Key Points• PD-L1/PD-1-mediated CD8T-cell dysfunction develops with CLL in different organs, and similarities to agingrelated immune defects exist.• PD-1 1 normal T cells have markedly different effector functions than PD-1 1 CLL T cells.T-cell defects, immune suppression, and poor antitumor immune responses are hallmarks of chronic lymphocytic leukemia (CLL), and PD-1/PD-L1 inhibitory signaling has emerged as a major immunosuppressive mechanism. However, the effect of different microenvironments and the confounding influence of aging are poorly understood. The current study uses the Em-TCL1 mouse model, which replicates human T-cell defects, as a preclinical platform to longitudinally examine patterns of T-cell dysfunction alongside developing CLL and in different microenvironments, with a focus on PD-1/PD-L1 interactions. The development of CLL was significantly associated with changes in T-cell phenotype across all organs and function. Although partly mirrored in aging wild-type mice, CLL-specific T-cell changes were identified. Murine CLL cells highly expressed PD-L1 and PD-L2 in all organs, with high PD-L1 expression in the spleen. CD3

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The bias of αβ T cells for MHC ligands has been proposed to be intrinsic to the T-cell receptor (TCR). Equally, the CD4 and CD8 coreceptors contribute to ligand restriction by colocalizing Lck with the TCR when MHC ligands are engaged. To determine the importance of intrinsic ligand bias, the germ-line TCR complementarity determining regions were extensively diversified in vivo. We show that engagement with MHC ligands during thymocyte selection and peripheral T-cell activation imposes remarkably little constraint over TCR structure. Such versatility is more consistent with an opportunist, rather than a predetermined, mode of interface formation. This hypothesis was experimentally confirmed by expressing a hybrid TCR containing TCR-γ chain germ-line complementarity determining regions, which engaged efficiently with MHC ligands. MHC restriction | TCRT cells expressing an αβ T-cell receptor (TCR) are MHCrestricted, recognizing self-and foreign peptide epitopes presented by MHC class I and II molecules during thymic development and peripheral activation, respectively. Two mechanisms are proposed to underlie this ligand bias. First, the CD8 and CD4 coreceptors have dual specificity for extracellular MHC and the intracellular proximal kinase Lck. Consequently, when MHC class I or II ligands are engaged, Lck is colocalized with the TCR/CD3 complex initiating signal transduction. The importance of this mechanism in disadvantaging non-MHC ligands is highlighted by the recovery of T-cell selection, where MHC and the coreceptors are both absent in comparison to the absence of MHC alone (1). In this setting, non-MHC ligands drive thymic positive selection and are recognized by peripheral T cells (2). The ability of αβ TCRs to recognize non-MHC ligands does not rule out an intrinsic bias of the TCR for MHC ligands. Indeed, evidence for such a hardwired bias is suggested by pairwise interactions between TCR-β germ-line complementarity determining regions 1 and 2 (CDR1/2) and the MHC α-helices observed in several structures (3-7). Recently, such recurrent interactions have been shown to be dependent on the partner TCR-α chain, which can impose distinct modes of TCR-β engagement, suggesting they may not drive MHC specificity (8). Although the relatively limited set of TCR/MHCpeptide structures reveals a semiconserved docking geometry, the angle of TCR engagement varies by more than 60°and the generally central docking position can shift toward the peptide aminoor carboxy-terminus (9). Likewise, conserved features of the MHC α-helices, including exposure of the polypeptide backbone and surface depressions, have been suggested to provide energetically favorable sites for CDR engagement (5). However, a crucial role for specific MHC residues in TCR docking has not emerged (10). The role of germ-line TCR structure in the bias to MHC ligands has thus been perplexing, especially given the structural variability of both components. To investigate this, we have applied a unique mutagenesis approach based on redirecting V(D)J recombina...