Modulation of leukocyte cyclic AMP phosphodiesterase activity by recombinant interferon-gamma: evidence for a differential effect on atopic monocytes

Phosphodiesterase-4 inhibitors in the treatment of inflammatory lung disease

Phosphodiesterases (PDE) belong to an important family of proteins that regulate the intracellular levels of cyclic nucleotide second messengers. Targeting PDE with selective inhibitors may offer novel therapeutic strategies in the treatment of various conditions, and in the context of respiratory disease these include asthma and chronic obstructive pulmonary disease (COPD). The rationale for such an approach stems, in part, from the clinical efficacy of theophylline, an orally active drug that is purportedly a nonselective PDE inhibitor. In addition, intracellular cyclic adenosine monophosphate (cAMP) levels regulate the function of many of the cells thought to contribute to the pathogenesis of respiratory diseases such as asthma and COPD, and these cells also selectively express PDE4. This has offered pharmaceutical companies the opportunity to selectively targeting these enzymes for the treatment of these diseases. Finally, the success of targeting PDE5 in the treatment of erectile dysfunction provides clinical proof of concept for the targeting of PDE in disease. Whether a 'Viagra' of the airways can be found for the treatment of asthma and COPD remains to be seen, but positive results from recent clinical studies examining the efficacy of selective PDE4 inhibitors such as cilomilast and roflumilast offer some optimism. However, one of the major issues to be resolved is the tolerability profile associated with this drug class that is a consequence of PDE4 inhibition. While cilomilast and roflumilast have low emetic potential they are not free from emesis and various strategies are being investigated in the hope of developing a PDE4 inhibitor without this adverse effect.

Reduced plasma c-AMP levels in children with atopic dermatitis

Plasma levels of cAMP and serum concentrations of IgE have been determined in children with acute atopic dermatitis (AD) and in a healthy control group, to illuminate the pathophysiological mechanisms that cause AD. There were significantly lower plasma levels of cAMP (P < 0.001) and significantly higher levels of serum IgE (P < 0.004) in children with AD, in comparison with a healthy control group. It is possible that defective control of c-AMP levels could contribute to the immunopathogenesis of AD and monitoring levels may be of value in the clinical evaluation of the disease.

Prostaglandin E2 control of T cell cytokine production is functionally related to the reduced lymphocyte proliferation in atopic dermatitis

Past studies of peripheral blood mononuclear cells (PBMC) from patients with atopic dermatitis (AD) have demonstrated reduced proliferation. We have studied phytohemagglutinin-induced lymphocyte proliferation in the context of interleukin-4 (IL-4), interferon-gamma (IFN-gamma), and prostaglandin E2 (PGE2) production in cultures of PBMC from patients without and with AD. The proliferation index was found to correlate proportionally to IFN-gamma production and inversely to T-cell IL-4 and monocyte PGE2 production. Assays in parallel cultures showed significantly increased PGE2 production by purified AD monocytes. The proliferation index in PBMC from persons with AD was significantly reduced compared with normal PBMC. This difference was normalized in the presence of extrinsic IFN-gamma but exaggerated when IL-4 was added. Increased AD monocyte production of inflammatory factors (e.g., PGE2) and cytokines appears to increase IL-4 production by Th2 while suppressing IFN-gamma production by Th1. Restoration of the normal proliferation of PBMC by the addition of IFN-gamma may represent one mechanism for the clinical efficacy of IFN-gamma treatment of AD.

Monocyte phosphodiesterase abnormalities and dysregulation of lymphocyte function in atopic dermatitis

The immunologic aberrations associated with atopic dermatitis include the paradox of reduced cell-mediated immune responses in the setting of increased cell-mediated immunity features that resemble allergic contact dermatitis. In this review, we present evidence that abnormalities in monocytes and Langerhans cells alter the function of T-helper-cell subpopulations to cause the immunologic defects associated with atopic dermatitis. Increased monocyte prostaglandin E2 production inhibits Th1 responses, accentuating interleukin (IL)-4 secretion by Th2 cells. Elevated prostaglandin E2 secretion correlates with abnormally increased cyclic adenosine monophosphate-phosphodiesterase activity in monocytes and this, along with other defective inflammatory cell responses, can be normalized in vitro by phosphodiesterase inhibitors. It appears that in addition to prostaglandin E2, IL-10 acts to regulate the balance between Th1 and Th2 functional responses accounting for many atopic features, including increased IL-4, IL-5, and IL-6 production by T cells; increased IgE synthesis; decreased interferon-gamma production; and impaired cell-mediated immune responses. All of these abnormalities can be related to increased phosphodiesterase activity in atopic monocytes, and inhibition of this key enzyme appears to reverse atopic dermatitis inflammatory abnormalities in vitro and in vivo.

Surface molecules involved in the adherence of recombinant interferon-gamma (rIFN-gamma)-stimulated human monocytes to vascular endothelial cells

During an inflammatory reaction, an increased number of circulating monocytes adhere to the endothelial cells (EC) of the vessel wall. This process is mediated by molecules located on the surface of monocytes and EC. Locally released inflammatory mediators can modulate monocyte-EC interaction. In an earlier study we reported that stimulation of monolayers of cultured venous EC with rIFN-gamma enhanced their adhesiveness for monocytes. The aim of the present study was to investigate the effect of rIFN-gamma on peripheral blood monocytes with regard to the expression of surface molecules and the binding to non-stimulated or cytokine-stimulated EC. Flow cytometric analysis demonstrated that monocytes stimulated with 500 U/ml rIFN-gamma for 24 h showed increased expression of CR3 (CD11b/CD18), p150,95 (CD11c/CD18) and Fc gamma RI (CD64); the expression of LFA-1 (CD11a/CD18), L-selectin, CD14 and VLA-4 (CD49d/CD29) did not change or was slightly reduced. Upon stimulation with rIFN-gamma monocytes showed an enhanced binding to both non-stimulated or rIFN-gamma-stimulated EC. This was even more pronounced when EC had been stimulated with rIL-1 alpha for 24 h. The increased binding of rIFN-gamma-stimulated monocytes to rIL-1 alpha-stimulated EC was further analysed. Studies with MoAbs against adhesion molecules on monocytes revealed that the binding of rIFN-gamma-stimulated monocytes, but not that of non-stimulated monocytes, to rIL-1 alpha-stimulated EC was inhibited by about 30-60% with MoAbs against CD11a, CD11b, CD18, L-selectin or CD14. MoAbs against CD11c or CD49d had little or no effect. From these results, we conclude that exposure of monocytes to rIFN-gamma enhances their adhesiveness to cytokine-stimulated EC by a mechanism which involves CD11a/CD18, CD11b/CD18, CD14 and L-selectin on monocytes.