Although it is true that hypervariable loops most commonly engage the peptide, germline loops (particularly CDR1) also frequently interact with the peptide and can contribute significantly to specificity [61C63]. receptor, affinity, protein design, structure, specificity, positive and negative design, molecular recognition 1.?Introduction T cell recognition of diseased cells is the cornerstone of cellular immunity. Recognition is facilitated by the T cell receptor (TCR), a heterodimeric biological sensor and signaling protein whose biochemistry has been intensely studied as immunotherapies for cancer, infectious disease, and autoimmunity have progressed from theoretical considerations to practical implementation. TCRs are similar to the antigen binding fragments of antibodies, but differ in key ways. One important difference is that whereas antibodies can be elicited against targets of seemingly unlimited structural and chemical diversity, TCRs in their normal function only recognize antigens bound and presented by proteins encoded by the major histocompatibility complex. This phenomenon, termed MHC restriction, is a fundamental feature of T cell biology (note that for simplicity we consider only peptide antigens and TCRs in this review, although the larger family of and TCRs recognize other antigens such as metabolites and lipids). Another key difference between TCRs and antibodies is the binding affinity these molecules have for their respective ligands. TCRs maintain relatively modest affinity for their ligands, typically in the mid-to-low micromolar range [1, 2]. Antibodies also bind with Talabostat mesylate modest affinities, but then undergo dramatic enhancements via the affinity maturation process. How MHC restriction and moderate binding affinities influence TCR specificity are major considerations for translating TCRs into new therapies. One consideration is the broad reactivity of T cells, and by extension, TCRs. Humans have a fixed number of TCRs, estimated to be in the tens of millions [3]. The universe of possible peptide antigens however is many orders of magnitude larger, resulting in a built-in requirement for cross-reactivity [4, 5]. The large number of potential targets relative to available receptors is evocative of the unmatured antibody repertoire. Indeed, keeping in mind the unique influences of MHC restriction and the added complexities of TCR signaling mechanisms, unmatured antibodies may be the closest analog to TCRs when considering molecular recognition properties. Indeed, a frequently heard adage that antibodies and TCRs differ when it comes to affinity and specificity is likely wrong – TCRs differ from antibodies, but in terms of ligand affinity and overall binding specificity, TCRs are quite similar to germline antibodies. In fact, some of the same language is used to describe both TCR and germline antibody molecular recognition [6C8]. Talabostat mesylate In this review, we discuss TCR affinity and specificity, focusing on their connection with the structural properties of TCRs and the interfaces they form with peptide/MHC complexes. Our review builds on other recent reviews Talabostat mesylate addressing TCR structural properties, TCR binding, and T cell function [9C13]. We conclude by emphasizing how the relationships between structure, affinity, and specificity might be exploited to help develop and optimize TCR-based therapeutics. 2.?Considerations Talabostat mesylate of affinity and specificity in clinical trials with TCR-gene modified T cells Pioneering studies from the 1980s to the early 2000s demonstrated that T cell specificity could be redirected via the transfer of TCR and genes [14C16]. These studies led to the proposal for TCR-based gene therapy of cancer [17], and were followed by several studies showing how anti-viral and anti-tumor immunity could be generated via gene-engineered T cells [18C21]. This work ultimately led to the first published clinical trial of TCR-based gene therapy for melanoma, in which patients received T cells transduced with genes encoding the DMF4 TCR targeting a MART-1 melanoma peptide presented SERPINE1 by the class I MHC protein HLA-A*0201 (HLA-A2) [22]. This first trial was followed by a second trial using the unrelated DMF5 TCR to target the same peptide [23]. Although the number of patients enrolled in the DMF4 and DMF5 trials was small, indications were that the DMF5 receptor showed greater promise, yet was also associated with incidences of autoimmune toxicity. An important difference between the DMF4 and DMF5 TCRs used in these early clinical trials was that the DMF5 TCR possessed stronger functional avidity (i.e., was more potent) in cellular experiments [24]. The stronger avidity of DMF5 was later shown to be associated with stronger TCR affinity towards the MART-1/HLA-A2 complex [25], consistent with earlier findings that T cell functional responses could be correlated with receptor binding affinity [11, 26C29]. Although outliers have always existed and some data suggest more nuanced interpretations (e.g., refs. [30, 31]), the clinical outcome seen.