Two-faced T cells in CLL

Ibrutinib combinations in CLL therapy: scientific rationale and clinical results

Ibrutinib has revolutionized the treatment of chronic lymphocytic leukemia (CLL). This drug irreversibly inhibits Bruton tyrosine kinase (BTK) by covalently binding to the C481 residue in the BTK kinase domain. BTK is a pivotal protein for B cell receptor signaling and tissue homing of CLL cells. Preclinical investigations have established the importance of the B cell receptor pathway in the maintenance and survival of normal and malignant B cells, underscoring the importance of targeting this axis for CLL. Clinical trials demonstrated overall and progression-free survival benefit with ibrutinib in multiple CLL subgroups, including patients with relapsed or refractory disease, patients with 17p deletion, elderly patients, and treatment-naïve patients. Consequently, ibrutinib was approved by the US Food and Drug Administration for newly diagnosed and relapsed disease. Ibrutinib has transformed the treatment of CLL; however, several limitations have been identified, including low complete remission rates, development of resistance, and uncommon substantial toxicities. Further, ibrutinib must be used until disease progression, which imposes a financial burden on patients and society. These limitations were the impetus for the development of ibrutinib combinations. Four strategies have been tested in recent years: combinations of ibrutinib with immunotherapy, chemoimmunotherapy, cell therapy, and other targeted therapy. Here, we review the scientific rationale for and clinical outcome of each strategy. Among these strategies, ibrutinib with targeted agent venetoclax results in high complete response rates and, importantly, high rates of undetectable minimal residual disease. Although we concentrate here on ibrutinib, similar combinations are expected or ongoing with acalabrutinib, tirabrutinib, and zanubrutinib, second-generation BTK inhibitors. Future investigations will focus on the feasibility of discontinuing ibrutinib combinations after a defined time; the therapeutic benefit of adding a third agent to ibrutinib-containing combinations; and profiling of resistant clones that develop after combination treatment. A new standard of care for CLL is expected to emerge from these investigations.

T-cell number and subtype influence the disease course of primary chronic lymphocytic leukaemia xenografts in alymphoid mice

Chronic lymphocytic leukaemia (CLL) cells require microenvironmental support for their proliferation. This can be recapitulated in highly immunocompromised hosts in the presence of T cells and other supporting cells. Current primary CLL xenograft models suffer from limited duration of tumour cell engraftment coupled with gradual T-cell outgrowth. Thus, a greater understanding of the interaction between CLL and T cells could improve their utility. In this study, using two distinct mouse xenograft models, we investigated whether xenografts recapitulate CLL biology, including natural environmental interactions with B-cell receptors and T cells, and whether manipulation of autologous T cells can expand the duration of CLL engraftment. We observed that primary CLL xenografts recapitulated both the tumour phenotype and T-cell repertoire observed in patients and that engraftment was significantly shorter for progressive tumours. A reduction in the number of patient T cells that were injected into the mice to 2-5% of the initial number or specific depletion of CD8(+) cells extended the limited xenograft duration of progressive cases to that characteristic of indolent disease. We conclude that manipulation of T cells can enhance current CLL xenograft models and thus expand their utility for investigation of tumour biology and pre-clinical drug assessment.

Antigen Selection Shapes the T-cell Repertoire in Chronic Lymphocytic Leukemia

PURPOSE

The role of antigen(s) in shaping the T-cell repertoire in chronic lymphocytic leukemia, although relevant for understanding malignant cell interactions with cognate T cells, is largely unexplored.

EXPERIMENTAL DESIGN

Here we profiled the T-cell receptor β chain gene repertoire in 58 chronic lymphocytic leukemia patients, focusing on cases assigned to well-characterized subsets with stereotyped clonotypic B-cell receptor immunoglobulins, therefore those cases most evidently selected by antigen (subsets #1, #2, and #4).

RESULTS

Remarkable repertoire skewing and oligoclonality were observed, and differences between subsets were noted regarding both T-cell receptor β chain gene usage and the extent of clonality, with subset #2 being the least oligoclonal. Longitudinal analysis of subset #4 cases revealed that although the repertoire may fluctuate over time, certain clonotypes persist, thus alluding to persistent antigenic stimulation. Shared ("stereotyped") clonotypes were found between different patients, reflecting selection by common antigenic elements. Cross-comparison of our dataset with public databases showed that some T-cell clonotypes may have expanded secondary to common viral infections; however, the majority of clonotypes proved to be disease-specific.

CONCLUSIONS

Overall, the T-cell receptor β chain repertoire in chronic lymphocytic leukemia is likely shaped by antigen selection and the implicated antigenic elements may concern epitopes that also select the malignant B-cell progenitors or, more intriguingly, chronic lymphocytic leukemia-derived epitopes.

Murine genetically engineered and human xenograft models of chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is a genetically complex disease, with multiple factors having an impact on onset, progression, and response to therapy. Genetic differences/abnormalities have been found in hematopoietic stem cells from patients, as well as in B lymphocytes of individuals with monoclonal B-cell lymphocytosis who may develop the disease. Furthermore, after the onset of CLL, additional genetic alterations occur over time, often causing disease worsening and altering patient outcomes. Therefore, being able to genetically engineer mouse models that mimic CLL or at least certain aspects of the disease will help us understand disease mechanisms and improve treatments. This notwithstanding, because neither the genetic aberrations responsible for leukemogenesis and progression nor the promoting factors that support these are likely identical in character or influences for all patients, genetically engineered mouse models will only completely mimic CLL when all of these factors are precisely defined. In addition, multiple genetically engineered models may be required because of the heterogeneity in susceptibility genes among patients that can have an effect on genetic and environmental characteristics influencing disease development and outcome. For these reasons, we review the major murine genetically engineered and human xenograft models in use at the present time, aiming to report the advantages and disadvantages of each.

Biology of chronic lymphocytic leukemia in different microenvironments: clinical and therapeutic implications

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of mature monoclonal B cells in peripheral blood, bone marrow, spleen, and lymph nodes. The trafficking, survival, and proliferation of CLL cells is tightly regulated by the surrounding tissue microenvironment and is mediated by antigenic stimulation, close interaction with various accessory cells and exposure to different cytokines, chemokines, and extracellular matrix components. In the last decade there have been major advances in the understanding of the reciprocal interactions between CLL cells and the various microenvironmental compartments. This article discusses the role of the microenvironment in the context of efforts to develop novel therapeutics that target the biology of CLL.

CD73-generated extracellular adenosine in chronic lymphocytic leukemia creates local conditions counteracting drug-induced cell death

Extracellular adenosine (ADO), generated from ATP or ADP through the concerted action of the ectoenzymes CD39 and CD73, elicits autocrine and paracrine effects mediated by type 1 purinergic receptors. We have tested whether the expression of CD39 and CD73 by chronic lymphocytic leukemia (CLL) cells activates an adenosinergic axis affecting growth and survival. By immunohistochemistry, CD39 is widely expressed in CLL lymph nodes, whereas CD73 is restricted to proliferation centers. CD73 expression is highest on Ki-67(+) CLL cells, adjacent to T lymphocytes, and is further localized to perivascular areas. CD39(+)/CD73(+) CLL cells generate ADO from ADP in a time- and concentration-dependent manner. In peripheral blood, CD73 expression occurs in 97/299 (32%) CLL patients and pairs with CD38 and ZAP-70 expression. CD73-generated extracellular ADO activates type 1 purinergic A2A receptors that are constitutively expressed by CLL cells and that are further elevated in proliferating neoplastic cells. Activation of the ADO receptors increases cytoplasmic cAMP levels, inhibiting chemotaxis and limiting spontaneous drug-induced apoptosis of CLL cells. These data are consistent with the existence of an autocrine adenosinergic loop, and support engraftment of leukemic cells in growth-favorable niches, while simultaneously protecting from the action of chemotherapeutic agents.