antigen processing and presentation of exogenous peptide antigen via MHC class II

Definition

Target type: biologicalprocess

The process in which an antigen-presenting cell expresses a peptide antigen of exogenous origin on its cell surface in association with an MHC class II protein complex. The peptide antigen is typically, but not always, processed from a whole protein. [GOC:add, ISBN:0781735149, PMID:15771591]

Exogenous antigen processing and presentation via MHC class II involves a complex series of steps that enable the immune system to recognize and respond to foreign proteins. This process begins with the uptake of exogenous antigens, such as bacteria, viruses, or toxins, by antigen-presenting cells (APCs). APCs, including macrophages, dendritic cells, and B cells, are specialized cells that can internalize and process antigens for presentation to T lymphocytes.

1. **Antigen Uptake:** Exogenous antigens are taken up by APCs through various mechanisms, such as phagocytosis (engulfment of large particles), endocytosis (uptake of smaller particles), or receptor-mediated endocytosis (specific binding of antigens to cell surface receptors).

2. **Antigen Processing:** Once internalized, exogenous antigens are processed within specialized compartments called endosomes. The endosome undergoes a series of maturation steps, becoming increasingly acidic and acquiring lysosomal enzymes. These enzymes degrade the antigen into smaller peptide fragments, typically 13-25 amino acids in length.

3. **MHC Class II Loading:** MHC class II molecules are expressed on the surface of APCs and are responsible for presenting processed antigens to CD4+ T lymphocytes. Newly synthesized MHC class II molecules are assembled in the endoplasmic reticulum (ER) and transported to endosomes. However, they are initially associated with a chaperone protein called invariant chain (Ii), which prevents premature loading of peptides.

4. **Invariant Chain Degradation:** As the endosome matures, Ii is degraded, leaving a small fragment called CLIP (class II associated invariant chain peptide) bound to the MHC class II molecule.

5. **Peptide Exchange:** The processed peptide fragments from the exogenous antigen compete with CLIP for binding to the MHC class II molecule. This exchange is facilitated by a specialized chaperone protein called HLA-DM.

6. **MHC Class II-Peptide Complex Formation:** Once a peptide binds to the MHC class II molecule, it forms a stable complex. This complex is then transported to the cell surface for presentation.

7. **T Cell Recognition:** The MHC class II-peptide complex on the APC surface is recognized by the TCR (T cell receptor) of CD4+ T lymphocytes. This interaction triggers activation of the T cell, leading to the production of cytokines and the differentiation of effector T cells, which can eliminate the antigen or activate other immune cells.'
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Proteins (15)

Compounds (129)