The human T-cell-specific surface glycoprotein CD28 is recombinant protein consists of a single glycosylated polypeptide chain with 376 amino acids (19-152aa).

The CD28 family of cell surface proteins includes co-stimulatory molecules (e.g., CD28, ICOS) and co-inhibitory molecules (e.g., CTLA-4, PD-1), essential for the induction and regulation of immune responses. CD28, a prototypical co-stimulatory molecule, provides the crucial secondary signal for effective immune responses to antigens presented by MHC molecules interacting with the T-cell receptor. Constitutively expressed on most CD4+ T-helper cells and major subsets of CD8+ T cells, CD28 binds to B7.1 and B7.2 on antigen- presenting cells, stimulating activation and differentiation of naïve T cells and guiding the development of memory/effector T-cell subsets. Upon prolonged activation, CD28 is downregulated, and CTLA-4 (CD152), with a higher affinity for B7.1 and B7.2, is mobilized to the cell surface, limiting T cell responses by blocking clonal expansion and cytokine production. CD28 co-stimulation is crucial for CD4+ T- cell proliferation, survival, interleukin-2 production, and T-helper type-2 development, and is necessary for the expansion and function of regulatory T cells.

Abbreviations:

T-cell-specific surface glycoprotein CD28 isoform 1, T-cell-specific surface glycoprotein CD28, TP44, CD antigen: CD28.