Identification of the immunosuppressive components of canine pulmonary surface active material.

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Abstract
Pulmonary surface active material (SAM), purified from canine lung lavage fluids, is a phospholipid-rich lipoprotein with potent immunosuppressive activity. Experiments were performed to identify those components of SAM that were responsible for this immunosuppressive effect. Results indicated that the lipid, and not the protein, fraction of SAM was immunosuppressive. Two phospholipids, phosphatidylglycerol and phosphatidylcholine, were identified as the predominant immunosuppressive components of the SAM-lipid fraction. lymphocyte proliferation in response to mitogenic or allogeneic stimulation was suppressed by intact SAM, SAM-lipid, phosphatidylglycerol, and phosphatidylcholine. Each of these preparations inhibited RNA, protein and DNA synthesis by mitogen-stimulated lymphocytes. Antioxidants consistently failed to diminish the immunosuppressive properties of SAM or its lipid components. The mechanism of this immunosuppressive action of SAM and its phospholipid components remains undefined. Our data indicate, however, that it is unlikely to be due either to cytotoxicity or to an artifact of lipid oxidation in vitro.