This antibody is a Golden Syrian Hamster monoclonal antibody that binds specifically to mouse CD28, and it can neutralize the bioactivity of mouse CD28.

Recombinant Mouse Anti-CD28 Antibody (5.11A1)

Anti-CD28 antibody is a Mouse antibody of IgG class that binds to a CD28.

What is CD28?

CD28 is a costimulatory molecule expressed on the surface of T lymphocytes, which plays an important role in the activation of T cells. It binds to B7 on APC (antigen-presenting cell), mediates the co-stimulation of T cells and promotes their survival, proliferation and cytokine production.The activation of resting T cells requires two signals: the first signal is that TCR-CD3 complex binds to the antigen peptide presented by MHCⅡ, which is antigen specific; the second signal, also known as costimulatory signal, is non antigen specific. In the absence of the second signal, T cells are not activated, instead it is in a non-reactive state, and even induced to apoptosis.Many co-receptors on the surface of T cells, such as CD2, CD4 and CD8, have been found to mediate costimulatory signals and promote cytokine secretion, but cannot prevent anergy from being induced. The combination of CD28 and anti-CD28 monoclonal antibody or ligand on cell surface can effectively prevent the induction of anergy, and promote the production of cytokines (such as IL-2) as well as the proliferation of T cells.More and more studies have shown that the binding of CD28 family members to B7 family molecules plays an important role in T cell activation and homeostasis of T cell population.

CD28 family

All CD28 like receptors are type I transmembrane glycoproteins and belong to immunoglobulin superfamily. The extracellular domain of CD28 family members has a single V-like domain, which is necessary for binding with ligands. CD28 family members’cytoplasmic segments contain regions for SH2 and SH3 binding, which are necessary for signal transduction. The first-order sequence similarity among family members is very poor (20% - 40%), but the third-order structure similarity is better (> 60%).In addition to CD28, the expression of other members of CD28 family will increase significantly after the cells are activated, which provides a mechanism for regulating receptor ligand binding, and it also has the function of facilitating the generation of appropriate effectors in response to specific stimulation.

The gene encoding CD28 is located in q33-34 region of human chromosome 2. CD28 is a kind of glycoprotein consists of 202 amino acid residues, linked by the disulfide bond between chains and existing on the surface of CD28 cells in the form of homologous dimer. The mature and fully glycosylated monomer molecular weight is about 44000, including extracellular region, transmembrane region and cytoplasmic region. There is a highly conserved peptide binding motif, MYPPY, in the three complementary domains of extracellular domain, which determines the specific binding between CD28 and B7 family.

The cytoplasmic region of human CD28 contains four tyrosine, four serine and two threonine residues, which are potential sites of protein tyrosine kinase (PKT) and serine/ threonine kinase. The protein with SH2 region can specifically bind to phosphorylated tyrosine residues, and its specificity is related to 3-4 amino acids at the carboxyl end of Yp.

The role of CD28 in immune response

CD28 and B7 act to transmit costimulatory signals necessary for T cell activation. CD28/B7 signal plays an important role in the initiation of immune response. At present, there is evidence that when T/APC cells are in contact, CD28 combines with Lck, CD3, TCR and some molecules on the cell surface to form supramolecular complexes, and through phosphorylation of ITAM, TCR related downstream signals are activated, finally T cells are activated.However, CD28/B7 co-stimulation signal defect will lead to T cell's anergy, which is an active response to exogenous stimulation. In the process of formation, there are protein synthesis, Ca²⁺ flow and protein kinase activation. Moreover, B7 can stimulate or add exogenous IL-2, IL-4, IL-7 and various cytokines that can activate JAK3 or the universal subunit γ chain that can cross link IL-2R to restore cells immune response.In addition, some experiments have proved that the defect of TCR signal will also lead to cell tolerance. Therefore, CD28/B7 co-stimulation is a necessary signal for T cells to start immune response. This signal modifies and supplements TCR signal, integrates multiple signals, and then starts immune response.

CD28 also plays an important role in immune maintenance and down regulation, which can regulate and maintain the survival of appropriate and functional T cells to ensure the specific immune response; meanwhile, it can maintain the immune balance and stability through the balance of cytokines and the activation of CTLA-4.CD28 regulates T cell activation by participating in the expression and function of negative regulatory factor CTLA4. And CD28 antagonists such as anti-b7-2 antibody can inhibit the up-regulation of CTLA4 expression, while moderate CD28 signal can assist the anti-CTLA4 antibody to inhibit the proliferation of activated T cells.

To sum up, CD28 is more like a regulatory hub. It forms a network of precise regulation of immune response with CTLA4 and B7 family molecules, which can ensure the immune homeostasis in vivo and generate specific response to foreign antigens. The regulation of CD28 involves the initiation, expansion, maintenance and down-regulation of immune response. In addition, it may also involve the maturation, differentiation of T cells and down-regulation of auto reactive cell responses that have escaped thymus monitoring. At present, recombinant and soluble CTLA4 Ig have been used to block CD28/B7 signals in order to inhibit body fluid immunity, immune rejection, transplant versus host disease and alleviate autoimmune disease.