CD28 Costimulation and Downstream Pathways

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

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.

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.

Records from Elicit search

• n = 200
• Papers screened out: n = 190
• Papers included for extraction: n = 10

Paper Search

We performed a semantic search across over 138 million academic papers from the Elicit search engine. 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: Study investigates CD28 costimulation blockade through methods (antibodies, genetic knockout, pharmacological inhibitors, blocking agents).
• Signaling Pathway Measurement: Study measures downstream signaling pathways, molecular cascades, or intracellular signaling events following CD28 blockade.
• Appropriate Controls: Study includes appropriate control groups (untreated, vehicle-treated, or isotype controls).
• Experimental Study Type: In vitro, ex vivo, or in vivo experimental study.
• Mechanistic Focus: Includes mechanistic signaling pathway data.
• Blockade vs Stimulation: Examines CD28 blockade.
• Publication Type: Full research article.
• Pathway Analysis Inclusion: Includes signaling pathway analysis.

Data Extraction

• CD28 Blockade Method: Extracts details about how CD28 costimulation was blocked, including specific agent used, dosage/concentration, timing of blockade, route of administration, and combinatorial treatments.
• Experimental System: Extracts the model system, disease/condition context, species, and specific experimental setup relevant to pathway analysis.
• Target Cell Types: Extracts which specific cell types were analyzed.
• Signaling Pathways Analyzed: Extracts all downstream signaling pathways that were measured, including specific pathway names, individual signaling molecules/proteins analyzed, measurement methods, time points of analysis, and pathway interactions.
• Pathway Changes Observed: Extracts specific alterations in downstream signaling pathways caused by CD28 blockade, including direction of change, magnitude of effect, kinetics of changes, comparison to appropriate controls, and any dose-response relationships.
• Mechanistic Insights: Extracts mechanistic explanations provided for how CD28 blockade leads to downstream signaling pathway changes.
• Functional Consequences: Extracts functional outcomes that result from the altered downstream signaling pathways after CD28 blockade.

Results

Characteristics of Included Studies

Ten studies examined downstream signaling pathway alterations following CD28 costimulation blockade. Study characteristics:

• G. Gorgun et al., 2007: Ex vivo MLR, Transplantation (GVHD/rejection), Human
• Koji Kishimoto et al., 2000: Cardiac transplant, Mouse (BALB/c, C57BL/6)
• G. Gorgun et al., 2008: Ex vivo MLR, Transplantation, Human
• A. Habicht et al., 2007: Skin transplant, Mouse (CD4−/−CD28−/−, CD8−/−CD28−/−)
• Hui Zhang et al., 2019: Human artery-NSG chimera, Vasculitis (GCA), Human/Mouse
• C. Olsson et al., 1999: In vitro blastoid T cells, Normal activation, Mouse
• J. Woodward et al., 1996: Cardiac transplant, Mouse (C57BL/6J→CBA/J)
• David V. Mathews et al., 2018: Transplant, Mouse, NHP
• L. Tuosto & O. Acuto, 1998: In vitro T cells, ex vivo TCR-Tg, Human (Jurkat), Mouse
• T. Zhang et al., 2011: Cardiac transplant, Mouse (C57BL/6, BALB/c)

Proximal TCR Signaling Pathways

CD28 blockade profoundly impaired the earliest TCR signaling events. TCR-induced tyrosine phosphorylation of the ζ chain and ZAP-70 was significantly reduced.

Transcription Factor Activation

CD28 blockade consistently suppressed critical transcription factors involved in T cell activation. NF-κB and AP-1 were potently suppressed by CTLA-4 ligation.

IL-27 Signaling Pathway

IL-27 signaling is a previously unrecognized target of CD28 costimulation blockade. This pathway's alterations contribute to Th1 differentiation.

Metabolic Signaling Pathways

CD28 blockade disrupted T cell metabolic fitness, reducing glycolytic enzyme expression and mitochondrial oxygen consumption.

Regulatory and Tolerance-Associated Pathways

CD28 blockade promoted regulatory pathways, increasing expression of markers such as Foxp3 and PD-1.

Alternative Costimulatory Pathways

Compensatory pathways like CD40-CD154 and CD122 signaling sustained T cell responses when CD28 signaling was eliminated.

Functional Consequences Across Studies

The downstream signaling alterations translated into consistent functional outcomes including suppression of T cell proliferation and modulation of cytokine production.