Visualization of immediate immune responses to pioneer metastatic cells in the lung

Tracing the fate of circulating tumour cells by intravital two-photon lung imaging shows that tumours produce microparticles as they arrive and these migrate along the lung vasculature and are mostly taken up by interstitial myeloid cells, in a process that contributes to metastatic seeding; a minor subset of microparticles is engulfed by conventional dendritic cells, which are thought to contribute to the initiation of an anti-tumour immune response in lung-draining lymph nodes. The interaction between a tumour and the patient's immune system during the first minutes and hours of lung metastasis is a critical point in the evolution of a cancer. This study traces the fate of circulating tumour cells by intravital imaging and shows that tumour-derived microparticles migrate along the lung vasculature. Most are taken up by interstitial myeloid cells, a process which contributes to metastatic seeding. A minor subset of particles is engulfed by conventional dendritic cells and thought to contribute to the initiation of an anti-tumour immune response in draining lymph nodes. Lung metastasis is the lethal determinant in many cancers1,2 and a number of lines of evidence point to monocytes and macrophages having key roles in its development3,4,5. Yet little is known about the immediate fate of incoming tumour cells as they colonize this tissue, and even less known about how they make first contact with the immune system. Primary tumours liberate circulating tumour cells (CTCs) into the blood and we have developed a stable intravital two-photon lung imaging model in mice6 for direct observation of the arrival of CTCs and subsequent host interaction. Here we show dynamic generation of tumour microparticles in shear flow in the capillaries within minutes of CTC entry. Rather than dispersing under flow, many of these microparticles remain attached to the lung vasculature or independently migrate along the inner walls of vessels. Using fluorescent lineage reporters and flow cytometry, we observed ‘waves’ of distinct myeloid cell subsets that load differentially and sequentially with this CTC-derived material. Many of these tumour-ingesting myeloid cells collectively accumulated in the lung interstitium along with the successful metastatic cells and, as previously understood, promote the development of successful metastases from surviving tumour cells3. Although the numbers of these cells rise globally in the lung with metastatic exposure and ingesting myeloid cells undergo phenotypic changes associated with microparticle ingestion, a consistently sparse population of resident conventional dendritic cells, among the last cells to interact with CTCs, confer anti-metastatic protection. This work reveals that CTC fragmentation generates immune-interacting intermediates, and defines a competitive relationship between phagocyte populations for tumour loading during metastatic cell seeding.