Frailty, Inflammation, and Immunity

Abstract

Frailty is an important geriatric syndrome that is characterized by multisystem dysregulation, leading to decreased physiological reserve and increased vulnerability for adverse health outcomes. A large number of studies have shown a heightened inflammatory state marked by elevated levels of inflammatory molecules, such as IL-6 and C-reactive protein (CRP), and increased counts of white blood cell (WBC) and WBC subpopulations in frail older adults. It has been postulated that this heightened inflammatory state, or chronic inflammation, may play an important role in the pathogenesis of frailty, directly or through its detrimental influence to other physiologic systems. Inflammatory and immune activation mediated by monocytes and macrophages demonstrated by upregulated expression of specific stress responsive inflammatory pathway genes and elevated neopterin levels may contribute, at least in part, to this chronically heightened inflammatory state in frailty. Decrease in lipopolysaccharide (LPS)-induced proliferation of the peripheral blood mononuclear cells (PBMCs), one of the functional readouts of the innate immune system, has also been observed in frail older adults. In the adaptive immune system, significant frailty-associated alterations have been identified in the T-cell compartment including expansion of CD8⁺ and CCR5⁺ T cells and loss of CD28 expression, above and beyond age-related senescent remodeling. Moreover, frailty is associated with impaired antibody responses to pneumococcal and influenza immunization and poor clinical protection against influenza infection in community-dwelling older adults. Taken together, these findings demonstrate significant inflammatory and immune dysregulation in frail older adults and highlight the need for strategies to improve the immune function for this vulnerable elderly population.

Frailty is an important and common geriatric syndrome. It is a clinical phenotype in old age with decreased physiologic reserve and increased vulnerability for subsequent morbidity and mortality [1–3]. It is also a syndrome characterized by multisystem dysregulation and a loss of complexity in resting dynamics, manifested by maladaptive response to stressors, leading to a vicious cycle towards functional decline and other serious adverse health outcomes [2,4]. The phenotypic characteristics of frail older adults, as described by Fried and colleagues [1], are now recognized to be a syndrome consisting of three or more of the following five items: weakness (by grip strength), low physical activity, slowed motor performance (by walking speed), exhaustion, and unintentional weight loss [1]. The aggregate of these syndromic characteristics independently predicts serious adverse health outcomes, including acute illness, falls, hospitalization, disability, dependency, and mortality, adjusting for comorbidities [1,5]. This phenotype definition has been favorably evaluated and compared to other proposed frailty criteria, and has been validated in a number of large older adult cohorts as well as in various clinical and cultural settings [6–9]. For example, the frailty index proposed by Rockwood and Mitnitski is a summary of accumulated deficits in multiple medical and functional domains [6,10–12]. Compared to this frailty index, the Fried’s criteria tend to be more focused on physical function and less cumbersome in operationalization, particularly in research settings. Based on Fried’s definition, the estimated prevalence of frailty is 7–10% among community-dwelling men and women age 65 and older, and up to one-third of those aged 80 years and older [1,13]. Because of the profound functional, medical, and socioeconomic consequences of the frailty syndrome, it is imperative to advance our understanding of the pathogenesis and physiologic impact of this syndrome and, with this information, to develop interventional strategies.

A large body of literature, most rapidly accumulated in the past few years, suggests that frailty-associated physiologic dysregulation involves multi-organ systems including the musculoskeletal, immune, endocrine, hematologic, and cardiovascular systems, to just name a few [14,15]. This article provides an overview of the most recent development of our knowledge about inflammation and immunity in frailty. As described below, chronic inflammation, or a heightened inflammatory state, appears to play an important role, directly or through other intermediary pathophysiologic processes, in the pathogenesis of frailty. Importantly, dysregulation in the innate and adaptive immunity likely also leads to chronic inflammation as well as impairment in vaccine-induced immune protection and increased susceptibility to and severity of infections in frail older adults.

Inflammation in Frailty

1) Molecular and cellular inflammatory markers in frailty

The aging immune system is characterized by a low grade, chronic systemic inflammatory state. This age related inflammatory state, or “InflammAging”, is marked by elevated inflammatory molecules, such as IL-6 and C-reactive protein [CRP], and is associated with increased morbidity and mortality in older adults [16–18]. Peripheral white blood cell (WBC) and its subpopulations are circulating immune cells and the principle cellular component of the inflammation system. Clinically, increase in WBC counts is recognized as an important cellular marker of systemic inflammation. Recent studies have provided a large body of evidence suggestive of a heightened inflammatory state in frail older individuals as marked by further increases of these molecular and cellular inflammatory markers compared to that observed in robust older adult controls.

IL-6 is a pro-inflammatory cytokine with increased circulating levels in older adults. Age-related increases in IL-6 levels are associated with several pathophysiologic processes, including atherosclerosis, osteoporosis, and sarcopenia, and with functional decline, disability, and all-cause mortality in older adults [19–22]. In addition, increased IL-6 levels are associated with lower muscle mass and strength even in well-functioning older men and women [22,23]. In a longitudinal study, Ferrucci and colleagues reported that elevated IL-6 levels at baseline predict a significantly higher risk for the development of physical disability and a steeper decline in muscle strength and motor performance during a follow-up period of 3.5 years in older women living in the community [24]. This study and others have shown that chronic systemic inflammation marked by elevated IL-6 levels is associated with decreased muscle strength and power and slowed walking speed, two central components of the frailty syndrome. Direct evidence supporting the relationship of this molecular inflammatory marker with frailty came first from a pilot study in which community-dwelling frail older adults had significantly higher IL-6 levels than non-frail controls with similar age [25]. A subsequent age, race, and sex-matched pair study has further demonstrated that frail older adults had significantly higher IL-6 production by the peripheral blood mononuclear cells (PBMCs), upon stimulation with lipopolysaccharide [LPS] than the non-frail controls [26]. These findings have been further confirmed in more recent studies of large cohorts of older women demonstrating that elevated IL-6 levels are independently associated with frailty [27,28]. In an IL-10 knockout mouse model for frailty, older mice with phenotype mimicking human frailty had elevated IL-6 levels compared to the control wild type mice [29]. These clinical, laboratory, and population studies have provided strong support for the association of this important proinflammatory cytokine with frailty in older adults living in the community.

CRP, discovered in 1930 as an acute phase reactant, is a classic circulating molecular marker of systemic inflammation [30]. Elevated CRP levels are associated with cardiovascular disease [31]. Clinically, CRP has now been integrated as part of the routinely measured panel of cardiovascular disease risk factors. Two large cohort studies have demonstrated the direct association of this molecular inflammatory marker with frailty. In the Cardiovascular Health Study [CHS], Walston and colleagues have shown that significant association of elevated CRP levels with frailty after excluding cardiovascular disease and diabetes and adjusting for basic demographic characteristics [32]. Data from the Longitudinal Aging Study Amsterdam [LASA] have further confirmed these findings [33]. However, no consistent associations were observed between IL-6 levels and prevalent or incident frailty in this study, suggesting different factors that may influence the production and regulation of IL-6 and CRP in the study population [33]. While frailty has not been extensively evaluated in centenarians, studies failed to show any associations of IL-6 or CRP levels with mortality in this extremely old population, which may be due to survival selection bias [34,35].

As part of the complete blood counts (CBC), WBC count is a standardized and widely available laboratory measurement. Acute and dramatic increase in total WBC counts (above the normal range) is recognized as a clinical indicator for systemic inflammation, frequently due to acute bacterial infections. A number of large cohort studies in older adults, have demonstrated that elevated WBC count, albeit within the normal range, is associated with cardiovascular and cerebrovascular events, cardiovascular and cancer mortality, as well as all-cause mortality in older adults [36–38]. Recent studies have demonstrated direct relationship of frailty with elevated counts of WBC as well as elevated counts of neutrophils and monocytes [27,39]. A potential synergistic interaction between WBC and IL-6 in their associations with frailty has also been suggested [27]. Such synergy is further supported by the laboratory study cited above in which the PBMCs, the isolated WBC subpopulations, from frail older adults had significantly higher LPS-induced IL-6 production than that from matched non-frail controls [26]. In addition, direct in vivo association between circulating IL-6 levels and WBC counts has been demonstrated in the same cohort of community-dwelling older women [40].

2) Molecular inflammatory pathway and immune activation mediated by monocytes and macrophages in frailty

Molecular evidence for inflammatory pathway activation in frailty has recently emerged through in-depth analyses of ex vivo expression of inflammatory pathway genes by purified monocytes [41,42]. Utilizing molecular and genetic techniques including pathway-specific gene array analysis and quantitative real time reverse transcriptase-polymerase chain reaction (RT-PCR), these studies have shown frailty-associated upregulation in monocytic expression of CXCL10 gene that encodes a potent pro-inflammatory chemokine [41]. Moreover, purified monocytes from frail older adults had consistent upregulation in ex vivo expression of seven stress-responsive inflammatory pathway genes upon LPS stimulation compared to those from matched non-frail older adult controls [42]. These genes encode transcription factors, signal transduction proteins, as well as chemokines and cytokines. Findings from these in-depth molecular analyses have demonstrated frailty-associated upregulation in the expression of specific inflammatory pathway genes by monocytes, a major cell type of the innate immune system. As a potential underlying molecular and immune mechanism, upregulated expression of these inflammatory pathway genes could lead to a heightened inflammatory state in frail older adults described above. This possibility is further supported by the correlation between frailty-associated CXCL10 upregulation and the elevation of serum IL-6 levels [41]. In addition, the identified inflammatory pathway genes are stress-responsive genes whose products are known to play a role in stress responses in various study settings [43–47]. This is consistent with the cardinal feature of frailty that frail elderly manifest increased vulnerability to stressors. Clinically, frail older adults may experience LPS exposure surge[s] during Gram [−] bacterial infections, such as urinary tract infection and urosepsis.

Neopterin, a guanosine triphosphate (GTP) metabolite primarily produced by human monocytes and macrophages, is a well known marker for chronic immune activation mediated by monocytes and macrophages [48,49]. Increased production and serum concentration of neopterin have been observed during aging [48–50]. Elevated neopterin levels are associated with increased mortality in older adults [51,52] and have been documented in conditions with significant immune activation, such as viral infections and autoimmune disease [53,54]. Data from a recent study in community-dwelling older adults have shown that elevated neopterin levels are associated with frailty, independent of IL-6 levels [55]. These findings suggest significant monocyte/macrophage-mediated immune activation marked by elevated neopterin levels in frail older adults.

3) Potential role of chronic inflammation in frailty

Although the consequences of this chronic activation is unclear, evidence is also emerging that chronic exposure to inflammatory mediators may be in part responsible for a host of tissue changes and susceptibility to the development of chronic disease states. As discussed above, the relationship between frailty and common molecular and cellular inflammatory markers are well documented. The critical question is whether this heightened inflammation plays a role in the pathogenesis of frailty. Individual inflammatory molecules, such as IL-6, may directly contribute to frailty or its central components (such as decreased muscle strength/power and slowed motor performance). In addition, frailty involves multiple physiologic organ systems, such as musculoskeletal system (sarcopenia, osteopenia), hematologic system (anemia), cardiovascular system (clinical or subclinical cardiovascular diseases), and endocrine system (decreased insulin-like growth factor-1 (IGF-1), decreased DHEA-S, and insulin resistance, etc.) [14,15,56]. It is conceivable that heightened inflammation contributes to frailty through its detrimental effects (functional impairment and/or structural damage) to these physiologic organ systems. In fact, studies have shown that circulating IL-6 levels have inverse associations with hemoglobin concentration and IGF-1 levels in frail older adults, but not in non-frail controls; low hemoglobin and IGF-1 levels are each independently associated with frailty, as well [25,56]. In addition, WBC counts have an inverse association with IGF- levels [57]. Other factors, such as low levels of micronutrients and vitamins including zinc have also been associated with frailty in older adults [58,59]. In addition to poor dietary intake, such nutritional dysregulation could again be caused by chronic inflammation in the elderly [60]. Taken together, it has been proposed that the heightened inflammatory state plays a key role in the pathogenesis of frailty, directly or through other intermediate pathophysiologic processes. In the Women’s Health Initiative Observational Study, however, use of statins which are known to have anti-inflammatory effect had no association with incident frailty [61]. These data suggest that factors other than chronic inflammation may also be important for the development of frailty in older women living in the community.

The Innate and Adaptive Immunity in Frailty

The innate immune system is the front line of defense against injury and infection in most living organisms. It provides an immediate response to external stressors, and as such is critical in the development and shaping of immune responses. The major cellular components of the innate immune system include neutrophils, monocytes and macrophages, as well as dendritic cells, although multiple other cell types such as fibroblasts and hepatocytes are also capable of mounting an inflammatory response to stressors. As discussed in the previous sections, studies have shown increased counts of total WBC and neutrophils and monocytes as well as significant monocyte/macrophage-mediated inflamematory and immune activation in the syndrome of frailty. Decrease in polyliposaccharide (LPS)-induced proliferation of the peripheral blood mononuclear cells [PBMCs], one of the functional readouts of the innate immune system, has also been observed in frail older adults [26]. These studies provide suggestive evidence for frailty-associated dysregulation in innate immunity.

Age-related immunosenescence has been extensively documented in the adaptive immune system. This includes age-related loss of CD28 expression, skewing immune repertoire to the memory phenotype, T cell clonal expansion, increased autoimmune antibody production, and altered cytokine expression. This is considered to be responsible, at least in part, for the inflammatory phenotype or “InflammAging”, poor immune response to vaccination, and overall immune functional decline observed in older adults. In the syndrome of frailty, increasing evidence supports significant alterations in the T cell compartment of the adaptive immune system. The first line of evidence comes from a post hoc analysis of the data from a nested-case control study evaluating the relationship between T cell subsets and mortality in community-dwelling older women [62]. The results showed that frail older women had significantly higher counts of CD8⁺ and CD8⁺CD28 T cells compared to non-frail older women (n=24) matched by age and major comorbidities (cancer, arthritis, diabetes, cardiovascular disease, hypertension, and hormone replacement therapy). While no difference was observed in CD4⁺ T cell frequencies between the two study groups, the frail group had significantly lower CD4⁺:CD8⁺ ratio compared to the non-frail group.

The second line of evidence comes from frailty studies in patients with acquired immune deficiency syndrome (AIDS) caused by human immunodeficiency virus (HIV) infection. For example, data from the Multi-center AIDS Cohort Study (MACS) showed that compared to HIV-uninfected controls, study participants with HIV infection were more likely to have a frailty-related phenotype (FRP) [63]. A follow-up study in the same cohort demonstrated that CD4⁺ T cell count predicted the development of FRP among HIV-infected subjects, independent of use of highly active antiretroviral therapies (HAAT) or plasma HIV viral load [64]. These findings suggest a role of CD4⁺ T cell dysregulation in the development of frailty in HIV infected patient population.

In addition, a pilot study in thirteen pairs of age, race, and sex-matched frail and non-frail older adults living in the community with mean age of 84 years (range: 72–94) has shown that frail participants had increased counts of T cells expressing chemokine CC receptor-5 (CCR5) compared to the matched non-frail controls [65]. The increase of CCR5⁺ T-cell frequencies in the frail elderly cannot be attributed to the frailty-associated CD8⁺ T-cell expansion as such an increase was also observed in the CD8⁺ T-cell compartment. In addition, there was a trend toward graded increase in CCR5⁺ T-cell counts across the frailty scores in the frail participants [65]. CCR5⁺ T cells have a type-1 pro-inflammatory phenotype and contribute significantly to several inflammatory conditions [66,67]. Moreover, CCR5 is a well known co-receptor for HIV; active development of anti-CCR5-based therapies for HIV infection and AIDS has shown promising results [68,69]. Therefore, findings from this pilot study, if validated, suggest that anti-CCR5-based strategies can potentially be developed for the prevention or delay and treatment of frailty in older adults.

Information about potential B-cell alteration in frailty is limited. Utilizing spectratype analysis of the immunoglobulin (CDR)3 region, a recent study evaluated and compared B-cell repertoire diversity between elderly participants from the Swedish NONA Immune Study and young adults [70]. The results showed an age-related decrease in B-cell diversity and a dramatic collapse of the B-cell repertoire in a subset of older individuals who were considered frail. However, details on how frailty was defined in that study were lacking. Importantly, emerging evidence suggest significant impact of frailty on antibody responses to immunizations in the elderly. For example, Ridda and colleagues have recently reported that frailty is associated with poor antibody response to pneumococcal vaccination [71]. Data from a more recent study indicate that frailty is associated with impaired antibody responses to influenza immunization and poor clinical protection against influenza infection in community-dwelling older adults [72]. These findings have significant clinical relevance as assessing frailty status in the elderly may identify those who are less likely to respond to pneumococcal and influenza immunizations and at higher risk for these common infections and their complications.

In summary, our current understanding of inflammation and immunity in frailty suggests multi-facet dysregulation in both innate and adaptive immune system in frail older adults. It provides a basis for further clinical and laboratory investigations into the pathogenesis of this important geriatric syndrome. It also highlights the need for strategies to improve the immune function for frail older adults.