Characterization of a novel lipoprotein expressed by Haemophilus ducreyi

Pooled sera from patients with chancroid contain antibodies to a Haemophilus ducreyi antigen with an approximate molecular weight of 28,000 (28K). Rabbit polyclonal serum that reacts to a 28K protein can be used to detect H. ducreyi in clinical samples. A monoclonal antibody, designated 5C9, bound to a 28K outer membrane protein and to 35 of 35 H. ducreyi isolates with diverse geographic origins and did not bind to many species of the families Pasteurellaceae, Neisseriaceae, and Enterobacteriaceae or to Corynebacterium and Candida species strains. A 5C9-reactive phage was recovered from a genomic library, and the gene encoding the 28K protein was localized to a 626-bp open reading frame, designated hlp, for H. ducreyi lipoprotein. Translation of hlp predicted a 23K polypeptide that contained a lipoprotein processing site. Escherichia coli transformed with a plasmid containing hlp expressed a novel, membrane-associated protein that could be labeled with [3H]palmitic acid. In H. ducreyi, processing of Hlp was inhibited by globomycin. Database searches found no homologies to hlp or to the predicted Hlp amino acid sequence. Restriction enzyme analysis indicated that hlp was conserved among H. ducreyi isolates. Serum samples from patients with chancroid and other genital ulcer diseases and from normal subjects contained antibodies that bound to purified, recombinant Hlp. Although monoclonal antibody 5C9 recognizes a species-specific epitope of a unique H. ducreyi lipoprotein, the presence of serum antibodies to Hlp may not indicate previous infection with H. ducreyi.