CD28 engagement by B7/BB-1 induces transient down-regulation of CD28 synthesis and prolonged unresponsiveness to CD28 signaling.

Costimulatory molecules on the APC regulate T cell growth by providing signals that regulate responses to TCR occupancy. One such molecule is B7/BB-1, which triggers a T cell activation pathway by binding the CD28 and/or CTLA-4 cell-surface molecules. Expression and signaling activity of CD28 have been shown to increase after T cell activation by various polyclonal activators. Here we show that CD28 expression and signaling activity in activated T cells decrease after ligand binding to CD28. Stimulation of CD28 on PHA- or PMA-activated T cells by cross-linked mAb 9.3 or by co-culture with B7+ Chinese hamster ovary (CHO) cells caused a marked reduction of CD28 mRNA levels within 4 h. The decrease in CD28 mRNA was transient, and by 24 h of CD28 stimulation, CD28 mRNA was found at approximately initial levels. In contrast, CTLA-4 mRNA levels were usually up-regulated by CD28 triggering. Cell-surface expression of CD28, but not CD2 or CD3, decreased by 12 to 24 h after addition of B7+ CHO cells, but returned to initial levels or higher by 48 h. The ability of CD28 cross-linking on PMA-activated CD4+ cells to trigger calcium mobilization was also reduced by treatment with B7+ CHO cells, and remained reduced even after cell-surface expression of CD28 returned to normal levels. Thus, engagement of the CD28 receptor by its natural ligand B7/BB-1 leads to a transient down-regulation of CD28 synthesis and a prolonged unresponsiveness to CD28 signaling. This represents a novel mechanism for regulation of costimulatory signals delivered by interactions of CD28 with the B7/BB-1 counter receptor.

CTLA-4 is a second receptor for the B cell activation antigen B7.

Functional interactions between T and B lymphocytes are necessary for optimal activation of an immune response. Recently, the T lymphocyte receptor CD28 was shown to bind the B7 counter-receptor on activated B lymphocytes, and subsequently to costimulate interleukin 2 production and T cell proliferation. CTLA-4 is a predicted membrane receptor from cytotoxic T cells that is homologous to CD28 and whose gene maps to the same chromosomal band as the gene for CD28. It is not known, however, if CD28 and CTLA-4 also share functional properties. To investigate functional properties of CTLA-4, we have produced a soluble genetic fusion between the extracellular domain of CTLA-4 and an immunoglobulin C gamma chain. Here, we show that the fusion protein encoded by this construct, CTLA4Ig, bound specifically to B7-transfected Chinese hamster ovary cells and to lymphoblastoid cells. CTLA4Ig also immunoprecipitated B7 from cell surface 125I-labeled extracts of these cells. The avidity of 125I-labeled B7Ig fusion protein for immobilized CTLA4Ig was estimated (Kd approximately 12 nM). Finally, we show that CTLA4Ig was a potent inhibitor of in vitro immune responses dependent upon cellular interactions between T and B lymphocytes. These findings provide direct evidence that, like its structural homologue CD28, CTLA-4 is able to bind the B7 counter-receptor on activated B cells. Lymphocyte interactions involving the B7 counter-receptor are functionally important for alloantigen responses in vitro.

Amylase synthesis as a simple model system for translation and hybrid arrest in Xenopus oocytes.

A human alpha-amylase-encoding cDNA has been cloned in a transcription vector. When messenger RNA (mRNA) made in vitro from this construct was injected into Xenopus oocytes, amylase (AMY) activity was detected both in oocyte homogenates and in the incubation medium, indicating that the oocyte machinery correctly translated and processed the protein. Because AMY activity is easy to detect with a blue-starch assay, this expression system was used to determine the parameters of antisense oligodeoxyribonucleotide (oligo) inhibition of translation in the oocytes. Unique oligos complementary to the AMY mRNA sequence were effective in arresting translation, at approximately stoichiometric levels. Mixed oligos also inhibited translation, at levels that suggest that some mismatches may be tolerated in the formation of DNA-RNA hybrids. The AMY system provides a convenient probe of oocyte protein synthesis and processing machinery and can serve as a control substrate in investigations of other mRNAs.