Activation kinetics and off-target effects of thymus-initiated cre transgenes

Methods to study tumor surveillance using tumor cell transplantation into genetically engineered mice

When a tumor evolves, there is constant crosstalk between the transformed cells and cells of the immune system. Transplantation of well-established tumor cell lines into genetically engineered mice is a valuable tool to study the contribution of a gene of interest to tumor surveillance. These methods bear several advantages: first, such cell lines are well characterized; second, much data for reference exist; and third, the impact of the immune system can be separated from tumor cell intrinsic effects. Here, we provide protocols for tumor cell transplantations to address the role of a specific gene product in tumor surveillance. We furthermore describe several approaches to define the impact of natural killer cells and T cells, such as cell depletion and adoptive transfer experiments or use of different genetically modified mice.

An Lck-cre transgene accelerates autoantibody production and lupus development in (NZB × NZW)F1 mice

Lupus is an autoimmune disease characterized by the development of antinuclear autoantibodies and immune complex-mediated tissue damage. T cells in lupus patients appear to undergo apoptosis at an increased rate, and this enhanced T cell apoptosis has been postulated to contribute to lupus pathogenesis by increasing autoantigen load. However, there is no direct evidence to support this hypothesis. In this study, we show that an Lck-cre transgene, which increases T cell apoptosis as a result of T cell-specific expression of cre recombinase, accelerates the development of autoantibodies and nephritis in lupus-prone (NZB × NZW)F1 mice. Although the enhanced T cell apoptosis in Lck-cre transgenic mice resulted in an overall decrease in the relative abundance of splenic CD4(+) and CD8(+) T cells, the proportion of activated CD4(+) T cells was increased and no significant change was observed in the relative abundance of suppressive T cells. We postulate that the Lck-cre transgene promoted lupus by enhancing T cell apoptosis, which, in conjunction with the impaired clearance of apoptotic cells in lupus-prone mice, increased the nuclear antigen load and accelerated the development of anti-nuclear autoantibodies. Furthermore, our results also underscore the importance of including cre-only controls in studies using the cre-lox system.

B Cell-Intrinsic Expression of the HuR RNA-Binding Protein Is Required for the T Cell-Dependent Immune Response In Vivo

The HuR RNA-binding protein posttranscriptionally controls expression of genes involved in cellular survival, proliferation, and differentiation. To determine roles of HuR in B cell development and function, we analyzed mice with B lineage-specific deletion of the HuR gene. These HuRΔ/Δ mice have reduced numbers of immature bone marrow and mature splenic B cells, with only the former rescued by p53 inactivation, indicating that HuR supports B lineage cells through developmental stage-specific mechanisms. Upon in vitro activation, HuRΔ/Δ B cells have a mild proliferation defect and impaired ability to produce mRNAs that encode IgH chains of secreted Abs, but no deficiencies in survival, isotype switching, or expression of germinal center (GC) markers. In contrast, HuRΔ/Δ mice have minimal serum titers of all Ab isotypes, decreased numbers of GC and plasma B cells, and few peritoneal B-1 B cells. Moreover, HuRΔ/Δ mice have severely decreased GCs, T follicular helper cells, and high-affinity Abs after immunization with a T cell-dependent Ag. This failure of HuRΔ/Δ mice to mount a T cell-dependent Ab response contrasts with the ability of HuRΔ/Δ B cells to become GC-like in vitro, indicating that HuR is essential for aspects of B cell activation unique to the in vivo environment. Consistent with this notion, we find in vitro stimulated HuRΔ/Δ B cells exhibit modestly reduced surface expression of costimulatory molecules whose expression is similarly decreased in humans with common variable immunodeficiency. HuRΔ/Δ mice provide a model to identify B cell-intrinsic factors that promote T cell-dependent immune responses in vivo.

Dual Function of the IRF8 Transcription Factor in Autoimmune Uveitis: Loss of IRF8 in T Cells Exacerbates Uveitis, Whereas Irf8 Deletion in the Retina Confers Protection

IFN regulatory factor 8 (IRF8) is constitutively expressed in monocytes and B cells and plays a critical role in the functional maturation of microglia cells. It is induced in T cells following Ag stimulation, but its functions are less well understood. However, recent studies in mice with T cell-specific Irf8 disruption under direction of the Lck promoter (LCK-IRF8KO) suggest that IRF8 directs a silencing program for Th17 differentiation, and IL-17 production is markedly increased in IRF8-deficient T cells. Paradoxically, loss of IRF8 in T cells has no effect on the development or severity of experimental autoimmune encephalomyelitis (EAE), although exacerbating colitis in a mouse colitis model. In contrast, mice with a macrophage/microglia-specific Irf8 disruption are resistant to EAE, further confounding our understanding of the roles of IRF8 in host immunity and autoimmunity. To clarify the role of IRF8 in autoimmune diseases, we have generated two mouse strains with targeted deletion of Irf8 in retinal cells, including microglial cells and a third mouse strain with targeted Irf8 deletion in T cells under direction of the nonpromiscuous, CD4 promoter (CD4-IRF8KO). In contrast to the report that IRF8 deletion in T cells has no effect on EAE, experimental autoimmune uveitis is exacerbated in CD4-IRF8KO mice and disease enhancement correlates with significant expansion of Th17 cells and a reduction in T regulatory cells. In contrast to CD4-IRF8KO mice, Irf8 deletion in retinal cells confers protection from uveitis, underscoring divergent and tissue-specific roles of IRF8 in host immunity. These results raise a cautionary note in the context of therapeutic targeting of IRF8.

MAZR and Runx Factors Synergistically Repress ThPOK during CD8+ T Cell Lineage Development

Th-inducing Pox virus and zinc finger/Krüppel-like factor (ThPOK) is a key commitment factor for CD4(+) lineage T cells and is essential for the maintenance of CD4 lineage integrity; thus, the expression of ThPOK has to be tightly controlled. In this article, we demonstrate that Myc-associated zinc finger-related factor (MAZR) and Runt-related transcription factor 1 (Runx1) together repressed ThPOK in preselection double-positive thymocytes, whereas MAZR acted in synergy with Runx3 in the repression of ThPOK in CD8(+) T cells. Furthermore, MAZR-Runx1 and MAZR-Runx3 double-mutant mice showed enhanced derepression of Cd4 in double-negative thymocytes and in CD8(+) T cells in comparison with Runx1 or Runx3 single-deficient mice, respectively, indicating that MAZR modulates Cd4 silencing. Thus, our data demonstrate developmental stage-specific synergistic activities between MAZR and Runx/core-binding factor β (CBFβ) complexes. Finally, retroviral Cre-mediated conditional deletion of MAZR in peripheral CD8(+) T cells led to the derepression of ThPOK, thus showing that MAZR is also part of the molecular machinery that maintains a repressed state of ThPOK in CD8(+) T cells.

TRAF2 regulates peripheral CD8(+) T-cell and NKT-cell homeostasis by modulating sensitivity to IL-15

In this study, a critical and novel role for TNF receptor (TNFR) associated factor 2 (TRAF2) is elucidated for peripheral CD8(+) T-cell and NKT-cell homeostasis. Mice deficient in TRAF2 only in their T cells (TRAF2TKO) show ∼40% reduction in effector memory and ∼50% reduction in naïve CD8(+) T-cell subsets. IL-15-dependent populations were reduced further, as TRAF2TKO mice displayed a marked ∼70% reduction in central memory CD8(+) CD44(hi) CD122(+) T cells and ∼80% decrease in NKT cells. TRAF2TKO CD8(+) CD44(hi) T cells exhibited impaired dose-dependent proliferation to exogenous IL-15. In contrast, TRAF2TKO CD8(+) T cells proliferated normally to anti-CD3 and TRAF2TKO CD8(+) CD44(hi) T cells exhibited normal proliferation to exogenous IL-2. TRAF2TKO CD8(+) T cells expressed normal levels of IL-15-associated receptors and possessed functional IL-15-mediated STAT5 phosphorylation, however TRAF2 deletion caused increased AKT activation. Loss of CD8(+) CD44(hi) CD122(+) and NKT cells was mechanistically linked to an inability to respond to IL-15. The reduced CD8(+) CD44(hi) CD122(+) T-cell and NKT-cell populations in TRAF2TKO mice were rescued in the presence of high dose IL-15 by IL-15/IL-15Rα complex administration. These studies demonstrate a critical role for TRAF2 in the maintenance of peripheral CD8(+) CD44(hi) CD122(+) T-cell and NKT-cell homeostasis by modulating sensitivity to T-cell intrinsic growth factors such as IL-15.

hCD2-iCre and Vav-iCre mediated gene recombination patterns in murine hematopoietic cells

Cre-recombinase mediated conditional deletion of Lox-P site flanked ("floxed") genes is widely used for functional gene annotation in mice. Many different Cre-transgenic mouse lines have been developed for cell-type specific gene disruption. But often, the precise tissue-patterns of Cre activity remain incompletely characterized. Two widely used transgenes for conditional gene recombination in hematopoietic cells are Vav-iCre driven from the murine Vav1 promotor, and hCD2-iCre driven from the human CD2 promotor. Vav-iCre expresses active Cre in fetal and adult hematopoietic stem cells and all descendants, hCD2-iCre in immature and mature B and T lymphocytes. To better characterize which hematopoietic cells contain hCD2-iCre activity, we compared EYFP fluorescence in hCD2-iCre+/- R26-stop-EYFP+/- and Vav-iCre+/- R26-stop-EYFP+/-mice. R26-stop-EYFP ubiquitously encodes EYFP preceded by a floxed stop cassette. By removing it, Cre activity induces measurable EYFP expression. Our results confirm the known activity patterns for both Cre transgenes and unveil additional hCD2-iCre mediated reporter gene recombination in common lymphoid progenitors, in natural killer cells and their progenitors, and in plasmacytoid and conventional dendritic cells. This supports previously proposed common lymphoid origins for natural killer cells and subsets of dendritic cells, and indicates the need to consider pleiotropic effects when studying hCD2-iCre mediated conditional knockout mice. Vav-iCre+/- R26-stop-EYFP+/-mice did not show the non-hematopoietic recombination in vascular endothelial cells seen in other Vav-Cre mouse lines, but displayed an unexpected Vav-iCre mediated recombination in a bone cell subset lacking hematopoietic markers. This pinpoints the need to consider stromal cell contributions to phenotypes of Vav-iCre mediated conditional knockout mice. Altogether, our data provide the first detailed assessment of hCD2-iCre and Vav-iCre mediated deletion of floxed genes during lymphocyte development from hematopoietic stem cells and open up novel applications for either Cre-transgenic mouse line.

ShcA regulates late stages of T cell development and peripheral CD4+ T cell numbers

T cell development in the thymus is a highly regulated process that critically depends upon productive signaling via the preTCR at the β-selection stage, as well as via the TCR for selection from the CD4(+)CD8(+) double-positive stage to the CD4 or CD8 single-positive stage. ShcA is an adapter protein expressed in thymocytes, and it is required for productive signaling through the preTCR, with impaired signaling via ShcA leading to a developmental block at the β-selection checkpoint. However, the role of ShcA in subsequent stages of T cell development has not been addressed. In this study, we generated transgenic mice (CD4-Cre/ShcFFF mice) that specifically express a phosphorylation-defective dominant-negative ShcA mutant (ShcFFF) in late T cell development. Thymocytes in CD4-Cre/ShcFFF mice progressed normally through the β-selection checkpoint, but displayed a significant reduction in the numbers of single-positive CD4(+) and CD8(+) thymocytes. Furthermore, CD4-Cre/ShcFFF mice, when bred with transgenic TCR mouse strains, had impaired signaling through the transgenic TCRs. Consistent with defective progression to the single-positive stage, CD4-Cre/ShcFFF mice also had significant peripheral lymphopenia. Moreover, these CD4-Cre/ShcFFF mice develop attenuated disease in CD4(+) T cell-dependent experimental autoimmune encephalomyelitis, a mouse model of multiple sclerosis. Collectively, these data identify an important role for the adapter protein ShcA in later stages of thymic T cell development and in peripheral T cell-dependent events.

Deletion of IFT20 in early stage T lymphocyte differentiation inhibits the development of collagen-induced arthritis

IFT20 is the smallest member of the intraflagellar transport protein (IFT) complex B. It is involved in cilia formation. Studies of IFT20 have been confined to ciliated cells. Recently, IFT20 was found to be also expressed in non-ciliated T cells and have functions in immune synapse formation and signaling in vitro. However, how IFT20 regulates T-cell development and activation in vivo is still unknown. We deleted the IFT20 gene in early and later stages of T-cell development by crossing IFT20(flox/flox) (IFT20(f/f) ) mice with Lck-Cre and CD4-Cre transgenic mice, and investigated the role of IFT20 in T-cell maturation and in the development of T cell-mediated collagen-induced arthritis (CIA). We found that both Lck-Cre/IFT20(f/f) and CD4-Cre/IFT20(f/f) mice were indistinguishable from their wild-type littermates in body size, as well as in the morphology and weight of the spleen and thymus. However, the number of CD4- and CD8-positive cells was significantly lower in thymus and spleen in Lck-Cre/IFT20(f/f) mice. Meanwhile, the incidence and severity of CIA symptoms were significantly decreased, and inflammation in the paw was significantly inhibited in Lck-Cre/IFT20(f/f) mice compared to Lck-Cre/IFT20(+/+) littermates. Deletion IFT20 in more mature T cells of CD4-Cre/IFT20(f/f) mice had only mild effects on the development of T cells and CIA. The expression of IL-1β, IL-6 and TGF-β1 were significantly downregulated in the paw of Lck-Cre/IFT20(f/f) mice, but just slight decreased in CD4-Cre/IFT20(f/f) mice. These results demonstrate that deletion of IFT20 in the early stage of T-cell development inhibited CIA development through regulating T-cell development and the expression of critical cytokines.

Cutting edge: CXCR4 is critical for CD8+ memory T cell homeostatic self-renewal but not rechallenge self-renewal

Central memory (CM) CD8(+) T cells "remember" prior encounters because they maintain themselves through cell division in the absence of ongoing challenge (homeostatic self-renewal), as well as reproduce the CM fate while manufacturing effector cells during secondary Ag encounters (rechallenge self-renewal). We tested the consequence of conditional deletion of the bone marrow homing receptor CXCR4 on antiviral T cell responses. CXCR4-deficient CD8(+) T cells have impaired memory cell maintenance due to defective homeostatic proliferation. Upon rechallenge, however, CXCR4-deficient T cells can re-expand and renew the CM pool while producing secondary effector cells. The critical bone marrow-derived signals essential for CD8(+) T cell homeostatic self-renewal appear to be dispensable to yield self-renewing, functionally asymmetric cell fates during rechallenge.

Conditional ablation of TYK2 in immunity to viral infection and tumor surveillance

Tyrosine kinase 2 (TYK2) has a pivotal role in immunity to infection and tumor surveillance. It is associated with several cytokine receptor chains including type I interferon (IFN) receptor 1 (IFNAR1), interleukin- (IL-) 12 receptor beta 1 (IL-12Rb1) and IL-10R2. We have generated a mouse with a conditional Tyk2 null allele and proved integrity of the conditional Tyk2 locus. TYK2 was successfully removed by the use of ubiquitous and tissue-specific Cre-expressing mouse strains. Myeloid TYK2 was found to critically contribute to the defense against murine cytomegalovirus. Ubiquitous TYK2 ablation severely impaired tumor immunosurveillance, while deletion in myeloid, dendritic or T cells alone showed no effect. The conditional Tyk2 mouse strain will be instrumental to further dissect TYK2 functions in infection, inflammation and cancer.

Hydrodynamic delivery of Cre protein to lineage-mark or time-stamp mouse hepatocytes in situ

Cre-responsive fluorescent marker alleles are powerful tools for cell lineage tracing in mice; however their utility is limited by regulation of Cre activity. When targeting hepatocytes, hydrodynamic delivery of a Cre-expression plasmid can convert Cre-responsive alleles without inducing the intracellular or systemic antiviral responses often associated with viral-derived Cre-expression vectors. In this method, rapid high-volume intravenous inoculation induces hepatocyte-targeted uptake of extracellular molecules. Here we tested whether hydrodynamic delivery of Cre protein or Cre fused to the HIV-TAT cell-penetrating peptide could convert Cre-responsive reporters in hepatocytes of mice. Hydrodynamic delivery of 2 nmol of either Cre or TAT-Cre protein converted the reporter allele in 5 to 20% of hepatocytes. Neither protein gave detectable Cre activity in endothelia, non-liver organs, or non-hepatocyte cells in liver. Using mice homozygous for a Cre-responsive marker that directs red- (Cre-naïve) or green- (Cre-converted) fluorescent proteins to the nucleus, we assessed sub-saturation Cre-activity. One month after hydrodynamic inoculation with Cre protein, 58% of hepatocyte nuclei that were green were also red, indicating that less than half of the hepatocytes that had obtained enough Cre to convert one marker allele to green were able to convert all alleles. For comparison, one month after hydrodynamic delivery of a Cre-expression plasmid with a weak promoter, only 26% of the green nuclei were also red. Our results show that hydrodynamic delivery of Cre protein allows rapid allelic conversion in hepatocytes, but Cre-activity is sub-saturating so many cells will not convert multiple Cre-responsive alleles.

Signal transducer and activator of transcription 3 (Stat3) contributes to T-cell homeostasis by regulating pro-survival Bcl-2 family genes

The naive T-cell pool in peripheral lymphoid tissues is fairly stable in terms of number, diversity and functional capabilities in spite of the absence of prominent stimuli. This stability is attributed to continuous tuning of the composition of the T-cell pool by various homeostatic signals. Despite extensive research into the link between signal transducer and activator of transcription 3 (Stat3) and T-cell survival, little is known about how Stat3 regulates homeostasis by maintaining the required naive T-cell population in peripheral lymphoid organs. We assessed whether the elimination of Stat3 in T cells limits T-cell survival. We demonstrated that the proportion and number of single-positive thymocytes as well as T cells in the spleen and lymph nodes were significantly decreased in the Stat3-deficient group as a result of the enhanced susceptibility of Stat3-deleted T lymphocytes to apoptosis. Importantly, expression of the anti-apoptotic Bcl-2 and Bcl-xL was markedly decreased in Stat3-deleted single-positive thymocytes and T lymphocytes, suggesting that Stat3 helps to maintain the T-cell pool in the resting condition by promoting the expression of Bcl-2 family genes. These findings suggest the importance of Stat3 in the integration of homeostatic cues for the maintenance and functional tuning of the T-cell pool.

Haematopoietic differentiation is inhibited when Notch activity is enhanced in FLK1(+) mesoderm progenitors

Notch signalling has been implicated during haematopoietic development in vivo and in the differentiation of haematopoietic cells from pluripotent cells in vitro. However interpretation of data from many of these studies has been complicated by the heterogeneous nature of cell populations under study and by the fact that the Notch pathway is active during embryogenesis prior to the development of the haematopoietic system. To define the role of Notch signalling in more precise cell populations during the early stages of haematopoietic development within the aorta-gonad-mesonephros (AGM) microenvironment we co-cultured differentiating ESCs on a stromal cell line derived from this region of the embryo. Our co-culture system had no effect on the production of FLK1(+) mesoderm progenitor cells but promoted their subsequent haematopoietic differentiation. We assessed the role of Notch signalling on haematopoietic differentiation of isolated FLK1(+) cells. Notch activity is dynamic and drops to basal levels as FLK1(+) cells commit to a haematopoietic fate. Further reduction of Notch activity by the inducible expression of dominant negative MAML had no functional consequences. In contrast, induction of Notch activity using an inducible NotchIC expression system had an inhibitory effect on haematopoietic differentiation. We used a Cre-mediated recombination strategy whereby NotchIC-expressing cells were marked with the hCD2 receptor and observed a reduction in the number of multi-lineage and myeloid colonies derived from NotchIC(+) compared to NotchIC(-) FLK1(+) cells isolated from the same culture. We believe that our culture system represents a good model for haematopoietic development within the AGM microenvironment and our data suggest that haematopoietic commitment of FLK1(+) cells in this setting occurs when Notch activity is below a specific threshold.

In vivo fate mapping identifies pre-TCRα expression as an intra- and extrathymic, but not prethymic, marker of T lymphopoiesis

A novel pre-TCRα (pTα) reporter mouse reveals that expression of pTα is confined to the T lineage and does not occur on prethymic progenitors.

Expression of the pre–T cell receptor α (pTα) gene has been exploited in previous studies as a molecular marker to identify tiny cell populations in bone marrow (BM) and blood that were suggested to contain physiologically relevant thymus settling progenitors (TSPs). But to what extent these cells genuinely contribute to thymopoiesis has remained obscure. We have generated a novel pTα^iCre knockin mouse line and performed lineage-tracing experiments to precisely quantitate the contribution of pTα-expressing progenitors to distinct differentiation pathways and to the genealogy of mature hematopoietic cells under physiological in vivo conditions. Using these mice in combination with fluorescent reporter strains, we observe highly consistent labeling patterns that identify pTα expression as a faithful molecular marker of T lineage commitment. Specifically, the fate of pTα-expressing progenitors was found to include all αβ and most γδ T cells but, in contrast to previous assumptions, to exclude B, NK, and thymic dendritic cells. Although we could detect small numbers of T cell progenitors with a history of pTα expression in BM and blood, our data clearly exclude these populations as physiologically important precursors of thymopoiesis and indicate that they instead belong to a pathway of T cell maturation previously defined as extrathymic.