Variant LTC(4) synthase allele modifies cysteinyl leukotriene synthesis in eosinophils and predicts clinical response to zafirlukast.

Clinical trials with leukotriene synthesis inhibitors and cysteinyl leukotriene (cys-LT) receptor antagonists have shown that cys-LTs (LTC4, LTD4, and LTE4) are the predominant mediators of bronchoconstriction in response to diverse asthma triggers including allergen, PAF, sulphur dioxide, and non-steroidal antiinflammatory drugs (NSAIDs). In addition, inhalation of a single dose of a cys-LT causes persistent eosinophilia in the bronchial mucosa and sputum of asthmatic patients, 3 and LT modifier drugs significantly reduce airway and blood eosinophilia in clinical asthma. We have shown that, in the asthmatic airway, eosinophils represent the majority of cells that express the terminal enzyme in the cys-LT pathway, LTC4 synthase, and hence have the capacity to generate cys-LT upon stimulation. 6 Inflammation and bronchoconstriction in the asthmatic airway may be sustained by a vicious cycle of cys-LT synthesis and eosinophil recruitment (fig 1). Anomalies in the genetic and cytokine factors that regulate cys-LT pathway enzymes in eosinophils may thus be central to the pathophysiology of asthma. Cys-LTs are synthesised from membrane derived arachidonic acid. During stimulusspecific cell activation, arachidonic acid released by phospholipases including cytosolic phospholipase A2 (PLA2)is translocated to the 5-lipoxygenase activating protein (FLAP) and converted in two steps to leukotriene (LT)A4 by 5-lipoxygenase (5LO). LTA4 is converted to LTB4 by cells expressing LTA4 hydrolase, and/or to LTC4 by cells expressing LTC4 synthase, which conjugates LTA4 to reduced glutathione. After carrier mediated cellular export of LTC4, the sequential cleavage of glutamate and glycine residues provides the extracellular receptor active metabolites LTD4 and LTE4, respectively. Cys-LTs act at specific cys-LT1 receptors on target cells including bronchial smooth muscle and inflammatory leucocytes. 8 The cys-LT pathway in eosinophils may be modulated by the cytokine microenvironment in the bone marrow, circulation, and airway. Maturation of eosinophils in vitro by the eosinophilopoietic cytokines interleukin (IL)-5 and IL-3 is accompanied by sequential expression of PLA2, 5-LO, FLAP, and LTC4 synthase, associated with acquisition of a cys-LT synthesising phenotype. In asthmatics, enhanced capacity of blood eosinophils to generate LTC4 may be caused by cytokine induction of 5-LO pathway enzyme expression, including FLAP. Induction of 5-LO pathway enzyme expression by the cytokine microenvironment may also be observable within the bronchial mucosa of mild asthmatics naturally exposed to a seasonal aeroallergen. The capacity for cys-LT synthesis of eosinophils may also be influenced by promoter polymorphism in the genes encoding cys-LT pathway enzymes. Promoter polymorphism in 5-LO is linked to modified gene transcription and a diminished clinical response to an LT synthesis inhibitor in a small proportion (∼5%) of asthmatics, but this cannot account for the large subgroup of poor responders to LT modifier drug therapy. In bronchial biopsy specimens of patients with aspirin intolerant asthma (AIA) who have a constitutively high baseline production of cys-LTs and generally good clinical responses to leukotriene modifier drugs, the numbers of cells expressing LTC4 synthase are four times more prevalent than in biopsy specimens from aspirin tolerant asthmatics (ATA) and 19 times more prevalent than in normal bronchial biopsy specimens. The increased prevalence of LTC4 synthase in biopsy specimens from patients with AIA is partly the result of an Figure 1 Schematic diagram illustrating that genetic and cytokine factors modulate the vicious cycle of cysteinyl leukotriene synthesis and eosinophil recruitment in the asthmatic airway. Predisposing factors