Activation and activation-induced death of human tonsillar B cells and Burkitt lymphoma cells: lack of CD95 (Fas/APO-1) ligand expression and function.

Activation of T cells was shown to up-regulate the Fas ligand (FasL) which binds to the CD95 (APO-1/Fas) antigen and mediates activation-induced cell death (AICD) of activated T cells and T lymphoma cells. A recent report showed that mouse B cells express the FasL upon activation with lipopolysaccharide (LPS). We therefore asked whether activation of human B cells induces expression of FasL and whether AICD is mediated, as in T cells, through autocrine production of the FasL. We used human tonsillar B cells and Burkitt lymphoma cell lines which were activated by CD40 ligand, surface (s)IgM cross-linking, or LPS. Northern and Western blot analysis failed to detect FasL during B cell activation or AICD of both normal and malignant B cells. Low-level expression of FasL was detected by reverse transcriptase-polymerase chain reaction. Functional experiments, however, showed that FasL is not functionally expressed upon activation. IgM-mediated AICD in the tonsillar or Burkitt lymphoma B cells could not be inhibited by FasL blocking. Thus, our data show that, in contrast to T cells, activation of normal or malignant human B cells does not lead to functional FasL expression.