Both invariant chain isoforms Ii31 and Ii41 promote class II antigen presentation.

The invariant chain (Ii) gene encodes two differentially spliced variants Ii31 and Ii41. The Ii31 isotype is the dominant form expressed in all antigen-presenting cells (APC). Ii41 is differentially expressed and can be found in large quantities in Langerhans and dendritic cells. While a functional role of Ii in class II antigen presentation is now well established, a distinct role of the Ii isotypes remains controversial. We tested Ii31 and Ii41 L cell transfectants for antigen presentation of hen egg lysozyme (HEL) to T cell hybridomas. The result indicates that both Ii chains promote antigen presentation equally well. To test other APC than transfected L cells, we introduced a recombinant Ii41 gene into anti-deficient mouse line. There the transgene induces about one-third of total li expression of wild-type mice. Surface expression of class II molecules and the CD4 compartment which are deficient in Ii knock-out mice are restored in Ii41 transgenic mice. B lymphocytes from Ii41 transgenic mice and Ii31-expressing B lymphocytes from wild-type mice were used as APC for presentation of keyhole limpet hemacyanin and ovalbumin to T cell hybridomas. The results show that both Ii chains facilitate antigen presentation equally well.