Modulation of inhibition of the human mitogen response by methylprednisolone

The in vitro response of human lymphocytes to phytohaemagglutinin was inhibited in a dose-related fashion by methylprednisolone added to the cell culture. The inhibition by steroid was 100-fold less than expected from published values of human lymphocyte receptor affinity. The dose response curve for inhibition was complex, frequently featuring a plateau region suggestive of a heterogeneous system, possibly caused by the presence of more than one subpopulation of cells. When lymphocytes were separated by unit velocity sedimentation, slow sedimenting lymphocytes were found to be highly steroid sensitive whereas rapidly sedimenting lymphocytes were steroid resistant. However, the addition of cultured macrophages rendered the steroid-sensitive fractions relatively steroid resistant. Similarly, unseparated peripheral blood lymphocytes at low concentrations were found to be highly steroid sensitive despite the addition of 2-mercaptoethanol. This sensitivity was lost by the addition of culture macrophages. Thus, macrophages play a vital role in the relative steroid resistance of human lymphocytes. No evidence was found that macrophages actually reduced steroid concentration in culture, and it is suggested that a cellular interaction is required to increase resistance to steroids.