Substance P, mast cells and basophils are involved in acute chest syndrome in sickle cell disease

A role for substance P in promoting neurogenic inflammation and pain has been described in sickle cell disease (SCD). However its origin and contribution to SCD pathophysiology remain unclear. We measured substance P level in plasma from 225 patients with SCD and observed the highest concentrations during acute chest syndrome (ACS). Therefore, we tested the hypothesis that substance P may induce ACS. In transgenic sickle mice, unlike control mice, intravenous injection of substance P caused lethal crises with dose-dependent acute lung injuries. Activation of Fc{varepsilon}R1 with MAR-1 had similar effects, suggesting a role for mast cell or basophil activation and degranulation. Pretreatment of sickle mice with cromolyn, a stabilizer of mast cells and basophils, prevented lethal crisis and lung injuries induced by substance P injection. In SCD patients, blood cellular histamine levels and increased histidine decarboxylase activity were consistent with an involvement of circulating basophils. Flow cytometry analysis revealed higher basophil counts with increased activation and degranulation markers in patients compared with healthy controls. During vaso-occlusive crisis, absolute basophil counts tended to decrease, suggesting their recruitment outside the vascular compartment. The same results were observed in sickle mice after hypoxia-reoxygenation, intravenous hemin injection or substance P injection. Immunohistochemistry revealed the presence of mast cells and basophils in the lungs of sickle mice, but not in control mice, with further basophil recruitment and degranulation after intravenous substance P injection. In SCD patients, we observed extremely high levels of substance P in the sputum collected during ACS, consistently with mast cell and basophil degranulation in the lungs. In vitro, substance P was shown to be a potent chemoattractant for basophils via NK1R. Gene expression analysis on sorted circulating basophils from SCD patients revealed an increased expression of several chemokine receptors, including CCR3 and FPR1, which was confirmed by spectral flow cytometry and could contribute to the recruitment of basophils in the lungs. The two substance P receptors, NK1R and MRGPRX2, were also overexpressed, promoting the vicious cycle of substance P release and pain in SCD patients. Our results reveal a novel mechanism that contributes to the understanding of ACS pathogenesis and highlights the potential role of mast cells and basophils in SCD pathophysiology.