CD28 Tp44, T44
Molecule TypeAntigen ExpressionMolecular Weight
Min / Max
Lineage Restricted Molecule
Type 1 glycoprotein
T Cell
Thymocyte
T Lymphocyte
Plasma Cell
44 / 44
90 / 90

Expression
CD28 is expressed on most T lineage cells with the exception of a small subset CD8+ cells.  Expression is in plasma cells and mature CD3+ thymocytes have higher levels of CD28 than the immature cells.  There are high levels on most peripheral T lymphocytes, all CD4+ cells and ~50% of human CD8+ cells are positive.  In general, activation of T cells leads to enhanced CD28 expression but ligation of CD28 leads to its transient downregulation. 

Structure
MOLECULAR FAMILY NAME: Belongs to the immunoglobulin supergene family.

CD28 is a single-pass type-1 disulfide-linked homodimeric 202 aa glycoprotein.  It contains an 134 aa extracellular domain which contains an Ig-likeV-type domain and 5 potential N-linked glycosylation sites, a 27 aa transmembrane domain and a 41 aa cytoplasmic domain containing a tyrosine-containing motif.  The extracellular domain shares a significant amino acid sequence homology with CD152 (CTLA-4), with a 31% identity.  The 2 genes are less than 150 kb apart, suggesting that they share a common ancestor in evolution and bind the same ligands.

MOLECULAR MASS
Molecule Type Unreduced Reduced
T lymphocytes ~ 90 kDa ~ 44 kDa
COS cells ~ 74 kDa ~ 39 kDa

POST-TRANSCRIPTIONAL MODIFICATION

There are 4 transcripts with 1.3 kb, 1.5 kb, 3.5 kb, and 3.7 kb are observed in T cells.  The longer pair of transcripts with 3.5/3.7 kb is generated by the use of an alternative polyadenylation signal.  The size difference between the 1.3 kb and 1.5 kb arise from alternative splicing within exon 2 extracellular domain.

POST-TRANSLATIONAL MODIFICATION

There are 5 potential N-linked glycosylation sites. 

Ligands
Like CD152, CD28 binds both CD80 and CD86 using a highly conserved motif MYPPPY in the CDR3-like loop.  CD28 binds CD80 with a low affinity of kDa 4 mM  and dissociates very rapidly of K off >1.6s-1.  Binding to CD86 may be even weaker.  The cytoplasmic domain interacts with phosphatidylinositol 3-kinase (PI-3k), the complex between GRB-2 and the guanine nucleotide exchange protein SOS (GRB-2/SOS), and the tyrosine kinase ITK.  SH2 domains in PI-3k and GRB-2/SOS mediate binding to the CD28 motif YMNMT, after it has been phosphorylated by Lck and Fyn.

LIGANDS AND MOLECULES ASSOCIATED WITH CD28
Molecule Comment
CD80 The CD80 molecule is also a ligand for CD152, which binds with high avidity.
CD86 The CD86 molecule is also a ligand for CD152, which binds with high avidity.
PI-3k The cytoplasmic region of CD28 binds to this intracellular signaling molecule.  The pYMNM motif binds to the SH2 domains of PI-3k and GRB-2.
ITK The cytoplasmic region of CD28 binds to this intracellular signaling molecule.
GRB-2-SOS (signaling complex) The cytoplasmic region of CD28 binds to this intracellular signaling molecule. The pYMNM motif binds to the SH2 domains of PI-3k and GRB-2.

 

Function
CD28 is considered a major co-stimulatory molecule, inducing T lymphocyte activation and IL-2 synthesis and preventing cell death.  Studies in vitro indicate that CD28 is the ligand on T cells by CD80 (B7-1) and CD86 (B7-2) on antigen presenting cells provides a co-stimulatory signal required for T cell activation but blocking this interaction causes functional inactivation of T cells.  CD28 costimulation is essential for CD4-positive T-cell proliferation, survival, IL-2 production and T-helper type-2 development.  Knockout mice lacking CD28 are able to mount effective T cell responses and are mainly defective in T cell-dependent antibody responses, suggesting that CD28 is mainly important for T and B cell interactions and humoral immunity.  The cytoplasmic domain binds intracellular signaling molecules  
PI 3-kinase, guanine nucleotide exchange protein (GRB-2-SOS)  via the PYMNM motif and binds tyrosine kinase ITK.  The motif requires phosphorylation by Lck and Fyn.  Intact CD28 antibodies co-stimulate T cell proliferation and cytokine production with the presence of suboptimal amount of  PHA, PMA, CD3 mAbs or CD2 mAbs.  

BIOCHEMICAL ACTIVITY: No information.

DISEASE RELEVANCE AND FUNCTION OF CD28 IN INTACT ANIMAL

The blockade of CD28 signal, using mAbs against CD80 and/or CD86 or CD152 Ig fusion protein, induces a functional inactivation of T cells in a number of in vivo experimental disease models.  CD28 deficient mice have normal T cell development and normal cell mediated immunity but T cell mediated humoral immunity was significantly reduced.  


Comments
MOLECULAR INTERACTIONS -
PROTEINS AND DNA ELEMENTS WHICH REGULATE TRANSCRIPTION OF CD28: No information.

SUBSTRATES: No information.

ENZYMES WHICH MODIFY CD28: No information.

ADDITIONAL INSIGHTS

A key challenge will be elucidation of the cellular and molecular mechanism which distinquish the function of CTL-4 from that of CD28 with which CTLA-4 shares aa sequence homology and common ligands, CD80/CD86.


Database accession numbers
AnimalPIRSWISSPROTEMGBL/GENBANK
 
HumanEntrezgene 940P10747
MouseA43523P31041M34563
RatS24413P31042X55288
Antibodies
2-4   View Reactivity
CD28.2   View Reactivity
YTH913.12   View Reactivity

Revised June 25, 2008


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