Introduction Cysteinyl leukotrienes CysLTs LTC LTD and LTE a
Cysteinyl leukotrienes (CysLTs), LTC4, LTD4 and LTE4, are 5-lipoxygenase pathway metabolites of arachidonic azacytidine and deeply involved in bronchial asthma via activation of CysLT1 receptors , . Specific CysLT1 receptor antagonists, including pranlukast ,  and montelukast , have widely been used as pharmacotherapy for bronchial asthma. Another receptor subtype of CysLTs, termed the CysLT2 receptor , , , , is a G-protein-coupled receptor with an amino acid sequence 38% identical to that of the CysLT1 receptor . CysLT2 receptor mRNA has been detected in lung macrophages, airway smooth muscle, and peripheral blood leukocytes . We have also demonstrated that CysLT2 receptors were expressed in lung specimens isolated from asthma subjects, and suggested that activation of CysLT2 receptors contribute to antigen-induced bronchoconstriction in certain asthma population . However, it is difficult to analyze the expression and functional roles of CysLT2 receptors in asthma using human lung tissue.
Guinea pig has been extensively used to assess the functional roles of CysLTs because the bronchial smooth muscle responds well to LTD4 and is constricted through activation of CysLT1 receptors . We developed a CysLT1/CysLT2 dual antagonist , and analyzed the role of CysLT2 receptors in guinea pig models of asthma by focusing on bronchoconstriction , , , . However, it is difficult to analyze molecular mechanisms using guinea pig models because various tools for molecular biological experiments are not available. For example, histological existence of CysLT2 receptors in normal and asthmatic lung tissues of guinea pigs is unclear.
Murine models have also been used to elucidate the cellular and molecular mechanisms underlying asthma pathogenesis. In particular, BALB/c mice have been utilized as an appropriate model for Th2-type airway inflammation. CysLT1 receptor antagonists suppress eosinophilic airway inflammation and airway remodeling induced by antigen challenges , , , , . The role of CysLT2 receptors in murine asthma has been analyzed using CysLT2 receptor-deficient mice, which revealed that CysLT2 receptors negatively regulate the development of CysLT1 receptor-dependent Th2-type airway inflammation . Further analysis is needed to elucidate the roles of CysLT2 receptors using specific receptor antagonists, to examine if CysLT2 receptors are histologically expressed in murine lung, and to ascertain whether expression is altered in asthmatic lungs.
The purpose of this study was to determine whether the CysLT2 receptor can be a molecular target for pharmacotherapy of asthma. Thus, murine experiments were designed to elucidate 1) whether CysLT2 receptors were expressed in the lung and if expression increased in asthmatic mice, and 2) whether CysLT2 receptors were functionally involved in allergic leukocyte infiltration into the lung and in the development of airway remodeling in asthmatic mice.
Materials and methods
Discussion We found that CysLT2 receptors were constitutively expressed in bronchial smooth muscle cells, and that CysLT2 receptor-expressing leukocytes, mainly consisting of macrophage/dendritic cells and eosinophils, infiltrated into the lung after multiple antigen challenges in sensitized mice. In addition, CysLT2 receptor activation may be functionally involved in allergic airway infiltration of eosinophils and mononuclear cells. To our knowledge, this is the first report demonstrating the existence of CysLT2 receptors in the histology of murine lung. Compared with CysLT2 receptors, CysLT1 receptor expression in the lung was not pronounced, even in bronchial smooth muscle. We used two different commercially available primary antibodies against CysLT1 receptors, but found only a limited number of antibody-positive areas. Low expression of CysLT1 receptors, even in airway smooth muscle, is consistent with the fact that mouse airway smooth muscle only weakly responds to CysLTs . It is impractical to directly compare expression between CysLT2 and CysLT1 receptors because different primary antibodies were used for detection. However, CysLT2 receptor expression in the murine lung was clearly detected in bronchial smooth muscle and in leukocytes infiltrating into the lung, implying that CysLT2 receptors exert functional roles in mouse lung pathophysiology.