Abstract
The continuous proliferation of Epstein-Barr virus (EBV)-immortalized B cells is enhanced by autocrine as well as paracrine growth factors. In the present study, the possibility that EBV-immortalized B cells might produce interleukin-6 (IL-6) proteins in an autocrine manner was examined. It was found that culture supernatants from EBV-transformed B cells, but not from Burkitt's lymphoma lines, augmented the proliferation of an IL-6-dependent murine hybridoma clone, MH60.BSF2. This growth-promoting activity for hybridoma cells found in culture supernatants of EBV-transformed B cells was specifically neutralized by rabbit anti-recombinant (r) IL-6 antibody. The IL-6 activity in culture supernatants of EBV-transformed B cells, though much less than that of lipopolysaccharide (LPS)-stimulated monocytes, was increased by the addition of phorbol myristate acetate. Western blot experiments using rabbit anti-rIL-6 antiserum demonstrated that supernatants from cultured EBV-transformed B cells contained the distinct forms of IL-6, with a peak of 23,000 MW. When examined by in situ hybridization analysis, it was found that IL-6 mRNA were expressed on EBV-transformed B cells. It was noted that a fraction, but not all, of these cells expressed IL-6 mRNA strongly, implying their cell cycle-dependent expression. In addition, it was shown that rIL-6 promoted the growth of EBV-transformed B cells at low cell densities. The results suggest that IL-6 serves as an autocrine growth factor in EBV-transformed B cells.