Population depletion activates autonomous CD154-dependent survival in biopsylike Burkitt lymphoma cells

Abstract

Population size is governed through cells reacting to a variety of intrinsic and extrinsic cues. Tumors, while liberated from many of the homeostatic constraints placed on physiologic counterparts, can nonetheless remain subject to both social and environmental control. Burkitt lymphoma cells faithful to the biopsy phenotype were used to model the reliance of the colony, if any, on an inbuilt population sensor. Below a normally suicidal threshold number of cells, low picomolar quantities of exogenous CD40 ligand (CD40L/CD154) were found to sustain the clone without the discernible shift in phenotype that accompanies high CD40L encounter. Although CD154 was undetectable in populous cultures, message was induced as numbers became limiting. Correspondingly, attempts to neutralize endogenous CD40L activity failed to perturb cells at optimal densities but resulted in their marked decline as the critical threshold was approached. These data reveal an auto-inducible survival mechanism seemingly regulated through the monitor- IntroductionAlthough progressive liberation from homeostatic control is a feature of tumor development, malignant cells are rarely emancipated completely from the multitude of environmental constraints that are placed on normal counterparts. 1 Indeed, before any progression toward metastasis, a dependency on specific tissue microenvironment is often observed in malignancy. For example, follicular lymphoma, one of the most common nonleukemic lymphoid tumors, develops within and is primarily limited to the lymphoid follicles that harbor their normal equivalents, the germinal center (GC) B cells. [2][3][4][5][6] Another tumor bearing a GC B-cell phenotype is Burkitt lymphoma (BL), although here development is not constrained to conventional follicles. [7][8][9][10] Nevertheless, BL presents with a classical "starry sky" histology reflecting the presence of large infiltrating macrophages that are there to scavenge tumor cells entering apoptosis. 9,11,12 This feature of spontaneous apoptosis is a vestige of the tumor's GC origin in which the constitutive B cells need to be selected by antigen during the development of a memory response. 6,13,14 Unlike their normal equivalents, however, the maintenance of the BL population proceeds independently of discernible antigen input and, despite their residual propensity for apoptosis, the balance is clearly in favor of survival.BL cells established in culture and kept at early passage remain remarkably faithful to their GC origins. 7,12,15,16 The L3055 cell line, derived from an Epstein-Barr virus (EBV)-negative case of sporadic BL, has been used extensively to model both GC B cells and BL itself. [17][18][19][20][21][22][23][24] These "biopsylike" cultures display a low background of apoptosis that can be accelerated by a number of factors, including B-cell receptor engagement, exposure to transforming growth factor ␤, cold shock, and serum deprivation. [17][18][19][20]24 With regards to survival factors, roles for both autocrine and paracrine factors have bee...

Constitutive activation of the CD40 pathway promotes cell transformation and neoplastic growth

“…Recent studies in Burkitt lymphoma cell lines and primary non-Hodgkin's lymphomas have demonstrated that the constitutive production of CD40L by the malignant cells provides an important signal for autonomous cell growth and oncogenic transformation (Challa et al, 2002;Pham et al, 2002). On the basis of these published reports and the data described in Figure 1, we reasoned that the constitutive engagement of the CD40 pathway may also confer oncogenic effects in nonhaemopoietic cells.…”

“…However, there is accumulating evidence to suggest that the level and duration of CD40 engagement may dictate the balance between stimulation and suppression of tumour cell growth. Thus, whereas treatment of L3055 Burkitt lymphoma cells with high concentrations of CD40L promotes substantial growth arrest, low-level engagement can favour proliferation and survival in the same cell line (Challa et al, 2002). Similarly, neutralization of the low-level constitutive production of CD40L in non-Hodgkin's lymphoma and chronic lymphocytic leukaemia cells inhibits their growth and induces apoptosis (Furman et al, 2000;Pham et al, 2002).…”

“…Low-level constitutive CD40 engagement by endogenously produced CD40L induces sustained proliferation in Burkitt lymphoma and non-Hodgkin's lymphoma cells (Challa et al, 2002;Pham et al, 2002). We therefore examined whether the coexpression of CD40 and CD40L results in increased epithelial cell growth.…”

“…In contrast, the effects of CD40 ligation on malignant B cells range from proliferation and rescue from apoptosis to growth inhibition and induction of cell death in vitro and in vivo (Costello et al, 1999). Moreover, CD40L has been detected in certain types of malignant B cells, such as chronic lymphocytic leukaemia, Burkitt lymphoma and non-Hodgkin's lymphoma, where it confers oncogenic effects via the constitutive engagement of the CD40 pathway (Furman et al, 2000;Challa et al, 2002;Pham et al, 2002). These differential effects of CD40 activation on B-cell growth may reflect differences in CD40 signalling in a cell-type and/or differentiation state-dependent manner.…”

Effects of Epstein-Barr virus infection on the development of multiple myeloma after liver transplantation

“…LMP1 plays an important role in B-cell transformation. However, B-cell multiplication in vitro cannot be maintained by the expression of LMP1 alone, and a second signal is required [5]. Cluster of differentiation 40 (CD40) is the main accessory of LMP1 signal transduction.…”