Phosphoantigen Presentation to TCR γδ Cells, a Conundrum Getting Less Gray Zones

Abstract

The mechanistic requirements of antigen recognition by T cells expressing a γδ TCR has revealed important differences with those of αβ TCR cells and, despite impressive new data generated in the very recent years, they remain poorly understood. Based on the structure of the TCR chains and the tissue distribution, γδ cells are represented in a variety of populations. The major subset of human peripheral blood γδ cells express Vγ9Vδ2 TCR heterodimers and are all stimulated by phosphorylated metabolites (commonly called phosphoantigens). Phosphoantigens are molecules with a very small mass and only stimulate Vγ9Vδ2 cells in the presence of antigen-presenting cells, suggesting a strict requirement for dedicated antigen-presenting molecules. Recent studies have identified butyrophilin (BTN) 3A1 as the molecule necessary to stimulate Vγ9Vδ2 cells. BTN3A1 extracellular, transmembrane, and cytoplasmic domains have different functions, including cognate interaction with the Vγ9Vδ2 TCR, binding of the phosphoantigens, and interaction with cytoplasmic proteins. This review mainly discusses the known molecular mechanisms of BTN3A1-mediated antigen presentation to γδ cells and proposes a model of phosphoantigen presentation, which integrates past and recent studies.

Keywords: antigen presentation; butyrophilin 3A1; infection control; tumor surveillance; γδ TCR.

Figure 1

Structure of the two most common phosphorylated metabolites stimulating Vγ9Vδ2 cells. The arrows indicate the parts of the molecules whose modifications significantly reduced immunogenicity.

Figure 2

Cladogram and schematic diagram of human BTN and BTNL family members. Protein sequences were aligned with ClustalW2 and the tree rendered using FigTree software (tree.bio.ed.ac.uk/software/figtree). Diagram shows the domain organization of members belonging to the BTN family consisting of two extracellular Ig-like domains, IgV and IgC, a single-pass transmembrane domain (TM) and an intracellular B30.2 domain. Exceptions include BTN1A1, consisting of a short IgV-like domain; BTN3A2, lacking a B30.2 domain; BTNL9, containing a single Ig-like domain; and BTNL2, consisting of four extracellular Ig-like domains and lacking a B30.2 domain.

Figure 3

Diagram of BTN3A1 topology. The extracellular Ig-like domains (green) and the intracellular B30.2 domains (orange) are illustrated here with available crystal structures (PDB IDs: 4F80 and 4N7U). The relative orientation of the domains is arbitrary as depicted by the dotted ovals. Site 1 and Site 2 represent the two identified binding sites of phosphoantigens.

Figure 4

Model of phosphoantigen presentation according to published data and mechanisms of phosphoantigen handling by APC. (1) Bacteria release HMBPP in the microenvironment or are internalized in phagosomes where HMBPP may be stored. (2) HMBPP passes phagosomal membranes and reaches BTN3A1 with unclear mechanisms. (3) Cytoplasmic IPP accumulated in the cytoplasm passes the membrane of endosomal compartments. (4) IPP may also pass the plasma membrane. (5) BTN3A1 traffics from plasma membrane to endosomal compartments where it encounters IPP. (6) IPP may also interact with the B30.2 cytoplasmic domain of BTN3A1. (7) Dimers of BTN3A1 containing IPP are formed and traffic to the plasma membrane. (8) The TCR Vγ9Vδ2 interacts with the BTN3A1 dimers and IPP facilitates this interaction.