Locally sourced: site-specific immune barriers to metastasis

Tumour cells migrate very early from primary sites to distant sites, and yet metastases often take years to manifest themselves clinically or never even surface within a patient’s lifetime. This pause in cancer progression emphasizes the existence of barriers that constrain the growth of disseminated tumour cells (DTCs) at distant sites. Although the nature of these barriers to metastasis might include DTC-intrinsic traits, recent studies have established that the local microenvironment also controls the formation of metastases. In this Perspective, I discuss how site-specific differences of the immune system might be a major selective growth restraint on DTCs, and argue that harnessing tissue immunity will be essential for the next stage in immunotherapy development that reliably prevents the establishment of metastases. A disseminated tumour cell will grow only if it arrives at a ‘fertile’ distant site, which as Ana Luísa Correia posits is determined largely by the immune context at the site. Site-specific differences in local immune cell types, ratios and spatial locations influence whether a disseminated tumour cell establishes metastases or is kept dormant.