CD4 and pancreatic ductal adenocarcinoma: In line with this, the majority of pancreatic ductal adenocarcinomas have poor immune cell infiltration [18] and the absence of a response in pancreatic cancer patients is thought to be due to the highly immunosuppressive TME which harbours a higher proportion of tumour-associated macrophages (TAMs), regulatory CD4+ T cells (Tregs), myeloid-derived suppressor cells (MDSCs), in addition to the development of a dense desmoplastic stroma that is thought to be a physical barrier to immune infiltration [19].