QL9 (QLSPFPFDL) is a high-affinity alloantigen for the 2C T cell receptor (TCR).
Mouse T cell clone 2C recognizes two different major histocompatibility (MHC) ligands, the self MHC Kb and the allogeneic MHC Ld. Two distinct peptides, SIY (SIYRYYGL) and QL9 (QLSPFPFDL), act as strong and specific agonists when bind to Kb and Ld, respectively. QL9 binding to MHC Ld is influenced by the majority of peptide side chains, distributed across the entire length of the peptide. Findings with both systems, but QL9-Ld in particular, suggest that many single-residue substitutions, introduced into peptides to improve their binding to MHC and thus their vaccine potential, could impair T cell reactivity due to their dual impact on TCR binding. T cell activation assays are performed to measure effects of peptide SIY and QL9 residues on T cell function[2].
[1]. Speir JA, et al. Structural basis of 2C TCR allorecognition of H-2Ld peptide complexes. Immunity. 1998 May;8(5):553-62. [2]. Bowerman NA, et al. Different strategies adopted by K(b) and L(d) to generate T cell specificity directed against their respective bound peptides. J Biol Chem. 2009 Nov 20;284(47):32551-61.