Tetramer assay

Tetramer stains usually analyze cytotoxic T lymphocyte (CTL) populations.[9] CTLs are also called CD8+ T-cells, because they have CD8 co-receptors that bind to MHC class I molecules. Most cells in the body express MHC class I molecules, which are responsible for processing intracellular antigens and presenting at the cell's surface. If the peptides being presented by MHC class I molecules are foreign—for example, derived from viral proteins instead of the cell's own proteins—the CTL with a receptor that matches the peptide will destroy the cell.[2][3] Tetramer stains allow for the visualization, quantification, and sorting of these cells by flow cytometry, which is extremely useful in immunology. T-cell populations can be tracked over the duration of a virus or after the application of a vaccine. Tetramer stains can also be paired with functional assays like ELIspot, which detects the number of cytokine secreting cells in a sample.[9]

An MHC tetramer binding to T-cell receptors (left), and an MHC molecule on the surface of an antigen presenting cell binding to T-cell receptors (right)

MHC tetramer molecules developed in a lab can mimic the antigen presenting complex on cells and bind to T-cells that recognize the antigen. Class I MHC molecules are made up of a polymorphic heavy α-chain associated with an invariant light chain beta-2 microglobulin (β2m). Escherichia coli are used to synthesize the light chain and a shortened version of the heavy chain that includes the biotin 15 amino acid recognition tag. These MHC chains are biotinylated with the enzyme BirA and refolded with the antigenic peptide of interest. Biotin is a small molecule that forms a strong bond with another protein called streptavidin. Fluorophore tagged streptavidin is added to the bioengineered MHC monomers, and the biotin-streptavidin interaction causes four MHC monomers to bind to the streptavidin and create a tetramer. When the tetramers are mixed with a blood sample, they will bind to T-cells expressing the appropriate antigen specific receptor. Any MHC tetramers that are not bound are washed out of the sample before it is analyzed with flow cytometry.[9]

Recent advancements within recombinant MHC molecules have democratised peptide MHC complex formulation and subsequent multimerisation. Highly active formulations of a broad range of MHC class I molecules[10] now allows non-experts users to make their own custom peptide-MHC complexes from day-to-day in any lab without special equipment.

Tetramers that bind to helper T-cells have also been developed.[9] Helper T-cells or CD4+ T-cells express CD4 co-receptors. They bind to class II MHC molecules, which are only expressed in professional antigen-presenting cells like dendritic cells or macrophages. Class II MHC molecules present extracellular antigens, allowing helper T-cells to detect bacteria,[11] fungi, and parasites.[2] Class II MHC tetramer use is becoming more common, but the tetramers are more difficult to create than class I tetramers and the bond between helper T-cells and MHC molecules is even weaker.[9][12]

Natural killer T-cells (NKT cells) can also be visualized with tetramer technology. NKT cells bind to proteins that present lipid or glycolipid antigens.[13] The antigen presenting complex that NKT cells bind to involves CD1 proteins, so tetramers made of CD1 can be used to stain for NKT cells.[9]