Arnav Mehta

MicroRNAs as regulatory elements in immune system logic

MicroRNAs (miRNAs) are crucial post-transcriptional regulators of haematopoietic cell fate decisions. They act by negatively regulating the expression of key immune development genes, thus contributing important logic elements to the regulatory circuitry. Deletion studies have made it increasingly apparent that they confer robustness to immune cell development, especially under conditions of environmental stress such as infectious challenge and ageing. Aberrant expression of certain miRNAs can lead to pathological consequences, such as autoimmunity and haematological cancers. In this Review, we discuss the mechanisms by which several miRNAs influence immune development and buffer normal haematopoietic output, first at the level of haematopoietic stem cells, then in innate and adaptive immune cells. We then discuss the pathological consequences of dysregulation of these miRNAs.

Conversion of danger signals into cytokine signals by hematopoietic stem and progenitor cells for regulation of stress-induced hematopoiesis.

During an infection, the body increases the output of mature immune cells in order to fight off the pathogen. Despite convincing evidence that hematopoietic stem and progenitor cells (HSPCs) can sense pathogens directly, how this contributes to hematopoietic cell output remains unknown. Here, we have combined mouse models with a single-cell proteomics platform to show that, in response to Toll-like receptor stimulation, short-term HSCs and multipotent progenitor cells produce copious amounts of diverse cytokines through nuclear factor κB (NF-κB) signaling. Interestingly, the cytokine production ability of HSPCs trumps mature immune cells in both magnitude and breadth. Among cytokines produced by HSPCs, IL-6 is a particularly important regulator of myeloid differentiation and HSPC proliferation in a paracrine manner and in mediating rapid myeloid cell recovery during neutropenia. This study has uncovered an important property of HSPCs that enables them to convert danger signals into versatile cytokine signals for the regulation of stress hematopoiesis.

Oncomir miR-125b regulates hematopoiesis by targeting the gene Lin28A

MicroRNA-125b (miR-125b) is up-regulated in patients with leukemia. Overexpression of miR-125b alone in mice causes a very aggressive, transplantable myeloid leukemia. Before leukemia, these mice do not display elevation of white blood cells in the spleen or bone marrow; rather, the hematopoietic compartment shows lineage-skewing, with myeloid cell numbers dramatically increased and B-cell numbers severely diminished. miR-125b exerts this effect by up-regulating the number of common myeloid progenitors while inhibiting development of pre-B cells. We applied a miR-125b sponge loss of function system in vivo to show that miR-125b physiologically regulates hematopoietic development. Investigating the mechanism by which miR-125b regulates hematopoiesis, we found that, among a panel of candidate targets, the mRNA for Lin28A, an induced pluripotent stem cell gene, was most repressed by miR-125b in mouse hematopoietic stem and progenitor cells. Overexpressing Lin28A in the mouse hematopoietic system mimicked the phenotype observed on inhibiting miR-125b function, leading to a decrease in hematopoietic output. Relevant to the miR-125b overexpression phenotype, we also found that knockdown of Lin28A led to hematopoietic lineage-skewing, with increased myeloid and decreased B-cell numbers. Thus, the miR-125b target Lin28A is an important regulator of hematopoiesis and a primary target of miR-125b in the hematopoietic system.

The microRNA-132 and microRNA-212 cluster regulates hematopoietic stem cell maintenance and survival with age by buffering FOXO3 expression.

MicroRNAs are critical post-transcriptional regulators of hematopoietic cell-fate decisions, though little remains known about their role in aging hematopoietic stem cells (HSCs). We found that the microRNA-212/132 cluster (Mirc19) is enriched in HSCs and is upregulated during aging. Both overexpression and deletion of microRNAs in this cluster leads to inappropriate hematopoiesis with age. Enforced expression of miR-132 in the bone marrow of mice led to rapid HSC cycling and depletion. A genetic deletion of Mirc19 in mice resulted in HSCs that had altered cycling, function, and survival in response to growth factor starvation. We found that miR-132 exerted its effect on aging HSCs by targeting the transcription factor FOXO3, a known aging associated gene. Our data demonstrate that Mirc19 plays a role in maintaining balanced hematopoietic output by buffering FOXO3 expression. We have thus identified it as a potential target that might play a role in age-related hematopoietic defects.

An NF-κB-microRNA regulatory network tunes macrophage inflammatory responses

The innate inflammatory response must be tightly regulated to ensure effective immune protection. NF-κB is a key mediator of the inflammatory response, and its dysregulation has been associated with immune-related malignancies. Here, we describe a miRNA-based regulatory network that enables precise NF-κB activity in mouse macrophages. Elevated miR-155 expression potentiates NF-κB activity in miR-146a-deficient mice, leading to both an overactive acute inflammatory response and chronic inflammation. Enforced miR-155 expression overrides miR-146a-mediated repression of NF-κB activation, thus emphasizing the dominant function of miR-155 in promoting inflammation. Moreover, miR-155-deficient macrophages exhibit a suboptimal inflammatory response when exposed to low levels of inflammatory stimuli. Importantly, we demonstrate a temporal asymmetry between miR-155 and miR-146a expression during macrophage activation, which creates a combined positive and negative feedback network controlling NF-κB activity. This miRNA-based regulatory network enables a robust yet time-limited inflammatory response essential for functional immunity.MicroRNAs (miR) are important regulators of gene transcription, with miR-155 and miR-146a both implicated in macrophage activation. Here the authors show that NF-κB signalling, miR-155 and miR-146a form a complex network of cross-regulations to control gene transcription in macrophages for modulating inflammatory responses.

Future Clinical Applications of Molecular Imaging: Nanoparticles, Cellular Probes, and Imaging of Gene Expression

Anatomical imaging has been the conventional paradigm for diagnosis since the advent of radiography a century ago. Computed tomography (CT), magnetic resonance (MR), and ultrasound (US) imaging have been at the forefront of anatomical imaging modalities. Under this conventional paradigm, pathology is identified by alteration of the typical anatomy, thus characterizing location and burden of disease. Changes in the morphological appearance have been used to assess response to treatments [1]. However, this convention provides little information of the physiological, molecular, and biochemical basis of disease.

A stochastic epigenetic switch controls the dynamics of T-cell lineage commitment

Cell fate decisions occur through the switch-like, irreversible activation of fate-specifying genes. These activation events are often assumed to be tightly-coupled to changes in upstream transcription factors, but could also be constrained by cis-epigenetic mechanisms at individual gene loci. Here, we studied the activation of Bcl11b, which controls T-cell fate commitment. To disentangle cis and trans effects, we generated mice where two Bcl11b copies are tagged with distinguishable fluorescent proteins. Quantitative live microscopy of progenitors from these mice revealed that Bcl11b turned on after a stochastic delay averaging multiple days, which varied not only between cells but also between Bcl11b alleles within the same cell. Genetic perturbations, together with mathematical modeling, showed that a distal enhancer controls the rate of epigenetic activation, while a parallel Notch-dependent trans-acting step stimulates expression from activated loci. These results show that developmental fate transitions can be controlled by stochastic cis-acting events on individual loci.

Validation of postoperative residual contrast-enhancing tumor volume as an independent prognostic factor for overall survival in newly diagnosed glioblastoma

Background In the current study, we pooled imaging data in newly diagnosed glioblastoma (GBM) patients from international multicenter clinical trials, single institution databases, and multicenter clinical trial consortiums to identify the relationship between postoperative residual enhancing tumor volume and overall survival (OS). Methods Data from 1511 newly diagnosed GBM patients from 5 data sources were included in the current study: (i) a single institution database from UCLA (N = 398; Discovery); (ii) patients from the Ben and Cathy Ivy Foundation for Early Phase Clinical Trials Network Radiogenomics Database (N = 262 from 8 centers; Confirmation); (iii) the chemoradiation placebo arm from an international phase III trial (AVAglio; N = 394 from 120 locations in 23 countries; Validation); (iv) the experimental arm from AVAglio examining chemoradiation plus bevacizumab (N = 404 from 120 locations in 23 countries; Exploratory Set 1); and (v) an Alliance (N0874) phase I/II trial of vorinostat plus chemoradiation (N = 53; Exploratory Set 2). Postsurgical, residual enhancing disease was quantified using T1 subtraction maps. Multivariate Cox regression models were used to determine influence of clinical variables, O6-methylguanine-DNA methyltransferase (MGMT) status, and residual tumor volume on OS. Results A log-linear relationship was observed between postoperative, residual enhancing tumor volume and OS in newly diagnosed GBM treated with standard chemoradiation. Postoperative tumor volume is a prognostic factor for OS (P < 0.01), regardless of therapy, age, and MGMT promoter methylation status. Conclusion Postsurgical, residual contrast-enhancing disease significantly negatively influences survival in patients with newly diagnosed GBM treated with chemoradiation with or without concomitant experimental therapy.

Post-chemoradiation volumetric response predicts survival in newly diagnosed glioblastoma treated with radiation, temozolomide, and bevacizumab or placebo.

Background In the current study we used contrast-enhanced T1 subtraction maps to test whether early changes in enhancing tumor volume are prognostic for overall survival (OS) in newly diagnosed glioblastoma (GBM) patients treated with chemoradiation with or without bevacizumab (BV). Methods Seven hundred ninety-eight patients (404 BV and 394 placebo) with newly diagnosed GBM in the AVAglio trial (NCT00943826) had baseline MRI scans available, while 337 BV-treated and 269 placebo-treated patients had >4 MRI scans for response evaluation. The volume of contrast-enhancing tumor was quantified and used for subsequent analyses. Results A decrease in tumor volume during chemoradiation was associated with a longer OS in the placebo group (hazard ratio [HR] = 1.578, P < 0.0001) but not BV-treated group (HR = 1.135, P = 0.4889). Results showed a higher OS in patients on the placebo arm with a sustained decrease in tumor volume using a post-chemoradiation baseline (HR = 1.692, P = 0.0005), and a trend toward longer OS was seen in BV-treated patients (HR = 1.264, P = 0.0724). Multivariable Cox regression confirmed that sustained response or stable disease was prognostic for OS (HR = 0.7509, P = 0.0127) when accounting for age (P = 0.0002), KPS (P = 0.1516), postsurgical tumor volume (P < 0.0001), O6-methylguanine-DNA methyltransferase status (P < 0.0001), and treatment type (P = 0.7637) using the post-chemoradiation baseline. Conclusions The post-chemoradiation timepoint is a better baseline for evaluating efficacy in newly diagnosed GBM. Early progression during the maintenance phase is consequential in predicting OS, supporting the use of progression-free survival rates as a meaningful surrogate for GBM.

Immunotherapy Resistance by Inflammation-Induced Dedifferentiation

A promising arsenal of targeted and immunotherapy treatments for metastatic melanoma has emerged over the last decade. With these therapies, we now face new mechanisms of tumor-acquired resistance. We report here a patient whose metastatic melanoma underwent dedifferentiation as a resistance mechanism to adoptive T-cell transfer therapy (ACT) to the MART1 antigen, a phenomenon that had been observed only in mouse studies to date. After an initial period of tumor regression, the patient presented in relapse with tumors lacking melanocytic antigens (MART1, gp100) and expressing an inflammation-induced neural crest marker (NGFR). We demonstrate using human melanoma cell lines that this resistance phenotype can be induced in vitro by treatment with MART1 T cell receptor-expressing T cells or with TNFα, and that the phenotype is reversible with withdrawal of inflammatory stimuli. This supports the hypothesis that acquired resistance to cancer immunotherapy can be mediated by inflammation-induced cancer dedifferentiation.Significance: We report a patient whose metastatic melanoma underwent inflammation-induced dedifferentiation as a resistance mechanism to ACT to the MART1 antigen. Our results suggest that future melanoma ACT protocols may benefit from the simultaneous targeting of multiple tumor antigens, modulating the inflammatory response, and inhibition of inflammatory dedifferentiation-inducing signals. Cancer Discov; 8(8); 935-43. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 899.