The Specificity of Conditional Gene Targeting: A Case for Cre Reporters

Variegate expression of Cre recombinase in hematopoietic cells in CD11c-cre transgenic mice

In this study, we performed an in-depth analysis of Cre expression in the widely used CD11c-Cre transgenic mice generated by the group of Boris Reizis. In contrast to previous observation, using the highly sensitive Rosa-26-floxed-tdTomato reporter mouse line, we show variegated expression of Cre in multiple hematopoietic linage cells starting in hematopoietic stem cells. Indeed, we found that in the CD11c-Cre driver mice, Cre is expressed in cDC linage cells and pDC starting from the myeloid dendritic cell precursor, as expected, but also in a substantial fraction of hematopoietic stem cells and common lymphoid progenitors and, consequently, in >50% of all leukocytes. Hence, this study indicates that the reporter mice used to characterize Cre expression in Cre-driver mice should be selected with caution and considering the sensitivity of the reporter system. This study also suggests that the interpretation of some reports using this CD11c-Cre transgenic mice may need to be re-considered based on a careful evaluation of the cell type-specificity of Cre-mediated in their model.

Reassessing endothelial-to-mesenchymal transition in mouse bone marrow: insights from lineage tracing models

Endothelial cells (ECs) and bone marrow stromal cells (BMSCs) play crucial roles in supporting hematopoiesis and hematopoietic regeneration. However, whether ECs are a source of BMSCs remains unclear. Here, we evaluate the contribution of endothelial-to-mesenchymal transition to BMSC generation in postnatal mice. Single-cell RNA sequencing identifies ECs expressing BMSC markers Prrx1 and Lepr; however, this could not be validated using Prrx1-Cre and Lepr-Cre transgenic mice. Additionally, only a minority of BMSCs are marked by EC lineage tracing models using Cdh5-rtTA-tetO-Cre or Tek-CreERT2. Moreover, Cdh5+ BMSCs and Tek+ BMSCs show distinct spatial distributions and characteristic mesenchymal markers, suggestive of their origination from different progenitors rather than CDH5+ TEK+ ECs. Furthermore, myeloablation induced by 5-fluorouracil treatment does not increase Cdh5+ BMSCs. Our findings indicate that ECs hardly convert to BMSCs during homeostasis and myeloablation-induced hematopoietic regeneration, highlighting the importance of using appropriate genetic models and conducting careful data interpretation in studies concerning endothelial-to-mesenchymal transition.

Pattern of tamoxifen-induced Tie2 deletion in endothelial cells in mature blood vessels using endo SCL-Cre-ERT transgenic mice

Tyrosine-protein kinase receptor Tie2, also known as Tunica interna Endothelial cell Kinase or TEK plays a prominent role in endothelial responses to angiogenic and inflammatory stimuli. Here we generated a novel inducible Tie2 knockout mouse model, which targets mature (micro)vascular endothelium, enabling the study of the organ-specific contribution of Tie2 to these responses. Mice with floxed Tie2 exon 9 alleles (Tie2floxed/floxed) were crossed with end-SCL-Cre-ERT transgenic mice, generating offspring in which Tie2 exon 9 is deleted in the endothelial compartment upon tamoxifen-induced activation of Cre-recombinase (Tie2ΔE9). Successful deletion of Tie2 exon 9 in kidney, lung, heart, aorta, and liver, was accompanied by a heterogeneous, organ-dependent reduction in Tie2 mRNA and protein expression. Microvascular compartment-specific reduction in Tie2 mRNA and protein occurred in arterioles of all studied organs, in renal glomeruli, and in lung capillaries. In kidney, lung, and heart, reduced Tie2 expression was accompanied by a reduction in Tie1 mRNA expression. The heterogeneous, organ- and microvascular compartment-dependent knockout pattern of Tie2 in the Tie2floxed/floxed;end-SCL-Cre-ERT mouse model suggests that future studies using similar knockout strategies should include a meticulous analysis of the knockout extent of the gene of interest, prior to studying its role in pathological conditions, so that proper conclusions can be drawn.

Dendritic cell functions in vivo: a user's guide to current and next generation mutant mouse models

Dendritic cells (DCs) do not just excel in antigen presentation. They orchestrate information transfer from innate to adaptive immunity, by sensing and integrating a variety of danger signals, and translating them to naïve T cells, to mount specifically tailored immune responses. This is accomplished by distinct DC types specialized in different functions and because each DC is functionally plastic, assuming different activation states depending on the input signals received. Mouse models hold the key to untangle this complexity and determine which DC types and activation states contribute to which functions. Here, we aim to provide comprehensive information for selecting the most appropriate mutant mouse strains to address specific research questions on DCs, considering three in vivo experimental approaches: (i) interrogating the roles of DC types through their depletion; (ii) determining the underlying mechanisms by specific genetic manipulations; (iii) deciphering the spatiotemporal dynamics of DC responses. We summarize the advantages, caveats, suggested use and perspectives for a variety of mutant mouse strains, discussing in more detail the most widely used or accurate models. Finally, we discuss innovative strategies to improve targeting specificity, for the next generation mutant mouse models, and briefly address how humanized mouse models can accelerate translation into the clinic. This article is protected by copyright. All rights reserved.

Optimization of tamoxifen-induced Cre activity and its effect on immune cell populations

Tamoxifen (TAM) inducible Cre recombinase system is an essential tool to study gene function when early ablation or overexpression can cause developmental defects or embryonic lethality. However, there remains a lack of consensus on the optimal route and dosage of TAM administration in vivo. Here, we assessed dosage and delivery of TAM for activation of Cre in immune cell subsets assessed longitudinally and spatially using transgenic mice with ubiquitously expressed Cre/ER and the Cre-inducible fluorescent reporter YFP. After comparing two TAM delivery methods (intraperitoneal versus oral gavage) and different doses, we found that 3 mg of TAM administered orally for five consecutive days provides maximal reporter induction with minimal adverse effects in vivo. Serum levels of TAM peaked 1 week after initiating treatment then slowly decreased, regardless of dosing and delivery methods. TAM concentration in specific tissues (liver, spleen, lymph nodes, and thymus) was also dependent on delivery method and dose. Cre induction was highest in myeloid cells and B cells and substantially lower in T cells, and double-positive thymocytes had a notably higher response to TAM. In addition to establishing optimal dose and administration of TAM, our study reveals a disparate activity of Cre in different cell immune populations when using Cre/ER models.

STOP floxing around: Specificity and leakiness of inducible Cre/loxP systems

Cre and CreER mouse strains are powerful tools that have proven invaluable for investigating the function of genes and for the fate-mapping of cell populations. The fidelity of these systems however are becoming more and more contested. In this issue of the European Journal of Immunology, Van Hove et al. and Chappell-Maor et al. carefully dissect the cellular specificities of two commonly used CreER mouse strains for the study of CNS macrophages; Cx3cr1^CreER and Sall1^CreER . Both studies elegantly highlight that CreER strains, as well as the "floxed" allele to be targeted, need to be carefully selected and properly characterized in order to ensure reproducible and robust data and interpretations. These studies are a cautionary tale for this technology, but also highlight that we must continuously question and improve our experimental approaches.

Selective reconstitution of IFN‑γ gene function in Ncr1+ NK cells is sufficient to control systemic vaccinia virus infection

IFN-γ is an enigmatic cytokine that shows direct anti-viral effects, confers upregulation of MHC-II and other components relevant for antigen presentation, and that adjusts the composition and balance of complex cytokine responses. It is produced during immune responses by innate as well as adaptive immune cells and can critically affect the course and outcome of infectious diseases, autoimmunity, and cancer. To selectively analyze the function of innate immune cell-derived IFN-γ, we generated conditional IFN-γ^OFF mice, in which endogenous IFN-γ expression is disrupted by a loxP flanked gene trap cassette inserted into the first intron of the IFN-γ gene. IFN-γ^OFF mice were intercrossed with Ncr1-Cre or CD4-Cre mice that express Cre mainly in NK cells (IFN-γ^Ncr1-ON mice) or T cells (IFN-γ^CD4-ON mice), respectively. Rosa26RFP reporter mice intercrossed with Ncr1-Cre mice showed selective RFP expression in more than 80% of the NK cells, while upon intercrossing with CD4-Cre mice abundant RFP expression was detected in T cells, but also to a minor extent in other immune cell subsets. Previous studies showed that IFN-γ expression is needed to promote survival of vaccinia virus (VACV) infection. Interestingly, during VACV infection of wild type and IFN-γ^CD4-ON mice two waves of serum IFN-γ were induced that peaked on day 1 and day 3/4 after infection. Similarly, VACV infected IFN-γ^Ncr1-ON mice mounted two waves of IFN-γ responses, of which the first one was moderately and the second one profoundly reduced when compared with WT mice. Furthermore, IFN-γ^Ncr1-ON as well as IFN-γ^CD4-ON mice survived VACV infection, whereas IFN-γ^OFF mice did not. As expected, ex vivo analysis of splenocytes derived from VACV infected IFN-γ^Ncr1-ON mice showed IFN-γ expression in NK cells, but not T cells, whereas IFN-γ^OFF mice showed IFN-γ expression neither in NK cells nor T cells. VACV infected IFN-γ^Ncr1-ON mice mounted normal cytokine responses, restored neutrophil accumulation, and showed normal myeloid cell distribution in blood and spleen. Additionally, in these mice normal MHC-II expression was detected on peripheral macrophages, whereas IFN-γ^OFF mice did not show MHC-II expression on such cells. In conclusion, upon VACV infection Ncr1 positive cells including NK cells mount two waves of early IFN-γ responses that are sufficient to promote the induction of protective anti-viral immunity.

Viral infections induce interferon (IFN) responses that constitute a first line of defense. Type II IFN (IFN-γ) is required for protection against lethal vaccinia virus (VACV) infection. To address the cellular origin of protective IFN-γ responses during VACV infection, we generated IFN-γ^OFF mice, in which the endogenous IFN-γ gene function can be reconstituted in a Cre-dependent manner. IFN-γ^OFF mice were intercrossed with Ncr1-Cre mice that express Cre selectively in Ncr1⁺ innate cell subsests such as NK cells. Surprisingly, VACV infected IFN-γ^Ncr1-ON mice mounted two waves of IFN-γ responses. Reconstitution of innate IFN-γ was sufficient to restore cytokine responses that supported normal myeloid cell distribution and survival upon VACV infection. In conclusion, IFN-γ derived from Ncr1⁺ innate immune cells is sufficient to elicit fully effective immune responses upon VACV infection. Our new mouse model is suitable to further address the role of Ncr1⁺ cell-derived IFN-γ also in other models of infection, as well as of autoimmunity and cancer.