INDUCTION OF BAX BY CD40 STIMULATION IN AGGRESSIVE HISTOLOGY B-CELL LYMPHOMAS

Abstract

CD40 is a 55kDa molecule that is a member of the tumor necrosis factor/nerve growth factor receptor superfamily. CD40 is present on B cells, dendritic cells, hematopoietic progenitor cells, and various malignant cells. Stimulation of CD40 on untransformed B cells results in proliferation and differentiation. CD40 and B cell Ig receptor activation lead to induction of phenotypic and functional germinal center features in the lymph node. It is also required for successful isotype switching. The ligand for CD40 (CD40L, gp39, CD154) is expressed primarily on activated CD4+ T cells, although it is present on natural killer cells, platelets and eosinophils. It is a member of the tumor necrosis factor superfamily. CD40L regulates B cell function as well as promotes isotype switching in the presence of various cytokines. Soluble recombinant human CD40L (srhCD40L) has been demonstrated to be an active ligand to CD40. It has been demonstrated previously that some signals of normal cell activation will inhibit the growth of transformed cells by activation induced cell death (AICD). AICD can occur by cell cycle arrest, apoptosis, and/or necrosis. An antibody to CD40 as well as srhCD40L inhibit aggressive histology human B-cell lymphomas in vitro and in vivo, supposedly by AICD. The mechanism by which CD40 causes AICD in these lymphomas is yet unknown. Stimulation of human B-cell lymphomas by either srhCD40L or anti-CD40 mAb's was able to significantly inhibit proliferation and induce apoptosis in the lines tested. Upon further investigation, it was shown that CD40 stimulation in these cell lines derived from tumors causes an increase in pro-apoptotic mRNA in the bcl-2 family of genes with a subsequent increase in Bax protein in the cytosol. After 48 hours, the Bax protein had translocated to the mitochondria, an event which has the potential to advance cell death. In vitro culture of the lymphoma cells with bax antisense resulted in a significant protection from CD40-mediated death. This indicates that CD40 mediates death in these lymphomas through bax. In agreement with the in vitro proliferation assays, in vivo treatment of immunodeficient mice with srhCD40L or anti-CD40 mAb bearing CD40 positive human B-cell lymphomas results in significant increases in survival. These results demonstrate that CD40 ligation on aggressive human B-cell lymphomas induces AICD through box and thus may be of potential clinical use in the treatment of lymphomas.