Normal immunosuppressive protein: in vitro inhibition of DNA synthesis in T and B lymphocytes and lymphoid cell lines.

The biologic activity of normal immunosuppressive protein (NIP) isolated from human plasma was studied. NIP was found to inhibit the proliferation of both T and B lympohcytes in vitro. It suppressed the DNA synthesis of normal mouse lymphocytes responding to the mitogens phytohemagglutinin and lipopolysaccharide, as well as the [3H]Thymidine and [3H]leucine uptake by T and B lymphoid cell lines of human and murine origin. The lymphoid specificity of NIP was demonstrated by showing that DNA and protein synthesis of normal and transformed fibroblasts and other nonlympohid cell lines was not affected by NIP treatment. Furthermore, by using lymphoid cell lines we were able to show that 1) NIP inhibits the process of ongoing DNA and protein synthesis; 2) the duration of the cells' exposure to NIP is crucial for obtaining optimal effect; and 3) the inhibitory effect of NIP is totally reversible.