Epitope expression of gonococcal lipooligosaccharide (LOS). Importance of the lipoidal moiety for expression of an epitope that exists in the oligosaccharide moiety of LOS.

Antigenic expression of lipooligosaccharide (LOS) of strain F62 of Neisseria gonorrhoeae, was investigated with mouse monoclonal IgM antibody 3F11. F62 LOS was modified in various ways in order to understand structural requirements for expression of the 3F11-defined epitope. When the LOS was partially deacylated by treating it with 50 mM NaOH at 80 degrees C for 20 min or with anhydrous hydrazine at 80 degrees C for 20 min, the binding of 3F11 to those deacylated LOS samples decreased significantly. Removal of phosphate groups by treatment of the LOS with HF (4 days at 4 degrees C) did not affect the antigenicity at all. Neither did reduction of carboxyl groups in the LOS molecule (by activation of carboxyl groups with a carbodiimide followed by treatment with NaBH4) alter epitope expression. On oxidation with NaIO4, the LOS lost its antigenicity completely. The presence of Mg2+ did not change the circular dichroism (CD) behavior of F62 LOS. However, the partially deacylated LOS samples showed significantly different CD patterns in the 190-200 nm region compared with F62 LOS, which suggests conformational changes of F62 LOS due to the loss of fatty acids in the lipoidal moiety. Oligosaccharide (OS) and lipoidal components obtained after hydrolysis of F62 LOS with 1% acetic acid, were not recognized by the antibody. The antigenicity of OS was not retained by non-stereospecific acylation of OS with decanoyl chloride. We conclude the following: (1) 3F11-defined epitope exists in the OS moiety of F62 LOS; however, for it to be expressed, the carbohydrate moiety must be in a certain conformation that is defined by an overall structure of the LOS molecule. This structure is significantly influenced by some of the fatty acids in the lipoidal moiety of the LOS molecule; (2) the presence of phosphate or 3-deoxy-manno-2-octulosonic acid (dOclA) is not essential for expression of the 3F11-defined epitope; (3) the presence of divalent cations does not affect epitope expression.