Epitope analyses of pneumococcal surface protein A: a combination of two monoclonal antibodies detects 94% of clinical isolates.

Immunisation of BALB/c mice with seven heat-treated Norwegian clinical isolates of Streptococcus pneumoniae of different serotypes elicited mainly monoclonal antibodies (mAbs) to pneumococcal surface protein A (PspA). It was remarkable that the fusions resulted only in a few mAbs directed against other protein antigens. Dot blot analysis with 16 mAbs using clinical isolates representing 23 different capsular types and the uncapsulated reference strain R36A showed that some of the mAbs bound to PspA epitopes expressed by a low number of strains whereas others bound to broadly distributed epitopes. On the basis of their reactivities, seven of these mAbs could be divided into two groups recognising different subsets of pneumococci. The three mAbs in the narrow reacting group bound to epitopes found in 21-25% of the strains whereas the four mAbs in the broad reacting group detected more than 57% of the analysed strains. The epitopes for these seven antibodies were surface exposed on live exponential phase grown pneumococci as shown by flow cytometry. The finding that a combination of mAb 180,C-1 (IgG2a) from the first group and mAb 170,E-11 (IgG2a) from the second group detected 94% of the examined strains is interesting because PspA has been reported by others to be a serological highly variable protein.