Chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) are tumors of mature B cells and are closely related biologically and in clinical behavior. Both are currently incurable with chemotherapy. Median survival for patients with CLL is 10 years and for MCL patients ranges between 5 and 6 years. BCR signaling has emerged as the pivotal pathway in the pathogenesis of CLL and MCL. A major contribution from my group has been the first demonstration of active BCR signaling in CLL patients in vivo. Furthermore, we showed that BCR signaling and the consequent activation of the NF-B pathway occurs primarily in the lymph node microenvironment rather than in the peripheral blood or bone marrow. Thus, in key aspects, the biology of CLL is shaped by its environment; an insight that changes our therapeutic approach, the design of correlative studies, and the development of model systems. To expand our previous studies, we conducted a clinical study using deuteriated water to label the proliferative fraction of the CLL clone in vivo (NCT01117142), which provided direct in vivo evidence for increased tumor proliferation in the lymph node. Furthermore, in collaboration with Dr. Chiorazzis group we could show that a distinct subpopulation of CLL cells in the lymph node contains the proliferative core of the disease. Having shown the importance of the lymph node microenvironment for CLL cells, we developed a mouse xenograft model using human CLL cells that engraft in the murine spleen where they proliferate and undergo activation of BCR and NF-B pathways, similar to what we have previously found in the human lymph node. We used this patient-derived xenograft model in conjunction with a transgenic murine CLL model to investigate the effects of kinase inhibitors and their combination on BCR signaling and anti-leukemic activity and showed that the combination of acalabrutinib (a second generation BTK inhibitor) and ACP-319 (a PI3K inhibitor) was superior to single-agent treatment in the murine model, warranting further investigation of this combination in clinical studies. Several small molecule inhibitors of BCR signaling and of PI3K have entered clinical development in CLL and other hematologic malignancies. The most promising clinical results have been achieved with ibrutinib. From January 2012 to January 2014 we accrued 86 patients into our single agent ibrutinib trial (NCT01500733). 51 patients had TP53 aberration (TP53 cohort) and 35 were enrolled for age 65 years (elderly cohort). Both cohorts included patients with treatment-nave (TN) and relapsed/refractory (RR) CLL. With the median follow-up of 4.8 years, 49 (57.0%) of 86 patients remain on study. Treatment was discontinued for progressive disease in 20 (23.3%) patients and for adverse events in 5 (5.8%). Atrial fibrillation occurred in 18 (20.9%) patients for a rate of 6.4 per 100 patient-years. No serious bleeding occurred. The overall response rate at 6 months, the primary study endpoint, was 95.8% for the TP53 cohort (95% CI, 85.7-99.5) and 93.9% in the elderly cohort (95% CI, 79.8-99.3). Depth of response improved with time; at best response, 14 (29.2%) of 48 patients in the TP53 cohort and 9 (27.3%) of 33 in the elderly cohort achieved a complete response. Median minimal residual disease (MRD) in peripheral blood was 3.8x10-2 at 4 years, with MRD negative (<10-4) remissions in 5 (8.3%) patients. In the TP53 cohort, the estimated 5-year progression-free survival (PFS) was 74.4% in TN-CLL compared to 19.4% in RR-CLL (P=.0002), and overall survival (OS) was 85.3% versus 53.7%, respectively (P=.023). In the elderly cohort, the estimated 5-year PFS and OS in RR-CLL were 64.8% and 71.6%, respectively, while no event occurred in TN-CLL. Long-term administration of ibrutinib was well tolerated and provided durable disease control for most patients. These results are much better than what can be achieved with current standard of care chemotherapy regimens and suggest that Ibrutinib may become the preferred agent for these patients. Some side effects may limit ibrutinib treatment for a subgroup of patients, especially for those who develop atrial fibrillation. We are currently conducting a clinical trial with aclabrutinib, a second generation BTK inhibitor, that seems to be equally efficacious but may have a different side effect profile. As of June 2018, we have enrolled all planed 48 patients and the trial is closed to further accrual. Because loss of function mutations in BTK causes a severe immune defect known as Brutons agammaglobulinemia, assessing the impact BTK inhibition on immune function is important. We found that immunoglobulin (Ig) G levels remained unchanged on treatment, while IgA increased. At the same time the frequency of infections decreased. Interestingly, patients with >50% improvement in IgA had fewer infections than patients with <50% improvement. We also conducted a pilot study using influenza vaccine and could show that patients on ibrutinib respond at least as well as untreated CLL patients to the vaccine, demonstrating that patients on ibrutinib can still mount a meaningful antibody response. Given that infections are a leading cause of death, this data have important preventive health implications. We are about to open a new study using Shingrix, a varicella-zoster vaccine, and Heplisav, a Hepatitis B vaccine to further investigate immune function in CLL patients and will compare treatment-nave patients to patients being treated with kinase inhibitors. Another area of investigation are mechanisms of drug resistance in CLL patients treated with ibrutinib. Disease progression in patients with chronic lymphocytic leukemia (CLL) treated with ibrutinib has been attributed to histologic transformation or acquired mutations in BTK and PLCG2. The rate of resistance and clonal composition of progressive disease are incompletely characterized. With median follow-up of 34 months, fifteen (17.9%) of 84 CLL patients treated with ibrutinib progressed. Relapsed/refractory disease at study entry, TP53 aberration, advanced Rai stage, and high -2 microglobulin were independent predictors of inferior progression-free survival (P<.05 for all tests). Histologic transformation occurred in 5 (6.0%) patients and was limited to the first 15 months on ibrutinib. In contrast, progression due to CLL in 10 (11.9%) patients occurred later, diagnosed at a median 38 months on study. At progression, mutations in BTK (Cys481) and/or PLCG2 (within the auto-inhibitory domain) were found in 9 (10.7%) patients, in 8 of 10 patients with progressive CLL and in one patient with prolymphocytic transformation. Applying high sensitivity testing (detection limit approximately 1 in 1,000 cells) to stored samples we detected mutations up to 15 months before manifestation of clinical progression (range 2.9-15.4 months). In 5 (6.0%) patients multiple subclones carrying different mutations arose independently leading to subclonal heterogeneity of resistant disease. Patients progressing with CLL had a median survival of 19.8 months from the time of progression. While these results are better than reported in previous series, progression on ibrutinib is a high-risk disease in need of better therapies.