The Flavobacterium psychrophilum OmpA, an outer membrane glycoprotein, induces a humoral response in rainbow trout

Aims: The purpose of this study was to characterize OmpA, a major glycoprotein isolated from the membrane fraction of Flavobacterium psychrophilum, and to evaluate its potential as antigenic unit in a possible vaccine. Methods and Results: The expression product of ompA is a 465- amino- acid protein precursor that contains a 21- amino acid signal peptide and has overall homology ( up to 60% identity) with similarly sized proteins of some bacteria belonging to the Flavobacteriaceae family. The carboxy- terminal region contains the ' OmpA / MotB' domain / signature and five putative ' Thrombospondin type 3 repeats' domains have been identified in the central region. OmpA was clearly detected in the outer membrane fraction and its surface exposure was demonstrated. OmpA is one of the immunodominant antigens and binding of specific anti- OmpA antibodies lead to cell lysis in the presence of complement. Fish immunized with OmpA emulsified with Freund's adjuvant developed a high antibody titter. Conclusions: Collectively, the data obtained here indicate that OmpA may be involved in Fl. psychrophilum / host cell interactions and appears to be a potential immunogen for a vaccine. Significance and Impact of the Study: This study is one step in the direction of understanding pathogenesis of Fl. psychrophilum and development of future vaccine.