Soluble factors produced by Mycoplasma arthritidis play an important role in the pathology of arthritis in rodents, which closely resembles human rheumatoid arthritis. At least one of the products of these microorganisms, M. arthritidis-T cell mitogen (MAM), has biological activities in common with superantigens. These superantigens activate T cells in a V beta-restricted fashion, and this response is strictly dependent on the presence of major histocompatibility complex (MHC) class II-positive cells. In the present study, we have examined the ability of MAM to induce proinflammatory monokine (interleukin 1 beta [IL-1 beta] and tumor necrosis factor alpha [TNF-alpha]) gene expression in the THP-1 monocytic cell line. Treatment of these cells (which express a very low level of HLA-DR molecules) with gamma interferon (INF-gamma) induced HLA-DR, -DQ, and -DP molecules and enabled them to respond to MAM in a dose-dependent manner, resulting in an increase in the level of steady-state mRNA for IL-1 beta and TNF-alpha. Stimulation of the U937 monocytic cell line (MHC class II-negative even after INF-gamma treatment) with MAM did not induce either IL-1 beta or TNF-alpha transcription. Moreover, MAM adsorption on Raji (MHC class II-positive) cells resulted in the loss of its cytokine-inducing activity to induce monokine gene expression. These findings demonstrate clearly that MAM induces monokine gene expression following interaction with MHC class II molecules. Pretreatment of INF-gamma-treated THP-1 cells with the transcription inhibitor actinomycin D prevented the induction of monokine mRNA, whereas cycloheximide superinduced mRNA after stimulation with MAM. Finally, our results, obtained with protein tyrosine kinase inhibitors and antiphosphotyrosine Western blotting (immunoblotting), indicate that protein tyrosine kinase is involved in MAM-induced IL-1 beta and TNF-alpha gene expression in the THP-1 monocytic cell line. The capacity of MAM to induce proinflammatory cytokine transcription in monocytes via MHC class II molecules can be one pathway of MAM contribution to autoimmune diseases.