Table 1.

Types of immune cells and their contribution to disseminated tumour cell progression in four frequent sites of metastasis

Cell type	Description	Function in DTC progression
Dendritic cell	Presents antigens on its surface	Suppression of bone165 and brain192 metastases
Group 1 innate lymphoid cell or tissue-resident natural killer cell	Generally weakly or non-cytotoxic; secretes cytokines in response to pathogens or cancer cells	IFNγ-mediated DTC dormancy in the liver20; suppression of liver metastases125
Group 2 innate lymphoid cell	Responds to large extracellular parasites and allergens	Suppression of brain metastases by enhancing dendritic cell antigen presentation and T cell responses192; pro-metastatic through natural killer cell suppression in the lung152
Inflammatory monocyte	Produces inflammatory molecules in response to infection and cancer	Establishment of a pre-metastatic niche in the liver132,133; support of DTC seeding and outgrowth in the lung through suppression of T cell function35,40,157
Neutrophil	Releases chemicals that act against pathogens and cancer cells	Establishment of a pre-metastatic niche in the lung36; awakening of dormant DTCs in the lung34,153; colonization of the lung38,39,154, bone165 and brain189
Natural killer cell	Kills pathogens and cancer cells through cytotoxic granules	DTC elimination in the lung149–151, liver150 and brain150,191; suppression of bone metastases165,170–172
Natural killer T cell	Shares properties of both T cells and natural killer cells; kills pathogens and cancer cells	IFNγ-mediated antimetastatic immunity in the liver226
Tissue-resident macrophage	Phagocytic cell that engulfs and destroys foreign pathogens and cancer cells	Clearance of DTCs upon liver entry130,131; establishment of a lung pre-metastatic niche through T cell suppression37 and neutrophil recruitment146; pro-metastatic in the lung147 and liver141 through acquisition of a lipid metabolism phenotype; promotion41,186,187 or suppression184,185 of brain metastases
Cytotoxic T cell	Processes specific immune responses to antigens; kills pathogens and cancer cells	Suppression of DTC outgrowth in the lung28,155,156,158, brain190 and bone165,171
γδ T cell	Promotes inflammatory responses through cytokine production	Pro-metastatic in the lung through suppression of cytotoxic T cells39
Regulatory T cell	Suppresses immune responses, to maintain peripheral tolerance, prevent autoimmunity and limit chronic inflammatory diseases	Promotion of bone165, lung148 and brain190 metastases

DTC, disseminated tumour cell; IFNγ, interferon-γ.