Contrasting effects of cyclophosphamide on anti‐CTL‐associated protein 4 blockade therapy in two mouse tumor models

Immune checkpoint blockade is a promising anticancer therapy, but must be used in combination with other anticancer therapies to increase its therapeutic efficacy. Cyclophosphamide (CP) is a chemotherapeutic drug that shows immune‐modulating effects. In this study, we examined the effect of CP on anti‐CTL‐associated protein 4 (CTLA‐4) blockade therapy in two mouse tumor models. Drastic tumor regression was observed in the CT26 colon carcinoma model after i.p. injection of CP (100 mg/kg) followed by anti‐CTLA‐4 antibody. However, administration in the reverse order increased apoptosis in tumor‐specific CD8(+) T cells. In the RENCA renal carcinoma model, the antitumor effect of combination therapy was marginal and the tumor‐bearing state reduced body weight with an increased serum level of interleukin‐6. Interestingly, although CP monotherapy increased myeloid‐derived suppressor cells (MDSCs) in the spleens of both models, subsequent anti‐CTLA‐4 therapy increased MDSCs only in RENCA‐bearing mice. Additionally, the serum levels of chemokine ligand 2 and C‐X‐C motif chemokine 10 were increased by the combination therapy only in RENCA‐bearing mice and in vivo depletion of Gr‐1(+) cells augmented the antitumor effect to some degree. These results reveal a contrasting effect of CP on anti‐CTLA‐4 therapy between the two mouse tumor models. Cyclophosphamide augments the antitumor effect of anti‐CTLA‐4 therapy in CT26‐bearing hosts, whereas CP after anti‐CTLA‐4 therapy attenuates this effect through induction of apoptosis in tumor‐reactive T cells. Alternatively, CP‐induced MDSCs can be increased by anti‐CTLA‐4 therapy only in RENCA‐bearing hosts with an elevated level of interleukin‐6.