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

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Abstract
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.