PDCD1 and Splenomegaly: This result demonstrated that 4T1‐bearing mice exhibited the cancer associated splenomegaly, which is a typical symptom of mice with hyper‐enhancing metastatic tumors than any other non‐pulmonary‐metastatic tumor models.[14] Additionally, monocytes formed in the enlarged spleen and migrated to the TME to promote immunosuppression.[22] Besides, the spleen from mice of NP+PD‐1 (+) groups appeared in the normal morphology (Figure S11, Supporting Information).