Respiratory pathogen to brain degeneration: a review of Chlamydia pneumoniae’s role in Alzheimer’s disease

Abstract

Alzheimer’s disease (AD) is a chronic neurodegenerative disorder with declining memory and cognitive impairment, largely mediated by extracellular amyloid-beta (Aβ). Although the amyloid cascade and tau protein hypotheses have long served as established frameworks for AD pathology, recent evidence suggests that long-term infections, particularly with Chlamydia pneumoniae (C. pneumoniae), may contribute to disease progression. A systematic search strategy was used to identify relevant literature using PubMed, Scopus, Google Scholar, and Web of Science. Keywords and Boolean operators such as “Chlamydia pneumoniae and Alzheimer’s disease,” “neuroinflammation,” “amyloid-beta,” and “tau protein” were applied, with filters for peer-reviewed articles, human and experimental studies, and publications from the past 25 years. Epidemiological and background data were supplemented by official sources, including the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO). This review examines the potential relationship of C. pneumoniae infection with AD pathogenesis. Studies have identified DNA and antigens of C. pneumoniae in AD-infected brain regions, often co-localized within Aβ plaques and neurofibrillary tangles (NFTs). Proposed mechanisms of CNS invasion include olfactory, hematogenous, and immune cell-mediated routes, leading to persistent glial activation, neuroinflammation, altered amyloid precursor protein processing, and tau protein hyperphosphorylation. Experimental models support these associations, with infected animals developing AD-like pathology. Diagnostic challenges persist due to the limitations of PCR and immunohistochemistry, though advanced approaches such as next-generation sequencing and TSPO-PET imaging are emerging. Potential therapeutic approaches include antimicrobial and immunomodulatory strategies, although human trials have shown mixed results. While current evidence suggests a possible link, causality remains unproven. Future research must prioritize large-scale, longitudinal, and mechanistic studies to clarify these relationships. Establishing a definitive role for C. pneumoniae in AD pathogenesis could reshape current understanding of disease etiology and inform the development of novel preventive and therapeutic strategies.

Keywords

Alzheimer’s disease / Chlamydia pneumoniae / amyloid-beta / neuroinflammation / neurodegeneration

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