In vitro characterization of a monoclonal IgGκ from a patient with planar xanthomatosis

Objective:  To characterize a monoclonal IgGκ (MAb) from a patient with planar xanthoma that precipitated with serum lipids at reduced temperature. Methods:  The molecular weight (Mr), sensitivity to proteases, and glycosylation of the purified MAb were analyzed. The specificity of the MAb was tested by measuring cryoprecipitation with pure high- (HDL) and low-density (LDL) lipoproteins. The effect of choline, phosphocholine, and glycerol 3-phosphate on the precipitation temperature of LDL by the MAb was studied. Results:  The MAb was larger than normal IgG due to hyperglycosylation of the MAb light chain. It formed cryoprecipitates with pure HDL and LDL as well as the lipids extracted from these lipoproteins. Fab fragments of the MAb lowered the temperature of its precipitation with LDL. Choline, phosphocholine, and glycerol 3-phosphate also lower the precipitation temperature. Conclusion:  This is the first human IgG reported with apparent specificity for phosphocholine antigens. Its precipitation with lipids at reduced temperature suggests that it recognizes conformations of phospholipid head groups that develop below core body temperature.