The CLIP-substituted invariant chain efficiently targets an antigenic peptide to HLA class II pathway in L cells

The presentation of antigenic peptides by major histocompatibility complex (MHC) class II to CD4+ T cells is crucial to initiate immune responses. We developed a new system for delivery of an antigenic peptide to the MHC class II pathway, using the invariant chain (Ii). We designed a mutated human p33-form Ii, CLIP-substituted Ii, in which streptococcal M12p55-68 (RDLEQAYNELSGEA) was substituted for CLIP (class II associated invariant chain peptide). We examined the peptide presenting function of this construct, in comparison with the previously reported C-terminal fused Ii, in which a cathepsin cleavage site and M12p54-68 was ligated to the C-terminus of Ii. Mouse L cell transfectants expressing either of these two mutated Ii along with HLA-DR4 could process and present M12p55-68 to the peptide specific and DR4-restricted CD4+ T cell clone. CLIP-substituted Ii was much more efficient in antigen presentation than was the C-terminal fused Ii. Similar to the wild-type Ii, the CLIP-substituted Ii was associated intracellularly with DR4 molecules. These results indicate that the peptide substituted for CLIP of Ii p33 bound to the groove of DR molecules in the same manner as CLIP and it was preferentially presented to the CD4+ T cell clone in the absence of HLA-DM molecules. This system may prove useful for immunotherapy with DNA vaccines or for construction of an antigen presenting cell library with diverse peptides.