Activation of the MHC class II transactivator CIITA by interferon-gamma requires cooperative interaction between Stat1 and USF-1.

CIITA is the mediator of MHC class II gene induction by interferon-gamma (IFNgamma). The CIITA gene is itself selectively activated via one of its four promoters (PIV). We show here that three cis-acting elements, the GAS, the E box, and the IRF-1-binding site, as well as the transacting factors Stat1 and IRF-1, are essential for activation of CIITA promoter IV by IFNgamma. Stat1 binds to the GAS site only in the presence of the ubiquitous factor USF-1, which binds to the adjacent E box. Indeed, Stat1 and USF-1 bind to the GAS/E box motif in a cooperative manner. The specificity for CIITA activation by IFNgamma is thus dictated by the GAS/E box motif and by the selective interaction of IFNgamma-activated Stat1 and USF-1. This clarifies the missing link in the overall pathway of IFNgamma activation of MHC-II expression.

CD20 monoclonal antibodies decrease interleukin-4-stimulated expression of the low-affinity receptor for IgE (Fc epsilon RII/CD23) in human B cells by increasing the extent of its cleavage.

CD20 monoclonal antibody (mAb) B1 is known to inhibit B cell proliferation. We show that B1 reduced both anti-mu + interleukin-4 (IL-4)-induced DNA synthesis and the concomitant expression of CD23 at the surface of human tonsillar B cells. B1 mAb had no effect on CD23 mRNA levels. The disappearance of CD23 molecule from the surface correlates with an increase of soluble CD23 fragments in the culture medium, indicating that CD20 mAb B1 stimulated the cleavage of the molecule. B1 also inhibits IgE production by peripheral blood mononuclear cells cultured in the presence of IL-4. Suppression of IgE synthesis and enhancement of CD23 cleavage are concomitant but appear not to be functionally related.

CD20 monoclonal antibodies stimulate extracellular cleavage of the low affinity receptor for IgE (Fc epsilon RII/CD23) in Epstein-Barr-transformed B cells.

This study demonstrates that monoclonal antibodies to the B cell-specific CD20 molecule down-regulate both constitutive and interleukin-4-induced CD23 expression on Epstein-Barr-transformed B cells. This effect of CD20 antibody B1 does not take place at the transcriptional level as shown by the lack of effect on the CD23 mRNA level. Incorporation of 35S-labeled amino acids into CD23 polypeptide chain is not affected either. In cycloheximide-treated cells, B1 increases the decline of CD23 from the cell surface. The disappearance of CD23 molecule correlates with an increase of soluble CD23 fragments detected in the culture medium. Taken collectively, these results indicate that CD20 mAb B1 stimulates the cleavage of the CD23 molecule at the surface of B cells.

Platelet-activating factor activates a Ca(2+)-dependent K+ channel which is not involved in c-fos expression in human B lymphoblastoid cells.

In this report, it is shown that the platelet-activating factor (PAF) induced, in human B lymphoblastoid cells, 86Rb+ influx and efflux suggesting that it activated a K+ channel. Opening of this channel was dependent on PAF-induced Ca2+ mobilization. Ionomycin and thapsigargin--a specific inhibitor of (Ca(2+)-Mg2+)-ATPase--mimicked the effect of PAF both on intracellular calcium and activation of the channel. This channel was inhibited by charybdotoxin, high doses of tetraethylammonium and barium but was insensitive to apamin, 4-aminopyridine. These features indicate that PAF activated a Ca(2+)-dependent K+ channel. In these cells, PAF also induced the expression of c-fos oncogene. This effect was not affected by charybdotoxin indicating that this channel is not involved in the control of early gene transcription.