Characterisation and protective capacity of monoclonal antibodies elicited in mice against protein epitopes of antibiotic-exposed Escherichia coli

The binding capacity and the protective activity of three monoclonal antibodies (MAbs) - ARM 1-4, ARM 1-7 and ARM 2-2 - obtained from spleen cells of mice immunised with Escherichia coli O6:K– pre-treated with sub-MIC of aztreonam were studied. The MAbs belonged to IgG1 isotype and showed different reactivity toward protein epitopes of E. coli in an immunoblotting assay. ARM 1-4 recognised epitopes on molecules of 30 kDa and 40 kDa. ARM 1-7 identified an epitope of a molecule of 41 kDa, and ARM 2-2 recognised epitopes of molecules of 15 kDa and 41 kDa. In ELISA the MAbs cross-reacted with E. coli O7:K-, E. coli O111:B4 and E. coli O128:K- with different binding affinity. Furthermore, the MAbs showed complement-dependent bactericidal activity. The MAbs displayed different protective capacities when given to mice 90 min before lethal challenges with 2 X LD50, 4 X LD50 and 8 X LD50 of E. coli strains. In all but one instance (ARM 1-4 versus E. coli O7:K–) it was not possible to correlate protective capacity with binding affinity of a MAb to a given bacterial cell. Therefore, the epitopes recognised by the MAb may be more closely associated with bacterial virulence than in binding to the bacterial cell.