Moraxella catarrhalis is an important bacterial cause of otitis media in children and respiratory tract infections in the elderly. Lipooligosaccharide (LOS), a major surface antigen of this bacterium, is a potential vaccine component against the organism. There are three major LOS serotypes (serotypes A, B, and C) in clinical isolates of M. catarrhalis. Our previous studies demonstrated that serotype A and B LOS-based conjugates were immunogenic in animals and elicited bactericidal antibodies. In this study, LOS from serotype C strain 26404 was isolated, detoxified, and conjugated to tetanus toxoid (TT) or the cross-reactive mutant (CRM) of diphtheria toxin to form detoxified LOS (dLOS)-TT, dLOS-CRM-1, and dLOS-CRM-2 vaccine candidates. The molar ratios (dLOS/protein) of the resulting conjugates were 47:1, 19:1, and 32:1, respectively, while the weight ratios were 0.94, 0.84 and 1.44, respectively. All conjugates were highly immunogenic in both mouse and rabbit models. Three subcutaneous injections of each conjugate formulated with the Ribi adjuvant elicited >700-fold increases in serum anti-LOS immunoglobulin G levels in mice (5 g of dLOS) and >2,000-fold increases in rabbits (50 g of dLOS). The resulting mouse and rabbit antisera showed complement-mediated bactericidal activity against the homologous strain. In addition, a representative rabbit antiserum showed bactericidal activity against 14 of 18 testable strains, and this bactericidal activity could be 100% inhibited by the serotype C or A LOS but only 30% inhibited by the serotype B LOS. These results indicate that the serotype C LOS-based conjugates can be used as vaccine components for further investigation in humans.Moraxella catarrhalis is a gram-negative aerobic dipolococcus and is currently the third most frequent cause of bacterial otitis media (OM) and sinusitis in children (3,14,21). More than 90% of all children develop OM before the age of 7 years (35); 15 to 20% of these middle ear infections are caused by M. catarrhalis (9, 31), and it is estimated that there are 3 to 4 million cases in the United States annually (23, 26). The chronic forms of OM in these young patients may result in hearing loss and are associated with developmental and learning problems as children reach school age (2, 22). Sinusitis, however, accounts for 5 to 10% of upper respiratory tract infections in early childhood (40). In addition, M. catarrhalis often causes lower respiratory tract infections in adults with chronic obstructive pulmonary disease (3, 14, 26) and occasionally causes severe infections, including pneumonia, endocarditis, septicemia, and meningitis (3, 5, 6). Currently, treatment of the diseases has relied mainly on antimicrobial agents. However, with growing antibiotic resistance observed in clinical isolates all over the world (15, 40), attention has been focused on the possibility of vaccination against M. catarrhalis infection (4, 27).The research on vaccine antigens is presently based on the hypothesis that humoral immunity provides protection due t...