Endospores of B subtilis are pyrogenic and activate Mono Mac 6 cells: importance of the CD14 receptor

The monocytic cell line Mono Mac 6 is sensitive to pyrogens and interleukin-6 secretion is induced after exposure to pyrogens. The aim of this study is to examine the pyrogenic activity and the interleukin-6-inducing capacity of the Gram-positive B. subtilis bacteria, endospores and isolated cell wall components. Furthermore the involvement of CD14 in activation of interleukin-6 release is investigated. All test substances are pyrogenic in the rabbit pyrogen test. The test substance is incubated with monocytic cells (Mono Mac 6) for 24 h and the secreted interleukin-6 is determined in a sandwich immunoassay. B. subtilis bacteria and endospores induce interleukin-6 in a dose-dependent manner. Endospores are less potent than bacteria. Lipoteichoic acid (LTA) isolated from B. subtilis induces interleukin-6 in a dose-dependent manner, whereas muramyl dipeptide (MDP) is unable to induce interleukin-6. Lipopolysaccharides (LPS) dose-dependently induce interleukin-6 release, but the curve differs from that of LTA both in shape and offset. The interleukin-6 secretion induced by LPS, LTA and B. subtilis bacteria can be blocked by 73-85% by an antibody directed against CD14, whereas the antibody only blocks 25% of B. subtilis endospores-induced interleukin-6 release. The results might indicate that B. subtilis endospores use an additional pathway to CD14 to activate mononuclear cells.