Xenografting and Xenotransplantation
The xenotransplant, also known as xenograft, is a tissue or an organ transplanted between members of different species
The limited availability of allografts motivates the extensive research occurring on xenotransplantation techniques. The preferred donor species for humans are pigs, due to the low risk of introducing new infections in the human population and for physiological compatibility; the widely used models for humans are baboons. Non-human primates are avoided as organ donors due to their close genetic relationship to humans, which increases the risk of transmitting infectious agents to human recipients. Xenotransplantation remains a critical area of research, primarily driven by the shortage of available human organs. Studies involving xenografts and non-human primate models yield essential insights into the mechanisms of rejection and strategies for prevention. Xenografts are a useful tool in tissue and organ failure and would save the lives of the patients that do not have the chance to find a compatible allograft donor. While the need for human organs is continuously increasing, researchers study the “humanization” of other species, mainly pigs, to try to solve the rejection problems with another useful tool (i.e. genetic engineering).
The key problem with xenografts is rejection caused by the recipient’s immune system. Hyperacute rejection occurs within minutes or hours from the transplant and is mediated by xenoreactive natural antibodies (XNAs). Acute vascular rejection occurs within two or three days, as an immune response of macrophages and Natural Killer (NK) cells . Cellular rejection is slower than the previous two types, and it is caused by the cellular immunity barrier, represented mainly by T cells and NK cells. Chronic rejection occurs very slowly and its cause may be arteriosclerosis. Xenograft studies are an important part of the development of novel therapeutics and remains the gold standard for oncology research. Xenograft studies are provided as a professional service by companies offering preclinical oncology research services (Altogen Labs). Xenograft studies provide unique data that are relevant to human care. When xenograft data is compared to cell-based in vitro studies, the in vivo xenograft studies provide a more accurate schema of both the development of a particular tumor as well as the efficacy of the drug.
In the preclinical phase development of new anti-cancer drugs and therapeutics, mouse models have been crucial to determining in vivo results for decades. Profiling a therapeutic agent can be accomplished through selecting the appropriate model, carrying out studies in different tumor types and evaluating efficacy with hematology analysis, histopathology, biomarker screening and pharmacokinetic/pharmacodynamic (PK/PD) analysis.
Xenograft services rely on the use of immunocompromised small animals (laboratory mice and rats). Severe combined immune deficient (SCID) mice and nude mice (nu) are effective as proxies for human subjects. The biologic scaffolds containing connective tissues and other proteins are used in surgical procedures, as well as in cancer research. According to their source, the biological scaffolds can be allografts, originated from a donor that belongs to the same species as the host, or xenografts, when the donor and the host do not belong to the same species. Although allografts are broadly used due to their biocompatibility with the host, the limited availability triggers extensive research for reducing the risk of rejection and for evaluating the risk of transferring viruses to the human population. The research on xenografts is extremely important due to availability. The scientific community is very optimistic regarding the success of genetic engineering “humanization” of other species.
Xenograft animal models
Human cancer causes are not fully understood, and an effective treatment for each of the 200 known cancer types needs extensive research. As the experimental therapeutics cannot be performed on humans, scientists utilize xenograft animal models in their studies. The tumor cells are transferred to immunocompromised or athymic nude mice, which allow the tumor development in several weeks, without a rejection caused by the immune system. The human tumors obtained this way can be used for testing new anticancer drugs and qualify them for human clinical trials, along with offering a better understanding of carcinogenesis.
Xenograft models are used in studies related to tumor development, signaling, inhibitors, novel therapeutics and drug interactions. These studies provided valuable knowledge to scientists and allowed the identification of some elements of carcinogenesis mechanism and the development of various drugs which can interfere with this process.
Cancer drugs were tested on mouse tumors starting in the 1950s, and in several decades it became evident that the studies were limited by the different response of human tumors and mouse tumors to similar treatments. By the mid-1980s, the xenotransplantation of human tumor cells in immunodeficient mice became possible.
The most widely used are the xenografts derived from the patient explant for predicting the response to a certain treatment, or the resistance to a certain drug. The xenograft cells are similar to the original tumor cells, and the tests proved a high reliability for some types of cancer. In such a research protocol, the subject of the study is a particular tumor similar to the tumor of a particular patient, and the developed treatment will be personalized.
The orthotopic human tumor xenografts are obtained by injecting the human tumor cells inside the similar organ in the immunodeficient mice. This technique provides more reliable results on human tumor response to a certain treatment. The researchers can qualify the treatment for human clinical trials if they obtain a 50% inhibition in the orthotopic human tumor as a response to that particular treatment and if regrowth of the tumor is not significant as compared to pre-treatment growth. The tumoral cells in these studies are not cultivated in Petri plates, and tumor stroma can be injected together with the tumor cells in order to obtain a reliable response as close as possible to the original tumor response.
The xenograft models used in oncology research are not perfect, but they provide useful information regarding tumor development and new treatments. They are used for crucial studies on tumor growth mechanism, carcinogenesis, tumor growth inhibitors, new treatments and for studying the interactions between various factors and drugs.
Research applications
The human tumor xenograft models are used for new drug discovery and development and are extremely useful tools in oncology. They can be used for general treatment research and for identifying the details of tumor growth and development. The xenograft models are also widely used for personalized cancer treatment set up and tests, as their cancerous cells are similar to those of the original tumor. The xenograft model techniques allow in vivo tests on human tumors, not on mouse tumors, which do not have the same response to a particular therapeutic or drug concentration.
Xenotransplantation is a very important technique. Due to low availability of human organs, research on xenografts and non-human primate models provides useful information regarding the rejection process and preventative measures. Xenografts are useful in cases of tissue and organ failure, offering a lifeline to patients unable to find compatible allograft donors. While the need for human organs is continuously increasing, researchers study the “humanization” of other species, mainly pigs, to try to solve rejection problems by incorporating another useful tool, i.e. genetic engineering.
History of xenograft cancer models
It was in 1894 that mice were first used for cancer research. At that time, mice were mostly employed for tumor transplantations within the same species as well as drug treatment studies. Later, the decision to test drug therapies on mice and rats was validated as research found that they possess a startling genetic similarity to humans. This, and the fact that they reproduce quite easily and quickly, have made mice the mammal of choice for genetic manipulation. Since then, scientists have bred many different types of mice, all with different features. All the inbreeding between the different types of genetically altered mice led to mice that were predisposed to getting cancerous tumors. The year was 1921, the inbred strains were disseminated among cancer researchers and a new era was ushered in.
A mutant mouse was discovered in 1962 as having very little immunity. A human tumor was transplanted into the mutant mouse and an important step forward for cancer research was taken. Another breakthrough in the early 1990’s led to the creation of transgenic mice. The genes in these mice could be altered to produce a distinctive feature or quality. This led the way to more detailed studies of oncogenes, i.e. genes that cause cancer.
Xenografts are an important part of creating personalized treatments. Today, scientists and doctors are only beginning to understand how to modify healthcare to a person’s exclusive genetic makeup. Altogen Labs xenograft services are an essential means helping basic science research to revolutionize personalized medicine.
How xenografts help find a cure for cancer
Our xenograft services use immunocompromised rodents as a model for human cancer. We have developed over 90 xenograft tumor models in immune-deficient mice, transgenic mice and conditional engineered mice to pinpoint the known tumor supporting pathways. A careful characterization of xenograft tissue and the mice lets us effectively determine the drug-target interaction. It also gives us the ability to test the forecasts regarding the pathway-targeted anti-tumor effectiveness of test compound. Tumor xenograft models have become an important tool in the fight against cancer.
The ethical implications of xenotransplantation are a significant aspect of ongoing research. Concerns include animal welfare, especially with regard to the genetic modification and use of pigs and primates, and the potential risks to human health, such as zoonotic infections. Ethical frameworks are being developed alongside scientific advances to ensure responsible research practices and societal acceptance of xenotransplantation therapies.
Recent advancements in genetic engineering, particularly through CRISPR-Cas9 technology, have made it possible to modify donor animals to reduce the likelihood of organ rejection. By knocking out specific genes that are responsible for hyperacute rejection, researchers aim to create ‘human-compatible’ organs that may one day be transplanted without triggering severe immune responses. These innovations are also exploring the addition of human proteins to animal tissues, potentially increasing compatibility and decreasing rejection.
Regulatory agencies such as the FDA and the EMA are closely monitoring xenotransplantation research to ensure its safety and effectiveness. Stringent guidelines focus on reducing risks of infection transmission, especially from porcine endogenous retroviruses (PERVs). Extensive safety protocols and long-term monitoring are required before xenotransplants can be considered for clinical application, ensuring that any potential public health risks are managed and minimized.
In addition to genetic modifications, researchers are investigating immunomodulatory strategies to improve xenograft survival. Immunosuppressive drugs, antibody treatments targeting specific immune pathways, and regulatory T-cell therapies are being explored to prevent both acute and chronic rejection. These therapies seek to create a balance between immunosuppression and immune function, reducing rejection while maintaining the host’s ability to fight infections.
Looking forward, the potential to use xenotransplantation to address the organ shortage crisis is promising. The goal of developing fully functional, human-compatible organs could transform transplant medicine, providing alternatives to those waiting for human donors. Continuous innovations in genetic engineering and immunology will likely drive this field forward, potentially making xenotransplants a viable option within the next decade. Additionally, advances in personalized medicine may tailor xenografts to individual patient needs, further optimizing outcomes.
While oncology research benefits greatly from xenograft models, their applications extend into other fields such as regenerative medicine, infectious disease research, and immunology. For instance, xenografts are being explored for use in liver failure studies, neurological disease research, and in modeling autoimmune disorders. This versatility highlights the broader impact of xenotransplantation research across multiple areas of human health.
Emerging technologies like organoid cultures, which are lab-grown organ-like structures derived from human cells, are complementing xenograft models. While xenografts provide a whole-organism context, organoids offer a controlled, human-specific environment for testing. Comparative studies are increasingly being conducted to understand how xenografts and organoids can be used together to provide comprehensive insights into cancer progression and drug response.
Long-term studies of xenotransplant recipients are crucial for understanding the prolonged effects and potential complications associated with xenotransplantation. Monitoring protocols are being developed to assess organ function, immune response, and the risk of zoonotic infections over time. These studies are essential for establishing safety profiles and understanding how xenotransplants integrate into human physiology.
Public perception and patient acceptance are also vital to the future of xenotransplantation. Research is increasingly recognizing the importance of patient and societal perspectives, as these can influence regulatory acceptance and healthcare policies. Education campaigns and transparent communication about the benefits, risks, and ethical considerations of xenotransplantation may help to foster acceptance of these emerging therapies.
References:
Xenotransplantation (2000). Nature Biotechnology, 18, IT53 – IT55 (2000) doi:10.1038/80100.
Scott, C.L., H.J. Mackay, and P. Haluska, Jr., Patient-derived xenograft models in gynecologic malignancies. Am Soc Clin Oncol Educ Book, 2014: p. e258-66.
Ramos, P. and M. Bentires-Alj, Mechanism-based cancer therapy: resistance to therapy, therapy for resistance. Oncogene, 2015. 34(28): p. 3617-26.
Leonard, S. M., Perry, T., & Kearns, P. (2014). Sequential Treatment with Cytarabine and Decitabine Has an Increased Anti-Leukemia Effect Compared to Cytarabine Alone in Xenograft Models of Childhood Acute Myeloid Leukemia.
Jardim-Perassi, B. V., Arbab, A. S., & Pires de Campo Zuccari, D. A. (2014). Effect of Melatonin on Tumor Growth and Angiogenesis in Xenograft Model of Breast Cancer.
Lu, Q. Y., Zhang, L, & Go, V. L. W. (2013). Determination of Rottlerin, a Natural Protein Kinases C Inhibitor, in Pancreatic Cancer Cells and Mouse Xenografts by RP-HPLC Method. Journal of Chromatography & Separation Technique. 2013 January; 4(1): 100062.