Structure of γδ T cell receptors and their recognition of non-peptide antigens

Abstract

The γδ T cell receptors (TCRs) and αβ TCRs are similar in both sequence and structure; however, γδ⁺ and αβ⁺ T cells are not merely similar lymphocytes with subtly different receptors. These cell types differ in several ways, including the types of antigens recognized, the mechanism of antigen presentation and recognition and the mechanism and kinetics of downstream signaling events. γδ TCRs can directly recognize antigens in the form of intact proteins or non-peptidic compounds, unlike αβ TCRs which recognize peptide antigens bound to major histocompatibility complex molecules (MHC). One of the major classes of human γδ⁺ T cells expresses Vγ9Vδ2 TCRs which recognize pyrophosphomonoester, alkylamine and aminobisphosphonate antigens. This review focuses on the recently determined structure of a Vγ9Vδ2 TCR, with emphasis on antigen recognition and receptor signaling.

Introduction

γδ T cell receptors (γδ TCRs), αβ T cell receptors (αβ TCRs) and antibodies are all products of gene rearrangement that provide the vertebrate immune system with the ability to recognize a diverse array of antigenic molecules. During lymphocyte maturation, rearrangement of the variable (V), diversity (D) and joining (J) gene segments creates an extensive array of antigen receptors. Combinations of V, D and J genes generate receptors with different amino-acid sequences and thus distinct molecular surfaces at their complementarity-determining regions (CDRs). The TCRs and antigen-binding fragments of antibodies (Fabs) are composed of two polypeptide chains each of which has two immunoglobulin-like domains: an amino-terminal variable (V) domain and a carboxy-terminal constant (C) domain (γδ TCR in Fig. 1). The γ and δ V domains are each composed of a “sandwich” of two β-sheets with five inner and four outer strands. The γ and δ CDR loops at the top of the V domains project out from the receptor. The γ and δ C domains are made up of two β-sheets with four inner and three outer strands.

Antigen recognition by γδ TCRs resembles recognition by antibodies (Hayday, 2000). Whereas αβ TCRs recognize peptide antigens bound to major histocompatibility complex molecules (MHC), γδ TCRs have been shown to recognize intact protein antigens and small, phosphate- or amine-containing compounds (Sciammas et al., 1994, Chien et al., 1996). Protein antigens include the murine nonclassical MHC class I molecule T22 (Crowley et al., 2000) and glycoprotein I from Herpes simplex virus (Sciammas et al., 1994) that do not appear to require presentation. Small-molecule antigens include pyrophosphomonoesters from Mycobacterium smegmatis, M. tuberculosis (Mtb) and Escherichia coli (Tanaka et al., 1995, Belmant et al., 1999, Hintz et al., 2001) and alkylamines from several natural sources (Bukowski et al., 1999). While MHC-presentation of these non-peptide antigens is not required, it has been shown that cell–cell contact is required for stimulation (Lang et al., 1995, Morita et al., 1995), suggesting either that non-MHC molecules may present small antigens to γδ TCRs or that costimulation from neighboring cells is required. As several reviews on γδ⁺ T cells have been published in recent years (Carding and Egan, 2000, Hayday, 2000, Kabelitz et al., 2000, Morita et al., 2000, Bendelac et al., 2001, Cai and Tucker, 2001), this review will focus primarily on the structure and biological implications of Vγ9Vδ2 TCRs which are stimulated by non-peptide antigens.

In humans and mice, there are fewer γδ TCR genes than αβ TCR genes (Table 1). Although, γδ TCRs have greater potential junctional diversity due to their ability to use multiple copies of their Vδ D genes, γδ TCRs generally show limited junctional diversity. In addition, these cells preferentially express tissue-specific, restricted sets of Vγ and Vδ genes that can serve as markers for different subsets of lymphocytes. For example, most intraepithelial γδ⁺ T cells of the nasal mucosa, small intestine and colon are Vδ1⁺. Similarly, about 5% of peripheral blood T cells bear γδ TCRs, most of which are Vγ9JγPVδ2⁺ and recognize non-peptide pyrophosphate- or amine-containing antigens, such as pyrophosphomonoesters from mycobacteria or isobutylamine from various sources (the nomenclature used here is from the Immunogenetics Database, http://imgt.cines.fr; in alternative nomenclature, Vγ9 is Vγ2 and JγP is Jγ1.2). Interestingly, there is no murine equivalent for the human Vγ9Vδ2⁺ subset of T cells. DeRosa et al., using multicolor fluorescence-activated cell sorting to determine the presence of several T cell surface markers, have concluded that the Vδ1⁺ and Vδ2⁺ T cell subsets represent distinct lineages with different developmental pathways (De Rosa et al., 2001).

γδ⁺ T cells have been implicated in several immunological roles, including both an immediate response to pathogenic invasion and long-term modulation of inflammation. Human Vγ9Vδ2⁺ T cells can exhibit a fast response, including the rapid release of pro-inflammatory chemokines (Cipriani et al., 2000) and provide a protective role against mycobacteria by directly killing infected macrophages (Dieli et al., 2000). Conversely, murine Vγ1Vδ6⁺ T cells are important for down-regulation of inflammation (Carding and Egan, 2000) and due to a lack of regulation of interferon-γ (IFN-γ) production, γδ T cell-deficient mice die from normally non-lethal doses of Listeria (Skeen et al., 2001). Several diseases also have been associated with γδ⁺ T cells, including systemic lupus erythematosus (Robak et al., 2001), multiple sclerosis and Guillain Barré syndrome (Poggi et al., 1999, Borsellino et al., 2000a, Borsellino et al., 2000b), myocarditis (Huber, 2000, Huber et al., 2000), and recurrent spontaneous abortion (Barakonyi et al., 1999, Polgar et al., 1999, Szekeres-Bartho et al., 1999).

Vγ9Vδ2⁺ T cells are able to kill both Mtb-infected macrophages and extracellular Mtb using granulysin, with aid from perforin for intracellular killing (Dieli et al., 2000, Dieli et al., 2001). In addition, blocking antibodies against either tumor necrosis factor-α (TNF-α) or Fas ligand (FasL) do not affect cytotoxicity (Dieli et al., 2000). However, pulmonary tuberculosis patients have reduced numbers of Mtb-reactive Vγ9Vδ2⁺ T cells as a result of activation-induced cell death that is mediated by Fas and FasL expression (Li et al., 1998a). Most mature Vγ9Vδ2⁺ T cells also express the inhibitory receptor CD94/NKG2 which recognizes the non-classical MHC class I HLA-E, thus increasing the TCR activation threshold (Battistini et al., 1997, Carena et al., 1997, Poccia et al., 1997).