From murine to human nude/SCID: the thymus, T-cell development and the missing link

Foxn1 overexpression promotes thymic epithelial progenitor cell proliferation and mTEC maintenance, but does not prevent thymic involution

The transcription factor FOXN1 is essential for fetal thymic epithelial cell (TEC) differentiation and proliferation. Postnatally, Foxn1 levels vary widely between TEC subsets, from low/undetectable in putative TEC progenitors to highest in differentiated TEC subsets. Correct Foxn1 expression is required to maintain the postnatal microenvironment; premature downregulation of Foxn1 causes a rapid involution-like phenotype, and transgenic overexpression can cause thymic hyperplasia and/or delayed involution. We investigated a K5.Foxn1 transgene that drives overexpression in mouse TECs, but causes neither hyperplasia nor delay or prevention of aging-related involution. Similarly, this transgene cannot rescue thymus size in Foxn1lacZ/lacZ mice, which undergo premature involution as a result of reduced Foxn1 levels. However, TEC differentiation and cortico-medullary organization are maintained with aging in both K5.Foxn1 and Foxn1lacZ/lacZ mice. Analysis of candidate TEC markers showed co-expression of progenitor and differentiation markers as well as increased proliferation in Plet1+ TECs associated with Foxn1 expression. These results demonstrate that the functions of FOXN1 in promoting TEC proliferation and differentiation are separable and context dependent, and suggest that modulating Foxn1 levels can regulate the balance of proliferation and differentiation in TEC progenitors.

Immuno-PET Imaging of Siglec-15 Using the Zirconium-89-Labeled Therapeutic Antibody, NC318

Objective. Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) is overexpressed in various cancers which has led to the development of therapeutic anti-Siglec-15 monoconal antibodies (mAbs). In these preclinical studies, the therapeutic mAb, NC318 (antihuman/murine Siglec-15 mAb), was labeled with zirconium-89 and evaluated in human Siglec-15 expressing cancer cells and mouse xenografts for potential use as a clinical diagnostic imaging agent. Methods. Desferrioxamine-conjugated NC318 was radiolabeled with zirconium-89 to synthesize [89Zr]Zr-DFO-NC318. Cancer cell lines expressing variable Siglec-15 levels were used for in vitro cell binding studies and tumor xenograft mouse models for biodistributions. [89Zr]Zr-DFO-NC318 biodistribution and PET imaging studies to determine tissue uptakes (tissue : muscle ratios, T : M) included pharmacokinetic evaluation in Siglec-15+tumor xenografts and immunocompetent mice, blocking with nonradioactive NC318 (20, 100, and 300 μg) and xenografts with low/negligible Siglec-15 expressing tumors. Results. [89Zr]Zr-DFO-NC318 exhibited high affinity ( $K_d$ ~4 nM) for Siglec-15 and distinguished between moderate and negligible Siglec-15 expression levels in cancer cell lines. The highest [89Zr]Zr-DFO-NC318 uptakes occurred in the spleen and lymph nodes of the Siglec-15+tumor xenografts at all time points followed by Siglec-15+tumor uptake which was lower although highly retained. In immunocompetent mice, the spleen and lymph nodes exhibited lower uptakes indicating that the athymic xenografts had increased Siglec-15+ immune cells. Specific [89Zr]Zr-DFO-NC318 binding to Siglec-15 was proven with NC318 blocking studies in which dose-dependent decreases in Siglec-15+tumor T : Ms were observed. Higher than expected, tumor T : Ms were seen in lower expressing tumors likely due to the contribution of murine Siglec-15+ immune cells in the tumor microenvironment as confirmed by immunohistochemistry. Siglec-15+tumors were identified on PET images whereas low/negligible expressing tumors showed lower uptakes. Conclusions. In vitro and in vivo [89Zr]Zr-DFO-NC318 uptakes correlated with Siglec-15 expression levels in target tissues. Despite uptake in immune cell subsets in the tumor microenvironment, these results suggest that clinical [89Zr]Zr-DFO-NC318 PET imaging may have value in selecting patients for Siglec-15-targeted therapies.

Engineered osteoclasts resorb necrotic alveolar bone in anti-RANKL antibody-treated mice

Medication-related osteonecrosis of the jaw (MRONJ) is a serious side effect of antiresorptive medications such as denosumab (humanized anti-RANKL antibody), yet its pathophysiology remains elusive. It has been posited that inhibition of osteoclastic bone resorption leads to the pathological sequelae of dead bone accumulation, impaired new bone formation, and poor wound healing in MRONJ, but this hypothesis has not been definitively tested. We previously engineered myeloid precursors with a conditional receptor activator of nuclear factor kappa-Β intracellular domain (iRANK cells), which differentiate into osteoclasts in response to a chemical inducer of dimerization (CID) independently of RANKL. In this study, we showed that CID-treated iRANK cells differentiated into osteoclasts and robustly resorbed mineralized surfaces even in the presence of anti-RANKL antibody in vitro. We then developed a tooth extraction-triggered MRONJ model in nude mice using anti-RANKL antibody to deplete osteoclasts. This model was used to determine whether reconstitution of engineered osteoclasts within sockets could prevent specific pathological features of MRONJ. Locally delivered iRANK cells successfully differentiated into multinucleated osteoclasts in response to CID treatment in vivo as measured by green fluorescent protein (GFP), tartrate-resistant acid phosphatase (TRAP), carbonic anhydrase II, matrix metallopeptidase 9 (MMP-9), and cathepsin K staining. Sockets treated with iRANK cells + CID had significantly more osteoclasts and less necrotic bone than those receiving iRANK cells alone. These data support the hypothesis that osteoclast deficiency leads to accumulation of necrotic bone in MRONJ.

The role of T-cells in neurobehavioural development: Insights from the immunodeficient nude mice

Mice homozygous for the nude mutation (Foxn1^nu) are hairless and exhibit congenital dysgenesis of the thymic epithelium, resulting in a primary immunodeficiency of mature T-cells, and have been used for decades in research with tumour grafts. Early studies have already demonstrated social behaviour impairments and central nervous system (CNS) alterations in these animals, but did not address the complex interplay between CNS, immune system and behavioural alterations. Here we investigate the impact of T-cell immunodeficiency on behaviours relevant to the study of neurodevelopmental and neuropsychiatric disorders. Moreover, we aimed to characterise in a multidisciplinary manner the alterations related to those findings, through evaluation of the excitatory/inhibitory synaptic proteins, cytokines expression and biological spectrum signature of different biomolecules in nude mice CNS. We demonstrate that BALB/c nude mice display sociability impairments, a complex pattern of repetitive behaviours and higher sensitivity to thermal nociception. These animals also have a reduced IFN-γ gene expression in the prefrontal cortex and an absence of T-cells in meningeal tissue, both known modulators of social behaviour. Furthermore, excitatory synaptic protein PSD-95 immunoreactivity was also reduced in the prefrontal cortex, suggesting an intricate involvement of social behaviour related mechanisms. Lastly, employing biospectroscopy analysis, we have demonstrated that BALB/c nude mice have a different CNS spectrochemical signature compared to their heterozygous littermates. Altogether, our results show a comprehensive behavioural analysis of BALB/c nude mice and potential neuroimmunological influences involved with the observed alterations.

The New "Wholly Trinity" in the Diagnosis and Management of Inborn Errors of Immunity

The field of immunology has a rich and diverse history, and the study of inborn errors of immunity (IEIs) represents both the "cake" and the "icing on top of the cake," as it has enabled significant advances in our understanding of the human immune system. This explosion of knowledge has been facilitated by a unique partnership, a triumvirate formed by the physician who gathers detailed immunological and clinical phenotypic information from, and shares results with, the patient; the laboratory scientist/immunologist who performs diagnostic testing, as well as advanced functional correlative studies; and the genomics scientist/genetic counselor, who conducts and interprets varied genetic analyses, all of which are essential for dissecting constitutional genetic disorders. Although the basic principles of clinical care have not changed in recent years, the practice of clinical immunology has changed to reflect the prodigious advances in diagnostics, genomics, and therapeutics. An "omic/tics"-centric approach to IEI reflects the tremendous strides made in the field in the new millennium with recognition of new disorders, characterization of the molecular underpinnings, and development and implementation of personalized treatment strategies. This review brings renewed attention to bear on the indispensable "trinity" of phenotypic, genomic, and immunological analyses in the diagnosis, management, and treatment of IEIs.

EXPRESSION OF E8^E2 IS REQUIRED FOR WART FORMATION BY MOUSE PAPILLOMAVIRUS 1 IN VIVO

Human papillomavirus (HPV) E1 and E2 proteins activate genome replication. E2 also modulates viral gene expression and is involved in the segregation of viral genomes. In addition to full length E2, almost all PV share the ability to encode an E8^E2 protein, that is a fusion of E8 with the C-terminal half of E2 which mediates specific DNA-binding and dimerization. HPV E8^E2 acts as a repressor of viral gene expression and genome replication. To analyze the function of E8^E2 in vivo, we used the Mus musculus PV1 (MmuPV1)-mouse model system. Characterization of the MmuPV1 E8^E2 protein revealed that it inhibits transcription from viral promoters in the absence and presence of E1 and E2 proteins and that this is partially dependent upon the E8 domain. MmuPV1 genomes, in which the E8 ATG start codon was disrupted (E8-), displayed a 10- to 25-fold increase in viral gene expression compared to wt genomes in cultured normal mouse tail keratinocytes in short-term experiments. This suggests that the function and mechanism of E8^E2 is conserved between MmuPV1 and HPVs. Surprisingly, challenge of athymic nude Foxn1^nu/nu mice with MmuPV1 E8- genomes did not induce warts on the tail in contrast to wt MmuPV1. Furthermore, viral gene expression was completely absent at E8- MmuPV1 sites 20 - 22 weeks after DNA challenge on the tail or quasivirus challenge in the vaginal vault. This reveals that expression of E8^E2 is necessary to form tumors in vivo and that this is independent from the presence of T-cells.IMPORTANCE HPV encode an E8^E2 protein which acts as repressors of viral gene expression and genome replication. In cultured normal keratinocytes, E8^E2 is essential for long-term episomal maintenance of HPV31 genomes, but not for HPV16. To understand E8^E2's role in vivo, the Mus musculus PV1 (MmuPV1)-mouse model system was used. This revealed that E8^E2's function as a repressor of viral gene expression is conserved. Surprisingly, MmuPV1 E8^E2 knock out genomes did not induce warts in T-cell deficient mice. This shows for the first time that expression of E8^E2 is necessary for tumor formation in vivo independently of T cell immunity. This indicates that E8^E2 could be an interesting target for anti-viral therapy in vivo.

Comparison of Genetically Engineered Immunodeficient Animal Models for Nonclinical Testing of Stem Cell Therapies

For the recovery or replacement of dysfunctional cells and tissue-the goal of stem cell research-successful engraftment of transplanted cells and tissues are essential events. The event is largely dependent on the immune rejection of the recipient; therefore, the immunogenic evaluation of candidate cells or tissues in immunodeficient animals is important. Understanding the immunodeficient system can provide insights into the generation and use of immunodeficient animal models, presenting a unique system to explore the capabilities of the innate immune system. In this review, we summarize various immunodeficient animal model systems with different target genes as valuable tools for biomedical research. There have been numerous immunodeficient models developed by different gene defects, resulting in many different features in phenotype. More important, mice, rats, and other large animals exhibit very different immunological and physiological features in tissue and organs, including genetic background and a representation of human disease conditions. Therefore, the findings from this review may guide researchers to select the most appropriate immunodeficient strain, target gene, and animal species based on the research type, mutant gene effects, and similarity to human immunological features for stem cell research.

Generation of foxn1/Casper Mutant Zebrafish for Allograft and Xenograft of Normal and Malignant Cells

Cell transplantation into immunodeficient recipients is a widely used approach to study stem cell and cancer biology; however, studying cell states post transplantation in vivo is inconvenient in mammals. Here, we generated a foxn1/Casper mutant zebrafish that is transparent and exhibits T cell deficiency. By employing the line for hematopoietic stem cell (HSC) transplantation (HSCT), we could achieve nonconditioned transplantation. Meanwhile, we found that fetal HSCs from 3 days post fertilization zebrafish embryos produce a better transplant outcome in foxn1/Casper mutants, compared with adult HSCs. In addition to HSCT, the foxn1/Casper mutant is feasible for allografts of myelodysplastic syndrome-like and muscle cells, as well as xenografts of medaka muscle cells. In summary, foxn1/Casper mutants permit the nonconditioned engraftment of multiple cell types and visualized characterization of transplanted cells in vivo.

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foxn1/Casper mutant zebrafish permit unconditioned and visualized cell transplantation

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Zebrafish fetal HSCs possess more robust engraftment ability than adult HSCs

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foxn1/Casper mutant zebrafish permit allogeneic MDS-like cell transplantation

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Allograft and xenograft of muscle cells can be monitored in foxn1/Casper mutant zebrafish

Successful Preclinical Development of Gene Therapy for Recombinase-Activating Gene-1-Deficient SCID

Recombinase-activating gene-1 (RAG1)-deficient severe combined immunodeficiency (SCID) patients lack B and T lymphocytes due to the inability to rearrange immunoglobulin and T cell receptor genes. Gene therapy is an alternative for those RAG1-SCID patients who lack a suitable bone marrow donor. We designed lentiviral vectors with different internal promoters driving codon-optimized RAG1 to ensure optimal expression. We used Rag1 -/- mice as a preclinical model for RAG1-SCID to assess the efficacy of the various vectors. We observed that B and T cell reconstitution directly correlated with RAG1 expression. Mice with low RAG1 expression showed poor immune reconstitution; however, higher expression resulted in phenotypic and functional lymphocyte reconstitution comparable to mice receiving wild-type stem cells. No signs of genotoxicity were found. Additionally, RAG1-SCID patient CD34+ cells transduced with our clinical RAG1 vector and transplanted into NSG mice led to improved human B and T cell development. Considering this efficacy outcome, together with favorable safety data, these results substantiate the need for a clinical trial for RAG1-SCID.

Athymic nude mice as an experimental model for cancer treatment

Athymic nude mice, a murine strain bearing spontaneous deletion in the Foxn1 gene that causes deteriorated or absent thymus (which results in inhibited immune system with reduction of number of T cells), represent a widely used model in cancer research having long lasting history as a tool for preclinical testing of drugs. The review describes three models of athymic mice that utilize cancer cell lines to induce tumors. In addition, various methods that can be applied in order to evaluate activity of anticancer agents in these models are shown and discussed. Although each model has certain disadvantages, they are still considered as inevitable instruments in many fields of cancer research, particularly in finding new drugs that would more effectively combat the cancer disease or enhance the use of current chemotherapy. Finally, the review summarizes strengths and weaknesses as well as future perspectives of the athymic nude mice model in cancer research.

The development of T cells from stem cells in mice and humans

T cells develop from hematopoietic stem cells in the specialized microenvironment of the thymus. The main transcriptional players of T-cell differentiation such as Notch, Tcf-1, Gata3 and Bcl11b have been identified, but their role and regulation are not yet completely understood. In humans, functional experiments on T-cell development have traditionally been rather difficult to perform, but novel in vitro culture systems and in vivo xenograft models have allowed detailed studies on human T-cell development. Recent work has allowed the use of human severe combined immunodeficiency stem cells to unravel developmental checkpoints for human thymocyte development.

Dysfunction of the thymus in mice with hypertension

The aim of this study was to evaluate thymus function in mice with hypertension. A total of 60 C57BL/6J mice were randomized into control, sham surgery and two-kidney, one-clip groups (n=20 in each). At 4 or 8 weeks after surgery, mice were sacrificed, and blood, spleens, kidneys and thymuses were harvested. The results of reverse transcription-quantitative polymerase chain reaction analysis revealed that the mRNA levels of Forkhead box protein N1 (Foxn1) and autoimmune regulator (AIRE) in the thymus tissue of mice from the HTN group were significantly lower than those from the control group at 4 and 8 weeks (P<0.05). Foxn1 and AIRE expression was also reduced in the sham surgery group at 4 weeks after surgery, but had recovered 4 weeks later. Similar results were observed for the expression of signal-joint T cell receptor excision circles and the percentages of T cell subsets. The present study indicates that impaired thymus function is associated with hypertension in mice, which suggests that thymus function may be a novel target for the treatment of patients with hypertension.

Protection against Acute Hepatocellular Injury Afforded by Liver Fibrosis Is Independent of T Lymphocytes

Collagen produced during the process of liver fibrosis can induce a hepatocellular protective response through ERK1 signalling. However, the influence of T cells and associated cytokine production on this protection is unknown. In addition, athymic mice are frequently used in hepatocellular carcinoma xenograft experiments but current methods limit our ability to study the impact of liver fibrosis in this setting due to high mortality. Therefore, a mouse model of liver fibrosis lacking T cells was developed using Foxn1 nu/nu mice and progressive oral administration of thioacetamide (TAA) [0.01–0.02%] in drinking water. Fibrosis developed over a period of 16 weeks (alpha-SMA positive area: 20.0 ± 2.2%, preCol1a1 mRNA expression: 11.7 ± 4.1 fold changes, hydroxyproline content: 1041.2 ± 77μg/g of liver) at levels comparable to that of BALB/c mice that received intraperitoneal TAA injections [200 μg/g of body weight (bw)] (alpha-SMA positive area: 20.9 ± 2.9%, preCol1a1 mRNA expression: 13.1 ± 2.3 fold changes, hydroxyproline content: 931.6 ± 14.8μg/g of liver). No mortality was observed. Athymic mice showed phosphorylation of ERK1/2 during fibrogenesis (control 0.03 ± 0.01 vs 16 weeks 0.22 ± 0.06AU; P<0.05). The fibrosis-induced hepatoprotection against cytotoxic agents, as assessed histologically and by serum AST levels, was not affected by the absence of circulating T cells (anti-Fas JO2 [0.5μg/g bw] for 6h (fibrotic 4665 ± 2596 vs non-fibrotic 13953 ± 2260 U/L; P<0.05), APAP [750 mg/kg bw] for 6 hours (fibrotic 292 ± 66 U/L vs non-fibrotic 4086 ± 2205; P<0.01) and CCl₄ [0.5mL/Kg bw] for 24h (fibrotic 888 ± 268 vs non-fibrotic 15673 ± 2782 U/L; P<0.001)). In conclusion, liver fibrosis can be induced in athymic Foxn1 nu/nu mice without early mortality. Liver fibrosis leads to ERK1/2 phosphorylation. Finally, circulating T lymphocytes and associated cytokines are not involved in the hepatocellular protection afforded by liver fibrosis.

Decline of FOXN1 gene expression in human thymus correlates with age: possible epigenetic regulation

BACKGROUND

Thymic involution is thought to be an important factor of age related immunodeficiency. Understanding the molecular mechanisms of human thymic senescence may lead to the discovery of novel therapeutic approaches aimed at the reestablishment of central and peripheral T cell repertoire.

RESULTS

As an initial approach, here we report that the decline of human thymic FOXN1 transcription correlates with age, while other genes, DLL1, DLL4 and WNT4, essential for thymopoiesis, are constitutively transcribed. Using a human thymic epithelial cell line (hTEC), we show that FOXN1 expression is refractory to signals that induce FOXN1 transcription in primary 3D culture conditions and by stimulation of the canonical WNT signaling pathway. Blockage of FOXN1 induceability in the hTEC line may be mediated by an epigenetic mechanism, the CpG methylation of the FOXN1 gene.

CONCLUSION

We showed a suppression of FOXN1 transcription both in cultured human thymic epithelial cells and in the aging thymus. We hypothesize that the underlying mechanism may be associated with changes of the DNA methylation state of the FOXN1 gene.

Severe combined immunodeficiency--an update

Severe combined immunodeficiencies (SCIDs) are a group of inherited disorders responsible for severe dysfunctions of the immune system. These diseases are life-threatening when the diagnosis is made too late; they are the most severe forms of primary immunodeficiency. SCID patients often die during the first two years of life if appropriate treatments to reconstitute their immune system are not undertaken. Conventionally, SCIDs are classified according either to the main pathway affected by the molecular defect or on the basis of the specific immunologic phenotype that reflects the stage where the blockage occurs during the differentiation process. However, during the last few years many new causative gene alterations have been associated with unusual clinical and immunological phenotypes. Many of these novel forms of SCID also show extra-hematopoietic alterations, leading to complex phenotypes characterized by a functional impairment of several organs, which may lead to a considerable delay in the diagnosis. Here we review the biological and clinical features of SCIDs paying particular attention to the most recently identified forms and to their unusual or extra-immunological clinical features.

A deletion in FOXN1 is associated with a syndrome characterized by congenital hypotrichosis and short life expectancy in Birman cats

An autosomal recessive syndrome characterized by congenital hypotrichosis and short life expectancy has been described in the Birman cat breed (Felis silvestris catus). We hypothesized that a FOXN1 (forkhead box N1) loss-of-function allele, associated with the nude phenotype in humans, mice and rats, may account for the syndrome observed in Birman cats. To the best of our knowledge, spontaneous mutations in FOXN1 have never been described in non-human, non-rodent mammalian species. We identified a recessive c.1030_1033delCTGT deletion in FOXN1 in Birman cats. This 4-bp deletion was associated with the syndrome when present in two copies. Percentage of healthy carriers in our French panel of genotyped Birman cats was estimated to be 3.2%. The deletion led to a frameshift and a premature stop codon at position 547 in the protein. In silico, the truncated FOXN1 protein was predicted to lack the activation domain and critical parts of the forkhead DNA binding domain, both involved in the interaction between FOXN1 and its targets, a mandatory step to promote normal hair and thymic epithelial development. Our results enlarge the panel of recessive FOXN1 loss-of-function alleles described in mammals. A DNA test is available; it will help owners avoid matings at risk and should prevent the dissemination of this morbid mutation in domestic felines.

Human mutations in methylenetetrahydrofolate dehydrogenase 1 impair nuclear de novo thymidylate biosynthesis

An inborn error of metabolism associated with mutations in the human methylenetetrahydrofolate dehydrogenase 1 (MTHFD1) gene has been identified. The proband presented with SCID, megaloblastic anemia, and neurologic abnormalities, but the causal metabolic impairment is unknown. SCID has been associated with impaired purine nucleotide metabolism, whereas megaloblastic anemia has been associated with impaired de novo thymidylate (dTMP) biosynthesis. MTHFD1 functions to condense formate with tetrahydrofolate and serves as the primary entry point of single carbons into folate-dependent one-carbon metabolism in the cytosol. In this study, we examined the impact of MTHFD1 loss of function on folate-dependent purine, dTMP, and methionine biosynthesis in fibroblasts from the proband with MTHFD1 deficiency. The flux of formate incorporation into methionine and dTMP was decreased by 90% and 50%, respectively, whereas formate flux through de novo purine biosynthesis was unaffected. Patient fibroblasts exhibited enriched MTHFD1 in the nucleus, elevated uracil in DNA, lower rates of de novo dTMP synthesis, and increased salvage pathway dTMP biosynthesis relative to control fibroblasts. These results provide evidence that impaired nuclear de novo dTMP biosynthesis can lead to both megaloblastic anemia and SCID in MTHFD1 deficiency.

miR-18b and miR-518b Target FOXN1 during epithelial lineage differentiation in pluripotent cells

MicroRNAs (miRNAs) regulate myriad biological processes; however, their role in cell fate choice is relatively unexplored. Pluripotent NT2/D1 embryonal carcinoma cells differentiate into an epithelial/smooth muscle phenotype when treated with bone morphogenetic protein-2 (BMP-2). To identify miRNAs involved in epithelial cell development, we performed miRNA profiling of NT2/D1 cells treated with BMP-2 at 6, 12, and 24 h, and on days 6 and 10. Integration of the miRNA profiling data with previously obtained gene expression profiling (GEP) data of NT2/D1 cells treated with BMP-2 at the same time points identified miR-18b and miR-518b as the top two miRNAs with the highest number of up-regulated predicted targets with known functions in epithelial lineage development. Silencing of miR-18b and miR-518b in NT2/D1 cells revealed several up-regulated TFs with functions in epithelial lineage development; among these, target prediction programs identified FOXN1 as the only direct target of both miRNAs. FOXN1 has previously been shown to play an important role in keratinocyte differentiation and epithelial cell proliferation. NT2/D1 and H9 human embryonic stem cells with silenced miR-18b and miR-518b showed up-regulation of FOXN1 and the epithelial markers CDH1, EPCAM, KRT19, and KRT7. A 3'UTR luciferase assay confirmed FOXN1 to be a target of the two miRNAs, and up-regulation of FOXN1 in NT2/D1 cells led to the expression of epithelial markers. Overexpression of the two miRNAs in BMP-2-treated NT2/D1 cells led to down-regulation of FOXN1 and epithelial lineage markers. These results show that miR-18b and miR-518b are upstream controllers of FOXN1-directed epithelial lineage development.

Thymic epithelial cell development and its dysfunction in human diseases

Thymic epithelial cells (TECs) are the key components in thymic microenvironment for T cells development. TECs, composed of cortical and medullary TECs, are derived from a common bipotent progenitor and undergo a stepwise development controlled by multiple levels of signals to be functionally mature for supporting thymocyte development. Tumor necrosis factor receptor (TNFR) family members including the receptor activator for NFκB (RANK), CD40, and lymphotoxin β receptor (LTβR) cooperatively control the thymic medullary microenvironment and self-tolerance establishment. In addition, fibroblast growth factors (FGFs), Wnt, and Notch signals are essential for establishment of functional thymic microenvironment. Transcription factors Foxn1 and autoimmune regulator (Aire) are powerful modulators of TEC development, differentiation, and self-tolerance. Dysfunction in thymic microenvironment including defects of TEC and thymocyte development would cause physiological disorders such as tumor, infectious diseases, and autoimmune diseases. In the present review, we will summarize our current understanding on TEC development and the underlying molecular signals pathways and the involvement of thymus dysfunction in human diseases.

Molecular evidence for a thymus-independent partial T cell development in a FOXN1-/- athymic human fetus

The thymus is the primary organ able to support T cell ontogeny, abrogated in FOXN1(-/-) human athymia. Although evidence indicates that in animal models T lymphocytes may differentiate at extrathymic sites, whether this process is really thymus-independent has still to be clarified. In an athymic FOXN1(-/-) fetus, in which we previously described a total blockage of CD4(+) and partial blockage of CD8(+) cell development, we investigated whether intestine could play a role as extrathymic site of T-lymphopoiesis in humans. We document the presence of few extrathymically developed T lymphocytes and the presence in the intestine of CD3(+) and CD8(+), but not of CD4(+) cells, a few of them exhibiting a CD45RA(+) naïve phenotype. The expression of CD3εεpTα, RAG1 and RAG2 transcripts in the intestine and TCR gene rearrangement was also documented, thus indicating that in humans the partial T cell ontogeny occurring at extrathymic sites is a thymus- and FOXN1-independent process.

Modulation of Signal Strength Switches Notch from an Inducer of T Cells to an Inducer of ILC2

Innate lymphoid cells (ILCs) are emerging key players of the immune system with close lineage relationship to T cells. ILC2 play an important role in protective immunity against multicellular parasites, but are also involved in the pathogenesis of type 2 immune diseases. Here, we have studied the developmental requirements for human ILC2. We report that ILC2 are present in the thymus of young human donors, possibly reflecting local differentiation. Furthermore, we show that uncommitted lineage⁻CD34⁺CD1a⁻human thymic progenitors have the capacity to develop into ILC2 in vitro under the influence of Notch signaling, either by stimulation with the Notch ligand Delta like 1 (Dll1) or by expression of the active intracellular domain of NOTCH1 (NICD1). The capacity of NICD1 to mobilize the ILC2 differentiation program was sufficiently potent to override commitment to the T cell lineage in CD34⁺CD1a⁺ progenitors and force them into the ILC2 lineage. As Notch is an important factor also for T cell development, these results raise the question how one and the same signaling pathway can elicit such distinct developmental outcomes from the same precursors. We provide evidence that Notch signal strength is a critical determinant in this decision: by tuning signal amplitude, Notch can be converted from a T cell inducer (low signal strength) to an ILC2 inducer (high signal strength). Thus, this study enhances our understanding of human ILC2 development and identifies a mechanism determining specificity of Notch signal output during T cell and ILC2 differentiation.

Repairing thymic function

PURPOSE OF REVIEW

Maintenance of T-cell function and modulation of tolerance are critical issues in organ transplantation. The thymus is the primary organ for T-cell generation, and a preserved thymic function is essential for a self-tolerant diverse T-cell repertoire. Transplant procedures and related immunosuppressive drugs may hinder thymic integrity and function. We review here the recent advances in understanding the regulation of the unique thymic microenvironment with relevance for the field of transplantation.

RECENT FINDINGS

Recent studies have assigned a role for IL-22 in the regeneration of thymic epithelium, and for microRNAs in the modulation of its survival and function. The interplay of key molecules in the cross-talk between thymic epithelial cells and thymocytes was depicted, opening new perspectives for the in-vitro recapitulation of T-cell development and for thymic transplantation. Additionally, the thymus was shown to be able to sustain thymocyte progenitor renewal.

SUMMARY

These findings open new venues of research toward therapeutic interventions in the endogenous thymus to modulate or reconstitute the immune system; thymic transplantation; and the future development of artificial thymus, which would represent an important tool to achieve tolerance across the histocompatibility barriers.

Antitumor activity of IL-32β through the activation of lymphocytes, and the inactivation of NF-κB and STAT3 signals

Cytokine and activation of lymphocytes are critical for tumor growth. We investigated whether interleukin (IL)-32β overexpression changes other cytokine levels and activates cytotoxic lymphocyte, and thus modify tumor growth. Herein, IL-32β inhibited B16 melanoma growth in IL-32β-overexpressing transgenic mice (IL-32β mice), and downregulated the expressions of anti-apoptotic proteins (bcl-2, IAP, and XIAP) and cell growth regulatory proteins (Ki-67 antigen (Ki-67) and proliferating cell nuclear antigen (PCNA)), but upregulated the expressions of pro-apoptotic proteins (bax, cleaved caspase-3, and cleaved caspase-9). IL-32β also inhibited colon and prostate tumor growth in athymic nude mice inoculated with IL-32β-transfected SW620 colon or PC3 prostate cancer cells. The forced expression of IL-32β also inhibited cell growth in cultured colon and prostate cancer cells, and these inhibitory effects were abolished by IL-32 small interfering RNA (siRNA). IL-10 levels were elevated, but IL-1β, IL-6, and tumor necrosis factor-alpha (TNF-α) levels were reduced in the tumor tissues and spleens of IL-32β mice, and athymic nude mice. The number of cytotoxic T (CD8(+)) and natural killer (NK) cells in tumor tissues, spleen, and blood was significantly elevated in IL-32β mice and athymic nude mice inoculated with IL-32β-transfected cancer cells. Constituted activated NF-κB and STAT3 levels were reduced in the tumor tissues of IL-32β mice and athymic nude mice, as well as in IL-32β-transfected cultured cancer cells. These findings suggest that IL-32β inhibits tumor growth by increasing cytotoxic lymphocyte numbers, and by inactivating the NF-κB and STAT3 pathways through changing of cytokine levels in tumor tissues.