CD28 Immune Checkpoint Molecule for Drug Development

Overview of CD28

CD28 is a differentiation antigen expressed on thymocytes and most mature T cells, including all CD4 T cells and CD8 T cells with cytolytic activity. It belongs to the immunoglobulin (Ig) superfamily and provides an important co-stimulatory signal. CD28 is a homodimeric glycoprotein composed of a disulfide-bonded 44 kDa subunit. Each CD28 monomer contains 134 extracellular amino acids with a single transmembrane domain and a short cytoplasmic tail. The extracellular domain of CD28 shows homology with Ig variable region domains. The CD28 gene contains four exons, with each exon defining a functional domain of the protein.

CD 28 overview. Fig.1 CD 28 overview. (Xia, 2020)

The CD28 Co-Receptor

CD28 is an important costimulatory receptor in the immune checkpoint pathway. Selective CD28 antagonists have drawn worldwide interest because they tip the co-stimulation/co-inhibition balance towards efficiently inhibiting effector T cells while promoting suppression by pre-existing regulatory T-cells. The extracellular portion of CD28 contains a single Ig-V-like domain incorporating an MYPPPY motif. This motif provides the structural specificity for interactions with CD80 (B7-1) and CD86 (B7-2). The higher affinity of related CTLA-4 for CD80/CD86 has, in turn, allowed the use of a CTLA-4-Ig fusion protein to out-compete CD28-CD80/86 binding in the treatment of autoimmune disorders. CD28 and CD80/CD86 pathway markedly enhances the production of lymphokines by helper T cells through transcriptional and posttranscriptional regulation of gene expression and can activate the cytolytic potential of cytotoxic T cells.

Signaling pathways downstream of CD28. Fig.2 Signaling pathways downstream of CD28. (Esensten, 2016)

CD28 Co-Signaling Pathways

CD28 binds to multiple signaling proteins that interact with numerous downstream pathways. The 41 aa cytoplasmic tail of human CD28 contains highly conserved tyrosine-based signaling motifs that are phosphorylated in response to TCR or CD28 stimulation and bind targets with SH2 domains in a phosphotyrosine-dependent manner. The membrane-proximal YMNM motif and the distal PYAP motif have been complex with several kinases and adaptor proteins. Some proteins can bind to either or both motifs via SH2 and/or SH3 domain interactions. These motifs are important for IL2 transcription mediated by the CD28-dependent activation of NFAT, AP-1, and NF-κB family transcription factors.

Clinical Significance

After demonstrating outstanding therapeutic efficacy in multiple models of autoimmunity, inflammation and transplantation, and safety in phase I studies in humans, selective CD28 antagonists are currently in early clinical development to treat different diseases. Significant progress has been made into CAR-T cell therapy incorporating costimulatory domains. It has led to the FDA approval of CAR-T cell therapy for relapsed or refractory acute lymphoblastic leukemia patients.

As a worldwide company with extensive scientific expertise, Creative Biolabs guides clients from drug discovery to approval and provides continuity for their entire program. Our highlighted immune checkpoint-related services include immune checkpoint antibody development, immune checkpoint assays, etc. For more services, please refer to our Services Overview for more information. Please do not hesitate to contact us for more details.


  1. Xia, S.; et al. CD28: A New Drug Target for Immune Disease. Current drug targets. 2020, 21(6): 589-598.
  2. Esensten, J. H.; et al. CD28 costimulation: from mechanism to therapy. Immunity. 2016, 44(5): 973-988.

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