Neoantigen
Summary
Neoantigens are tumor-specific peptide antigens generated by somatic mutations — SNVs producing altered amino acid sequences, frameshift insertions/deletions creating novel reading frames, and chromosomal rearrangements generating fusion peptides. They are the molecular basis by which the adaptive immune system distinguishes cancer cells from normal cells: processed and presented on MHC class I molecules, neoantigens mark tumor cells for CD8+ T-cell recognition and clearance. Neoantigen burden is a double-edged sword in cancer evolution — it increases immune visibility (favoring elimination) but also provides substrate for immune editing (selecting for clones that evade recognition through HLA LOH or antigen loss). The relationship with chromosomal-instability is particularly complex: CIN generates neoantigens through frameshifts and aberrant proteins, but CIN also enables immune escape through CNA-mediated HLA loss and neoantigen-coding segment deletion (Turajlic et al., 2019).
Definition and Origin
Neoantigens arise from somatic alterations that produce protein sequences not present in the normal proteome:
SNV-derived neoantigens. A non-synonymous point mutation alters a single amino acid in a protein. The mutant peptide, when processed and loaded onto MHC-I, presents a novel epitope not encountered during thymic selection. SNV-derived neoantigens are the most studied class and the primary target of personalized cancer vaccines.
Frameshift neoantigens. Insertions or deletions that shift the translational reading frame produce entirely novel peptide sequences downstream of the mutation. Frameshift neoantigens are more immunogenic than SNV-derived neoantigens because they are more foreign — they share no sequence homology with self-peptides and are not subject to central tolerance.
Fusion-gene neoantigens. Chromosomal rearrangements (translocations, inversions) produce fusion proteins with novel junctional peptide sequences spanning the breakpoint. These are particularly relevant in cancers driven by gene fusions (e.g., BCR-ABL in CML, EML4-ALK in NSCLC).
CNA-driven neoantigens. CIN and catastrophic events (chromothripsis) generate frameshift peptides and aberrant protein products at scale — orders of magnitude more than SNV mutagenesis alone. This is the basis for the immune-visibility mechanism at high CIN: more chromosomal rearrangements → more aberrant protein products → higher neoantigen burden → better immune recognition (chromosomal-instability).
Role in Immune Surveillance and Editing
Neoantigens are the molecular link between genetic ITH and immune-mediated selection:
Immune surveillance. CD8+ T cells recognize neoantigen-MHC-I complexes via their T-cell receptors. Clones bearing immunogenic neoantigens are eliminated — a form of negative selection acting at the level of the tumor cell population. This is the cancer-immunity cycle: mutation generates neoantigens, neoantigens are presented, T cells recognize and eliminate the presenting cells.
Immune editing. The selective pressure of immune surveillance shapes ITH through three phases (the three E’s of cancer immunoediting):
- Elimination — immunogenic clones are destroyed
- Equilibrium — surviving clones accumulate further mutations but are held in check by residual immune pressure
- Escape — clones with immune-evasion mechanisms (HLA LOH, neoantigen loss, PD-L1 upregulation) expand
CNA-mediated neoantigen loss. CNA provides a particularly efficient route to immune escape: deletion of the chromosomal segment containing a neoantigen-coding mutation eliminates the antigen without requiring a second SNV. This is one reason CNA-based immune escape is recurrently observed — a single copy-number event can delete multiple neoantigens simultaneously (Turajlic et al., 2019).
Neoantigens and ITH
Neoantigen burden is positively correlated with genetic ITH: more mutations → more neoantigens → more immune visibility. This creates a tension:
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High ITH → high neoantigen burden → good immunotherapy response. Tumors with high mutational burden (and therefore high neoantigen burden) respond better to immune checkpoint inhibitors (anti-CTLA4, anti-PD1). This is the basis for the ITH-immunotherapy paradox: while standard population-genetic theory predicts worse outcome with higher ITH (more substrate for adaptation), immunotherapy inverts the relationship because the same diversity that fuels adaptation also marks the tumor for immune destruction. The compression-entrenchment hypothesis explicitly excludes immunotherapy-treated patients from its primary U-curve prediction for this reason (intratumor-heterogeneity §5.1).
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Immune selection shapes ITH. Clones bearing strongly immunogenic neoantigens are eliminated early (clonal neoantigens are more conserved, less immunogenic). Subclonal neoantigens are enriched for recently-arisen, potentially more immunogenic epitopes that have not yet been eliminated. The neoantigen landscape at any snapshot reflects the history of immune selection, not the raw mutational output.
Limitations
Neoantigen prediction is computationally intensive and incomplete. Identifying which mutations produce neoantigens requires: (1) calling somatic mutations, (2) predicting which mutant peptides bind the patient’s MHC-I alleles, (3) predicting which peptide-MHC-I complexes are recognized by T cells. Steps 2 and 3 rely on machine learning models trained on in vitro binding data; their performance on in vivo presentation and recognition is imperfect.
Not all neoantigens are equally immunogenic. The same neoantigen may be recognized in one patient but ignored in another, depending on T-cell repertoire, MHC-I allele, and tumor microenvironment context. Neoantigen burden is a necessary but not sufficient condition for immune-mediated tumor control.
Immune evasion is multi-factorial. Neoantigen loss explains only one route to immune escape. PD-L1 upregulation, immunosuppressive cytokine secretion, regulatory T-cell recruitment, and defective MHC-I presentation machinery all contribute independently.
Current wiki coverage of immune-evasion mechanisms is limited. This page focuses on the genetic generation and loss of neoantigens. The broader immunoediting framework (elimination-equilibrium-escape), the role of the tumor microenvironment in immune suppression, and the interaction with immunotherapy are referenced in immune-evasion (page pending) and intratumor-heterogeneity §5.1 but are not yet fully developed in the wiki.