Inside these “ectopic lymphoid structures,” growth-promoting proteins fuel the development of cancer progenitor cells. From a certain point onwards, the liver cancer cells supply themselves with their own fuel and migrate into liver tissue, where they grow into tumors.
In experiments with mice, chemical agents that targeted the growth-promoting proteins slowed down the carcinogenic process. The researchers report that they are now able to use a specific genetic signature to identify individuals with a high risk for liver cancer. Their work appears in the journal “Nature Immunology.”
They are found in rheumatism and multiple sclerosis, hepatitis C and tuberculosis, as well as in breast and bowel cancers: small structures in the diseased organs and tissues that are made up of immune cells and resemble lymph nodes in structure and composition.
Until now, scientists have not known exactly how and why these ectopic lymphoid structures (ELSs) form. In bowel, breast and lung cancers, their presence is associated with a more favorable prognosis. Mathias Heikenwälder from the German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ) in Heidelberg, in collaboration with researchers from Hebrew University in Jerusalem and further colleagues from Germany, Switzerland and the USA, has now shown that this does not hold true for liver cancer.
Liver cancer (hepatocellular carcinoma) is the second-most common cause of cancer deaths worldwide. In most cases, the disease develops as a consequence of chronic inflammation of the liver, which results from viral hepatitis or inflammatory fatty liver. Lymphoid-like follicles are a hallmark of chronic liver inflammations.
Heikenwälder and his colleagues counted the lymphoid-like follicles in cancer-free liver tissue from 82 liver cancer patients. They discovered that the more of these immune structures are present in a patient’s liver, the higher the risk for a recurrence of cancer after treatment.
As a cause for the formation of ectopic lymphoid structures, the researchers identified a notorious growth driver for immune cells and inflammatory reactions: the nuclear protein NF-kappa B. Mice in whose livers NF-kappa B signals were permanently active soon developed large numbers of ELSs in the liver. Without exception, these animals developed liver cancer by the age of 20 months at the latest.
In young animals, however, the investigators found tumor progenitor cells exclusively within the ELS, and not in the hepatic tissue itself. Only at some later point did advanced cancer cells start migrating from the ELS, in some cases over a period of several months. The researchers also found cancer progenitor cells inside the immune structures in liver tissue samples from liver cancer patients.
Ectopic lymphoid structures are made up of many types of cells from the innate immune system (macrophages, dendritic cells) and others indicative of adaptive immunity (T and B cells). The ELSs also contain blood vessels that enable immune cells to enter the follicles. Mice whose cells are unable to form T and B cells do not produce ELS even when NF-kappa B signaling is permanently active, and they hardly ever develop liver cancer.
Withdrawal of fuel reduces cancer growth
Why are ELSs such fertile breeding sites for liver cancer cells? The scientists found high levels of signaling molecules typical for the immune system, in particular lymphotoxins, in the ectopic lymphoid structures. In the early stage of carcinogenesis, only the immune cells in the ELSs produce these cancer-promoting proteins.
“The liver cancer progenitor cells are literally addicted to lymphotoxins,” Heikenwälder says. “Then at some point they reach a stage where they are able to produce their own ‘fuel’. Only then do they start leaving the ELS and proliferating in liver tissue. We were able to detect this in tissue investigations.” Heikenwälder, who recently started building a research department at the DKFZ, previously pursued research at the Helmholtz Zentrum München (German Research Center for Environmental Health).
The scientists treated mice with an agent that inhibits lymphotoxins, thus depriving the cancer progenitor cells of their fuel. As anticipated, the animals developed fewer tumors as a result. However, this only happened if the agent was administered early enough, i.e., while the cancer progenitors still depended entirely on the lymphotoxins produced by the ELSs. Once they were able to produce their own fuel, the therapy was no longer effective.
“We now have identified the gene activity pattern that leads to abnormally high activation of NF kappa B, thus contributing to the formation of ectopic lymphoid structures and the transformation of healthy hepatic cells into tumor progenitor cells,” Heikenwälder says. “We can use this ‘gene signature’ to assess whether an individual patient with chronic liver inflammation has a high risk of developing liver cancer.” In a next step, the researchers will investigate the clinical relevance of this gene signature in more patient groups. Heikenwälder and his colleagues plan to investigate whether blocking the growth-promoting lymphotoxins can also prevent the development of liver cancer in high-risk human patients.
Shlomi Finkin, Detian Yuan, Ilan Stein, Koji Taniguchi, Achim Weber, Kristian Unger, Jeffrey L Browning, Nicolas Goossens, Shigeki Nakagawa, Ganesh Gunasekaran, Myron E Schwartz, Masahiro Kobayashi, Hiromitsu Kumada, Michael Berger, Orit Pappo, Klaus Rajewsky, Yujin Hoshida, Michael Karin, Mathias Heikenwälder, Yinon Ben-Neriah & Eli Pikarsky: Ectopic lymphoid structures function as microniches for tumor progenitor cells in hepatocellular carcinoma. Nature Immunology 2015, DOI: 10.1038/ni.3290
The German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ) with its more than 3,000 employees is the largest biomedical research institute in Germany. At DKFZ, more than 1,000 scientists investigate how cancer develops, identify cancer risk factors and endeavor to find new strategies to prevent people from getting cancer. They develop novel approaches to make tumor diagnosis more precise and treatment of cancer patients more successful. The staff of the Cancer Information Service (KID) offers information about the widespread disease of cancer for patients, their families, and the general public. Jointly with Heidelberg University Hospital, DKFZ has established the National Center for Tumor Diseases (NCT) Heidelberg, where promising approaches from cancer research are translated into the clinic. In the German Consortium for Translational Cancer Research (DKTK), one of six German Centers for Health Research, DKFZ maintains translational centers at seven university partnering sites. Combining excellent university hospitals with high-profile research at a Helmholtz Center is an important contribution to improving the chances of cancer patients. DKFZ is a member of the Helmholtz Association of National Research Centers, with ninety percent of its funding coming from the German Federal Ministry of Education and Research and the remaining ten percent from the State of Baden-Württemberg.