Protein D from nontypeable Haemophilus influenzae (Protein D)

Protein D is a highly conserved surface lipoprotein (approximately 42 kDa) from all Haemophilus influenzae strains, including nontypeable (unencapsulated) strains[5][1]. It is anchored in the outer membrane and is exposed on the bacterial surface[1]. Protein D functions primarily as a virulence factor: it binds human immunoglobulin D, aiding the bacteria in evasion of the host immune system[1]. In addition, it possesses glycerophosphodiester phosphodiesterase activity, supporting a role in nutrient acquisition and metabolism[2]. Its strong conservation and surface accessibility have made Protein D a key antigen in vaccine design, used as a carrier protein in certain pneumococcal conjugate vaccines such as Synflorix™[2][5]. It contributes to colonization of the human respiratory tract and is involved in the pathogenesis of diseases like otitis media and chronic obstructive pulmonary disease exacerbations; thus, it is considered an important therapeutic and preventive target[5][2][1]. There is no record of Protein D being the target of classic small-molecule drugs, but it is directly targeted by the immune system and by Protein D–based or conjugate vaccines[2][5][1].

Other names

Immunoglobulin D-binding protein DIgD-binding protein DH. influenzae Protein D

As a vaccine carrier, augments the immune response against conjugated polysaccharide antigens by providing a protein component for T-cell–dependent immune responses[2]. As a bacterial virulence factor, binds immunoglobulin D and facilitates colonization and immune system evasion[1]. Enzymatic activity as a glycerophosphodiester phosphodiesterase may be involved in nutrient acquisition for the bacterium[2].