Abstract. The article concerns interactions between immunoglobulin A and recombinant P6, P7, P8 polypeptides, designed on the basis of externally localized Bac protein of the Group B streptococci, possessing IgA-binding activity.There is a current demand for immunochemical reagents that are strictly specific for IgA, in order to develop antigenic standards for detection of IgA levels in biological fluids, as well as for affinity purification of IgA and its fragments.To analyze an opportunity of the abovementioned application ways for these proteins, a special study was performed to assay an interaction capability of recombinant P6, P7, P8 polypeptides binding to Fc regions of different IgA forms (serum IgA, secretory IgA, subclasses of serum IgA – IgA1, IgA2). Selectivity of ligand binding was specially confirmed.It was found out that, among three presented polypeptides, the structure of recombinant P6 derivative proved to be optimal for IgA-binding ability of Bac protein.Structural features of IgA-binding fragments of Bac protein, i.e., binding site position on the IgA molecule (proximity to epitopes for three monoclonal antibodies), variability of the site structure, as well as resistance of binding site for P6, P7, P8 in IgA molecule against partial disulfide bonds reduction. (Med. Immunol., vol. 10, N 4-5, pp 327-336).

Recombinant polypeptides corresponding to the conservative N-terminal area of Bac surface protein of group B streptococci (GBS) and having the human IgA binding-site were obtained and evaluated in terms of Immunogenic and protective properties. GBS strain 219, serotype 1 bc, served as the source of chromosomal DNA used for cloning. Three polypeptides P1, P5 and P6 were constructed. Polypeptides P1 and P5 with a molecular weight of 22 kD were coded by practically identical DNA fragments (685 nucleotide pairs) and differed in two aminoacids In their central part. Polypeptide P6 had a molecular weight of 35 kD, The fragment of recombinant Bac protein with a molecular weight of 35 kD was found to have protective properties: when used for the subcutaneous immunization of animals, it stimulated the development of immune response capable, in case of challenge, to induce accelerated elimination of streptococci and the prevention of the mice death. Recombinant fragment P6 of GBS protein Bac may be regarded as a candidate for inclusion into GBS-specific vaccine.

A comparative study of immunogenic and antigenic features of recombinant polypeptides based on the surface expressed Group В streptococcal protein Вас was accomplished. Polypeptides P6, P1, P5 with molecular mass 35, 22, 22 kDa were constructed by cloning of different DNA fragments corresponding to 3' part Вас, using expression vectors. P5 was different from PI due to substitution of two amino acids in the potential antigenic domain region. The immune response on laboratory mice demonstrated that intraperitoneal injection of the polypeptides under study induces different levels of occurrence of specific IgG. Titers of immunoglobulines reached 1 : 100 000; 1 : 50 000 and 1 : 12 000 for P6, P1 и Р5, consequently. Immune response was different by affinity levels of antibodies. Binding constant (Kc) varied from 108 M-1 to 1013 M-1 for P6 and from 106 M1 to 1012 M-1 for P1 and P5. Investigation of immunochemical cross reactions of polypeptides P6 with P1 and P5 and mutual adsorption of antisera demonstrated that P6 and P1 have identical antigenic domains. Polypeptide P5 was generating antibodies of two types one of which could not recognize P6. Comparative study of antigenic and immunogenic features of recombinant polypeptides based on Вас protein allowed concluding that polypeptide P6 expressed the highest level of antibodies with high affinity with long period of circulation. Polypeptide P6 can be considered a good potential candidate for part of the complex peptide vaccine against GBS. (Med. Immunol., 2004, vol. 6, N 6, pp 493-498)