Soluble CD23 is also found in the saliva of Sjögren’s syndrome

patients41,42 and in the plasma of patients with systemic lupus erythematosus,41,42 though in the case of systemic lupus erythematosus the effect of sCD23 is likely to be mediated via its interaction with CD21 on autoimmune B cells rather than via integrins on monocytic cells.43 The finding of high sCD23 levels in such syndromes has made both sCD23 protein itself and its various receptors attractive targets for therapeutic intervention. This aspiration is supported by data from rodent systems where anti-CD23 mAbs have been shown to both prevent initial and ameliorate existing NVP-BKM120 clinical trial arthritic disease,25,26 and by the success of Lumiliximab, a humanized macaque anti-CD23 antibody, in treatment of B chronic lymphocytic leukaemia,44 a disease characterized by strikingly high plasma sCD23 levels.45 A different strategy, employing a CD23-binding peptide identified by phage display technology, also shows promise in preventing onset of adjuvant-induced arthritis

and reducing severity of established disease in rats.46 The identification of αVβ3 as an sCD23 receptor linked to TNF-α release in human monocytes18 suggested that antibodies to this integrin might be useful in autoimmune inflammatory disease.47 The Etaracizumab Dorsomorphin mAb (Abergrin, Vitaxin),48,49 a humanized form of the LM609 anti-αVβ3 reagent, was shown to be potent in inhibiting angiogenesis.50,51 However, Etaracizumab was also assessed in psoriatic arthritis but was not found to have a therapeutic effect and this is potentially explained by the fact that the parent LM609 mAb does not inhibit sCD23-driven TNF-α release from monocytes,18 a finding that implies that the mAb does not influence the site on the integrin responsible for control of cytokine release. Our data that showed LM609 did not induce cytokine production from either THP-1 or U937 cells (Fig. 3) were also in agreement with this

suggestion. Etaracizumab retains significant Vasopressin Receptor promise, however, and is currently in trials for therapy of metastatic melanoma.52 It is important to bear in mind that most previous studies on integrin function have been performed in adherent cells. The possibility of an alternative mode of integrin signalling illustrated by sCD23 is particularly interesting in the context of haematopoietic cells, including monocytes, which are non-adherent cells, but nonetheless express a wide range of integrins, and are the precursors of a number of adherent, terminally differentiated cells, such as macrophages and osteoclasts. The differentiation of monocytes into adherent counterparts is the result of paracrine or autocrine signalling in response to cytokines, such as those released by the interaction of sCD23 with integrins.