Secretory component-dependent hepatic transport of IgA antibody-antigen complexes

The ability of the liver to transport antigen-antibody complexes containing polymeric IgA was tested in a model system using the isolated perfused rat liver and soluble complexes formed between trinitrophenylated (TNP) antigens and MOPC 315, a polymeric mouse IgA protein with anti-TNP activity. A double-label strategy (125I and 131I) was used to separately follow antigen and antibody during isolation and transport by the isolated perfused liver. Complexes formed in antigen excess and isolated by gel filtration were added to the perfusate. The quantity of antigen or antibody transported was determined by counting the radioactivity in collected bile fractions. TNP-human albumin (TNP-HSA) complexed to polymeric IgA antibody was transported from blood to bile while the same antigen complexed to IgG antibody was not. The transport of IgA-(TNP-HSA) complexes was inhibited by preincubation with human secretory component (SC), which indicated that transport of such complexes proceeds though an SC-dependent mechanism previously described for uncomplexed polymeric IgA antibody. Complexes (m.w. congruent to 970,000) of trinitrophenylated bovine thyroglobulin (TNP-TG) and polymeric IgA were transported less well than IgA-(TNP-HSA) complexes (m.w. less than or equal to 460,000), even though both types of complexes bound SC. The possibility that the poor transport of IgA-(TNP-TG) complexes reflected a size restriction on hepatic transport from blood to bile is discussed.