Truncated D Protein as a New Vaccine Candidate Against Nontypeable Haemophilus influenza

Nonencapsulated, nontypeable Haemophilus influenzae (NTHi) is considered an important cause of acute otitis in children and respiratory diseases among adults. The bacteria express several outer membrane proteins, some of which have been studied as vaccine candidates. Protein D is a highly conserved surface lipoprotein, which has been found in strains of both encapsulated and nonencapsulated H. influenzae.
Opsonin and protective antibodies against protein D play major roles in the prevention of infections caused by NTHi. Since NTHi can be also an intracellular organism, it needs to be removed through immune-mediated mechanisms.
In this study, groups of BALB/c mice were subcutaneously immunized with recombinanat truncated D protein and an aluminum hydroxide (alum) adjuvant or protein D combined with an outer membrane vesicle (OMV) adjuvant from Neisseria meningitidis bacteria. Then, opsonophagocytic activities of antibodies were measured in serum samples collected on days 14, 28, and 42. The levels of interleukin-4 (IL-4), IL-10, and interferon-gamma were measured in splenocyte cultures of immunized mice.
The opsonophagocytic activity of antibodies significantly increased in the immunized mice, compared to the control group, particularly on day 42. In addition, the secretion level of cytokines significantly increased in both groups of immunized mice, compared to the control group.
The results of this study demonstrated that recombinant truncated D protein can induce specific immune responses against nonencapsulated H. influenzae. It was suggested that the recombinant truncated D protein, as a vaccine candidate against the nonencapsulated strains of H. influenza, is essential in protection and development of sustainable immunity against infections caused by this bacterium.