Acquired immunity: immunosenescence and physical activity

Abstract

Several lines of evidence indicate that infectious diseases, cancer, and autoimmune disorders occur more frequently in elderly people, thus suggesting that altered function of immune organs and cells, such as thymus and T and B lymphocytes are of primary importance in the pathogenesis of these diseases. Furthermore, old subjects are less responsive to vaccine than younger because of immune changes. The most common changes accompanying the adaptive immune system include decrement of T and B cells proliferation, repertoire degeneracy, increase of the memory cell type, decreased numbers of naive cells, and shift from T helper₁ (Th1) to T helper₂ (Th2) response. Regular exercise in the elderly may improve the alterations in acquired immunity which follow the physiological process of aging, allowing a major resistance against external pathogens and a better quality of life.

Introduction

Aging represents an important challenge to public health. It is associated with weakening of the organism and a decreased performance of the physiological system, including the immune system. Thus, the term immunosenescence designates a sort of remodeling of the immune response [45]. Whether the alterations in immune responses contribute to morbidity and mortality in elderly still remain a tangled question.

Elderly subjects display decline of absolute lymphocyte counts [5]. They have various suppressed immune parameters [15], such as decreased proliferative lymphocyte responses or anergic lymphocytes [55]. The alterations of the immune system in the elderly are linked principally to replicative senescence, which may limit T cell clonal expansion and alterations associated with the thymus involution. In addition, following stimulus with antigens, mitogens, or anti-CD3 antibody, irregularities of apoptosis, changes in the peripheral lymphocyte pool, in the cytokine production profile, in the signaling function, and in the replicative ability have been described (Table 1).

Table 1 T lymphocytes in elderly

The most common changes accompanying the adaptive immune system include decrement of T- and B-cell proliferation, repertoire degeneracy, increase of the memory cell type, decreased numbers of naive cells, and shift from T helper₁ (Th1) to T helper₂ (Th2) response. It is noteworthy that the T helper₁ phenotype is characterized by a pro-inflammatory status and a resistance to infectious agents, whereas the T helper₂ phenotype is characterized by an anti-inflammatory status.

Previously, we reported that a combination of immune status index, including high T cell proliferation, high CD4⁺ cells in relation to CD8⁺ cells, can predict survival over a 2-year follow-up in very old people [46]. In our previous review, we have evaluated the changes in physical activity-induced innate immunity [47]. The aim of the present review is to evaluate the changes that physical activity has on adaptive immunity and examining the alterations of acquired immunity both with aging and physical activity.

Immunity in the elderly

Thymus

The thymus is a central lymphoid organ that lies within the superficial region of the superior mediastinum. It is responsible for T lymphocytes development [22, 76]. The involution aging dependent of thymus is characterized by a progressive reduction in size and weight, about 3% per year until middle age and 1% per year for the rest of its life [21]. The loss of thymocytes and epithelial cells is replaced by adipose tissue. The decreased proliferation and differentiation of thymocyte is dependent on the reduced production by thymic epithelial cells (TECs) of thymic–hormonal factors, such as thymulin, thymopentin, and thymosines [52].

The molecular mechanism underlying thymus involution has not been fully resolved. It has been hypothesized as a correlation between aging of TECs with reduced expression of MHC molecules and decreased production of thymopoietic cytokines [2, 16]. As a consequence, the output of new T cells to the peripheral T cell pool declines with age. However, the phenotypes of all expected thymic T-cells intermediates are present; there is evidence of cell proliferation as determined by Ki 67 antigen expression and of rearrangement of T-cell receptor gene [72, 73]. Therefore, the age-related loss in thymic tissue is quantitative rather than qualitative.

Post-thymic differentiation involves changes in the expression of CD45 isoforms, cell surface glycoproteins with tyrosine phosphatase function, as well as other markers that discriminate between naïve and memory T cells. Moreover, several studies addressed to the effect of thymectomy on peripheral T-cell numbers in humans, demonstrated that thymectomy in adults had no effect upon absolute numbers of CD4⁺ and CD8⁺ T cells or upon the ratio of naïve and memory T cells [31, 78]. Additionally, defective Fas signaling play a crucial role in the progression of thymic atrophy with aging. Thymic involution was experimentally prevented following the insertion of a Fas transgene abolishing the resistance to apoptosis of T cells in aged mice and prevents thymic involution [85]. Furthermore, thymic involution can be affected by other microenvironmental factors including hormones and cytokines. Interestingly, melatonin, the main neurohormone of the pineal gland can correct many of the age-related declines of thymic endocrine activity as well as immune dysfunction observed in thymocytes, peripheral blood lymphocytes, and spleen cells [51]. Recent studies demonstrated that IL-12 play a crucial role in the inhibition of thymic involution affecting the thymocyte-negative selection process [43]. It has been observed that IL-12, in combination with IL-2 and IL-7, allows to an increased proliferative response, induced by the latter cytokines on thymocytes [41].

T lymphocytes

The major changes in immunosenescence occur in T-cell populations, leading to reduced cellular-mediated responses [65].

The decrease of the immunological functions in the elderly could be caused by a decline in the number of T cells or by a deterioration of T-cell functions (Table 2). Analytical investigations reveal age-related changes in the peripheral T cell pool, in the predominant phenotype, cytokine production profiles, signaling function, and in replicative ability following stimulus with antigen, mitogens or anti-CD3 antibody [64]. Counts of T-cell subpopulations give controversial results; however, it is more possible to find an increased number of CD8⁺.

Table 2 Function of T cells in elderly

As mentioned before, the changes in the behavior of peripheral T cells are thought to be causally linked to an age-associated thymic involution. Early thoughts on age-associated immune dysfunction are centered on the number of T lymphocytes. Although from this decreased output one would expect a reduced number of cells in the T-cell pool, a comparison to the total numbers of T cells in old and young individuals failed to show any significant difference [34].

In humans, aging leads to a decline in T-cell proliferation and in the production of and response to IL-2 [23, 56]. Several evidence show that shifts in T-cell subsets, particularly shifts from the naïve to the subset of previously activated memory T cells, may contribute to age-related changes in T-cell function [6].

The reduced cell-mediated response characterizing elderly subjects seems to be dependent on altered phenotypes of T cells. It has been observed that there is a conspicuous decrease in naïve T cells (CD45RA); meanwhile, the proportion of memory T cells (CD45RO⁺) increases [48]. The functional alterations of the T cells depend on the increase of the CD45RO⁺, which are generally thought to have a poor capacity of activation [69], although others suggested that these cells may have an improved capacity of activation [44], as well as a different production of cytokines [74].

The decline of T-cell responses might be related to an increased activity of indoleamine 2–3 dyoxigenase, an enzyme located on APC, which is responsible for decreased levels of tryptophan surrounding T lymphocytes. Higher activity, found in nonagenarians, is likely due to infectious and inflammatory stimuli, and it could be considered as a predictor of mortality [67].

Regarding the CD4⁺ T cells, some authors sustain that serum IL-7 levels, especially decreased levels, and IL-7R expression played a remarkable role on high frequency of CM (central memory) CD4⁺ cells and a decreased frequency of EM (effector memory) CD4⁺ cells. This change may affect, in aging, the development of long memory CD4⁺ T-cell immune responses to pathogens in vivo, as an altered T cell immune response to influenza vaccine [37].

The increased proportion of memory T cells might be due either to loss in the regulation of the cell cycle and DNA repair or to a missing apoptotic activity caused by the p53 deficiency [63] as well as the decreased expression of the Fas antigen (CD95) [1].

Recently, it has been observed that the increased apoptosis in the naïve CD8⁺ cells was associated with increased cleavage of both caspase-8 and caspase-3 compared to young subjects [30].

The total activation of T cells occurs along primarily two distinct but synergistic signals [36]. The first signal delivered through the T-cell antigen receptor is provided by antigen itself and is responsible for the specificity of the immune response. The second signal, or co-stimulatory signal, is not antigen-related. Many T-cell molecules may serve as receptors for co-stimulatory signals; the CD28 molecule is the best characterized of these molecules [42, 53]. In the elderly, a wide pool of CD28⁻ CD8⁺ T cells is present. In some of these people, the pool could increase, reaching more than 50% of CD8⁺ total pool. This presence may allow the appearance of latent infections, such as VZV (shingles) and EBV (lymphomas), as well as a reduced control over acute infection with a repeatedly encountered virus (influenza), well documented in elderly persons [14].

A topic that deserves emphasis is the critical role of co-stimulation in T-cell responses. In the absence of co-stimulatory signals, a T cell encountering an antigen undergoes abortive activation, short of proliferation and production of appreciable amounts of cytokines and becomes unresponsive to appropriate stimulation for up to several weeks [24] or undergoes programmed cell death [54].

B lymphocytes

In contrast to numerous studies of age-associated alterations of T-cell function, those involving B-lymphocyte function or humoral immunity are relatively few. However, aging influences both the quantity and the quality of humoral immunity (Table 3).

Table 3 B lymphocytes in elderly

Quantitative changes include an altered number of immunoglobulin and antigen-specific Ig-secreting B cell.

Qualitative changes include shift in the number and activity of B-cell subsets as well as shift in the antibody repertoire with respect to the specificity, isotype, and idiotype [19]. However, some authors found neither evidence for a global loss of B-cell function nor the total number of B cells or of Ig-secreting B cells which were decreased [84].

A similarity, as observed with T-cell antigen receptor (TCR), has been found for the repertoire of antibody response by B cells, associated with age-related changes in the germline immunoglobulin repertoire and a marked decline of somatic mutation in germinal centers of aged mice [50, 77, 83]. Somatic mutation designates the process of gene rearrangement that takes place in somatic maturing B cells but not in germline cells, followed by selective survival of those B cells, whose binding immunoglobulin-mutated surfaces have developed high affinity toward the corresponding antigen. Additionally, aged mice fail to express B7-2, a co-stimulatory molecule normally expressed by B cells in the germinal centers of young mice in response to T-cell-dependent antigens [49].

In aging mice, these intrinsic defects of B cells, coupled with the alterations of T-helper cells provide some clues to the abnormalities in the antibody response of aged animals.

Recent evidence shows that the CD 154 (CD40L) expression, essential for the cognate function, was reduced, compared to the younger models [32]. This alteration reflects impaired humoral immune responses, with defects in B-cell proliferation, isotype switching, germinal center formation, and antibody secretion [18].

The first evidence that the humoral immunity changed with age was the report that the quantity of serum antibodies, specific for foreign antigens, declined with age [33]. The antibody response to foreign antigens is also lower in old individuals compared to the youngest, whereas the number of B cells secreting antibodies is enhanced [12]. The B-cell repertoire changes with age and determines an altered ability to recognize antigen [12]. In fact, vaccination seems to stimulate production of antibodies that cross-react with self-antigens in old but not in young mice [7].

The changes in the quality and quantity of antibody contribute to the increased susceptibility and sensitivity of the elderly to infection. It is likely that the B-cell clonal expansions that occur in midlife are the precursors of late life.

Immunoglobulins

B lymphocytes have been investigated in longitudinal studies, and they are numerically reduced [25]. It has been suggested that aging is associated with the appearance of B-cell clonal expansion, which not only limits the diversity of the B-cell repertoire but likely induces monoclonal serum immunoglobulins and B-cell neoplasm [39, 40]. In humans, the serum concentration of IgM, IgA, and IgG also increased with age, although the concentration of IgD decreased [13]. In contrast to the increase in the steady level of serum IgG and IgM with age, the number of antigen-specific IgG and IgM-producing cells stimulated by immunization is decreased in old mice compared to young [28]. These studies showed that there was a greater loss of IgG-producing compared to IgM-producing cells. The preferential loss of IgG antigen-specific antibody reflects an age-associated defect in isotype switching as a consequence of impaired T-cell function required for the generation of germinal centers and for isotype switching [79]. The ability of B cells to generate antibody responses changes with age, although much of this is related to declining T-cell function [20]. Some murine studies have shown that the antibodies produced by old mice has a lower affinity for its target and it is less effective in preventing infections [33, 57]. In another experience, performed on nonagenarians, serum levels of IgM, IgG, and IgA have been measured. The last two were highest and related to greater mortality. Particularly, IgA, reactive protein C (RPC), and IL-6 are index of intestinal inflammation and/or a defect in mucosal defence [35].

Physical activity and acquired immunity

Physical activity leads to several changes in leucocyte subpopulations, involving both innate immunity [47] and acquired immunity.

It is of remarkable note to know the type, duration, and intensity in any kind of exercise, because these peculiarities have much influence on immunity alterations.

After intensive running for 2.5–3 h, leucocytosis has been shown in venous blood of young adults, with a peak 3 h post-run, then leucocytes returned to baseline level by the next morning. The increased values were mostly due to granulocytes and, in a lesser percentage, to monocytes [59].

Lymphocytes, instead, came out from the vascular compartment. T lymphocytes and natural killer (NK) cells were mainly responsible for lymphocitopenia, instead of B lymphocytes which do not have any role [59].

During and immediately after a marathon, it has been observed that there is an increase in the absolute number of T lymphocytes, but they were lower 30 min after the end of a treadmill exercise [10].

The recruitment of all lymphocytic subsets (CD4⁺ T, CD8⁺ T, CD16⁺ NK, CD 56⁺ NK, CD19⁺ B) raised the lymphocyte blood levels [66]. In the recovery period after intensive exercise, immune changes as decreased lymphocytes count, neutrophilia, and suppressed T and NK cells function are likely due to the release of glucocorticoids during exhausting and long duration of physical activity. In fact, if an exercise lasts lesser than 90 min, the adrenaline recruits lymphocytes into the blood compartment, instead of 2.5–3 h of intensive running, which leads to unchanged lymphocytes levels because of a prevalent concentration of cortisol during the recovery period [62].

Another aspect to point out is the function of T and B cells. The human lymphocytes are tested in vitro with some mitogens able to induce proliferative response, as the antigens do in vivo.

Lymphocytes exhibited different sensibility to the most used mitogens, thus, T cells were stimulated primarily by PHA (phytoaemagglutinin), Con-A (concanavalin-A) and, in a lesser degree, by poke weed mitogen (PWM). B cells were more responsive to stimulation by PWM [58].

Increased values during intensive training were found by Bay et al. [4] in athlete lymphocytes, which were stimulated with PHA and anti-CD3 monoclonal antibody, but elite cyclists during low training periods have shown some differences with values found in non-athletes. These data were not confirmed by Nieman et al. [61], which did not demonstrate differences between runners and non-athletes, after mitogen stimulation.

Tvede et al. [75] found that athletes did not show mitogen-induced lymphocyte proliferation during high or low training as non-athlete subjects.

After stimulation with Con-A, marathoners had a fall of 30–40% for 3 h after 2.5 h of intensive running; furthermore, in endurance race events, it was measured and found to have a greater decrease [62].

The exercise is responsible in alterations of total lymphocyte count and in distribution of lymphocyte subpopulations, which has a great influence in lymphocyte proliferative capacity [58].

The diminished responsiveness to PHA and Con-A was largely imputable to CD4+ and CD8+ fractions that fell with exercise [58]. However, CD16+ NK fraction increased during exercise, leading to increased lymphocyte proliferation after stimulation with IL-2 [58].

Physical activity in elderly people

Also in the elderly, exercise programs could interfere with the modifications aging induced to the immune system (Table 4).

Table 4 Variations of acquired immunity in the elderly through the physical activity

In male and female elderly athletes, undergone to endurance activity training, it has been reported that there was an increase in lymphocyte proliferation following mitogen stimulation [60, 70].

Active elderly people, practising regularly long term and moderate physical activity, showed a higher responsiveness than sedentary control group, when lymphocytes were stimulated with anti-CD3 monoclonal antibody [29], which, as well as PHA and PWM, shows a significant influence on lymphocytes of active elderly people [70].

In contrast, the study carried out in elderly women by Flynn et al. [17] reported no significant changes on lymphocyte proliferation after mitogen stimulation or immediately at the end of the exercise of resistance training neither during recovery period. Furthermore, in the experience of Cedda et al., the proliferative responses at most doses of Con-A and PHA was lower than the young control group, and even the elderly had high percentages of CD3+ cells [11]. The total number of leucocytes in exercising elderly people increases as that which occurs in the in the young adults [3]. The levels were measured both immediately after the exercise and 4 h post-run, exhibiting highly significant increase [3], even though elderly subjects who underwent the maximal exercise treadmill test, showed pre-exercise values after 20 min by the end of exercise [11]. The lymphocytic population raised lesser than leucocytes, immediately after the exercise [3]. The subsets of T lymphocytes were greatly influenced by physical activity. It was remarkable that there was an increase in the CD4⁺ cells, while CD8⁺ cells were unchanged [3], thus CD4/CD8 ratio was also found to be increased [3, 9, 70]. In the sedentary control group, it has been found that there were alterations in adaptive immunity as an imbalance between T and B lymphocytes due to decreased T CD3⁺, increased CD20 B cells [82]. Furthermore, the release of cytokines owning to Th2 pattern stimulated B cell clone proliferation, which was responsible of enhanced antibody production and the following occurring in autoimmune disorders [82].

These alterations were not present in active elderly people, who enjoy practicing swimming, cycling, or jogging, thereby T-cell/B-cell ratio had unvaried results. [82].

Extremely important is the effect of exercise relative to naïve and memory cell subsets, which is mainly responsible for the response to infectious diseases. Woods et al. reported an increased percentage and number of CD40- and CD8-naïve cells (CD45RA+) and a decrease in CD4+ memory cells (CD45RO+) after 6 months of moderate exercise training [80], though these results were found in the elderly control group, as well. Furthermore, they demonstrated that CD4 and CD8 memory cells were reduced in the spleen of mice that underwent exercise, implying a raise in naïve/memory cell ratio. Likely, the exercise resulted in apoptosis of memory T cells [81].

An experience carried out on thymus mice has shown that exercise in old mice did not modify the aging-related changes, as thymic weight, percentages of CD4⁻/CD8⁻, CD4⁺/CD8⁺, and single positive cells [81].

The major susceptibility to infections, found in elderly people, is clearly due to a reduced capacity of the immune system to generate a strong response to novel antigenic stimuli, as well as the impairment of immunoglobulin (Ig) response, making the vaccination unsuccessful [26, 68]. Although the data on the changes in serum Ig after physical activity are scanty, Smith et al. [71] have given, through their experience in old men, useful information. They demonstrated that in physically active old men, the response to novel protein antigen keyhole-limpet hemocyanin (KLH) was enhanced. They measured the anti-KLH IgM, IgG, and IgG1 serum levels in old people and in old active men, finding increased values in the last group, independent of the total concentration of antibodies. Furthermore, Kohut et al. [38] sustained that trained elderly people had a better anti-influenza response to influenza vaccination. Unlike, Bruunsgaard et al. [8], data reported no changes in tri-athletes, after a race, in response to diphtheria, tetanus, and pneumococcal vaccination, suggesting that humoral immunity was not affected by intensive exercise.

The antibody isotype showed any changes after exercise, in fact, elderly people, following long-term regular exercise displayed increased levels of IgA and IgM [9]. Unlike what has been observed in elite swimmers, who trained for 7 months, IgA, IgM, and IgG levels were 10% lower than the clinical norms [27].

Conclusions

The regressing pathway of the thymus in every human being influences negatively the essential function of adaptive immunity.

The fragility of the elderly is more evident when they encounter a novel antigenic challenge, showing a great difficulty to face it up, likely enhancing, in certain cases, the risk of mortality (e.g., influenza pneumonia).

In elderly subjects practicing regularly moderate exercise training, some aspects of immunosenescence are attenuated or improved, both innate immunity [47] and acquired immunity, as the reduction of memory cells and the increase of naïve T cells [79, 80] increase further the production of primary antibody response [71].

It is much clear that regular exercise could help in maintaining the well-being in elderly people. However, this point of view is not entirely inquired nowadays; thus, it deserves further experiences. In fact, with the increasing number of older people every way, giving an enhancement of immune function could improve the quality of life during aging. Thereby, the physical activity may represent the “gold standard”, given that it is not very expensive, and it allows socializing, improving mental and physical conditions at last.