Plasma Amyloid-β in Relation to Antibodies Against Herpes Simplex Virus,Cytomegalovirus,and Chlamydophila pneumoniae

INTRODUCTION

Alzheimer’s disease (AD) is a terminal condition that affects millions of people worldwide. The etiology of sporadic AD is not completely understood, but a growing body of evidence points to an infectious component in AD development [1, 2]. Herpes simplex virus type 1 (HSV1), cytomegalovirus (CMV), and Chlamydophila pneumoniae are all pathogens that have been associated with AD [3–10].

HSV1 is ubiquitous and found in approximately 80–90% of the adult population [11]. After prim-ary infection, the virus persists latently in sensory neurons, most commonly in the trigeminal ganglion, and can subsequently reactivate. HSV1 has been detected in a high proportion of brains from AD patients and co-localizes with amyloid plaques in particular [12–16]. In epidemiological studies, carrying HSV1 has been associated with increased risk of AD, especially among genetically susceptible individuals [3–7, 17–21]. Similar to HSV1, the seroprevalance of CMV is high, reaching 80–90% in adults [11, 22]. Following primary infection, the virus remains latent in T lymphocytes and can be reactivated repeatedly without causing symptoms [23]. Co-infection of CMV and HSV1 has been reported to increase the risk of developing AD [24, 25]. C. pneumoniae is a bacterium that can cause respiratory symptoms. Asymptomatic infection also occurs and approximately 60% of the adult population has serological evidence of C. pneumoniae infection [26].

The hypothesis of infectious involvement in AD corresponds well with the finding that amyloid-β (Aβ) has potent antimicrobial activity [27–29]. Cultured cells accumulate Aβ when infected with a broad spectrum of viruses (including HSV1 and CMV), bacteria, and fungi [9, 29–33]. In addition, both HSV1 and C. pneumoniae infection lead to the formation of amyloid plaques in murine models [28, 34–36]. Mice overexpressing Aβ have greater survival rates compared to non-transgenic mice after brain infection [28, 29], suggesting a potential link between high levels of Aβ and resistance to infection.

Aβ is primarily produced in the brain. However, there are potential extracerebral sources of Aβ, such as platelets [37, 38] and fibroblasts [9]. Aβ containing 40 amino acids (Aβ₄₀) is the most common variant of the Aβ peptide in the central nervous system. The longer Aβ₄₂ fragment is the most amyloid-prone and is thus considered more neurotoxic than Aβ₄₀ [39]. Soluble plasma concentrations of both variants might vary as a consequence of pathogens infecting peripheral tissues. A previously published study showed that increased levels of anti-HSV IgM were associated with lower plasma levels of both Aβ₄₀ and Aβ₄₂ [40]. The correlation between plasma Aβ levels and antibodies against CMV and C. pneumoniae has not been previously investigated.

Relationships between Aβ₄₀ and Aβ₄₂ in plasma and antibodies against different pathogens might indirectly support earlier findings that Aβ has antimicrobial properties. Therefore, the aim of this study was to analyze the potential associations between plasma Aβ levels and circulating antibodies against HSV, CMV, and C. pneumoniae.

METHODS

RESULTS

The study enrolled 339 cases with AD and 339 matched controls (76.4% females). The mean age at plasma sampling was 61.3 years in both groups and the mean age at diagnosis was 70.8 years (Table 1). AD cases had significantly higher seroprevalence of anti-HSV IgG compared to their controls (94.4% versus 89.7%, respectively, p = 0.030; Table 1). Otherwise, no differences between the groups were observed in terms of Aβ plasma concentrations and the presence of anti-HSV1 IgG, anti-HSV2 IgG, anti-CMV IgG, or anti-C. pneumoniae IgG antibodies (Table 1).

Neither Aβ₄₂ nor Aβ₄₀ levels were, after adjustment for multiple comparisons, associated with presence or levels of anti-HSV1 IgG, anti-HSV2 IgG, anti-CMV IgG and anti-C. pneumoniae IgG (Tables 2 and 3).

Plasma concentrations of Aβ₄₂ and Aβ₄₀ in relation to follow-up time among AD patients were not affected by age (p = 0.367 and p = 0.911, respectively) or sex (p = 0.618 and p = 0.997, respectively). There were no significant interactions between follow-up time and the presence of anti-HSV1 IgG, anti-HSV2 IgG, anti-CMV IgG, and anti-C. pneumoniae IgG in association with Aβ₄₂ and Aβ₄₀ after adjustment for multiple comparisons (Table 4).

DISCUSSION

This nested case-control study included a large number of thoroughly diagnosed AD cases and closely matched controls, with plasma samples taken on average almost 10 years prior to diagnosis. The study material was therefore well suited to investigate whether biochemical markers for AD correlate with infectious agents. Here, we specifically investigated the prevalence and titers of IgG towards possible AD-related pathogens and the concentrations of free, soluble Aβ₄₂ and Aβ₄₀.

We found that Aβ concentrations did not correlate with antibodies against the different pathogens investigated. While the unadjusted analyses indica-ted a few associations, no clear patter emerged. None of these associations were significant after adjustment for multiple comparisons. Our negative results, therefore, do not indicate that Aβ is directly involved in managing AD-associated infections in the periphery. An early hypothesis was that the preclinical AD patients would react differently concerning Aβ concentrations in response to infection, possibly revealing altered ability to control peripheral infections, but no clear pattern was observed as to differences in the associations between AD cases and their controls. Importantly, we measured IgG antibodies, which can be considered markers of latent or chronic infection. Presence of IgG directed towards a specific pathogen could indicate carriage, while higher IgG levels could possibly reflect a more active or recently triggered immune response. Latent HSV1 is mainly localized to the trigeminal ganglia. At this locale, non-reactivating virus may not be exposed to the peripheral immune system, which could explain the lack of an association between anti-HSV1 IgG and plasma Aβ. It should also be noted that concentrations of free plasma Aβ do not necessarily correlate with concentrations of Aβ in the central nervous system and our results are not representative of intracerebral processes [42].

In the present study, AD cases had a significantly higher proportion of HSV seropositives compared to controls. This corresponds well with the growing evidence for an association between HSV1 and AD development [1, 3].

A limitation of the study is the small number of anti-HSV IgM seropositive individuals, making it difficult to investigate whether IgM antibodies might correlate with Aβ concentrations, an association which has been detected in a previous study [40]. Another limitation was that follow-up samples were available only for a subset of the AD cases, making it impossible to compare the groups of cases and controls with respect to the presence of antibodies and Aβ concentrations over time.

In conclusion, this large nested case-control study indicates that plasma Aβ concentrations are not associated with antibodies against AD-related pathogens (viruses and bacteria). Investigations of other cohorts will be needed to confirm the findings of this study as well as to determine the predictive value of serum biomarkers for later AD development.

CONFLICT OF INTEREST

The authors have no conflict of interest to report.