MHC (Major Histocompatibility Complex) is a major histocompatibility complex, which is a highly polymorphic family of cell surface proteins, also known as HLA in humans. MHC can bind to peptide fragments of intracellular antigens to form MHC-peptide complexes, which are then transported to the cell surface and recognized by the corresponding T cell receptor (TCR) to initiate an immune response.
In humans, the main types of MHC involved in antigen presentation are MHC I and MHC II. MHC I, after binding antigen peptides, is recognized by CD8+ T cells, while MHC II, after binding peptides, is recognized by CD4+ T cells. MHC-peptide complexes represent a category of intracellular antigen targets. Their unique binding pattern with TCR plays a crucial role not only in adaptive immune processes but also holds significance for TCR-related therapies such as TCR-T development.
MHC Class I (MHC-I)
MHC-I molecules play a crucial role in the immune system by presenting peptide antigens to cytotoxic T cells. These heterotrimers consist of a transmembrane heavy chain, a light chain known as β2-microglobulin (β2m), and an 8-10 peptide antigen. The heavy chain contains two peptide binding domains (α1 and α2), an immunoglobulin-like domain (α3), and a transmembrane region. The folding of the α1 and α2 domains forms a groove where peptide antigens bind to the MHC-I molecule. β2m stabilizes the peptide binding groove and MHC I presentation.
A single nucleated cell expresses 105 copies of each MHC I molecule, presenting a variety of peptides simultaneously on the cell surface to CTLs. Accumulating genomic mutations in cancers result in the production of tumor-specific antigens or neoantigens, which can be presented by MHC I molecules of tumor cells to CTLs.
MHC Class 1 complex (MHC-I) containing an α-chain spanning the cell membrane and an extracellular β2m (β2-microglobulin) connected to this chain.
MHC Class II (MHC-II)
MHC class II molecules are pivotal components of the immune system, playing an instrumental role in initiating and modulating immune responses, particularly in the context of extracellular pathogenic invasions. These molecules are primarily expressed on the surface of antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells, which seize, process, and present antigens derived from extracellular pathogens to CD4+ T helper cells. The MHC class II molecule is a heterodimer, composed of an alpha (α) and a beta (β) chain, each contributing to the formation of the peptide-binding groove that accommodates antigenic peptides.
Once an extracellular pathogen, such as bacteria, is ingested by an APC, its proteins are broken down into smaller peptide fragments within the endosomal compartments. These antigenic peptides are then loaded onto the MHC class II molecules and transported to the cell surface. Subsequently, the MHC class II-peptide complex is recognized by the T-cell receptor (TCR) on CD4+ T cells, culminating in a cascade of immune responses that include the activation and clonal expansion of the T cells, the production of antibodies by B cells, and the orchestration of an immune response tailored to eliminate the pathogenic threat. Thus, MHC class II molecules are crucial in bridging innate and adaptive immunity, safeguarding the organism against a myriad of microbial threats while also being implicated in several immunological disorders and autoimmune diseases due to their role in shaping immune responses.
T-Cell Interactions with MHC Tetramers
Measuring T-cell responses is essential for better understanding how new vaccines and treatments work, and analyzing disease development or recovery. In immune system research, class I and class II MHC Tetramers, widely used for studying T cell responses, have become crucial in various disease and vaccine studies.
T-cell antigen receptors (TCR) are crucial, as they identify peptides presented on antigen-presenting cells. Cytotoxic T lymphocytes (CTLs) identify antigenic peptides presented through MHC class I molecules on cell surfaces, which leads to the lysis of the recognized target cells. Therefore, detecting such antigen-specific T cells becomes critically important, especially during viral infections and post-vaccination studies.
Peptide-MHC Tetramers are complexes composed of four fluorophore-labeled MHC molecules, with each monomer binding to a specific peptide. Given their tetrameric structure, they can bind a T-cell with up to three of its four MHC monomeric units, thereby enhancing its total binding avidity to the targets. These complexes enable the direct visualization and quantification of antigen-specific T cells in MHC tetramer assays.
Binding Interactions of Peptide-MHC Monomers vs Peptide-MHC Tetramers with TCRS
Figure 2. (Left) Peptide-MHC Monomers demonstrate weak binding affinity with TCRs. (RIght) Peptide-MHC tetramers bind multiple monomeric units to TCRs to increase binding affinity. Image created with biorender.com
Applications and Advancements of MHC Tetramers
In practical applications, analyzing samples like PBMCs or whole blood mixed with MHC-peptide tetramers via flow cytometry enables the detection and sorting of CD4+ or CD8+ T cells specific to a peptide antigen of interest. This analysis also sheds light on the presence and strength of certain cell-mediated immune responses to specific pathogens from which the antigen peptide is derived. The use of tetramers has allowed the incorporation of a broader spectrum of HLA molecules, which has improved their integration with functional assays.
Since the inception of MHC Tetramers, they have been universally utilized for quantifying antigen-specific T cell responses. They are now synonymous with the term “tetramers” due to the prevalent use of biotin-labeled pMHC displayed on streptavidin molecules, forming tetravalent complexes. When combined with methods predicting peptide binding to MHC molecules, tetramer analysis proves exceedingly effective for identifying T cell epitopes, especially in studies of tumor antigens and self-antigens.
KACTUS MHC Complex Products
KACTUS offers a selection of high quality MHC complex products including:
- → Monomers
- → Tetramers
- → Various Alleles
- → Biotinylated
- → Fluorophore-labeled tetramers for flow cytometry
- → Human, Mouse, Cynomolgus, Rhesus macaque
- → Chimeric MHCs
MHC Product Features
- → > 95% as determined by Tris-Bis PAGE
- → > 95% as determined by HPLC
- → Low Endotoxin (< 1EU per μg by the LAL method)
- → His-Avi tag