Sera from patients with acute viral hepatitis B were found to inhibit the in vitro proliferation of normal lymphocytes induced by different mitogens and antigens. In addition, an effect on concanavalin A-induced T suppressor cell activity and pokeweed mitogen-stimulated IgG and IgM synthesis was demonstrated.
Studies concerning the kinetics of serum immunosuppressive effects indicated that serum immunosuppressive factor (SIF) interfered with the intermediate phase of mitogen-induced lymphocyte activation which was defined by protein and RNA synthesis. Thus, when SIF-positive sera were added to lymphocytes, which were already activated by phytohemagglutinin, for 8, 12, or 18 hr, the inhibitory effect decreased in relation to the duration of lymphocyte activation. No inhibition could be demonstrated when SIF-positive sera were added 24 hr after initiation of mitogen stimulation. Furthermore, similar inhibitory effects were found measuring either uptake of [3H]uridine (RNA synthesis) or [3H]leucine (protein synthesis) in a 24 hr culture of phytohe-magglutinin-stimulated lymphocytes or [3H]thymidine uptake (DNA synthesis) after 48 hr.
These results indicate that SIF act(s) like an antiactivator and may belong to immunoregulatory physiologic serum factors.