Immunodominance

Immunodominance is the quantitative and hierarchical phenomenon in adaptive immunology wherein the T-cell or B-cell immune response against a complex pathogen, tumor proteome, or multi-determinant protein focuses on a remarkably small subset of antigenic peptides or conformational surface sites—termed immunodominant epitopes—even though the pathogen encodes thousands of other potentially immunogenic sequences. First systematically characterized for helper T-cell responses to lysozyme and myoglobin by Eli E. Sercarz, Jay A. Berzofsky, and Ronald H. Schwartz in the late 1970s and 1980s, and quantified for antiviral CD8⁺ cytotoxic T lymphocyte (CTL) hierarchies by Jonathan W. Yewdell and Jack R. Bennink (1999), immunodominance partitions all candidate epitopes within an antigen into a three-tier hierarchy: immunodominant epitopes (which command 50% to 90% of the primary clonal expansion), subdominant epitopes (which elicit weak or delayed responses in the presence of the dominant epitope, but expand robustly if the dominant epitope is deleted), and cryptic epitopes (which fail to be processed from the intact native protein in vivo, yet can stimulate T cells if administered directly as synthetic pre-cleaved peptides, or emerge secondarily during chronic tissue damage via epitope spreading). Understanding the biochemical filters that govern immunodominance is central to rational vaccine engineering against mutable pathogens such as Influenza A, HIV, and SARS-CoV-2.

A typical viral genome such as vaccinia virus (~200 open reading frames, ~60,000 amino acids) or herpes simplex virus encodes tens of thousands of overlapping 8- to 10-mer octapeptides and nonapeptides. Yet in an inbred mouse strain such as C57BL/6 (H-2^b) infected with Influenza A virus (whose proteome contains ~4,500 amino acids), over 60% of the entire pulmonary CD8⁺ CTL response is directed against just two peptides: the nucleoprotein peptide NP_366–374 (ASNENMETM) presented on H-2D^b and the acidic polymerase peptide PA_224–233 (SSLENFRAYV) presented on H-2D^b.

Quantitative mass-balance analyses by Assarsson, Sette, Grey, and Yewdell (2007) established that a series of multiplicative intracellular biochemical bottlenecks eliminates more than 99.8% of all theoretical viral nonapeptides before they ever reach the immunological synapse:

1. The DRiP Hypothesis and Immunoproteasome Remodeling

Formulated by Jonathan Yewdell, Ulrich Schubert, and Jack Bennink (1996, 2000), the Defective Ribosomal Products (DRiPs) hypothesis explains how infected cells present viral peptides on major histocompatibility complex class I within 30 to 60 minutes of infection, even when the parent viral structural protein has a metabolic half-life of several days. Approximately 20% to 30% of newly translated polypeptide chains fail to fold properly, stall on ribosomes, or are mistranslated; these nascent DRiPs are co-translationally ubiquitinated and degraded by the 26S proteasome, coupling MHC-I peptide supply directly to the rate of viral mRNA translation rather than steady-state protein abundance.

When cells are exposed to interferon-gamma (IFN-γ) or TNF-α during infection, the three catalytic β subunits of the constitutive 20S proteasome core (β1, β2, β5) are replaced by the inducible immunoproteasome subunits β1i (LMP2 / PSMB9), β2i (MECL-1 / PSMB10), and β5i (LMP7 / PSMB8). This subunit swap suppresses caspase-like cleavage after acidic residues (Asp, Glu) and amplifies chymotrypsin-like and trypsin-like cleavage after hydrophobic (Leu, Ile, Val, Phe, Tyr) and basic (Lys, Arg) amino acids—precisely the C-terminal anchor residues required by both the TAP1/TAP2 transporter and the F-pocket of human HLA-A, HLA-B, and murine H-2 class I molecules.

2. Tapasin Editing, Kinetic Stability (t_1/2), and DM Editing for MHC Class II

As demonstrated by Alessandro Sette (1994) for MHC-I and Andrea Sant for MHC-II, although an equilibrium binding affinity threshold of K_d ≤ 500 nM is necessary for a peptide to be immunogenic, the dissociation half-life (t_1/2, kinetic stability) of the peptide–MHC complex on the surface of a migrating dendritic cell is an even stronger predictor of immunodominance than equilibrium affinity alone:

• MHC-I Editing by Tapasin and TAPBPR: Inside the ER peptide-loading complex, the chaperone tapasin (and its PLC-independent homolog TAPBPR) pries open the α₂-helix of MHC-I, accelerating the off-rate of suboptimal, fast-dissociating peptides until a peptide with a slow off-rate (t_1/2 > 3 hours) binds and locks the α₁/α₂ groove closed for Golgi export.
• MHC-II Editing by HLA-DM (H-2M): For exogenous antigens processed in late endosomal MIIC (MHC class II compartments), the non-classical heterodimer HLA-DM catalyzes the release of the invariant-chain fragment CLIP and continuously ejects low-stability peptides from the open-ended MHC-II groove at pH 5.0. Only peptides capable of forming a cooperative hydrogen-bond network with the MHC-II backbone and resisting HLA-DM-mediated release survive to prime immunodominant CD4⁺ follicular helper (T_FH) and T_H1 responses.

Even among peptides that are abundantly presented on dendritic cells with high kinetic stability, clear immunodominance hierarchies emerge due to properties of the T-cell repertoire:

1. Naive TCR Precursor Frequency and Public Clonotypes: Using peptide–MHC tetramer magnetic enrichment across the entire lymphoid compartment of unimmunized mice, Marc K. Jenkins, Steven C. Jameson, and colleagues (Moon et al., 2007; Obar et al., 2008) discovered that the number of naive T cells specific for different single epitopes in an unprimed mouse varies by more than 20-fold, ranging from ~15 to 1,200 naive precursors per mouse (roughly 1 in 10⁶ to 1 in 2 × 10⁴ total T cells). Epitopes recognized by large naive precursor pools—often featuring "public" TCRβ complementarity-determining region 3 (CDR3) sequences generated with zero or minimal TdT N-nucleotide additions during V(D)J recombination—enjoy a numerical head start during the early exponential growth phase of clonal expansion. Conversely, viral peptides that resemble self-peptides lose their highest-affinity naive precursors to thymic negative selection (central tolerance), relegating them to subdominance.
2. Immunodomination (Active Suppression of Subdominant Clones): Coined by Yewdell and Bennink, immunodomination refers to the active competitive suppression of subdominant T-cell clones by immunodominant T-cell clones crowding the same antigen-presenting dendritic cell. High-frequency or rapidly dividing dominant CD8⁺ T cells outcompete subdominant clones for access to the dendritic cell surface, physically strip pMHC-I complexes and costimulatory B7 ligands from the dendritic cell membrane via trogocytosis, sequester paracrine IL-2, and—once differentiated into early effector CTLs—lyse antigen-bearing dendritic cells via perforin, truncating the duration of antigen presentation available to slower-recruited subdominant clones.

In humoral immunity inside the germinal center, B-cell immunodominance is frequently exploited by rapidly mutating RNA viruses as an evolutionary decoy strategy:

• Influenza Hemagglutinin (HA) Head vs. Stalk: On the trimeric Influenza A surface glycoprotein hemagglutinin (HA), the globular HA1 head domain projects outward from the viral envelope, making it sterically accessible to naive B-cell receptors, whereas the membrane-proximal HA2 stalk (stem) domain is sterically hindered and packed tightly on the virion surface. Consequently, natural infection and seasonal vaccination elicit > 85% of antibodies against the immunodominant, hypervariable HA head, which tolerates high rates of point mutation (antigenic drift). Meanwhile, the subdominant, functionally conserved HA2 stalk (which mediates pH-dependent viral membrane fusion and cannot mutate without losing infectivity) elicits weak antibody titers.
• Antigenic Seniority (Original Antigenic Sin / Immune Imprinting): First described by Thomas Francis Jr. (1960), when an individual is sequentially exposed to antigenically drifted variants of a virus (such as childhood H1N1 influenza followed decades later by drifted H1N1 or pandemic strains), pre-existing high-frequency memory B cells specific for epitopes conserved from the primary childhood strain outcompete lower-frequency naive B cells specific for novel epitopes on the variant strain.
• Rational Vaccine Strategies to Overcome Immunodominance: Modern vaccinology overcomes decoy immunodominance through four structural designs: (1) "Headless" or Chimeric HA Stalk Immunogens developed by Peter Palese, Adolfo García-Sastre, and Florian Krammer, which sequentially immunize hosts with exotic avian HA heads (cH5/1, cH8/1) grafted onto a conserved human H1 or H3 stalk to selectively boost subdominant stalk-reactive germinal center B cells; (2) Glycan Masking, which engineers N-linked glycosylation sites (Asn-X-Ser/Thr) over immunodominant decoy loops on HIV Env gp120 or influenza HA to redirect B-cell binding toward conserved neutralizing sites; (3) Germline-Targeting Immunogens (such as eOD-GT8 for HIV VRC01-class broadly neutralizing antibodies) designed by William Schief and Shane Crotty to prime rare subdominant naive B-cell precursors and guide their somatic hypermutation; and (4) Mosaic Polyvalent T-Cell Vaccines that physically separate subdominant epitopes into distinct anatomical injection sites or nanoparticles to prevent inter-clonal immunodomination on the same dendritic cell.

1. Yewdell, J. W., & Bennink, J. R. (1999). "Immunodominance in major histocompatibility complex class I-restricted T lymphocyte responses." Annual Review of Immunology, 17, 51–88. https://doi.org/10.1146/annurev.immunol.17.1.51
2. Assarsson, E., Sidney, J., Oseroff, C., Pasquetto, V., Bui, H.-H., Frahm, N., Brander, C., Peters, B., Grey, H., & Sette, A. (2007). "A quantitative analysis of the variables affecting the repertoire of T cell specificities recognized after vaccinia virus infection." Journal of Immunology, 178(12), 7890–7901. https://doi.org/10.4049/jimmunol.178.12.7890
3. Moon, J. J., Chu, H. H., Pepper, M., McSorley, S. J., Jameson, S. C., Kedl, R. M., & Jenkins, M. K. (2007). "Naive CD4+ T cell frequency varies for different epitopes and predicts repertoire diversity and response magnitude." Immunity, 27(2), 203–213. https://doi.org/10.1016/j.immuni.2007.07.007
4. Angeletti, D., Gibbs, J. S., Angel, M., Kosik, I., Hickman, H. D., Frank, G. M., Das, S. R., Wheatley, A. K., Prabhakaran, M., Leggat, D. J., McDermott, A. B., & Yewdell, J. W. (2017). "Defining B cell immunodominance to viruses." Nature Immunology, 18(4), 456–463. https://doi.org/10.1038/ni.3680