IMPORTANCE OF CYCLOPHOSPHAMIDE-INDUCED BYSTANDER EFFECT ON T-CELLS FOR A SUCCESSFUL TUMOR-ERADICATION IN RESPONSE TO ADOPTIVE IMMUNOTHERAPYIN MICE

Cyclophosphamide (CTX) increases the antitumor effectiveness of adopti ve immunotherapy in mice, and combined immunotherapy regimens are now used in some clinical trials. However, the mechanisms underlying the s ynergistic antitumor responses are still unclear. The purpose of this study was (a) to evaluate the antitumor response to CTX and adoptive i mmunotherapy in mice bearing four different syngeneic tumors (two resp onsive in vivo to CTX and two resistant); and (b) to define the mechan ism(s) of the CTX-immunotherapy synergism. Tumor-bearing DBA/2 mice we re treated with a single injection of CTX followed by an intravenous i nfusion of tumor-immune spleen cells. In all the four tumor models, a single CTX injection resulted in an impressive antitumor response to t he subsequent injection of spleen cells from mice immunized with homol ogous tumor cells independently of the in vivo response to CTX alone. Detailed analysis of the antitumor mechanisms in mice transplanted wit h metastatic Friend leukemia cells revealed that (a) the effectiveness of this combined therapy was dependent neither on the CTX-induced red uction of tumor burden nor on CTX-induced inhibition of some putative tumor-induced suppressor cells; (b) the CTX/immune cells' regimen stro ngly protected the mice from subsequent injection of FLC, provided the animals were also preinoculated with inactivated homologous tumor tog ether with the immune spleen cells; (c) CD4(+) T immune lymphocytes we re the major cell type responsible for the antitumor activity; (d) the combined therapy was ineffective in mice treated with antiasialo-GM(1 ) or anti-IFN-alpha/beta antibodies; (e) spleen and/or bone marrow cel ls from CTX-treated mice produced soluble factors that assisted in pro liferation of the spleen cells. Altogether, these results indicate tha t CTX acts via bystander effects, possibly through production of T cel l growth factors occurring during the rebound events after drug admini stration, which may sustain the proliferation, survival, and activity of the transferred immune T lymphocytes. Thus, our findings indicate t he need for reappraisal of the mechanisms underlying the synergistic e ffects of CTX and adoptive immunotherapy, and may provide new insights into the definition of new and more effective strategies with chemoth erapy and adoptive immunotherapy for cancer patients.