Science Eventually Cracked The Code For Immortality

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Abstract

Non-specific innate and antigen-specific adaptive immunological memories are vital evolutionary adaptations that confer long-lasting protection against a wide range of pathogens. Adaptive memory is established by the memory of T and B lymphocytes following the recognition of an antigen. On the other hand, innate immune memory, also called trained immunity, is imprinted in innate cells such as macrophages and natural killer cells through epigenetic and metabolic reprogramming. However, these mechanisms of memory generation and maintenance are compromised as organisms age. Almost all immune cell types, both mature cells, and their progenitors go through age-related changes concerning numbers and functions. The aging immune system renders the elderly highly susceptible to infections and incapable of mounting a proper immune response upon vaccinations. Besides the increased infectious burden, older individuals also have heightened risks of metabolic and neurodegenerative diseases, which have an immunological component. This review discusses how immune function, particularly the establishment and maintenance of innate and adaptive immunological memory, regulates and is regulated by epigenetics, metabolic processes, gut microbiota, and the central nervous system throughout life, with a focus on old age. We explain in-depth how epigenetics and cellular metabolism impact immune cell function and contribute to or resist the aging process. cistanche benefícios Microbiota is intimately linked with the immune system of the human host, and therefore, plays an important role in immunological memory during both homeostasis and aging. The brain, which is not an immune-isolated organ despite former opinion, interacts with the peripheral immune cells, and the aging of both systems influences the health of each other. With all these in mind, we aimed to present a comprehensive view of the aging immune system and its consequences, especially in terms of immunological memory. The review also details the mechanisms of promising anti-aging interventions and highlights a few, namely, caloric restriction, physical exercise, metformin, and resveratrol, that impact multiple facets of the aging process, including the regulation of innate and adaptive immune memory. We propose that understanding aging as a complex phenomenon, with the immune system at the center role interacting with all the other tissues and systems, would allow for more effective anti-aging strategies.

Keywords Immune memory.Immunosenescence.Aging.Trained Immunity.Metabolism.Microbiota

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introduction

Human beings, like all organisms, inevitably age and die. Even if science eventually cracked the code for immortality, that would not end the need to understand the mechanisms of aging and the efforts to slow or revert it. If anything,it will be even more critical to maintain the health of all cells and organs throughout a long life. Tackling aging is always a worthwhile effort to improve the quality of life for the middle-aged and elderly populations, especially since the human population over 60 years of age is expected to reach two billion by 2050 [1]. Infectious diseases of the elderly, especially in low-income countries, represent a significant social and economic burden. The immune system undergoes numerous changes as humans age, leaving older individuals more prone to disease [2]. Cistanche Extract Anti Radiation The age-related dysregulations in the immune system are collectively referred to as"immunosenescence" and include accumulating tissue dam-age, a low-grade chronic systemic inflammation termed "inflammaging, "impaired immune cell function, inadequate response to vaccination, and increased vulnerability to infections [3].

The importance of immune memory has perhaps never been more evident than during the ongoing coronavirus dis-ease 2019(COVID-19) pandemic, which disproportionately affected the elderly population due to the altered function of their immune system [4]. Thanks to the outstanding collaborative effort of governments and scientists, 7 vaccines generating effective immune response and protection against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)have been authorized for emergency use by World Health Organization (WHO)-recognized authorities as of June 2021, and many more are in use with authorizations by national regulatory agencies[5]. Due to the increased vulnerability of the elderly, they are the priority group in COVID-19 vaccination rollouts.

Besides the morbidities caused by infection, the elderly also present an increased incidence of metabolic diseases such as type 2 diabetes and obesity [6], and neurodegenerative disorders such as Alzheimer's and Parkinson's diseases [7]. However, the development of these age-related conditions is not separated from their aging immunity. All systems and organs exchange signals with and are influenced by the immune system. Combining all the accumulating insights from different lines of research is critical to drawing up a comprehensive view of aging.

In this review focusing on immune memory, we first outline how memory is developed and maintained. Next, we delve into metabolic and epigenetic mechanisms, their roles in immune memory, how they change with age, and the implications for age-related pathologies. As two examples of the far-reaching impacts of an aging immune system, we highlight the interplay of immune memory with the gut microbiota and the brain. We end the review by presenting the current preventative and therapeutic strategies against aging, approaching from the alternative points of view of epigenetic modulation, metabolic intervention, microbiota reconstitution, and neuroprotection.

Adaptive Immune Memory

Infections have been one of the primary selective forces throughout evolution, so immunological memory has evolved to ensure survival when an organism is exposed to a pathogen that it encountered before [8]. Until the discovery of non-specific innate immune memory in the last decade, the antigen-specific memory established by T and B lymphocytes has been getting all the credit for long-term protection against pathogens.

T Cells: Thymus-Derived Troops of Immunity

Immunological memory against infections and tumors requires the intervention of T cells. T cells can recognize both self and non-self antigens through their T cell receptors (TCRs) and mount self-tolerance or immunological memory. Different subsets of T cells include naive T cells that recognize new antigens and memory T cells that are formed upon former exposure to antigen and assure long-lasting immunity.

T Cell Development

T cells are derived from the hematopoietic stem cells(HSCs) in the bone marrow but mature in the thymus. Most mature T cells reside in lymphoid tissues, but they are ubiquitously present throughout the body. After lymphoid progenitors migrate from bone marrow to the thymus, TCR gene rearrangement occurs, and CD4+ CD8+ double-positive cells expressing both co-receptors are generated. Then, these cells undergo positive selection based on TCR-antigen interactions and differentiate into naive single positive CD4t helper or CD8t cytotoxic T cells, which are released into the periphery [9].

Most of our knowledge of T cell development originates from mouse studies. However, there are substantial differences between mice and humans. For instance, although the peripheral naive T cell pool is almost exclusively provided by the thymus in mice, humans primarily sustain it by peripheral cell division [10].

When a naive cell recognizes an antigen presented by antigen-presenting cells(APCs) such as dendritic cells (DCs)and macrophages, they proliferate and develop into effector cells that can clear the source of the antigen, likely a pathogen. A small portion of these effector cells later become memory cells to establish long-term immunity that can last multiple decades, while the rest die by apoptosis [11]. Early in life, before exposure to many antigens, naive T cells constitute most of the T cell pool[12]. Meanwhile, regulatory T cells (Treg)are critical for the development of tolerance for innocuous antigens in the environment [13].

Around 5% of all adult CD4+ T cells are Tregs that are able to suppress the immune response [12]. Tregs are produced in the thymus but can also derive from peripheral naive T-cells by acquiring Forkhead Box P3(FOXP3)expression in response to environmental cues [13]. Recently, Tregs were shown to acquire memory characteristics, mostly against self-antigens, to prevent unwanted inflammation [14].

Memory T cells are divided into three subtypes which are central memory(TCM), effector memory(TEM), and stem cell memory (TSCM). Compared to TEMs, TCMs have more proliferation capacity and are closer to naive T cells in gene expression profiles [15]. TEMs can perform effector functions such as cytokine production. TSCMs are a stem cell-like, less differentiated cell type with high self-renewal capacity and the ability to differentiate into effector T cells, TEMs, or TCMs[16]. Following TCR stimulation, they are able to secrete interferon-gamma(IFN-y)and interleukin 2(IL-2). The long-lasting, multipotent TSCMs might help protect the organism against infections later in life when thymic output is low.

Although 90-95% of the effector T cells die after an infection resolves, a population of terminally differentiated effector cells regaining the naive T cell marker CD45RA, termed TEMRA cells, remain in circulation. These senescent-like cells have defects in telomerase expression and proliferation; however, they are capable of cytokine production and cytotoxicity, unlike exhausted cells [17].

In many tissues such as lungs, intestines, and spleen, TEMs are the predominant T cell type[18,19]. Moreover, discrete tissue-resident memory T cell populations (TRM)are identified with enhanced expression of adhesion markers and homing receptors, lower proliferative capacity, and higher production ability of pro-inflammatory and anti-inflammatory cytokines [20]. They can quickly react upon tissue injury or infection while also restricting the inflammatory damage. Establishing TRMs is a promising approach to consider in vaccine design, boosting and prolonging vaccine-mediated protection [21-24].

Effects of Aging on TCells

Lineage differentiation dynamics of HSCs in the bone marrow are altered with age. They skew towards myeloid differentiation, leading to lower numbers of lymphoid cells in the elderly [25]. HSCs also accumulate DNA damage throughout life and differentiate into leukocytes with chronic DNA damage response [26]. cistanche herba This triggers cellular senescence, which contributes to chronic inflammation by inducing a senescence-associated secretory phenotype(SASP), impacting neighboring immune and non-immune cell types. Another way that DNA damage can contribute to inflammation is the activation of DNA-dependent protein kinase catalytic subunits (DNA-PKcs) that can promote NFkB and inflammasome activity [27,28].

Involution of the thymus is one of the critical age-dependent changes in the immune system [29]. It is an evolutionarily conserved phenomenon in all vertebrates, starting before puberty, where the total mass, volume, and cellular content of the thy-mus shrink [30]. Thymic activity does not entirely cease, at least until the sixth decade of life, but thymopoiesis strikingly decreases with age [31,32]. Thymic epithelial cells gradually lose the ability to produce IL-7, which is crucial to support thymopoiesis[33,34]. Low thymic output in the elderly is associated with increased vulnerability to infections [35].In a young adult, the thymus provides around 16% of the naive Tcell pool, the rest of which derives from peripheral proliferation[36].In the elderly, this number falls below 1%, causing them to entirely rely on the proliferation of existing naive T-cells.

Cistanche can anti-aging

The decline in the number of naive T cells and the accumulation of terminally differentiated cells are two of the hallmarks of T cell aging [36]. CD4+ and CD8+ naive cell pools, although more markedly for CD8+ T cells, contract in the elderly. Maintenance of naive T cells through peripheral proliferation is more successful for CD4+T cells, but CD8+T cells are largely lost. Interestingly, while this is mostly the case in cytomegalovirus(CMV)+individuals in women, it is observed in men irrespective of the CMV status[37]. Also, CMV+individuals of both sexes have a higher proportion of late-differentiated senescent T cells than CMV individuals.

Chronic CMV infection affects most adults, with an 83%global seroprevalence rate [38]. Even though it usually does not cause active symptoms and is mainly unrecognized, CMV presence significantly shapes the T cell compartments and accelerates immunosenescence. Accumulation of terminally differentiated T cell types such as TEMs and TEMRAs occurs faster in CMV+individuals throughout their lifespan [39]. Expansion of CD8+ TEMRA cells is related to impaired antibody production upon influenza vaccination in the elderly [40]. cistanche penis growth Latent CMV infection is also associated with inadequate CD4+ T cell response against influenza antigens [41]. Moreover, CMV positivity is associated with a higher risk of all-cause mortality [42]. Of note, CMV+young adults displayed higher antibody responses to influenza vaccination, compared to CMV-young individuals [43].In the early stages of the infection, CMV might be potentiating immune responses before the accumulation of CMV-induced senescent cells pass a certain threshold and causes functional impairments.

Not just the numbers but also the receptor diversity of naive T cells are compromised in aged organisms. Naive T cells of a young adult carry around 100 million different TCR sequences; however, this repertoire diversity is reduced up to tenfold in the elderly [44]. Moreover, memory T cells experience a narrowing of TCR repertoires [45], and the proliferative capacity of senescent T cells following TCR engagement is defective [46]. Activated CD8+ cells of elderly individuals also produce lower levels of cytotoxins such as granzyme B and perforin [47]. On the other hand, CD4+ naive T cells of the elderly seem to maintain their differentiation and subsequent cytokine production capacities [48].

Lastly, differentiation of non-Treg cells into Tregs and proliferation of existing Tregs can maintain the Treg pools throughout life, despite reduced thymic output with aging. However, the balance between Tcell subsets is altered: as in other T cell types, the naive subset declines with age while memory Tregs increase [49].

B Cells: Bone Marrow-Born Battlers

B cells are a vital part of the adaptive immune memory. They have several immunological functions, including antibody and cytokine production, antigen presentation, and regulation of T cell responses [50]. Most vaccines mainly target and rely on B cell activation by inducing long-lived plasma and memory B cell proliferation[51]. However, aging affects the functional capacity of existing B cell subsets drastically, which is evident from the susceptibility to diseases and poor vaccine responses [52].

B Cell Development

B cells continuously arise from the hematopoietic stem cells (HSCs)and develop in the bone marrow(BM)[53]. HSCs generate multipotent progenitors that eventually diverge into common lymphoid progenitors(CLPs). Certain environmental cues, transcription factors(TFs), cytokines, and chemokines lead CLPs to differentiate into B-cell lineage. Following differentiation, cells undergo a rearrangement in the variable regions of the immunoglobulin (Ig) genes and start to express B-cell receptors (BCRs)and IL-7 receptors (IL-7R)[54]. Each B cell has a unique BCR with a different specificity to antigens.

B cells that finish their developmental process in the bone marrow are called transitional (TR)B cells. They make 4%of all B lymphocytes in healthy individuals [55] and are found in several places, including the bone marrow, peripheral blood, and secondary lymphoid tissues. Transitional B cells become either marginal zone(MZ) or mature follicular (FO) cells partly based on the strength of their BCR signal-ing. Cells with more robust signaling tend to develop into follicular types, while weaker signaling drives them to be MZ cells [56]. FO B cells have a broad immunoglobulin repertoire and are located in the follicles close to T cell zones [57]. Therefore, they are suited for getting T-cell help and becoming short-lived plasma cells. On the other hand, MZB cells can get activated easier than FO B cells, which quickly allows them to produce immunoglobulin M(IgM) or induce class switching without T-cell help [58].

The third naive B cell subset is B-1 cells, which are considered part of the innate immune system [59,60]. cistanche salsa benefits Apart from the other B cell subsets developed in the bone marrow, B-1 B cells originate from a distinct progenitor in the fetal bone marrow [61]. They are mainly found in peritoneal and pleural cavities; however, low numbers can also be located in secondary lymphoid organs. During an infection, they act by producing non-specific antibodies that are crucial for early defense [62, 63].

Advancing age alters the entire course of B cell development, the abundance of distinct B cell subsets, and their function. Furthermore, a B cell subset emerging with increasing age influences immune responses in the elderly.

Effects of Aging on B Cell Development

B cell development and the influence of old age in this process are extensively studied in mice. First of all, the differentiation capacity of long-term HSCs(LT-HSCs)reduces with advanced age [64]. The genes driving lymphoid cell differentiation and function are downregulated in LT-HSCs, while the genes mediating myeloid cell development are upregulated. Numbers and percentages of early B-cell lineage pro-genitors decrease as C57BL/6 mice age [65]. Furthermore, these populations exhibit declined I-7 responsiveness, indicating an impaired B lymphopoiesis.

Following progenitor differentiation, the development of B cells in the bone marrow is also influenced by aging. In different groups of old mice, a severe decrease with more than 80% loss of pre-B cells and 50% loss of pro-B cells, or a moderate decrease with 20-80% loss of pre-B cells were observed [66]. TFs regulating B cell development are altered by age, influencing the abundance of developing B cells [66-68]. Among them, the E2A gene encodes for two proteins, E47 and E12. Transcription and DNA-binding capacity of E47 were shown to decline in aged mice[66]. As E47 is a vital TF in B cell development during the pro- to pre-B cell stage[69], lower numbers of pre-and pro-B cells in old mice could partly be explained by the decreased function and expression of E47.PAX5 is another TF regulating early B-cell development that is lower in the elderly [70]. Lastly, BCR expression and diversity are altered upon aging [71, 72], although a study suggested that the changes were not evident until 70 years of age [73].

The Emergence of Age-Associated B Cells

In 2011, a new subset of B cells was described in aged mice [74,75]. This mature B cell population is named age-associated B cells(ABCs)since it progressively accumulates with increasing age. The origins of ABCs are not exactly known; however, differentiated FO, MZ, and B-1 cells are thought to contribute to the heterogeneous ABC pool[76]. Although studies define ABCs using different markers, they agree that ABCs are mature B cells with memory characteristics. Unlike the other B cell subtypes, ABCs express the transcription factor T-bet and a unique surface marker combination [77]. Therefore, their activation requirements, functions, and survival conditions are remarkably different. BCR engagement induces FO and MZ B cell proliferation, while Toll-like receptor 9(TLR9)or TLR7 signaling with or without BCR ligation drives proliferation in ABCs[76]. In vitro studies showed that TLR stimulation leads to IL-10and IFNy production from ABCs, and an in vivo study reported that they also produce tumor necrosis factor-alpha (TNFα)[78].

ABCs are engaged in both protective and autoreactive immune responses, although their protective role seems scarce. Furthermore, they are linked with autoinflammatory and autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis[75, 79,80], making ABCs a potential underlying reason for the increased incidence of autoimmune diseases in the elderly.

ABCs contribute to immune dysfunctions observed during the aging process. For instance, TNFα produced by ABCs has direct and indirect effects on pro-B cell numbers: ABCs directly induce pro-B cell apoptosis and lead to their loss by altering the bone marrow microenvironment [78]. Besides, increased abundance of ABCs was significantly correlated with the loss of B cell precursors in the bone marrow of aged mice.

ABCs express considerably high major histocompatibility complex II (MHC-II), CD80, and CD86 compared to FO B cells; therefore, they are better inducers of T cell activation and antigen presentation [81]. However, the same study associated these properties of ABCs with autoimmune diseases in an autoimmune-prone mice strain. Besides, considering that they make the bone marrow environment more inflammatory via the production of TNFα and robustly produce IL-6 and IFNy upon TLR7 and TLR9 engagement [74, 78], it is plausible to propose that ABCs contribute to inflammation. Lastly, a study reported that humoral response depends more on TLR signaling and less on CD4+ T cell help due to decreased FO B cells and increased ABCs in aged mice [82]. This eventually resulted in impaired production of IgG and long-lived plasma cells.

Abundance and Functions of B Cells in the Elderly

Several studies reported a decrease of mature B cell subsets in humans with aging, although the extent of these changes varies depending on the subsets, experimental approaches, and cohorts of people [53,83,84]. For instance, Muggen et al.reported that the numbers and relative abundance of several B cell subsets including transitional B cells, memory cells, and plasmablasts reduced with aging, particularly in individuals older than 70 years old [73]. Plasma and memory B cell percentages in the circulation and bone marrow decline, while naive and immature B cells remain relatively stable in older people [85]. The abundance of B-1 cells, along with their ability to produce IgM, decreases with age [63]. A study found significantly low switched memory B cells, but high naive and double-negative memory B cells in people over 65 years of age compared to younger adults [86]. The authors concluded that double-negative or so-called late-exhausted memory B cells express senescence markers and are associated with poor immune responses against the influenza vaccine. Of note, switched memory B cells play a role in antibody production upon re-infection, generating a rapid response compared to naive B cells[84]; therefore, a lower abundance of switched memory B cells is another evidence of impaired humoral immune response in the elderly.

Not only the numbers but also the functions of B cells are diminished with aging. Poor antibody responses in the elderly after influenza vaccination are due to low binding and neutralization capacity of antibodies, decreased class switch recombination, hypermutations of the antibody variable regions, and a higher abundance of inflammatory B cells [87,88]. Besides, antigen-specific antibody production decreases with age, while self-reactive antibodies become more abundant, rendering old individuals more susceptible to developing autoimmune diseases [89]. All these defects in the humoral immune response lead to increased susceptibility to diseases and reduced efficiency of vaccines [90].

Trained Immunity: a De facto Innate Immune Memory

Although immune memory had been attributed only to the adaptive immune system for a long time, growing evidence consistently shows the existence of memory-like characteristics in innate immune cells [91-94]. Certain infections, vaccinations, or molecules can reprogram innate immune cell types to exhibit increased responsiveness against a secondary insult. This phenomenon is termed trained immunity and is mediated through extensive epigenetic and metabolic changes.

Over the last couple of years,innate immune cells, including monocytes [95], natural killer(NK)cells [96],innate lymphoid cells (ILCs)[97], DCs[98],and neutrophils [99], have been reported to exhibit trained immunity response. As innate immune cells can only recognize microbial patterns via their pattern recognition receptors(PRRs), their memory-like response is not specific to pathogens but can work against a wide range of antigens. Thus far, vaccines, such as the tuberculosis vaccine Bacillus-Calmette Guérin (BCG)[100], measles [101], and oral polio vaccine [102]; microbes/microbial patterns,e.g.,β-glucan [91], Candida albicans; oxidized low-density lipoprotein (oxLDL)[103]; and metabolites such as fumarate [104] have been reported to induce heterologous protection through trained immunity.

Epidemiological studies reporting decreased all-cause mortality after certain vaccinations suggested the existence of an innate immune memory [105]. The existence of trained immunity was first depicted in monocytes with an in vitro model and in vivo in mice, where C.albicans and β-glucan induced enhanced cytokine productions after the second microbial stimulation [91]. In parallel, BCG vaccination was reported to induce higher TNFα and IL-1β production against unrelated pathogens, even 3 months after the vaccination [100]. Further research demonstrated that trained immunity could persist up to l year and possibly even longer [106]. Considering that monocytes have a half-life of around 1-2 days in the circulation [107], the programming of progenitor cells could be involved in sustaining the memory-like phenotype. Indeed, β-glucan administration leads to the expansion of myeloid lineage progenitors in the bone marrow of mice [108]. Increased myelopoiesis is associated with upregulated IL-1β and granulocyte-macrophage colony-stimulating factor(GM-CSF) signaling, besides alterations in glucose and cholesterol metabolism. Another mouse study demonstrated increased myelopoiesis following BCG vaccination, which is associated with enhanced protection against M.tuberculosis infection [109]. These findings align with a recent study on humans, showing that BCG vaccination leads to the upregulation of myeloid and granulocyte-lineage genes in HSCs [110].

Trained Immunity in the Elderly

Low-grade chronic inflammation occurring in the elderly is associated with poor innate and adaptive immune responses [111]. Koken et al.recently reported that BCG vaccination reduces systemic inflammation, and a lower abundance of circulating inflammatory proteins at baseline is correlated with trained immunity response 3 months after vaccination in males[12]. Therefore, BCG vaccination could alleviate inflammation while providing non-specific protection via trained immunity induction in the elderly. On the other hand, since the cell differentiation capacity of HSCs in the bone marrow changes and is skewed toward myelopoiesis with aging, inducing trained immunity could lead to unfavorable outcomes by further expanding the myeloid cell production in older people.

Nevertheless, a double-blinded placebo-controlled clinical trial demonstrated that trained immunity could be safely induced in the elderly by BCG vaccination, evident from the increased cytokine production compared to the participants who received a placebo [113]. Remarkably, the trial showed that BCG prolongs the time until infection and reduces the risk of all new infections and respiratory infections by 45% and 79% compared to the placebo group, respectively. In line with this, other trials reported a decrease in acute upper respiratory tract infections and pneumonia in older people vaccinated with BCG [114,115]. However, more research is needed to explore the strength and longevity of trained immunity responses in older individuals compared to adults. BCG's ability to confer protection against heterologous infections has attracted a lot of attention during the COVID-19 pandemic, which disproportionally affects the elderly. BCG is being tested in more than 20 randomized control trials to investigate if it has a protective effect against SARS-CoV-2 infection[116]. Promisingly, a recently published study from Greece reported a 68% risk reduction for COVID-196months after BCG vaccination[117]. Another study revealed that even an early history of BCG vaccination is associated with decreased incidence and symptoms of COVID-19 among healthcare workers [118]. Therefore, induction of trained immunity by BCG vaccination may be utilized as a preventive measure against COVID-19, especially in the vulnerable elderly group.

Aging as a Multisystem Malady

Aging leaves no part of the body unscathed. Besides tissue-specific damage occurring with advanced age, the aging immune system impacts many other systems and processes. Even the organs that were once thought to be devoid of immune cells, such as the brain, are now known to harbor tissue-resident immune cells and interact extensively with the peripheral immune system. The last few decades have also witnessed a boom in research on the microbiota, the collection of up to 100 trillion microorganisms residing in human bodies, mainly in the gut [119]. The microbiota has close interactions with the host immune system and is also prone to age-related disruptions.

In the following chapters, we discuss the interplay of microbiota and the brain with the aging immune system, mainly focusing on immune memory. We especially approach this body of research from a metabolic perspective, describing various cellular metabolic programs and their impact on immune memory in aging and age-related diseases. Additionally, we point out the role of epigenetic regulation underlying all the topics discussed. By providing such a comprehensive view, visualized in Fig.1, we aim to strengthen the notion of aging as a multisystem problem and accordingly inform counteractive efforts.

This article is Clinical Reviews in Allergy & Immunology https://doi.org/10.1007/s12016-021-08905-x