Moraxella catarhalis Vaccines
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Moraxella catarhalis
Moraxella catarrhalis is a Gram-negative, aerobic, diplococcus that exclusively resides in the upper respiratory tract of the human body. The otitis media in infants and children caused by this bacterium accounts for 15-20% of cases of acute otitis media. In addition, the bacterium is also a common pathogen causing lower respiratory tract infections and sometimes it is able to cause pneumonia, meningitis, bacteremia, and sinusitis in adults and children. M. catarrhalis can also trigger invasive infections in the elderly and immunocompromised people. The emergence of antibiotic-resistant bacteria is a serious problem at present. Today, approximately 90%, or even 100%, of M. catarrhalis strains are resistant to ampicillin by being able to produce beta-lactamase (BRO-1, BRO-2, and BRO-3). Recent reports have also pointed out that the resistance of the bacteria to cotrimoxazole increased by 82.5% and 18.5% in India and Taiwan, respectively.
Pathogenicity of Moraxella catarhalis
In the past, M. catarrhalis was considered to be a non-pathogenic commensal bacterium, but subsequent studies confirmed its relevance to disease and some mechanisms for its interaction with the host. Adherence is a key link in attaching bacteria to host epithelial cells. M. catarrhalis can express a variety of adhesins including UspA (ubiquitous surface protein A family), OMP CD (the outer membrane protein CD), LOS (lipooligosaccharide), hag/MID (the human erythrocyte agglutinin/Moraxella immunoglobulin D-binding protein), and Macp (M. catarrhalis adherence protein) to achieve bacterial adhesion. These binding molecules facilitate the survival of M. catarrhalis in different parts of the human respiratory tract and under different conditions. UspA1, LOS and some outer membrane components may regulate the invasion of respiratory epithelial cells by M. catarrhalis. It is speculated that bacterial invasion of epithelial cells can help bacteria escape from killing of the host immune system and the extracellular antibiotics. The formation of biofilm is one of the pathogenic factors of respiratory pathogens. The Hag/MID and UspA families regulate biofilm formation, and data show that biofilm formation is related to age, and children can form more extensive biofilms. The ability of M. catarrhalis to evade host immune responses is an important cause of its survival in the host, and its ability to resist complement is its most important virulence factor. The bacterium is able to participate in the defense of complement through the outer membrane proteins such as UspAs, CopB, OMP CD and OMP E, as well as the surface structure LOS. Moreover, UspA2 of M. catarrhalis is able to bind to complement component C3 and cause the alternative complement pathway failed to be activated.
Development of the Moraxella catarrhalis Vaccines
Acute otitis media and chronic obstructive pulmonary disease are serious diseases and cause economic stress. The development of the M. catarrhalis vaccine is an effective measure to alleviate disease and economic stress. The design of the M. catarrhalis vaccine is based on the pathogenesis of the bacteria. As an adhesion protein of bacteria, OMP CD has been shown to be capable of eliciting functional antibodies in animal models in vaccine evaluation. Mucosal and systemic immunized with recombinant OMP CD protein enhance bacterial clearance after M. catarrhalis challenge. MID706-1194 is a conserved sequence of MID that induces strong antibody levels in COPD patients. Moreover, the inoculation with truncated MID showed a protective response in the mouse lung clearance model. McaP is not only an adhesion molecule but also an autotransporter with phospholipase activity. The protein is highly conserved and studies have shown that it is capable of inducing the production of functional antibodies and inhibiting bacterial adhesion to human epithelial cells. UspA1 and UspA2 were once the preferred antigens for the M. catarrhalis vaccine, but due to the variability of their sequences, they are not the first choice for vaccine targets.
Proteins related to nutrient acquisition have also been studied as vaccine antigens. OppA is able to mediate bacterial uptake of peptides and mediate bacterial adaptation in the respiratory tract. OppA is highly conserved at the genetic level, and vaccine studies have shown that it is able to clear bacteria in the lungs of mice after immunization. TbpB and LbpB are the transferrin receptor and lactoferrin receptor in iron acquisition, respectively. The former detected the corresponding antibodies in both children and adult sera while the latter detected antibodies only in adult serum. CopB is associated with iron utilization and serum resistance, antibodies are detected in both serum and mucosa of children and adults, and it also enhances the ability of the lungs of mice to clear bacteria after immunization. OMP E binds and transports fatty acids, and its sequence is also highly conserved at the genetic level. Related antibodies have also been detected in mucosal and serum of adults. Other outer membrane proteins such as OMP G1a, Msp22, -75, -78 and M35 are also highly conserved, and studies on these antigens have shown some vaccine potential.
It is relatively conservative with LOS which produces an inflammatory response and is associated with adhesion of bacteria to epithelial cells and serum resistance. Related antibodies are also detected in the serum and mucosa of adults. Bactericidal antibodies were detected in the serum of rabbits and mice after immunization with the antigen, and enhanced ability to remove bacteria was also found in the lungs of mice.
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