Signals from the sea: the structural peculiarity of lipid A and weak immunostimulatory lipopolysaccharide from Rheinheimera japonica†

Lipopolysaccharides (LPSs) isolated from marine bacteria represent a valuable resource for biomedical innovation. Here, we report the first structural elucidation of the lipid A moiety and a preliminary immunological assessment of the full LPS from the marine Gram-negative Rheinheimera japonica KMM 9513^T. Using MALDI-TOF mass spectrometry (MS) and tandem MS, we show that the lipid A from R. japonica KMM 9513^T exhibits a heterogeneous architecture, composed of mono- and bis-phosphorylated tetra- and penta-acylated species with variations in the acyl chain length, saturation, branching, and positional isomerism. Functionally, the full LPS was found to be immunologically silent toward TLR4-mediated NF-κB activation in HEK-Blue™ hTLR4 cells and triggered only modest, dose-dependent responses in differentiated human THP-1 macrophages. Strikingly, the R. japonica LPS was able to antagonize E. coli LPS-induced TLR4 activation, even at low doses. Overall, this study uncovers a structurally and functionally atypical marine LPS with a dual profile, inactive towards TLR4 yet capable of modulating LPS-induced signaling. These findings offer a promising basis to consider R. japonica LPS as a source of structural inspiration for the design of synthetic derivatives with controlled immunological properties.