(A) Schematic representation of the purification of HLA class II peptides. HLA class II complexes are purified from cell lysates and plasma using either L243-conjugated resin (HLA DR) or SPVL3-conjugated resin (HLA DQ). After acid elution of HLA class II complexes, peptides are separated from proteins by stepwise elution from C18 resin and analyzed by LC-MS/MS. (B, C) Length distribution of identified peptides from HLA class II purifications. The number of peptides between 6 and 30 amino acids is plotted in relation to their length. Length distributions for HLA class II DR peptides (B) and for HLA class II DQ peptides (C) are shown. Left panel Maver-1, right panel DOHH2, respectively (D-G) Definition of HLA-specific motifs from HLA class II DR and DQ purifications. HLA class II-specific motifs were created from all unique peptide sequences with a minimum length of 9 amino acids isolated from Maver-1 (D, F) or DOHH2 cells (E, G) using the GibbsCluster-2.0 Server.