CD40 ligand induces an antileukemia immune response in vivo

Leukemia cells may express tumor specific antigens in association with Class I and II major histocompatability complex (MHC) molecules. However, lack of expression of conventional costimulator molecules means that these cells tend to induce specific T-cell anergy rather than activation. CD40 ligand (CD40L) is a costimulator molecule that directly activates T cells and may promote antigen presentation by CD40-expressing cells, which include professional antigen presenting cells and B-acute lymphoblastic leukemia (ALL) cells from many patients. We determined whether transgenic expression of CD40L could enhance an antileukemia immune response using a CD40⁺ murine lymphoblastic (A2O) leukemia and a CD40 myeloblastic (WEHI-3) leukemia in a tumor treatment model. Injection of otherwise nonimmunogenic A20 cells in the presence of CD40L induced an immune response active against preexisting A2O tumor at a distant site. Moreover, concomitant local secretion of transgenic interleukin-2 (IL-2) further amplified the antileukemic response induced and increased protection against preexisting tumor. In ex vivo studies, CD40 activation of A20 cells enhances the antigen presenting potential of A20 cells by upregulating expression of B7.1 (CD80), Class I and II MHC molecules, and increases expression of fas antigens. The importance of CD40 activation to the resulting antitumor response is further emphasized by the failure of transgenic CD40L to protect against the CD40^- WEHI myeloblastic leukemia. Depletion studies showed the protective effects against A20 cells to be mediated by a combination of CD4⁺ and CD8⁺ T lymphocytes and by natural killer (NK) cells. These results suggest a means by which CD40⁺ leukemia cells may be rendered immunogenic in vivo.