Seed and soil
In the autumn of 1889, a London surgeon named Stephen Paget sat alone with a stack of autopsy records and a growing sense how cancer determined its next move. He had collected the postmortem reports from 735 women who had died of breast cancer. As he worked through them, a pattern emerged that defied the contemporary concept. The liver was almost always involved; the ovaries were disproportionately so; certain bones were frequently impacted. But the spleen — large, vascular, perpetually awash in blood — was nearly always clean. Rudolf Virchow, the towering figure of nineteenth-century pathology, had argued that cancer spread by a simple mechanical logic: cells broke free from the primary tumor, entered the circulation, and lodged wherever the current deposited them. If that were true, the spleen made no sense, and the distribution in his observation could not be explained.
Paget reached for a botanical metaphor, as Victorians were inclined to do. "When a plant goes to seed," he wrote in The Lancet, "its seeds are carried in all directions; but they can only live and grow if they fall on congenial soil." The breaking-off tumor cell was the seed, and the distant organ was the soil. And metastasis, Paget proposed, was not a matter of chance or anatomy but of compatibility. It is a specific, almost ecological relationship between a travelling cell and the tissue it hoped to inhabit. It was an beautiful idea but also controversial. For the following decades, the debate between Paget's biological hypothesis and the anatomical hypothesis was waged almost entirely with statistics drawn from autopsies. What the field needed was an experiment. The argument needed to wait for almost a century to be resolved.
In the 1960s, a young Israeli, Isaiah J. Fidler, came to the United States to study in school of veterinary medicine and then received a Ph.D. When starting his own laboratory at NCI-Frederick, Maryland, in 1970s, he focused his research on metastasis. NCI-Frederick was no ordinary campus. It was an autonomous hub for translational research, home to one of the largest mouse-study facilities in the country, and the place where landmark chemotherapy drugs, including paclitaxel, had been developed. For a scientist of Fidler's style and background — restless, rigorous, always asking for physiological and pathological relevance — it was exactly the right place.
One of his earliest acts there was to establish a cell line. The B16 melanoma had originated from a spontaneously arising tumor in an aged C57BL/6 mouse at the Jackson Laboratory, where it had been maintained by serial transplantation in vivo. Fidler made it a cell line that can be expanded stably in culture dish instead of in mice, so it became a reproducible scientific instrument. His first question came in 1973: are tumor cells were interchangeable? If, like the contemporary idea, any cell in a growing mass was as capable of spreading and growing in the metastatic sites as any other. Using the B16 melanoma, he designed an experiment of almost irritating simplicity: inject cells into mice intravenously, wait for lung metastases to appear, harvest those colonies, culture them, and inject them again, cycle after cycle. What he found was that with each passage, the resulting population became more capable of forming metastases, more lethal as growing faster in the new site (Fidler, 1973). This result clearly demonstrated that metastatic site selected better fitting cells in each cycle.
Nevertheless, how the cells were selected was debatable. It could be explain by two opposite ways. On one hand, the better fitting subclones might pre-exist in the tumor cell population. On the other hand, some subclones may randomly adapt the environment of the metastatic site, and their adaptability was enhanced in each cycle of transplantation. The person who pushed Fidler toward an answer was his wife, immunologist Margaret Kripke, who had joined him at NCI-Frederick. Could all the cells in a culture dish truly be considered identical? The question rang a bell- how similar is this to Delbrück's question of origin of bacterial resistance to phages!
In 1943, at the dark climax of fascism with no sight of light at the end of the tunnel, Max Delbrück and Salvador Luria were thinking about an important biological question in their quiet office in Tennessee. Bacteria develop resistance to phages due to genetic mutations. Do such mutations preexist or occur spontaneously after their exposure to the bacteriophages? In other words, is the resistance caused by the Darwinian selection or Lamarckian adaptation?
At that time, the structure of DNA had yet to be discovered, genes are just concepts, invisible and intangible. Luria suggested that the inconsistency of the frequency of the resistance can be used to distinguish pre-existed or induced mutation. Inspired by Luria’s insight, Delbrück established a statistical model of mutation distribution models to predict the frequency of phage-resistant bacteria. Luria designed experiments based on Delbrück's statistical model. The results confirmed that gene mutations occurred spontaneously in bacteria before their exposure to phages. This is the first time in the history of biology that Darwin's theory of evolution by natural selection was proved experimentally. The dynamic duo were awarded the Nobel Prize in 1969. Today this model is well known as the "Luria-Delbrück distribution".
Years later, Fidler recalled that he immediately thought of “go Delbrück”.
Together, Fidler and Kripke generated 10 distinct sublines, each the descendant of a single cell from the B16 parental line. These were injected separately into groups of mice via the tail vein. The logic was straightforward: if metastatic ability were a pre-existing trait, different sublines would behave very differently from one another. If it arose through adaptation, they would converge on the same behavior. The results sided with Darwinism unambiguously: most sublines produced few metastases at all, and a handful generated hundreds of lung colonies. This was the Luria-Delbrück distribution described in the textbook, proving that the metastatic cells had pre-existed in the primary tumor (Fidler & Kripke, 1977). The paper was as concise as just two pages, but its conclusion could not be clearer.
The discoveries of selection by the metastatic site and pre-existing tumor heterogeneity were still not sufficient to prove Paget's hypothesis. Why did the melanoma colonize the lung and not the kidney? Why the liver and not the spleen? It demanded the evidence of the soil; that is, certain organ environments specially equipped to receive melanoma cells. The anatomical hypothesis had always countered that drainage patterns alone could account for such preferences.
Fidler, working now with Ian R. Hart, devised a strategy to test the organ environment. They transplanted small fragments of embryonic mouse lung and kidney tissue into the flanks of syngeneic mice (ectopic grafts). They were relocated from their anatomical position, but vascularized and viable. Then they injected B16 melanoma cells into the bloodstream and watched where the tumors grew. The cells colonized the native lungs, as expected; but they also colonized the grafted lung tissues sitting in the flank. In contrast, B16 cells did not appear in the ectopic kidney grafts sitting right beside the lung grafts (Hart & Fidler, 1980). Paget's century-old conjecture, formed over Victorian autopsy tables, had been confirmed in the mouse flank of a laboratory in Frederick, Maryland. It is the organ microenvironment, its physiological character, biochemical hospitality, and the signals it sent and received, that determined where cancer grew.
In 1983, Fidler and Kripke left NCI-Frederick for the University of Texas MD Anderson Cancer Center, where he became the founding chair of the Department of Cancer Biology. He would spend the rest of his career there, training scientists, extending the framework, and continuing to demonstrate that metastasis was not a mystery, but a problem to be solved. As he wrote in a 2003 review in Nature Reviews Cancer, the metastatic potential of any tumor cell is ultimately determined by its interactions with the homeostatic machinery of the distant organ (Fidler, 2003). The soil was not passive but an active participant.
The descendants of this insight are everywhere in modern oncology. Researchers have since shown that primary tumors send molecular emissaries ahead of themselves — secreted factors and extracellular vesicles that remodel distant organs before a single metastatic cell arrives, preparing the soil for the seed through what are now called pre-metastatic niches (Kaplan et al., 2005). Therapies targeting the tumor microenvironment, rather than the tumor cell alone, are a direct product of this intellectual lineage. The vocabulary has grown more molecular, the mechanisms more intricate — but the essential picture is still the one Paget drew in 1889, and the one Fidler spent his career proving: cancer spreads not by accident but by affinity. “In essence, everything I had done confirmed Paget’s hypothesis,” Dr. Fidler later said.
On May 8, 2020, MD Anderson announced the passing of Dr. Isaiah "Josh" Fidler. His legacies included not only the discoveries in the nature of cancer metastasis, but also his experimental approaches whose impacts on cancer research continue to these days. In vivo cycling has become a standard procedure to generate organ-specific metastatic cancer cell sublines, widely used in the study of organotropism of metastases. When researchers consider to study intratumoral heterogeneity, their first thought is to derive sublines from single cells of parental lines. In the years that followed, B16 cell line and its sublines would become perhaps the most widely used mouse cancer cell lines in the history of biomedical research — a reagent so fundamental that decades later, Jim Allison would use B16 sublines to validate the anti-CTLA-4 immune checkpoint theory that would earn him the Nobel Prize in Physiology or Medicine in 2018.
I used to ask Dr. Fidler, what does F in B16F0-10 sublines really stands for? "Fidler." He smirked. To this day, I could not tell whether he was joking. Nevertheless, he did not have to mark his name on B16 sublines to let us remember his achievements. Cancer researchers have always told their trainees, "do a Fidler experiment".
References
- Paget, S. (1889). The distribution of secondary growths in cancer of the breast. The Lancet, 133(3421), 571–573.
- Fidler, I. J. (1973). Selection of successive tumour lines for metastasis. Nature New Biology, 242(118), 148–149.
- Fidler, I. J., & Kripke, M. L. (1977). Metastasis results from preexisting variant cells within a malignant tumor. Science, 197(4306), 893–895.
- Hart, I. R., & Fidler, I. J. (1980). Role of organ selectivity in the determination of metastatic patterns of the B16 melanoma. Cancer Research, 40(7), 2281–2287.
- Fidler, I. J. (2003). The pathogenesis of cancer metastasis: the 'seed and soil' hypothesis revisited. Nature Reviews Cancer, 3(6), 453–458.
- Kaplan, R. N., et al. (2005). VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche. Nature, 438(7069), 820–827.