Ectopic germline recombination activity of the widely used Foxp3-YFP-Cre mouse: a case report

Iron capture through CD71 drives perinatal and tumor-associated Treg expansion

Besides suppressing immune responses, regulatory T cells (Tregs) maintain tissue homeostasis and control systemic metabolism. Whether iron is involved in Treg-mediated tolerance is completely unknown. Here, we showed that the transferrin receptor CD71 was upregulated on activated Tregs infiltrating human liver cancer. Mice with a Treg-restricted CD71 deficiency spontaneously developed a scurfy-like disease, caused by impaired perinatal Treg expansion. CD71-null Tregs displayed decreased proliferation and tissue-Treg signature loss. In perinatal life, CD71 deficiency in Tregs triggered hepatic iron overload response, characterized by increased hepcidin transcription and iron accumulation in macrophages. Lower bacterial diversity, and reduction of beneficial species, were detected in the fecal microbiota of CD71 conditional knockout neonates. Our findings indicate that CD71-mediated iron absorption is required for Treg perinatal expansion and is related to systemic iron homeostasis and bacterial gut colonization. Therefore, we hypothesize that Tregs establish nutritional tolerance through competition for iron during bacterial colonization after birth.

High frequency of germline recombination in Nestin-Cre transgenic mice crossed with Glucagon-like peptide 1 receptor floxed mice

The Cre-loxP strategy for tissue-specific gene inactivation has become a widely employed tool in several research studies. Conversely, inadequate breeding and genotyping without considering the potential for non-specific Cre-recombinase expression may lead to misinterpretations of results. Nestin-Cre transgenic mice, widely used for the selective deletion of genes in neurons, have been observed to have an incidence of Cre-line germline recombination. In this study, we attempted to generate neuron-specific Glucagon-like peptide 1 receptor (Glp1r) knock-out mice by crossing mice harboring the Nestin-Cre transgene with mice harboring the Glp1r gene modified with loxP insertion, in order to elucidate the role of Glp1r signaling in the nervous system. Surprisingly, during this breeding process, we discovered that the null allele emerged in the offspring irrespective of the presence or absence of the Nestin-Cre transgene, with a high probability of occurrence (93.6%). To elucidate the cause of this null allele, we conducted breeding experiments between mice carrying the heterozygous Glp1r null allele but lacking the Nestin-Cre transgene. We confirmed that the null allele was inherited by the offspring independently of the Nestin-Cre transgene. Furthermore, we assessed the gene expression, protein expression, and phenotype of mice carrying the homozygous Glp1r null allele generated from the aforementioned breeding, thereby confirming that the null allele indeed caused a global knock-out of Glp1r. These findings suggest that the null allele in the NestinCre-Glp1r floxed breeding arose due to germline recombination. Moreover, we demonstrated the possibility that germline recombination may occur not only during the spermatogenesis at testis but also during epididymal sperm maturation. The striking frequency of germline recombination in the Nestin-Cre driver underscores the necessity for caution when implementing precise breeding strategies and employing suitable genotyping methods.

The Atoh1-Cre Knock-In Allele Ectopically Labels a Subpopulation of Amacrine Cells and Bipolar Cells in Mouse Retina

The retina has diverse neuronal cell types derived from a common pool of retinal progenitors. Many molecular drivers, mostly transcription factors, have been identified to promote different cell fates. In Drosophila, atonal is required for specifying photoreceptors. In mice, there are two closely related atonal homologs, Atoh1 and Atoh7 While Atoh7 is known to promote the genesis of retinal ganglion cells, there is no study on the function of Atoh1 in retinal development. Here, we crossed Atoh1Cre/+ mice to mice carrying a Cre-dependent TdTomato reporter to track potential Atoh1-lineage neurons in retinas. We characterized a heterogeneous group of TdTomato+ retinal neurons that were detected at the postnatal stage, including glutamatergic amacrine cells, AII amacrine cells, and BC3b bipolar cells. Unexpectedly, we did not observe TdTomato+ retinal neurons in the mice with an Atoh1-FlpO knock-in allele and a Flp-dependent TdTomato reporter, suggesting Atoh1 is not expressed in the mouse retina. Consistent with these data, conditional removal of Atoh1 in the retina did not cause any observable phenotypes. Importantly, we did not detect Atoh1 expression in the retina at multiple ages using mice with Atoh1-GFP knock-in allele. Therefore, we conclude that Atoh1Cre/+ mice have ectopic Cre expression in the retina and that Atoh1 is not required for retinal development.

A role of Achaete-scute complex homolog 2 in T follicular regulatory cell development

T follicular regulatory (Tfr) cells, a subset of CD4⁺ Foxp3⁺ regulatory T (Treg) cells, locate to the lymphoid follicle and germinal center (GC) and regulate antibody responses. Tfr cells express the functional molecules of follicular helper T (Tfh) cells, including CXCR5 and Bcl6. CD25^- mature Tfr cells differentiate from CD25⁺ Treg cells through CD25⁺ immature Tfr cells. Others and we have shown that Achaete-scute complex homolog 2 (Ascl2) plays a role in Tfh cell development; however, the role of Ascl2 in the development of Tfr cells remains unclear. Here, we found that Ascl2 was highly and preferentially expressed in CD25⁺ Tfr cells and CD25^- Tfr cells, and that the differentiation from CD25⁺ Tfr cells to CD25^- Tfr cells was impaired by the absence of Ascl2. Furthermore, the forced Ascl2 expression in Treg cells downregulated CD25 expression and suppressed IL-2-induced phosphorylation of STAT5, which is known to suppress CD25^- Tfr cell development. Finally, we found that the downregulation of CD25 by Ascl2 in Treg cells is independent of Bach2, which also regulates CD25 downregulation in CD25⁺ Tfr cells. These results suggest that Ascl2 plays a vital role in developing Tfr cells, possibly by downregulating CD25 expression in a Bach2-independent mechanism.

Glucocorticoid signaling and regulatory T cells cooperate to maintain the hair-follicle stem-cell niche

Maintenance of tissue homeostasis is dependent on the communication between stem cells and supporting cells in the same niche. Regulatory T cells (Treg cells) are emerging as a critical component of the stem-cell niche for supporting their differentiation. How Treg cells sense dynamic signals in this microenvironment and communicate with stem cells is mostly unknown. In the present study, by using hair follicles (HFs) to study Treg cell-stem cell crosstalk, we show an unrecognized function of the steroid hormone glucocorticoid in instructing skin-resident Treg cells to facilitate HF stem-cell (HFSC) activation and HF regeneration. Ablation of the glucocorticoid receptor (GR) in Treg cells blocks hair regeneration without affecting immune homeostasis. Mechanistically, GR and Foxp3 cooperate in Treg cells to induce transforming growth factor β3 (TGF-β3), which activates Smad2/3 in HFSCs and facilitates HFSC proliferation. The present study identifies crosstalk between Treg cells and HFSCs mediated by the GR-TGF-β3 axis, highlighting a possible means of manipulating Treg cells to support tissue regeneration.

A Cre-driven allele-conditioning line to interrogate CD4+ conventional T cells

CD4⁺ T cells share common developmental pathways with CD8⁺ T cells, and upon maturation, CD4⁺ T conventional T (Tconv) cells lack phenotypic markers that distinguish these cells from FoxP3⁺ T regulatory cells. We developed a tamoxifen-inducible ThPOK^CreERT2.hCD2 line with Frt sites inserted on either side of the CreERT2-hCD2 cassette, and a Foxp3^{Ametrine-FlpO} strain, expressing Ametrine and FlpO in Foxp3⁺ cells. Breeding these mice resulted in a CD4conv^{iCreERT2-hCD2} line that allows for the specific manipulation of a gene in CD4⁺ Tconv cells. As FlpO removes the CreERT2-hCD2 cassette, CD4⁺ Treg cells are spared from Cre activity, which we refer to as allele conditioning. Comparison with an E8I^iCreERT2.GFP mouse that enables inducible targeting of CD8⁺ T cells, and deletion of two inhibitory receptors, PD-1 and LAG-3, in a melanoma model, support the fidelity of these lines. These engineered mouse strains present a resource for the temporal manipulation of genes in CD4⁺ T cells and CD4⁺ Tconv cells.

Piezo1 channels restrain regulatory T cells but are dispensable for effector CD4+ T cell responses

T lymphocytes encounter complex mechanical cues during an immune response. The mechanosensitive ion channel, Piezo1, drives inflammatory responses to bacterial infections, wound healing, and cancer; however, its role in helper T cell function remains unclear. In an animal model for multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), we found that mice with genetic deletion of Piezo1 in T cells showed diminished disease severity. Unexpectedly, Piezo1 was not essential for lymph node homing, interstitial motility, Ca²⁺ signaling, T cell proliferation, or differentiation into proinflammatory T helper 1 (T_H1) and T_H17 subsets. However, Piezo1 deletion in T cells resulted in enhanced transforming growth factor-β (TGFβ) signaling and an expanded pool of regulatory T (T_reg) cells. Moreover, mice with deletion of Piezo1 specifically in T_reg cells showed significant attenuation of EAE. Our results indicate that Piezo1 selectively restrains T_reg cells, without influencing activation events or effector T cell functions.

Droplet digital PCR or quantitative PCR for in-depth genomic and functional validation of genetically altered rodents

Gene targeting and additive (random) transgenesis have proven to be powerful technologies with which to decipher the mammalian genome. With the advent of CRISPR/Cas9 genome editing, the ability to inactivate or modify the function of a gene has become even more accessible. However, the impact of each generated modification may be different from what was initially desired. Minimal validation of mutant alleles from genetically altered (GA) rodents remains essential to guarantee the interpretation of experimental results. The protocol described here combines design strategies for genomic and functional validation of genetically modified alleles with droplet digital PCR (ddPCR) or quantitative PCR (qPCR) for target DNA or mRNA quantification. In-depth analysis of the results obtained with GA models through the analysis of target DNA and mRNA quantification is also provided, to evaluate which pitfalls can be detected using these two methods, and we propose recommendations for the characterization of different type of mutant allele (knock-out, knock-in, conditional knock-out, FLEx, IKMC model or transgenic). Our results also highlight the possibility that mRNA expression of any mutated allele can be different from what might be expected in theory or according to common assumptions. For example, mRNA analyses on knock-out lines showed that nonsense-mediated mRNA decay is generally not achieved with a critical-exon approach. Likewise, comparison of multiple conditional lines crossed with the same CreER^T2 deleter showed that the inactivation outcome was very different for each conditional model. DNA quantification by ddPCR of G0 to G2 generations of transgenic rodents generated by pronuclear injection showed an unexpected variability, demonstrating that G1 generation rodents cannot be considered as established lines.

T reg-specific insulin receptor deletion prevents diet-induced and age-associated metabolic syndrome

Adipose tissue (AT) regulatory T cells (T regs) control inflammation and metabolism. Diet-induced obesity causes hyperinsulinemia and diminishes visceral AT (VAT) T reg number and function, but whether these two phenomena were mechanistically linked was unknown. Using a T reg–specific insulin receptor (Insr) deletion model, we found that diet-induced T reg dysfunction is driven by T reg–intrinsic insulin signaling. Compared with Foxp3^cre mice, after 13 wk of high-fat diet, Foxp3^creInsr^fl/fl mice exhibited improved glucose tolerance and insulin sensitivity, effects associated with lower AT inflammation and increased numbers of ST2⁺ T regs in brown AT, but not VAT. Similarly, Foxp3^creInsr^fl/fl mice were protected from the metabolic effects of aging, but surprisingly had reduced VAT T regs and increased VAT inflammation compared with Foxp3^cre mice. Thus, in both diet- and aging-associated hyperinsulinemia, excessive Insr signaling in T regs leads to undesirable metabolic outcomes. Ablation of Insr signaling in T regs represents a novel approach to mitigate the detrimental effects of hyperinsulinemia on immunoregulation of metabolic syndrome.

A retrospective view of 2020: A unique year for Immunology

2020 was a year unlike any other for Immunology. Through the SARS-CoV-2 pandemic, with fantastic support from the global immunology community, we worked together to reach new heights. Here, we look back at some of the highlights for Immunology in a challenging and memorable year.

Tracing goes viral: Viruses that introduce expression of fluorescent proteins in chemically-specific neurons

Over the last century, there has been great progress in understanding how the brain works. In particular, the last two decades have been crucial in gaining more awareness over the complex functioning of neurotransmitter systems. The use of viral vectors in neuroscience has been pivotal for such development. Exploiting the properties of viral particles, modifying them according to the research needs, and making them target chemically-specific neurons, techniques such as optogenetics and chemogenetics have been developed, which could lead to a giant step toward gene therapy for brain disorders. In this review, we aim to provide an overview of some of the most widely used viral techniques in neuroscience. We will discuss advantages and disadvantages of these methods. In particular, attention is dedicated to the pivotal role played by the introduction of adeno-associated virus and the retrograde tracer canine-associated-2 Cre virus in order to achieve optimal visualization, and interrogation, of chemically-specific neuronal populations and their projections.

Transcriptional Control of Regulatory T Cells in Cancer: Toward Therapeutic Targeting?

Extensive research in the past decades has highlighted the tight link between immunity and cancer, leading to the development of immunotherapies that have revolutionized cancer care. However, only a fraction of patients display durable responses to these treatments, and a deeper understanding of the cellular and mechanisms orchestrating immune responses to tumors is mandatory for the discovery of novel therapeutic targets. Among the most scrutinized immune cells, Forkhead Box Protein P3 (Foxp3)+ Regulatory T cells (Treg cells) are central inhibitors of protective anti-tumor immunity. These tumor-promoting functions render Treg cells attractive immunotherapy targets, and multiple strategies are being developed to inhibit their recruitment, survival, and function in the tumor microenvironment. In this context, it is critical to decipher the complex and multi-layered molecular mechanisms that shape and stabilize the Treg cell transcriptome. Here, we provide a global view of the transcription factors, and their upstream signaling pathways, involved in the programming of Treg cell homeostasis and functions in cancer. We also evaluate the feasibility and safety of novel therapeutic approaches aiming at targeting specific transcriptional regulators.

BACH2 drives quiescence and maintenance of resting Treg cells to promote homeostasis and cancer immunosuppression

Regulatory T (Treg) cell populations are composed of functionally quiescent resting Treg (rTreg) cells which differentiate into activated Treg (aTreg) cells upon antigen stimulation. How rTreg cells remain quiescent despite chronic exposure to cognate self- and foreign antigens is unclear. The transcription factor BACH2 is critical for early Treg lineage specification, but its function following lineage commitment is unresolved. Here, we show that BACH2 is repurposed following Treg lineage commitment and promotes the quiescence and long-term maintenance of rTreg cells. Bach2 is highly expressed in rTreg cells but is down-regulated in aTreg cells and during inflammation. In rTreg cells, BACH2 binds to enhancers of genes involved in aTreg differentiation and represses their TCR-driven induction by competing with AP-1 factors for DNA binding. This function promotes rTreg cell quiescence and long-term maintenance and is required for immune homeostasis and durable immunosuppression in cancer. Thus, BACH2 supports a "division of labor" between quiescent rTreg cells and their activated progeny in Treg maintenance and function, respectively.

Should we be more cre-tical? A cautionary tale of recombination

The cre-loxP system has been revolutionary in the field of immunology. The technology enables genetic deletion in mice with unprecedented precision. It is therefore now widely used to investigate gene functions in animal models of disease, and in fundamental studies of the immune response. This widespread adoption of cre-loxP technology has allowed a thorough investigation of its strengths and weaknesses. Here we highlight an important paper which not only describes potential problems with the commonly-used screening procedures used when identifying offspring of the correct genotype, but also describes how this screening can be improved to ensure that the right animals are produced.