Decline of immune responsiveness: A pathogenetic factor in Alzheimer’s disease?

Journal of Psychiatric Research 39 (2005) 535–543

JOURNAL OF PSYCHIATRIC RESEARCH www.elsevier.com/locate/jpsychires

Decline of immune responsiveness: A pathogenetic factor in AlzheimerÕs disease? Elke Richartz a,*, Elke Stransky a, Anil Batra a, Perikles Simon b, Piotr Lewczuk c, Gerhard Buchkremer a, Mathias Bartels a, Klaus Schott a a

Department of Psychiatry and Psychotherapy, University of Tuebingen, Osianderstr. 24, D-72076 Tuebingen, Germany b Department of Pathology, University of Tuebingen, Calwer Str. 7, D-72076 Tuebingen, Germany c Department of Psychiatry and Psychotherapy, University of Erlangen-Nuremberg, Erlangen, Germany Received 21 September 2004; received in revised form 10 December 2004; accepted 20 December 2004

Abstract The involvement of immunological alterations in the pathogenesis of AlzheimerÕs disease (AD) is widely discussed. Hitherto, findings on systemic immunological alterations are inconsistent. We measured the concentrations of the pro-inflammatory cytokines IL1b, IL-2, IL-6, and TNF-a, and of the soluble receptors sIL-2r, sIL-6r, and sTNF-ar, in cerebrospinal fluid (CSF) and serum of 20 Alzheimer patients and 21 controls. Moreover, we studied levels of the pro-inflammatory IL-6, Il-12, IFN-c, and TNF-a, and of the anti-inflammatory IL-5 and IL-13 in stimulated blood cell cultures from 27 AD patients and 25 controls. The levels in CSF and serum were diminished in AD or under detection limit. In mitogen-stimulated blood cultures we found a significant decrease of pro- and anti-inflammatory cytokines in the AD group. Our data suggest a general decline of immune responsiveness in AD. Based on the recent research, an impaired immune response may be considered as a pathogenetically relevant factor in AD. With respect to the putative role of ageing in AD, we assume a premature immunosenescence contributing to the AlzheimerÕs pathology. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: AlzheimerÕs disease; Peripheral cytokine release; Impaired immune function; Immunosenescence

1. Introduction Among various factors involved in the pathogenesis of AlzheimerÕs disease (AD), a large body of evidence supports the hypothesis of a direct contribution of the inflammatory response to the neurodegeneration associated with AD. The putative relevance of inflammatory processes is shown by over 20 epidemiological studies suggesting a potential benefit of anti-inflammatory intervention (Akiyama et al., 2000; McGeer and McGeer, 1999). Further indication of a pathophysiological role of inflammation in AD is given by the presence *

Corresponding author. Tel.: +49 7071 2983442; fax: +49 7071 294141. E-mail address: [email protected] (E. Richartz). 0022-3956/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpsychires.2004.12.005

of inflammatory mediators in AD brains, including proinflammatory cytokines, acute phase proteins and the full complement cascade (Mrak et al., 1995; Hu¨ll et al., 1996; Tarkowski et al., 1999). In summary, data available suggest that the AD brain undergoes chronic inflammatory process mediated by activated glial cells, targeted on the destruction of senile plaques, but lethal to surrounding neurons (McGeer and McGeer, 2003). The particular roles of the cytokines found in the AD brain are virtually unknown, for they exert protective as well as detrimental effects. As to their origin, it seemed reasonable to postulate a link between the cytokine profile in the blood stream and that in the brain, because there is an active and highly regulated communication between the brain and the immune system (Huberman et al., 1994). On this background, several

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studies on inflammatory markers in serum and CSF in AD patients have been carried out, in attempt to find a premortem diagnostic marker for AD. First, it seemed consequent that the local inflammatory processes would be associated with systemic inflammatory signs. However, data remained inconsistent and, hitherto, do not allow drawing definite conclusions. Guided by cerebral findings, numerous studies focused on the peripheral secretion of proinflammatory cytokines. In CSF, increased levels of proinflammatory cytokines (BlumDegen et al., 1995; Bagli et al., 2003), unchanged levels (Ma¨rz et al., 1997; Lanzrein et al., 1998; Tarkowski et al., 1999) and decreased levels (Singh, 1994; Yamada et al., 1995) have been found in AD. Of similar inconsistence are the findings in serum: Some working groups report elevated levels of proinflammatory cytokines (Kalman et al., 1997; Singh and Guthikonda, 1997; Lombardi et al., 1999; Licastro et al., 2000), other do not see any changes (Esumi et al., 1991; Androsova et al., 1995; Lanzrein et al., 1998), while several find a decrease of proinflammatory cytokine secretion (Cacabelos et al., 1994; De Luigi et al., 2001; Paganelli et al., 2002; Sala et al., 2004). These discrepancies have mostly been attributed to technically different approaches and to different criteria to choose patient groups as well as control groups. Moreover, most of the studies report very low cytokines levels nearby their detection limit, so that statistical evaluation is restricted. However, within the confusing variety of systemic findings it is becoming increasingly substantiated that AD patients exhibit systemic immunological alterations, which do not just reflect the inflammatory processes in the brain. It has been stated that the neuroinflammatory events found in the brain and CSF of AD patients seem to be limited to the CNS without direct association of a peripheral inflammation (Blum-Degen et al., 1995). Own studies were carried out on the hypothesis, that AD patients display systemic immunological alterations in terms of a dysregulation or impairment of the immune response, which do not only reflect an epiphenomenon, but may causally be related to the AlzheimerÕs pathology. On the assumption that various immune functions, not only of the proinflammatory response, are hampered in AD, we investigated the cytokine secretion of TH 1 cells, TH 2 cells, as well as of the macrophage/moncyte system. In a preliminary study, we measured the concentrations of the proinflammatory cytokines IL-1b, IL-2, IL-6, and TNF-a, as well as of the soluble receptors sIL-2r, sIL-6r, and sTNF-ar in cerebrospinal fluid (CSF) and in serum of Alzheimer patients and controls. With respect to the low concentration values, we then stimulated whole blood cell cultures with mitogens, leading to higher cytokine levels. After mitogenous stimulation, we measured the increase of cytokine levels above basal levels of the proinflammatory cytokines IL-6,

IL-12, IFN-c and TNF-a, and of the anti-inflammatory cytokines IL-5 and IL-13.

2. Material and methods 2.1. Patients and controls Recruitment of AD patients was done at the University Clinic for Psychiatry Tuebingen. The diagnosis of probable AD was performed according to the NINCDS-ADRDA criteria (McKhann et al., 1984). Control subjects for CSF and serum investigations were chosen from the Department of Neurology, Goettingen. Lumbar punction was carried out either in patients with questionable disc prolapse, who underwent radiological examination with contrast medium, or in patients suspected of having an inflammatory or other CNS disease. Their CSF status was normal as regards cell count, albumin and IgG, as were all measured serum parameters. Any organic CNS disease was excluded in all of these persons. For cell cultures, control blood was gained by healthy aged persons, who were recruited through advertisement in the local press. A comprehensive somatic, psychiatric, and sociodemographic history was taken of all persons. All subjects underwent thorough psychiatric and neurological examination including EEG and neuroimaging (CT or NMR). Cognitive decline was measured by the MiniMental-State Test (MMST, Folstein et al., 1974). Total blood count and blood chemistry including C reactive protein, thyroid function, vitamin B12, Folic acid, Borrelia and Lues serology was evaluated. Patients with a psychiatric, neurological, inflammatory or infectious disease or with a history of immunological or malignant disease were excluded, as well as persons with abnormal white blood cell count, C reactive protein or signs of malnutrition. Further exclusion criteria were the intake of immunologically relevant or psychotropic drugs and a positive family history for dementia. All control subjects underwent the same clinical examinations including MMST and laboratory tests as the AD patients. The same exclusion criteria were applied. MMST of controls had to be normal. In vivo concentrations of cytokines and soluble receptors in CSF and serum were determined in twenty patients with probable AD (16 female and 4 male, 60–88 years, median 72 years). The MMST score was in the range of 10–23, with a median of 16. As controls, we investigated CSF and serum samples from 21 subjects (7 female, 14 male, 59–82 years, median 68 years). For studying cytokine production in stimulated blood cell cultures, further twenty-seven patients, 18 of them females, 9 males, with probable AD and 23 healthy aged volunteers, 16 females and 7 males, were included. The median age of the Alzheimer patients was 70 years

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(63–84 years), of the control persons 68 years (59–77 years). The MMSE score ranged between 11 and 21 in the patient group (median: 17.3). Mean of AD duration was 2.5 years (1.5–3.4 years). The groups for native and stimulated cytokine investigations were comparable with respect to age and disease duration. The investigation was carried out in accordance with the Declaration of Helsinki. Written informed consent was given from all subjects or their relatives following full explanation of the procedure. The study was carried out after approval by the local ethics committee.

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Table 1 Cytokine concentrations in CSF and serum (pg/ml) CSF

IL-1b IL-2 sIL-2r IL-6 sIL-6r TNF-a sTNF-ar

Serum

AD

Controls

AD

Controls

19.6 (2.0) – 47.6 (1.85) 4.6 (0.48) 575 (38.70) 14.0 (0.37)* 681 (33.78)

23.3 (2.1) – 55.6 (2.97) 10.6 (4.44) 767 (22.43) 19.3 (0.43) 667 (34.04)

– – 421 (35.57) 4.7 (2.4)* 21.03 (1.89) – 1.527 (1.88)

– – 447 (37.55) 16.1 (3.04) 24.08 (1.36) – 1.94 (0.27)

Mean and standard error of the mean (SEM) (in parentheses); *, p < 0.005 (Bonferroni adjustment); ‘‘–’’, levels under detection limit.

2.2. Methods Samples were collected at routine venipuncture between 8:00 and 9:00 am in order to take in account the circadian rhythm. For in vivo cytokine measurement, blood samples were centrifuged and the serum frozen at 20 °C until analysis. CSF was obtained by lumbar punction, centrifuged and frozen at 20 °C until analysis. For blood cell stimulation, whole blood samples were cultured following the Lu¨beck protocol (Kirchner et al., 1982). Peripheral blood cells were stimulated with LPS and PHA, for 48 and 96 h, respectively. After centrifugation supernatants were stored at 80 °C until measurement. Cytokine concentrations were determined using commercially available ELISA kits (IL-1b, IL-6, TNF-a, IFN-c, sIL-2r: Milenia, Bad Nauheim, Germany; IL-5, IL-12, IL-13, sIL-6r, sTNFar: R&D Systems, Wiesbaden, Germany). Based on preliminary experiments, for each cytokine the time of stimulation was chosen according to the time of maximal induction. IL-5, IL-6, IL-13, and TNF-a were measured after 48 h of stimulation, IL-12 after 72 h, IFN-c after 96 h of stimulation. For statistical analysis, the differences between the patients and control groups were analyzed by Wilcoxon rank sum test and v2 test. The Bonferroni adjustment for multiple comparisons was applied.

was no effect of gender (Kendall tau b correlation) and age (Pearson correlation). The diminished levels were not correlated with disease duration (MMST values) or severity. 3.2. Production of cytokines in stimulated blood cell cultures We determined the ability of blood cells to produce the pro-inflammatory cytokines IL-6, IL-12, TNF-a and IFN-c, and the T-helper (TH)-2-cell derived antiinflammatory cytokines IL-5 and IL-13. As illustrated in Figs. 1 and 2, the AD group shows reduced levels of all cytokines after mitogen-induced whole blood stimulation in comparison with the control group. On account of Bonferroni adjustment (6 investigations), a p-value of less than 0.008 was considered statistically significant. Thus, a high significance was shown for the decrease of IL-6 (p < 0.001), IFN-c (p < 0.0002), TNFa (p < 0.0005) and of IL-5 (p < 0.001). IL-12 was decreased with p < 0.019, IL-13 with p < 0.023. The results remained significant also after stepwise regression control to exclude the possible influence of age and sex. No correlation was found between the cytokine levels and duration of disease or severity of disease, respectively.

3. Results 4. Discussion 3.1. In vivo concentrations of cytokines and soluble receptors in CSF and serum The data of this study are compiled in Table 1. The concentration of IL-2 in CSF as well as serum levels of IL-1b, IL-2 and TNF-a were too low to reach detection limit. Regarding the other values, we found a decrease of all parameters in CSF and serum of the AD patients compared with the control group. Considering a p-value of less than 0.005 after Bonferroni adjustment (10 investigations), there was seen a statistically significant decrease of TNF-a in CSF (p < 0.0001) and of IL-6 in serum (p < 0.0012) of the AD patients. There

The role of the immune system in the pathogenesis of AD has widely been discussed. Since AD has no longer been regarded as a single unified condition but as a complex syndrome, it has been postulated that the presence of different clinical subgroups may imply a differential involvement of the immune system (Huberman et al., 1994; Licastro et al., 2000). 4.1. Cytokine measurement in AD Reports on peripheral cytokine secretion in AD are various and inconsistent (see p. 1–2). Obvious

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IL-12

IL-6 3

9000 8000

2.5

7000 2

pg/ml

pg/ml

6000 5000 4000

1.5 1

3000 2000

0.5 1000 0

0

AD

Ctrl

AD

IFN-y

Ctrl

TNF-α 700

250000

600 500 150000

pg/ml

pg/ml

200000

100000

400 300 200

50000

100 0

0 AD

AD

Ctrl

Ctrl

Fig. 1. Release of pro-inflammatory cytokines (in pg/ml, with SEM) in mitogen-stimulated whole blood cell cultures from AD-patients (AD) and controls (Ctrl).

IL-5

IL-13

120

140

100

120 100

pg/ml

pg/ml

80 60 40

80 60 40

20

20 0

0 AD

Ctrl

AD

Ctrl

Fig. 2. Release of anti-inflammatory cytokines (in pg/ml, with SEM) by mitogen-stimulated whole blood cell cultures from AD-patients (AD) and controls (Ctrl).

methodological differences among studies, including inclusion criteria and technical variations contribute to the great variability of data. Sample sizes show considerable differences, and patient groups differ with respect to stage of dementia, further pathological conditions and drug intake. Moreover, varying cytokine levels

may also be due to genetic polymorphisms (Bagli et al., 2003). Therefore, the measurement of a single cytokine does not allow any conclusions on disease dependent effects. Rather, an overlapping set of cytokines as presented in this study may give more information. Most importantly, cytokine production is highly depen-

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dent on the health status. Previously reported higher levels of pro-inflammatory cytokines in aged persons as well as in AD may reflect an underlying but undiagnosed disease state (Beharka et al., 2001). Against this background, we excluded each person with the slightest sign of infection or another medical disease in our study, because any comorbidity could influence the cytokine production. What is more, treatment with acetylcholinesterase inhibitors can modulate cytokine expression (Reale et al., 2004). Therefore, patients were included only before starting anti-dementive therapy. The measurement of cytokine secretion was carried out using stimulated whole blood cell cultures. Whole blood cultures resemble more closely the in vivo situation because manipulation, prestimulation, and possible selection of PBMC are minimized, and the role of plasma factors is included. We observed diminished levels of proinflammatory cytokines in CSF and serum and of the soluble receptors in the AD group compared with healthy, aged controls. In summary however, these in vivo concentrations have been shown to be very low. Critical parameters influencing cytokine levels in CSF are, e.g., the relatively large volume and the dynamics of the CSF system, the brain CSF barrier as well as the distance of the liquor system from the relevant brain regions (Ma¨rz et al., 1997). Similarly, some native cytokine concentrations in serum were near or under the detection limit. In contrast to our results, some other investigators detected higher cytokine levels (cf. p. 2). This discrepancy can be explained by undiagnosed comorbidity or intake of drugs leading to altered cytokine secretion. More important are the technical differences, particularly concerning origin, structure and sensitivity of the antibodies applied in the different ELISA kits. Findings in stimulated blood cell cultures are much more evident, since cytokine levels are markedly higher, and differences between groups are depicted more clearly. Additionally, the relative increase of cytokine levels upon stimulation reflects the functional responsiveness of the particular immune cells on inflammatory stimuli. In our study, the increase of all measured cytokines, i.e., IL-5, IL-6, IL-12, IL-13, TNF-a and IFN-c in whole blood cell cultures stimulated with mitogens, was significantly lower in AD patients than the increase of cytokine levels in the control group. The observation of an unidirectional decrease of all measured cytokines points to a general dysfunction of the cellular immune response to stimulating agents. The main source of IL6, IL-12, TNF-a and IFN-c is the monocyte/macrophage system. Moreover, IFN-c, and to a lower degree TNF-a, are also expressed by TH-1 cells. TH-1cells play a central role in the activation of the monocyte system. Besides, they induce B cells to produce opsonizing antibodies. Opsonizing, again, promotes phagocytosis. Thus, a diminished production of these cytokines may

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be associated with an impaired phagocytic activity. As phagocytosis is essential for the removal of foreign bodies, debris and dysfunctional proteins, impairment can lead to the accumulation of amyloid proteins as is the case in a number of local and systemic amyloid diseases (Linke, 1996). In contrast, IL-5 and IL-13 derive from TH-2 cells and act as anti-inflammatory immune mediators. Interestingly, in our research their expression was found to be significantly decreased as well. All in all, we see a generally blunted secretory response of immune cells to activating stimuli in AD. This observation is in contrast to the protective effect of anti-inflammatory drugs when taken over a long period before the onset of AD, as seen in several epidemiological studies. However, up to now a therapeutic effect of anti-inflammatory substances has not been confirmed in prospective clinical studies. Furthermore, the histopathological evidence of proinflammatory molecules in the diseased brain is not necessarily in contrast to the assumption of an underlying general immune depression. A decline of phagocytic activity may constitute an early event in the pathogenetic chain. The local overproduction of inflammatory markers has been attributed to a secondary reaction to the accumulating amyloid burden (McGeer and McGeer, 2003) obviously overtaxing the phagocytic capacities of the AD brain. Finally, the mechanism of the anti-inflammatory drug effect in AD has not yet been clarified. Possibly, they do not act via inhibition of the prostaglandin synthesis, but through reduction of the amyloid burden (Cirrito and Holtzman, 2003). 4.2. Consistent findings indicating an immune dysfunction in AD Several studies point to an impairment of the immune system in AD. A decreased production of TNF-a in mild stages of AD has been interpreted as a sign of defective immune functions (Huberman et al., 1994). Phytohemagglutinin (PHA)-stimulated proliferation and IL-2 production of nonadherent monocytes in AD patients has been shown to be significantly reduced (Fujiwara, 1996). The lack of proliferative responsiveness to APP peptides in AD led to the assumption of a ‘‘T cell anergy’’ in AD (Trieb et al., 1996). A generally decreased in vitro T-cell-activation to a number of stimuli in AD has been described, and an increase of acute reactants is interpreted as a compensatory reaction to in vivo functional alterations of leukocytes (Dickson et al., 1996). Other studies have shown imbalances of cellular immunity and immunoregulatory T cells and a reduced T cell response to various antigenic determinants suggesting a defect of the T-cell mediated immunity in AD (Giubilei et al., 2003; Streit, 2001). Accordingly, a decrease of proliferative activity of AD lymphocytes has been reported, subsequently resulting in the

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impairment of immune functions in AD (Zhang et al., 2003). These functional defects have been attributed to oxidative damage of DNA in lymphocytes from AD patients (Mecocci et al., 1998) and an altered calcium response of peripheral T lymphocytes in AD (Sulger et al., 1999). Most interestingly, an accelerated telomere shortening in lymphocytes has been found as an underlying cause of the impaired lymphocyte function in AD (Panossian et al., 2003; Zhang et al., 2003). 4.3. Putative causal connection of immune dysfunction and AD The question of apathogenetic role of the immune dysfunction in AD is matter of the ongoing discussion. One hypothesis suggests that a peripheral immune impairment is an epiphenomenon, secondary to the central immune activation seen in AD. Via the hypothalamic pituitary axis the cerebral inflammation can lead to an increased production of cortisol, resulting in a peripheral immunodepression (Woiciechowsky et al., 1999). Indeed, a mild hypercortisolemia has been seen in AD patients (Hartmann et al., 1997). On the other hand, a causal role of an underlying general impairment of the immune response in AD seems conceivable with respect to three major aspects: 4.3.1. Microglial dysfunction in AD The role of immunological and inflammatory processes in the pathogenesis of AD is widely understood in terms of ‘‘bystander damage hypothesis’’ (Streit, 2002). Accordingly, the neurodegeneration in AD is result of the bystander damage by autoaggressive microglial cells that produce neurotoxins in response to continued A b exposure (Akiyama et al., 2000; McGeer and McGeer, 2001). However, the primary function of microglia is to support neuronal survival and regenerative processes including phagocytosis (Rogers et al., 2002; Streit, 2002). The role of microglia in the degradation and clearance of cell debris as well as of amyloid proteins is meanwhile well established (Popovic et al., 1998; Streit, 2001). Microglia are descended from the same stem cells as monocytes and have been observed to undergo similar functional impairment in AD as assumed for the peripheral monocytes of AD patients (Streit, 2001; Fiala et al., 2002). Histopathological studies on AD microglia have shown an altered morphology indicating a functional impairment (De Witt et al., 1998; Sasaki et al., 1997). The long-term presence of activated microglia around b-amyloid plaques has been assigned to their inability of phagocytosing and clearing senile plaque cores (Apelt and Schliebs, 2001). Microglial dysfunction can become manifest in a number of ways, including a decreased ability to produce neurotrophic factors, a decreased phagocytic capacity, as well as increased neurotoxicity (Streit,

2002). These alterations may be of pathogenetic relevance in AD. Deficient phagocytosis promotes inflammation and can lead to immune-mediated tissue degeneration (Wyss-Coray and Mucke, 2002). Presumably, the chronic struggle of microglia to remove Abcontaining plaque material promotes inflammatory processes in AD (Lue and Walker, 2002). These changes are assumed to be age-related, but they are especially pronounced in AD. To conclude, findings of a systemic attenuation of cellular immune response may be related to the cerebral pathology in AD in terms of insufficient phagocytosis of amyloid proteins and resulting neurotoxic effects. 4.3.2. Decrease of amyloid burden through immune stimulation The assumption of a causal significance of an immunological impairment in AD is even more intriguing in the light of the studies on immunization with b-amyloid. Vaccination of transgenic mice with b-amyloid leads to an enhanced removal of amyloid deposits in the brain (Schenk et al., 1999) by promoting microglial phagocytosis. While the exact mechanisms are still point of discussion, also peripheral mechanisms have been considered (Lemere et al., 2003). Peripheral immune cells have been shown to invade the brain of adult mice as well as the brain of AD patients (Eglitis and Mezey, 1997; Fiala et al., 2002). Possibly, immunization leads to a peripheral immune response, which via penetration of T cells and macrophages into the brain will enhance phagocytosis of local Ab. Furthermore, immune stimulation with LPS results in reduction of b-amyloid plaques in APP PS1 transgenic mice. This effect has been observed by direct intrahippocampal injection (DiCarlo et al., 2001) as well as by systemic administration of LPS (Quinn et al., 2003). In view of a putatively underlying immune deficit and impaired phagocytotic activity in AD, the effect of immunization or immune stimulation – leading to a decrease of the cerebral amyloid burden- seems consistent and conceivable. 4.3.3. Role of aging Finally, there seems to be an obvious association between aging processes and the immune alterations seen in AD. Immune-aging phenomena constitute a major risk factor for AD (Gasiorowski and Leszek, 1997; Blasko and Grubeck-Loebenstein, 2003). Several epidemiological studies have identified advanced age as the only consistent risk factor for AD. The age-dependent decrease of immune functions does not only involve the adaptive immunity (Blasko and Grubeck-Loebenstein, 2003), but the innate immune system as well. T-cell derived cytokine production decreases with aging (Gillis et al., 1981; Esumi et al., 1991), and in vitro lymphocyte responsiveness to activating agents (e.g. lectins) is

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reduced in elderly humans (Marx et al., 1998; DiCarlo et al., 2001). Macrophages, as well, underlie age-associated functional alterations (Lloberas and Celada, 2002). Obviously, the immunological alterations in AD patients are more pronounced than the age-related changes in healthy persons. The T-cell observations in AD patients are characteristic of T cells that reach a state of high replicative senescence after multiple cycles of antigen-induced cell-divisions (Effros, 1998; Panossian et al., 2003). Besides, in comparison with healthy aged people, AD patients show increased mitochondrial DNA mutations and genomic DNA damage which can lead to dysfunction and decline of PBMC (De la Monte et al., 2000). Against this background, the observations of a blunted T cell-response in AD patients can be understood as a sequel of a premature immunosenescence, presumably being one important factor within the multifactorial etiopathogenesis of AD. This assumption is substantiated by the parallels between AD patients and patients with Down syndrome (DS). DS patients suffer from progerie and are of high risk to develop AD. Interestingly, DS patients show similar signs of advanced immunological senescence as observed in AD, such as telomere shortening (Park et al., 2000; Zhang et al., 2003) and altered intracellular calcium responses of T cells. This can negatively influence the T cell help required to generate an effective antibody response to A b (Grossmann et al., 1993). This study is limited due to the small amount of data and the heterogeneity of patients in terms of age, disease duration and severity. However, the present data support alternative views on the hypothesis of a mere inflammation-mediated pathogenesis, particularly since trials with anti-inflammatory agents have not yet shown a clear benefit in preventing or delaying the onset of the disease. Our hypothesis of a premature immunosenescence as a pathogenetically relevant factor in AD is in line with a ‘‘gerocentered’’ view rather than an ‘‘amyloidocentered’’ approach in understanding the etiology of AD (Joseph et al., 2001). Conclusively, the development of therapeutic strategies which stimulate the general immune responsiveness seems to be a promising challenge for future research.

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