Although metastasis is the leading cause of cancer-related death, it is not clear why some patients with localized cancer develop metastatic disease after complete resection of their primary tumor. Such relapses have been attributed to tumor cells that disseminate early and remain dormant for prolonged periods of time; however, little is known about the control of these disseminated tumor cells. Here, we have used a spontaneous mouse model of melanoma to investigate tumor cell dissemination and immune control of metastatic outgrowth. Tumor cells were found to disseminate throughout the body early in development of the primary tumor, even before it became clinically detectable. The disseminated tumor cells remained dormant for varying periods of time depending on the tissue, resulting in staggered metastatic outgrowth. Dormancy in the lung was associated with reduced proliferation of the disseminated tumor cells relative to the primary tumor. This was mediated, at least in part, by cytostatic CD8 + T cells, since depletion of these cells resulted in faster outgrowth of visceral metastases. Our findings predict that immune responses favoring dormancy of disseminated tumor cells, which we propose to be the seed of subsequent macroscopic metastases, are essential for prolonging the survival of early stage cancer patients and suggest that therapeutic strategies designed to reinforce such immune responses may produce marked benefits in these patients. IntroductionMetastatic disease is the major cause of death by cancer (1, 2). Metastasis is a complex multistage process that requires cancer cells within the primary tumor to invade the local tissue and enter the blood or lymphatic vessels. Tumor cells need to survive in the circulation and migrate across vessel walls in order to colonize new sites and grow to form secondary tumors (3). The traditional view has been that tumor cell dissemination occurs late in cancer development (4-6); however, this notion has recently been challenged. Several expression profiling studies (7-10) suggest that the propensity of cancer cells to metastasize is acquired relatively early during tumor progression (reviewed in ref. 11). In addition, examination of bone marrow from early stage cancer patients without overt metastases (reviewed in refs. 12 and 13) and tumorbearing mice (14) revealed that disseminated tumor cells (DTCs) are present at much earlier time points than expected. We now need to understand the significance of these DTCs. Specifically, we must determine how early DTCs contribute to clinically relevant macrometastases and identify the mechanisms involved in the development, maintenance, and breakdown of dormancy.Transplanted tumor models in rodents are often used to study metastasis, with most of our current knowledge of cancer cell dissemination being drawn from xenograft models. However, these models often fail to recapitulate the gradual process of tumorigenesis that is observed in humans, and, in the case of immuno-

In order to metastasize, cancer cells need to acquire a motile phenotype. Previously, development of this phenotype was thought to rely on the acquisition of selected, random mutations and thus would occur late in cancer progression. However, recent studies show that cancer cells disseminate early, implying the existence of a different, faster route to the metastatic motile phenotype. Using a spontaneous murine model of melanoma, we show that a subset of bone marrow-derived immune cells (myeloid-derived suppressor cells or MDSC) preferentially infiltrates the primary tumor and actively promotes cancer cell dissemination by inducing epithelial-mesenchymal transition (EMT). CXCL5 is the main chemokine attracting MDSC to the primary tumor. In vitro assay using purified MDSC showed that TGF-β, EGF, and HGF signaling pathways are all used by MDSC to induce EMT in cancer cells. These findings explain how cancer cells acquire a motile phenotype so early and provide a mechanistic explanation for the long recognized link between inflammation and cancer progression.

The immune system has multiple, complex, and sometimes opposing roles during cancer progression. While immune-compromised individuals have a higher incidence of cancers, inflammation is also associated with increased risk of disease progression. It is becoming apparent that simple measures of immune responses in the blood are of limited use in cancer. Instead, the importance of the exact identity and functional characteristics of tumor-infiltrating immune cells is increasingly recognized. This realization has led to recent studies that have revealed a critical role for chemokine expression in the tumor microenvironment and suggested a therapeutic potential of manipulating intratumoral expression of chemokines to alter the local immune milieu.

Metastasis is a key step in cancer progression, and was traditionally attributed to the accumulation of genetic and epigenetic changes within individual cancer cells. These changes promoted invasiveness, immune evasion and survival at distant sites. However, recent studies reveal that metastasis is not achieved by the cancer cell in isolation, but requires intervention from the immune system. The myeloid cell population in particular is now implicated in many aspects of metastasis. Here, we bring together the evidence for the importance of various myeloid cell sub-populations throughout the metastatic process, from initiation of cancer cell invasiveness, to priming the tissue site for colonization.