CD4 and neoplasm: This property has led researchers to utilize IL-21 to generate “younger” T cells, characterized by superior proliferation potential and enhanced antitumor capacity.20,21 Meanwhile, IL-21 also supports the effector function of cytotoxic CD8+ T cells by activating the Basic Leucine Zipper ATF-Like Transcription Factor pathway.21,22 Within the tumor microenvironment, IL-21 is secreted by tumor-specific Tfh cells to bolster the effector function of tumor-infiltrating CD8+ T cells23 and is regarded as a pivotal cytokine in the CD4+ T and CD8+ T cell interaction.