Neutralizing ability of monoclonal antibodies directed to epitopes of functionally significant Streptococcus pneumoniae pneumolysin domain

Background. Pneumolysin (Ply) – bacterial cholesterol-dependent cytolysin produced by almost all strains of Streptococcus pneumoniae. Antibodies induced to pneumolysin for some extend are able to neutralize its pathogenic effect on cells.

Aim. Investigation of the effect of monoclonal antibodies obtained to native (nPly) and recombinant (rPly) forms of Ply on the neutralization of the cytolytic activity of bacterial Ply in vitro and their preventive properties on mice challenged with a virulent strain of S. pneumoniae.

Materials and methods. mAbs to the nPly and rPly forms of Ply was obtained from the ascitic fluid of mice. Polyclonal antibodies pAbs (rPly) were obtained from the serum of rabbits immunized with rPly. The inhibition of erythrocyte hemolysis was assessed by the minimum concentration of mAbs (nPly), pAbs (rPly) and pAbs (rPly), which prevents lysis of erythrocytes induced by nPly. The neutralizing ability of antibodies was evaluated on the ovarian cells of the Chinese hamster CHO-K1 by the minimum concentration of antibodies necessary to protect cells from 4 cytopathogenic doses of nPly. The protective properties of the antibodies were studied with a single intraperitoneal injection of 200 µg of antibodies to mice, followed by challenge with a lethal dose of S. pneumoniae.

Results. It was shown that 24 hours after intraperitoneal administration of nPly to mice, the body weight of the animals decreased and degenerative forms of red blood cells appeared in the blood. In in vitro experiments, nPly caused hemolysis of erythrocytes and destruction of CHO-K1 cells by the mechanism of apoptosis. mAbs (nPly), mAbs (rPly) and pAbs (rPly) inhibited erythrocyte hemolysis and neutralized the destruction of CHO-K1 cells. The highest neutralizing activity was shown by mAbs (rPly) and pAbs (rPly). A single injection of the studied antibodies to mice caused a decrease of their death when infected with the S. pneumoniae strain. There were no differences in the protective activity of antibodies.

Conclusion. Methods based on neutralizing the toxic effect of Ply S. pneumoniae with the help of antibodies can be considered as a tool to reduce the pathogenicity of pneumococci.