Antigenic Characterization of Haemophilus somnus Lipooligosaccharide

Lipooligosaccharide (LOS) is the major outer membrane component of many Gram-negative bacteria inhabiting the mucosal membranes, including pathogenic species of Haemophilus and Neisseria. LOS phase variation is one mechanism by which some of these bacteria avoid the host immune response. To better understand LOS phase variation as a virulence mechanism of H. somnus, knowledge of the antigenic diversity of LOS epitopes must be increased. Monoclonal antibodies (MAbs) to H. somnus LOS were produced and used with cross-reacting MAbs to H. aegyptius LOS (MAb 5F5) and Neisseria gonorrhoeae LOS (MAb 3F11) in an ELISA to investigate LOS heterogeneity among forty-five strains of H. somnus. Using three MAbs, thirty-nine of these H. somnus strains were grouped into six antigenic types. Three groups, associated solely with the cross-reacting MAbs 5F5 and 3F11, included the majority (76%) of H. somnus strains. The anti-H. somnus LOS MAb 5D7 recognized a low frequency epitope associated with each of the remaining three groups, which included 11% of the H. somnus strains. Six strains (13%) were not recognized by any of these MAbs.

Inhibition ELISA experiments showed that the MAb 5F5 epitope contained phosphocholine (PCho) and this epitope was present in 56% of the strains tested. The MAb 5F5 epitope is phase variable in H. somnus LOS. How PCho negative variants could allow for systemic infection after initial colonization of the mucosa by PCho positive variants is discussed.