CD28 Costimulation and Downstream Pathways

What downstream signaling pathways are altered when CD28 costimulation is blocked?

CD28 costimulation blockade disrupts proximal TCR signaling (ζ chain, ZAP-70, Ca2+/calcineurin, ERK/MAPK, JNK), suppresses transcription factors (NF-κB, AP-1), reduces IL-27 pathway signaling (STAT3, pSTAT1), impairs metabolic pathways (AKT-mTOR-HIF-1α axis, glucose uptake, glycolysis), and when selective, upregulates regulatory pathways (Foxp3, PD-1, IDO, CTLA-4).

Abstract

CD28 costimulation blockade disrupts multiple downstream signaling pathways across a coordinated cascade. At the proximal level, CD28 blockade impairs TCR-induced tyrosine phosphorylation of the ζ chain and ZAP-70, which cascades to strongly diminished Ca2+/calcineurin, ERK/MAPK, and JNK pathway activation. Transcription factors critical for T cell activation, particularly NF-κB and AP-1, are potently suppressed, with reduced IκB-α phosphorylation indicating impaired NF-κB regulation. CD28:B7 signaling is required for monocyte IL-27 production, and blockade decreases IL-27 pathway components (p28, EBI3, IL-27R, STAT3, pSTAT1) in both monocytes and CD4+ T cells. Metabolically, CD28 blockade disrupts the AKT-mTOR-HIF-1α axis, reducing glucose transporter Glut1 expression, glycolytic enzyme levels by 30-50%, and mitochondrial oxygen consumption.

Selective CD28 blockade while preserving CTLA-4 promotes regulatory pathways, upregulating Foxp3, PD-1, IDO, and CTLA-4 expression, which contribute to tolerance induction. However, when CD28 is eliminated, alternative costimulatory pathways including CD40-CD154, CD134-CD134L, and CD122 signaling can sustain T cell responses, particularly in memory CD8+ T cells and stringent rejection models. These pathway alterations translate into 36-73% inhibition of T cell proliferation, decreased type 1 cytokines (IL-2, IFNγ), increased regulatory cytokines (IL-10), and functional outcomes including donor-specific tolerance and prolonged allograft survival in transplantation models.

Methods

We analyzed 10 sources from an initial pool of 200, using 8 screening criteria. Each paper was reviewed for 7 key aspects that mattered most to the research question.

Screening Criteria

• CD28 Blockade Intervention: Does the study investigate CD28 costimulation blockade through any method (antibodies, genetic knockout, pharmacological inhibitors, or other blocking agents)?
• Signaling Pathway Measurement: Does the study measure downstream signaling pathways, molecular cascades, or intracellular signaling events following CD28 blockade?
• Appropriate Controls: Does the study include appropriate control groups (untreated, vehicle-treated, or isotype controls)?
• Experimental Study Type: Is this an in vitro, ex vivo, or in vivo experimental study?
• Mechanistic Focus: Does the study include mechanistic signaling pathway data (not solely clinical outcomes)?
• Blockade vs Stimulation: Does the study examine CD28 blockade (not solely CD28 agonism or stimulation)?
• Publication Type: Is this a full research article (not a conference abstract, case report, or editorial)?
• Pathway Analysis Inclusion: Does the study include signaling pathway analysis (not only phenotypic or functional outcomes)?

Data extraction

We asked a large language model to extract each data column below from each paper.

• CD28 Blockade Method: Extract details about how CD28 costimulation was blocked, including:
  • Specific agent used (e.g., anti-CD28 antibody, selective CD28 blockers, CTLA-4 ligands)
  • Dosage/concentration
  • Timing of blockade (when initiated, duration)
  • Route of administration
  • Any combinatorial treatments
• Experimental System: Extract the experimental model and context where CD28 blockade effects on signaling pathways were studied, including:
  • Model system (in vitro cell culture, animal model, ex vivo assays)
  • Disease/condition context (transplantation, autoimmunity, normal activation, etc.)
  • Species (human, mouse, etc.)
  • Specific experimental setup relevant to pathway analysis
• Target Cell Types: Extract which specific cell types were analyzed for downstream signaling pathway changes after CD28 blockade, including:
  • Primary cell type studied (CD4+ T cells, CD8+ T cells, monocytes, etc.)
  • Cell activation state (naive, activated, memory, etc.)
  • Any cell subsets specifically examined
  • Tissue location if relevant (blood, lymphoid organs, transplanted tissue, etc.)
• Signaling Pathways Analyzed: Extract all downstream signaling pathways that were measured after CD28 blockade, including:
  • Specific pathway names (NF-κB, AP-1, AKT, mTOR, metabolic pathways, etc.)
  • Individual signaling molecules/proteins analyzed
  • Measurement methods (Western blot, flow cytometry, gene expression, etc.)
  • Time points of analysis
  • Any pathway interactions or crosstalk studied
• Pathway Changes Observed: Extract the specific alterations in downstream signaling pathways caused by CD28 blockade, including:
  • Direction of change (upregulated, downregulated, no change)
  • Magnitude of effect (fold-change, percentage change, statistical significance)
  • Kinetics of changes (immediate, delayed, sustained, transient)
  • Comparison to appropriate controls
  • Any dose-response relationships
• Mechanistic Insights: Extract any mechanistic explanations provided for how CD28 blockade leads to downstream signaling pathway changes, including:
  • Proposed mechanism of action
  • Key intermediary molecules or steps
  • Relationship between CD28 and other costimulatory/inhibitory pathways
  • Any evidence for direct vs. indirect effects
  • Regulatory loops or feedback mechanisms identified
• Functional Consequences: Extract functional outcomes that result from the altered downstream signaling pathways after CD28 blockade, including:
  • Changes in T cell proliferation, differentiation, or survival
  • Cytokine production alterations
  • Metabolic changes
  • Effects on immune responses (tolerance, rejection, etc.)
  • Any therapeutic implications mentioned

Results

Characteristics of Included Studies

Ten studies examined downstream signaling pathway alterations following CD28 costimulation blockade. Three studies used human cells or tissues, one used both human and mouse cells, and six used murine systems exclusively. The majority focused on transplantation contexts, with one examining vasculitis and two studying normal T cell activation.

The results indicate CD28 blockade profoundly impaired the earliest TCR signaling events, suggesting CD28 enhances TCR signaling capacity during the earliest stages of T cell-APC interaction.

Conclusion

The studies reveal a coordinated cascade of signaling disruptions initiated by CD28 blockade. At the proximal level, CD28 enhances TCR signaling through ζ chain and ZAP-70 phosphorylation, which when blocked, impairs all downstream pathways including Ca2+/calcineurin, ERK/MAPK, and JNK. This proximal defect propagates to transcription factor suppression (NF-κB, AP-1), cytokine signaling alterations (IL-27 pathway), and metabolic reprogramming (AKT/mTOR/glycolysis). Several mechanistic themes emerge, emphasizing the importance of context in the effects of CD28 blockade.