We describe a novel method for generating double humanized BLT-mice that feature a functional human immune system and a stable engrafted human-like gut microbiome. This protocol can be followed without the need for germ-free mice or gnotobiotic facilities.
Humanized mice (hu-mice) that feature a functional human immune system have fundamentally changed the study of human pathogens and disease. They can be used to model diseases that are otherwise difficult or impossible to study in humans or other animal models. The gut microbiome can have a profound impact on human health and disease. However, the murine gut microbiome is very different than the one found in humans. There is a need for improved pre-clinical hu-mice models that have an engrafted human gut microbiome. Therefore, we created double hu-mice that feature both a human immune system and stable human-like gut microbiome. NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) mice are one of the best animals for humanization due to their high level of immunodeficiency. However, germ-free NSG mice, and various other important germ-free mice models are not currently commercially available. Further, many research settings do not have access to gnotobiotic facilities, and working under gnotobiotic conditions can often be expensive and time consuming. Importantly, germ-free mice have several immune deficiencies that exist even after the engraftment of microbes. Therefore, we developed a protocol that does not require germ-free animals or gnotobiotic facilities. To generate double hu-mice, NSG mice were treated with radiation prior to surgery to create bone-marrow, liver, thymus-humanized (hu-BLT) mice. The mice were then treated with broad spectrum antibiotics to deplete the pre-existing murine gut microbiome. After antibiotic treatment, the mice were given fecal transplants with healthy human donor samples via oral gavage. Double hu-BLT mice had unique 16S rRNA gene profiles based on the individual human donor sample that was transplanted. Importantly, the transplanted human-like microbiome was stable in the double hu-BLT mice for the duration of the study up to 14.5 weeks post-transplant.
Humanized mice (hu-mice) have transformed the study of many aspects of human health and disease including hematopoiesis, immunity, cancer, autoimmune disease, and infectious disease1,2,3,4,5,6,7,8,9. These hu-mice have the distinct advantage over other mouse models in that they have a functional human immune system and can be infected with human specific pathogens. Nevertheless, the importance of the gut microbiome has been demonstrated by its role in many human diseases such as obesity, metabolic syndrome, inflammatory diseases, and cancer10,11,12,13. The mucosal immune system and gut microbiome are reciprocally regulated to maintain gut and systemic homeostasis. The immune system is shaped by antigens presented by the gut microbiome and reciprocally the immune system plays an important regulatory role in promoting commensal gut bacteria and eliminating pathogens14,15,16. However, the gut microbiome of hu-mice has not been well characterized and the murine gut microbiome differs substantially in composition and function from humans17. This is due to evolutionary, physiological, and anatomical differences between the murine and human gut as well as other important factors such as diet, which may influence the experimental results of hu-mice disease models18. Therefore, beyond classification of murine gut microbiome of hu-mice, an animal model featuring both a human immune system and human gut microbiome is needed to study the complex interactions of human disease in vivo.
The study of human diseases directly in human subjects is often impractical or unethical. Many animal models cannot be used to study human pathogens like human immunodeficiency virus type 1 (HIV-1). Non-human primate models are genetically outbred, very expensive, and are not susceptible to many human pathogens. Mice that have been derived as germ free (GF) and reconstituted with human-like gut microbiomes have been widely used to study human health and disease19,20. However, these animals do not have a human immune system and working with GF animals requires specialized facilities, procedures, and expertise. Therefore, there is a need for improved pre-clinical models to study the complex relationship of the gut microbiome and the human immune system. Many strains of mice, such as NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG), are not commercially available as GF. GF animals also may suffer from long-lasting immune deficiencies that are not completely reversed by the engraftment of microbes21. Therefore, we created a double hu-mice featuring both a functional human immune system and stable human-like gut microbiome under specific pathogen free (SPF) conditions. To generate double hu-mice, surgery was performed on NSG mice to create bone-marrow, liver, thymus humanized mice (hu-BLT). The hu-BLT mice were then treated with broad spectrum antibiotics and then given fecal transplants with a healthy human donor sample. We characterized the bacterial gut microbiome of 173 fecal samples from 45 double hu-BLT mice and 4 human fecal donor samples. Double hu-BLT mice have unique 16S rRNA gene profiles based on the individual human donor sample that is transplanted. Importantly, the transplanted human-like microbiome was stable in the mice for the duration of the study up to 14.5 weeks post-transplant. In addition, the predicted metagenomes showed that double hu-BLT mice have different predicted functional capacity than hu-mice that is more similar to the human donor samples.
All methods described here were conducted in accordance with Institutional Animal Care and Research Committee (IACUC)-approved protocols at the University of Nebraska-Lincoln (UNL). The IACUC at UNL has approved two protocols related to generating and using hu-BLT mice, including double hu-mice. Additionally, the Scientific Research Oversight Committee (SROC) at UNL has also approved the use of human embryonic stem cells and fetal tissues, which are procured from the Advanced Bioscience Resources for humanized mice studies (SROC# 2016-1-002).
1. Mice housing and maintenance
2. Generation of humanized BLT mice
NOTE: Generation of hu-BLT mice has been described previously22,23,24.
3. Antibiotic treatment
4. Donor samples and fecal transplant
5. Fresh fecal sample collection
Figure 1 shows an outline of the methods used to create double hu-BLT mice and briefly describes the process of adding a functional human immune system and stable human-like gut microbiome to the NSG mice. Figure 2 shows an example of flow cytometry analysis of peripheral blood from a humanized BLT-mouse 10 weeks post-surgery. Figure 3 shows the relative abundance of the human fecal donor samples used to transfer a gut microbiome to create double hu-mice. Figure 4 shows the phenotypic changes induced by antibiotic treatment to the spleen and cecum, similar to what is observed in germ-free animals. Figure 5 shows a principal component analysis (PCA) plot of the 16S rRNA sequencing data revealing double hu-mice have human-like gut microbiomes that are unique to the human donor sample.
Figure 1: Creating double humanized BLT-mice. Creating double hu-BLT mice is a two-step process. The first step is to engraft the human immune system to the NSG mice. On the day of surgery, NSG mice are given irradiation to create a niche for stem cells. The mice are then implanted with human fetal liver and thymus tissues and injected with human hematopoietic stem cells. Human immune cell reconstitution is checked around 10 weeks post-surgery. The second step is to engraft the human gut microbiome. Mice are treated with antibiotics to reduce the pre-existing murine gut bacteria. Mice are then given fecal transplants to provide the human gut microbiome. Please click here to view a larger version of this figure.
Figure 2: Testing human immune cell reconstitution in double humanized BLT-mice. An example of flow cytometry analysis of a humanized BLT-mouse peripheral blood 10 weeks post-surgery. The figure shows the gating strategy used to identify the lymphocyte population, mCD45- hCD45+ cells, CD19+ B cells, CD3+ T cells, CD4+ T cells, and CD8+ T cells. Please click here to view a larger version of this figure.
Figure 3: Human donor fecal sample profiles. Relative abundance of the 3 human donor and mixed (all donors) samples shown at the family level. Please click here to view a larger version of this figure.
Figure 4: Antibiotic treated mice resemble germ-free phenotypes. Hu-mice were sacrificed after 9 days of antibiotic treatment (Antibiotics) or no antibiotic treatment (Control). After antibiotic treatment, the phenotype of the humanized mice begins to resemble those seen in germ-free animals. As a result of antibiotic treatment there is a reduction in the size of the spleen (left) and the cecum is enlarged (right). Please click here to view a larger version of this figure.
Figure 5: Double humanized BLT-mice feature fecal donor specific gut microbiomes. PCA plot of 16S rRNA sequencing data show after human fecal transplant the double hu-BLT mice feature gut microbiomes that are unique to the individual human fecal donor. Please click here to view a larger version of this figure.
The protocol described here is for the creation of double hu-BLT mice that feature both a functional human immune system and a stable human-like gut microbiome. This protocol can be adapted to other humanized or non-humanized mice models without the need for GF animals and gnotobiotic facilities. While the methods described here are relatively simple, there are several critical details that are important for the successful creation of double hu-BLT mice. NSG mice are extremely immunodeficient and preventing infections is key to long-term survival of the mice. We took following measures to prevent infection. First, animals were housed in individual microisolator cages with HEPA filters (0.22 μm) in a rack system with air exchange rate management in a dedicated suite. The air handler for the rack contained pre-filters along with HEPA filtered (0.22 μm) supply and exhaust air, as well as real-time on-line monitoring of cage exhaust air temperature and relative humidity. Second, everyone who entered the animal room had to shower and wear clean scrubs and shoes as well as put on gloves, disposable tyvek suit, booties, hair bonnet, and face mask. Third, all procedures, including cage changes and addition of food and water, were performed within a Class II Type A2 biological safety fume hood that was pre-sterilized with 70% ethanol and UV light. Fourth, aseptic surgical technique was used during survival surgery, which included the surgeon and assistants wearing an additional layer of protection including a surgery gown and gloves. Surgery was conducted in the disinfected fume hood, using only sterile instruments, gauzes, and wound closure materials, while maintaining the sterility of gloves and instruments throughout the surgery. Finally, to prevent infection and ensure the stability of the engrafted human-like gut microbiome, all food and water given to the mice was sterile. All food should be irradiated and all water should be autoclaved. To minimize pain and distress, we administered long-acting buprenorphine subcutaneously to mice before surgery. The combination of Ketamine and Xylazine for mouse surgical anesthesia is very reliable and can last about for 30 min. If that is not long-enough, we give isoflurane gas to further anesthetize the mice. It is also very important to maintain mouse body temperature post-surgery. We put the cage on the heated warming pad until the hematopoietic stem cell injection via the tail vein. At that time, the mice are recovered from anesthesia and returned to rack.
To deplete the murine gut microbiome and prepare for human fecal transplant, it is important to always use freshly prepared antibiotics and to change antibiotic supplemented water and cages daily. This will use many microisolator cages throughout the 14-day antibiotic treatment, but it ensures the mice are not being re-inoculated through coprophagia. During antibiotic treatment, it is also critical to monitor the body weight and health of the mice. After fecal transplant, the mice quickly regain any lost weight. It is important to minimize any freeze-thaw cycles for the fecal transplant material and to make sure to use an anaerobic chamber if aliquoting samples is needed. While creating double hu-BLT mice it is important to minimize handling and stress induced on the mice. This helps to prevent infection and improves long-term survival.
We initially tried to pre-treat mice with anti-fungal amphotericin B but found the mice did not tolerate the treatment very well and it is no longer used. We also experimented with different durations of antibiotic treatment. We found that while a majority of murine gut bacteria appear to be depleted after 7 days of antibiotics, the level of donor engraftment is much higher after 14 days of treatment. We also tried administering antibiotics through twice-daily oral gavage. However, we found that this method was too invasive for our hu-mice. We switched to a single daily gavage schedule but the mice still appeared to be stressed and unhealthy. We found that providing antibiotics in the drinking water was the best method. It reduced the amount handling and stress to the mice while still adequately reducing the murine gut bacteria. We provided grape sugar sweetened drink mix in the drinking water to ensure the mice received an adequate dosage of antibiotic and to prevent dehydration. The mice do experience a reduction in body weight during the first 3-4 days of antibiotic treatment, but providing extra fluids via intraperitoneal injection does not increase body weight. After fecal transplant, the mice quickly regain the lost weight.
While this method is able to reproducibly generate double hu-BLT mice, there are some limitations to the model. The first thing to consider is hu-mice have less organized lymphoid structure, including the germinal center, leading to reduced antibody class switching and limited affinity maturation. However, NSG hu-BLT mice have systemic immune reconstitution and translatable T cell responses and can be used to model many human diseases. Another issue is the potential development of graft-versus-host disease (GVHD) in some hu-mice after several months of excessive human immune reconstitution. We and others have observed GVHD manifestations such as blepharitis, alopecia, weight loss, and malocclusion that must be carefully monitored25.
There are several documented regimens for depleting gut bacteria in mice with antibiotics26,27,28,29,30,31. We chose our cocktail of antibiotics due to their known capacity to target a broad range of bacteria in the gut and because we found several examples of successful bacterial depletion in the literature. Many published cases use a far less rigorous course of antibiotics but in our study, we found that 14 days is needed for optimal engraftment of a human-like gut microbiome. While we initially tried a protocol based on Hintze et al., we found that oral gavage was too invasive and that anti-fungal treatment was detrimental to mice26. We believe that NSG hu-BLT mice are unique and less invasive procedures are preferred compared to other more robust mice. We did not use GF animals in our study. The use of GF mice to study the effects of the gut microbiome have been well-documented, however, these animals do not have a human immune system19,32. Further, we admit that working with a GF NSG hu-BLT mice model would create interesting opportunities for research. For one, studying human immune cell reconstitution and the pathogenesis of human specific pathogens like HIV-1 without the presence of the gut microbiome could provide interesting results. Further, GF models may allow for a more complete reconstitution of a human-like gut microbiome following fecal transplant. However, GF mice have long-lasting immune deficiencies, even after gut microbiome reconstitution21. Our model has the advantage of using SPF housing conditions, which are widely available and less expensive compared to GF facilities. Our model also has the advantage of not perturbing the normal procedures of surgically generating hu-BLT mice because there is no need for a completely GF environment.
We believe that this double hu-BLT mouse model is unique in that it not only can be used to study human immune function and human diseases, but also determine the impact of the gut microbiome on disease pathogenesis and treatment in vivo. With this protocol, we can reproducibly create double hu-BLT mice with human donor specific gut microbiome profiles. Therefore, we believe that using double hu-BLT mice will be beneficial to future personalized medicine applications designed to test the impact of the gut microbiome on treatments for various human diseases like HIV-1 and cancer. In summary, our double hu-BLT mice model is an important and novel pre-clinical model that features both a functional human immune system and a stable human-like gut microbiome to study human health and disease.
The authors have nothing to disclose.
We would like to thank Yanmin Wan, Guobin Kang, and Pallabi Kundu for their assistance in generating BLT-humanized mice. We would like to acknowledge the UNMC Genomics Core Facility who receives partial support from the Nebraska Research Network In Functional Genomics NE-INBRE P20GM103427-14, The Molecular Biology of Neurosensory Systems CoBRE P30GM110768, The Fred & Pamela Buffett Cancer Center – P30CA036727, The Center for Root and Rhizobiome Innovation (CRRI) 36-5150-2085-20, and the Nebraska Research Initiative. We would like to thank University of Nebraska – Lincoln Life Sciences Annex and their staff for their assistance. This study is supported in part by the National Institutes of Health (NIH) Grants R01AI124804, R21AI122377-01, P30 MH062261-16A1 Chronic HIV Infection and Aging in NeuroAIDS (CHAIN) Center, 1R01AI111862 to Q Li. The funders had no role in study design, data collection and analysis, preparation of the manuscript or decision for publication.
Animal Feeding Needles 18G | Cadence Science | 9928B | |
Clidox-s Activator | Pharmacal Research Laboratories | 95120F | |
Clidox-s Base | Pharmacal Research Laboratories | 96125F | |
DGM 108 cage rack | Techniplast | ||
Flat Brown Grocery Bag 3-5/8"D x 6"W x 11-1/16"L | Grainger | 12R063 | |
FMT Upper Delivery Microbiota Preparations | OpenBiome | FMP30 | |
Grape Kool-Aid | Kraft Foods Inc. | ||
hCD19-PE/Cy5 | Biolegend | 302209 | |
hCD3-PE | Biolegend | 300408 | |
hCD4-Alexa 700 | Biolegend | 300526 | |
hCD45-FITC | Biolegend | 304006 | |
hCD8-APC/Cy7 | Biolegend | 301016 | |
Lactate Buffered Ringer's Solution | Boston BioProducts Inc | PY-906-500 | |
mCD45-APC | Biolegend | 103111 | |
Microvette 100 K3E | Microvette | 20.1278.100 | |
Neosporin First Aid Antibiotic/Pain Relieving Ointment | Neosporin | ||
NSG mice (NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ) | The Jackson Laboratory | 005557 | |
PrecisionGlide 25 G Needle | BD | 305127 | |
RS200 X-ray irradiator | RAD Source Technologies | ||
Sealsafe Plus GM500 microisolator cages | Techniplast | ||
Sterile Non-woven Gauze | Fisherbrand | 22-028-558 | |
Teklad global 16% protein irradiated mouse chow | Teklad | 2916 |