How aging impacts vaccine efficacy: known molecular and cellular mechanisms and future directions

Abstract

Aging leads to a gradual dysregulation of immune functions, one consequence of which is reduced vaccine efficacy. In this review, we discuss several key contributing factors to the age-related decline in vaccine efficacy such as alterations within the lymph nodes where germinal center reactions take place, alterations in the B cell compartment, alterations in the T cell compartment and dysregulation of innate immune pathways. Additionally, we discuss several methods currently used in vaccine development to bolster vaccine efficacy in older adults. This review highlights the multifactorial defects that impair vaccine responses with aging.

Aging Diminishes Vaccine Efficacy

One of the consequences of aging is the gradual dysregulation of the immune system, a phenomenon known as immunosenescence (see Glossary) [1]. Immunosenescence leads to impaired functions of the immune system, such as reduced production of T cells from the thymus, as well as enhanced functions, such as inflamm-aging, the increase of sterile, low-grade, chronic inflammation with aging. Consequences of immunosenescence include increased susceptibility to viral infections such as influenza virus [1,2] and respiratory syncytial virus [2,3] and bacterial infections such as Streptococcus pneumoniae [4] and uropathogenic Escherichia coli [5]. In addition to host defense, immunosenescence has been associated with increased rates of tumorigenesis and the development of autoimmune diseases such as multiple sclerosis [6,7].

Another important consequence of immunosenescence is an impaired response to vaccination characterized by reduced titers of vaccine-specific antibodies, shorter duration of measurable vaccine-specific antibodies, and a reduced quality and affinity of the antibody response. For example, following both the first and second dose of the BNT162b2 (Pfizer-BioNTech) COVID-19 vaccine, older adults (i.e., ≥80 years old) exhibited decreased IgG antibody titers against the SARS-CoV-2 spike protein compared to younger adults (<60 years old) [8]. Additionally, older adults exhibited lower levels of neutralizing antibodies against the SARS-CoV-2 virus [8], and reduced somatic hypermutation and class-switching in B cell receptor (BCR) repertoire [8].

While numerous clinical studies have shown that aging alters the immune response to vaccination, a definitive mechanistic understanding for this phenomenon is lacking. Understanding the mechanisms underlying age-related dysregulation of the immune response is critical for the development of new vaccines with improved efficacy in older adults. Since the main purpose of vaccines is to prevent and reduce the severity of infections, the development of high-efficacy vaccines for older adults is essential for limiting the burden of pathogenic infections and increasing the health-span of older individuals.

Due to the complexities of the immune system, one should not focus on one or even a handful of cell types when trying to decipher how aging alters immunity. Even a seemingly straightforward response to vaccination, such as antibody production, entails the coordinated orchestration of immune and stromal cells and dysregulation at any step can lead to a weakened vaccine response. In this review, we outline how aging impacts vaccine responses and highlight several current strategies to boost these responses in older adults.

The Adaptive Immune Response to Vaccination

In Box 1 we briefly describe the processes by which the adaptive immune system is activated and forms immunological memory following vaccination (Figure 1). A more in-depth discussion on the formation of immunological memory can be found in other review articles [9]. In this review, we will primarily focus on the age-related changes of the complex cellular interactions within lymphoid tissues that lead to long-lasting immunity following vaccination.

Box 1. The Adaptive Immune Response to Vaccination.

Following vaccine administration, the antigen is taken up by antigen presenting cells (APCs) such as dendritic cells (DCs). The DCs then migrate from the site of vaccine administration to the draining lymph node (dLN) [82]. In the dLN, the DCs present peptides of the antigen by MHC molecules to activate T cells through their T cell receptor (TCR) [82]. Once activated, T cells will proliferate and differentiate into effector T cells or memory T cells. For example, CD4 T cells can differentiate into various CD4 effector subsets such as Th1, Th2, Th17, Treg, and T follicular helper (Tfh).

B cells, unlike T cells, can be activated by antigen through their B cell receptor (BCR) without the need for MHC presentation. Following BCR activation, B cells uptake the antigen on their BCRs and present them on MHC II molecules to Tfh cells at the T/B border of the germinal center. Tfh cells provide survival, differentiation and proliferation signals to the B cells, such as through the CD40-CD40L interaction, in a process known as T-cell dependent B cell response [83]. After the B cells receive survival and mitogenic signals from Tfh, they migrate to the dark zone of the germinal center in the LN to undergo somatic hypermutation. The B cells then return to the light zone of the germinal center for selection of B cells with improved BCR affinity for the antigen in a process known as affinity maturation. The cycling of B cells undergoing selection between the light and dark zones of the germinal center occurs for several iterations. Additionally, cytokines made by Tfh determine the class-switching fate of the immunoglobulin B cells. After undergoing affinity maturation and class switching, some B cells will differentiate into antibody-producing plasma cells or memory B cells.

Plasma cells migrate to the bone marrow where they will continuously produce high levels of secreted antibodies to form long-lasting humoral immunity. Both memory T cell and memory B cells will remain in the LN or migrate to the tissues where they can be quickly reactivated following antigen-specific activation.

Figure 1: The Adaptive Immune Response to Vaccination.

Following vaccination, T cells are activated by dendritic cells (DCs) that present antigen on MHC molecules. Once activated, T cells can differentiate into memory T cells or effector T cells. For example, activated CD4 T cells may differentiate into T helper subsets such as Th1, Th2, Th17, Tfh, or Treg cells. B cells are activated by binding to their cognate antigen through the B cell receptor. Tfh cells provide survival, proliferation and differentiation signals to activated B cells in the germinal center. B cells can differentiate into memory B cells or antibody-secreting plasma cells and exit the germinal center. This figure was created with BioRender.com.

Aging-Related Dysfunction of Innate Immunity and Impacts on Vaccine Response

The innate immune system plays instrumental roles in the vaccine response. In Box 2, we briefly describe the processes by which innate immunity supports the vaccine response and how aging mechanistically impacts these pathways.

Box 2. Aging of Innate Immunity Impairs the Vaccine Response.

Robust innate immune responses are critical for a strong vaccine response. Adjuvants used in vaccine formulation typically target various innate immune pathways to boost a sufficient inflammatory innate immune response. For example, aluminum salts (alum), the most widely used adjuvant in human vaccines, has been shown to promote IL-1β through the NLRP3 inflammasome pathway [84]. Likewise, the adjuvant system 04 (AS04), used in the Hepatitis B (rDNA) vaccine (Fendrix), contains the TLR4 agonist 3-O-desacyl-4’-monophosphoryl lipid A (MPL) and aluminum salts. MPL has been shown to induce TLR4, leading to NF-κB activation of human peripheral blood mononuclear cells (PBMCs) [85]. Once innate immune cells, such as DCs and monocytes, are activated at the site of vaccination, they uptake antigen and express the co-stimulatory molecules CD80 and CD86. They then migrate to the dLN to activate T and B cells.

Importantly, age-associated dysregulation to innate immune signaling pathways can hinder humoral and memory responses following vaccination. Aging limits the production of inflammatory cytokines such as TNF-α, IL-6 and IL-12 of human DCs following activation with TLR agonists, which may be in part due to reduced TLR expression with age [86]. Moreover, TLR-induced production of cytokines positively correlates with the seroconversion rate and seroprotection rate following influenza vaccination [86]. TLR-induced expression of co-stimulatory molecules CD80 and CD86 are decreased with aging, shown in human PBMCs, further impairing subsequent adaptive immune responses [87].

Baseline expression of inflammatory markers such as TNF-α, IL-6, and C-reactive protein (CRP) is increased with aging, contributing to ‘inflamm-aging’, the age-associated low level, chronic, sterile inflammation. While an inflammatory response from innate immune cells is necessary for a robust vaccine response, inflamm-aging is detrimental to vaccine responses. Increased expression of innate inflammatory signatures at baseline negatively correlate with antibody responses following influenza vaccination [88] and hepatitis B vaccination [89]. The use of p38 mitogen-activated protein kinase (p38 MAPK) inhibitors in older humans reduces inflamm-aging as measured by CRP, IL-6 and TNF-α in the serum, and monocyte secretion of IL-6 and TNF-α [90–92]. Importantly, treatment with losmapimod, a p38 MAPK inhibitor, improved varicella zoster virus (VZV) antigen recall responses, indicating that inflamm-aging can potentially be pharmaceutically targeted [90].

Alterations to the Vaccine Response With Aging

Lymph nodes

Vaccine-specific antibodies are generated from two main pathways: the extrafollicular response and the germinal center (GC) reaction. The extrafollicular response forms an early and short-lived humoral response in the extrafollicular foci of the spleen and the medullary cords of lymph nodes (LNs) [10]. The GC reaction leads to a long-lived and high-affinity humoral and adaptive-memory response [10]. Due to the vital role of GCs in secondary lymphoid organs, such as LNs, in the production of a long-lived antigen-specific adaptive memory and humoral responses, understanding how aging affects the GC response is crucial to understanding the age-impaired response to vaccines.

Critically, the size of GCs and the magnitude of GC responses are diminished with aging. The area of the GCs within various LNs, such as the mesenteric, inguinal and axillary, is reduced with age in humans [11]. Aged mice showed a reduced number and area of GCs in the spleen following influenza infection [12] and reduced number of GCs in the draining LN (dLN) following West Nile infection [13]. The decreased size of lymph nodes with aging is consistent with the decrease of infiltrating immune cells, such as T cells and B cells, to the LN in mice, non-human primates and humans [11,13–15]. Alterations to LN architecture with aging, such as the loss of high endothelial venules (HEVs) to the LN, impairs the influx of immune cells [14]. With aging, the LN of mice show increased levels of collagen and α-smooth muscle actin, which can further inhibit homing of immune cells [16]. Aging also increases the occurrence of fibrosis and lipomatosis in the LN of humans [14], which may further impede immune cell migration to the LNs. Defects of various immune cells, such as reduced expression of CD62L on CD4 T cells [17], and reduced phosphoinositide 3-kinase signaling and CCR7 signaling in dendritic cells (DCs) [18,19], also limit the migration of immune cells to the LNs.

A reduction in LN stromal cells contributes to the decreased size of LNs. Within the mesenteric LN [16] and the spleen [20], there is a reduced number of fibroblast reticular cells (FRCs), cells critical for LN remodeling and organization and the generation of chemokines, cytokines and growth factors to promote proper immune cell activation [21,22]. Aging also limits the number of follicular dendritic cells (FDCs), cells critical for the organization of the light zone and dark zone of the GC, B cell trafficking through CXCL13 and CXCL12 production, and B cell survival through BAFF production [23,24]. A recent study showed that aging impairs the activation and expansion of lymphoid stromal cells that express mucosal addressin cell adhesion molecule-1 (MAdCAM-1), a molecule critical for immune cell homing to the LN, in response to vaccination [24,25]. A young/aged mouse parabiosis model, where the blood circulation of two mice is surgically connected, showed that exposure to young circulating immune cells failed to rescue the age-associated impaired activation and expansion of MAdCAM-1⁺ lymphoid stromal cells [24,25]. These results suggest that age-associated impairments in the LN stroma are not due to defects within the immune cell compartment. Interestingly, the addition of a toll-like receptor (TLR) 4 agonist with vaccination improved the activation and expansion of MAdCAM-1⁺ lymphoid stromal cells, suggesting that with appropriate adjuvants the vaccine response in aged hosts can be rejuvenated [25].

The interaction of T and B cells at the T/B border, located at the interface between the T cell zone and the B cell follicle, are essential for a proper GC response, including differentiated T cell responses, somatic hypermutation and isotype switching of BCRs and differentiation of B cells. During the initiation of a GC response, CXCL13 produced by FDCs and CCL19 and CCL21 produced by FRCs ensure proper localization of T and B cells and the formation of the T/B border. With the reduced number of FRCs and FDCs in aged LNs of mice, there is a subsequent reduction in the overall expression of CXCL13, CCL21 and CCL19 [13,16,24]. However, the effects of aging on the per cell output of these chemokines is unknown. The reduction in the homeostatic chemokines in the LN may explain the disintegration of the T/B border and reduced density of the T and B cell zones as seen in various models of aging such as non-human primates, mice and humans [15,24,26].

B cell responses

With aging, there is a decline in the number of total CD19⁺ B cells in the peripheral blood and a decrease in the production of new B cells from the bone marrow (BM), leading to a contraction of the BCR repertoire and limiting the B cell response to novel antigens [27,28]. The limited production of B cells with aging is due to both the reduced response of pro-B cells to IL-7 and the limited secretion of IL-7 from BM stromal cells [29,30]. The reduced production of B cells is also partially due to enhanced myeloid-lineage biasing of hematopoietic stem cells within the aged BM in both humans and mice [31]. Enhanced myelopoiesis of the aged BM is due to increased inflammatory signals in the BM microenvironment [32–35]. A study suggests that an accumulation of plasma cells in the BM of aged mice promote a pro-inflammatory microenvironment through the secretion of cytokines such as IL-1 and TNF-α, leading to increased myelopoiesis [34]. Hematopoietic stem cells from aged mice show increased expression of genes associated with stress and inflammation, which may be a consequence of the inflammatory microenvironment of the aged BM [36].

Aging gives rise to a unique subset of B cells known as age-associated B cells (ABCs) in both mice and humans. ABCs are characterized by the expression of T-bet and CD11c [37,38]. Interestingly, ABCs are not typically activated via the BCR but instead via innate immune receptors such as TLRs [37,38]. Importantly, the role of ABCs on vaccine responses remains understudied and consequently our knowledge of the role of ABCs in vaccine effectiveness is incomplete. T-bet expression in B cells may be important for memory responses and a long-lasting antibody response as mice lacking T-bet expression in B cells showed reduced levels of both IgG antibodies and neutralizing antibodies against the viral protein hemagglutinin (HA) at day 40 post influenza infection [39]. ABCs may be important precursors to plasma cells as B cell expression of T-bet has been shown to be required for the formation of plasma cells and HA-specific antibodies following influenza infection, but not parasitic infection, in mice [40]. This discrepancy between the role of T-bet in the humoral response of viral versus parasitic infections may indicate that B cells require a virally-induced (Th1) rather than a parasitic-induced (Th2) cytokine milieu, as shown by the IFN-γ-dependent upregulation of T-bet in B cells [40]. Therefore, vaccine strategies aimed at utilizing ABCs should include adjuvants that prime for Th1 responses. However, whether utilization of ABCs is a viable vaccination strategy remains to be determined. It is unknown if the antibody response produced by ABC-derived plasma cells is broadly protective with similar rates of class-switching, affinity maturation, and neutralization ability as plasma cells of young hosts. Reports also indicate that ABCs may contribute to autoimmune diseases such as lupus and rheumatoid arthritis, suggesting that these cells may play pathogenic and inflammatory roles in certain contexts [41,42]. Additionally, the majority of the research on ABCs has been done in the context of autoimmunity or viral infection; therefore, the role of ABCs in vaccine response requires further investigation.

It is unclear if aging leads to defects in B cell differentiation into plasma cells. In vitro experiments using B cells isolated from young (18-36 years) and older (65-75 years) patients show that aging does not alter the number of plasma cells generated following T-cell dependent activation of B cells, suggesting that there is no age-associated cell-intrinsic defect in differentiation [43]. However, these experiments were conducted using total B cells from peripheral blood and did not account for age-associated differences in the number of peripheral naïve and memory B cells [27]. Additionally, the microenvironmental factors in the GCs and BM, such as the impairment of T follicular helper (Tfh) cells, may lead to limited B cell activation and differentiation to plasma cells in vivo [12,44]. Overall, more research is needed to understand how aging impacts the development of antibody-producing plasma cells.

Aging impairs class switching and somatic hypermutation of B cells in the GC. In addition to alterations in the LNs and in the T cell compartments, these impairments can be partially attributed to B-cell intrinsic defects, such as the reduced expression of activation-induced cytidine deaminase (AID), a critical enzyme for DNA recombination [45,46]. AID induction in B cells has been correlated with affinity maturation in humans following influenza vaccination [47]. Upregulation of tris-tetraprolin, a negative regulator of mRNA stability, and the degradation of E47, a transcription factor leading to expression of AID, have been implicated in the age-associated decline of AID induction in B cells [46,48,49]. Overall, the age-associated defects in class switching and somatic hypermutation of B cells lead to an inability to mount a diverse and high-affinity B cell response following vaccination.

T cell response

Age-associated thymic involution (i.e., the shrinking of the thymus) leads to the reduced production of naïve T cells with aging. Additionally, the continuous lifelong exposure to antigens causes an expansion of memory T cells. Overall, the ratio of memory T cells to naive T cells increases in older adults [50]. This progressive increase of the memory/naïve T cell ratio leads to an overall contraction of the T cell repertoire and limits the ability of the T cell compartment to respond to new antigens. Involution of the thymus has been shown to correlate with disorganization of the epithelial thymic structure with aging [51,52]. Maintenance of the peripheral naïve T cell population in humans, but less so in mice, is also dependent on homeostatic proliferation of naïve T cells induced by low-level T cell receptor (TCR)) activation and IL-7 and IL-15 cytokine signaling [53]. Interestingly, in mice homeostatic proliferation of naive T cells is reduced with age and this impairment is largely due to defects in the secondary lymphoid environment preventing T cell homing to the secondary LNs, therefore limiting access to IL-7 in the LNs [54]. Homeostatic proliferation of a limited TCR repertoire coupled with a limited production of new T cells produced by the thymus further limits the TCR repertoire with aging. With a contracted TCR repertoire, fewer CD4 T cells are activated in response to immunization, leading to a restriction of Tfh cells that provide costimulation and differentiation signals to B cells in the GC.

Cognate functions of CD4 T cells are attenuated with aging, leading to reduced antigen-specific B cell expansion and differentiation. The transfer of young or aged TCR transgenic CD4 T cells into CD4 KO mice show that aged CD4 T cells exhibit a diminished capacity to promote an antigen-specific B cell response and antibody titers following immunization [55]. These observations may be explained by the reduced expression of CD40L (CD154), the costimulatory ligand to CD40 expressed on B cells, by aged CD4 T cells [55]. The decreased expression of CD40L might be due to impaired activation of NF-kB, the transcription factor critical for CD40L expression and T cell activation [55,56]. However, these experiments were conducted using transgenic TCR CD4 T cells and the implications of the study for a heterogenous polyclonal T cell response, where antigen specific CD4 T cells are a significantly smaller population of the overall CD4 T cells, are unknown. Furthermore, adoptive transfer experiments of aged CD4 T cells into young mice show that aged CD4 T cells have a reduced capacity to promote the GC response and somatic hypermutation of GC B cells following immunization [57], highlighting that T-cell intrinsic age-associated defects can limit the vaccine response.

Activation of CD4 and CD8 T cells from aged mice and older humans show reduced IL-2 production and signaling [58], reduced proliferation [58,59], impaired generation of effector functions [58,59] and decreased expression of the activation marker CD69 [59,60]. Likewise, activation of T cells from old mice shows diminished levels of NF-kB activity, an essential transcription factor for T cell activation [56]. These defects in aged T cells can be partially attributed to impaired formation of the immunological synapse and impaired TCR-mediated activation [59]. However, even bypassing the TCR complex with the use of PMA/ionomycin failed to overcome the activation defect in aged T cells, indicating that alternative pathways, downstream of the TCR, hinder the activation of aged T cells. A potential mechanism for these observations is the decline of miR-181a expression with aging in T cells, which in turn promotes the expression of DUSP6 [60]. DUSP6 negatively regulates ERK signaling, an activating cellular signal upstream of NF-kB, following TCR activation, leading to an increased threshold for proper T cell activation [60].

Tfh cells play essential roles in the GC to provide selection, costimulation and differentiation signals to B cells through the production of cytokines such as IL-21 and IL-4 and through CD154 (CD40L) [61]. As such, Tfh are critical for the differentiation of B cells to undergo affinity maturation and differentiation into memory B cells and plasma cells. The differentiation of activated CD4 T cells into mature antigen-specific Tfh in GCs is impaired with aging in mice and humans post-vaccination [12,44]. These defects in Tfh generation can be partially attributed to the aged microenvironment as adoptive transfer studies show that the differentiation of young CD4 T cells into Tfh cells in an aged mouse are hindered [62]. One such microenvironmental defect is impaired antigen presentation, costimulatory signaling and type I interferon-induced activation of conventional dendritic cells 2 (cDC2) in the LN of aged mice. This limits cDC2s in providing sufficient activation and survival signals to both naïve CD4 T cells and Tfh cells [44]. Additionally, the effector function of Tfh in aged mice following vaccination exhibit a T follicular regulatory-like phenotype with the increased expression of immunosuppressive molecules such as IL-10 and PD-1 [12,63] and decreased expression of IL-2 [12], which may further impair immune processes in the LN following vaccination. Additionally, these aged Tfh show reduced expression of Tfh markers such as ICOS and CXCR5 [12,63] compared to their young counterparts, further highlighting an age-associated change in the phenotype and function of Tfh. Overall, these impairments limit Tfh differentiation and function and lead to a reduced capacity of Tfh to provide help to B cells.

Limitations of Murine Models in Studying Vaccine Responses

While significant work has been done to uncover the mechanisms underlying impaired vaccine responses with aging, many of these experiments have been conducted in aged inbred mice housed in specific-pathogen free (SPF) facilities. However, these models show substantial limitations as these mice do not accurately represent the genetic heterogeneity of humans or immunological experience of humans [64]. Additionally, aged murine models typically do not account for frailty, which has been shown to negatively correlate with vaccine effectiveness in older humans [65], comorbidities that are more prevalent in older humans, which may also impact vaccine immune responses [66], or varied longevity among inbred strains [67]. In Box 3, we discuss how the use of SPF-housed inbred mice may limit the translatability of murine vaccine studies and potential alternative murine models.

Box 3. Immune System of Inbred SPF Mice.

When choosing an appropriate murine model for experimentation, it is important to consider the immune cell composition of the mice. The immunological makeup of inbred C57BL/6 mice differs from outbred Swiss Webster mice, with the C57BL/6 having about half the number of circulating white blood cells compared to the Swiss Webster mice [93]. These differences in the immunological makeup may explain the differences in the antibody responses between outbred Swiss Webster and the inbred C57BL/6 mice [94]. Even between inbred mouse strains, such as C57BL/6 compared to BALB/c, there are differences between both the innate and adaptive immune cellular compartments and immune functions that can lead to different immune responses to and alter interpretation of murine experimental data [95,96].

Additionally, humans are exposed to a lifetime of pathogens and antigens, however, in contrast, mice in SPF facilities are protected from the large majority of infectious pathogens, limiting exposure to antigen and training of the immune system [97]. Repeated exposure to antigen can also train how the immune system responds to vaccine by a phenomenon known as antigenic seniority, in which the development of the memory immune response to the first antigenic exposure can condition the immune response to subsequent exposures of similar antigens [98]. Limited exposure to antigen and pathogens also leads to improperly formed immune systems, as shown by immunological makeup of SPF-raised lab mice compared to humans and non-SPF mice. For example, there are fewer numbers of circulating, memory CD8 T cells and tissue-resident CD8 T cells in SPF-raised C57BL/6 mice compared to adult humans, feral mice, and pet store mice [75]. Furthermore, co-housing lab mice (C57BL/6) with pet store mice leads to an increase in the number of CD4 T cells, innate lymphoid cells and B cells in various tissues such as the lung, spleen, and skin, indicating that the exposure to microbes and antigens is crucial for the proper expansion of the immune system [99]. Overall, these studies show that repeated exposure to antigen trains the immune responses and further complicates the experimental murine models necessary to properly replicate the immune systems of older individuals. The use of outbred mice such as the UM-HET3 [100], the Swiss Webster mice [101] and non-SPF mice, such as pet store-purchased mice [102], to verify prominent findings in mouse vaccination studies may offer more credibility and increase possibility of translation of mouse studies to humans.

Strategies to Improve Vaccine Response in Older Adults

The need for improved humoral and memory immune response to vaccines in older adults has driven the development of novel vaccine therapies, even though a full mechanistic understanding of age-impaired vaccine efficacy has not been achieved. In this next section we will discuss some in-use and under-development methods to boost the age-associated impairment of the vaccine response.

Increasing antigen load

Influenza burden and influenza-related complications in older adults continues to be of high public health concern [1]. A currently used strategy to improve the vaccine response to the seasonal influenza vaccination in older adults is to increase the amount of antigen delivered. The high-dose quadrivalent seasonal influenza vaccine (Fluzone), which contains 60μg of HA per strain compared to the standard dose of 15μg, has been approved for use by the Food and Drug Administration (United States) since 2009. The high dose influenza vaccine is more effective than the standard dose vaccine in protecting older individuals from influenza infection and influenza-related hospitalization, irrespective of the circulating strain or antigenic match of the vaccine to the dominant circulating strain of the season [68,69].

The high dose influenza vaccine is thought to be more immunogenic compared to the standard dose vaccine, providing more antigen to activate both the innate and adaptive arms of the immune system. The high-dose influenza vaccination produces higher induction of antibody-dependent cell-mediated cytotoxicity (ADCC)-inducing antibodies [70], HA-inhibiting antibodies [71], and virus neutralizing antibodies [71]. Increased antigen load, leading to higher immunogenicity, can lead to higher levels of T cell activation and B cell activation. With the high-dose vaccination, but not with the standard-dose vaccination, there is a positive correlation between Tfh cell induction at day 7 post vaccination and IgG titers at 30 days post vaccination [70]. This potentially indicates that the high-dose vaccine better supports Tfh activation, ultimately lead to increased generation of antibody-producing plasma cells. The use of high-dose vaccination is further supported by the higher induction of plasmablasts, precursors to plasma cells, at 7 days post vaccination, in older individuals that receive the high-dose vaccine compared to those that receive the standard-dose vaccine [72]. Overall, the high-dose vaccination leads to a more substantial antibody response and improved protection against influenza infection in older individuals relative to the standard-dose vaccination.

Improved adjuvants

Another strategy to improve the immunogenicity of vaccines is the addition of adjuvants that activate innate immune pathways. Several adjuvants targeting the TLR pathways have shown promising results. For instance, the MAdCAM-1+ lymphoid stromal cell response, which is impaired in aged mice, is improved by the use of TLR4 agonists during vaccination [25]. The recombinant herpes zoster vaccine (Shingrix), which utilizes the TLR4 agonist-containing adjuvant AS01B, shows a 89.8% vaccine efficacy versus placebo control in patients 70 years or older [73]. The addition of a TLR7 agonist improves both the cDC2 and Tfh response in aged mice [44]. Clinical trials with the TLR7 agonist imiquimod given in conjunction with an intradermal trivalent influenza vaccine show improved responses in both young and older adults [74,75]. In older adults, the use of imiquimod improved the seroconversion rate from 32.3% to 86.7% measured at 1 year post-vaccination [75]. Overall, these studies show that the age-associated defects in the vaccine response are not irreversible, and the development of improved adjuvants can boost the vaccine response in older adults. Interestingly, the use of combined TLR4, TLR7 and TLR9 adjuvants increases antibody responses in mice following ovalbumin vaccination [76]. However, it remains to be determined if combined adjuvant vaccinations can improve vaccine responses in older individuals.

Multivalent vaccines

Multivalent vaccines contain antigens from multiple different strains/serotypes of a pathogen in order to provide a broader range of coverage against antigenically variable pathogens [77]. An example of a multivalent vaccine is the pneumococcal 20-valent conjugate vaccine (Prevnar20), which protects against 20 different serotypes of Streptococcus pneumoniae, the causing agent of pneumococcal disease [78].

A study comparing the use of multivalent and monovalent vaccines against rotavirus in mice suggest that the use of multivalent vaccines induce stronger protection against homotypic and heterotypic strains [79]. A similar study comparing trivalent versus monovalent influenza vaccination in mice showed that the multivalent vaccination was able to induce broader heterotypic, cross-reactive protection without compromising antibody avidity or affinity to the individual three antigens in the vaccine [80]. These studies highlight the use of multivalent vaccines to protect against a wide variety of pathogenic strains as a potential ‘universal’ vaccine.

Concluding Remarks

The cause of the decline in vaccine efficacy with aging is multifactorial. Age-associated alterations in the innate immune system, LNs, GCs, B cells and T cells all contribute to the reduced humoral response following vaccination (Figure 2). However, the impaired response to vaccination is not irreversible, as indicated by the use the of adjuvants and higher antigen doses in vaccine formulations to induce a more robust and long-lasting antibody response in older adults (see Clinician’s Corner).

Figure 2: Age-Associated Alterations in the Immune System Limit the Response to Vaccination.

(A) Aging leads to a reduction of lymph node size that is at least in part due to reduced number of FDC and FRCs and decreased immune cell influx. There are also fewer and smaller germinal centers in aged lymph nodes. The reduced demarcation of the T/B border with age is associated with declines in chemokines, such as CXCL13, CCL19 and CCL21, that coordinate proper homing of B cells and T cells. (B) B cell production from the bone marrow is reduced with aging, leading to an decrease of naïve B cell numbers, as well a decrease in total B cell numbers, in the periphery. This decrease in B cell production is due to reduced IL-7, increased inflammation and increased myelopoiesis of the bone marrow. Additionally, aging leads to an accumulation of ABCs whose role in the vaccine response has yet to be fully elucidated. A decrease of the enzyme AID within B cells with age leads to reduced somatic hypermutation and class switching. (C) Thymic involution leads to a decreased production of naïve T cells with age. Aging leads to a diminished ability of T cells to become activated, proliferate, and produce IL-2 following TCR-mediated activation. Additionally, aging limits the ability of T cells to provide costimulatory signals such as CD40L to B cells. This figure was created with BioRender.com.

Clinician’s Corner.

• Various alterations in the stromal and immune compartments associated with aging lead to an impaired vaccine response in older adults.

• The age-dysregulated immune response to vaccination is not irreversibly impaired with aging, indicating that novel therapies can be developed to improve vaccine efficacy in older adults.

• New vaccine designs and the development of novel adjuvants may improve protective immunity in older adults. Although future strategies, such as combining adjuvants may boost immunity further in older people, will have to be balanced against side effects and safety of such a potential approach.

Most biological and immunological studies approach the experimental method by focusing on one or a few cell types. However, this limited view may mask the coordinated efforts of the immune system, which is a complicated integrated network of cells and pathways. Future vaccination studies using a systems biology approach, allowing for the integration various levels of data such as immune phenotyping, single cell gene expression, proteomics and epigenetics, will provide a more rigorous approach to answering remaining questions in the field (see Outstanding Questions). A recent study has applied system biology methods to analyze the immune response in the blood and lymph nodes of humans following a Hepatitis B vaccination [81], highlighting the feasibility of a future study utilizing a systems biology approach to discern the impact of aging on the immune response to vaccination.

Outstanding Questions.

• Do age-associated B cells (ABCs) play a beneficial or detrimental role in the vaccine response? Can ABCs be utilized to improve vaccine efficacy in older adults?

• Does aging lead to defects in the generation of plasma cells following B cell activation?

• Does the use of outbred mouse models for vaccine studies better predict the outcomes seen in human vaccine studies?

• How does prior antigen and pathogen exposure affect the age-impaired vaccine response? Does the use of aged non-SPF mice, who have had lifelong exposure to various pathogens unlike SPF-housed mice, better model the age-associated alterations of the human immune system?

• Does a combination of adjuvants (i.e., combining TLR4 and TLR7 adjuvants) boost the vaccine response in older people as compared to single adjuvants?

Highlights.

• The vaccine response, as measured by titers of vaccine-specific antibodies, the duration of measurable vaccine-specific antibodies and the quality and affinity of the antibody response, is reduced with age.

• Age-associated alterations in lymph nodes, such as a reduced number of germinal centers and the decline in the number of follicular dendritic cells (FDCs) and fibroblastic reticular cells (FRCs), contribute to the age-associated decline in vaccine efficacy.

• The reduced production of B cells from the bone marrow and impaired affinity maturation of B cells contribute to the age-associated decline in vaccine efficiency.

• Aging impairs the production and activation of T cells, leading to reduced number and impaired function of T follicular helper cells that provide costimulatory, survival, and differentiation signals to B cells in the germinal centers.

Glossary

Dark Zone

a specialized region within a germinal center where B cells migrate to undergo proliferation, class switching and somatic hypermutation

Germinal Center

a specialized microstructure in the B cell follicle of secondary lymphoid tissues, such as lymph nodes, that provides the necessary microenvironment for the activation, proliferation, selection and differentiation of B cells

High Endothelial Venules

blood vessels that specialize in lymphocyte trafficking and are found in secondary lymphoid organs such as lymph nodes

Humoral Response

the antibody-mediated immune response formed through the secretion of antibodies by plasma cells

Immunosenescence

the gradual age-associated dysregulation, rather than overall impairment, of the immune system that leads a wide-range of age-associated outcomes such as impaired host-defense, increased tumorigenesis, increased prevalence of certain auto-immune diseases and impaired vaccine response

Light Zone

a region within the germinal center where B cells migrate to undergo selection by competing for survival signals from FDCs and TFh cells

Plasma Cell

fully differentiated B cell that secretes large quantities of antibodies

T cell receptor (TCR)

a protein receptor complex that is located on the surface of T cells and is necessary for T cell activation through the recognition of cognate-antigen presented on an MHC molecule

T follicular helper cells (Tfh)

a specialized subset of CD4 T cells that are found in secondary lymphoid organs such as the lymph nodes and that play important roles in providing costimulation, survival and proliferation signals to B cells

T/B Border

located within the secondary lymphoid organ at the interface of the T cell zone and the B cell follicle, where activated B cells receive signals from T cells to further undergo proliferation, differentiation, and germinal center reactions