PDCD1 and cancer: Even though tumors have an immunosuppressive environment where cancer cells can inhibit the activation of immune cells through a variety of mechanisms, including the recruitment of TAMs, MDSCs, and Tregs, the attenuation of MHC class I expression, and the use of the PD-1/PD-L1 axis, the immune response against TICs can be strengthened by modulating the TME by using inhibitors of immune checkpoint molecules or reducing the recruitment of immunosuppressive cells.