Immunological functions are known to decline with age in animal models and humans [1–3]. An understanding of immunological functions at an individual level is clinically important because immunological decline is accompanied by various ailments such as infections, cancer, vascular diseases, and possibly autoimmune phenomena in the aged.

The concept of immunosenescence reflects changes in both cellular and humoral immune responses. Accumulating evidence obtained mainly from animal models has shown that age-related immunological decline occurs primarily in T cell-dependent immune functions, and it is mainly caused by thymic involution that begins in the early phase of life [3]. As a result, T cells are central to the immune response, and their function is significantly altered with increasing age. In humans, data on immunological functions are mainly obtained from blood serum and blood cells. Serum contains immunoglobulin, complements, and cytokines. The levels of serum IgG and IgA generally tend to increase slightly with age [4], while the level of serum complements does not change remarkably with age. The level of cytokines in healthy old people is generally reduced. In contrast, the level of white blood cells (WBCs) remarkably changes with disease and aging. WBCs are constituted of lymphocytes, monocytes, and neutrophils. There are various subpopulations of lymphocytes with different functions. Data on age-related changes in human lymphocytes and their functions are limited.

Besides aging, the fact that gender influences the immune system has long been recognised. In postmenopausal women, changes in the immune system have been attributed to oestrogen deprivation. Women are at higher risk for developing autoimmune diseases, which suggests that these diseases are somehow mediated by sex steroids, with oestrogens as enhancers, at least with humoral immunity. There is an increase in proinflammatory serum markers, an increasing response of the body’s cells to cytokines, a decrease in CD4⁺ T and B lymphocytes, and in the cytotoxic activity of natural killer (NK) cells [5]. One cellular process related to lifespan, which differs according to sex, is the rate at which the protective telomere chromosome caps are lost. In humans, men have shorter lifespans and greater telomere shortening than do women. This has led to speculation in the medical literature that sex-specific telomere shortening is one cause of sex-specific mortality [6].

Therefore, the purpose of this study is to obtain data on age-related changes and gender differences in the number and function of peripheral blood lymphocytes and their subpopulations obtained from 162 healthy male and 194 healthy female volunteers, ranging in age from 20 to 90 years. The results show that the rate of age-related immunological decline is less in women than in men, and this could be a contributory factor to women living longer than men in Japan.

The age-related decline of the immune response has a major impact on the health status of the elderly [7] and is a determinant of longevity [8]. It is well known that aging is associated with a decline in the normal function of the immune system, leading to increased susceptibility to various diseases and shortened longevity; however, specific dysfunctions in the immune system directly responsible for this have yet to be identified. Among the important factors, T cells are central to the immune response, and their function is significantly altered with increasing age. The present study confirms and extends an earlier study by Utsuyama et al. [1].

Previously, Strindhall et al. [9] reported that a reduced number of CD8⁺CD28^- T cells and a higher CD4⁺ T cell/CD8⁺ T cell ratio are associated with populations that survive until the age of 100 years. Moreover, Xie and McElhaney [10] reported that susceptibility to influenza infection in older adults is associated with an increase in the number of CD8⁺CD28^- T cells. In this study, we found that the number of CD8⁺CD28⁺ cells decreased with age, and this was associated with a decrease in the T cell proliferative response (MTS-OD₄₉₀) and TCPI. Furthermore, the rate of decline was significantly less in women than in men.

Although CD4⁺CD25⁺ regulatory T cells are crucial to tolerance in normal individuals, the factors regulating this cell population and its function are largely unknown. In the present study, we demonstrated that the number of CD4⁺CD25^high regulatory T cells (TREGs) showed an increasing trend with age. Further studies are required because it is still not clear whether the age-related increase in the number of TREGs is responsible for immune dysfunction in the elderly [11].

Olsson et al. (2000) reported that a decrease in the CD4⁺ T cell/CD8⁺ T cell ratio was an important indicator of the immune risk phenotype. In the present survey, a contrasting feature was observed between CD4⁺T cells and CD8⁺T cells. The results presented here show that the number of CD4⁺ T cells was maintained at a relatively steady level or had an increasing trend with age, whereas the number of CD8⁺ T cells significantly decreased with age; therefore, the CD4⁺ T cell/CD8⁺ T cell ratio showed a distinct age-related increase. A higher percentage of CD8⁺ T cells and a decreased CD4⁺ T cell/CD8⁺ T cell ratio were observed in the Saudi male population than that in the Caucasian population [12]. Hence, it is possible that race is a factor influencing these parameters.

T cell proliferative activity is a value obtained by a determined number of T cells in vitro, and it does not directly reflect a value at the individual level. In order to convert the T cell proliferative activity to the individual level, we developed a new parameter, TCPI, which is calculated by the T cell proliferative activity and the number of T cells/mm³. Both T cell proliferative activity and TCPI showed age-related decreases in men and women. However, statistically significant differences between men and women were observed in TCPI, not in T cell proliferative activity.

In addition, we developed another novel immunological parameter, T cell immune score, which was calculated by combining 5 T cell-related parameters: number of T cells, CD4/CD8 T cell ratio, number of naïve T cells, naïve to memory T cell ratio, and TCPI. Each parameter was given a score from 3 to 1 according to the database: 3 (high level), 2 (moderate level), and 1 (low level). The summation of 5 scores was employed as the T cell immune score. It is interesting to note that the T cell immune score significantly decreased with age in both men and women, and the decline was significantly greater in men than in women.

The absolute number of total B lymphocytes increases about 3-fold from the baseline in the first year of life and progressively decreases until adulthood [13]. The present study showed that the number of B cells decreased gradually throughout life from 20 to 90 years of age, and this decline was significantly greater in men than in women.

NK cells are a key component of innate immunity and are involved in the regulation of the immune response by producing many cytokines that can activate other cellular components of innate and adaptive immunity. Although many studies have investigated the effects of age on NK cells, their conclusions remain controversial [14]. In the present study, an age-related increase was observed in the number of NK cells, and this increase was significantly higher in men than in women. It is interesting to note that Lee et al. [15] reported a higher percentage of NK cells in the Asian populations than that in the Caucasian population.

Although aging is accompanied by a progressive increase in proinflammatory cytokines, little is known about the development of age-dependent modifications in circulating cytokines. The serum levels of cytokines are usually too low to detect. Thus, in the present study, we measured the concentration of cytokines secreted into the culture supernatant by T cells in the presence of anti-CD3 monoclonal antibody. Many reports [16–18] employed the stimulation of lymphocytes by PMA and ionomycin to monitor cytokine production. Because stimulation by the anti-CD3 monoclonal antibody is more routinely used than PMA and ionomycin, we employed the former method. It is known that a shift from Type 1 to Type 2 cytokine production occurs with age; however, we could not detect such a shift. Garner and Murasko [19] reviewed over 60 studies and reported that the age-associated changes in cytokine production are inconsistent. We observed that both Th-1 type cytokines (IL-2 and IFNγ) and Th-2 type cytokines (IL-6) showed an age-related declining trend. A statistically significant decline was observed only in IL-6 in men. The rate of age-related decline in IL-10 was lesser in women than in men. Because IL-10 is known to act as an immune suppressor, a relatively lesser level of IL-10 in women could be consistent with the fact that the age-related decline of various immunological parameters is less pronounced in women than in men.

Sex hormones influence both humoral and cell-mediated immune responses, and oestrogen is one of the potential factors in this immunological dimorphism [20]. These environments are established by the cytokines that are released by immune cells, particularly T helper (Th) lymphocytes. In this regard, Pietschmann et al. [16] reported that the pattern of cytokine production is gender specific. It is possible that oestrogen deficiency after menopause induces a disturbance of peripheral tolerance through CD8⁺ T cells bearing the suppressor phenotype. In contrast, it was shown that mitochondrial oxidative stress is higher in men than in women and that the higher levels of oestrogens in women protects them against aging by upregulating the expression of antioxidants and longevity-related genes [21]. It was also demonstrated that oestrogens upregulate the expression of antioxidant enzymes via the oestrogen receptor and MAPK activation, which in turn activate the NF-kB signalling pathway, resulting in the upregulation of longevity-related gene expression [21]. The role of gender in the regulation of longevity may be linked to gender specific genetic differences, including the expression of sex hormone patterns and the changes in these patterns during an individual’s lifetime.

In humans, cytomegalovirus (CMV) infection has an enormous impact on biomarkers associated with aging of the immune system [22]. In Japan, an extensive survey of blood serum by enzyme immunoassays revealed that the percentage of CMV antibody positivity is approximately 87.2% in the whole population, showing an age-related increase. However, gender difference was not observed with regard to CMV antibody positivity and titre in each age group [23].

The essential function of T cells is the recognition of antigens and the following proliferative response. The present study confirmed that a statistically significant age-related decline was observed in T cell proliferative activity and TCPI. TCPI was the value of individual levels calculated from T cell proliferative activity, and it showed a statistically significant gender difference that was greater decline in men than in women. The lesser rate of decline in TCPI indicates that the immunological functions were relatively well preserved in elderly women than in elderly men; this finding may be consistent with the fact that women live longer than do men. As a reference, the mean lifespan of the Japanese population reported by the Japanese Ministry of Health, Labour, and Welfare is 85.5 years in women and 79.0 years in men [24].

This approximately 6 year difference in the mean lifespan may raise the question: Is this difference due to the different biological ages of the men and women? To answer the question, we compared an immunological parameter, T cell immune score, between groups of men 6 years younger than women. In other words, by removing men over 70 years and women younger than 33 years, the mean age of men was 48.2 years and that of women was 54.7 years. Then, we analysed the parameter by comparing groups of men 6 years younger than women. A significant difference was observed when comparing the original male and female groups (p < 0.012); the p-value was 0.157 when comparing groups between men 6 years younger than women. This result obtained by rearranging the groups suggested that the different life expectancy between men and women could be due to differences in biological age, and immunological parameters could be used as a marker of the biological age.