Invariant chain regulates endosomal fusion and maturation through the SNARE Vti1b

Invariant chain (Ii) is an important multifunctional player in the regulation of adaptive immune responses and is responsible for several cellular functions related to MHCI and MHCII antigen loading and antigen presentation. While regulating endosomal trafficking of MHCII and other proteins that bind to Ii, this molecule is able to influence the endosomal pathway delaying the maturation of endosomes to the late endosomal loading compartments. When expressed in cells Ii is found to increase endosomal size, but the mechanisms for this is not known. We used RNAi silencing to identify SNARE proteins controlling Ii induced increase of endosomal size and delay of the endosomal pathway. Ii was found to interact with the SNARE protein Vti1b. Vti1b localized at the contact sites of fusing Ii positive endosomes and a tailless Ii was able to relocate Vti1b to the plasma membrane. Furthermore, silencing Vti1b, abrogated the delay in endosomal maturation induced by Ii expression. In conclusion, Ii interacts with Vti1b and this interaction is fundamental for Ii-mediated alteration of the endosomal pathway. We propose that Ii, by interacting with SNAREs, in particular Vti1B in the biosynthetic pathway of antigen presenting cells, is able to assemble SNARE directed fusion partners in the early part of the endosomal pathway that lead to a slower endosomal maturation for efficient antigen processing and antigen loading.