CD28

CD28 is a 44 kDa costimulatory receptor expressed on the surface of T lymphocytes that augments antigen receptor signaling to promote T cell activation, survival, and differentiation. Discovered in studies of T cell hybridomas and monoclonal antibodies, CD28 interacts with B7 family ligands on antigen-presenting cells to influence adaptive immunity, tolerance, and immunopathology. Investigations of CD28 have informed therapeutic strategies in transplantation, autoimmunity, and oncology, and have linked its signaling to intracellular kinases, phosphatases, and transcriptional programs.

Structure and expression

CD28 is a type I transmembrane glycoprotein of the immunoglobulin superfamily with an extracellular V-set domain, a single-pass transmembrane helix, and a cytoplasmic tail containing conserved motifs critical for signaling. The extracellular domain mediates homodimerization and binding to the B7 ligands B7-1 and B7-2, while the cytoplasmic region contains a YMNM motif and proline-rich regions that recruit signaling adaptors and kinases such as PI3K and Grb2. Expression is constitutive on most peripheral CD4+ and a subset of CD8+ T cells in adult humans and varies across developmental stages in the thymus, where interactions influence positive and negative selection events described in work from institutions such as National Institutes of Health laboratories and academic centers like Harvard Medical School. CD28 expression is modulated in specialized populations including regulatory T cells studied at centers like Stanford University and tissue-resident memory T cells characterized in research from universities including University of Oxford.

Function and signaling

CD28 provides a critical second signal that synergizes with T cell receptor engagement to amplify intracellular cascades including PI3K–AKT, RAS–MAPK, and NF-κB pathways. Recruitment of PI3K to the YMNM motif leads to generation of phosphatidylinositol (3,4,5)-trisphosphate and activation of AKT1, while Grb2 and SOS1 link CD28 to RAS activation and downstream ERK phosphorylation characterized in signaling studies at laboratories such as Cold Spring Harbor Laboratory. The cytoplasmic tail also engages kinases like LCK and ITK and phosphatases whose balance governs thresholds of activation; seminal biochemistry on these interactions originated from groups at institutions like Max Planck Society and Wellcome Trust. CD28 signaling enhances transcriptional programs mediated by factors including NF-κB, AP-1, and NFAT, promoting cytokine gene expression and metabolic reprogramming described in research from consortia including those at National Cancer Institute.

Role in T cell activation and immune responses

During antigen presentation, simultaneous engagement of the T cell receptor and CD28 lowers the threshold for clonal expansion, augments interleukin-2 production, and increases survival via anti-apoptotic molecules identified in studies from Memorial Sloan Kettering Cancer Center. CD28 costimulation shapes helper T cell differentiation into subsets first mapped in foundational work at institutions like Pasteur Institute and University of California, San Francisco, influencing responses in infections with pathogens studied at Centers for Disease Control and Prevention and in tumor immunity characterized by researchers at Dana-Farber Cancer Institute. CD28 signaling is also essential for the development and maintenance of regulatory T cell suppressive function as shown in investigations from Imperial College London and impacts memory T cell formation reported by teams at Fred Hutchinson Cancer Center. Dysregulated CD28-mediated activation contributes to immunopathology in models studied by groups at Karolinska Institutet and has been central to understanding cytokine release syndromes observed in clinical cases from major hospitals like Johns Hopkins Hospital.

Clinical significance and therapeutic targeting

CD28 and its ligands have been targeted therapeutically with monoclonal antibodies and fusion proteins to modulate immune responses in transplantation, autoimmunity, and oncology. The development of CTLA-4–Ig fusion proteins to block B7–CD28 interactions emerged from translational work at companies and centers including Bristol-Myers Squibb and University of Pennsylvania and is used to prevent graft rejection and treat autoimmune diseases in trials reported by Food and Drug Administration–regulated programs. Superagonistic anti-CD28 antibody trials highlighted safety risks and guided regulatory scrutiny following high-profile adverse events involving research clinics in Europe, prompting revisions to clinical trial design by regulatory agencies like European Medicines Agency. In cancer immunotherapy, modulation of costimulatory pathways is combined with checkpoint blockade strategies pioneered at institutions such as Memorial Sloan Kettering Cancer Center and Yale University, and synthetic approaches to engage CD28 are being explored in engineered T cell therapies developed at translational centers including National Cancer Institute and biotech firms.

Genetic variation and regulation

The CD28 gene locus exhibits polymorphisms and alternative splicing events that influence expression and function; genome-wide association studies led by consortia at institutions such as Wellcome Trust and Broad Institute have associated variants near the locus with autoimmune conditions characterized in cohorts from University of Cambridge and Karolinska Institutet. Transcriptional regulation involves promoters and enhancers bound by factors studied in chromatin work from Massachusetts Institute of Technology and Cold Spring Harbor Laboratory, with epigenetic modifications modulated in health and disease contexts investigated at centers like Salk Institute. MicroRNA-mediated regulation and post-translational modifications such as phosphorylation and ubiquitination alter CD28 signaling dynamics, areas of active investigation in research programs at institutions including University of Tokyo and ETH Zurich.

Category:Immune system proteins