T helper cell
CD80 can be found on the surface of various immune cells including B cells, monocytes and antigen-presenting cells (APCs) such as dendritic cells, and also T-cells. CD80 is the receptor for the proteins CD28 (for autoregulation and intercellular association) and CTLA-4 (for attenuation of regulation and cellular disassociation) found on the surface of T-cells. However, the bond with CTLA-4 has up to 2500 fold higher avidity than with CD28. CD80 binds to CD28 and CTLA-4 with lower affinity and fast binding kinetics (Kd =4 μM), allowing for quick interactions between the communicating cells. This interaction results in an important costimulatory signal in the immunological synapse between antigen-presenting cells, B-cells, dendritic cells and T-cells that result in T and B-cell activation, proliferation and differentiation. When stimulated by CD80 T helper cell preferentially differentiate into Th1 cells. CD80 is an especially important component in dendritic cell licensing and cytotoxic T-cell activation. When the major histocompatibility complex class II (MHC class II)- peptide complex on a dendritic cell interacts with the receptor on a T helper cell, CD80 is up-regulated, licensing the dendritic cell and allowing for interaction between the dendritic cell and CD 8+ T-cells via CD28. This helps to signal the T-cell differentiation into a cytotoxic T-cell. The expression of CD80, as well as CD86, is increased by presence of microbes and cytokines that reacts to the presence of microbes. This mechanism ensures that costimulatory molecules for T-cells are present at the right time.