might be partly caused or aggravated by changes in microbial proportions [
3], such as high plasma proinflammatory cytokine (IL-6 and IL-8) levels, which are associated with the presence of bacteria belonging to the phylum
Proteobacteria [
32].
In the middle-aged group, a significantly positive correlation was observed between the presence of phylum Bacteroidetes and IgG levels, which were positively correlated with IgM levels; whereas the presence of Firmicutes was negatively correlated with IgM levels. Bacteroidetes/Firmicutes ratio was also positively correlated with serum IgG and IgM levels. Antibodies, such as IgG and IgM, as the key components of host humoral immunity can play an important role in protecting the host from various infections. Our findings indicated that ageing-related decrease in Bacteroidetes levels and Bacteroidetes/Firmicutes ratio could be associated with the dysregulation of host humoral immunity.
In terms of cellular immunity, the presence of phylum Bacteroidetes was positively correlated with the percent of CD8+ T cells and negatively correlated with CD4+/CD8+ ratio. CD8+ T cells are also known as cytotoxic T cells because they express the CD8 glycoprotein on their surfaces, destroy virus-infected cells and tumour cells and are also implicated in transplant rejection. Our findings indicated that Bacteroidetes might stimulate host cellular and humoral immunity. Therefore, these results indicate that ageing-related decrease in Bacteroidetes observed in the present study could be associated with the dysregulation of host cellular immunity.
In the elderly group, the presence of phylum
Verrucomicrobia was positively correlated with serum IgA levels and the percent of CD8
+ T cells but negatively correlated with the percent of CD4
+ T cells and CD4
+ to CD8
+ ratio.
A. muciniphila is the only species of
Verrucomicrobia that shows anti-inflammatory effects in humans, and studies have shown that the presence of this species is associated with the development of irritable bowel syndrome, obesity and type-2 diabetes [
33‐
36]. Therefore, our findings indicate that the microbes taxonomically belonging to
Verrucomicrobia may be a group of microbes that benefit the host. Therefore,
Verrucomicrobia could be considered as an important biomarker to monitor host immune status in the elderly.
In the present study, the percent of CD8+ T cells decreased with decrease in the diversity (Simpson index) of gut microbiota in the middle-aged group, whereas decrease in diversity was not significant in the elderly group. In contrast, serum IgA levels decreased with decrease in the richness (Chao index and observed species) of gut microbiota in the elderly group. Therefore, our findings indicate that ageing-related decrease in gut microbial diversity could be associated with the dysregulation of host immunity with various faces dependent on the different ageing process. These results demonstrate that gut microbiota could be closely associated with the immune status of the elderly and that they could act as potent pathogens to cause or influence various ageing-associated diseases and disorders.