Dextramers

Dextramers are dextran-based multimeric reagents used to detect antigen-specific T cells by flow cytometry and related techniques. They consist of a dextran backbone to which multiple peptide–MHC (major histocompatibility complex) complexes are attached, creating a multivalent reagent that can engage T cell receptors with higher avidity than monomeric pMHC. The MHC class I dextramers present peptides to CD8+ T cells, while class II dextramers target CD4+ T cells; many products also carry a fluorophore for detection by fluorescence cytometry.

Construction typically involves synthesizing a defined molecular weight dextran scaffold, functionalizing it with reactive groups, attaching biotinylated or directly conjugated pMHC complexes, and labeling with a fluorescent dye. Some designs use a streptavidin-based assembly to achieve high valency. The exact valency and labeling vary by product.

Applications include enumeration and characterization of antigen-specific T cell populations in blood or tissue samples, monitoring immune responses to vaccines, infections, or cancer immunotherapies, and basic research into T cell recognition. Dextramers are particularly useful for detecting T cells with low-affinity TCRs that may be missed by lower-valency reagents.

Interpretation notes: proper controls and HLA matching are essential to minimize non-specific binding. Because high valency can increase background, experiments should include irrelevant peptide controls and unstained or single-color controls. Dextramers are complementary to tetramer reagents rather than a universal replacement.