Invariant chain is a new chaperone for TLR7 in B cells.

The innate immune system provides the first barrier against pathogens. Intracellular Toll-like receptors (TLR3, 7 and 9) localise in endosomes and sense nucleotides from viruses and bacteria. This recognition induces their conformational changes resulting in the production of proinflammatory cytokines and MHC class II (MHCII) antigenic presentation. In the absence of stimulation, TLRs are retained in the endoplasmic reticulum. Upon stimulation, they relocate to the endo-lysosomal compartment, allowing the recruitment of the adaptor molecules, MyD88 or TRIF. Increasing evidences describe a cross talk between proteins that regulate both innate and adaptive immune responses. For example, proteolytic enzymes which are required for breaking down exogenous antigen to generate suitable peptides for MHCII molecules are also essential to activate endosomal TLRs and MHCII molecules were recently described to regulate TLR signalling. But other proteins are possibly involved and regulated differentially between cell types. We have observed that intracellular TLR trafficking and signalling in B cells are different from dendritic cells and macrophages and involved the MHCII chaperone molecule, the invariant chain (Ii).