COMPLEX SEROLOGY AND IMMUNE-RESPONSE OF MICE TO VARIANT HIGH-MOLECULAR-WEIGHT O-POLYSACCHARIDES ISOLATED FROM PSEUDOMONAS-AERUGINOSA SEROGROUP O2 STRAINS

The O antigen of the Pseudomonas aeruginosa lipopolysaccharide is the optimal target for protective antibodies, but the unusual and complex nature of their sugar substituents has made it difficult to define the range of these structures needed in an effective vaccine. Most clinic al isolates of P. aeruginosa can be classified into 10 O-antigen serog roups, but slight chemical differences among O polysaccharides within a serogroup give rise to subtype epitopes, These epitopes could impact the reactivity of O-antigen-specific antibodies, as well as the susce ptibility of a target strain to protective, opsonic antibodies. To def ine parameters of serogroup and subtype-epitope immunogenicity, antige nicity, and surface expression on P. aeruginosa cells, we prepared hig h-molecular-weight O-polysaccharide vaccines from strains of P. aerugi nosa serogroup O2, for which eight structurally variant O antigens exp ressing six defined subtype epitopes (O2a to O2f) have been identified . A complex pattern of immune responses to these antigens was observed following vaccination of mice, The high-molecular-weight O polysaccha rides were generally more immunogenic at low doses (1 and 10 mu g) tha n at a high dose (50 mu g) and usually elicited antibodies that opsoni zed the homologous strain for phagocytic killing. Some of the individu al polysaccharides elicited cross-opsonic antibodies to a variable num ber of strains that express all of the defined serogroup O2 subtype ep itopes, Combination into one vaccine of two antigens that individually elicited cross-reactive opsonic antibodies to most members of the O2 serogroup inhibited, instead of enhanced, the production of antibodies broadly reactive with most serogroup O2 subtype strains. Thus, immune responses to P. aeruginosa O antigens may be restricted to a limited range of epitopes on structurally complex O antigens, and combining mu ltiple related antigens into a single vaccine formulation may inhibit the production of those antibodies best able to protect against most P . aeruginosa strains within a given O-antigen serogroup.