Immunological studies on the localization of phosphatidylglycerol in the membranes of Mycoplasma hominis.

Phosphatidylglycerol is the main component (87%) of the membrane phospholipids of Mycoplasma hominis. It is immunologically active. Antibodies directed against phosphatidylglycerol were detected in rabbits intravenously immunised with native M. hominis or isolated M. hominis membranes. The intravenous method of immunisation was chosen in order to select for a response to surface antigenic determinants. Anti-phosphatidylglycerol antibodies were induced in rabbits by intravenously injecting the flocculated complexes of methylated bovine serum albumin and a phosphatidylglycerol/phosphatidylcholine/cholesterol mixture. These antibodies were specifically bound to intact M. hominis, as shown by complement fixation and Coombs tests. Native M. hominis were not agglutinated by anti-phosphatidylglycerol antibodies; but after partial digestion of the membrane proteins with Pronase, the mycoplasmas were heavily agglutinated by the anti-phosphatidylglycerol antibodies. The same amount of anti-phosphatidylglycerol antibodies was bound to intact M. hominis, containing 600 mug of phosphatidylglycerol as to 6 mug of phosphatidylglycerol in the optimal configurational arrangement of a mixed phosphatidylglycerol/phosphatidylcholine/cholesterol micelle. It is concluded that the major part of the phosphatidylglycerol in native M. hominis membranes is masked, probably by membrane proteins, and is not accessible to the anti-phosphatidylglycerol antibodies.