Cross-Presentation

Cross-presentation is the specialized immunological antigen-processing pathway by which professional antigen-presenting cells—principally conventional type 1 dendritic cells (cDC1)—internalize exogenous protein antigens from extracellular virions, intracellular bacteria, transplanted allogeneic tissues, or dying tumor cells, process those antigens into 8- to 10-amino-acid peptides, and load them onto major histocompatibility complex class I (MHC-I) molecules for recognition by naive CD8⁺ cytotoxic T lymphocytes (cross-priming) or for the induction of peripheral CD8⁺ T-cell deletion and anergy (cross-tolerance). First discovered in 1976 by Michael J. Bevan through minor histocompatibility congenic mouse bone-marrow chimera experiments, cross-presentation resolves a central physiological paradox of adaptive immunology: according to the classical paradigm of MHC restriction, MHC-I molecules present endogenous cytosolic peptides to CD8⁺ T cells, whereas exogenous endocytosed proteins are routed to MHC class II (MHC-II) molecules to activate CD4⁺ helper T cells. Because naive CD8⁺ T cells circulate through secondary lymphoid organs via high endothelial venules and require costimulatory priming by lymphoid-resident or migratory dendritic cells—yet many parenchymal viruses (such as papillomaviruses, herpesviruses, and hepatotropic viruses) and non-hematopoietic solid tumors never directly infect dendritic cells—cross-presentation provides the indispensable bridge that allows uninfected dendritic cells to sample infected or malignant tissue debris and activate antigen-specific cytotoxic T-cell immunity across the immunological synapse.

In 1976, Michael Bevan immunized F₁ hybrid mice (H-2^b×d) with splenocytes from congenic mice that shared the same minor histocompatibility antigens (such as H-Y or minor H antigens) on only one parental MHC haplotype (H-2^b). Unexpectedly, the recipient mice generated vigorous CD8⁺ cytotoxic T lymphocyte (CTL) responses restricted not only to the donor H-2^b allele, but also to the host's second MHC allele (H-2^d), which was absent from the injected donor cells. Bevan deduced that host bone-marrow-derived antigen-presenting cells had engulfed the exogenous donor cells, transferred donor minor histocompatibility proteins into their own MHC-I processing machinery, and cross-primed host H-2^d-restricted CD8⁺ T cells.

Subsequent work by William R. Heath, Francis R. Carbone, and Kenneth M. Murphy demonstrated that the physiological outcome of cross-presentation depends on the maturation state of the dendritic cell and the presence of innate pathogen-associated or damage-associated molecular patterns (PAMPs and DAMPs):

• Immunogenic Cross-Priming (Infected or Necrotic Tissue): When dendritic cells engulf viral debris or necrotic tumor cells alongside Toll-like receptor (TLR3, TLR9), cGAS–STING, or RIG-I ligands—and receive licensing signals from CD4⁺ helper T cells via CD40L–CD40 engagement—they upregulate the costimulatory ligands CD80 (B7-1), CD86 (B7-2), and CD70, secrete interleukin-12 (IL-12) and type I interferons, and drive clonal expansion and effector differentiation of naive CD8⁺ T cells into cytotoxic effectors focused on immunodominance hierarchies.
• Tolerogenic Cross-Tolerance (Steady-State Apoptotic Self-Tissues): Under homeostatic conditions in the absence of infection or inflammation, immature cDC1s continuously phagocytose parenchymal cells undergoing normal physiological apoptosis (such as pancreatic β-islet cells, intestinal epithelial cells, or keratinocytes) and cross-present tissue-restricted self-peptides on MHC-I without costimulation (Signal 1 without Signal 2). Naive self-reactive CD8⁺ T cells that escaped thymic central tolerance and recognize these self-peptides on steady-state dendritic cells undergo abortive proliferation followed by BIM-dependent clonal deletion or peripheral tolerance via PD-1 and CTLA-4 checkpoint engagement.

In mice and humans, the ontological lineage with the highest intrinsic efficiency for cross-presentation in vivo is the conventional type 1 dendritic cell (cDC1) subset, whose development from bone-marrow common dendritic cell progenitors depends on the transcription factors BATF3, IRF8, and ID2. As shown by Kenneth Murphy and colleagues (2008), Batf3^−/− mice selectively lack lymphoid-resident CD8α⁺ cDC1s and peripheral tissue-migratory CD103⁺ cDC1s (homologous to human CD141⁺ [BDCA-3⁺] XCR1⁺ CLEC9A⁺ cDC1s); these mice fail to cross-present viral antigens and fail to reject immunogenic syngeneic fibrosarcomas or respond to immune checkpoint blockade.

cDC1s possess three evolutionary adaptations tailored for dead-cell capture and phagosomal antigen preservation:

1. CLEC9A (DNGR-1) Necrotic Cell Sensing: cDC1s selectively express the C-type lectin receptor CLEC9A (DNGR-1), engineered to bind filamentous F-actin–myosin II complexes exposed when the plasma membrane of necrotic or virally lysed cells ruptures. Revealed by Caetano Reis e Sousa and colleagues, CLEC9A signals through its hemITAM motif and Syk kinase not to trigger pro-inflammatory NF-κB gene transcription, but rather to divert internalized necrotic cargo into non-degradative recycling endosomes and induce localized phagosomal membrane rupture via NADPH oxidase, accelerating cytosolic antigen export for cross-presentation.
2. XCR1 Chemotactic Positioning: All mammalian cDC1s express the chemokine receptor XCR1, which binds XCL1 (lymphotactin) secreted by recently activated CD8⁺ T cells and natural killer cells, stabilizing prolonged ternary clusters of cDC1s, CD4⁺ helper T cells, and naive CD8⁺ T cells inside the T-cell paracortex of lymph nodes.
3. Attenuated Phagosomal Proteolysis via NOX2 and Rab27a: Whereas macrophages and neutrophils rapidly acidify their phagolysosomes to pH 4.5–5.0 via the vacuolar H⁺-ATPase (V-ATPase)—activating lysosomal cathepsins that digest engulfed proteins down to single amino acids and destroy potential MHC-I epitopes—Sebastian Amigorena and colleagues (Savina et al., 2006) discovered that cDC1s actively maintain a near-neutral phagosomal pH (pH 7.0 to 7.5) for several hours after phagocytosis. Upon cargo engulfment, the small GTPase Rab27a recruits the NOX2 (gp91^phox/Cybb) NADPH oxidase complex to the phagosomal membrane. NOX2 transfers electrons from cytosolic NADPH to luminal O₂ to generate superoxide anions (O₂^•−), which dismutate with luminal protons (2 O₂^•− + 2 H⁺ → H₂O₂ + O₂), consuming H⁺ ions to buffer phagosomal acidification. Simultaneously, the SNARE protein Sec22b recruits endoplasmic reticulum (ER) membrane components while limiting V-ATPase recruitment, preserving polypeptide fragments long enough for export or controlled trimming.

Once exogenous antigens are internalized into the alkaline phagosome or endosome of a dendritic cell, loading onto MHC-I proceeds through two distinct intracellular trafficking pathways:

1. The Cytosolic (Phagosome-to-Cytosol) Pathway

Accounting for the majority of physiological cross-presentation of viral and tumor antigens in vivo, the cytosolic pathway requires the transfer of partially unfolded polypeptide antigens across the phagosomal lipid bilayer into the cytoplasm:

• Sec22b-Mediated ER–Phagosome Tethering: Resident at the ER-Golgi intermediate compartment (ERGIC), the R-SNARE Sec22b pairs with the plasma-membrane Q-SNARE Syntaxin-4 on nascent phagosomes to deliver ER-resident proteins—including the Peptide-Loading Complex (TAP1/TAP2, tapasin, calreticulin, ERp57) and ER-associated degradation (ERAD) components—directly to the phagosome without full membrane fusion.
• Cytosolic Export and Proteasomal Cleavage: Polypeptides are exported into the cytosol through a combination of AAA+ ATPase p97 (VCP) extraction across retrotranslocation channels (Sec61 or Hrd1) and transient phagosomal membrane destabilization driven by NOX2-derived reactive oxygen species (lipid peroxidation) and pore-forming proteins. In the cytosol, ubiquitinated substrates are cleaved by the 26S proteasome or immunoproteasome into oligopeptides bearing hydrophobic or basic C-termini suited for the MHC-I F-pocket.
• TAP Re-Import and IRAP/ERAP1 Trimming: The resulting peptides are transported by TAP1/TAP2 either into the canonical perinuclear ER lumen (where ERAP1 trims their N-termini) or directly back into the Rab14⁺ Sec22b⁺ phagosomal lumen (where the endosomal aminopeptidase IRAP trims their N-termini), followed by loading onto MHC-I–β₂m heterodimers chaperoned by tapasin and export to the plasma membrane.

2. The Vacuolar (Endosomal) Pathway

Identified by Kenneth L. Rock and colleagues (Shen et al., 2004), the vacuolar pathway operates independently of cytosolic export, the proteasome, and TAP1/TAP2. Instead, exogenous antigens are cleaved inside the endophagosomal compartment primarily by the thiol protease Cathepsin S (which remains catalytically active at near-neutral pH). Concurrently, pre-existing cell-surface MHC-I molecules are internalized from the plasma membrane into Rab11a⁺ ERC (endosomal recycling compartment) vesicles via phosphorylation of a conserved cytoplasmic tail tyrosine residue and recruited to the phagosome in a TLR-MyD88-dependent manner (Nair-Gupta et al., 2014), where Cathepsin S-generated peptides exchange directly onto MHC-I before returning to the cell surface.

Because non-replicating subunit protein vaccines, mRNA vaccines, and dying cancer cells rely on cross-presentation to elicit tumor-infiltrating CD8⁺ T cells, pharmacologically enhancing cDC1 recruitment and cross-priming is a cornerstone of modern oncology:

1. Intratumoral NK–cDC1 Axis and Flt3L Expansion: Solid tumors frequently evade immunity by secreting prostaglandin E₂ (PGE₂), which downregulates the chemokines CCL5 and XCL1 produced by intratumoral natural killer (NK) cells, excluding XCR1⁺ cDC1s from the tumor microenvironment (Böttcher et al., 2018). Clinical trials combining systemic Flt3 ligand (Flt3L, CDX-301)—the growth factor that expands bone-marrow cDC1 progenitors by 10- to 20-fold—with intratumoral poly-ICLC (TLR3 agonist) and anti-PD-1 checkpoint blockade restore cross-priming of tumor neoantigens and convert immunologically "cold" tumors into T-cell-infiltrated responders.
2. Targeted Antibody–Antigen Conjugates: Monoclonal antibodies engineered to fuse viral or tumor antigens directly to anti-CLEC9A (DNGR-1), anti-DEC-205 (CD205), or anti-XCR1 single-chain variable fragments selectively deliver vaccine payloads into the cross-presenting endosomes of cDC1s at picomolar doses.
3. Immunogenic Cell Death (ICD) Inducers: Anthracycline chemotherapeutics (doxorubicin, oxaliplatin) and radiotherapy promote cDC1 cross-presentation by inducing cell-surface translocation of calreticulin ("eat-me" signal binding LRP1/CD91), extracellular release of HMGB1 (TLR4 agonist), and autocrine type I interferon signaling via the cGAS–STING pathway.

1. Bevan, M. J. (1976). "Minor H antigens introduced on H-2 different stimulating cells cross-react at the cytotoxic T cell level during in vivo priming." Journal of Immunology, 117(6), 2233–2238. https://doi.org/10.4049/jimmunol.117.6.2233
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