Purification and partial characterization of a C1q inhibitor from the membranes of human peripheral blood lymphocytes

An activity that is capable of precipitating highly purified C1q and inhibiting the C1q-dependent biologic reactions was solubilized from the membranes of 1 x 10¹⁰ normal human peripheral B lymphocytes, and has been designated membrane-derived C1q inhibitor (M-C1q INH). M-C1q INH has an apparent m.w. of 1.0 to 2 x 10⁶ as assessed by gel filtration on Sepharose 6B, and an electrophoretic mobility of -6.7 x 10^-5 cm²V^-1sec^-1 on cellulose acetate at pH 8.6. Monospecific antiserum raised in rabbits to isolated M-C1q INH as well as polyvalent antiserum raised in rabbits to purified peripheral blood B lymphocytes were found to react with unfractionated, solubilized membrane proteins as well as C1q INH derived from both lymphocytes and lymphoblastoid (Raji) cells. Upon immunoelectrophoresis of M-C1q INH and normal human serum, the antisera were found to detect a material that migrated with an electrophoretic behavior similar to that of isolated M-C1q INH. Purified M-C1q INH was able to bind and precipitate highly purified C1q in solution or in 0.8% agarose, at pH 7.2. Such binding was optimal under conditions of low ionic strength. Preincubation of purified C1q with M-C1q INH at 37°C for 60 min abrogated C1q hemolytic activity in a dose-dependent fashion. No inhibition was observed, however, when the inhibitor was first pretreated with the monospecific antiserum raised against isolated M-C1q INH before incubation with the C1q. In addition, uptake of ¹²⁵I C1q by B lymphocytes was effectively blocked by prior treatment of the labeled C1q with the inhibitor, suggesting the inhibitor binds to the region of the C1q molecule through which it binds to cellular receptors. Preliminary chemical analyses of M-C1q INH suggests it consists of 23% uronic acid and 21% galactosamine. Comparison of amino acid data seems to indicate that the serum C1q INH and M-C1q INH might not be identical. Furthermore, digestion of M-C1q INH with chondroitinase ABC did not affect its properties. Thus, the evidence presented seems to indicate that the M-C1q INH could represent a cell membrane receptor for human C1q.