CTLA4 and cancer: The tumor microenvironment plays a crucial role in the tumorigenesis and progression of tumor cells, which consists of cancer cells, stromal cells (fibroblasts, endothelial cells and mesenchymal stem cells), immune and inflammatory cells (lymphocytes, macrophages and myeloid cells), extracellular matrix as well as secreted factors and their receptors, such as immune checkpoints (PD-1/L1 and CTLA4), chemokines, cytokines and growth factors [12].