Antigen Processing and Presentation (PART I): MHC I Antigen Presentation pathway (FL-Immuno/25)

Summary

This video explains the concept of antigen processing and presentation, focusing on how B and T cells recognize different types of antigens. B cells can recognize various macromolecules, while T cells only recognize protein antigens. The video delves into the role of MHC molecules—class 1 presenting intracellular pathogen peptides, and class 2 presenting extracellular pathogen peptides. It explores how viral antigens are processed in infected cells, broken down by proteasomes, and presented to T cells through MHC class 1 molecules. The T cells recognize these MHC-peptide complexes with their receptors and coreceptor CD8.

Takeaways

• 😀 B cells and T cells recognize different types of antigens. B cells can recognize proteins, polysaccharides, nucleic acids, and lipids, while T cells can only recognize protein antigens in the form of peptides.
• 😀 T cells require the presentation of peptide fragments through MHC (Major Histocompatibility Complex) molecules to recognize antigens.
• 😀 MHC Class 1 molecules present peptides derived from intracellular pathogens (e.g., viruses), which are displayed on the surface of infected cells.
• 😀 MHC Class 2 molecules present peptides derived from extracellular pathogens (e.g., bacteria, toxins, parasites), and they are typically displayed by antigen-presenting cells (APCs).
• 😀 Antigen presenting cells (APCs) internalize extracellular pathogens, degrade them, and present peptide fragments using MHC Class 2 molecules to T cells.
• 😀 Antigen processing involves the degradation of pathogens and their products into peptide fragments, which are then presented by MHC molecules to T cells. This process is essential for immune recognition.
• 😀 The process by which pathogens and their products are degraded to produce peptide antigens is called antigen processing, while the presentation of these antigens to T cells is called antigen presentation.
• 😀 The antigen presentation pathway for intracellular antigens involves the processing of viral proteins by proteasomes, and the resulting peptides are transported to the rough endoplasmic reticulum.
• 😀 Peptides enter the rough endoplasmic reticulum through a protein complex known as TAP (Transporter Associated with Antigen Processing). The peptides then bind to MHC Class 1 molecules.
• 😀 The MHC Class 1 molecules, once bound to the peptides, move through the Golgi apparatus to the cell surface, where they are displayed for recognition by T cells.
• 😀 T cells recognize the MHC peptide complex on the surface of infected cells using their T cell receptors (TCR), and CD8 receptors interact specifically with the MHC Class 1 molecule.

Q & A

What is the key difference between B cells and T cells in terms of antigen recognition?

B cells can recognize a wide range of macromolecules, including proteins, polysaccharides, nucleic acids, and lipids, whereas T cells can only recognize protein antigens, specifically in the form of peptides.

What is required for T cells to recognize antigens?

For T cells to recognize antigens, the peptide fragments must be presented to them in the form of an MHC peptide complex.

What are the two classes of MHC molecules, and how do they differ in antigen presentation?

MHC class 1 molecules present peptides derived from intracellular pathogens, such as viruses, while MHC class 2 molecules present peptides from extracellular pathogens, such as bacteria and parasites.

What role do antigen-presenting cells (APCs) play in immune response?

Antigen-presenting cells (APCs) internalize extracellular pathogens, break them down into peptide fragments, and then present these fragments on MHC class 2 molecules for recognition by T cells.

What is antigen processing?

Antigen processing is the process by which pathogens and their products are degraded to produce peptide antigens that can be presented to T cells.

What happens during antigen presentation?

During antigen presentation, the MHC peptide complex formed inside the cell travels to the cell surface, where it displays the peptide fragment for recognition by T cells.

How are intracellular or endogenous antigens processed and presented to T cells?

In the case of intracellular pathogens like viruses, viral proteins are synthesized in the cytoplasm of the infected cell, degraded by the proteasome into peptide fragments, and then transported to the rough endoplasmic reticulum (ER). These peptides bind to MHC class 1 molecules and are presented on the cell surface for T cell recognition.

What is the function of the proteasome in antigen processing?

The proteasome degrades cytosolic proteins, including viral proteins, into peptides that are typically 8 to 15 amino acids long, which are then used in antigen presentation.

What is TAP, and how does it contribute to antigen processing?

TAP (Transporters Associated with Antigen Processing) is a protein complex that facilitates the transport of peptide fragments from the cytosol into the rough endoplasmic reticulum, where they bind to MHC class 1 molecules.

How do T cells recognize the MHC peptide complex on the surface of infected cells?

T cells recognize the MHC peptide complex using their T-cell receptors, which interact with the peptide. Additionally, CD8 coreceptors on cytotoxic T cells specifically recognize the Alpha 3 domain of MHC class 1 molecules, facilitating the immune response.