The role of the cytoplasmic tail of antigen-presenting cell surface molecule CD80 in delivery of T cell costimulation

Thesis (Ph. D.)--University of Washington, 1997CD80 is a highly glycosylated surface protein of 45-60 kDa found on activated B cells, T cells, monocytes, and dendritic cells. Based upon the CD80 cDNA sequence, the molecular weight of the protein core is 30 kDa. The remaining 15-30 kDa of the molecular weight is N-linked carbohydrate attached at eight possible sites in the extracellular region. CD80, and its structural and functional homolog CD86, both bind to the T cell molecules CD28 and CTLA-4. CD28 is found on resting and activated T cells, and is a major costimulatory molecule in the immune response. Ligation of CD28 increases both cytokine gene transcription and mRNA stability, and up-regulates cytokine receptor expression. CTLA-4 is expressed on T cells and B cells only after activation. Its role in T cell immunity is less clear than that of CD28. Initially, antibodies to CTLA-4 were shown to enhance CD3-and CD28-mediated T cell proliferation, but it has become clear recently that CTLA-4 is a major negative regulatory protein, limiting T cell expansion. CD28 crosslinking is necessary to provide costimulation; thus, monomeric CD80 or CD86 should not be able to induce CD28-mediated costimulation. We found that cells expressing a mutant form of CD80 lacking the cytoplasmic tail are incapable of inducing CD28-mediated signals, and do not undergo actin-dependent antibody-mediated redistribution. Furthermore, we found that there are at least two sites in the cytoplasmic tail of CD80 which are required for normal CD80 redistribution and CD28-mediated costimulation. Chimeric CD80 molecules containing the transmembrane and cytoplasmic domains of CD2 or CD54 can induce CD28-dependent T cell proliferation. CD2 and CD54 interact with the tubulin-based and the actin-based cytoskeleton, respectively. These results suggest there is no requirement for CD80 to interact specifically with the actin-dependent cytoskeleton to induce costimulation. We identified a protein of approximately 30 kDa that interacts with the cytoplasmic tail of CD80 in cells treated with calcium ionophore and phorbol esters. The interaction of CD80 with this protein may regulate the redistribution of CD80 on antigen-presenting cells (APC), and thus regulate the ability of CD80 to induce CD28-mediated costimulation