Two classes of CysLT receptor have been identified and can be distinguished by their relative affinities for LTC₄ (CysLT1R=CysLT2R) and LTD₄ (CysLT1R>CysLT2R). In addition, they are distinguished by their antagonism by the selective CysLT1 receptor antagonist (MK571) and the non-selective antagonist Bay-u9773. These receptors are widely expressed on immune cells; however, their functions have not been characterized. The current studies investigated the expression and function of each class of CysLT receptor on mononuclear phagocytic cells.

Monocytes were enriched from human peripheral blood via negative selection using Miltenyi Biotech beads. Expression of receptors was evaluated by flow cytometry using rabbit anti-human CysLT1 and CysLT2 receptor antibodies (Cayman) and Zenon^® PE rabbit IgG labeling kit (Molecular Probe). LPS-activated monocytes were exposed to a range of concentrations of LTC₄ and LTD₄ and evaluated for cytokine production and surface expression of CD80 (B7.1), MHC class II, and CD11b (CR3) without or with MK571 or Bay-u9773.

Monocytes demonstrated high levels of expression of both CysLT1 and CysLT2R although CysLT2R expression was consistently greater. Exposure of monocytes to CysLT stimulated cytokine synthesis in a dose-dependent fashion, however, LTC₄ was more potent than LTD₄. For IL-10, LPS produced 29.1 ± 6.2 pg/ml, which increased to 49.4 ± 19.6 and 60.8 ± 17.8 in the presence of LTC₄and LTD₄, respectively. Cytokine production was significantly inhibited by Bay-u9773 but not by MK571. Finally, CysLTs were shown to stimulate expression of CD80, MHC class II, and CD11b on monocytes.

CysLTs potentiate pro-inflammatory functions of monocytes through a CysLT2 receptor-dependent mechanism.