T cell receptor CDR3 loops influence alphabeta pairing

T cell receptor transfer is an attractive strategy for the generation of antigen specific T cells to target infection and malignancy. Cross pairing of the transduced and endogenous TCR chains produces new and potentially auto-reactive specificities and dilutes the therapeutic TCR. This is further complicated as the efficiency of pairing for each alphabeta pair is unpredictable and the factors which influence it are not well characterized. Complementarity determining region 3 (CDR3) loops are the main sources of TCR alpha and beta diversity due to nucleotide insertion and deletion at V(D)J junctions. Given the variability in composition and length of these non-germ line encoded structures, it is likely that structural strain may occur during formation of some TCR hetero-dimers contributing to the observed pairing restrictions. The beta chain of the HY specific T cell receptor C6 is such an example. Despite pairing efficiently with the C6 alpha chain, it pairs poorly with many other alpha chains. To investigate whether the long, C6 beta CDR3 region underlies this effect, it was replaced with a short, artificial CDR3 region that restored efficient pairing with the endogenous alpha chain repertoire. Molecular modelling is consistent with the beta chain CDR3 region causing steric incompatibility. Despite poor pairing and low surface expression, the WT C6 beta chain mediates positive selection in retrogenic mice.