CD4 and neoplasm: In addition, studies have also shown that the Hippo signaling pathway not only directly regulates immune cells through activities such as affecting the differentiation of CD4 + helper T cells but also plays a regulatory role in the tumor microenvironment, such as recruiting additional type II macrophages and MDSCs and upregulating expression of the PD-L1 protein in tumor cells (Bhandoola 2020; Wang 2015; Janse van Rensburg 2018).