Immune cell populations such as granulocytic and monocytic myeloid-derived suppressor cells (MDSCs), regulatory T (Treg) cells, IL-22+CD4+ T helper 22 (TH22) cells, IL-22+ innate lymphoid cells (ILCs) and plasmacytoid dendritic cells (pDCs) can promote tumour growth. These cells are recruited to the tumour microenvironment in response to different chemokines that are expressed in the tumour microenvironment (the relevant receptors and ligands are shown). Pro-tumour immune cells may inhibit antitumour immune responses, and may also promote and maintain cancer stemness and angiogenesis, leading to cancer progression. CCL, CC-chemokine ligand; CCR, CC-chemokine receptor; CXCL, CXC-chemokine ligand; CXCR, CXC-chemokine receptor.