Ag Processing and Presentation Flashcards
(13 cards)
Cytosilic - Endogenous Pathway Introduction
Most nucleated cells express MHC class I molecules and can function as target cells by presenting cytosolic peptides to CD8+ T cells. Endogenous antigens — including viral proteins, tumour antigens, or defective self-proteins — are degraded via the ubiquitin-proteasome system. Proteins tagged with ubiquitin are directed to the 26S proteasome, composed of a 20S catalytic core and 19S regulatory caps. In the proteasome’s central chamber, ATP-dependent cleavage generates peptides 8–10 amino acids in length.
Overview/Introduction
Antigen (Ag) processing and presentation are critical for adaptive immunity, allowing T cells to recognize pathogen-derived peptides only when presented in the context of self-major histocompatibility complex (MHC) molecules — a phenomenon termed MHC restriction. This concept was defined through pioneering work by Zinkernagel and Doherty (Nobel Prize, 1996), who showed that T cell responses require both a specific antigenic peptide and recognition of self-MHC. CD8+ T cells detect endogenous peptides via MHC I; CD4+ T cells respond to exogenous peptides via MHC II. The distinction between these pathways is foundational to understanding immune responses to viruses, bacteria, and cancer.
T cell receptors (TCRs) engage both the antigenic epitope and polymorphic residues of the presenting MHC molecule. During thymic maturation, T cells are positively selected for reactivity to self-MHC, and thus are “educated” to recognize antigens only in that context. Antigenic peptides are generated and loaded onto MHC molecules for surface display.
Immunoproteosome
In response to interferon-γ (IFNγ), cells form the immunoproteasome, incorporating alternative catalytic subunits (LMP2/β1i, LMP7/β5i, and MECL-1/β2i), which generate peptides with carboxy-terminal hydrophobic residues optimal for MHC class I binding. IFNγ also induces expression of the PA28 proteasome-activator complex, a ring structure that binds the 20S core and enhances peptide output.
ER + TAP
Peptides generated in the cytosol are translocated into the endoplasmic reticulum (ER) by the TAP1/2 transporter, a heterodimeric ATP-binding cassette (ABC) protein complex which favors 8–10mer peptides with hydrophobic C-termini. Within the ER, peptides associate with nascent MHC class I molecules, which consist of a polymorphic α-chain and invariant β2-microglobulin. Proper folding and assembly are aided by chaperones: initially, calnexin stabilizes the α-chain; after β2m binding, calreticulin, tapasin, and ERp57 form a peptide-loading complex (PLC) that facilitates high-affinity peptide loading. Tapasin tethers the MHC molecule to TAP and acts as a peptide editor. Once loaded with peptide, the stable MHC I complex dissociates from the PLC and is transported via the Golgi to the cell membrane.
ERAP1 + BLS
ERAP1 (an aminopeptidase), induced by IFNγ, trims peptides to optimal MHC I length. Mutations in TAP or MHC assembly components can cause bare lymphocyte syndrome, a condition characterized by impaired CD8+ responses and chronic infections.
HLA-E/CD94:NKG2A
An important regulatory mechanism linked to MHC class I presentation involves the non-classical MHC molecule HLA-E, which presents leader sequence peptides derived from classical MHC class I proteins. CD94:NKG2A receptor, a disulfide-linked heterodimer expressed on NK cells and some CD8+ T cells monitors HLA-E expression. Binding of CD94:NKG2A to HLA-E delivers an inhibitory signal, preventing the cytotoxic killing of cells that maintain normal MHC class I expression. Thus, this receptor acts as a safeguard, allowing immune cells to detect when pathogens or tumours attempt to evade immune surveillance by downregulating MHC I, as the resulting decrease in HLA-E leads to loss of inhibition and NK cell activation.
Endocytic - Exogenous Pathway Introduction
MHC class II expression is limited to professional antigen-presenting cells (APCs) — dendritic cells (DCs), macrophages, and B cells — which activate CD4+ T cells. These cells internalize extracellular antigens through phagocytosis, pinocytosis, or receptor-mediated endocytosis (including via surface-bound antibody or complement receptors). B cells internalize antigen specifically via their membrane-bound immunoglobulin.
Peptide formation
Following uptake, antigens enter the endocytic pathway, where they are processed in progressively acidified compartments: early endosomes (pH 6–6.5), late endosomes/MIICs (pH 5–6), and lysosomes (pH 4.5–5). In these vesicles, antigens are degraded by proteases into peptides of 13–18 amino acids. Protease inhibitors and pH modulators can block processing, confirming that low pH and proteolysis are essential. Different APCs process antigens differently, influencing T cell specificity.
Invariant Chain + CLIP
Class II MHC molecules are synthesized in the ER, where their peptide-binding grooves are protected by the invariant chain (Ii/CD74). This trimeric chaperone directs MHC II molecules into endocytic vesicles via cytoplasmic targeting signals. As MHC II-Ii complexes traverse the pathway, Ii is degraded by lysosomal proteases, leaving a small fragment called CLIP (class II-associated invariant chain peptide) in the binding groove.
HLA-DM/HLA-DO + Presentation
For antigenic peptides to bind, CLIP must be exchanged, a process catalyzed by the non-classical MHC molecule HLA-DM. HLA-DM stabilizes empty MHC II molecules and facilitates high-affinity peptide loading. HLA-DO, another non-classical MHC molecule, can inhibit HLA-DM and modulate peptide exchange. Once peptide binding occurs, the stable MHC II complex moves to the cell surface to present antigen to CD4+ T cells. Like MHC I, this complex requires peptide for structural stability at the neutral pH of the plasma membrane.
Cross-presentation Introduction
A unique exception to the pathway segregation is cross-presentation, predominantly mediated by dendritic cells. In this process, exogenous antigens are presented on MHC class I to activate CD8+ T cells, a mechanism essential for antiviral and antitumor immunity when pathogens do not infect APCs directly. Cross-presentation occurs via cytosolic or vacuolar routes, allowing exogenous antigens to access MHC I.
Defects/Clinical
Defects in peptide loading, chaperone function, or MHC expression can lead to immunodeficiency or autoimmunity. Conversely, many pathogens evade immunity by inhibiting TAP, downregulating MHC expression, or producing MHC mimics. On the clinical side, manipulating antigen presentation is central to vaccine design, cancer immunotherapy, and tolerance induction in autoimmune diseases or transplantation.
Conclusion/Historical Context
In conclusion, antigen processing and presentation are central to the coordination of innate and adaptive immunity. MHC presentation enable the immune system to detect infections, eliminate malignant cells, and maintain self-tolerance. The historical insights of immunologists such as Unanue, Shevach, and Townsend have uncovered the intricacies of these processes, setting the stage for therapeutic advances rooted in fundamental immunobiology.
