The LipidA from Rhodopseudomonas palustris Strain BisA53 LPS Possesses a Unique Structure and Low Immunostimulant Properties

The search for novel lipidA analogues from any biological source that can act as antagonists, displaying inhibitory activity towards the production of pro-inflammatory cytokines, or as immunomodulators in mammals, is a very topical issue. To this aim, the structure and immunological properties of the lipopolysaccharide lipidA from the purple nonsulfur bacterium Rhodopseudomonas palustris strain BisA53 have been determined. This lipidA displays a unique structural feature, with a non-phosphorylated skeleton made up of the tetrasaccharide Manp--(14)-GlcpN3N--16-GlcpN3N--(11)--GalpA, and four primary amide-linked 14:0(3-OH) and, as secondary O-acyl substituents, a 16:0 and the very long-chain fatty acid 26:0(25-OAc), appended on the GlcpN3N units. This lipidA architecture is definitely rare, so far identified only in the genus Bradyrhizobium. Immunological tests on both murine bone-marrow-derived and human monocyte-derived macrophages revealed an extremely low immunostimulant capability of this LPS lipidA.