# 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. More on methods. ## Paper search We performed a semantic search across over 138 million academic papers from the Elicit search engine, which includes all of [Semantic Scholar](https://www.semanticscholar.org/) and [OpenAlex](https://openalex.org/). We ran this query: "What downstream signaling pathways are altered when CD28 costimulation is blocked?" The search returned 200 total results from Elicit. ## Screening We screened in sources based on their abstracts that met these 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)? We considered all screening questions together and made a holistic judgement about whether to screen in each paper. ## Data extraction ### CD28 Blockade Method - Specific agent used: Various antibodies and methods - Dosage/concentration: As reported in individual studies - Timing of blockade: Variable across studies - Route of administration: Variable across studies - Combinatorial treatments: Variable across studies ### Experimental System - Model system: In vitro, ex vivo, and animal models - Disease/condition context: Transplantation, autoimmune conditions - Species: Human, mouse, and others ### Target Cell Types - Specific cell types analyzed: CD4+ T cells, CD8+ T cells, monocytes, etc. - Cell activation state: Varied across studies ### Signaling Pathways Analyzed - Pathway names: NF-κB, AP-1, AKT, mTOR, metabolic pathways - Measurement methods: Western blot, flow cytometry, gene expression ### Pathway Changes Observed - Direction of change: Upregulated or downregulated as noted in studies ### Mechanistic Insights - Proposed mechanism of action: Based on specific cases in reviewed studies ### Functional Consequences - Changes in T cell activation, proliferation, cytokine production, metabolic alterations, and implications for therapy. ## 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. ### Functional Consequences Across Studies The downstream signaling alterations translated into consistent functional outcomes across experimental systems. T cell proliferation was suppressed in all studies that measured it, ranging from 36-73% inhibition. Cytokine production patterns shifted toward a less inflammatory profile, with decreased type 1 cytokines (IL-2, IFNγ, IL-15) and increased regulatory cytokines (IL-10). In transplantation models, CD28 blockade induced donor-specific tolerance, prolonged allograft survival, prevented chronic rejection, and reduced alloantibody production. The vasculitis model demonstrated reduced tissue-infiltrating T cells, suppressed vasculitis, and prevention of vessel wall remodeling, establishing therapeutic potential beyond transplantation. ## Synthesis 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). Multiple mechanistic themes emerge from this research, emphasizing the interconnectedness of signaling pathways and their impact on immune responses.