DIMERIC AND TETRAMERIC IGA ARE TRANSCYTOSED EQUALLY BY THE POLYMERIC IG RECEPTOR

Polymeric IgA (pIgA) is transcytosed across epithelial cells and into external secretions by the polymeric Ig receptor (pIgR). Binding of di meric IgA (dIgA) to the pIgR stimulates transcytosis of the pIgR. The pIgA in secretions is found as dimers (dIgA) and higher polymers, such as tetramers (tIgA), but little is known of the functional significan ce of the different sizes. Here we compared the ability of dIgA and tI gA to perform three functions that are essential to their transport in to mucosal secretions. 1) Equilibrium binding studies showed that ther e were twice as many binding sites for tIgA as dIgA at the basolateral cell surface, but that the affinity of these sites for tIgA was one-h alf of that for dIgA. 2) Both dIgA and tIgA were rapidly transcytosed by the pIgR, although transcytosis of tIgA was slower. 3) Both dIgA an d tIgA could stimulate transcytosis of the pIgR, although tIgA was les s effective. The possible implications of these findings for the relat ive biologic roles of dIgA and tIgA are discussed.