IgE and IL-33-mediated triggering of human basophils inhibits TLR4-induced monocyte activation

Basophils are circulating granulocytes, best known as effector cells in allergic reactions. Recent studies in mice suggest that they might also participate in the suppression of chronic inflammation. The aim of this study was to assess the ability of purified human basophils to modulate monocyte responses upon IL-33 and IgE triggering. Activation of human basophils with IL-33 induced the production of IL-4 and the release of histamine, and enhanced their IgE-mediated activation. In addition, basophils triggered with IL-33 and anti-IgE significantly suppressed the LPS-induced production of the proinflammatory cytokine TNF-alpha and the upregulation of the costimulatory molecule CD80 by monocytes. These effects were mainly explained by the release of histamine, as they could be inhibited by the histamine receptor 2 antagonist ranitidine, with a smaller contribution of IL-4. In contrast, basophil-derived IL-4 and histamine had opposing effects on the expression of the inhibitory Fc gamma receptor IIb and the production of IL-10 by monocytes. Our data show that basophils can influence monocyte activation and suggest a previously unrecognized role for human basophils in the modulation of monocyte-mediated immune responses, through the balanced secretion of histamine and IL-4.