Demonstration of a dog liver immunosuppressive factor active in vitro

These studies define the presence of an immunosuppressive factor in dog liver that is active in vitro. A crude dog liver extract was prepared by saline extraction and ultracentrifugation. In vitro immunosuppressive activity of the extract was assayed by its influence on standard phytohemagglutinin mitogen stimulation assays (PHA), two-way mixed lymphocyte culture (MLC) reactions, and primed lymphocyte culture (PLC). In six experiments with four extracts, the mean percentage of inhibition (MPI) of dog lymphocytes in PHA mitogen stimulation assays was 102.0 +/- 3.6. In two human MLC experiments, the extract produced an MPI of 102.3 +/- 0.3. In one canine MLC the MPI was 102.5. In three PLC experiments the MPI was 92.7 +/- 8.0, indicating that the extract inhibited sensitization. Cytotoxicity of the extract was shown not to be the mechanism of proliferation inhibition by restimulation of cells washed free of extract, concurrent eosin viability studies, and documentation of normal base proliferation of cells after extract was washed from them. We concluded that there is a naturally occurring immunosuppressive agent in dog liver that is active in vitro as demonstrated by inhibition of PHA MLC, and PLC cellular proliferation assays. The activity is not attributable to cytotoxicity.