The purpose of the present disclosure is to develop a new
combination therapy that exerts a superior antitumor effect as compared to administration of an anti-CD26
antibody alone. In order to assess both the function of an
immune checkpoint inhibitor and the antitumor effect of an anti-CD26
antibody, it is necessary to be provided with both a
binding site on a human CD26 molecule and an
immune system of the same lineage of a
cancer cell. Thus, in order to acquire data of an
immune checkpoint inhibitor and an anti-CD26
antibody, it is essential to perform analysis in a human
immune system and a
human tumor cell line. The present inventors have made an attempt to produce a human
immune system mouse to be used as a model animal for confirming the effect of the combination between ICI and an anti-CD26 antibody, and have successfully produced a model mouse superior for generating human T-cells. Next, the present inventors have studied the effect of combining an
immune checkpoint inhibitor and an anti-CD26 antibody in a tumor bearing model by using said mouse. As a result, it was found that an agent using said combination has a synergistic effect that is stronger than when the respective agents are used individually.