Background: The clinical successes of BTK inhibitor Ibrutinib and Ofatumumab (GA101), a humanized, glycoengineered mAb against CD20, suggests that a combination therapy may have clinical potential. Our previous study demonstrated that Ibrutinib antagonizes ADCC in CD20⁺ B-cell lymphoma due to Ibrutinib's irreversible binding to interleukin-2 inducible tyrosine kinase (ITK), which is required for FcR-stimulated NK cell function. (Kohrt et al Blood 2014, 123;12:1957). As GA101 is superior to Rituximab in induction of NK cell mediated ADCC, we hypothesized that-Ibrutinib may not antagonize GA101 and inhibition of NK cell function by Ibrutinib could be overcome by CD137 agonistic mAb enhanced NK cell function. Methods: Purified NK cells were isolated from healthy peripheral blood mononuclear cells and cultured with Ibrutinib for 4 hours together with GA101-coated DHL4 lymphoma cells. For NK cell activation studies, the cells were primed with GA101 coated cells for 20 hours prior to CD137 mAb addition. Secretion ofIFN-gamma, CD107a mobilization and cytotoxicity were assessed. Results: The results showed that Ibrutinib inhibited GA101 induced NK cell mediated lysis of lymphoma target cells and degranulation, although IFN gamma release was unaffected. Anti-CD137 mAb enhanced functional GA101 induced NK cell IFN gamma release, restored and enhanced degranulation and lysis of target cells that was impaired by Ibrutinib. Similar results were obtained when autologous peripheral blood mononuclear cells were cultured with CLL cells obtained from circulating tumor cells. The therapeutic efficacy of Ibrutinib and GA101 with anti-CD137 mAb was evaluated in a xenotransplant model of DHL4 lymphoma. Following tumor inoculation, athymic mice received GA101 on day 14 and oral dose of Ibrutinib twice daily and anti-CD137 mAb on day 15 with each treatment repeated weekly for a total of three weeks. Our results demonstrate that GA101 retains its potency in vivo in combination therapy with Ibrutinib and anti-CD137 mAb. Conclusions: Our results support a novel approach to overcome Ibrutinib inhibition of NK cell function by activation of the host immune system by CD137 stimulation that warrants clinical investigation.