In vivo experiments involving secretory component in the rat hepatic transfer of polymeric IgA from blood into bile.

Human or rat purified secretory IgA (sIgA) injected intravenously (i.v.) into rats is transferred to bile much less (seven to twenty-four times) than human or rat polymeric IgA (pIgA) devoid of secretory component (SC). A polymeric Fc alpha (pFc alpha) fragment of a human IgAl myeloma protein, obtained by IgA-protease digestion, bound in vitro to rat SC and was actively transferred in vivo into bile, in contrast to the corresponding Fab alpha. The IgA recovered in bile was not degraded, as judged by sedimentation in density gradients. Purified rabbit IgG anti-rat SC antibody was also efficiently transported in vivo into bile, about forty times more than normal rabbit IgG. The biliary transport of anti-Sc antibody could be reduced and retarded by the simultaneous i.v. injection of purified rat SC or human pIgA. The transfer of rat 125I-pIgA into bile was also significantly reduced and retarded by the concomitant i.v. injection of purified rat or human SC. Moreover, i.v. injection of purified rat or human SC induced a marked and prolonged decrease of the sIgA level in the bile. Rat SC was more effective than human SC in this respect. All these in vivo experiments confirm the in vitro findings of Orlans, Peppard, Fry, Hinton : Mullock, (1979) and Socken, Jeejeebhoy, Bazin & Underdown, (1979) showing that Sc is the IgA-receptor on the hepatocyte membrane for the transfer of pIgA from rat plasma into bile.