OK‐432, penicillin‐killed Streptococcus pyogenes, is used in treating lymphangiomas and carcinomas. We have studied proinflammatory interleukin (IL) secretion following OK‐432 stimulation of total blood, peripheral blood mononuclear cell (PBMC) and purified monocytes in vitro. OK‐432 stimulation of purified monocytes gave IL‐1β, IL‐1RA, IL‐6, IL‐12p40 and tumour necrosis factor (TNF)‐α response. OK‐432 stimulation of cells within blood did, however, not yield TNF‐α secretion. When PBMC or monocytes were cultured in low‐attachment wells a decreased IL secretion was observed compared to adherent cells. Inhibition of Syk kinase with piceatannol, only at high, non‐specific doses, but not PI3 kinase inhibition with LY294002 or Wortmannin, decreased monocyte IL response to OK‐432. This shows that β1–3‐integrin receptor function is not necessary for monocyte OK‐432‐stimulated TNF‐α secretion. Direct blockage of the β2‐integrin (CD18) receptor by anti‐CD18 antibody was also unable to prevent the stimulating effects of OK‐432 in human monocytes. On the other hand, Syk phosphorylation is elevated upon adherence of monocytes and this is further increased by OK‐432 stimulation, as shown by Western blot. The Fc‐receptor was also ruled out as a main receptor of the OK‐432 monocyte response. In conclusion, TNF‐α secretion is only found in monocytes removed from blood. This TNF‐α secretion is not mediated through the β1–3‐integrin receptors. OK‐432 may act as a target‐seeking substance whereby only monocytes adhered, e.g. to a tumour cell, become cytotoxic in part explaining why OK‐432 is well suited as a cancer treatment drug.

Background: OK-432, penicillin-killed Streptococcus pyogenes, is used in treating lymphangiomas and carcinomas. We have studied in vitro the role of mononuclear phagocytes (MNPs), including purified monocytes (MOs), in the immune response to OK-432. MIP-1α/β and MCP-1 secretions were assessed in whole blood (WB), peripheral blood mononuclear cells (PBMCs) and purified MOs, after in vitro stimulation with OK-432 with or without adherence for 24 hours.