Peptide-specific, TCR-alpha-driven, coreceptor-independent negative selection in TCR alpha-chain transgenic mice

The T-cell receptor is not hardwired to engage MHC ligands

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.

T-cell receptor retrogenic mice: a rapid, flexible alternative to T-cell receptor transgenic mice

The T-cell receptor (TCR) is unique in its complexity. It determines not only positive (life) and negative (death) selection in the thymus, but also mediates proliferation, anergy, differentiation, cytotoxicity and cytokine production in the periphery. Through its association with six CD3 signalling chains (εγ, δε and ζζ), the TCR is capable of recognizing an extensive variety of antigenic peptides, from both pathogens and self-antigens, and translating these interactions into multiple signalling pathways that mediate diverse T-cell developmental and functional responses. The analysis of TCR biology has been revolutionized by the development of TCR transgenic mice, which express a single clonotypic T-cell population, with diverse specificities and genetic backgrounds. However, they are time consuming to generate and characterize, limiting the analysis of large numbers of TCR over a short period of time in multiple genetic backgrounds. The recent development of TCR retrogenic technology resolves these limitations and could in time have a similarly important impact on our understanding of T-cell development and function. In this review, we will discuss the advantages and limitations of retrogenic technology compared with the generation and use of TCR transgenic mice for studying all aspects of T-cell biology.

Self-tolerance checkpoints in CD4 T cells specific for a peptide derived from the B cell antigen receptor

Linked recognition of Ag by B and T lymphocytes is ensured in part by a state of tolerance acquired by CD4 T cells to germline-encoded sequences within the B cell Ag receptor (BCR). We sought to determine how such tolerance is attained when a peptide from the BCR variable (V) region is expressed by small numbers of B cells as it is in the physiological state. Mixed bone marrow (BM) chimeras were generated using donor BM from mice with B cells that expressed a transgene (Tg)-encoded κ L chain and BM from TCR Tg mice in which the CD4 T cells (CA30) were specific for a Vκ peptide encoded by the κTg. In chimeras where few B cells express the κTg, many CA30 cells were deleted in the thymus. However, a substantial fraction survived to the CD4 single-positive stage. Among single-positive CA30 thymocytes, few reached maturity and migrated to the periphery. Maturation was strongly associated with, and likely promoted by, expression of an endogenous TCR α-chain. CD4(+) CA30 cells that reached peripheral lymphoid tissues were Ag-experienced and anergic, and some developed into regulatory cells. These findings reveal several checkpoints and mechanisms that enforce a state of self-tolerance in developing T cells specific for BCR V region sequences, thus ensuring that T cell help to B cells occurs through linked recognition of foreign Ag.