Role of human forkhead box P3 in early thymic maturation and peripheral T-cell homeostasis.

BACKGROUND: Forkhead box P3 (FOXP3) is a key transcription factor in regulatory T (Treg) cell function. FOXP3 gene mutations cause immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome, a fatal autoimmune syndrome. FOXP3 has also been proposed to act in effector T (Teff) cells, but to date, this role has not been confirmed. OBJECTIVE: We sought to evaluate the effect of reduced FOXP3 expression on human Treg and Teff cell development and correlate it with IPEX syndrome immune pathology. METHODS: We developed a model of humanized mice (huMice) in which the human hematopoietic system is stably knocked down or knocked out for the FOXP3 gene (knockdown [KD]/knockout [KO] huMice). RESULTS: Because FOXP3-KD/KO was not 100% effective, residual FOXP3 expression in hematopoietic stem progenitor cells was sufficient to give rise to Treg cells with normal expression of FOXP3. However, numerous defects appeared in the Teff cell compartment. Compared with control mice, FOXP3-KD/KO huMice showed altered thymocyte differentiation, with KD/KO thymocytes displaying significantly reduced T-cell receptor (TCR) signaling strength and increased TCR repertoire diversity. Peripheral KD/KO Teff cells were expanded and showed signs of homeostatic proliferation, such as a significantly contracted TCR repertoire, a severely reduced naive compartment, decreased telomeric repeat-binding factor 2 expression, and a skew toward a TH2 profile, resembling an aged immune system. Consistent with results in FOXP3-KD/KO huMice, analysis of patients with IPEX syndrome provided evidence of defects in the Teff cell compartment at both the thymic and peripheral levels. CONCLUSIONS: These findings support an intrinsic role for human FOXP3 in controlling thymocyte maturation and peripheral expansion of Teff cells and reveal a previously undescribed pathogenic mechanism through an altered Teff cell compartment in patients with IPEX syndrome.

KAP1 regulates gene networks controlling mouse B-lymphoid cell differentiation and function.

Chromatin remodeling is fundamental for B-cell differentiation. In the present study, we explored the role of KAP1, the cofactor of KRAB-ZFP transcriptional repressors, in this process. B-lymphoid-specific Kap1-KO mice displayed reduced numbers of mature B cells, lower steady-state levels of Abs, and accelerated rates of decay of neutralizing Abs after viral immunization. Transcriptome analyses of Kap1-deleted B splenocytes revealed an up-regulation of PTEN, the enzymatic counteractor of PIK3 signaling, and of genes encoding DNA-damage response factors, cell-cycle regulators, and chemokine receptors. ChIP/seq studies established that KAP1 bound at or close to several of these genes and controlled chromatin status at their promoters. Genome wide, KAP1 binding sites lacked active B cell-specific enhancers and were enriched in repressive histone marks, further supporting a role for this molecule in gene silencing in vivo. Likely responsible for tethering KAP1 to at least some of these targets, a discrete subset of KRAB-ZFPs is enriched in B lymphocytes. Our results therefore reveal the role of KRAB/KAP1-mediated epigenetic regulation in B-cell development and homeostasis.

KAP1 regulates gene networks controlling T-cell development and responsiveness.

Chromatin remodeling at specific genomic loci controls lymphoid differentiation. Here, we investigated the role played in this process by Kruppel-associated box (KRAB)-associated protein 1 (KAP1), the universal cofactor of KRAB-zinc finger proteins (ZFPs), a tetrapod-restricted family of transcriptional repressors. T-cell-specific Kap1-deleted mice displayed a significant expansion of immature thymocytes, imbalances in CD4(+)/CD8(+) cell ratios, and altered responses to TCR and TGFß stimulation when compared to littermate KAP1 control mice. Transcriptome and chromatin studies revealed that KAP1 binds T-cell-specific cis-acting regulatory elements marked by the H3K9me3 repressive mark and enriched in Ikaros/NuRD complexes. Also, KAP1 directly controls the expression of several genes involved in TCR and cytokine signaling. Among these, regulation of FoxO1 seems to play a major role in this system. Likely responsible for tethering KAP1 to at least part of its genomic targets, a small number of KRAB-ZFPs are selectively expressed in T-lymphoid cells. These results reveal the so far unsuspected yet important role of KAP1-mediated epigenetic regulation in T-lymphocyte differentiation and activation.

Aryl hydrocarbon receptor activity downstream of IL-10 signaling is required to promote regulatory functions in human dendritic cells.

Interleukin (IL)-10 is a main player in peripheral immune tolerance, the physiological mechanism preventing immune reactions to self/harmless antigens. Here, we investigate IL-10-induced molecular mechanisms generating tolerogenic dendritic cells (tolDC) from monocytes. Using genomic studies, we show that IL-10 induces a pattern of accessible enhancers exploited by aryl hydrocarbon receptor (AHR) to promote expression of a set of core genes. We demonstrate that AHR activity occurs downstream of IL-10 signaling in myeloid cells and is required for the induction of tolerogenic activities in DC. Analyses of circulating DCs show that IL-10/AHR genomic signature is active in vivo in health. In multiple sclerosis patients, we instead observe significantly altered signature correlating with functional defects and reduced frequencies of IL-10-induced-tolDC in vitro and in vivo. Our studies identify molecular mechanisms controlling tolerogenic activities in human myeloid cells and may help in designing therapies to re-establish immune tolerance.

HIV-1-mediated insertional activation of STAT5B and BACH2 trigger viral reservoir in T regulatory cells.

HIV-1 insertions targeting BACH2 or MLK2 are enriched and persist for decades in hematopoietic cells from patients under combination antiretroviral therapy. However, it is unclear how these insertions provide such selective advantage to infected cell clones. Here, we show that in 30/87 (34%) patients under combination antiretroviral therapy, BACH2, and STAT5B are activated by insertions triggering the formation of mRNAs that contain viral sequences fused by splicing to their first protein-coding exon. These chimeric mRNAs, predicted to express full-length proteins, are enriched in T regulatory and T central memory cells, but not in other T lymphocyte subsets or monocytes. Overexpression of BACH2 or STAT5B in primary T regulatory cells increases their proliferation and survival without compromising their function. Hence, we provide evidence that HIV-1-mediated insertional activation of BACH2 and STAT5B favor the persistence of a viral reservoir in T regulatory cells in patients under combination antiretroviral therapy.HIV insertions in hematopoietic cells are enriched in BACH2 or MLK2 genes, but the selective advantages conferred are unknown. Here, the authors show that BACH2 and additionally STAT5B are activated by viral insertions, generating chimeric mRNAs specifically enriched in T regulatory cells favoring their persistence.

Kruppel-associated box (KRAB) proteins in the adaptive immune system.

The ability of adaptive immune system to protect higher vertebrates from pathogens resides in the ability of B and T cells to express different antigen specific receptors and to respond to different threats by activating distinct differentiation and/or activation pathways. In the past 10 years, the major role of epigenetics in controlling molecular mechanisms responsible for these peculiar features and, more in general, for lymphocyte development has become evident. KRAB-ZFPs is the widest family of mammalian transcriptional repressors, which function through the recruitment of the co-factor KRAB-Associated Protein 1 (KAP1) that in turn engages histone modifiers inducing heterochromatin formation. Although most of the studies on KRAB proteins have been performed in embryonic cells, more recent reports highlighted a relevant role for these proteins also in adult tissues. This article will review the role of KRAB-ZFP and KAP1 in the epigenetic control of mouse and human adaptive immune cells.

Gene/cell therapy approaches for Immune Dysregulation Polyendocrinopathy Enteropathy X-linked syndrome.

Immune dysregulation, Polyendocrinopathy, Enteropathy, X-linked (IPEX) syndrome is a rare autoimmune disease due to mutations in the gene encoding for Forkhead box P3 (FOXP3), a transcription factor fundamental for the function of thymus-derived (t) regulatory T (Treg) cells. The dysfunction of Treg cells results in the development of devastating autoimmune manifestations affecting multiple organs, eventually leading to premature death in infants, if not promptly treated by hematopoietic stem cell transplantation (HSCT). Novel gene therapy strategies can be developed for IPEX syndrome as more definitive cure than allogeneic HSCT. Here we describe the therapeutic approaches, alternative to HSCT, currently under development. We described that effector T cells can be converted in regulatory T cells by LV-mediated FOXP3-gene transfer in differentiated T lymphocytes. Despite FOXP3 mutations mainly affect a highly specific T cell subset, manipulation of stem cells could be required for long-term remission of the disease. Therefore, we believe that a more comprehensive strategy should aim at correcting FOXP3-mutated stem cells. Potentials and hurdles of both strategies will be highlighted here.

Global and stage specific patterns of Krüppel-associated-box zinc finger protein gene expression in murine early embryonic cells.

Highly coordinated transcription networks orchestrate the self-renewal of pluripotent stem cell and the earliest steps of mammalian development. KRAB-containing zinc finger proteins represent the largest group of transcription factors encoded by the genomes of higher vertebrates including mice and humans. Together with their putatively universal cofactor KAP1, they have been implicated in events as diverse as the silencing of endogenous retroelements, the maintenance of imprinting and the pluripotent self-renewal of embryonic stem cells, although the genomic targets and specific functions of individual members of this gene family remain largely undefined. Here, we first generated a list of Ensembl-annotated KRAB-containing genes encoding the mouse and human genomes. We then defined the transcription levels of these genes in murine early embryonic cells. We found that the majority of KRAB-ZFP genes are expressed in mouse pluripotent stem cells and other early progenitors. However, we also identified distinctively cell- or stage-specific patterns of expression, some of which are pluripotency-restricted. Finally, we determined that individual KRAB-ZFP genes exhibit highly distinctive modes of expression, even when grouped in genomic clusters, and that these cannot be correlated with the presence of prototypic repressive or activating chromatin marks. These results pave the way to delineating the role of specific KRAB-ZFPs in early embryogenesis.

APOBEC3G-depleted resting CD4+ T cells remain refractory to HIV1 infection.

BACKGROUND: CD4+ T lymphocytes are the primary targets of HIV1 but cannot be infected when fully quiescent, due to a post-entry block preventing the completion of reverse transcription. Chiu et al. recently proposed that this restriction reflects the action of APOBEC3G (A3G). They further suggested that T cell activation abrogates the A3G-mediated block by directing this protein to a high molecular mass complex. METHODOLOGY/PRINCIPAL FINDINGS: In the present work, we sought to explore further this model. However, we found that effective suppression of A3G by combined RNA interference and expression of HIV1 Vif does not relieve the restrictive phenotype of post-activation resting T cells. We also failed to find a correlation between HIV resistance and the presence of A3G in a low molecular complex in primary T cells. CONCLUSIONS/SIGNIFICANCE: We conclude that A3G is unlikely to play a role in the HIV restrictive phenotype of quiescent T lymphocytes.

Promoter trapping reveals significant differences in integration site selection between MLV and HIV vectors in primary hematopoietic cells.

Recent reports have indicated that human immunodeficiency virus (HIV) and murine leukemia virus (MLV) vectors preferentially integrate into active genes. Here, we used a novel approach based on genetic trapping to rapidly score several thousand integration sites and found that MLV vectors trapped cellular promoters more efficiently than HIV vectors. Remarkably, 1 in 5 MLV integrations trapped an active promoter in different cell lines and primary hematopoietic cells. Such frequency was even higher in growth-stimulated lymphocytes. We show that the different behavior of MLV and HIV vectors was dependent on a different integration pattern within transcribed genes. Whereas MLV-based traps showed a strong bias for promoter-proximal integration leading to efficient reporter expression, HIV-based traps integrated throughout transcriptional units and were limited for expression by the distance from the promoter and the reading frame of the targeted gene. Our results indicate a strong propensity of MLV to establish transcriptional interactions with cellular promoters, a behavior that may have evolved to enhance proviral expression and may increase the insertional mutagenesis risk. Promoter trapping efficiency provides a convenient readout to assess transcriptional interactions between the vector and its flanking genes at the integration site and to compare integration site selection among different cell types and in different growth conditions.