CTLA4 and cancer: Furthermore, cancer-associated fibroblasts (CAF) can impact the immune response by secreting cytokines, like IL-6, Granulocyte Colony-stimulating Factor (G-CSF), and Macrophage Colony-stimulating Factor (M-CSF), or inhibiting CD8+ T cells by either expressing PD-L1 or promoting expression of PD-1 and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) on CD8+ T cells (13).