Cyclophosphamide inhibition of anti-CD40 monoclonal antibody-based therapy of B cell lymphoma is dependent on CD11b+ cells.

Monoclonal antibody (mAb)-based immunotherapy is now established as an important option for treating some cancers. The antitumor effects may be further enhanced by combining mAb with conventional chemotherapy. Certain novel immunomodulatory mAbs such as anti-CD40 have shown significant activity in preclinical models. We therefore assessed the efficacy of combining anti-CD40 mAb, known to elicit CTL responses against murine lymphoma models with the commonly used cytotoxic drug, cyclophosphamide. Using the syngeneic tumor model, BCL1, we have shown that timing of cyclophosphamide relative to mAb is critical to therapeutic outcome. Pretreatment with cyclophosphamide 7 to 10 days prior to mAb results in markedly reduced survival levels, similar to that achieved with cyclophosphamide alone. Conversely, when anti-CD40 is given before cyclophosphamide, the level of tumor protection was moderately increased. In vivo tracking experiments reveal that pretreatment with cyclophosphamide leads to diminished CTL expansion, as well as an increased number of CD11b+ cells that display an activated phenotype. These latter cells are able to inhibit T-cell proliferation, at least in part via production of nitric oxide, but do not induce T-cell apoptosis. Furthermore, adoptive transfer of the induced CD11b+ cells is sufficient to inhibit anti-CD40 therapy in tumor-bearing recipients. We have shown that the timing of cyclophosphamide relative to mAb administration is critical to the therapeutic outcome, and although the combination can improve survival, cyclophosphamide given prior to immunotherapy may generate a population of myeloid cells that can interfere with CTL responses and compromise the therapeutic outcome.