Chronic lymphocytic leukemia (CLL) is an indolent malignancy of CD5+ B lymphocytes. CLL cells express CD40, a key regulator of B cell proliferation, differentiation, and survival. In nonmalignant B cells, CD40 ligation results in nuclear translocation and activation of NF-κB proteins. Based on observations that in some CLL cases, the tumor cells express both CD40 and its ligand, CD154 (CD40 ligand), we proposed a model for CLL pathogenesis due to CD40 ligation within the tumor. To evaluate this issue, we used freshly isolated CLL B cells to examine constitutive and inducible NF-κB activity by electrophoretic mobility shift assay. We consistently observed high levels of nuclear NF-κB-binding activity in unstimulated CLL B cells relative to that detected in nonmalignant human B cells. In each case examined, CD40 ligation further augmented NF-κB activity and prolonged CLL cell survival in vitro. The principle NF-κB proteins in stimulated CLL cells appear to be quite similar to those in nonmalignant human B cells and include p50, p65, and c-Rel. In a CD154-positive case, blocking CD154 engagement by mAb to CD154 resulted in inhibition of NF-κB activity in the CLL cells. The addition of anti-CD154 mAb resulted in accelerated CLL cell death to a similar degree as was observed in cells exposed to dexamethasone. These data indicate that CD40 engagement has a profound influence on NF-κB activity and survival in CLL B cells, and are consistent with a role for CD154-expressing T and B cells in CLL pathogenesis. The data support the development of novel therapies based on blocking the CD154-CD40 interaction in CLL.

The mechanisms underlying the autonomous accumulation of malignant B cells remain elusive. We show in this study that non-Hodgkin’s lymphoma (NHL) B cells express B cell-activating factor of the TNF family (BAFF) and a proliferation-inducing ligand (APRIL), two powerful B cell-activating molecules usually expressed by myeloid cells. In addition, NHL B cells express BAFF receptor, which binds BAFF, as well as transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) and B cell maturation Ag (BCMA), which bind both BAFF and APRIL. Neutralization of endogenous BAFF and APRIL by soluble TACI and BCMA decoy receptors attenuates the survival of NHL B cells, decreases activation of the prosurvival transcription factor NF-κB, down-regulates the antiapoptotic proteins Bcl-2 and Bcl-xL, and up-regulates the proapoptotic protein Bax. Conversely, exposure of NHL B cells to recombinant or myeloid cell-derived BAFF and APRIL attenuates apoptosis, increases NF-κB activation, up-regulates Bcl-2 and Bcl-xL, and down-regulates Bax. In some NHLs, exogenous BAFF and APRIL up-regulate c-Myc, an inducer of cell proliferation; down-regulate p53, an inhibitor of cell proliferation; and increase Bcl-6, an inhibitor of B cell differentiation. By showing that nonmalignant B cells up-regulate BAFF and APRIL upon stimulation by T cell CD40 ligand, our findings indicate that NHL B cells deregulate an otherwise physiological autocrine survival pathway to evade apoptosis. Thus, neutralization of BAFF and APRIL by soluble TACI and BCMA decoy receptors could be useful to dampen the accumulation of malignant B cells in NHL patients.

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