MHC Multimers: Central Hub and Activation Mechanism in DNA Vaccine-Induced Immune Response

Vaccine-encoded endogenous antigenic proteins undergo covalent ubiquitination in the cytosol, followed by proteasomal degradation into 8-12 amino acid antigenic peptides. These peptide fragments are selectively transported into the endoplasmic reticulum lumen via the Transporter associated with Antigen Processing (TAP), where they assemble with newly synthesized MHC class I molecules to form stable peptide-MHC-I complexes. This complex is subsequently trafficked through the Golgi apparatus to the cell membrane surface, enabling activation of CD8+ cytotoxic T lymphocytes (CTLs). Upon recognition, these CTLs initiate specific cytotoxic immune responses to eliminate infected or tumor cells.

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The immune response to DNA vaccines can be explained through three core pathways:
  1. MHC Class I Cross-Presentation Pathway
    • Endogenous antigenic proteins encoded by the vaccine covalently bind to ubiquitin in the cytosol and are degraded by proteasomes into antigenic peptides. These peptides are transported into the endoplasmic reticulum via TAP, forming a stable complex with MHC class I molecules. After being transported to the cell membrane by the Golgi apparatus, this complex activates CD8+ cytotoxic T lymphocytes (CTLs), triggering an immune response against infected or cancerous cells.
  2. MHC Class II Dedicated Presentation Pathway
    • Some antigenic proteins are phagocytosed by antigen-presenting cells (APCs) and enter the endosomal-lysosomal system. Here, they are hydrolyzed by cathepsins to form antigenic peptides. These peptides bind to newly synthesized MHC class II molecules in endosomes under acidic conditions. The resulting complex is transported to the cell membrane and recognized by CD4+ helper T cells (Th cells), initiating Th cell activation and the secretion of cytokines like IL-2 and IFN-γ. This process induces B cell differentiation and antibody class switching, forming a humoral immune response.
  3. Direct B Cell Presentation Pathway
    • Under specific conditions, antigenic peptides can directly activate B cells without T cell help. When the antigen concentration reaches a threshold, B cells internalize the antigen via BCR receptors. After processing, the antigenic peptide is presented on MHC class II molecules, forming a "dual recognition" signal. This triggers autonomous B cell activation, leading to their differentiation into plasma cells and the secretion of specific antibodies, complementing the rapid humoral immune response.
These three pathways form an immune network through T-B cell interactions. Activated Th cells enhance B cell responses via the CD40L-CD40 axis and secrete cytokines (IL-4 drives IgE switching, IL-21 promotes plasma cell differentiation), creating a positive feedback loop that coordinates cellular and humoral immunity.
DNA vaccine technology has expanded into applications for HIV, Zika, influenza, MERS-CoV, and cancer immunotherapy. Many candidates have shown promising immunogenicity and safety in animal and early clinical studies. However, transitioning from research to clinical use remains challenging. Despite three COVID-19 vaccines entering clinical trials, full approval requires rigorous multi-phase trials. Vaccine development follows strict scientific principles, with each stage strictly evaluating efficacy and safety. Historical examples, like the HVTN702 trial, highlight the high-risk nature of vaccine research.
The Coalition for Epidemic Preparedness Innovations (CEPI) has analyzed the challenges of accelerated vaccine development during pandemics, addressing how to maintain scientific rigor while shortening timelines and balancing emergency needs with long-term safety. These insights are crucial for global vaccine strategies.
Current COVID-19 vaccine development faces five core challenges:
  1. Scientific and Technological Bottlenecks
    • Antigen Target Optimization: Choosing between full-length spike protein and receptor-binding domain involves trade-offs between broad immune responses and precision targeting, requiring structural vaccinology to balance immunogenicity and broad protection.
    • ADE Risk Mitigation: Historical data show suboptimal antibody responses may enhance infection via Fc receptor-mediated endocytosis. A multidimensional monitoring system, especially for vulnerable populations, is needed.
    • Unknown Immune Durability: Data only confirms antibody levels up to 12 months post-vaccination. The longevity of memory B cells and tissue-resident memory T cells remains unclear, and single-dose strategies need validation for sustained immune memory.
  2. Industrialization Implementation Barriers
    • Vaccine Development Economics: Vaccine development is capital-intensive (over $1 billion per candidate) with high phase III trial participant requirements (30,000 - 50,000 subjects). Historical success rates from preclinical to approval are below 10%, creating sustainability challenges for small biotechs.
    • Global Equity Dilemma: Risk-stratified vaccination strategies require seroepidemiological surveys to identify susceptible populations. Current global health governance struggles with cross-border data sharing, and initiatives like the G7's "vaccine allocation index" remain untested.
The recent approval of the third mRNA vaccine marks progress in diversified technology approaches. However, the accelerated timeline from gene sequencing to emergency use authorization must not ignore the scientific process. A 18 - 24-month observation period is still needed to confirm real-world effectiveness. It is essential to maintain scientific rigor in assessments to avoid misleading public health decisions.
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This article is reviewed and published by the technical expert team of UA

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