Allelic diversity of the two transferrin binding protein B gene isotypes among a collection of Neisseria meningitidis strains representative of serogroup B disease: Implication for the composition of a recombinant TbpB-basedvaccine

The distribution of the two isotypes of tbpB in a collection of 108 serogro up B meningococcal strains belonging to the four major clonal groups associ ated with epidemic and hyperendemic disease (the ET-37 complex, the ET-5 co mplex, lineage III, and cluster A4) was determined. Isotype I strains (with a 1.8-kb tbpB gene) was less represented than isotype II strains (19.4 ver sus 80.6%). Isotype I was restricted to the ET-37 complex strains, while is otype II was found in all four clonal complexes. The extent of the allelic diversity of tbpB in these two groups,vas studied by PCR restriction analys is and sequencing of 10 new tbpB genes. Four major tbpB gene variants were characterized: B16B6 (representative of isotype I) and M982, BZ83, and 8680 (representative of isotype II). The relevance of these variants was assess ed at the antigenic level by the determination of cross-bactericidal activi ty of purified immunoglobulin G preparations raised to the corresponding re combinant TbpB (rTbpB) protein against a panel of 27 strains (5 of isotype I and 22 of isotype II). The results indicated that rTbpB corresponding to each variant was able to induce cross-bactericidal antibodies. However, the number of strains killed with an anti-rTbpB serum was slightly lower than that obtained with an anti-TbpA(+)B complex. None of the sera tested raised against an isotype I strain was able to kill an isotype II strain and vice versa. None of the specific antisera tested (anti-rTbpB or anti-TbpA(+)B c omplex) was able to kill all of the 22 isotype II strains tested. Moreover, using sera raised against the C-terminus domain of TbpB M982 (amino acids 352 to 691) or BZ83 (amino acids 329 to 669) fused to the maltose-binding p rotein, cross-bactericidal activity was detected against 12 and 7 isotype I I strains, respectively, of the 22 tested. These results suggest surface ac cessibility of the C-terminal end of TbpB. Altogether, these results show t hat although more than one rTbpB will be required in the composition of a T bpB-based vaccine to achieve a fully cross-bactericidal activity, rTbpB and its C terminus were able by themselves to induce cross-bactericidal antibo dies.