CD28

Signaling

CD28 possesses an intracellular domain with several residues that are critical for its effective signaling. The YMNM motif beginning at tyrosine 170 in particular is critical for the recruitment of SH2-domain containing proteins, especially PI3K, Grb2 and Gads. The Y170 residue is important for the induction of Bcl-xL via mTOR and enhancement of IL-2 transcription via PKCθ, but has no effect on proliferation and results a slight reduction in IL-2 production. The N172 residue (as part of the YMNM) is important for the binding of Grb2 and Gads and seems to be able to induce IL-2 mRNA stability but not NF-κB translocation. The induction of NF-κB seems to be much more dependent on the binding of Gads to both the YMNM and the two proline-rich motifs within the molecule. However, mutation of the final amino acid of the motif, M173, which is unable to bind PI3K but is able to bind Grb2 and Gads, gives little NF-κB or IL-2, suggesting that those Grb2 and Gads are unable to compensate for the loss of PI3K. IL-2 transcription appears to have two stages; a Y170-dependent, PI3K-dependent initial phase which allows transcription and a PI3K-independent second phase which is dependent on formation of an immune synapse, which results in enhancement of IL-2 mRNA stability. Both are required for full production of IL-2.

CD28 also contains two proline-rich motifs that are able to bind SH3-containing proteins. Itk and Tec are able to bind to the N-terminal of these two motifs which immediately succeeds the Y170 YMNM; Lck binds the C-terminal. Both Itk and Lck are able to phosphorylate the tyrosine residues which then allow binding of SH2 containing proteins to CD28. Binding of Tec to CD28 enhances IL-2 production, dependent on binding of its SH3 and PH domains to CD28 and PIP3 respectively. The C-terminal proline-rich motif in CD28 is important for bringing Lck and lipid rafts into the immune synapse via filamin-A. Mutation of the two prolines within the C-terminal motif results in reduced proliferation and IL-2 production but normal induction of Bcl-xL. Phosphorylation of a tyrosine within the PYAP motif (Y191 in the mature human CD28) forms a high affinity-binding site for the SH2 domain of the src kinase Lck which in turn binds to the serine kinase PKC-θ.

Structure

The structure of the human CD28 protein contains 220 amino acids, encoded by a gene consisting of four exons. It is a glycosylated, disulfide-linked homodimer of 44 kDa expressed on the cell surface. The structure contains paired domains of the V-set immunoglobulin superfamilies (IgSF). These domains are linked to individual transmembrane domains and cytoplasmic domains that contain critical signaling motifs. As CTLA4, CD28 share highly similar CDR3-analogous loops. In the CD28-CD80 complex, the two CD80 molecules converge such that their membrane proximal domains collide sterically, despite the availability of both ligand binding sites for CD28.

CD28 family members

CD28 belongs into group members of a subfamily of costimulatory molecules that are characterized by an extracellular variable immunoglobulin-like domain. Members of this subfamily also include homologous receptors ICOS, CTLA4, PD1, PD1H, and BTLA. Nevertheless, only CD28 is expressed constitutively on mouse T cells, whereas ICOS and CTLA4 are induce by T cells receptor stimulation and in response to cytokines such as IL-2. CD28 and CTLA4 are very homologous and compete for the same ligand – CD80 and CD86. CTLA4 binds CD80 and CD86 always stronger than CD28, which allows CTLA4 to compete with CD28 for ligand and suppress effector T cells responses. But it was shown that CD28 and CTLA4 have opposite effect on the T cells stimulation. CD28 acts as a activator and CTLA4 acts as inhibitor. ICOS and CD28 are also closely related genes, but they cannot substitute from one another in function. The opposing roles of CD28 and ICOS compared to CTLA4 cause that these receptors act as a rheostat for the immune response through competitive pro- and anti-inflammatory effects.

As a drug target

The drug TGN1412, which was produced by the German biotech company TeGenero, and unexpectedly caused multiple organ failure in trials, is a superagonist of CD28. Unfortunately, it is often ignored that the same receptors also exist on cells other than lymphocytes. CD28 has also been found to stimulate eosinophil granulocytes where its ligation with anti-CD28 leads to the release of IL-2, IL4, IL-13 and IFN-γ.

It is known that CD28 and CTL4 may be critical regulators of autoimmune diseases in mouse model. But there is less data from patients on the role of CD28 in human diseases.

Other potential drugs in pre-clinical development are agonist CD28 aptamers with immunostimulatory properties in a mouse tumor model, a monoclonal anti-CD28 Fab´ antibody FR104, or an octapeptide AB103, which prevents CD28 homodimerization.

Interactions

CD28 has been shown to interact with:

• GRAP2,
• Grb2, and
• PIK3R1.

References

References

1. (September 2002). "Expression of CD28 by bone marrow stromal cells and its involvement in B lymphopoiesis". Journal of Immunology.
2. (2011). "Loss of surface antigens is a conserved feature of apoptotic lymphocytes from several mammalian species". Cellular Immunology.
3. (May 2016). "CD28 Costimulation: From Mechanism to Therapy". Immunity.
4. (November 2014). "CD28 negative T cells: is their loss our gain?". American Journal of Transplantation.
5. (March 1994). "T-cell antigen CD28 interacts with the lipid kinase phosphatidylinositol 3-kinase by a cytoplasmic Tyr(P)-Met-Xaa-Met motif". Proceedings of the National Academy of Sciences of the United States of America.
6. (April 1995). "T cell antigen CD28 binds to the GRB-2/SOS complex, regulators of p21ras". European Journal of Immunology.
7. (October 2011). "A motif in the V3 domain of the kinase PKC-θ determines its localization in the immunological synapse and functions in T cells via association with CD28". Nature Immunology.
8. (April 2002). "The B7 family of ligands and its receptors: new pathways for costimulation and inhibition of immune responses". Annual Review of Immunology.
9. (March 2003). "Crystal structure of the receptor-binding domain of human B7-2: insights into organization and signaling". Proceedings of the National Academy of Sciences of the United States of America.
10. (March 2005). "Crystal structure of a soluble CD28-Fab complex". Nature Immunology.
11. (April 2013). "Molecular mechanisms of T cell co-stimulation and co-inhibition". Nature Reviews. Immunology.
12. (July 1990). "T-cell antigen CD28 mediates adhesion with B cells by interacting with activation antigen B7/BB-1". Proceedings of the National Academy of Sciences of the United States of America.
13. (July 2006). "CTLA-4 overexpression inhibits T cell responses through a CD28-B7-dependent mechanism". Journal of Immunology.
14. (August 1995). "CD28 and CTLA-4 have opposing effects on the response of T cells to stimulation". The Journal of Experimental Medicine.
15. (August 1994). "CTLA-4 can function as a negative regulator of T cell activation". Immunity.
16. (February 2009). "Roquin differentiates the specialized functions of duplicated T cell costimulatory receptor genes CD28 and ICOS". Immunity.
17. (August 1999). "Expression of CD28 and CD86 by human eosinophils and role in the secretion of type 1 cytokines (interleukin 2 and interferon gamma): inhibition by immunoglobulin a complexes". The Journal of Experimental Medicine.
18. (October 2002). "Human eosinophils express and release IL-13 following CD28-dependent activation". Journal of Leukocyte Biology.
19. (April 2000). "B7/CD28 costimulation is essential for the homeostasis of the CD4+CD25+ immunoregulatory T cells that control autoimmune diabetes". Immunity.
20. (November 1995). "Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regulatory role of CTLA-4". Immunity.
21. (June 2013). "CD28 aptamers as powerful immune response modulators". Molecular Therapy. Nucleic Acids.
22. (October 2012). "Preclinical efficacy and immunological safety of FR104, an antagonist anti-CD28 monovalent Fab' antibody". American Journal of Transplantation.
23. (2014-07-23). "Single administration of p2TA (AB103), a CD28 antagonist peptide, prevents inflammatory and thrombotic reactions and protects against gastrointestinal injury in total-body irradiated mice". PLOS ONE.
24. (June 2000). "GRID: a novel Grb-2-related adapter protein that interacts with the activated T cell costimulatory receptor CD28". Journal of Immunology.
25. (August 1998). "Grb2 forms an inducible protein complex with CD28 through a Src homology 3 domain-proline interaction". The Journal of Biological Chemistry.
26. (January 1996). "Signal transduction by CD28 costimulatory receptor on T cells. B7-1 and B7-2 regulation of tyrosine kinase adaptor molecules". The Journal of Biological Chemistry.
27. (April 1996). "Two distinct intracytoplasmic regions of the T-cell adhesion molecule CD28 participate in phosphatidylinositol 3-kinase association". The Journal of Biological Chemistry.