CD23 shedding: requirements for substrate recognition and inhibition by dipeptide hydroxamic acids.

CD23 (low affinity IgE receptor, FcepsilonRII) is expressed as a Type II extracellular protein on a variety of cells such as B cells, monocytes and macrophages and is cleaved from the cell surface to generate several distinct fragments. The expression of CD23 on the cell surface as well as the generation of soluble fragments of CD23 has been shown to be involved in regulation of IgE synthesis. CD23 is released from the cell surface by a metalloprotease, analogous to the cleavage of other cell surface molecules such as TNF-alpha. This activity has been extensively studied with respect to biochemical characterization and ability to cleave specific mutants of CD23. Both local sequence and distal domains have been shown to affect cleavage of CD23. Selective dipeptide hydroxamic acid inhibitors of CD23 processing have been identified and demonstrated to very potently and selectively inhibit CD23 processing.

Inhibition of CD23 processing correlates with inhibition of IL-4-stimulated IgE production in human PBL and hu-PBL-reconstituted SCID mice.

BACKGROUND: CD23, the low affinity serum immunoglobulin E (IgE) receptor, is upregulated on B cells following interleukin (IL)-4 stimulation and is concomitantly cleaved to generate soluble CD23 (sCD23) fragments with cytokine-like activity. OBJECTIVE: Compounds that selectively inhibit the proteolytic release of CD23 to generate sCD23 were assessed for their ability to inhibit IgE production in order to evaluate the contribution of sCD23 in the production of human IgE as well as the ability of such compounds to block IgE production. METHODS: IgE production was measured in IL-4-stimulated human peripheral blood lymphocytes (PBL) and PBL-reconstituted SCID mice in the presence of a broad-spectrum matrix metalloprotease (MMP) inhibitor, a compound selective for inhibition of CD23 processing over MMPs and an anti-CD23 mAb, MHM6. RESULTS: The two compounds were equipotent in inhibiting IgE production without inhibition of IgG production by IL-4/anti-CD40-stimulated PBL. Soluble CD23 release was also shown to precede IgE accumulation in the cell-free medium. Addition of compound at later times other than day 0 in the 14 day assay resulted in progressively less inhibition of both IgE and sCD23, and exactly paralleled the effect of an anti-CD23 mAb, MHM6 on IgE levels. Both compounds also inhibited the release of CD23 from human RPMI 8866 cells adoptively transferred i. p. to mice. Doses required for inhibition of CD23 correlated well with the doses required for inhibition of IgE production in IL-4-challenged hu-PBL-SCID mice. IgE was selectively inhibited over total IgG in the SCID mice as well. CONCLUSIONS: Inhibition of CD23 processing alone is sufficient to inhibit IL-4-stimulated IgE production both in vitro and in vivo.

CD23 (FcepsilonRII) release from cell membranes is mediated by a membrane-bound metalloprotease.

CD23 (low-affinity IgE receptor, FcepsilonRII) is expressed as a Type II extracellular protein on a variety of cells such as B-cells, monocytes and macrophages and is cleaved from the cell surface to generate several distinct fragments. The expression of CD23 on the cell surface as well as the generation of soluble fragments of CD23 has been shown to be involved in the regulation of IgE synthesis. Here we report that the release of CD23 from the cell surface is mediated by a metalloprotease. An assay utilizing purified CD23 and an neo-epitope antibody specific for one of the known cleavage products is described and used to demonstrate unambiguously the cleavage of CD23 by a distinct protease. Characterization of the mechanism of CD23 processing shows that the protease exists as an integral membrane protein with a functional molecular mass of approx. 63 kDa as determined by gel-filtration chromatography. The CD23-cleaving activity found in enriched plasma membranes from RPMI 8866 cells is inhibited by the metalloprotease inhibitors 1, 10-phenanthroline and imidazole and by the matrix metalloprotease inhibitor batimastat, but not by inhibitors of cysteine proteases, serine proteases or acid proteases. The same or a similar activity that cleaves CD23 to the known 33 kDa fragment and is inhibited by batimastat is present in diverse cell types such as unstimulated fibroblasts and monocytic cell lines not expressing CD23, as well as in the Epstein-Barr virus-transformed B-cell line, RPMI 8866, which constitutively expresses CD23.

SB 203347, an inhibitor of 14 kDa phospholipase A2, alters human neutrophil arachidonic acid release and metabolism and prolongs survival in murine endotoxin shock.

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 fatty acyl group [predominantly arachidonic acid (AA)] of membrane phospholipids, the products of which are further metabolized, forming a variety of eicosanoids and/or platelet-activating factor. PLA2 activity is significantly enhanced during inflammation and therefore offers an intriguing target in designing anti-inflammatory drugs. SB 203347 (2-[2-[3,5-bis (trifluoromethyl) sulfonamido]-4- trifluoromethylphenoxy] benzoic acid) potently inhibits rh type II 14 kDa PLA2 (IC50 = 0.5 microM) but exhibits a 40-fold weaker inhibition of 85 kDa PLA2 (IC50 = 20 microM) using [3H]-AA E. coli as substrate. A specific interaction with rh type II 14 kDa PLA2 was confirmed both by observing the pH dependence of its IC50 and by demonstrating linear inhibition in a "scooting" kinetic model using radiolabeled phospholipid reporter substrate in a 1,2-dimyristoyl phosphatidylmethanol vesicle. Before evaluating the effect of SB 203347 on AA metabolism in intact human neutrophil, we showed that it fully inhibits PLA2 activity in acid extracted-intact human neutrophil homogenate (IC50 = 4.7 microM). SB 203347 inhibited A23187-induced intact human neutrophil AA mass release in a concentration-dependent manner (IC50 = 1 microM), which coincided with reductions in the biosynthesis of platelet-activating factor (IC50 = 1.5 microM) and leukotriene B4 (IC50 = 2.3 microM). Finally, SB 203347 prolonged survival in a mouse model of endotoxin shock delivered i.p. Taken together, the data support a role of cellular 14 kDa PLA2 in the formation of AA-derived proinflammatory lipid mediator.(ABSTRACT TRUNCATED AT 250 WORDS)