Limited diversity of the protein D gene (hpd) among encapsulated and nonencapsulated Haemophilus influenzae strains.

Protein D is a surface-exposed lipoprotein of the gram-negative bacterium Haemophilus influenzae with affinity for human immunoglobulin D myeloma protein. The gene encoding protein D (hpd) in a serotype b strain of H. influenzae was cloned. Escherichia coli carrying the hpd gene bound human myeloma immunoglobulin D. Nucleotide sequence analysis identified an 1,092-bp open reading frame that was more than 99% identical to the hpd gene from a nontypeable H. influenzae strain. In the deduced amino acid sequences for protein D, only 2 of 364 amino acid residues differed. The restriction fragment length polymorphism of the hpd region in different strains was analyzed by Southern blot analyses of PstI- or EcoRI-digested genomic DNA from 100 H. influenzae strains. The analysis was performed by using isolated fragments of the cloned hpd gene, originating from the nontypeable H. influenzae 772, as probes. All strains tested had DNA sequences with a high degree of homology to the hpd probes. The analysis also showed that restriction endonuclease sites within the gene were more conserved than sites adjacent to the hpd gene. An interesting difference between type b strains and unencapsulated strains was observed. The majority of type b strains seem to have a 1.4-kbp DNA fragment upstream of the hpd gene that is absent in nontypeable strains. On the basis of the high degree of conservation of the hpd gene among H. influenzae strains, we conclude that protein D is a possible vaccine candidate.