Structure of the Lipopolysaccharide from Paenalcaligenes hominis: A Chemical Perspective on Immune Recognition

Gram-negative bacterium Paenalcaligenes hominis, which is increasingly prevalent in elderly individuals, is associated with cognitive decline and gut–brain axis dysfunction. Here, we present a comprehensive structural characterization of P. hominis lipopolysaccharide (LPS), a key modulator of immune recognition and the main component of its outer membrane. Using a multidisciplinary approach combining chemical, spectroscopic, spectrometric, biophysical and computational methods, we unveil a unique O-antigen characterized by a trisaccharide repeating unit containing rhamnose and glucosamine, displaying nonstoichiometric O-acetylation and a terminal methylated rhamnose capping the saccharide chain. Furthermore, we disclose a short core oligosaccharide and a Lipid A composed of penta- to tetra-acylated species. Notably, this LPS exhibits reduced activation of Toll-Like Receptor-dependent signaling compared to the highly immunostimulatory Escherichia coli LPS and elicits a poor pro-inflammatory cytokine response. Moreover, P. hominis LPS exhibits selective binding to immune lectins such as Ficolin-3 and Galectin-4, as shown by the microarray assays. This raises the possibility that lectin-mediated recognition may represent an alternative route of immune engagement, which could help explain altered immune responses observed in elderly individuals. These findings provide a molecular basis for further exploring the role of P. hominis LPS in microbiota-induced immune modulation and its possible impact on age-related inflammatory and neurodegenerative conditions.