In vitro immunosuppressive activity of gram-positive (Streptococcus pyogenes) peptidoglycan on mouse lymphocytes

A peptidoglycan fraction prepared from group-A streptococcus was assayed for in vitro mitogenicity on mouse lymphocytes. This fraction reduced considerably the uptake of radioactive thymidine both on unstimulated cell suspensions and on suspensions stimulated by T(PHA)- or B(LPS)-mitogens. The immunosuppression was induced by relatively moderate doses of the fraction, and was dose-dependent. Several experiments ruled out the possibility that these results could be due to a cytotoxicity of the fraction, or to a non-specific interference with the uptake or metabolism of the radioactive precursor. These results are coherent with the observations made in vivo on the mouse and previously published. It is suggested that the mechanism of the immunosuppression could be in relation with the capacity of this fraction to stimulate the reticulo-macrophagic system.