A comparative study of the biliary secretion of human dimeric and monomeric IgA in the rat and in man.

In the rat, dimeric immunoglobulin A (dIgA) is cleared rapidly from the systemic circulation into bile by vesicular transport through the hepatocyte. Whether such transfer of dIgA occurs in man is controversial. The fate of dIgA and monomeric IgA (mIgA) was studied in rats with biliary drainage, and in parallel in 4 patients, 3 of whom had biliary drainage. Human dIgA and mIgA were prepared from myeloma sera and labeled with radioisotopes of iodine. Ten microcuries each of 125I-dIgA and 131I-mIgA (2 to 4 microgram protein) were given i.v. simultaneously. In the four patients, 125I-dIgA disappeared more rapidly from the serum than did 131I-mIgA. Biliary recovery of 125I-dIgA (expressed as per cent total dose given) was only 0.2 to 0.9% in 8 8 hr while that of 131I-mIgA was 0.1 to 0.2%. In contrast, biliary recovery over the same period in rats was 21 to 32% for 125I-dIgA and 3.0 to 4.6% for 131I-mIgA. The data show that in man after injection of a trace amount of human myeloma IgA, rapid transport of dIgA into bile, as observed in the rat, was not seen. Although selective transport of dIgA over mIgA into bile occurred in man, the total amount of dIgA transported was small, and it is suggested that under physiological conditions, the major part of human biliary IgA is derived from local synthesis.