Exogenous Antigen Presentation Pathway via MHC II Processing

T lymphocyte activation requires three essential signals: antigen recognition via TCR-MHC binding, co-stimulation from antigen-presenting cells, and cytokines from multiple sources. MHC class II presents antigens to CD4+ T cells, while MHC class I presents to CD8+ T cells. CD4+ responses derive from dendritic cells processing ingested microorganisms or vaccine antigens, while CD8+ responses arise from cross-presentation of intracellular pathogens. Co-stimulators are critical second signals; without them, T cells become anergic or apoptotic. The CD28-B7 pathway is central: CD28 binds B7 molecules (CD80/CD86) on activated APCs. B7 expression is tightly regulated—constitutive low levels with upregulation upon activation. CD28 has high affinity for B7 and couples when B7 levels are low, while CTLA4 couples when B7 levels are high, creating a balance that initiates or terminates responses. The CD40-CD40L pathway provides positive feedback: activated T cells express CD40L binding to CD40 on APCs, stimulating B7 expression and IL-12 secretion. Upon activation, T cells upregulate CD69 (retaining them in lymphoid organs), downregulate S1PR1 (enabling migration), and express CD44 (facilitating tissue retention).

T cell activation requires two essential signals: the first signal involves T cell receptor (TCR) binding to peptide-MHC complexes on antigen-presenting cells, while the second signal comes from co-stimulation via CD28 interacting with B7 proteins (CD80/CD86); these signals trigger intracellular cascades involving Lck, ZAP-70, LAT, and SLAP-76 proteins that activate PLC, MAPK, and Vav pathways, ultimately leading to T cell proliferation and differentiation into effector subsets through transcription factors like AP-1 and NF-kB.

T cells originate from bone marrow hematopoietic stem cells but mature in the thymus, undergoing positive selection (retaining self-MHC recognizing cells) and negative selection (eliminating self-reactive cells). Classified as CD4+ (helper) or CD8+ (cytotoxic) based on surface markers, T cells express TCR for antigen recognition, co-receptors (CD4 binds MHC II, CD8 binds MHC I), CD3 for signal transduction, and CD28 for co-stimulation. Full activation requires three signals: TCR-MHC binding (Signal 1), CD28-B7 interaction (Signal 2), and cytokine signaling (Signal 3). Dendritic cell-released cytokines determine differentiation: IL-12 drives Th1 (IFN-γ production, macrophage activation), while IL-4 drives Th2 (B cell support via IL-4, IL-5, IL-13).

T helper cells (CD4+) require two signals for full activation: the first signal is antigen binding to the T cell receptor (TCR), and the second signal is co-stimulation through B7 molecules on antigen-presenting cells interacting with CD28 receptors on T cells; upon receiving both signals, the selected T cell clone undergoes rapid mitosis (clonal expansion) and secretes interleukin-2 to amplify the immune response.

T cell activation requires two signals: the first signal involves TCR binding to antigen-MHC complex with CD4 coreceptor binding to MHC II (for CD4+ T cells) or CD8 coreceptor binding to MHC I (for CD8+ T cells), while the second signal (co-stimulation) involves B7/CD80 on antigen-presenting cells interacting with CD28 on T cells; without co-stimulation, T cells enter anergy, but with both signals, T cells proliferate and differentiate into effector and memory cells.

Cross-presentation is the process by which exogenous antigens are presented on MHC Class I molecules to cytotoxic T cells. This allows cytotoxic T cells to recognize and respond to extracellular pathogens that have been internalized by antigen presenting cells. Cross-presentation is important for initiating immune responses against viruses and tumors, as it enables cytotoxic T cells to respond to antigens that would otherwise only be presented on MHC Class II molecules.

Cross-presentation is the process by which dendritic cells present exogenous antigens on MHC class I molecules to activate CD8+ cytotoxic T cells, rather than presenting them on MHC class II molecules to CD4+ helper T cells; this specialized mechanism allows dendritic cells to sample antigens from nearby infected or dead cells and quickly trigger cytotoxic T cell responses against viral infections, though it requires careful regulation to prevent autoimmune reactions when self-antigens might be presented.

Exogenous antigens enter cells through endocytosis or phagocytosis and are degraded in acidic endosomal compartments; MHC class II molecules, generated in the ER and stabilized by the invariant chain (which blocks premature peptide binding), undergo CLIP removal by HLA-DM in the MHC class II loading compartment (M2C) to load antigenic peptides, with HLA-DO serving as a negative regulator, while dendritic cells uniquely perform cross-presentation by redirecting exogenous antigens into the cytoplasm for MHC class I presentation to CD8 T cells.

ER-resident proteins can be presented on MHC class I through a specialized export pathway. Additionally, cross-presentation allows exogenous antigens (from phagocytosed material) to be presented on MHC class I molecules. This occurs in dendritic cells and macrophages that phagocytose dead cells, with antigens escaping lysosomes into the cytoplasm for proteasomal processing. Cross-presentation is essential for generating CD8+ T cell responses against pathogens that do not infect antigen-presenting cells directly, such as liver-tropic viruses that infect hepatocytes rather than dendritic cells.

Intracellular antigens (from viruses, abnormal proteins) are degraded by the proteasome into peptides. These peptides are transported into the endoplasmic reticulum by TAP (Transporter associated with Antigen Processing) proteins. In the ER, peptides bind to newly synthesized MHC Class I molecules, which are then transported to the cell surface for presentation to CD8+ cytotoxic T cells. Cross-presentation allows dendritic cells to present extracellular antigens via MHC Class I molecules to CD8+ cytotoxic T cells. This process is crucial for initiating immune responses against viruses and tumors. Normally, extracellular antigens are presented via MHC Class II, but cross-presentation enables CD8+ T cells to recognize and destroy cells infected with intracellular pathogens. T cells undergo 'education' in the thymus where they are exposed to self-antigens presented by thymic epithelial cells. This process eliminates autoreactive T cells that could attack the body's own tissues. Successful education ensures T cells only respond to foreign antigens, preventing autoimmune diseases.

MHC II epitope prediction involves analyzing peptide sequences to identify binding regions that can activate T-cells, using algorithms that break sequences into segments, align them with known MHC II receptors, and assign binding affinity scores; the prediction process includes filtering results by percentage scores (typically 0-10% for high-affinity binders) and removing redundant sequences to select diverse, non-redundant epitopes for vaccine design.

The MHC II antigen presentation pathway processes extracellular pathogens by internalizing them through endocytosis or phagocytosis into endosomes that fuse with lysosomes to form endolysosomes, where proteolytic enzymes digest pathogens into 11-15 amino acid peptides; simultaneously, MHC II molecules synthesized in the rough endoplasmic reticulum associate with invariant chains that block premature peptide binding, and after transport to vesicles where invariant chains are degraded, the resulting CLIP fragment is displaced by antigen-derived peptides which then bind to MHC II molecules before being transported to the cell surface for recognition by CD4+ T cells via T cell receptors.

The exogenous pathway (MHC Class II) processes extracellular proteins via endocytosis, while the endogenous pathway (MHC Class I) processes intracellular proteins via the proteasome. Many viruses evade immune detection by inhibiting MHC Class I presentation at multiple stages, including blocking TAP transporter function and preventing peptide loading.

MHC class I pathway: Intracellular proteins (viral, misfolded, or tumor proteins) are tagged with ubiquitin and degraded by the proteasome. Antigens are transported to the endoplasmic reticulum via TAP protein, loaded onto MHC class I with tapasin assistance, then transported to the cell surface via Golgi. CD8+ T cells recognize these complexes and kill infected cells. Viruses can evade detection by preventing MHC maturation or antigen processing. MHC class II pathway: Only on professional APCs. After phagocytosis, the phagosome fuses with lysosome, breaking down bacteria into antigens. MHC class II is produced in the ER bound to invariant chain (with CLIP at the end). In the Golgi, HLA-DM replaces CLIP with antigen, then MHC class II-antigen complex travels to the cell surface where CD4+ T cells recognize it and activate B cells.

Intracellular pathogens have evolved sophisticated survival strategies within host cells. While spending most of their life cycle inside host cells, they must temporarily exit to extracellular fluid when moving between cells, becoming vulnerable to phagocytosis. Antigen-presenting cells then display pathogen fragments on MHC Class II molecules, activating CD4+ T helper cells. Some pathogens, like Mycobacterium tuberculosis and Listeria monocytogenes, can survive within phagosomes by blocking lysosome fusion. These phagosomal pathogens still trigger MHC Class II presentation, demonstrating how pathogen localization determines immune recognition pathways.