β-Amyloid (Aβ) and Human Cathelicidin LL-37: Two Sides of the Same Coin?

Physiologically produced circulating β-amyloid (Aβ) exerts critical physiological functions. Although Aβ is a key player in Alzheimer’s disease (AD), it may initially be beneficial at the onset of infection. As an evolutionary conserved antimicrobial peptide (AMP), Aβ contributes to innate immune defense against pathogens. Host defense peptides such as Aβ and human cathelicidin (LL-37) not only kill pathogens through their antimicrobial activity but also exhibit high affinity for bacterial lipopolysaccharides (LPSs) and membrane receptors. LL-37, which is upregulated in the brain, binds to Aβ, modulating its aggregation; Aβ and LL-37 are protective under physiological conditions, but during chronic infection or dysregulation, their interaction becomes toxic and contributes to AD pathology. Similarly to Aβ, LL-37 can induce neuroinflammation by stimulating human microglia to release inflammatory cytokines, such as TNF-α and IL-6. Neuroinflammation is essential for protecting the brain from pathogens—when prolonged, it drives pathological processes underlying AD, Parkinson’s disease (PD), and other neurodegenerative disorders.