Major histocompatibility complex class I alpha 3 domain and beta-2 microglobulin (MHC I α3/B2M)

The Major Histocompatibility Complex (MHC) class I α3 domain and beta-2 microglobulin (B2M) form the non-polymorphic, constant region of the MHC class I molecule, which is vital for the adaptive immune response (UniProt P61769, P04439) [1, 2]. While the α1 and α2 domains are responsible for presenting intracellular peptides, the α3 domain acts as the essential binding site for the CD8 co-receptor on cytotoxic T cells, facilitating the stabilization of the T-cell receptor (TCR)-MHC interaction (Salter et al., 1990) [3]. B2M is a small protein that non-covalently associates with the MHC I heavy chain, a process necessary for the proper folding and transport of the complex to the cell surface (Bjorkman et al., 1987) [4]. Without B2M, MHC I molecules cannot be stably expressed, leading to a failure in antigen presentation. In clinical oncology, the downregulation or loss of B2M is a well-documented mechanism of immune escape, as it prevents the surface expression of MHC I and allows tumor cells to avoid detection by CD8+ T cells. Additionally, soluble B2M serves as a significant biomarker in hematologic malignancies, such as multiple myeloma, and its accumulation in patients with chronic renal failure can lead to dialysis-related amyloidosis (StatPearls) [5]. Therapeutic strategies targeting this complex include the use of antibodies to stabilize MHC I expression in "cold" tumors or the development of agents like Ab-02 that target B2M to treat amyloidosis (Plieth, 2016) [6]. Other approaches involve modulating the α3-CD8 interface to treat autoimmune disorders or prevent organ transplant rejection.

Other names

HLA class I alpha 3 domainBeta-2 microglobulinB2MMHC-I constant regionCD8 binding domainMHC class I light chain

The α3 domain and B2M complex facilitates the binding of the CD8 co-receptor to the MHC class I molecule, which stabilizes the immunological synapse and enhances T-cell receptor (TCR) signaling for cytotoxic activity.