Mast cells play a critical role in the development of the allergic response. Upon activation by allergens and IgE via the high affinity receptor for IgE (FcεRI), these cells release histamine and other functional mediators that initiate and propagate immediate hypersensitivity reactions. Mast cells also secrete cytokines that can regulate immune activity. These processes are controlled, in whole or part, by increases in intracellular Ca2+ induced by the FcεRI. We show here that N-(4-(3,5-bis(Trifluoromethyl)-1H-pyrazol-1-yl)phenyl)-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2), a pyrazole derivative, inhibits activation-induced Ca2+ influx in the rat basophil cell line RBL-2H3 and in bone marrow-derived mast cells (BMMCs), without affecting global tyrosine phosphorylation of cellular proteins or phosphorylation of the mitogen-activated protein kinases Erk1/2 and p38. BTP2 also inhibits activation-induced degranulation and secretion of Interleukin (IL)-2, IL-3, IL-4, IL-6, IL-13, Tumor Necrosis Factor (TNF)-α, and Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) by BMMCs, which correlates with the inhibition of Nuclear Factor of Activated T cells (NFAT) translocation. In vivo, BTP2 inhibits antigen-induced histamine release. Structure-activity relationship analysis indicates that substitution at the C3 or C5 position of the pyrazole moiety on BTP2 (5-triflouromethyl-3-methyl-pyrazole or 3-triflouromethyl-5-methyl-pyrazole, respectively) affected its activity, with the trifluoromethyl group at the C3 position being critical to its activity. We conclude that BTP2 and related compounds may be potent modulators of mast cell responses and potentially useful for the treatment of symptoms of allergic inflammation.

Store-operated calcium channels are plasma membrane Ca2+ channels that are activated by depletion of intracellular Ca2+ stores, resulting in an increase in intracellular Ca2+ concentration, which is maintained for prolonged periods in some cell types. Increases in intracellular Ca2+ concentration serve as signals that activate a number of cellular processes, however, little is known about the regulation of these channels. We have characterized the immuno-suppressant compound BTP, which blocks store-operated channel mediated calcium influx into cells. Using an affinity purification scheme to identify potential targets of BTP, we identified the actin reorganizing protein, drebrin, and demonstrated that loss of drebrin protein expression prevents store-operated channel mediated Ca2+ entry, similar to BTP treatment. BTP also blocks actin rearrangements induced by drebrin. While actin cytoskeletal reorganization has been implicated in store-operated calcium channel regulation, little is known about actin binding proteins that are involved in this process, or how actin regulates channel function. The identification of drebrin as a mediator of this process should provide new insight into the interaction between actin rearrangement and tore-operated channel mediated calcium influx.

A lthough a number of antiviral drugs are effective inhibitors of Epstein-Barr virus (EBV) replication and are used empirically, none is of proven effectiveness for treatment of EBV infection (1, 2). Maribavir (MBV), which is in late-stage clinical trials for use against human cytomegalovirus (HCMV) infection in allogeneic stem cell and bone-marrow transplant recipients (3,4), is of special interest because it is also a potent inhibitor of EBV replication (5-7). In stem-cell and organ transplant recipients, EBV infection poses the hazard of generating B-cell lymphomas that are ultimately fatal. While no drugs are currently approved for treatment of EBV disease, several that inhibit EBV are available, and these can be divided into two main classes: those that target the viral DNA polymerase and those that function independently of it (8-12). Acyclic nucleoside and phosphonated nucleotide analogs, as well as pyrophosphate analogs, all target the viral polymerase.A new class of HCMV inhibitors, benzimidazole compounds, with more specific antiviral properties and fewer adverse side effects, blocked HCMV DNA maturation and encapsidation processes and led to the design of 1-H-␤-L-ribofuranoside-2-isopropylamino-5,6-dichlorobenzimidazole (maribavir [MBV]) (13)(14)(15)(16)(17)(18)(19)(20). Unlike its parent compound, which inhibits HCMV replication but not EBV replication, MBV inhibits both (7,21). Inhibitory effects of MBV are produced mainly through inhibition of the HCMV and EBV protein kinases (PK) (21-24). Previous phase 3 studies with a dosage of 100 mg twice a day (BID) did not have sufficient activity to prevent HCMV disease, but the safety profile and data from case studies suggested that higher doses would be clinically active (3). MBV is now in new phase 2 trials at doses of 400, 800, and 1,200 mg BID (3).Maribavir selectively inhibits the HCMV protein kinase, UL97, determined by direct inhibition of kinase activity in vitro and by genetic mapping of the MBV-resistant phenotype (21). MBV also inhibits the EBV protein kinase (BGLF4), resulting in inhibition of phosphorylation of the EBV DNA processivity factor BMRF1, but does not seem to act directly on the EBV kinase in vitro (7,24).We have recently found that MBV also inhibits expression of multiple EBV transcripts, in contrast to acyclovir (ACV), which has little effect on EBV RNAs. Thus, MBV has a unique dual effect on viral DNA transcription as well as replication (25). In this study, we find that the inhibitory profile of MBV transcripts is similar to that produced by mutant EBV in which PK expression and activity have been knocked out (26). Thus, the results suggest that MBV largely affects EBV transcript levels through inhibition of BGLF4.To determine if the profile of viral transcripts produced by MBV is mediated by the viral kinase, we utilized BGLF4 knockout (KO) (dBGLF4/NeoST) and revertant (dBGLF4/NeoSt/R) viruses constructed and characterized by Murata et al. (26). 293 cells maintaining wild-type (WT), BGLF4 knockout, and revertant EBV genomes (2...

Mast cells play critical roles in allergic responses, and calcium signaling controls the function of these cells, and a role for actin in regulating calcium influx into cells has been suggested. We have previously identified the actin reorganizing protein Drebrin as a target of the immunosuppressant BTP, which inhibits calcium influx into cells. We show here that Drebrin−/− mice exhibit reduced IgE-mediated histamine release and passive systemic anaphylaxis and Drebrin−/− mast cells also exhibit defects in FcεRI-mediated degranulation. Drebrin−/− mast cells exhibit defects in actin cytoskeleton organization and calcium responses downstream of the FcεRI, and agents that relieve actin reorganization rescue mast cell FcεRI induced degranulation. Our results indicate that Drebrin regulates the actin cytoskeleton and calcium responses in mast cells, thus regulating mast cell function in vivo.