Age-associated alterations in human natural killer cells. 1. Increased activity as per conventional and kinetic analysis

The impact of ageing on natural killer cell function and potential consequences for health in older adults

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Roles are emerging for natural killer (NK) cells beyond removing transformed cells.

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These include immune regulation and the elimination of senescent cells.

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Human ageing is associated with a decline in NK cell function.

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We propose some aspects of human ageing are due in part to reduced NK cell function.

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These include reduced vaccination efficacy and delayed resolution of inflammation.

Forming the first line of defence against virally infected and malignant cells, natural killer (NK) cells are critical effector cells of the innate immune system. With age, significant impairments have been reported in the two main mechanisms by which NK cells confer host protection: direct cytotoxicity and the secretion of immunoregulatory cytokines and chemokines. In elderly subjects, decreased NK cell activity has been shown to be associated with an increased incidence and severity of viral infection, highlighting the clinical implications that age-associated changes in NK cell biology have on the health of older adults. However, is an increased susceptibility to viral infection the only consequence of these age-related changes in NK cell function? Recently, evidence has emerged that has shown that in addition to eliminating transformed cells, NK cells are involved in many other biological processes such as immune regulation, anti-microbial immune responses and the recognition and elimination of senescent cells, novel functions that involve NK-mediated cytotoxicity and/or cytokine production. Thus, the decrease in NK cell function that accompanies physiological ageing is likely to have wider implications for the health of older adults than originally thought. Here, we give a detailed description of the changes in NK cell biology that accompany human ageing and propose that certain features of the ageing process such as: (i) the increased reactivation rates of latent Mycobacterium tuberculosis, (ii) the slower resolution of inflammatory responses and (iii) the increased incidence of bacterial and fungal infection are attributable in part to an age-associated decline in NK cell function.

Cigarette smoking is associated with the reduction of lymphokine-activated killer cell and natural killer cell activities

Lymphokine-activated killer (LAK) cell and natural killer (NK) cell activities were determined in a group of healthy individuals with differing smoking habits. The study population consisted of 54 Japanese males, including 23 smokers, 8 ex-smokers and 23 non-smokers. Peripheral blood mononuclear cells (PBMC) were isolated and used as effector cells. LAK cells were generated by incubation of PBMC with interleukin-2 for 72 h. LAK cell activity against NK-resistant Raji cells and NK cell activity against NK-sensitive K562 cells were examined by 4-h51Cr-release assay. LAK cell activity in the smokers was significantly lower than that in the nonsmokers. The smokers showed significantly lower NK cell activity than the nonsmokers, whereas NK cell activity of the ex-smokers was comparable to that of the non-smokers. The proportion of NK cells (CD3-16+56-, CD3-16-56+, or CD3-16+56+ cells) in the smokers was significantly lower than that in the nonsmokers. The present study demonstrates for the first time that cigarette smokers have lower LAK cell activity than nonsmokers.

Cell division and aging of the organism

The capacity to regenerate cell compartments through cell proliferation is an important characteristic of many developed metazoan tissues. Pre- and post-natal development proceeds through the modifications occurring during cell division. Experiments with cultivated cells showed that cell proliferation originates changes in cell functions and coordinations that contribute to aging and senescence. The implications of the finite cell proliferation to aging of the organism is not the accumulation of cells at the end of their life cycle, but rather the drift in cell function created by cell division. Comparative gerontology shows that the regulation of the length of telomeres has no implications for aging. On the other hand there are interspecies differences in regard to the somatic cell division potential that seem to be related with the "plasticity" of the genome and with longevity, which should be viewed independently of the aging phenomenon. Telomeres may play a role in this plasticity through the regulation of chromosome recombination, and via the latter also in development.

The role of exercise on the innate immunity of the elderly

The increased life span in human population has shown that some diseases, as infections, cancer and autoimmune phenomena, occur more frequently in the elderly than in the younger. We describe the ageing process involving the innate immune system and the improvement given by moderate physical activity. In addition, we discuss the altered neutrophil granulocytes function, the role of macrophages and natural killer cells, besides the influence of cytokines and secretory IgA. The acquired information help us to explain how these changes could favor the onset of diseases in the elderly and how they may boost their immune function.

The immune system in extreme longevity

Recent observations indicate that immunosenescence is not accompanied by an unavoidable and progressive deterioration of the immune function, but is rather the result of a remodeling where some functions are reduced, others remain unchanged or even increased. In addition, it appears that the ancestral/innate compartment of the immune system is relatively preserved during aging in comparison to the more recent and sophisticated adaptive compartment that exhibit more profound modifications. The T-cell branch displays an age-dependent decline of the absolute number of total T-cells (CD3+), involving both CD4+ and CD8+ subsets, accompanied by an increase of NK cells with well-preserved cytotoxic function and by a reduction of B-cells. One of the main characteristics of the immune system during aging is a progressive, age-dependent decline of the virgin T-cells (CD95-), which is particularly profound at the level of the CD8+ subpopulation of the oldest old subjects. The progressive exhaustion of this important T-cell subpopulation dedicated primarily to the defense against new antigenic challenges (viral, neoplastic, bacterial ones), could be a consequence of both the thymic involution and the lifelong chronic antigenic stimulation. The immune function of the elderly, is therefore weakened by the exhaustion of CD95- virgin cells that are replaced by large clonal expansions of CD28- T-cells. The origin of CD28- cells has not been completely clarified yet, but it is assumed that they represent cells in the phase of replicative senescence characterized by shortening telomers and reduced proliferative capacity. A major characteristic of the immune system during aging is the up-regulation of the inflammatory responses which appears to be detrimental for longevity. In this regard, we have recently observed a progressive age-dependent increase of type 1(IL-2, IFN-gamma, TNF-alpha) and type 2 (IL-4, IL-6, IL-10) positive CD8+ T-cells; in particular, type 1 cytokine-positive cells significantly increased, with age, in all CD8+ subsets particularly among effector/cytotoxic and memory cells. A major force able to drive a chronic pro-inflammatory state during aging may be represented by persistent viral infections by EBV and CMV. Therefore, we have determined the frequency and the absolute number of viral antigen-specific CD8+ T-cells in subjects older than 85 years, who were serologically positive for CMV or EBV. In the majority of these subjects we detected the presence of T lymphocytes positive for epitopes of CMV or EBV. In all subjects the absolute number of CMV-positive CD8+ cells outnumbered that of EBV-positive ones. In addition, the majority of CMV+ T cells were included within the CD28- subpopulation, while EBV+ T cells belonged mainly to the CD28+ subset. These data indicate that the chronic antigenic stimulation induced by persistent viral infections during aging bring about important modifications among CD8+ subsets, which are particularly evident in the presence of CMV persistence. The age-dependent expansions of CD8+CD28- T-cells, mostly positive for pro-inflammatory cytokines and including the majority of CMV-epitope-specific cells, underlines the importance of chronic antigenic stimulation in the pathogenesis of the main immunological alterations of aging and may favour the appearance of several pathologies (arteriosclerosis, dementia, osteoporosis, cancer) all of which share an inflammatory pathogenesis.

Might DHEA be considered a beneficial replacement therapy in the elderly?

Dehydroepiandrosterone (DHEA) [prasterone] is typically secreted by the adrenal glands and its secretory rate changes throughout the human lifespan. When human development is completed and adulthood is reached, DHEA and DHEA sulphate (DHEAS) [PB-008] levels start to decline so that at 70-80 years of age, peak DHEAS concentrations are only 10-20% of those in young adults. This age-associated decrease has been termed 'adrenopause', and since many age-related disturbances have been reported to begin with the decline of DHEA/DHEAS levels, this provides a potential opportunity for use of DHEA as replacement therapy. For these reasons, use of DHEA as a replacement therapy in aging men and women has been proposed and this paper outlines the reported beneficial effects of such treatment in humans. Many interesting results have been obtained in experimental animals suggesting that DHEA positively modulates most age-related disturbances. However, renewed interest in DHEA has arisen as a result of recent studies suggesting that DHEA appears to be beneficial in hypoandrogenic men as well as in postmenopausal and aging women. Menopause is the event in a woman's life that induces a dramatic change in the steroid milieu, and use of DHEA as 'replacement treatment' has been reported to restore both the androgenic and estrogenic environment and reduce most of the symptoms of this change. As menopause is the beginning of the biological transition of women towards senescence, it is of great interest to better understand how DHEA might help to solve and/or overcome the problems of this complex stage of life. In men with adrenal insufficiency and hypogonadism without androgen replacement, DHEA administration results in a significant increase in circulating androgens. Though most data are suggestive for use of DHEA as hormonal replacement treatment, more defined and specific clinical trials are needed to uncover all of the 'secrets' and features of this steroid before it can be used as a standard treatment. Furthermore, DHEA is perceived differently around the world, being considered only a 'dietary supplement' in the US, while in many European countries it is considered a 'true hormone' that has not been approved for use as a hormonal treatment by the European health authorities. This overview offers some points of view on use of DHEA as an experimental hormonal replacement therapy.

Beneficial effects of the Nishino breathing method on immune activity and stress level

OBJECTIVES

Immunologic activity and stress level in practitioners of the Nishino breathing method were assayed before and after 90 minutes of an exercise class, in order to assess the value of the method.

BACKGROUND

The Nishino Breathing Method was developed by Kozo Nishino, a former ballet choreographer and a master of martial arts, in the 1970s in an attempt to develop Ki (an internal life energy or a spiritual energy). Although he developed his method independently of the Chinese qigong practice, there are certain similarities between them, in that both involve visualization of internal energy flow, slow body movements, and the emission of life energy from one's hand.

METHODS

Twenty one (21) practitioners were tested. Their blood was withdrawn before and after a class, and the activity of natural killer (NK) cells and their cell numbers were assayed. The level of stress was also measured using the Lorish face scale method. The heart rate was also monitored.

RESULTS

The NK cell activity of 17 of 21 (76%) subjects increased after attending a class. The mean NK cell activity of the whole group of subjects increased after the practice (p < 0.03). The mean stress level was significantly reduced after the class (p < 0.001).

CONCLUSIONS

These results demonstrated that the Nishino Breathing Method can effectively enhance the immunoregulatory system and reduce stress even after one class. This indicates that the method would help improve the health of its practitioners if they continue to practice it.

Level of NK cytotoxic activity in the elderly aged more than 80 years

A group of healthy volunteers aged 60-93 years, was evaluated with respect to their NK activity. These people have been under careful medical observation for 3 years and were selected according to Senieur Protocol criteria. NK activity of each person was checked at least three times at regular 12-month intervals with the chromium release test and twice at the same interval of time with a single cell cytotoxicity assay. NK activity of 60-79-year-old females was in a similar range of values as that of young females (20-35 years) and was lower than that of the males of the corresponding age groups. The females aged 80-88 years revealed the highest NK activity. The chronically ill 80-88-year-old females, on the other hand, were characterized by a marked decrease of NK activity in relation to their age counterparts. NK activity of both the females and males significantly declined at the age of 89-93 years. The significance of NK activity for survival is discussed.

Hematopoiesis in the elderly

The effect of age on the hematopoietic system has always been an area of clinical interest. Alterations in lymphocyte immunophenotype and function with age have been clearly demonstrated as has a decrease in neutrophil function. However, controversy continues to surround the significance of unexplained anaemia in the elderly patient and the extent to which this could be a physiological occurrence. The weight of evidence from animal and human studies would suggest that anaemia is not a physiological occurrence but may have a multifactorial pathogenesis. Are older patients therefore, appropriate candidates for high dose therapy, including autologous stem cell transplantation? The loss of telomeric DNA from hematopoietic progenitor cells with aging implies that stem cell collections from an older patient may have compromised replicative capacity with a reduced response to hematopoietic growth factors. Recent studies, however, show that age is not an obstacle for the collection of a stem cell product, which is capable of restoring normal hematopoietic function. A study of autologous stem cell transplantation has shown comparable neutrophil and platelet recovery times between younger and older patients.

Impact of nutritional status and nutrient supplements on immune responses and incidence of infection in older individuals

With advancing age there is a progressive decline in immune responses although this is not inevitable. The impairment in immunocompetence is noticeable as early as 35-40 years in many individuals. At the same time, some persons even in the 80s may show a vigorous immune system comparable with that of the young adult. Nutrient deficiencies are frequent in older populations. A variety of nutrients are affected: zinc, iron, beta-carotene, Vitamins B6, B12, C, D and E, ad folic acid. The causal interaction between nutritional deficiencies and impaired immunity has been known in children; a similar relationship has been postulated in the elderly. In the last 25 years, many studies employing different designs have examined the role of diet, nutritional status, and nutrient supplements in the immune responses of older individuals. Some nutrients, for example zinc and Vitamin E, have been shown to increase selected immune responses but have not been beneficial in terms of reduction in infectious morbidity. A growing consensus indicates that the use of a multinutrient containing optimum amounts of essential trace elements and vitamins is likely to result in enhanced immune responses and reduction in the occurrence of common infections. These findings have considerable fundamental, clinical and public health significance.

Pharmacology And Therapeutic Effects of Dehydroepiandrosterone In Older Subjects

Man and higher primates have adrenals that secrete large amounts of dehydroepiandrosterone (DHEA) [prasterone] and its sulphate (DHEAS) [PB 008]. A remarkable feature of plasma DHEAS levels in humans is their great decrease with aging. Researchers have postulated that this age-related decline of DHEAS levels may explain some of the degenerative changes associated with aging. Moreover, administration of DHEA to laboratory animals has demonstrable beneficial effects such as prevention of diabetes mellitus, obesity, cancer, heart disease and positive immunomodulator effects. However, in rodents DHEA(S) circulating levels are so low that it is impossible to detect any significant age-related decrease. Therefore results from rodent experiments are not relevant to human beings. Three mechanisms of action of DHEA(S) have been identified. DHEA and DHEAS are precursors of testosterone and estradiol, DHEAS is a neurosteroid which modulates neuronal excitability via specific interactions with neurotransmitter receptors and DHEA is an activator of calcium-gated potassium channels. Randomised, placebo-controlled clinical trials which included healthy individuals aged 60 years and over treated with (near) physiological doses of DHEA (50-100 mg/day) have yielded very few positive results. Impact of DHEA replacement treatment was assessed on mood, well being, cognitive and sexual functions, bone mass, body composition, vascular risk factors, immune functions and skin. The major limitations of these trials were their short duration (maximum 1 year) and the low number of study participants involved (maximum 280). Many elderly people in western countries take DHEA without medical supervision. In the US, DHEA is even classified as food supplement. At present there is no scientific evidence to recommend DHEA replacement in the elderly. Further studies are needed to form conclusions about the efficacy and the safety of DHEA replacement in elderly, and to better understand the mechanisms of action of DHEA at the molecular and cellular levels.

Specific and nonspecific immune responses to fasting and refeeding differ in healthy young adult and elderly persons

BACKGROUND

Undernutrition is a main cause of immunodeficiency. Many confounding factors limit the interpretation of immune function in hospitalized elderly patients.

OBJECTIVE

We compared the effects of short-term fasting and refeeding on lymphocyte subset distribution and neutrophil function in healthy subjects.

DESIGN

Seven young adult (x +/- SE age: 24 +/- 2 y) and 8 elderly (71 +/- 3 y) subjects were fed standardized diets (1.6 x predicted resting energy expenditure; 16% protein) for 7 d. They then fasted for 36 h and were refed for 4 h (42 kJ/kg). Lymphocyte subsets were quantified by using fluorochrome-conjugated monoclonal antibodies. Neutrophil chemotactic migration was evaluated by using a 2-compartment chamber. Neutrophil reactive oxygen species production was measured by using a luminol-amplified chemiluminescence assay and oxidation of 2'7'-dichlorofluorescein diacetate.

RESULTS

Baseline total and cytotoxic T lymphocyte subpopulations were lower in elderly than in adult subjects (P < 0.01). Nutritional state had a significant effect (P < 0.05) on total, helper, and cytotoxic T and B lymphocyte counts in all subjects, and the response of lymphocyte subpopulations to nutritional fluctuations was significantly affected by age. The chemotactic index was lowered by fasting in both groups (P < 0.05 compared with basal values). After refeeding, neutrophil migration was restored in adult but not elderly subjects. The superoxide anion production rate increased with fasting and reverted to prefasting values with refeeding in both groups (P < 0.05). Fasting induced a significant decrease in hydrogen peroxide production in stimulated neutrophils that was reversed by refeeding in adult but not elderly subjects.

CONCLUSION

The lack of response of lymphocyte subpopulation counts and neutrophil function to nutritional changes may help to explain the proneness of elderly persons to infection.

Functional alteration of granulocytes, NK cells, and natural killer T cells in centenarians

The immune system in centenarians was characterized as elevated levels in the proportion and number of granulocytes, NK cells, and extrathymic T cells (including NKT cells) in the peripheral blood. Conventional T cells, abundant in youth, were decreased in proportion and number. In addition to this numerical change in centenarians, the function was significantly altered in comparison with that in middle-aged subjects. The phagocytic function and cytokine production of granulocytes in centenarians increased whereas the production of superoxides from granulocytes decreased. This tendency was almost the same in both healthy and unhealthy centenarians. IFN gamma production by NK and extrathymic T cells in centenarians seemed to be augmented and resulted in an elevated level of serum IFN gamma. Possibly due to the effect of this endogenous IFN gamma, the proportion of CD64(+) (Fc gamma RI) cells among granulocytes was elevated. The expansion of CD64 antigens on granulocytes is known to be regulated by IFN gamma and to be associated with their induction of phagocytosis. These results suggest that the immune system of centenarians is not merely impaired, but altered in terms of the number and functions of granulocytes, NK cells, NKT cells.

Exercise and the immune system--influence of nutrition and ageing

In essence, the immune system is enhanced during moderate and severe exercise, and only intense long-duration exercise is followed by impairment of the immune system. The latter includes suppressed concentration of lymphocytes, suppressed natural killer cell activity, lymphocyte proliferation and secretory IgA in saliva. During the time of immune impairment, referred to as "the open window", microbial agents, especially viruses may invade the host and infections may be established. One reason for the "overtraining effect" seen in elite athletes could be that this window of opportunism for pathogens is longer and the degree of immunosuppression more pronounced. Alterations in metabolism and metabolic factors may contribute to exercise-associated changes in immune function. Reductions in plasma-glutamine concentrations, altered plasma-glucose level, free oxygen radicals and prostaglandins (PG) released by the elevated number of neutrophils and monocytes may influence the function of lymphocytes and contribute to the impaired function of the later cells. Thus, nutritional supplementation with glutamine, carbohydrate, anti-oxidants or PG-inhibitors may, in principle, influence exercise-associated immune function. Although several intervention studies have been performed, it is premature to make recommendations regarding nutritional supplementation to avoid post-exercise impairment of the immune system.

Exercise and immune function: effect of ageing and nutrition

Strenuous exercise is followed by lymphopenia, neutrophilia, impaired natural immunity, decreased lymphocyte proliferative responses to mitogens, a low level of secretory immunoglobulin A in saliva, but high circulating levels of pro- and anti-inflammatory cytokines. These exercise-induced immune changes may provide the physiological basis of altered resistance to infections. The mechanisms underlying exercise-induced immune changes are multifactorial and include neuroendocrinological and metabolic mechanisms. Nutritional supplementation with glutamine abolishes the exercise-induced decline in plasma glutamine, but does not influence post-exercise immune impairment. However, carbohydrate loading diminishes most exercise effects of cytokines, lymphocyte and neutrophils. The diminished neutrophilia and elastase (EC 3.4.21.37) responses to eccentric exercise in elderly subjects were enhanced to levels comparable with those of young subjects by fish oil or vitamin E supplements. However, although vitamin C supplementation may diminish the risk of contracting an infection after strenuous exercise, it is not obvious that this effect is linked to an effect of vitamin C on exercise-induced immune changes. In conclusion, it is premature to make recommendations regarding nutritional supplementation to avoid post-exercise impairment of the immune system.

Granulocyte and natural killer activity in the elderly

The deterioration of the immune system in ageing, 'immunosenescence', is thought to contribute to increased morbidity and mortality from infections and possibly autoimmune diseases and cancer. The most profound changes involve effector and immunoregulatory T-cell functions. Immunosenescence appears also to be related to changes in non specific immunity as well. In the present study we have assessed superoxide production, chemotaxis and the expression of the apoptosis-related molecule APO1/Fas (CD95) on neutrophils (PMN) from young and old subjects. Furthermore, we have measured the basal natural killer (NK) activity of young and elderly subjects and we have compared the number of CD16+ cells found in these two groups. We observed a significant decrease age-related both of formation of O2- and chemotaxis whereas no significant correlation between age and the expression of CD95 on granulocyte membrane was demonstrated, suggesting that an increase age-related of CD95-linked apoptosis of PMN should be not an important determinant in the decreased PMN function. We also observed a significant correlation between age and NK activity. The decreased NK cell function was not due to a decreased number of NK cells in effector cell preparations since the number of CD16+ cells was significantly increased in old subjects. In conclusion, our results show that in the elderly there is also a deficit of the aspecific immunity that might play a role in the pathogenic mechanisms of the immunosenescence.

Relationship between plasma IGF-I levels, in vitro correlates of immunity, and human senescence

Insulin-like growth factor-I (IGF-I) is a polypeptide mitogen which is regulated by growth hormone (GH). IGF-I mediates many of the biological functions of GH, including the maintenance of lymphoid mass and functions. Since GH secretion declines with age, we asked whether changes in the availability of IGF-I might contribute to age-associated alterations in immune functions. As a first step, we examined relationships between plasma levels of IGF-I and in vitro correlates of immunity in young and elderly subjects. Heparinized plasma and lymphocytes were collected from the peripheral blood of 34 healthy young (aged 27 +/- 0.9 years, mean +/- SEM) and 41 elderly (79 +/- 1.3 years) volunteers (31 males and 44 females in total). Plasma levels of IGF-I, measured by radioimmunoassay after the removal of IGF-I-binding proteins, were reduced among elders compared to young controls (138 +/- 8.7 ng/mL vs 80.2 +/- 4.7 ng/mL, P < 0.001). The number of circulating lymphocytes did not change with age. The proliferative response ([3H]thymidine uptake into DNA) of T-cells to concanavalin A and B-cells to pokeweed mitogen were reduced among elders (P < 0.05). An increased spontaneous antitumor natural killer (NK) activity (P < 0.001) was accompanied by a higher percentage of CD16(+) NK cells among lymphocytes in older subjects (P < 0.001). The NK cell number was positively related to IGF-I levels in young volunteers but not among elders. Correlation analysis demonstrated a highly significant relationship between plasma IGF-I levels and T-cell (but not B-cell) proliferative response during aging (r = 0.492, P < 0.001). Our results imply that reduced immunocompetence may be one of the consequences of reduced IGF-I levels in human aging. Among the three types of immune cells tested, the T-cells were most sensitive to fluctuations in IGF-I levels. Reduced IGF-I availability may be one of the determinants of the decline in T-cell-mediated immune function in the elderly. To our knowledge, this is the first report presenting correlative data on concurrent changes in IGF-I levels and immune parameters in human aging.

Increased natural killer cell cytotoxicity in Alzheimer's disease may involve protein kinase C dysregulation

Increased cytokine-mediated cytotoxic natural killer (NK) cell activity has recently been demonstrated in patients with senile dementia of the Alzheimer's type (SDAT). In the present study, we evaluated whether protein-kinase C (PKC), a main regulatory enzyme involved in the mechanism of exocytosis by NK cells, has a role in the cytotoxic response of NK cells (during IL-2 and IFN-beta exposure) from SDAT patients. Our data demonstrate the presence of an increased cytotoxic response by NK cells to IL-2 (mean increase +102%) and IFN-beta (mean increase +132%) in SDAT patients in comparison with healthy elderly subjects (+75% and +88% for IL-2 and IFN-beta, respectively). A smaller suppression of NK cytotoxicity after cortisol was also observed in SDAT (mean decrease -24%) than in the control group (-44%). The NK cell activity of SDAT patients was inversely correlated with the cognitive status as evaluated by the analysis of MMSE (Mini Mental State Examination) score. A comparison of young and elderly healthy subjects revealed no variations in NK cell activity. A physiological decrease in cytosolic PKC activity was demonstrated in healthy old subjects after IL-2 and IFN-beta incubation, but not in SDAT patients, while no variations in kinase activity were observed after cortisol incubation. The decreased activity with cytokines was associated with reduced levels of PKC alpha and betaII isoforms. An alteration in cytokine-mediated NK cell activity associated with PKC dysregulation is therefore suggested to occur in patients with SDAT. These changes may indicate the existence of an immunological component to the pathogenesis and progression of the disease.

Modulation of immune cell function by the autonomic nervous system

This review discusses some of the major findings implicating the autonomic nervous system in the regulation of immune function. The sympathetic nervous system, the primary focus of this line of research, directly innervates the major lymphoid organs, and physiological release of sympathetic neurohormones at these sites has been documented. Leukocytes have been shown to express receptors for catecholamines, as well as neuropeptide Y, and studies in vitro and in vivo have indicated that occupation of these receptors by the appropriate ligands produces functional changes in immunological cells. Finally, altered sympathetic regulation may underlie some of the immunological abnormalities observed in chronic stress, clinical depression, and ageing.

Numerical and functional characteristics of lymphocyte subsets in centenarians

The immune system in the aged is a very interesting subject for study. In this study, analysis was extended to extrathymic T cells as well as NK cells and "conventional" T cells (i.e., thymus-derived T cells) in terms of their constitution and function in both healthy and unhealthy centenarians. Middle-aged persons were used as controls. Healthy and unhealthy centenarians showed lower levels in the proportion and absolute number of lymphocytes. The major change in the constitution of lymphocyte subsets was increased levels in the proportion of NK cells (CD56+/CD57+) and extrathymic T cells (CD3+CD57+). Inversely, conventional T cells decreased in proportion and function (i.e., proliferative response to mitogen). Although NK cells increased in centenarians, NK activity by whole lymphocytes and the purified NK fraction decreased. The difference between healthy and unhealthy centenarians was small in all parameters, the only difference being a lower level of expression of CD56 antigens on CD57+ T cells in unhealthy centenarians. These results indicate that there is a major shift in lymphocyte population from conventional T cells to NK cells and extrathymic T cells with aging. Concerning the age-associated increases in CD56+ T and CD57+ T cells, these cells correspond to NK1+ T cells in mice.

Natural killer cells in the late decades of human life

We examined the lymphocyte subsets and indices of natural killer (NK) cell activity (lytic unit (LU), index of absolute NK cell activity in vivo (ALU), and NK cell activity on a per-cell basis (PCNK)) in 82 people (age, 30-99 years) who were immunologically normal. Although the number of NK cells was maintained throughout the examined age range, the ALU and PCNK values correlated negatively with age. We then examined whether any of the various immunologic parameters, including the function and cell counts of NK cells, T cells, and neutrophils, related to past infectious episodes and death in the follow-up period in 44 elderly subjects (age, 63-98 years). Only low ALU and PCNK values correlated with a past history of severe infection, while low LU, ALU, and PCNK values were the only parameters which correlated with death due to infection during the follow-up period. We propose that human NK cells do not escape the aging process and that a low NK cell function relates to the development of severe infections, which may be fatal, in elderly subjects.

Senescence and cytokines modulate the NK cell expression

We have been investigating senescence-related changes in human peripheral blood natural killer (NK2) cells. Data accumulated so far consistently and clearly show that both basal and cytokine (IL-2 and interferon alpha and gamma) induced antitumor MHC-unrestricted cytotoxic activity of NK cells are well-preserved in the healthy elderly. To investigate if the non-cytotoxic functions of NK cells are also spared from the influence of senescence, recombinant IL-2-inducible secretion of IFN-gamma, which serves as a first line of defense, was examined. The amount of IFN-gamma secreted by purified, 18 h activated NK cells from the elderly was only 25 percent of that released by the cells from the young. Thus, the type 1 cytokine-inducible cytotoxic and cytokine secretory functions appear to be dissociable properties of NK cells, at least in the elderly. However, this aging-related early phase secretory deficit could be overcome by chronic stimulation with IL-2 (7 day culture). Since different subsets could perform different functions, we analyzed the NK subsets by flow cytometry. A minor CD56bright subset and a major CD56dim subset could be distinguished based on the density of expression of the cell surface CD56 molecule (N-CAM). We inquired if immunosenescence is likely to impact the steady-state level of circulating NK subsets. A significant decrease (P < 0.01) in percent CD56bright among CD56+ cells was observed in the elderly with a relative sparing of the CD56dim subset. The CD56bright/CD56dim ratio, perhaps representing NK cell maturity status, declined with age. This maturation-related subset redistribution and partial loss during aging of high affinity IL-2 alpha beta gamma receptor bearing 'immature' CD56bright NK cells has not been reported before. It could be a consequence of the decline in the level of IL-2 during aging. It is concluded that post-adolescent age-associated modulations in human NK cells are not expressed uniformly; they are pronounced in some, subtle in others but negligible in yet other biological parameters.

Cytokine sensitivity of human NK cells during immunosenescence. 2. IL2-induced interferon gamma secretion

A majority of natural killer (NK) cells constitutively express intermediate affinity IL2 receptors made of beta gamma chains and respond by way of enhanced cytokine secretion. However, alterations in cytokine sensitivity of purified NK cells with respect to cytokine secretion during immunosenescence have not been examined before. In view of the major immunomodulatory role of IL2 and the anti-tumor effects of interferon gamma (IFN-gamma), we have investigated the recombinant human IL2-induced NK cell secretion of IFN-gamma in vitro. After an 18-h activation period, the secretion of IFN-gamma by the peripheral blood NK cells from the elderly was severely impaired at 80 U/ml of IL2 and above. At an optimal dose of IL2 (960 U/ml). NK cells from young and elderly showed a mean 11- and 3-fold increase in secretion, respectively. Under similar conditions, purified T cells did not respond to IL2. However, the sensitivity of NK cells to the same inductive ligand, IL2, towards a different function i.e., cytotoxic activity, was not significantly impaired in the aged. It is possible that the cytokine secretory deficiency of senescent NK cells might be an additional mechanism which could favor the establishment of tumors and viral infections in the elderly. However, as discussed here, our findings do offer alternate explanations and a potential target for experimental immunotherapy.

Natural killer cells and cancer

BACKGROUND

Natural cytotoxicity, mediated by natural killer (NK) cells and cell with lymphokine-activated killer (LAK) activity, is believed to play an important role in host anti-cancer mechanisms.

METHODS

The authors critically review recent publications on the role of natural cytotoxicity in patients with cancer.

RESULTS

In patients with cancer, several studies have noted variations in the numbers and activity of NK and cells with LAK activity in different body compartments. NK cell activity in the peripheral blood lymphocytes (PBLs) is higher than that found in lymph nodes and within tumors, and this appears to be due to the presence of suppressor factors. The natural cytotoxicity of PBLs in patients with different types of cancers varies. However, there appears to be a trend for natural cytotoxicity to be reduced in certain cancer patients, possibly related to tumor volume or dissemination. Anti-cancer treatments (e.g., surgery, hormonal modulation, radiotherapy and chemotherapy) can also result in suppression of natural cytotoxicity, although the long-term effect on response to treatment and development of metastases is at present unknown.

CONCLUSIONS

NK and LAK cells, through the use of immune biologic modifiers, have been demonstrated to have a therapeutic role in the treatment of human cancers. Further studies are required to determine the optimal dosages and combinations of chemotherapeutic agents, the timing of surgery, and the adjuvant use of immune biologic response modifiers. An increasing awareness and understanding of this field, may allow for the future development of anti-cancer therapies.

The effect of age on hemopoiesis

Although several workers have described numerous changes affecting the hemopoietic system during senescence, the existence of univocal "hematological disease" closely related to the elderly is controversial. Many of the hematological changes described, such as sideropenic or megaloblastic anemia, are frequently the consequence of the different pathological conditions which often affect elderly patients. This review will consider the most important alterations of hemopoiesis and coagulation in the elderly, the causes capable of influencing hematological changes in old people, and their pathogenesis. Some of the major diagnostic problems encountered in the management of elderly subjects with hematological changes are also addressed. In the presence of an elderly patient with hematological alteration, it is necessary to follow a precise diagnostic schedule, which should first of all exclude the presence of a primary hematological disorder, and consider the different extrahematological conditions which frequently occur in elderly subjects (malignancies, malnutrition, chronic infections from immunological abnormalities, hormonal changes, deficiencies of various organs and systems etc.) and are responsible for many different hematological changes. These must be tackled rationally so that treatment may not only be symptomatic, but may also directly intervene on the cause of the disorder.

Natural killer cell cytotoxicity in elderly humans after influenza immunization

Previous studies have reported that human natural killer (NK) cytotoxicity can be augmented by either in vitro stimulation with influenza virus antigens or in vivo administration of killed influenza vaccine. The study demonstrating the latter conclusion reported an increase in NK cytotoxicity lasting for 4 weeks postvaccination in young subjects. We initiated our study to determine if a similar increase in NK activity was observed in an elderly population after immunization with the 1992-1993 influenza vaccine. NK activity of 34 elderly (mean age, 77.3 years) was determined at 3 time points: prevaccination, 4 to 6 weeks postvaccination, and 5 to 6 months after vaccination. In contrast to the results of the previous study, the NK cytotoxicity of our elderly subjects was not augmented by the influenza vaccine at any time tested. We also determined the number of CD56+ cells in whole-blood samples at each of the time points and found that there is no change in NK cell number after influenza vaccination.

Immune cell circulating subsets are affected by gonadal function

Influence on the immune system activity by sex hormones has been widely reported. Fertile women are proner to the onset of autoimmune diseases than men, but this increased susceptibility disappears after menopause. The hormonal changes are very likely to be responsible for this event, but precise correlations between sex hormone levels and immune functions have not been defined. For this reason we have analyzed phenotype and natural cytotoxicity of peripheral blood lymphocytes (PBL) from 35 women in menopause, comparing them with the same parameters of 28 fertile and 8 postmenopausal women and correlating them with the hormonal pattern of each group. We have also considered 8 women with premature menopause. Hormonal levels have been detected by radioimmune assays, while PBL phenotype has been studied by immunofluorescence and FACS analysis. The natural killer (NK) cell activity has been calculated on the basis of a chromium release assay. Postmenopausal women showed a reduction of the number of total lymphocytes (1650 +/- 215 cells/mmc) in comparison to fertile women (2081 +/- 200 cells/mmc, P < 0.01). The decrease mainly involved B and CD4+ T lymphocyte subpopulations (P < 0.05 and P < 0.01, respectively). Women with premature menopause had lower percentage of CD4 lymphocytes (34% vs 47%, P < 0.01) and higher percentage of CD8 (30% vs 22%, P < 0.02) and NK cells (32% vs 14%, P < 0.009) than fertile women of the same age. The percentage of circulating lymphocytes expressing HLA class II antigens also resulted as being increased (22% vs 9%, P < 0.01). The number of total, CD2, CD4 T lymphocytes, B and NK cells correlated positively with LH and negatively with FSH serum levels (P < 0.05 and P < 0.002, respectively). PRL positively influenced CD2, CD4 and B lymphocyte numbers (P < 0.001). FSH and 17 beta-estradiol inversely affected CD8 and B lymphocyte numbers (P < 0.005 and P < 0.02, respectively). In conclusion, the increase of FSH and the decrease of PRL levels appear to be involved in the reduction of B and CD4 T lymphocytes thus lowering the risk for the onset of autoimmune diseases during and after menopause. Generalized activation of the immune system (raised expression of HLA class II antigens) with elevated numbers of cytotoxic subpopulations (CD8 and NK lymphocytes) is present in women affected by premature menopause suggesting the involvement of autoimmune dysregulation in the pathogenesis of this syndrome.

Elderly high NK responders are characterized by intensive proliferative response to PHA and Con A and optimal health status

In this paper we verify a hypothesis emerging from recent immunogerontological studies assuming that an elevated level of NK activity in the elderly may be regarded as a factor compensating for a decreased reactivity of T cells. According to this assumption a high NK activity should be associated with a poor reactivity of T cells. In our study the mitogenic responses of lymphocytes of the elderly and the young high and low NK responders to phytohemagglutinin (PHA) and Concanavalin A (Con A) were measured. A high level of NK activity was not found, as would be expected from the hypothesis, to be associated with a poor reactivity to PHA and Con A. In contrast, the mitogenic response to PHA and Con A was intensive in both the young and old high NK responders while weak proliferative responses to T cell mitogens were characteristic of the young and old low NK responders. Thus, our results do not support the assumption of an association between a high NK activity and a low proliferative response to T cell mitogens in the elderly. We also found that both the elderly and the young high NK responders were characterized by an optimal health status.

Low natural killer cell function in disseminated aspergillosis

We investigated whether immune deficiency was associated with a fatal case of invasive cerebral aspergillosis due to Aspergillus fumigatus infection. The lymphocyte proliferative capacity to T- and B-cell mitogens was comparable to that of healthy controls. However, the natural killer (NK) cell activity of the patient was 2-6-fold lower than the activity expressed by a paired control and age-sex matched healthy controls (n = 20). No specific abnormalities were evident in the capacity to transduce either proliferative signals or cytokine (interleukin 2) secretory capacity of in vitro activated lymphocytes from the patient. The specific reduction in NK cell activity might either indicate that she was a low NK responder, or that the low NK cell activity was primarily or secondarily associated with the fungal infection caused by Aspergillus fumigatus in humans.

Preferential accumulation of mature NK cells during human immunosenescence

The major histocompatibility complex-unrestricted, cell-mediated, constitutive anti-tumor cytotoxic function of natural killer cells is highly preserved in healthy elderly. A study of the dynamics of expression of natural killer cell-associated phenotypes during immunosenescence shows that selective, bidirectional, and disproportionate changes in certain natural killer cell subset number and ratio take place during aging. The mean natural killer cell subset ratio (%CD16+CD57+ over %CD56+CD57-) gradually increases from a young adult level of 0.7 to 4.6 with advancing age predominantly due to a tripling of %CD16+57+ cells as opposed to a moderate decrease (-54%) in %CD56+57- phenotype. The parallel increase in natural killer phenotype ratio and cytotoxic activity might represent a shift in the maturity status of these cells. Based on these findings, a model of natural killer cell immunosenescence is proposed. It is concluded that not all immunosenescent changes need be detrimental; some may even improve the potential for survival and represent an adaptational immunosenescent change.

The impairment of natural killer function in the healthy aged is due to a postbinding deficient mechanism

In order to study the fine mechanisms that underlie the impairment of non-MHC-restricted cytolytic activity which occurs during human aging, we examined by multiparametric flow cytometry the binding and lytic activities of human natural killer cells. The flow analysis revealed a striking increase of the CD16+8- subset, together with a significant decrease of CD8bright cells and total T cells (CD3+). Aging had no influence on the CD8dim subset. The total lytic activity expressed by PBL as well as their binding efficiency to K562 targets were moderately but not significantly increased in the elderly. In contrast, the cytotoxicity of the single target-bound natural killer cell (i.e., lytic efficiency) was deeply impaired in aged subjects, suggesting that the NK functional impairment observed in aging is located at postbinding level.

Negative modulation of human NK cell activity by purinoceptors. 2. Age-associated, gender-specific partial loss of sensitivity to ATP

Aging is known to modulate the affinity and sensitivity of receptors for hormones and regulatory molecules. We have shown previously that exogenous adenosine triphosphate (ATP), perhaps acting as a purinoceptor agonist, can down-regulate the cell-mediated anti-tumor natural cytotoxic activity of human peripheral blood natural killer (NK) cells. We have extended these studies to investigate whether this effect is modulated during immunosenescence, and if so, whether it is gender-restricted or NK subset-associated. While the inhibitory effect is demonstrable in most individuals, there is a gender-restricted, age-associated transition in the sensitivity of NK cell activity to inhibition by ATP at 2.5 x 10(-5) to 80 x 10(-5) M in vitro. Data from both suboptimal (100 microM) and optimal (800 microM) inhibitory doses of ATP support this conclusion. The ID20ATP were 10.2 x 10(-5) and 17.8 x 10(-5) M for the young (less than 40 years) and elderly (greater than 70 years) females, respectively (P = 0.02). The frequency distribution curve of ATP sensitivity shifts to the left in the elderly, i.e., the sensitivity to be inhibited at 50% or more by ATP was expressed by one-half of young and one-fifth of elderly female donors. Linear regression analysis suggests an inverse relationship between percentage CD57+ and percentage CD16+57+ (but not percentage CD16+) NK subsets and sensitivity to down-regulation by ATP. The mean percentage of the above NK cell phenotypes among lymphocytes from young and old female donors differ significantly (less than 0.0001). The data suggest that the presence of CD57 antigen-positive cells may render NK cells relatively more resistant to the action of purinoceptor agonists such as ATP. Thus in females, immunosenescence results in a diminished ability of NK cells to transduce those signals that may normally mediate ATP-induced suppression of NK cytolytic activity. Such a diminished ability may be an immunobiological advantage to aging NK cells since they can be kept at a higher steady-state level of (anti-tumor cytotoxic) activity through a protection from negative modulators. These findings have an implication on the lower rate of mortality due to cancer seen in older women compared to that in older men. It is suggested that the ATP-NK cell interaction through the P2 purinoceptor may serve as a potentially useful model to study immunosenescence, ontogenic, or gender-specific changes in NK cells at the cell surface level.

Immunosenescence of human NK cells: effects on tumor target recognition, lethal hit and interferon sensitivity

Previous results from this laboratory have shown the preservation of non-MHC-restricted, constitutive oncolytic activity of human peripheral blood NK cells in the elderly as assessed by the chromium release assay which quantitates the lytic endpoint at the cell population level. We have now addressed this senescence-related change at single-cell level using 101 blood samples. Both the efficiency of the initial tumor target binding step i.e., recognition of K562, the NK-sensitive erythroleukemia cell line, as well as the ability of NK cells to deliver lethal hit are highly conserved during healthy aging. In fact, the elderly exhibit a statistically significant, moderately higher frequency of active killers among circulating lymphocytes. Analyzed in another way, a majority of "high NK responders" were found to be older donors, while none in the "low NK responders" group were > 70 years old. Gamma interferon, a gene product as well as an autocrine activator of NK cells, is effective in converting non-lytic "pre-NK" cells to active killers at single-cell level. This in vitro cytokine sensitivity of NK cells is unaltered during immune senescence. The intactness of the NK cell's capacity to be modulated may be vital in both tumor resistance and host viral defenses of aged humans.

Healthy lifestyles are associated with higher natural killer cell activity

BACKGROUND

To investigate the association of individual lifestyle with immune response against tumors, we assayed peripheral blood lymphocytes of 62 healthy males ranging in age from 30 to 60 years for natural killer (NK) cell activity, which is the first line of defense against tumors, and for frequencies of NK cell subsets.

METHODS

The NK cell activity was determined by 51Cr release assay, and NK cell subsets were counted using surface antigens (CD16, CD57) which recognize NK cell subgroups with different cytolytic potentials. Subjects were classified into groups reporting good, moderate, and poor lifestyles according to their responses on a questionnaire regarding eight health practices (tobacco smoking, alcohol consumption, hours of sleep, physical exercise, eating breakfast, balanced nutrition, hours of work habits, and mental stress).

RESULTS

Individuals reporting good lifestyle habits were found to have the highest NK cell activity and it was significantly higher than the NK cell activity in those reporting poor lifestyle habits. Those reporting good health practices regarding smoking and physical exercise showed significantly higher NK cell activity at an effector-to-target-cell ratio of 40:1.

CONCLUSIONS

The higher NK cell activity among individuals reporting good lifestyle practices may reflect an increase in the cytolytic potential of NK cell activity within the CD16+, CD57- subset.

Negative modulation of human NK cell activity by purinoceptors. 1. Effect of exogenous adenosine triphosphate

A variety of receptors, including purinergic receptors (for adenosine and adenosine nucleotides), are present on lymphoid cells. We have investigated the negative modulatory action of ATP, a high-energy purinergic compound, on the human peripheral blood natural killer (NK) cells. Micromolar doses of exogenous ATP produced a strong, direct, reversible, and dose-dependent (2.5 x 10(-5) to 8 x 10(-4) M) inhibition of NK cell-mediated cytotoxic activity in 98% of the 131 healthy donors tested. The inhibitory effect of ATP could be demonstrated in unseparated total lymphocytes and enriched NK cells (Percoll density gradient or negative selection by complement lysis) in 2- or 16-hr NK assays, at various effector:target ratios, at suboptimal and saturating concentrations of the K562 targets. The inhibitory constants ID20 and ID50 max for ATP (2 hr), which were 5.6 x 10(-5) and 8 x 10(-5) M, respectively, appear to be a relatively stable feature of an individual. The mechanism of action of ATP was also explored. It is not due to a simple chelation by ATP of the divalent cations Mg2+ and Ca2+ which play a role in the binding/lytic reactions during NK cell-mediated oncolysis. ATP treatment of lymphocytes neither nonspecifically decreased the number of viable cells nor specifically eliminated the NK cells, including their subsets, CD57+16-, CD57+16+, and CD57-16+ as revealed by immunofluorescence staining. Use of an enzyme-like kinetic approach to define the nature of ATP-induced inhibition suggests that it may be an "uncompetitive" type (Lineweaver-Burke reciprocal plot), because both Vmax (maximal velocity of oncolysis) and Km (Michaelis constant) were reduced simultaneously from 1.53 x 10(4) and 2.64 x 10(4) to 0.44 x 10(4) and 0.73 x 10(4), respectively, in presence of ATP. Other methods (Eadie-Hofstee's and Hane's equations) confirmed that ATP does elicit a qualitatively and quantitatively similar shift in these kinetic parameters. Finally, an 18-hr preactivation of effectors by interleukin 2 (IL2) resulted in partial protection from the negative modulation of NK activity by ATP, while the presence of ATP during preincubation did not interfere with the desensitizing effect of IL2. The biological relevance of these findings are discussed and it is concluded that by interacting with appropriate (non-FcRIII?) cell-surface binding sites such as P2 purinergic receptors, ATP is capable of negatively modulating the antitumor cytotoxic activity of human NK cells. This down-regulation can be partially overcome by IL2 that up-regulates NK cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunomodulation by 9-amino-1,2,3,4-tetrahydroacridine (THA): 1. Down-regulation of natural cell-mediated cytotoxicity in vitro

THA (Tacrine), a drug used in the experimental therapy of dementia of Alzheimer's disease type, and whose biochemical site of action is believed to be the neural cholinesterase, is shown, for the first time, to be an immunosuppressant in vitro on normal human peripheral blood lymphocytes in microgram quantities. THA down-regulates non-MHC restricted natural killer (NK) cell activity without affecting the general viability of cells. This down-regulation can be demonstrated at all effector and target (K562) concentrations, in purified resting NK cells as well as in lymphokine (interleukin 2) activated killer cells in 3- or 16-h NK assays and in all the blood samples tested. Kinetic analysis shows that the Vmax (maximal cytotoxic potential) and Km of NK cell-mediated cytolysis are also attenuated. Single cell assays using agarose matrix reveal that THA moderately interferes with tumor target binding/recognition events and strongly abrogates the delivery of lethal hit, thus lowering the frequency of active killer cells among THA-treated lymphocytes. THA down-regulates NK cells upon direct interaction and does not require the help of non-NK cells. The THA sensitive site(s) on NK cells does not appear to be perturbed significantly either by their proliferative status or by membrane modulations that may be normally induced by interleukin 2. The in vitro immunomodulatory pharmacological properties of THA reveal that the biological site of action of THA extends to non-neural cells also. Such non-neural models may be helpful in exploring the pathophysiological neuroimmunomodulatory properties of THA at cellular and molecular levels.

Deficient interleukin-2-activated killer cell cytotoxicity in patients with systemic lupus erythematosus

This study was undertaken to determine if the cytotoxic activity of interleukin-2 (IL-2)-activated killer (LAK) cells, which is induced by the direct activation of lymphocytes by IL-2, is defective in systemic lupus erythematosus (SLE). The killer cell activity of SLE patients, whether it be generated in autologous plasma or serum-free media, was significantly less than the controls against three different target cells. It was observed, by incubating control lymphocytes in 10% fresh SLE plasma, that soluble factors were responsible for a portion of the reduced generation of LAK cell cytotoxicity (P less than 0.05). Suppression ranges from 4 to 70% with a mean of 33%. However, when SLE LAK cells were generated in serum-free media, a greater reduction of generated LAK cell activity was observed (P less than 0.005). Using a disease activity score, it was determined that SLE cytotoxicity negatively correlated with disease activity only when the cells were cultured in serum-free media. Irrespective of the culture conditions, lytic activity did not correlate with levels of anti-DNA antibodies, anti-Sm antibodies, circulating immune complexes, or prednisone therapy.

Biology of natural killer cells

Studies of cytotoxicity by human lymphocytes revealed not only that both allogeneic and syngeneic tumor cells were lysed in a non-MHC-restricted fashion, but also that lymphocytes from normal donors were often cytotoxic. Lymphocytes from any healthy donor, as well as peripheral blood and spleen lymphocytes from several experimental animals, in the absence of known or deliberate sensitization, were found to be spontaneously cytotoxic in vitro for some normal fresh cells, most cultured cell lines, immature hematopoietic cells, and tumor cells. This type of nonadaptive, non-MHC-restricted cellmediated cytotoxicity was defined as “natural” cytotoxicity, and the effector cells mediating natural cytotoxicity were functionally defined as natural killer (NK) cells. The existence of NK cells has prompted a reinterpretation of both the studies of specific cytotoxicity against spontaneous human tumors and the theory of immune surveillance, at least in its most restrictive interpretation. Unlike cytotoxic T cells, NK cells cannot be demonstrated to have clonally distributed specificity, restriction for MHC products at the target cell surface, or immunological memory. NK cells cannot yet be formally assigned to a single lineage based on the definitive identification of a stem cell, a distinct anatomical location of maturation, or unique genotypic rearrangements.

Age-associated alterations in human natural killer cells. 2. Increased frequency of selective NK subsets

We have analyzed the peripheral blood lymphocytes from healthy volunteers (20 to 94 years) for the expression of natural killer (NK) cell surface markers, NK activity, and B-cell proliferative response. An increase (2- to 3.5-fold) in relative percentage and absolute number of lymphocytes expressing Leu-7 (HNK-1) or Leu-11a (CD 16) antigen was found in the elderly group (greater than 80 years) as compared to young adults (less than 40 years). A two-color immunofluorescence analysis revealed that the age-associated increment was both progressive and selective; the actual increase occurred in Leu-7+11a+ and Leu-7+11a- populations (subsets with variable and weak NK activity) but not in the Leu-7-11a+ (most active) subset. There is a corresponding decrease in the 7-11a- cells. The ratios of 7+11a+/7-11a+ and 7+11a-/7-11a+ cells doubled with advancing age. Linear regression analysis suggests that the 7-11a+ cells are highly preserved through human senescence and the ratio of 7+11a- cells to the most stable subset, 7-11a+, could expand nearly 100-fold from birth to old age. Further analysis of Leu-7+ cells for the coexpression of Leu-11c (an epitope of Leu-11a) confirmed a similar pattern of changes in 7+11c+ and 7+11c- NK subsets with advancing age. The frequency of Leu-11+ (epitopes 11a+ or 11c+), but not of the subsets of 7+ phenotype (7+11a- or 7+11c-), correlates well with the NK activity (spontaneous killing of K562 tumor cell line). The 7+11c+ cells may directly or indirectly be responsible for the increase in NK activity observed with a majority of aged donors. The inverse relationship observed between the mitogenic response of lymphocytes to pokeweed mitogen (PWM) and the initial frequency of 7+11a-, but not other phenotypes, raises a potential functional significance for the expansion of the 7+11a-(7+11c-) subset. These age-associated NK phenotypic changes provide a cellular basis for our observations on age-associated increase in NK activity and decrease in mitogenic response to PWM.