Suppression of cytotoxic responses by a supernatant factor derived from Mycoplasma hyorhinis-infected mammalian cell lines

A class of interleukin-2-dependent T-cell clones, isolated from a murine fetal thymus, was previously shown to suppress the induction of cytotoxic responses to alloantigens (H.-S. Teh, M. Ho, and W. R. McMaster, J. Immunol. 135:1582-1588, 1985). In that article, the immunosuppressive properties of these T-cell clones were shown to be a direct consequence of infection by Mycoplasma hyorhinis. Suppression of cytotoxic responses was mediated by both the mycoplasmas and a 200-kilodalton factor present in supernatants of infected cultures. This factor was sensitive to proteases but was resistant to heating to 60 degrees C for 1 h and to incubation on ice at pH 2 or pH 14 for 4 h. The production of suppressor factor in infected cultures was independent of the viability or the protein synthesis capability of the mammalian cells, suggesting that it was produced by M. hyorhinis. The factor was most suppressive when it was added during the early stages of the cytotoxic response. Its suppressive effects on cytotoxic responses were not reversed by the addition of an excess of recombinant interleukin-2. This factor also suppressed mitogenic responses to lipopolysaccharide. However, it is not a growth inhibitor since it did not affect the proliferation of tumor cell lines. A simple method for detecting M. hyorhinis in the infected T-cell clones and for eliminating it is described.