immuno gerontology

ELSEVIER

Experimental Gerontology, Vol. 32, Nos. 4/5, pp. 401-413, 1997 Copyright © 1997 Elsevier Science Inc. Printed in the USA. All rights reserved

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PII S0531-5568(96)00152-0

THE AGING IMMUNE SYSTEM: PRIMARY AND SECONDARY ALTERATIONS OF IMMUNE REACTIVITY IN THE ELDERLY

GEORG WICK 1'2 and BEATRIX GRUBECK-LOEBENSTEIN 1

qnstitute for Biomedical Aging Research, Austrian Academy of Sciences, Innsbruck, Austria, and 2Institute for General and Experimental Pathology, University of Innsbruck,

Medical School, Innsbruck, Austria

AbstractmThe function of the immune system is known to decline in the elderly. The present communication will show that, similar to the situation in children, it is necessary to distinguish between primary and secondary alterations of the immune reactivity in the elderly. The primary immunological change in the elderly is the age-dependent intrinsic decline of immune responsiveness that is also observed in healthy persons, i.e., those selected according to the criteria of the SENIEUR Protocol of EURAGE (European Community Concerted Action on Aging). Secondary immunological changes are due to underlying diseases and various environmental factors, including diet, drug intake, physical activity, etc. While primary immunodeficiencies of the elderly are not, or only to a very minor extent, presently influenced by therapeutic measures, secondary alterations of the immune function offer further possibilities for corrective measures. Clinically, the consequences of impaired immune function in the elderly include increased susceptibility to infectious disease, the emergence of tumors, and increased autoimmune reactions, the latter albeit often without concomitant autoimmune disease. In fact, autoimmune diseases in generally begin to develop at younger ages, but their consequences are major factors affecting the quality of life of the elderly. Investigations of basic molecular and cellular aspects of the aging immune system are, therefore, of theoretical, clinical and socioeconomic interest. © 1997 Elsevier Science Inc.

Key Words: aging, immune system, Aizheimer's disease, atherosclerosis, vaccination

INTRODUCTION

THERE IS a general consensus, supported by experimental data and clinical observations, that the function of the immune system declines with age. Thus, on a worldwide basis, infections are still the main cause of death in the elderly, a situation that mirrors that found in childhood. In children, immunodeficiencies have long been classified as being primary or secondary, the

Correspondence to: Georg Wick, Institute for Biomedical Aging Research, Austrian Academy of Sciences, Rennweg 10, 6020 Innsbruck, Austria

401

402 G. WICK and B. GRUBECK-LOEBENSTEIN

former including genetically determined conditions such as a sex-linked hypogammaglobulin- emia Bruton, DiGeorge, and Nezelof syndromes, selective IgA deficiency, and many less severe intrinsic alterations (Buckley, 1993). While replacement of lacking immune system components and even hematopoietic stem cells have been successful in children, this option is not yet often used in the elderly for a variety of reasons, including logistic and economic considerations. On the other hand, the causes for secondary immunodeficiencies, i.e., those due to underlying diseases, inadequate living conditions, drugs, etc., can be identified and eliminated in the elderly, allowing improvement of the altered immune reactivity, Thus, the main reasons for deficient immune reactivity in the elderly of less-developed countries is still undernourishment, while the opposite, i.e., diminished blood supply to immunocompetent organs as a result of cardiovascular disease is one cause of impaired immune function in the elderly population of developed countries. Other examples of secondary immunodeficiencies are those due to immu- nosuppressive drugs, for example, steroids, suppression of the immune system by tumors, and endocrine disturbances. It is now clear that a multitude of interactions between the three major communication systems of the body--the nervous, endocrine, and immune systems--occur. It is beyond the scope of this brief review to extensively cover interactions of this kind, such as age-dependent decreased production of growth hormone or prolactin, which both exert an immunostimulatory effect (Berczi and Nagy, 1987). We will rather attempt to briefly discuss evidence for the existence of primary and secondary age-dependent alterations of immune reactivity in humans and exemplify these issues with experimental data from our own work.

PRIMARY ALTERATIONS OF IMMUNE REACTIVITY IN THE ELDERLY

General phenomena associated with declining immune function in older age

The first and most striking age-dependent alteration affecting the immune system is thymus involution after puberty. Consensus of experimental data in mice and humans is that the T cell response is most severely affected in the elderly, while B cell function is impaired to a very minor extent, if at all. Interestingly, very little information is available on possible age- dependent changes of antigen-processing and -presenting cell function, respectively, and this question certainly deserves intensified attention.

Table 1 summarizes the major changes in immune reactivity in older individuals. The task of the immune system is to guarantee the identity and integrity of the organism. It

achieves this aim by the capacity to distinguish "self ' from "nonself," high specificity and immunologic memory. All three of these characteristics are impaired in the aging immune system, as shown in the following examples.

While immune reactivity against foreign antigens drops significantly in older age, giving rise to a higher frequency of infections, autoreactivity paradoxically rises (Globerson et al., 1993). Thus, autoantibodies increase later in life, but without simultaneous clinical manifestations. This increased autoreactivity seems to be due to a decreased potential Ibr downregulation of the immune reaction (e.g., by a shift from Thl to Th2 cells with autoreactive potential) and/or the fact that the immune system also fulfills a "pitting" function, removing old and damaged autologous material. In any case, autoimmune disease does not become more frequent in the elderly because genetic factors that render target cell structures more susceptible to autoimmune attack are unaltered. Figure 1 schematically shows that old mice immunized with allogenic lymphoid cells not only give a lower reaction against these alloantigens compared to histocom- patible young controls, but a less specific immune reaction manifested in crossreactions with unrelated third-party cells and even autologous major histocompatibility complex (MHC)

T H E A G I N G I M M U N E S Y S T E M

T A B L E 1. H A L L M A R K S O F I M M U N O S E N E S C E N C E

1. Lymphocyte depletion and hypoplasia 2. Memory 3. Signal transduction in T cells 4. Cellular immune reaction

• CD4 ~, • T cell inducers of suppression ,I,

(oral tolerance less effective in elderly) • T cell mitogen response • T cell antigen response • IL-2 production • IL-3 production

5. Humoral immune reaction • Hyper ~/-globulinemia • Autoantibodies • B cell mitogen response --*

6. APC Number --* Function?

403

antigens. An example for the loss of specificity including the emergence of autoreactivity, is given in Fig. 2. This figure shows data circumstantially obtained in a control experiment from studies designed to analyze age-related effects on the fine specificity of cytotoxic T cells. Spleen cells of individual young (three months, Fig. 2a) and old (24 months, Fig. 2b) male (C3Heb/ FeJ × C57B1/6J)F 1 (H-2 k/b) mice were stimulated in vitro for five days with irradiated syngeneic control cells. Each responder was tested at different cell doses (1 × 102-5 × 105 per culture with a constant number of stimulators, 15 × 105 cells per culture). After stimulation the cultures were split and cytotoxic T lymphocyte (CTL) activities measured in a 51Cr release assay against the relevant (R = in this case, the syngeneic cell type used for immunization) and nonrelevant (NR = third-party allogeneic) target cells (2 × 104 cells per culture). While aged responder cells showed far lower cytotoxic responses to allogeneic stimulators (data not shown) there was a consistent slight, but significant, cytotoxic activity towards syngeneic targets. No such effect was seen with cells from young donors (Wick et al., 1989).

- Decrease of immune response and loss of specificity during aging - a a

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404 G. WICK and B. GRUBECK LOEBENSTEIN

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FIG. 2. In vitro induction of "autoimmune" cytotoxic cells in old (b), but not young (a) mice. R, cpm = Cytotoxic T cell (CTL) response to syngeneic (H-2 km relevant) targets; NR, cpm = CTL response to allogeneic (H-2 q = nonrelevant allogeneic) targets; for details see text. (Reproduced with permission from Wick et al., 1989.)

Loss of memory is reflected by the difficulties in obtaining satisfactory protective immune responses after infection or vaccination of elderly people, a clinically relevant problem that will be discussed below.

Finally, factors derived from the other communication systems, such as the modulatory effect of neuroendocrine hormones, may also contribute to the intrinsic decline in immune function. A good example of such defects are increased baseline levels of glucocorticoid hormones in the elderly, stress hormones known to suppress immune function (Sapolsky et al., 1984; Wick et al., 1993). This increased baseline glucocorticoid concentration deprives the elderly of a certain buffering capacity when the immune system is modulated by the immunoendocrine interaction via the hypothalamo-pituitary-adrenal (HPA) axis (Sapolsky et al., 1986, Wick et al., 1992).

The SENIEU R protocol

To understand the changes occurring in the aging immune system, attempts have been made to clearly differentiate primary from secondary processes. A working party of EURAGE has

THE AGING IMMUNE SYSTEM 405

developed the so-called SENIEUR protocol, which contains criteria for the identification of "healthy" old and young control probands for immunogerontological studies of intrinsic, nondisease-related changes (Ligthart et al., 1984; Reibnegger et al., 1988). Selection according to SENIEUR criteria provides information about life circumstances, laboratory values, and drug intake. For example, steroid hormones as treatment of rheumatoid diseases in the elderly or as contraceptives in young women would exclude them from study. In our laboratory, investigations on immunoreactivity in "Healthy Aging" were always performed on the basis of the SENIEUR protocol, which we have extended by including important parameters missing in the original version, such as laboratory lipid values (Traill et al., 1985; Reibnegger et al., 1988). In our geographical area, only 30% of male and female volunteers of all ages who consider themselves healthy fulfilled SENIEUR criteria. Using this protocol, we performed cross- sectional studies of immune function in normal, healthy elderly over the past decade by establishing cell and serum banks frozen in liquid nitrogen. Our main interest focussed on elucidating the possible role of an altered lipid metabolism in age-dependent changes of immune reactivity (Traill et al., 1990; Wick et al., 1991). In addition to providing data relating to this research topic, the project also provided several interesting methodological results that have proven useful beyond immunogerontology in that we established age-related normal values for a large number of laboratory parameters for an Austrian population (Stulnig et al., 1993). Furthermore, we introduced the fluorescent-activated cell sorter (FACS) as a useful tool for the determination and characterization of lipoprotein receptors using fluorescently labeled ligands (Jiirgens et al., 1989). FACS was also used for single cell assessment of plasma membrane viscosity by fluorescence depolarization (Brck et al., 1989). Finally, the availability of the above-mentioned cell banks allowed for a comparative study of intrinsic age-dependent changes of mononuclear cell subpopulations in healthy old people (Stulnig et al., 1995a). Thus, applying the SENIEUR protocol was not only helpful in addressing our specific scientific concerns, but also gave us a good idea about primary immunodeficiencies in the elderly and provided a good basis for later studies on secondary immunologic alterations.

Proportional changes in peripheral blood mononuclear cell composition in the elderly

Various lymphocyte subpopulations change during aging, but age-related reference limits from healthy reference probands are scarce. In a well-controlled study on SENIEUR protocol- compatible healthy young (20-32 years) and elderly (65-74 years) subjects, we determined commonly analyzed lymphocyte subpopulations (Stulnig et al., 1995a). The most striking age-related changes included increased HLA-DR+ T lymphocytes and a shift in expression of CD45 isoforms from the CD45RA+ CD45RO- to the CD45RA- CD45RO+ subset. Both alterations occurred in both the CD4+ and CD8+ subpopulations and most were present in the relative and absolute number of lymphocyte subsets. In contrast to other studies (Goto and Nishioka, 1989; Utusyama et al., 1992), we found no age-dependent alterations of CD16+ NK cell values, a finding attributable to our rigid selection criteria for blood donors.

T cell reactivity in the elderly

T cells from aged individuals are impaired in their response to mitogens such as phytohemagglutinin (PHA), Concanavalin A (Con A), and the anti-CD3 monoclonal antibody OKT3 (Song et al., 1993). Reduced proliferation in response to stimulation is associated with diminished production of interleukin-2 (IL-2) and decreased responsiveness to growth factors (Nagel et al., 1988; Schwab et al., 1989). The latter quality appears to be due, at least partly, to failure

406 G. WICK and B. GRUBECK-LOEBENSTEIN

of a large proportion of activated T cells from aged individuals to express IL-2 receptors (Miller, 1994), a defect that seems to result from reduced signalling, possibly due to age-dependent plasma membrane alterations (Huber et al., 1991). Thus, it was postulated that changes in lipid composition of the membrane might affect ligand binding to their receptors as well as receptor crosslinking processes. As a consequence, aged T cells would no longer be able to respond to activation signals transmitted through the binding of antigen/MHC complexes to the T cell receptor. Decreased expression of costimulatory molecules, such as CD28 (Effros et aL, 1994), changes in cytoskeletal structure (Rao et al., 1992), and retarded/decreased transcription of various mRNAs (Nagel et al., 1988; Gamble et al., 1990; Wu et al., 1986) may provide additional explanations for decreased T cell reactivity in the elderly.

Clonal senescence o f t cells

The final consequence of T cell activation is the entry of the cell into the cell cycle. The expression of proto-oncogenes such as c-myc and c-myb genes, as well as the production of IL-2 and its corresponding receptor, are prerequesites for this process. Decreased expression of these parameters may inhibit cell progression from GO to G1, and from GI to S, and result in the occurrence of nonresponsive lymphocytes, as described above (Pieri et al., 1992; Perillo et al., 1993). T cells capable of entering the cell cycle start replicating upon stimulation until a substantial population of one specificity is generated. This process is called clonal expansion. Due to the limited replicative capacity of nontransformed cells, proliferation ceases after a certain number of population doublings (Perillo et al., 1989). The period of slow growth preceding the final elimination of a T cell population by apoptosis has been named clonal senescence. At this stage, T cells characteristically decrease their expression of CD25 (Grubeck- Loebenstein et al., 1994) as well as the concentration of the apoptosis-suppressing molecule, bcl-2 (Salmon et al., 1994). At the same time, they fail to downregulate CD95 (fas, APO-1), an important apoptosis-inducing molecule, at the end of their cell cycle (Lechner et al., 1996). Clonal senescence has been shown to occur earlier, after a lower number of population doublings in T cell lines and clones from old individuals (McCarron et al., 1987; Walford et al., 1987; Grubeck-Loebenstein et al., 1994), suggesting that T cells have a reduced expansion potential in old age. Failure to fight long-term infections as well as shortened immunological memory could be the consequence (Ahmed and Gray, 1996). The latter assumption is supported by the finding that vaccination has a shorter lasting effect in aged individuals (Ruben et al., 1978).

Age-dependent changes o f antigen processing and presentation

The functional capacity of antigen presenting cells in old age is still a matter of debate. Despite a vast amount of literature on the decline of T cell function, relatively few and controversial studies have been published on antigen-presenting cells (APC). Peritoneal macrophages from aged mice have been reported by some groups to have reduced antigen- presenting capacity (De la Fuenta, 1985; Vetvicka et al., 1985), but were described as unimpaired by others (Perkins et al., 1982; Komatsubara et al., 1986). Decreased density and functional activity of Langerhans cells has been observed in aged mouse skin (Sprecher et al., 1990). Peripheral blood monocytes from aged humans have been investigated and their function has mainly been reported to be normal (Schwab et al., 1985; Fagiolo et al., 1993), although a reduced activity has also been suggested (Beckman et aI., 1990; Rich et al., 1993). In a recent study from our laboratory, we demonstrated that functionally intact dendritic cells could be


derived from the peripheral blood of aged individuals (Steger et al., 1996a). These cells, which were numerous and relatively resistant to injury, had an unimpaired capacity to present antigen to T cells and could even reinduce proliferation in senescent T cell clones otherwise destined to perish. Dendritic cells may thus protect T cells from clonal elimination and provide a useful tool for immunotherapy in the aged.

The role of lipids in altered immune responsiveness of the elderly

All cells, including those of the immune system, need cholesterol for proper function and survival. Specifically, intraceUular and plasma membranes consist of a lipid bilayer rich in cholesterol. Various protein molecules, such as receptors, are anchored in the plasma membrane, the function of which has also been shown to depend on the microviscosity of the surrounding lipid mosaic domains. The molar ratio of free cholesterol/phospholipids (C/P1) is considered to be one of the most important factors influencing membrane microviscosity. Because surface- exposed protein molecules must be mobile to fulfill their function, the rigidity or fluidity, respectively, of the intracellular and plasma membranes are also crucial for cells of the immune system (Rivnay et al., 1980).

It has been shown previously that mitogen response inversely correlates to overall plasma membrane viscosity as determined by fluorescence depolarization in total cell suspensions assessed in cuvettes (Lyte and Shinitzky, 1989). Because monocytes are the main contributors to the viscosity values served in such bulk determinations, we adapted our single-cell techniques to reassess the possible correlation between membrane viscosity and function of purified lymphoid cells. Under these conditions, a small but significant increase of membrane viscosity correlated with decreased response to T, but not B cell, mitogens (Brck et al., 1989). Increased membrane viscosity was associated with an increased C/P1 and "fluidization" of the plasma membrane by removal of cholesterol ameliorated mitogen responsiveness that could be reversed by reintroduction of cholesterol and subsequent increase of the C/P1 ratio (Wick et al., 1991). In addition, several less pronounced changes affecting the lipid composition of cell membranes, including shifts in free fatty acid content, etc., were demonstrated (Huber et al., 1991). The therapeutic relevance of these in vitro studies to the in vivo situation, such as a possible immunostimulatory effect of appropriate dietary regimens and/or lipid lowering drugs, respectively, remains to be determined.

The lipid metabolism of each nucleated cell is subject to finely tuned homeostatic regulation based on a delicate balance between intracellular cholesterol production and uptake from the environment in the form of so-called low-density lipoproteins (LDL). The rate-limiting enzyme for intercellular cholesterol biosynthesis is 3'hydroxy 3'methylglutaryl-coenzymeA-reductase (HMG-CoA-R), the activity of which is upregulated under conditions of low environmental LDL values and downregulated when high amounts of cholesterol are taken up via the cell surface. Each cell possesses LDL surface receptors (LDL-R) that bind and internalize native LDL as an external cholesterol source. Some cells, such as macrophages and vascular smooth muscle cells, have an additional "scavenger" LDL-R that can bind chemically modified (e.g., oxidized) LDL and, in contrast to native LDL-R, is not downregulated by high concentrations of the ligand. We analyzed lymphoid cells from SENIEUR-compatible probands and demonstrated a paradoxically increased expression of LDL-R on the surface of lymphoid cells from SENIEUR compatible elderly donors compared to young controls (Traill et al., 1987). This increase, demonstrable at both the mRNA and protein levels, was unexpected, because these

4 0 8 G, WICK and B. GRUBECK-LOEBENSTE1N

elderly persons displayed the known age-dependent increase of serum LDL that should lead to LDL-R downregulation.

Thus, in essence, we demonstrated that the known decreased response to T cell mitogens in the elderly correlated with increased plasma membrane viscosity and increased serum LDL levels, and was associated with a paradoxically increased density of LDL-R. Subsequent studies using our cell banks revealed that the increased LDL-R expression was not due to a higher concentration of chemically modified (oxidized) LDL in the sera of these blood donors, and also not due to a quantitative or qualitative deficiency of HMG-CoA-R in the cytoplasm. We were able to show that achieving the same degree of downregulation of LDL-R required incubating lymphocytes from healthy old donors with significantly higher LDL concentrations compared to young controls (Stulnig et al., 1995b). Interestingly, HDL cholesterol also led to a downregulation of LDL-R mRNA expression. Thus, it could be speculated that the known protective effect of HDL in development of atherosclerosis is not only due to a reverse cholesterol transport from the cell, but also to an decreased LDL-R expression. This abnormal downregulation of LDL-R by lymphocytes in the elderly was confirmed on the mRNA level by quantitative reverse transcription polymerase chain reaction (RT-PCR) (Stulnig et al., 1995b). Therefore, we concluded that the lowered mitogen response of lymphocytes from healthy elderly persons is partly due to a disturbed regulation of LDL-R expression altering the homeostasis of the cellular lipid metabolism and leading to increased plasma membrane viscosity and thus malfunctioning of surface receptors.

SECONDARY ALTERATIONS OF IMMUNE RESPONSIVENESS IN THE ELDERLY

Possible role o f an immune reaction in the development o f Alzheimer's disease

Several lines of evidence have led to the hypothesis that inflammatory reactions play a role in the pathogenesis of Alzheimer's disease (Vandenabele and Fiers, 1991). It has been demonstrated that [3-amyloid deposits are closely associated with various complement components such as C 1 q, C3, C4d, C5, and with activated microglial cells in which production of TNFo~ and IL-I is elevated (Griffin et al., 1989). TNFo~ has been shown to mediate neurotoxicity (Meda et al., 1995), while IL-1 stimulates production of the Alzheimer amyloid precursor protein (APP) and induces the generation of its amyloidogenic metabolites (Vasilakos et al., 1994). These and related findings led to the belief that inflammatory factors might be involved in the development of plaques and the propagation of the pathology in the surrounding tissue. A new therapeutic approach has, therefore, been launched in which patients with Alzheimer's disease are being treated with antiinflammatory agents and the results of these clinical trials are awaited with great interest.

However, there are also indications that the immune system may play a protective role against the development of Alzheimer's disease. Microglial cells have been shown to degrade [3- amyloid even when the peptide is in its immobilized, fibrillar form (Shaffer et al., 1995). Recent evidence from our laboratory additionally suggests that soluble [3-amyloid can be presented to T lymphocytes and induces IL-2 receptor expression and proliferation (Trieb et al., 1996). It is of interest that this reaction was more pronounced in old vs. young, SENIEUR protocol- compatible healthy individuals. However, lymphocytes from the majority of patients with Alzheimer's disease did not proliferate when stimulated with APP peptides, while their proliferative response to anti-CD3 was unimpaired. This lack of proliferative responsiveness might reflect T cell anergy, because it was accompanied by unimpaired IL-2 receptor expression, Our results suggest that autoreactive T cells may be relevant to the elimination of these potentially


amyloidogenic APP metabolites. Interferon-~/(IFN~) secretion by activated T cells could also be beneficial, because IFN~/has recently been shown to inhibit the production and metabolism of APP (Schmitt et al., 1996). Thus, it seems reasonable to think that the immune system represents a natural line of defense against the accumulation of dangerous amyloidogenic substances. This mechanism could be of particular importance in old age when the production of amyloid is highest (Vangool et al., 1994). Impairment of this immunological defense mechanisms may be a permissive agent in the development of Alzheimer's disease.

Atherosclerosis as an autoimmune disease

Atherosclerosis is a multifactorial disease for which numerous risk factors have been established, including genetic and epigenetic components. Specifically, three main hypotheses for the development of atherosclerosis have been formulated: "response-to-injury," "altered lipoprotein," and "oligoclonal smooth muscle proliferation." Ample clinical and experimental support- ing evidence has been provided for each of these theories. But little attention has been paid to the fact that signs of inflammation can be observed very early in the disease process. When this fact has been mentioned, it was considered to play a secondary rather than primary role.

Based on clinical and pathohistological evidence and appropriate animal experiments, our group established a new "immunologic concept" for the pathogenesis of atherosclerosis during the past few years that postulates an autoimmune reaction against stress proteins (heat shock proteins--HSP) as the initiating event. Specifically, we have shown that (IL-2 receptor positive, MHC class II antigen positive) activated T cells are the first cells found in the arterial intima at predilection sites of atherosclerosis, i.e., those areas subjected to major haemodynamic stress, such as the aortic arch or the branching sites of larger vessels. In fact, investigations in very young, healthy children provided evidence that resident lymphocytes can be found at those sites prior to any signs of atherosclerosis or even fatty streaks that are the lesions preceding development of atherosclerotic plaques. For these intravascular accumulations of lymphoid cells, we have coined the term vascular-associated lymphoid tissue (VALT), analogous to the mucosa-associated lymphoid tissue (MALT) (Wick et al., 1995). VALT may have a monitoring function similar to MALT (for review, see Ogra et al., 1994) for potentially dangerous antigenic material contained in the blood.

Phenotypic investigations of the lymphoid cells emerging in early atherosclerotic lesions showed that the majority expressed the T cell receptor ct/13 (TCR ct/13), but a surprisingly high proportion (over 10%) was TCR ~/~ positive, again paralleling the situation in the MALT. Functional analyses of intralesional T cell lines showed a preferential reactivity of these cells with HSP 60. HSPs constitute a phylogenetically highly conserved group of proteins and cognates thereof that play important biological roles in protein folding and transport. Some of them also have a protective function (chaperonines), preventing the denaturation of other intracellular proteins after stress, such as heat, mechanical pertubation, toxins, cytokines, etc. (for review, see Van Eden and Young, 1995). Because human HSP 60 shows an over 50% amino acid homology with its bacterial homologue, for example, mycobacterial HSP 65, a protective immune reaction against the latter often must be "paid for" by an autoimmune crossreaction against the former. Immunization of normocholesterolemic rabbits with complete Freund's adjuvant alone (containing heat killed mycobacteria) results in the development of atrteriosclerotic inflammatory lesions at the characteristic predilection sites, albeit without the occurrence of foam cells. Additional feeding of these immunized rabbits with a diet high in

410 G. WICK and B. GRUBECK LOEBENSTEIN

cholesterol leads to the development of atherosclerotic lesions closely paralleling those in humans, i.e., including foam cells.

With respect to humoral immunity, we found significantly increased titers of antibodies to HSP 65/60 in clinically healthy volunteers with sonographically demonstrable atherosclerotic lesions in their carotid arteries (Xu e t al. , 1993). These antibodies are not only of diagnostic value, but may also play a pathogenetic role because they are able to lyse stressed, but not unstressed, endothelial cells in a complement-mediated fashion or by antibody-dependent cytotoxicitiy (ADCC) (Schett et al. , 1995).

Atherosclerosis thus may be a paradigm lor an immunological ly mediated disease that develops early in life, the sequelae of which, however, become increasingly evident with age.

VACCINATION IN OLDER AGE

Aged inividuals are susceptible to infectious disesases for a number of different reasons, in particular their decline in T cell immunity. Infectious diseases can be prevented or ameliorated by appropriate vaccination. In the aged, vaccinations against pneumococcal pneumonia, influenza, and tetanus are of special importance (Begg, 1994). Elderly subjects may, however, have an impaired response to primary as well as secondary immunization (Stein, 1994). Cohort studies demonstrated a decline in the immune response with age as measured by peak antibody titers. Decreased specific T cell activity was also reported (Miller, 1980; Huang e t al . , 1992). Due to the early involution of the thymus, this problem may be particularly relevant when aged individuals are immunized with new antigens such as Yellow fever virus, which they have not come in contact with before. Determinations of antibody titers should be made before an elderly person is considered to be satisfactorily protected against such pathogens. Antibody measure- ments also seem to be indicated to assess the duration of protection after vaccination. A recent study demonstrates that 40% of a population of SENIEUR-compatible Austrians were not protected against Tetanus (Steger et al. , 1996b). Fifty percent of these individuals had been vaccinated within the last 10, 25% within the last five years. This and similar examples support the notion that general vaccination strategies cannot be uncritically applied in the elderly.

Acknowledgments--The original work of the authors was supported by grants from the Austrian Science Fund (G.W., Project No. 10677, and B.G.-L., Project No. 9296), the Sandoz Research Foundation (G.W.), a grant from the State of Tyrol (G.W.), and the Austrian National Bank (B.G.-L., project No. 5568).

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