In vivo metabolism of a monoclonal IgG cryoglobulin.

The metabolism of a monoclonal IgG cryoglobulin was studied in a patient with primary idiopathic cryoglobulinaemia under two different clinical conditions. Early in the illness, cryoprotein levels were diminished to 770 mg% by plasmapheresis and plasma disappearance assayed while serum levels of cryoprotein increased to 1100 mg%, i.e. the study was performed during a period of unstable cryoprotein synthesis. A second study was performed 1 year later when serum levels of cryoglobulin were stable at 1500 mg% which allowed a synthetic rate of 64 mg/kg of body weight per day to be computed. This was the upper limit of normal IgG synthesis. The final slopes of the plasma disappearance curves were nearly identical for both studies, indicating that the same fractional percentage of the cryoglobulin serum pool was degraded regardless of its serum level. T1/2 of 15·5 and 17·0 days were obtained when plasma levels were unstable and stable, respectively. For comparative purposes, the plasma disappearance of pooled normal IgG, IgG3 myeloma proteins and a highly aggregated IgG3 myeloma were also studied when cryoprotein levels were 1500 mg%. Normal IgG synthesis was suppressed and only 8·5 mg/kg body weight per day, but its plasma disappearance curve showed a final slope nearly identical to those for the cryoprotein, indicating that the ability to catabolize normal IgG was unimpaired. IgG3 myelomas were cleared at their normal accelerated rate, while aggregated IgG3 was totally cleared from the circulation within 2 days, indicating that the patient was able to catabolize circulating immune complexes. As controls, the catabolism of cryoprotein was shown to be identical to that of pooled normal IgG in two volunteers. The data support the concept that the build up of circulating IgG cryoprotein in the patient studied was due to an increase in cryoprotein synthesis and not to a lack of ability to catabolize it.