PHASE VARIATION AND CONSERVATION OF LIPOOLIGOSACCHARIDE EPITOPES IN HAEMOPHILUS-SOMNUS

The bovine-specific pathogen Haemophilus somnus is capable of undergoi ng structural and antigenic phase variation in its lipooligosaccharide (LOS) components after in viva and in vitro passage, However, commens al isolates from the reproductive tract have not been observed to vary in phase (T. J. Inzana, R. P. Gogolewski, and L. B. Corbeil, Infect, Immun, 60:2943-2951, 1992), We now report that specific monoclonal ant ibodies (MAbs) to the LOSs of Haemophilus aegyptius, Neisseria gonorrh oeae, and Haemophilus influenzae, as well as H. somnus, reacted,vith s ome phase-variable epitopes in H., somnus LOS, All reactive MAbs bound to LOS components of about 4.3 kDa in the same H. somnus isolates, in cluding a non-phase-varying strain, Following in vitro passage of a cl onal variant of strain 738 that,vas nonreactive with the MAbs, 11.8% o f young colonies shifted to a reactive phenotype, A digoxigenin-labell ed 5'-CAATCAATCAATCAATCAATCAATCAAT-3' oligonucleotide probe hybridized to genomic DNA from strain 738 but did not react with DNA from a non- phase-varying strain, Sequence analysis of the gene containing 5'-CAAT -3' tandem sequences revealed 48% amino acid homology with the lex-2B gene-encoded protein of H. influenzae type b., Our results indicate th at some LOS epitopes are conserved between H. somnus and other Haemoph ilus and Neisseria species, that LOS phase variation may occur at a hi gh rate in some strains of H. somnus, and that phase variation may, in part, be due to 5'-CAAT-3' tandem sequences present in H. somnus gene s.