Adaptive Immunity in Older People

Vaccination is the best strategy to prevent this fact, but older people present a less efficient response, as their immune system is weaker due mainly to a phenomenon known as immunosenescence. The adaptive immune system is constituted by two types of lymphocytes, T and B cells, and the function and fitness of these cell populations are affected during ageing.

adaptive immunity
immunosenescence
aging
vaccines
t cells
b cells

1. Introduction

Ageing is one of the main health challenges worldwide, and promoting healthy ageing is a key global priority. The health of older people is threatened by their increased susceptibility to infectious disease and associated complications, which are related to many factors, especially the dysregulation of immunity generally termed “immunosenescence”. This is believed to adversely affect the efficacy of vaccines, thus reducing the protection provided by most current vaccines in older people. Specifically, a recent metanalysis indicated that the influenza vaccine’s effectiveness was 51% among people aged 18 and 64 years and 43% among people aged over 65 years [1]. Identifying the key factors responsible for reduced vaccination efficiency in older adults and devising countermeasures to solve this problem are essential for improving the outcomes of vaccination. This will allow for better protection against infections in this growing segment of the population [2].

To restore immunity in older people, several approaches have been assessed, including higher doses of antigens, new adjuvants and different routes of administration of antigens. Nevertheless, although stronger responses have been achieved by some of these means, the net result is still unsatisfactory [3,4]. Vaccine efficacy is usually based on antibody responses developed following immunization. However, cellular responses are also critical for long-term protection. Accordingly, T cells are the most important effectors in the cellular immune system, which are activated and mobilized by myeloid antigen-presenting cells (APCs) [5]. It is, therefore, of primary importance to design vaccination strategies specifically focused on older people. Developing countries recommend four vaccines for older individuals: influenza, pneumococcal infection, zoster and the combination against tetanus, diphtheria and pertussis [6]. All these formulations rely on a B cell response that is dependent on T cells, except for the Streptococcus (S.) pneumoniae infection, as it has a polysaccharide base [7].

2. Immunosenescence of Adaptive Immunity and Vaccine Failure in Older People

Some of the features of immunosenescence have been associated with vaccine failure in older individuals. In general, changes in T cell function and sub-population shifts correlate with the antibody responses to the influenza vaccine [93]. In addition, antibody production is associated with lymphocyte infiltration but not with monocyte infiltration [94]. In the case of influenza, yellow fever and hepatitis B, there is a lower response when the baseline state of inflammation is high [95]. Furthermore, individuals with decreased CD28 expression in T cells show diminished T cell proliferation and weaker responses after vaccination. This is coupled with decreased TCR repertoire diversity, which indicates increased susceptibility to new pathogens and the reduced efficacy of vaccination programs [96]. Additionally, diminished CD4/CD8 ratios correlate with poor antibody titres against influenza. The effect of the presence of HCMV-specific T cells or HCMV infection in older individuals is unclear. While some studies indicate that HCMV positive older individuals present worse vaccine responses, this is not the case in younger individuals. Other studies report better vaccine responses in the seropositive individuals [97,98]. On the other hand, a single-nucleotide polymorphism has been found to be associated with reduced transcriptional transactivation of CD39, which, in turn, correlates with better responses to influenza and varicella zoster virus immunizations [61].

Effective vaccines need to elicit good T cell responses. Memory T cells have a broad specificity against internal and conserved pathogen epitopes due to their longevity and residence in circulation and peripheral sites [99]. For example, for tuberculosis and smallpox vaccines, T cell responses are essential [100,101]. In addition, BCG vaccines were reported to generate adequate CD4 and CD8 T cell responses [102]. In fact, most immunosenescence research has been focused on T cell-mediated immunity [103]. DNA vaccines in particular generate good T cell responses, as the antigen is expressed from a gene inside the target cells, leading to a strong T cell response, which in turn culminates in good antibody responses. However, outcomes from clinical trials were not encouraging [104]. Viral vectors contain a genetic load that is introduced into the target cells at the vaccination sites, eliciting strong transgene expression leading to adequate T cell responses [105,106].

The efficacy of influenza immunization in the adult population is about 59%, while this percentage decreases to 39% in individuals over 65 years old [107]. Immunity against influenza includes CD8⁺ T lymphocytes that recognize internal viral antigens as well as antibodies towards surface proteins such as hemagglutinin. In addition, the presence of effector-memory and effector T cells in the respiratory tract is important for protection against re-infections [108]. While the expression of genes associated with T and B lymphocyte function correlates positively with antibody responses, the genes associated with inflammation and monocytic-lineage have a negative impact [109]. It should be noted that aged CD8⁺ T cells demonstrated decreased proliferative capacity, and this reduction in influenza-specific CD8⁺ T cells negatively affects viral clearance in older patients [110]. Moreover, older individuals show an impaired CD4 response after vaccination [111]. The number of Th1 cells that secrete inflammatory cytokines is reduced in the lungs of aged mice [112]. Furthermore, CD8 TCR diversity in aged mice is lower, which is reflected in a reduced response against epitopes from the nucleoprotein of the influenza virus [113]. Regarding pneumococcal immunization, the efficacy is around 40–65% for older people, contrasted to the 60–70% efficacy for younger individuals [114]. The pneumococcal vaccine is a polysaccharide-based antigen formulation, which is independent of T cells. However, T cells are important in the natural immunity against these bacteria. Accordingly, CD4⁺ T cells secreting IL-17 mediate the adaptive response against S. pneumoniae [115]. On the other hand, varicella zoster virus reactivates throughout life, but individuals are asymptomatic due to T cell-dependent cellular immunity [116]. In natural infections, antigen-specific CD4⁺ T cells are generated, and clinical trials show that the CD4 response persists for three years after vaccination [117]. Specifically, a vaccine with the adjuvant AS01B has been developed that favours T cell responses in animal models [118]. Notably, virus attenuated vaccines have been shown to generate an adequate TCR repertoire in CD4⁺ T cells [119].

Although most studies attempting to explain the role of immunosenescence in vaccine response have centred on immunosenescence T, immunosenescence B also plays a role. Humoral immunity is known to play an important role in preventing influenza virus transmission and infection, and immunogenicity of influenza vaccines is usually measured by hemagglutinin (HA) inhibition (HAI) assay, which quantifies antibodies specific for the virus HA. A greater number of older adults fail to seroconvert, i.e., to have the four-fold increase in post-vaccination antibody titre, relative to their younger counterparts, which is one of the WHO criteria for assigning responsiveness, with seroconversion rates ranging from 10 to 30% in older adults compared to 50–75% in younger subjects (although it can sometimes be the case that older people already have a high antibody titre due to previous exposures, and thus cannot increase titres four-fold, resulting in erroneous classification as non-responders). In particular, older adults may fail to generate protective HAI antibody titres compared to younger adults. Cellular immunity is also strongly associated with protection against influenza, as some older adults have been shown to remain protected against infection even in the absence of robust antibody responses. However, further studies are needed in order to fully understand the effects of immunosenescence on cellular immunity to influenza [108,120,121].

As reported by Frasca and Bloomberg [86], several studies have identified B cells’ intrinsic defects that account for sub-optimal antibody responses of older people. They are: (i) the decrease in class switch recombination, responsible for the generation of a secondary response; (ii) the decrease in de novo somatic hypermutation of the antibody variable region, responsible for the failure of high affinity antibodies; (iii) decreased binding and neutralization ability, as well as binding specificity, of the secreted antibodies; (iv) increased epigenetic changes associated with lower antibody responses and (v) increased frequencies of inflammatory B cell subsets.

Human studies have found that people > 65 years old have significantly lower antibody titres against many of the common pneumococcal serotypes and diminished opsonisation activity compared to younger adults. Therefore, antibody titres wane over time, and there may also be functional deficiencies in antibody responses against pneumococcal antigens. While humoral immunity is primarily thought to mediate protection from disease, there are conflicting reports regarding age-related changes in T cell responses against pneumococcal infection [121].

It was demonstrated that inflamm-ageing plays an important role in compromising the immune responses by way of inducing the high expression of some microRNAs that interfere with B cell activation. In vitro, this drives TNF production and inhibits B cell activation. Increased serum levels of TNF are also linked to a defective T cell response, in part due to reduced expression of CD28 on T cells. Moreover, in monocytes, the pre-vaccination expression of genes related to inflammation and innate immune response is negatively correlated to vaccination-induced activation of influenza-specific antibody responses [94,122,123].

Nowadays, we are immersed in a pandemic caused by the SARS-CoV-2 virus, and older people constitute one of the main risk groups affected (74.3% of deaths in the US) [124]. Remarkably, some species from the coronavirus family induce thymic involution, and thus the hypothesis that this might also occur with SARS-CoV-2 virus has been considered [125]. In addition, T cell response is critical for immune protection against SARS-CoV-2, since it is essential for viral clearance, prevention of infection and recognition of viral variants [126]. In this regard, severe COVID-19 disease has been associated with lower TCR diversity against SARS-CoV-2 epitopes, accompanied by reduced T cell responses [110]. Specifically, an increase in TEMRA CD8⁺ T cells (see Section 2.1) is associated with worse memory responses against SARS-CoV-2, as well as a compromised antibody response [127]. In general terms, antibody responses are decreased in older individuals who are prone to mild and moderate adverse events [128]. It is noteworthy that SARS-CoV-2 is demonstrated to reduce the number of CD8⁺ T lymphocytes, a fact that is associated with poor survival of COVID-19 patients [129]. Accordingly, impaired cytotoxic CD8 T cell responses have been reported in older COVID-19 patients [130]. Although SARS-CoV-2-cross reactive CD8⁺ T cells have been detected in unexposed individuals, this population was found to be decreased in older individuals [131]. SARS-CoV-2 was also reported to decrease CD4⁺ helper T cells, specifically in older patients suffering severe COVID-19 disease [132]. Vaccines have been developed and approved to prevent the severity of the infection, but, unfortunately, an aged immune system compromises their efficacy. For mRNA-based vaccines, the BNT162b2 formulation first claimed that 18–98 years old individuals benefited with a 94% efficacy within the >65 years old group [133,134]. However, a later study demonstrated the time-dependent lessening of both cellular and antibody responses in older people compared with younger adults and a relationship with inflammaging [135]. The second dose of the BNT162b2 mRNA vaccine showed no differences in neutralization potency against the B.1.1.7 (Alpha), B.1.351 (Beta) and P.1. (Gamma) variants of concern in comparison with the wild type virus in older individuals. However, SARS-CoV2-spike specific T cells from older participants produced less IFN-γ and IL-2 [136]. Alternatively, the mRNA-1273 vaccine generates adequate antibody titres independently of the age of the vaccines [137]. Finally, the adenovirus-based vaccine AZD1222 has been demonstrated to be a safe and tolerated alternative, with a degree of immunogenicity comparable to younger individuals [138].

3. Strategies to Reverse Immunosenescence of Adaptive Immunity in Older People

A significant effort has been made to modulate T cell senescence by a wide variety of strategies (Table 1). First, a group of approaches target altered molecules in the senescent T cells. On TCR signalling, genetic inhibition of DUSP6 or DUSP4 recovers T cell signalling in aged T lymphocytes [139]. CD4 responses can also be improved by DUSP6 kinase repression by its natural inhibitor miR-181a, or specific siRNA [50]. On the other hand, pharmacologic inhibition of SHP-1 allows increased secretion of IL-2 and proliferation of CD4⁺ T cells [52]. The function of the TCR can also be recovered by targeting the p38 MAPK pathway. The MAPK p38 blockade reverses CD8 senescence by a process independent of mTOR [140]. Moreover, the simultaneous inhibition of this MAP kinase and PD-1 favours the proliferation of effector-memory CD8⁺ T cells that re-express CD45RA (TEMRA) [141]. In addition, the proliferation of highly-differentiated human T cells is recovered by inhibiting the macromolecular complex made of protein kinase AMP-activated (AMPK), TGF-beta activated kinase 1 (MAP3K7) binding protein 1 (TAB1) and p38, either with AMPK or with p38 inhibitors [53]. In aged mice, T cell activity is recovered upon knockout of the sestrin-MAPK activation complex (sMAC), thus increasing the efficacy of the influenza vaccine FLUAD. In addition, TCR signalling is restored by genetic blockade of sestrins [54,55]. MAPK p38 inhibitors have been used to reduce the inflammation generated by the antigens from a purified derivative of tuberculin, Candida albicans and Varicella virus [142]. Another strategy that restores TCR sensitivity and prevents immuno-aging is DJ-1 inhibition at a young age [58]. Other membrane receptors are compromised in aged T cells, and thus, their modulation has also been evaluated. Accordingly, PD-1 suppression increases cytokine production [7]. Furthermore, the inhibition of tumour necrosis TNF-α or its receptor postpones the CD28 down-regulation characteristic of T cell replicative senescence [143].

Table 1. Strategies to reverse immunosenescence of adaptive immunity in older people.

Strategy		Effect
Senescent T cell	DUSP6 inhibition	Recovery of T cell signalling
	DUSP4 inhibition	Recovery of T cell signalling
	SHP-1 inhibition	Increased secretion of IL-2 and proliferation of CD4⁺ T cells
	MAPK p38 inhibition	Reversion of CD8⁺ T cell senescence
	MAPK p38 and PD-1 inhibition	Proliferation of TEMRA CD8⁺ T cells
	AMPK-TAB1-MAPK p38 complex inhibition	Proliferation of highly-differentiated T cells
	Sestrins–MAPK complex inhibition	Recovery of T cell activity Increase in influenza vaccine efficacy in mice
	Sestrins inhibition	Recovery of TCR signalling
	DJ-1 inhibition	Restoration of TCR
	PD-1 inhibition	Increase of cytokine production
	TNF-alpha inhibition	Postponement of CD28 downregulation
	mTOR inhibition	Improvement in immune response after influenza and SARS-CoV 2 vaccination
	mTOR and PI3K inhibition	Control of infection
	VPS39 inhibition	Higher levels of memory T cells
	Autophagy inhibition	Expansion of antigen specific CD8⁺ t cells
	cPLA2 inhibition	Prevention of T cell decline
	PTEN/PI3K-NRF2 axis activation	Loss of senescence markers expression
	AMPK activation	Decrease of Th17 differentiation and increase in Tregs
	Senolytic drugs	Depletion of senescent cells
	Thymosin	Increase in CD4⁺ and CD8⁺ T cells in older COVID-19 patients
Adjuvants	Lipophilic adjuvants and TLR4 agonist	Improvement of T follicular responses to malaria vaccines in mice
	AS01 adjuvant	Increase in CD4⁺ T cells for herpes zoster virus vaccination
	MF59	Persistence of B cell and CD4+ T cell responses
	AS03	Persistence of B cell and CD4+ T cell responses
	Flagellin	Increase in IFN-γ producing memory CD4+ T cells
	GLA-SE	Th1-biased T cell responses and enhances cytokine and granzyme B secretion
	Imiquimod	Increase in IFN-y expression and IgG isotype switching
Lifestyle	Exercise	Decrease in the number of senescent lymphocytesIncreased levels of IL-7 and IL-15 Apoptosis of exhausted T cells
	Zinc	Increase in CD4 and CD8 numbers
	Vitamin E	IL-2 production Naïve T cell activation and proliferation
	Vitamin C	Reduction of inflammaging T helper maturation
	Carotenoid	Mature T cell phenotype
	Polyphenols	Increase in IL-2 and IFN-gamma
	Polyunsaturated fatty acids	Proliferation of T lymphocytes
Adoptive T cell therapy	Stem cell memory cells
	Virus-specific T cells
	Mesenchymal stem cells	Reduction in the expression of senescent markers in CD4⁺ T cells Moderation of inflammaging in mice
B cell	Induction of autophagy	Improvement in B cell response
B cell	Depletion	Rejuvenation of B cell population

Targeting cell signalling is an interesting strategy to reverse T cell immunosenescence. Thus, mTOR inhibition improves general immune response after vaccination of older people. Accordingly, clinical trials using the mTOR inhibitor everolimus showed a better immune function and lower frequencies of PD-1 T lymphocytes after immunization against influenza [144]. Moreover, the combination with PI3K inhibitors increased the control of infection in older people. Inhibition of the mTOR upstream activator VPS39, a protein that may promote clustering and fusion of late endosomes and lysosomes, ameliorates the expansion of antigen-specific T cells, generating higher levels of memory T cells [145]. mTOR inhibitors have also been tested in combination with SARS-CoV-2 vaccines in immunocompromised individuals. The combination treatment showed better antibody and T cell responses compared to the absence of mTOR inhibition [146].

Another approach relies on the use of the autophagy activator spermidine, which favours the expansion and functions of antigen specific CD8⁺ T cells in aged mice [89].

It has been demonstrated that the endogenous polyamine metabolite, spermidine, induces autophagy in vivo, hence, rejuvenating memory B cell responses [147]. Data from mice and humans indicate that spermidine has the potential to be safe for testing its epigenetic-dependent and independent effects on human health span. Spermidine post-translationally modifies the translation factor eIF5A, essential for the synthesis of the autophagy transcription factor TFEB that coordinates expression of lysosomal hydrolases, membrane proteins and genes involved in autophagy. Spermidine is depleted in older people and its supplementation restored TFEB expression and autophagy, hence, improving the responses of B cells from older people. Taken together, these results reveal an unexpected autophagy regulatory mechanism at the translational level, which can be used to block and/or reverse human immunosenescence.

Furthermore, the restoration of lipid metabolism by blockade of cPLA2, or the use of drugs that favour lipid catabolism, prevent T cell decline [71,72]. The blockade of TNF-α, or its receptor, postpones CD28 down-regulation in replicative senescent T cells [143]. Alternatively, exosomes derived from placenta mesenchymal stem cells (MSC) containing miR-21 induce loss of expression of senescence markers in CD4⁺ aged T cells by activating the phosphatase and tensin homolog (PTEN)/PI3K-NFE2-like bZIP transcription factor 2 (Nrf2) signalling axis [148]. The AMPK agonist metformin could be considered an interesting treatment to reverse T cell immunosenescence, as it reduces inflammation by decreasing Th17 differentiation and increasing Tregs [149].

A second group of approaches does not directly target senescent T lymphocytes. However, some of these methods accomplish suitable T cell functions in vaccination, as mentioned above. For example, the combination of lipophilic adjuvants and toll-like receptor 4 (TLR4) agonists improves T follicular responses to malaria vaccines in mice [150]. The addition of the adjuvant AS01 in the vaccine formulation for herpes zoster virus increases the number of CD4⁺ T cells in older individuals [151]. A systematic review from 2022 suggests that adjuvanted influenza vaccines are preferable over conventional vaccines for older individuals [152]. MF59 and AS03 adjuvants have been used for influenza vaccines for older adults with great effectiveness [153]. Influenza vaccines that contain MF59 adjuvant showed better persistence of B cell and CD4⁺ T cell responses, and similar effects have been described for AS03 [154,155]. Flagellin from Salmonella typhimurium has also shown promising results in influenza vaccine formulations with an increased number of IFN-y producing memory CD4⁺ T cells [156]. AS02, GLA-SE and CpG are TLR4 and TLR9 agonists that generate better humoral immunity in pneumococcal vaccines [157]. GLA-SE induces Th1-biased T cell responses and enhances cytokine and granzyme B secretion [154]. On the other hand, Imiquimod, a TLR7/8 agonist that improves influenza vaccines’ protection, was shown to increase IFN-y expression and IgG isotype switching [158].

Interestingly, immune fitness can be modulated by lifestyle. Thus, exercise is associated with fewer senescent lymphocytes, better function of these cells and improved response after vaccination [159,160]. With respect to the effects of exercise effects, one potential may be increase in thymic mass and the resulting output of naïve T-cells in older people through increased levels of IL-7 and/or growth hormone synthesis. IL-15 released from muscle, now considered an important regulating organ for the immune system, can improve NK cell cytotoxicity and cytokine secretion, helping to maintain blood T and NK cell numbers. Skeletal muscle produces myokines, proteins with anti-inflammatory and immune response enhancing effects [29]. Moreover, apoptosis of exhausted T cells could be caused by frequent bouts of exercise. Notably, the consumption of specific nutrients is also beneficial. Accordingly, zinc deficiency generates a decrease in timulin, a peptidic hormone that enhances the expression of T cell activation markers. Indeed, zinc supplementation reduces infection incidence and increases CD4 and CD8 numbers in older individuals [161]. Vitamin E intake is associated with IL-2 production and naïve T cell activation and proliferation [162]. Moreover, vitamin C might counteract inflammaging and help T cell maturation [163]. On the other hand, carotenoid supplementation in older people allows T cells to express a mature phenotype. Thus, high doses of β-carotene increase CD4⁺ T cells [164]. Polyphenols have been described to increase IL-2 and IFN-gamma and correlate with improved immune responses. Accordingly, CD4⁺ T cell numbers are increased in aged rats after resveratrol dietary intake [165]. The consumption of polyunsaturated fatty acids induces the proliferation of T lymphocytes [89].

Adoptive T cell therapy has also been considered for reversal of immunosenescence. Stem cell memory cells or virus-specific T cells might be expanded ex vivo and transferred to older people [166]. Another adoptive cell therapy is based on mesenchymal stem cell (MSCs). MSCs are multipotent progenitor cells with the ability to reduce the inflammation. In addition, these cells express transforming growth factor beta 1 (TGF-beta), a molecule that promotes the generation of CD8 and Tregs. This approach has been effective in disease models associated with an impaired effector T cell response or immune regulation mediated by Tregs [167]. For example, exosomes derived from placental MSCs reduce the expression of senescence markers in aged CD4⁺ T cells. This outcome relied on the miR-21, a microRNA that activates the PTEN/PI3K-Nrf2 axis, a signalling pathway that has been implicated as an important regulator of the proliferation, differentiation and apoptosis in a variety of cell types [148]. Thymic negative selection can be restored by the transplantation of these extracellular vesicles from young mice to older animals, leading to moderation of inflammaging [168]. In addition, older COVID-19 patients treated with thymosin alpha 1, a polypeptide hormone secreted by epithelial thymic cells, experienced an increase in CD4⁺ and CD8⁺ T cells [169]. On the other hand, the so-called “anti-aging drugs”, senolytics, promote removal of senescent immune cells that accumulate with age [170]. Specifically, in old mice, it has been demonstrated that senolytic drugs can impact CD4⁺ T cells, most likely by modulating the microenvironment, which can then positively influence T cell differentiation during the response to influenza infection [171].

Finally, another study proposed a different strategy to enhance immune responsiveness in aged mice and older humans, through rejuvenation of the B lineage upon B-cell depletion. The authors used old and young mice to deplete blood B cells, analysed B cell subgroups, their repertoire and cell functions in vitro and immune response in vivo. Depletion of B cells in aged mice resulted in a rejuvenated B cell population generated de novo in the bone marrow. Rejuvenated B cells exhibited a “youthful” repertoire and cellular reactivity to immune stimuli in vitro. However, the treated mice did not increase antibody responses to immunization in vivo, nor did they survive longer than the control mice in a “dirty” environment. Older patients, previously treated with rituximab, healthy older and younger subjects were vaccinated against hepatitis B (HBV) after undergoing a detailed analysis for B-cell compartments. Consistent with the results obtained in models, B cells from older depleted patients showed a “young”-like repertoire, population dynamics and cellular responsiveness to stimulus. However, the response rate to HBV vaccination was similar between depleted and nondepleted patients, although antibody titres were higher in depleted patients [172]. Further studies are necessary to apply this approach for enhancing humoral immune responsiveness of older people.

This entry is adapted from the peer-reviewed paper 10.3390/ijms23179797

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