Peptide–MHC class II complex (pMHC-II)

The peptide–MHC class II complex is formed when a processed peptide, typically derived from extracellular proteins, is loaded into the peptide-binding groove of a major histocompatibility complex class II (MHC-II) molecule[1][4][5]. This complex is expressed on the surface of professional antigen-presenting cells (APCs) such as dendritic cells, B cells, and macrophages, as well as thymic epithelial cells. Its primary biological role is the presentation of peptide antigens to CD4+ T lymphocytes, initiating and regulating adaptive immune responses[3][4][1]. The structure of the complex consists of an MHC-II heterodimer (α and β chains) with an open-ended peptide-binding groove, which allows binding of peptides generally 15-24 amino acids in length[1][3][4]. Peptide binding stabilizes the MHC-II molecule and is essential for T cell receptor (TCR) recognition. The interaction between the pMHC-II complex and TCR is the molecular basis for T cell activation, tolerance, and immunity. MHC-II allelic variants (notably HLA-DR, -DP, and -DQ in humans) determine which peptides can be presented, influencing disease susceptibilities, transplant compatibility, and vaccine responses[1][5][7]. pMHC-II complexes are implicated as therapeutic targets in autoimmunity, transplantation, and infection. Targeted drugs often do not interact with the complex directly but modulate its function or its downstream path via inhibition of costimulatory signals or altered antigen presentation. The broad expression and polymorphism of MHC-II pose challenges for targeted therapy, with concerns about unwanted immunosuppression or unintended activation of harmful immune responses[1][5].

Other names

pMHC class IIpeptide-loaded MHC class IIpeptide–MHC-II complexMHC class II–peptide complexHLA class II–peptide complex (for humans: HLA-DR/DP/DQ–peptide complex)

Blockade of T cell receptor recognition of pMHC-II complex (indirect, through costimulatory molecule inhibition); Inhibition or modulation of antigen presentation/peptide loading; Induction of T cell anergy or regulatory T cell responses by altered peptide ligand presentation