The immune system has a near limitless capacity for detecting abnormalities. This remarkable ability for selfinterrogation is achieved by the related structures of two molecules, immunoglobulins and T cell receptors (TCR). The TCR, a defining structure of T cells, is a transmembrane heterodimer consisting of either an alpha and beta chain or delta and gamma chain linked by a disulphide bond. Within these chains are complementary determining regions (CDRs) which determine the antigen to which the TCR will bind. TCRs activate the T cells in which they reside leading to a plethora of immune responses. Harnessing the power of this response and of TCR specificity is leading to a new generation of extremely promising immunotherapies.
An overview of T cell receptors Mini-review
Initial descriptions of TCRs were made 30 years ago primarily through similarity with immunoglobulin DNA sequences. From this beginning, a clearer picture of TCRs as a pair of clone-specific, heterodimeric polypeptide chains consisting of both constant and variable regions has developed (Clambey et al. 2014). In humans, the majority of T cells express a TCR composed of alpha (α) and beta (β) chains (95%), and a smaller subset of T cells express a TCR with gamma (γ) and delta (δ) chains.
The TCR, through its CDRs, endows the T cell with the ability to recognize and respond to foreign or “non self” material. Antigen presenting cells (APCs) digest pathogens and display their fragments on major histocompatibility complex (MHC) molecules. This MHC/antigen complex binds to the TCR while other co-stimulatory molecules (e.g. CD28) are activated leading to T cell activation, proliferation, differentiation, apoptosis, or cytokine release (Samelson 2011). MHC/antigen complexes are, however, not the only molecules capable of interaction with TCRs. Non-peptide antigens such as lipids can interact with TCRs via some of the five isoforms of CD1 (a-e) (Mori and De Libero 2012), and several studies describe TCRs binding to metabolic intermediates bound to the MHC like molecule MR1 (Reantragoon et al. 2012).
TCRs have been described as the most intricate receptor structures of the mammalian immune system (Clambey et al. 2014). In the following sections, we describe how these unique structures are developed, the mechanisms used to drive immune responses via TCR signaling, the effects of their dysfunction and their exploitation as therapeutics.
Fig. 1. Diagram of TCR engagement with the peptide antigen MHC complex using the CD4 T cell as an example.
The TCR is a disulfide-linked membrane bound heterodimeric protein normally consisting of the highly variable α and β chains expressed as part of a complex with the invariant CD3 chain molecules. T cells expressing these two chains are referred to as α:β (or αβ) T cells, though a minority of T cells express an alternate receptor, formed by variable γ and σ chains, referred as γσ T cells. TCR development occurs through a lymphocyte specific process of gene recombination, which assembles a final sequence from a large number of potential segments. This genetic recombination of TCR gene segments in somatic T cells occurs during the early stages of development in the thymus. The TCRα gene locus contains variable (V) and joining (J) gene segments (Vβ and Jβ), whereas the TCRβ locus contains a D gene segment in addition to Vα and Jα segments. Accordingly, the α chain is generated from VJ recombination and the β chain is involved in VDJ recombination. This is similar for the development of γδ TCRs, in which the TCRγ chain is involved in VJ recombination and the TCRδ gene is generated from VDJ recombination (Figure 2). Recombination is temporally regulated by the access of recombinant activating gene (RAG) 1 and RAG2 to recognizable sequences. TCR recombination occurs at two stages during the process of T cell development. First, the β chain gene undergoes Dβ – Jβ rearrangement before Vβ – DJβ recombination in the double negative cells of the thymus. Rearrangement of the α chain gene takes place in double positive thymocytes. RAG1/2 bind to and introduce double strand breaks at recombination signal sequences (RSS), which flank all TCR gene segments. DNA repair machinery completes the recombination reaction. RAG1/2 are expressed by all lymphoid progenitors and immature T and B cells.
The TCR α chain gene locus consists of 46 variable segments, 8 joining segments and the constant region. The TCR β chain gene locus consists of 48 variable segments followed by two diversity segments, 12 joining segments and two constant regions. The D and J segments are located within a relatively short 50 kb region while the variable genes are spread over a large region of 1.5 mega bases (TCRα) or 0.67 megabases (TCRβ). Therefore, the process of VDJ recombination requires contraction of the entire locus by looping. In precursor cells, the gene loci are localized at the periphery of the nucleus where gene transcription is less active. However the proximity allows for D-J recombination at this stage. As differentiation progresses the gene locus moves to a more central nuclear position which likely promotes chromatin opening. As the cells commit to a particular lineage, the variable (V) gene loci undergo a contraction that places usually distant V genes next to the already arranged D-J segments. The contraction produces a looping that allows the different V genes access to the already combined DJ segment with similar frequency.
Fig. 2. Simplified overview of VJ & V(D)J recombination of alpha and beta chains of TCRs
A: TCR gene locus showing the (V) variable, (J) joining and (C) constant regions. Red triangles denote the RSS at the edge of each gene segment. DJ recombination of the beta chain occurs.
B: Following DJ recombination in the precursor cells. The locus has moved to a more central location. The V gene segments undergo the looping allowing each at chance at combination with the DJ segments
C: Combination of the selected V regions with DJ segment
D: Mature TCR transcripts showing the approximate location of complementary determining regions (CDRs)
The ultimate goal of the recombination process is diversity among the CDRs. CDRs are the antigen binding sections of TCRs and a diverse recognition capability leads to efficient protection against pathogens and the generation of optimal immune responses. VDJ recombination, along with the addition or deletion of nucleotides at the junctions between gene segments helps to generate an astounding amount of TCR diversity. The diversity generated by VDJ recombina