QL9 (QLSPFPFDL) TFA is an alloantigen that exhibits high affinity for the 2C T cell receptor (TCR).

The mouse T cell clone 2C recognizes both the self MHC molecule K^b and the allogeneic MHC molecule L^d. Peptides SIY (SIYRYYGL) and QL9 (QLSPFPFDL) serve as potent, specific agonists when bound to K^b and L^d, respectively. The binding of QL9 to L^d is notably affected by most of its peptide side chains along its entire length. Research, particularly with the QL9-L^d system, indicates that single-residue modifications designed to enhance peptide-MHC binding and vaccine efficacy could detrimentally alter T cell reactivity by concurrently affecting TCR interaction. Activation assays are utilized to assess the impact of SIY and QL9 residues on T cell function [2].