CD28

CD28 is one of the molecules expressed on T cells that provide co-stimulatory signals, which are required for T cell activation. CD28 is the receptor for B7.1 (CD80) and B7.2 (CD86). When activated by Toll-like receptor ligands, the B7.1 expression is upregulated in antigen presenting cells (APCs). The B7.2 expression on antigen presenting cells is constitutive. CD28 is the only B7 receptor constitutively expressed on naive T cells.

Stimulation through CD28 in addition to the TCR can provide a potent co-stimulatory signal to T cells for the production of various interleukins (IL-2 and IL-6 in particular).

ignalling

CD28 possesses an intracellular domain with several residues that are critical for its effective signalling. 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 second phase which is dependent on formation of an immune synapse and PI3K-independent 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.

CD28 as a drug target

The drug TGN1412, which was produced by the German biotech company TeGenero and which unexpectedly caused multiple organ failure in trials, is a superagonist of CD28. The first elucidation of the molecular structure of CD28 was achieved in 2005 by the T-cell biology group at the University of Oxford [ [http://www.t-cellbiology.org t-cellbiology.org ] ] , in complex with the Fab fragment of an antibody provided by TeGenero. [ [http://www.tegenero.com/documents/pr_tegenero_mar_15.pdf] Dead link|date=March 2008] 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. [

cite journal |author=Woerly G, Roger N, Loiseau S, Dombrowicz D, Capron A, Capron M |title=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 |journal=J Exp Med |volume=190 |issue=4 |pages=487-95 |year=1999 |pmid=10449520 |url=http://www.jem.org/cgi/content/full/190/4/487 ] [cite journal |author=Woerly G, Lacy P, Younes A, Roger N, Loiseau S, Moqbel R, Capron M |title=Human eosinophils express and release IL-13 following CD28-dependent activation |journal=J Leukoc Biol |volume=72 |issue=4 |pages=769-79 |year=2002 |pmid=12377947]

References

ee also

* List of human clusters of differentiation

Further reading

PBB_Further_reading
citations =
*cite journal | author=Linsley PS, Ledbetter JA |title=The role of the CD28 receptor during T cell responses to antigen. |journal=Annu. Rev. Immunol. |volume=11 |issue= |pages= 191-212 |year= 1993 |pmid= 8386518 |doi= 10.1146/annurev.iy.11.040193.001203
*cite journal | author=Lenschow DJ, Walunas TL, Bluestone JA |title=CD28/B7 system of T cell costimulation. |journal=Annu. Rev. Immunol. |volume=14 |issue= |pages= 233-58 |year= 1996 |pmid= 8717514 |doi= 10.1146/annurev.immunol.14.1.233
*cite journal | author=Greenfield EA, Nguyen KA, Kuchroo VK |title=CD28/B7 costimulation: a review. |journal=Crit. Rev. Immunol. |volume=18 |issue= 5 |pages= 389-418 |year= 1998 |pmid= 9784967 |doi=
*cite journal | author=Chang TT, Kuchroo VK, Sharpe AH |title=Role of the B7-CD28/CTLA-4 pathway in autoimmune disease. |journal=Curr. Dir. Autoimmun. |volume=5 |issue= |pages= 113-30 |year= 2002 |pmid= 11826754 |doi=
*cite journal | author=Bour-Jordan H, Blueston JA |title=CD28 function: a balance of costimulatory and regulatory signals. |journal=J. Clin. Immunol. |volume=22 |issue= 1 |pages= 1-7 |year= 2002 |pmid= 11958588 |doi=
*cite journal | author=Greenway AL, Holloway G, McPhee DA, "et al." |title=HIV-1 Nef control of cell signalling molecules: multiple strategies to promote virus replication. |journal=J. Biosci. |volume=28 |issue= 3 |pages= 323-35 |year= 2004 |pmid= 12734410 |doi=
*cite journal | author=Bénichou S, Benmerah A |title= [The HIV nef and the Kaposi-sarcoma-associated virus K3/K5 proteins: "parasites"of the endocytosis pathway] |journal=Med Sci (Paris) |volume=19 |issue= 1 |pages= 100-6 |year= 2003 |pmid= 12836198 |doi=
*cite journal | author=Tolstrup M, Ostergaard L, Laursen AL, "et al." |title=HIV/SIV escape from immune surveillance: focus on Nef. |journal=Curr. HIV Res. |volume=2 |issue= 2 |pages= 141-51 |year= 2004 |pmid= 15078178 |doi=
*cite journal | author=Anderson JL, Hope TJ |title=HIV accessory proteins and surviving the host cell. |journal=Current HIV/AIDS reports |volume=1 |issue= 1 |pages= 47-53 |year= 2005 |pmid= 16091223 |doi=
*cite journal | author=Li L, Li HS, Pauza CD, "et al." |title=Roles of HIV-1 auxiliary proteins in viral pathogenesis and host-pathogen interactions. |journal=Cell Res. |volume=15 |issue= 11-12 |pages= 923-34 |year= 2006 |pmid= 16354571 |doi= 10.1038/sj.cr.7290370
*cite journal | author=Stove V, Verhasselt B |title=Modelling thymic HIV-1 Nef effects. |journal=Curr. HIV Res. |volume=4 |issue= 1 |pages= 57-64 |year= 2006 |pmid= 16454711 |doi=

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