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Sumario Nº 1 > Immune System

The Immune System as a Marker of Health and Longevity

Dr. Mónica De la Fuente
Professor of Physiology. Faculty of Biological Sciences, Complutense University
SUMMARY
 

The function of the immune system is to recognize the "self" or identity of each individual and to destroy foreign microorganisms and tumor cells. It has been shown that a well preserved function of the immune cells is an excellent marker of health and, accordingly, we have confirmed that the functional competence of leucocytes, determined by means of a battery of parameters that change with age, is a marker of "biological age" in human subjects as well as in laboratory animals. Further, we have found that in our model of premature ageing in mice the immune function biomarkers are good predictors of longevity. Thus, the mice showing immune parameters characteristic of animals of older chronological age have a lower life expectancy than those which as regards their immune function are "biologically younger". In relation to the above, it is proper to consider why the immune functions are altered in the aged organisms. In our opinion, the senescence of immune cells, like that of other cell types, is probably due to the oxidative stress (linked to raised production of free radicals and decreased antioxidant defense) that occurs with age. Moreover, we suggest that the immune system, because of its need to produce free radicals (FR) and other oxidant and inflammatory compounds in order to support its functions, is very involved in the oxidation/inflammation process that underlies senescence. This view justifies present attempts to prevent age-related oxidative stress by diet supplementation with antioxidants. Accordingly, as regards the immune system a number of studies, performed both in human subjects and in experimental animals, has shown that the ingestion of antioxidants by aged individuals changes the functional parameters of leucocytes, bringing them to levels similar to those of the adult. This immune "rejuvenation", that is accompanied by an increased longevity in the experimental animals, supports the oxidation/inflammation theory of ageing and the useful role of the investigated leucocyte functions as markers of life expectancy and health.

Key words: Immune System. Aging. Oxidative stress. Antioxidants.

   
THE IMMUNE SYSTEM
  Since we are born we are continuously exposed to suffer infectious and carcinogenic processes that might result in death if the organism did not have at its disposal a complex physiological system that protects us against those processes, namely the immune system. This system is composed by a large variety of cells and molecules able to recognize and eliminate a great number of different agents foreign to the organism, which comprise not only the invading microorganisms but also our own cells that are continuously exposed to tumorigenic mutations. The immune cells have a wide functional competence and present multiple and complex forms of communication. The mechanisms used by the leucocytes in order to carry out their functions, the so-called "immune response", are based on the following steps: (a) recognition of the foreign, i.e., the antigen. (b) Activation against this antigen, that is very well regulated, since an uncontrolled activation of the immune system may be the cause of pathological processes and mortality. (c) Destruccion of the foreign agents (infectious microorganisms or cells that have suffered a malignant transformation).

In view of the above, the immune system is uniquely suited to recognize our integrity, i.e., our own self and thus be able to defend us from foreign pathogenic agents. On the basis of the above, since it is obvious that an adequate leucocyte function is essential to insure a correct organismic function, the functional competence of leucocytes has been recommended as one of the best biomarkers of the health of a subject and therefore of his life expectancy (1). Moreover, since the immune system has a key role in the preservation of homeostasis, this system is presently considered a genuine regulatory system, comparable to the "classic" regulatory systems, namely the nervous and the endocrine systems (2-4).
   
THE NEUROENDOCRINE-IMMUNE SYSTEM
  It is worth mentioning that the immune system (IS) does not work in isolation but it functions in close relationship with the other regulatory systems of the organism, namely the nervous and the endocrine system. This bidirectional communication between the regulatory systems was confirmed in the seventies by Besedowski et al., who found that the glucocorticoid levels, which increased during the immune response, had a suppressor effect on that response (5). Further work by Besedowsky and other workers confirmed the above mentioned system connections (6-8). Thus it was accepted that the IS represents a system of reception of information of non-cognition related stimuli that appear in the organism (infections, tumor cells or other types of foreign cells) and response to those stimuli, accompanied by transfer of that information (by means of the cytokines produced by I.S. cells) to the neuroendocrine system. On the other hand, the neuroendocrine system is a receptor of cognitive stimuli (light, sound, stress situations, etc.) to which it responds, and its mediators (neurotransmitters and hormones) rech the I.S to inform it about the situation. Thus, there is a neuroendocrine-immune system that allows the preservation of organismic homeostasis and therefore of health. The scientific confirmation of this communication has allowed to understand, on the basis of the experimental data, a number of facts of everyday life. Thus it is well known that the situations of depression, emotional stress or anxiety, provoked for instance by the loss of the job or of a close relative, are accompanied by a greater vulnerability to conditions ranging from infectious processes to cancer or autoimmune diseases, which agrees with the concept that the IS is impaired, which results in worse health and a shorter life span. By contrast, pleasurable situations and an "optimistic outlook" on life help us to overcome IS-related diseases and enjoy a better overall health. Conversely, it has been shown that IS changes such as found in infectious processes alter nervous system functions, which can even lead to psychotic disorders. Presently it is accepted that the three above mentioned regulatory systems share receptors and therefore any influence exerted on the IS will have an effect on the nervous and endocrine systems and viceversa.
   
AGE-RELATED CHANGES IN THE IMMUNE SYSTEM
  The impairment of the IS with age, i.e. the immunosenecence, exerts a great influence on the increasing morbility and mortality observed in aging human subjects (1). In fact, it is well known that with the passage of time there is a decrease in the resistance to infections, and an increase in autoimmune processes and cancer, which indicates the pressence of a less competent I.S. Moreover, the increased death rate found in aged populations is due in great proportion to infectious processes (9-10). In view of the key role of an optimum immune function in the preservation of health, it seems logical that one of the theories on the cause(s) of aging, namely the "immunological theory", maintains that the responsability for the changes that take place in the organism with the passage of time lies on the impairment of the immune defense system (11,12). Nevertheless, despite the fastly-increasing amount of data on immunosenescence, the changes in the immune functions with age as well as the specific role played by the IS in organism ageing is not well understood. This may be due to the great complexity of both organismic ageing and of the aging process of the IS, with its diverse cell populations and subpopulations, and the interactions among themselves as well as with other physiological systems.

Although the immune cells change their functional competence with increasing age, not all immune cell types show a significant impairment. In fact, several cell types are more functional with age whereas other types do not show substantial age-related changes. Because of the conflicting observations, there is no general agreement on the immune response changes that occur during senescence, although most data support the view that aging is associated with a restructuration involving each component of the IS, as well as their interactions (2,9,13-15).

In order to establish the reference values, our group has performed a study on the age-related changes both in laboratory animals (mice) and in human subjects in the most representative immune cell types, i.e. the phagocytes (peritoneal macrophages from mice and neutrophiles from peripheric human blood), lymphocytes (from peritoneum and the immunocompetent organs of laboratory animals and from human peripheric blood) and "natural killer" (NK) from the same locations that the lymphocytes. In these three cell types we have analyzed different functions, which are listed in Table 1. In the phagocytes, the following functions linked to the phagocytic process have been investigated: (a) Adherence to tissues. (b) Mobility to infectious focus or chemotaxis. (c) Ingestion or phagocytosis of foreing material and (d) digestion capacity of this material through the production of intracellular free radicals, namely superoxide anion. In lymphocytes the functions analyzed have been: (a) Adherence to tissues. (b) Migration ability that allows arrival to the site of antigen recognition. (c) Prolipherative response to the "foreign", antigens or mitogens. (d) Production of cytokines needed to support proliferation, such as the IL-2. In the NK cells, the study has focused on its cytotoxic action against tumour cells.

It has been pointed out that human immunosenescence may differ from that of mice because of the difference in many physiological characteristics between the two species. Nevertheless, the experimental data from several laboratories, including our own, show similar age related immune changes in humans and mice, although, because of their great differences in life span, aproximately 100 years for humans and 2 years for mice. The senescent changes of the above mentioned functions are summarized in Table 1. It seems that some functions (like adherence) increase continuously with age, whereas other (such as lymphoproliferative response, production of IL-2 and NK) increase in the adult, with respect to the young, and decrease significantly in the aged. On the other hand there are functions, like chemotaxis and phagocytosis that decline progressively since youth to old age. This explains that for a function that follows the same kinetics that lymphoprolipheration, when a comparison is performed between the values found in very young subjects (2-3 months old mice, which are usually used in immunological studies) and the values obtained in old mice (22 months of age), no significant differences are found. Likewise, the comparison of the values obtained in very young mice and those from "mature" mice (l4-17 months old, which some researchers may consider old animals) could lead us to accept that this immune function increases with age. The above may explain many of the contradictory results, wich as previously noted are obtained in the study of immunosenescence.

It is also very interesting that, in human subjects, the proliferative response of lymphocytes to the mitogen fitohemaglutinin, PHA (a typical mitogen for human T lymphocytes) and the production of IL-2 (two fundamental functions which are the most sensitive to the psychological and physiological factors that influence the immune system and health) shows the greatest response at the age of 30-39 years in men and at 40-49 years in women. Afterwards there is a significant decline in these functions, in both genders, until the seventies. It is worth noting that in this decade when the maximum mortality rate occurs in the developed countries (in which mean life expectancy is 72 and 78 years for men and women, respectively). Another interesting finding is that the above mentioned two functions, the lymphoproliferative response and the production of IL-2, are found at the same level in human subjects in the age range 80-104 year old than in the adult. This confirms the opinion already offered by some authors that the individuals who reach that advanced age are those endowed with a very adequate IS, and more especifically have lymphocytes T showing a very good functional competence. This may be due to the fact that these cells are better preserved, by themselves or as the result of other factors that influence their correct functioning. In any case, these results confirm that the IS is an excellent marker of health and longevity.

In short, we can conclude that with the passage of time the IS "reestructures" itself, as suggested by some researchers. It is interesting that, in general, the age-related changes in the IS are expressed, on the one hand in a lower response in those aspects that could be of benefit, and on the other hand in an exaggerated response in those activities that, although initially have a defensive role, become detrimental if they are produced in excess (2,9,15). Thus, in senescence, immune functions such as adherence to tissue substrates, and production of cytoquines of the proinflammatory type like TNFa, are those that show a stimulation (Table 1). The activated functions are precisely those more markedly related to an oxidative state of the subject (16,17), in agreement with the hypothesis that the aged organism is more oxidized (18), as we will discuss below.
   
THE IMMUNE SYSTEM AS A MARKER OF BIOLOGICAL AGE
  The concept of "biological age" or "functional age" arises as a consequence of the different rate of the physiological changes in every member of a population of the same chronological age (19). This concept, first introduced by the insurance companies of the USA, is useful to assess the level of aging experienced by each individual, and therefore his life expectancy. In order to calculate the "biological age", a number of parameters should be determined. Aging is associated with a great number of changes at all levels of biological organization, influencing in different ways the diverse systems of each subject and the diverse individuals of each species. Therefore, there is a need to select a number of biochemical, physiological and psychological parameters that change with age and can be subjected to statistical analysis to reveal the relations between "biological age", chronological age, health loss and life expectancy. The most exhaustive investigation on these subjects has been performed by Borkan and Norris (20) on about one thousand men enrolled in the study of human aging of the Gerontological Center of Baltimore. This study showed that, although it is not possible to calculate an "integrated biological age" for an individual (since each physiological system may have a particular biological age different of that of the other systems), the finding that certain parameters or biomarkers are "more aged" than those of the majority of the subjects of the same chronological age is linked to a greater probability to die prematurely. These biomarkers include those related to respiratory function, systolic arterial tension and reaction times in psychometric tests. The Baltimore study did not include the above mentioned immune parameters, which presently are considered essential and very representative of the "true" biological age of a subject. Thus, a positive relation has been shown between a good function of the T cells or the NK and longevity. Accordingly, a fact that confirms the key role of the IS on health and longevity is that the centenarians who reach that very advanced age in good health are those showing a perfect preservation of the immune cell functions, that show values like those of adult subjects (21), as already noted.

In relation to the above, our group planned several years ago to test if the above immune parameters, standardized in mice and humans, can be used as markers of biological age. This requires that the parameters show a relation with life expectancy, which could be only demonstrated in mice (because of its short life span of approximately two years). In order to carry out this project, we have relied on a model of premature aging , in the mouse, in which is quite evident the relation betwen the immune competence of a subject and its life span. This model, that provides another proof of the relation between the nervous and the immune system, relies on the differences in performance among mice of the same sex and chronological age when subjected to a behavioral (exploration) test in a simple T-maze. We have shown that the animals that fail the test, are"biologically older", i.e. suffer premature senescence. This concept is in agreement with the fact that these mice have a prematurely aged IS, with the diverse functional parameters investigated (phagocytes, NK cells and lymphocytes) showing values characteristic of animals of an older chronological age. Moreover, these prematurely aging mice also showed higher levels of anxiety and emotionality and a neurochemistry similar to that of an older chronological age. The confirmation that such parameters were markers of biological age was provided by the fact that the mice showing a prematurely aged behavioral competence had a significantly decreased life span (22-28).
   
WHY DOES IMMUNOSENESCENCE OCCUR?
  It has been demonstrated that aging is accompanied by changes in the immune function and that these changes are excellent markers of biological age and therefore of probable longevity. This justifies many studies, including our own, to identify the mechanisms responsible for the age-related immune dysfunctions. It is obvious that when these mechanisms are elucidated it will be possible to develop strategies to retard immunosenescence and thereby to preserve health and obtain a satisfactory longevity. Before reviewing the mechanisms of imnunosenescence, we will summarize the main facts that underly the aging process.

Aging is linked to a decline of the homeostasis mechanisms of the body with concomitant functional changes, which trigger diverse pathologies that are quite frequent in old age and may even cause death. Of the about 300 theories that, according to Medvedev (29) have been enunciated to explain why takes place the general deterioration of aging, that of the "free radicals", proposed by Harman in 1956 (30), and developed by him (31) and other researchers, among whom stands out Miquel (32-33), probably is the most widely accepted. According to this theory, the progressive deleterious oxidation, that is a result of the use of oxygen in respiration to support the life-maintaining metabolic processes, leads to the functional decline linked to aging. The oxygen free radicals (FR) produced in our cells are highly reactive, and therefore they injure all kind of biomolecules, i.e.: lipids, proteins and genetic materials. Since the greatest production of FR and reactive oxygen species (ROS) takes place in the cellular organelles that carry out respiration, i.e. the mitochondria, these organelles are probably the main target of the respiration-linked oxygen stress. In agreement with this view, a fast-increasing amount of data shows that the mitochondrial damage caused by the FR results in a loss of bioenergetic competence that leads to the aging and death of cells and therefore of the organism (33).

In order to protect themselves against oxygen toxicity, the cells have developed a variety of antioxidant mechanisms that prevent the formation of FR or neutralize them after they are produced. However, these defensive systems are not perfect and thus when the formation of ROS exceeds the antioxidant protection there is an oxidative stress, with resulting cell injury (34).

Despite the above, we should consider that oxygen is essential for life and that ROS, in certain amounts, are needed for many physiological processes that are essential for our survival (16-18). Therefore, the functions of our organisms are based on a perfect balance between the levels of pro-oxidants (ROS) and those of antioxidants. It is the loss of this balance, because of an excess in the production of the first or an insufficient availability of the last, what leads to the oxidative stress that underlies ROS-related disease and aging (35).

Despite the yet scant work on this subject, but based on it and on the work of our laboratory, it can be maintaned that immunosenescence is produced by the same mechanisms responsible for aging of all cellular components of the organism, i.e.: the oxidation resulting from the necessary use of oxygen and the damaging effects of FR in uncontrolled amounts (36).

The IS provides a good example of the need to maintain oxidation under control to preserve an adequate functional state. In order to carry out a great proportion of their functions, the immune cells must produce ROS, with the activated leucocytes being a very important source of oxidation (16-18). Moreover, it should be considered that these cells are especially vulnerable to oxidation because of the high content of polyunsaturated fatty acids of their membranes, the key role of intracellular signalling related with these membranes and the gene expression required for their defensive function. In view of this, if it is important to preserve in any cell the mentioned antioxidant/oxidant balance, it is even more important to preserve that balance in the cells of our defensive system, since that equilibrium conditions their functional competence. The changes that occur in the function of the immune cells with aging are due in great proportion to the "chronic oxidative stress" to which they are exposed in the course of time. We have been able to confirm this in a study of the evolution experimented by the immune functions, testing them along the months of life of the mice in the hours of survival to an "acute oxidative stress" caused by a septicemia (Table 1). Thus, in animals in which an "endotoxic shock" is induced by injection of baterian lipopolisacharide (LPS, from E. coli), which show a 100% mortality in 30 hours after provoking the infection, the kinetics of the investigated function is basically the same observed in the study of aging (17). It is well known that the endotoxic shock, one of the main causes of death in the intensive care units, is the oxidative stress produced by immune cells in their attempt to protect us against infections, that leads to death of the patient. Moreover, recent studies of our group have widened this parallelism not only to the immune functions, but also to the levels of oxidant and proinflammatory compounds as well as of antioxidants in the cells of the defensive system (Table 2), which evolve in a similar fashion in the hours after LPS injection than in several months of normal aging.
   
INVOLVEMENT OF THE IMMUNE SYSTEM IN THE PROCESSES OF AGING: THEORY OF THE OXIDATION/INFLAMMATION
  A logical question, once the role of the immune system is known, is if the age-related changes and the cause of these changes, namely a "chronic oxidative stress" are only one more of the results of the oxidative alterations that take place with the passage of time or if they can be an important cause of those changes.

We must remember that the immune cells in order to fullfill their defensive function show an inflammatory response, producing factors (such as TNFa and ROS) which support the inflammation and oxidation processes that allow the eliminatioon of the "foreign". Since, as already pointed out, the oxidant and pro-inflammatory factors are increased with age, a new theory of aging, namely the "inflammation theory", is being developed. In fact, a transcription factor as ubiquituous as the NF-kB, which is involved with the expression of genes of oxidant and inflammatory compounds (such as the TNFa, the enzymes of the inducible nitric oxide synthase type (iNOS, productor of nitric oxide, or the cyclooxigenase 2 (COX-2), show a great activation in the immune cells in situations of oxidative stress (16-18), as it happens in aging (37). This could lead to a "vicious circle" that would stimulate even more the oxidative stress. A personal view of the facts in support of this inflammation /oxidation theory of aging is that our IS, with the passage of time, has had to face numerous foreign agents, which has resulted in a "wear-and-tear" condition and concomitant "chronic oxidative stress", which although generally present in all cells of the organism, would be more striking in those of the IS because of their role as producers of oxidant and inflammatory factors to support their everyday work. With the passage of time, this increase in inflammatory and oxidant factors would involve all cells of the organism, with the differentiated-postmitotic populations being the most vulnerable.

On the other hand, as a result of the oxidative injury that immune cells suffer with age, these cells would lose some of their capacity to regulate their own redox balance, which would result in the above mentioned vicious circle or, more precisely, in "a vicious spiral", since as a matter of fact the situation becomes progressively worse and there is no possibility of returning to the initial oxidative-inflammatory homeostasis. Those senescent changes of the immune cells could also become evident as an altered intracellular signalling, that makes them respond in a diffeerent way to the incoming stimuli (9). Therefore, the communication between the regulatory systems would be impaired, in agreement with a concept proposed at the beginning of the past decade as one of the most logical theories of aging, according to which senescence is associated with a failure of the neuroendocrine-immune communication (38). In agreement with this theory, the experimental data confirm that with age not only there is an alteration of the responses of the nervous, the endocrine and the immune system, but also of the ability to communicate between them (39-40). Our group has confirmed that the functional impairment of immune cells with aging renders them "deaf" to the messages from the nervous system that reaches them (37-39). It seems probable that the cause of this impairment of the regulatory systems and their communication is the oxidative stress that results in the above mentioned changes and associated homeostatic failure leading to the age-related increase in morbility and mortality.
   
STRATEGIES TO REVITALIZE THE IMMUNE FUNCTION IN AGING: INGESTION OF ANTIOXIDANT COMPOUNDS
  Since there is already some understanding of the mechanisms responsible for immunosenescence, it is possible to develop procedures to modulate them in order to preserve a better immune function in the aging organism. In this respect, one of the most adequate strategies relies on the use of antioxidant compounds. Presently, there is a wealth of experimental data suggesting that indeed the administration of antioxidants, many of which also possess anti-inflammatory properties, can normalize the balance between the levels of oxidation and inflammation and those of the antioxidant defenses, thus decreasing the oxidative stress.
   
THE ANTIOXIDANTS IN THE AGING IMMUNE SYSTEM
  The antioxidant compounds, which are able to prevent the production of ROS or to neutralize them, can be endogenous or exogenous. The first are present in our organism in order to insure the presence of the right levels of ROS compatible with adequate physiological functions, preventing both an excessive production or accumulation of ROS and the pathological processes triggered by them (41). When there is a decrease in the levels of endogenous antioxidants, often caused because they are spent neutralizaing an excess of ROS, the antioxidant defenses can be raised by administering in the diet the adequate amounts of endogenous or exogenous antioxidant compounds.

Among the exogenous antioxidants, probably the best known are the vitamins C and E and the carotenes, although others such as the flavonoids and the lipoid acid and other thiolic compounds (which raise the intracellular levels of reduced glutathione, GSH) are being incorporated to the already long list of antioxidants used in gerontological research (41).

Vitamin C or ascorbic acid and GSH, the two most important intracellular water-soluble antioxidants, which act in collaboration, are very effective against the growth of virus, and play a key role in the defense system, protecting the organism against infections, cancers and many other diseases. Vitamin E, the main antioxidant in cell membranes (to which it protects against peroxidation damage), works synergistically with ascorbic acid and GSH in the defense against virus infections and protects the circulating LDL against an excessive oxidation (42). The carotenoids also have a protective role against cancer and coronary disease. Similar functions have the flavonoids, which act inhibiting the enzymes implicated in the metabolism of arachidonic acid, thus preventing many pathological processes like cancers and vascular diseases (41). A group of compounds like taurine, thioproline and N-acetylcisteine, among others, provide GSH to our organism, showing important favorable effects on health owing to both their ability to increase the GSH levels, and to its direct antioxidant and anti-inflammatory action, which results in preservation of mitochondrial function (41) and improved cell survival. Considering that all cell functions depend to a high degree on the redox reactions of the thiol compounds, the preservation of adequate levels of GSH or of other thiols during aging is essential for an adequate activity of cells in general and especially of those of the IS, and therefore for health of the aging subjects.

In agreement with the above, the favorable action on the aging process of antioxidants like vitamins C and E, the thiolic precursors of GSH and the flavonoids is precisely their ability to raise the reducing power, thereby protecting against the oxidative stress associated with aging. In fact it has been shown that the organelles and cells of the aged animals contain less GSH than those of the young, and this decrease becomes more striking at the age when mortality shows a marked increase. Thus the administration of antioxidants able to increae life expectancy in experimental animals may be effective because of their protective effect against oxidative stress and more especifically against the effects of this stress on the DNA of mitochondria and the GSH loss in these organelles (35). These favorable effects, at least as regards the vitamins C and E are obtained with the administration of doses much higher than those indicatd in the RDA and present in the usual vitamin nutritional supplements (37).

Many research groups, including our own, have confirmed that these antioxidants are consumed to support the functions of our immune system. Therefore, in the performance of their function the immune cells may exhaust their reserves of antioxidants (43). This could explain, both in experimental animals and in human subjects, the improvement of the functional competence of the IS, in the adult age, after the in vitro incorporation or the supplementation in vivo of several exogenous antioxidants such as vitamin C, vitamin E, and thiols like thioproline or N-acetylcisteine (41-48).

If we consider that aging is associated with a production of higher amounts of ROS, often accompanied by nutritional defficiencies and significant declines in the levels of antioxidant defense (35,41), is seems evident that diet supplementation with these kind of compounds could have a favorable effect on the neutralization of oxidative stress, thus restoring the lost oxidant/antioxidant balance. This justifies the present studies to find out if the administration of antioxidants might have a stimulating effect on the functions of our defense system in old age. This line of work has provided very encouraging results (Table 1 and 2), with improvement of health conditions and prevention of many pathological conditions linked to oxidative stress (49-52). This is agreement with the fact that one the most convincing observations that support the oxidation theory of aging is that there is an increase in the life expectancy of some laboratory animals which consumed antioxidant-supplemented diets (28,53). Moreover the positive effects of the antioxidants is more evident in the immune cells of aged subjects than in those of the adult, with larger doses being required to observe the same favorable effect with increasing age (54). Thus, in a Spanish population the ingestion of vitamin C and vitamin E improved significantly the functional competence of immune cells in aged subjects (37,48-49). Indeed, the antioxidant supplementation brought up the levels of immune function to the values found in 30-35 yers old individuals (at which age humans show the most competent IS function), with the supplementation being able to stimulate those functions which were depressed and to depress those that were overstimulated (Table 1). The regulatory action of these vitamins lasted approximately six months, with most values obtained in the aged subjects returning to the initial levels after spending that period of time without the antioxidant supplementation. The modulating action of the antioxidants on the immune functions is more striking in those subjects in which it is more impaired, as ascertained by us both in humans and in experimental animals. In elderly subjects showing depression or cardiopathy, diet supplementation with antioxidant vitamins was more effective for IS function improvement than in subjects of the same age, who, although having the impaired immune function characteristic for that age, could be considered quite healthy (49). On the other hand, the thiolic antioxidants showed a more significant favorable effect on the immune functions of mice suffering an infectious process after injection of LPS (48,55), increasing their survival, as well as in mice showing premature aging, in which the antioxidant supplementation of the diet increased their longevity (28).

Since the positive action of the antioxidants on the IS is expressed in an increase of the functions that are depressed and a decrease of those that are excessively active, the antioxidants can not be considered general immunostimulants. In fact they may bring each immune function to its optimum level in situations in which it is impaired by oxidative stress, thus acting as immunomodulators (48). This modulating ability appears to be focused at the level of the ubiquituous intracellular factors implicated in oxidation and inflammation, such as the NF-kB (37). This regulatory role would be performed not only by the IS, but also by the other regulatory systems. As a matter of fact it is accepted that the antioxidants play a role in the recovery of a great number of nervous functions (35,41). In addition, in the prematurely aging mice the ingestion of antioxidants decreases the oxidative stress (Table 2), improves the behavioral response and increases longevity. This suggests that the oxidative stress that appears to play a fundamental role in the aging of both the IS and the nervous system, can be counteracted to certain degree by antioxidant administration and that antioxidant diet supplementation may be a useful procedure to neutralize or retard the age-related homeostatic impairment, which would provide an explanation for its favorable role in reducing the morbidity and mortality of aging populations.
   
CONCLUSIONS
  In view of the esential role of the immune system in the preservation of health and functional longeviy, it is assumed that the usefulness of diet supplementation with antioxidants for preventing pathological aging and increasing longevity may be due to the fact that this supplementation causes a "rejuvenation" of the immune function. It may be that a beter immune system is the cause of a greater longevity, or on the other hand it is also possible that the better immune functions may be only a consequence of an improved general homeostasis of the organisms. In any case there is no doubt that preserving a functionally "young" immune system, despite the passing years, is an excellent strategy for preserving the quality of life.

Acknowledgements
This work has been supported in part by MCYT (BFI2001-1218) and CM (08.5/0061/2001) grants. I wish to thank Dr. Miquel for his help in the preparation of this manuscript.
   
 
TABLE 1. Changes with ageing in different functions of immune cells. Effects of a diet supplemented with antioxidants
Cells
Function
Effects of Ageing
Effects of Antioxidants*
1. Phagocytes   Adherence
  Chemotaxis
  Phagocytosis
  Digestion
>
<
<
<

<
>
>
>
2. Lymphocytes   Adherence
  Migration
  Proliferation
  IL-2 release
>
<
<
<
<
>
>
>
3. NK   Cytotoxicity
<
>
 >: Increase. <: Decrease. * The diet supplementation with antioxidants counteracts the age-related
     changes in the immune functions bringing them to the normal adult values.
   
 
TABLE 2. Changes with ageing, in leucocytes, of different oxidant/inflammatory parameters and antioxidant defences. Effects of a diet supplemented with antioxidants
 
Parameter
Effects of Ageing
Effects of Antioxidants*
1. Oxidants   Extr.Superox. a.
  PGE2
  TNFa release
  GSSG

>
>
>
>

<
<
<
<
2. Antioxidants   GSH
  SOD
  CAT
  GPx
  Gr
<
<
<
<
<

>
>
>
>
>
>: Increase. <: Decrease.  Extr. Superox. a.= Extracellular Superoxide anion. PGE2 = Prostaglandine E2.. GSSG = oxidate glutathione. SOD = superoxide dismutase. CAT = catalase. GPx = glutathione peroxidase. Gr = glutathione reductase. * The diet supplementation with antioxidants counteracts the age-related changes in the oxidative stress parameters bringing them to the normal adult values.
   
   
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