Analysis of the Effects of the Bruton's tyrosine kinase (Btk) Inhibitor Ibrutinib on Monocyte Fcγ Receptor (FcγR) Function

The irreversible Bruton's tyrosine kinase (Btk) inhibitor ibrutinib has shown efficacy against B-cell tumors such as chronic lymphocytic leukemia and B-cell non-Hodgkin lymphoma. Fc gamma receptors (Fc gamma R) on immune cells such as macrophages play an important role in tumor-specific antibody-mediated immune responses, but many such responses involve Btk. Here we tested the effects of ibrutinib on Fc gamma R-mediated activities in monocytes. We found that ibrutinib did not affect monocyte Fc gamma R-mediated phagocytosis, even at concentrations higher than those achieved physiologically, but suppressed Fc gamma R-mediated cytokine production. We confirmed these findings in macrophages from Xid mice in which Btk signaling is defective. Because calcium flux is a major event downstream of Btk, we tested whether it was involved in phagocytosis. The results showed that blocking intracellular calcium flux decreased Fc gamma R-mediated cytokine production but not phagocytosis. To verify this, we measured activation of the GTPase Rac, which is responsible for actin polymerization. Results showed that ibrutinib did not inhibit Rac activation, nor did the calcium chelator 1,2-bis(2-aminophenoxy) ethane-N, N, N', N'-tetraacetic acid tetrakis( acetoxymethyl ester). We next asked whether the effect of ibrutinib on monocyte Fc gamma R-mediated cytokine production could be rescued by IFN gamma priming because NK cells produce IFN gamma in response to antibody therapy. Pretreatment of monocytes with IFN gamma abrogated the effects of ibrutinib on Fc gamma R-mediated cytokine production, suggesting that IFN gamma priming could overcome this Btk inhibition. Furthermore, in monocyte-natural killer cell co-cultures, ibrutinib did not inhibit Fc gamma R-mediated cytokine production despite doing so in single cultures. These results suggest that combining ibrutinib with monoclonal antibody therapy could enhance chronic lymphocytic leukemia cell killing without affecting macrophage effector function.