Bromodomain and extraterminal (BET) protein inhibition of IgG/IgE production in murine B cells is counter‐balanced by a strong Th2 bias

OBJECTIVES: Inhibitors of bromodomain and extra terminal domain (BET) proteins are a new and growing class of anti‐cancer drugs, which decrease oncogene expression by targeting superenhancers. Antibody production is another physiological process relying on superenhancers, and it remains to be clarified whether potential immunomodulatory properties of BET inhibitors might impact humoral immunity and allergy. METHODS: We thus evaluated humoral immune responses and their Th2 context in vitro and in vivo in mice following treatment with the classical BET‐inhibitor JQ1. We quantified immunoglobulin (Ig) and antibody production by B cells either stimulated in vitro or obtained from immunised mice. JQ1 effects on class switching and activation‐induced deaminase loading were determined, together with modifications of B, T follicular helper (Tfh) and T helper 2 (Th2) populations. JQ1 was finally tested in B‐cell‐dependent models of immune disorders. RESULTS: Bromodomain and extra terminal domain inhibition reduced class switching, Ig expression on B cells and antibody secretion and was correlated with decreased numbers of Tfh cells. However, JQ1 strongly increased the proportion of GATA3(+) Th2 cells and the secretion of corresponding cytokines. In a mouse allergic model of lung inflammation, JQ1 did not affect eosinophil infiltration or mucus production but enhanced Th2 cytokine production and aggravated clinical manifestations. CONCLUSION: Altogether, BET inhibition thus interweaves intrinsic negative effects on B cells with a parallel complex reshaping of T‐cell polarisation which can increase type 2 cytokines and eventually promote B‐cell‐dependent immunopathology. These opposite and potentially hazardous immunomodulatory effects raise concerns for clinical use of BET inhibitors in patients with immune disorders.