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DISEASE MODELS Modeling human lung infections in mice Respiratory pathogens and the human immune response they elicit can now be studied in humanized mice. Hergen Spits and Julien Villaudy

Human BLT-L: esearch on human pathogens is often lung tissue hampered by the lack of appropriate Middle East respiratory coronavirus Respiratory syncytial virus animal models that can recapitulate Zika virus R Human cytomegalovirus a normal infection cycle and pathogenesis. Mycobacterium bovis BCG Many human pathogens show preferential, often exclusive, tropism for human cells and do not replicate in experimental animals. 1 In this issue, Wahl et al. provide mouse HSPC models to study respiratory pathogens, which are the leading cause of death by BLT and HIS mice: Kidney HIV infectious disease globally and the fourth HuHEP: capsule Epstein–Barr virus most common cause of death overall. virus Human T- cell leukemia virus virus Fetal The mice carry both human fetal lung tissue and Dengue virus Salmonella Typhi and a humanized , allowing (+ human erythrocytes) the study of pathogen replication in human cells from the respiratory tract and of human immune responses to the pathogens in an animal model. They are especially suited to the investigation of pathogens Fig. 1 | Humanized mice to study human-tropic pathogens. Various human cell types have been targeting cells from the lower respiratory transplanted into immunodeficient mice to study diverse pathogens and diseases. Human immune tract, particularly pathogens for which no system (HIS) mice are engrafted with hematopoietic stem cells. –liver–thymic (BLT) relevant animal models exist, such as human mice are engrafted with hematopoietic stem cells, fetal liver and fetal thymus from the same donor. cytomegalovirus (HCMV), which replicates HuHEP mice are engrafted with human liver cells to study hepatotropic viruses and, when combined only in humans. with human erythrocytes, the replication cycle of the malaria parasite. BLT-L mice, now developed 1 Wahl et al.1 build on decades of work by Wahl et al. , are engrafted with lung cells, with or without autologous immune cells, to study on developing immunodeficient mice that respiratory viruses. allow engraftment of human immune cells and tissues (Fig. 1). Several mouse strains are available that support engraftment of by simply injecting the mice with human leukemia virus. To study pathogens that human umbilical , mobilized hematopoietic stem cells4–7. In these mice, infect non-hematopoietic cells, researchers blood, bone marrow and fetal tissues such as human T cells develop in the mouse can transplant human tissues along with liver and thymus2,3. Human hematopoietic thymus. Bone marrow–liver–thymic (BLT) hematopoietic cells. For example, mice stem cells are typically engrafted in strains mice8, a more complex model, require engrafted with human immune cells and that lack or have inactivating in transplantation of human fetal liver and human hepatocytes (HIS-HuHEP mice)10 genes required for rearrangement of thymus under the kidney capsule, followed have been used to study immune reactions and receptor genes (e.g., recombinase by injection of bone marrow hematopoietic against human liver trophic viruses like activating genes (Rag) or the DNA-dependent stem cells from the same donor. The hepatitis B virus11. serine/threonine protein kinase (Prkdc)) and presence of human liver (as a source The new study by Wahl et al.1 in the γ common chain of the of stem cells) and thymus in BLT mice substantially extends the number of (IL)-2 receptor (IL2R), which is critical supports more robust T cell development human pathogens that can be modeled for development of natural killer cells. than observed in HIS mice, whereas in humanized mice. The authors first All these strains lack mouse T cells, co-injection of hematopoietic stem cells transplant pieces of human fetal lung B cells and innate lymphoid cells, including optimizes development of B cells and other subcutaneously in the back of NSG natural killer cells. A frequently used hematopoietic cells. mice to create ‘human lung-only’ (LoM) strain, which is also used by Wahl et al.1, In both HIS and BLT mice, cells from mice. Using an elegant technology called is a non-obese diabetic–severe combined the different human hematopoietic lineages, acoustic angiography, they demonstrate the (NOD-SCID) strain with including B cells and T cells, are present in occurrence of active angiogenesis in and a Prkdc and an IL2Rγ deficiency, primary and secondary lymphoid organs, around the implants. Interestingly, although called NSG mice. as well as in peripheral blood2,3,9. Both the lung implants are ectopic and therefore These highly immunodeficient strains are excellent models for studying human- not ventilated, their structural features are serve to create two types of mice with specific pathogens that target hematopoietic highly similar to those of normal human a humanized immune system. Human cells, such as human immunodeficiency lung, including ciliated epithelium, alveolar immune system (HIS) mice are generated virus, Epstein–Barr virus and human T-cell structures, blood vessels and cartilage.

Nature Biotechnology | VOL 37 | OCTOBER 2019 | 1127–1130 | www.nature.com/naturebiotechnology 1129 news & views

Wahl et al.1 show that the grafts support The authors provide convincing evidence It will be important to analyze the robust replication of clinically relevant that the immune response to HCMV in B cell response in the BLT-L mice in detail. human respiratory pathogens that have no BLT-L mice is functional and can control If the mice indeed generate not or limited tropism to non-human cells. viral replication. Plasma levels of pro- and only class-switched but also affinity- These pathogens—including Middle anti-inflammatory cytokines including matured antibodies, their utility East respiratory coronavirus, Zika virus, interferon-γ, IL-6, – would be greatly enhanced. For instance, respiratory syncytial virus and HCMV— colony stimulating factor (GM-CSF) and this would allow testing of new vaccine represent a risk for human health globally. chemokines such as IL-8 and interferon-γ- strategies, antiviral drugs and antibiotics. The authors also study Mycobacterium bovis induced protein 10 (IP10) were increased It would also be possible to generate bacillus Calmette–Guerin, which is used to after the infection, indicative of an ongoing broadly reacting, highly neutralizing vaccinate against M. . Moreover, immune response. The adaptive immune antibodies against clinically relevant the authors measure the expression of viral response in these mice includes both antigen- pathogenic viruses. ❐ genes during lytic replication of HCMV specific, cytokine-producing T cells and in vivo for the first time. virus-specific antibodies. HCMV-reactive Hergen Spits 1* and Julien Villaudy2 Because LoM mice lack an immune immunoglobulin M is detected 14 days after 1Amsterdam Infection & Immunity Institute, system, pathogen replication is uncontrolled. primary infection. Class switching to HCMV- Department of Experimental Immunology, To mimic infection as it occurs in healthy specific immunoglobulin G is not observed Amsterdam UMC, Amsterdam, the Netherlands. humans, the authors continue their upon single exposure but is detected in nearly 2AIMM Terapeutics BV, Amsterdam, remarkable study by combining LoM and 75% of the mice re-exposed to HCMV. the Netherlands. BLT models to create mice with both a Wahl et al.1 did not investigate whether *e-mail: [email protected] lung transplant and an immune system, there is affinity maturation of the HCMV- called BLT-L mice. For this, NSG mice are specific IgGs. If the mice support somatic Published online: 10 September 2019 first transplanted with human fetal liver hypermutation, this would represent an https://doi.org/10.1038/s41587-019-0269-x and thymus under the kidney capsule, important advance because thus far high- followed by implantation of autologous lung affinity, class-switched, antigen-specific References tissue, and finally injection of autologous antibodies have rarely been produced in 1. Wahl, A. et al. Nat. Biotechnol. https://doi.org/10.1038/s41587- liver-derived hematopoietic stem humanized mouse models. There are several 019-0225-9 (2019). cells. possible reasons for this. One is that the host 2. Shultz, L. D., Brehm, M. A., Garcia-Martinez, J. V. & Greiner, D. L. The level of repopulation with human strains of HIS and BLT mice have only a few Nat. Rev. Immunol. 12, 786–798 (2012). 3. Rongvaux, A. et al. Annu. Rev. Immunol. 31, 635–674 (2013). hematopoietic cells outside the lung secondary lymph nodes owing to a lack of 4. Gimeno, R. et al. Blood 104, 3886–3893 (2004). transplants is as robust in BLT-L as in BLT IL2Ry-dependent lymphoid tissue inducer 5. Traggiai, E. et al. Science 304, 104–107 (2004). mice. The lung grafts also have a substantial cells. Another is that there is a suboptimal 6. Ito, M. et al. Blood 100, 3175–3182 (2002). 7. Shultz, L. D. et al. J. Immunol. 174, 6477–6489 (2005). population of human immune cells; whether interaction of B cells and T cells caused by a 8. Melkus, M. W. et al. Nat. Med. 12, 1316–1322 (2006). this population was present in the lung lack of cross-reactivity on the part of critical 9. Ito, R., Takahashi, T., Katano, I. & Ito, M. Cell. Mol. Immunol. 9, grafts before implantation or whether it cytokines involved in this interaction. Also, 208–214 (2012). 10. Strick-Marchand, H. et al. PLoS One 10, e0119820 (2015). derives from the injected hematopoietic cells non-hematopoietic cells important for B cell 11. Dusséaux, M. et al. Gastroenterology 153, 1647–1661.e9 (2017). is unclear, but its cell composition—notably, maturation are of mouse origin and do not the relative proportions of CD4 T cells, CD8 appropriately interact with maturing B cells. Competing interests T cells and tissue-resident memory T cells— This may also be a reason for the lack of J.V. is an employee of and stockholder in AIMM closely recapitulates that of hematopoietic structured germinal centers in immunized Therapeutics. H.S. owns stock in AIMM Therapeutics and cell populations in human lungs. HIS and BLT mice. consults for GlaxoSmithKline.

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