Loss of transforming growth factor β₁ receptors and its effects on the growth of EBV-transformed human B cells

Transforming growth factor-β (TGF-β) is a potent negative regulator of normal human B cell growth mediated by exogenous signals, including IL-2 and low m.w. B cell growth factor 12 KDa (BCGF-12 kDa). In the present study, we investigated the regulatory linkage between viral or nonviral transformation of human B cells and the growth inhibitory effects of TGF-β₁. A panel of EBV⁺ and EBV B cell lines, derived either by in vitro EBV B cell transformation, or from cases of lymphoma was used to quantitate the negative growth effects of TGF-β₁. The proliferative response of three EBV^- B cell lines to rBCGF-12 kDa or serum was inhibited by low concentrations of TGF-β₁ (0.2-0.5 ng/ml for 50% maximal effect), as measured by tritiated thymidine uptake and viable cellular recovery. In contrast, rBCGF-12 kDa or serum mediated proliferation of three EBV⁺ B cell lines was refractory to the growth inhibitory effects of TGF-β₁. In an attempt to understand the mechanism(s) for this differential growth control in EBV⁺ and EBV^- B cells, we studied the expression of TGF-β₁, c-myc, and TGF-β₁ receptors. No correlation was observed between the expression of TGF-β₁ or c-myc gene and growth inhibition by TGF-β₁ in the cell lines studied. Our results indicate that sensitivity or resistance to TGF-β₁ correlated with the presence or absence (loss) of high affinity receptors for TGF-β₁. EBV B cell lines expressed levels of high affinity receptors similar to those found on activated normal B or T cells. In contrast, EBV⁺ B cell lines showed no detectable high affinity receptors. Chemical cross-linking studies with a bifunctional reagent, dissuccinimidyl suberate revealed a normal expression of type I (65-70 kDa), type II (85-90 kDa), and type III (280-300 kDa) TGF-β₁ high affinity receptors on EBV^- B cell lines. In contrast, EBV⁺ B cell lines did not express type I and type II receptors, whereas type III receptors were expressed but could not be inhibited by unlabeled TGF-β₁. To directly determine whether changes in receptor expression in EBV⁺ B cells coincided temporally with acquisition of growth resistance to TGF-β, an EBV^- Burkitt's lymphoma cell line, BL30, was in vitro exposed to wild-type EBV strain B95-8, and the effects on TGF-β₁ receptor expression and responsiveness to TGF-β₁ were examined in the parental cell line and its EBV⁺ clone. Unlike BL30 cells, BL30-B95-8 cells failed to express TGF-β₁ receptors that correlated with acquisition of the TGF-β unresponsive phenotype. It is proposed that in vitro or in vivo EBV transformation of human B cells results in a loss or alteration of TGF-β high affinity receptors rendering cells resistant to TGF-β.