Characterization of bone marrow tumor associated macrophages in patients with chronic lymphocytic leukemia treated with Ibrutinib

Introduction. Complete remission (CR) is observed in 9-26% of patients with chronic lymphocytic leukemia (CLL), with virtually no patient converting to CR beyond 2 years of treatment. Residual bone marrow (BM) disease is typically observed in patients not achieving CR, increasing interest about the specific effects of ibrutinib on the CLL BM microenvironment. Ibrutinib markedly increases the number and function of T cells, and suppresses regulatory B-cell function. However, it may induce M2 polarization of tumor-associated macrophages (TAM), as suggested by limited studies conducted on peripheral blood. Methods. In order to characterize the specific effect of ibrutinib on the CLL BM microenvironment, we identified patients with previously untreated CLL, receiving frontline single agent ibrutinib for more than 2 years, for whom BM biopsy samples were available at baseline and after 1 and 2 years of treatment. All samples were analyzed by immunohistochemistry and the number of positive cells per high power field (HPF)(200X) was assessed; CD68 was used as a general TAM marker, and CD163, PD-L1 and arginase-1 as M2 markers. Negative BM samples, collected as part of lymphoma staging, were used as normal controls. Results. Thirteen patients were included in the study; 85% had unfavorable FISH abnormalities (including deletion 11q and deletion 17p) and 77% had unmutated IGHV. After > 2 years of treatment, 3 (23%) patients had achieved CR, and 10 (77%) partial remission (PR), characterized by residual BM disease. BM samples were available for 11 patients at baseline, 12 patients at 1 year of treatment, and 9 patients at 2 years of treatment. PD-L1 and arginase-1 were negative in BM samples from all patients at all time points and in all 3 normal BMs. The median number of CD68-positive cells/HPF was 3 (range, 3-5) at baseline, 5 (range, 3-20) at 1 year, and 5 (range, 3-10) at 2 years; no differences in CD68 expression was observed between patients who achieved PR and those who achieved CR. All patients with normal BM had five CD68-positive cells/HPF. The median number of CD163-positive cells/HPF was 5 (range, 5-15) at baseline, 15 (range, 10-30) at 1 year, and 20 (range, 15-20) at 2 years; a more pronounced increase in CD163-positive cells was observed among patients who achieved PR as compared to patients who achieved CR. All patients with normal BM had approximatively ten CD163-positive cells/HPF. Conclusions. Ibrutinib increases BM CD68 (TAM) in patients with CLL, reaching a plateau after 1 year of treatment, and BM CD163 (TAM M2), with no plateau observed at 2 years of treatment. The increase in TAM M2 is more pronounced in patients who do not achieve CR after > 2 years of treatment, representing a potential therapeutic target for consolidation strategies. Additional M2 markers, including CSF1R, and digital analysis are being applied, and results will be presented at the meeting. Citation Format: Paolo Strati, Ellen Schlette, Luisa Solis Soto, Daniela Duenas, Ignacio Wistuba, Jan Burger. Characterization of bone marrow tumor associated macrophages in patients with chronic lymphocytic leukemia treated with Ibrutinib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3099.