Introduction and Overview.

As sequencing technologies improved, new classes of genes were uncovered. Initially, many of these were considered non-functional given their low protein-coding potential but have now emerged as important regulators of biological processes. One of the new classes of genes are called long noncoding RNAs (lncRNAs). LncRNAs are the largest group of transcribed RNA. As their name suggests, they are non-protein coding genes. To differentiate them from other smaller, noncoding RNAs, lncRNAs are transcripts whose length are greater than 200 nucleotides. According to GENCODE Release 38, there are approximately 18,000 lncRNAs, of which only 4% have a known function. Of the lncRNAs characterized, many of them play regulatory roles in many biological processes, including regulation of gene expression, alternative splicing, chromatin modification, protein activity, and posttranscriptional mechanisms. Compared to protein coding genes, lncRNAs show high cell type specificity. Many lncRNAs have been shown to be expressed in distinct immune cell populations and play RNA-mediated immune-regulatory roles. Many aspects of the immune response, including the duration, magnitude, and subsequent return to homeostasis are carefully controlled. Dysregulation of lncRNAs can result in an uncontrolled immune response, which can lead to a variety of immune-related diseases. This introduction aims to summarize the chapters highlighting the discovery of lncRNAs, their role in the immune response, and their functional characterization, either through interaction with DNA, RNA, and/or proteins in distinct immune cell populations or cells implicated in immune-related diseases. Additionally, the immune regulatory role of lncRNAs will be covered, and how lncRNA localization, sequence and secondary structure can inform function. Delving into this largely unexplored field can identify lncRNAs as potential therapeutic targets.

Generation of an isoform-level transcriptome atlas of macrophage activation.

RNA-seq is routinely used to measure gene expression changes in response to cell perturbation. Genes upregulated or downregulated following some perturbation are designated as genes of interest, and their most expressed isoform(s) would then be selected for follow-up experimentation. However, because of its need to fragment RNA molecules, RNA-seq is limited in its ability to capture gene isoforms and their expression patterns. This lack of isoform-specific data means that isoforms would be selected based on annotation databases that are incomplete, not tissue specific, or do not provide key information on expression levels. As a result, minority or nonexistent isoforms might be selected for follow-up, leading to loss in valuable resources and time. There is therefore a great need to comprehensively identify gene isoforms along with their corresponding levels of expression. Using the long-read nanopore-based R2C2 method, which does not fragment RNA molecules, we generated an Isoform-level transcriptome Atlas of Macrophage Activation that identifies full-length isoforms in primary human monocyte-derived macrophages. Macrophages are critical innate immune cells important for recognizing pathogens through binding of pathogen-associated molecular patterns to toll-like receptors, culminating in the initiation of host defense pathways. We characterized isoforms for most moderately-to-highly expressed genes in resting and toll-like receptor-activated monocyte-derived macrophages, identified isoforms differentially expressed between conditions, and validated these isoforms by RT-qPCR. We compiled these data into a user-friendly data portal within the UCSC Genome Browser (https://genome.ucsc.edu/s/vollmers/IAMA). Our atlas represents a valuable resource for innate immune research, providing unprecedented isoform information for primary human macrophages.

A conserved long noncoding RNA, GAPLINC, modulates the immune response during endotoxic shock.

Recent studies have identified thousands of long noncoding RNAs (lncRNAs) in mammalian genomes that regulate gene expression in different biological processes. Although lncRNAs have been identified in a variety of immune cells and implicated in immune response, the biological function and mechanism of the majority remain unexplored, especially in sepsis. Here, we identify a role for a lncRNA-gastric adenocarcinoma predictive long intergenic noncoding RNA (GAPLINC)-previously characterized for its role in cancer, now in the context of innate immunity, macrophages, and LPS-induced endotoxic shock. Transcriptome analysis of macrophages from humans and mice reveals that GAPLINC is a conserved lncRNA that is highly expressed following macrophage differentiation. Upon inflammatory activation, GAPLINC is rapidly down-regulated. Macrophages depleted of GAPLINC display enhanced expression of inflammatory genes at baseline, while overexpression of GAPLINC suppresses this response. Consistent with GAPLINC-depleted cells, Gaplinc knockout mice display enhanced basal levels of inflammatory genes and show resistance to LPS-induced endotoxic shock. Mechanistically, survival is linked to increased levels of nuclear NF-κB in Gaplinc knockout mice that drives basal expression of target genes typically only activated following inflammatory stimulation. We show that this activation of immune response genes prior to LPS challenge leads to decreased blood clot formation, which protects Gaplinc knockout mice from multiorgan failure and death. Together, our results identify a previously unknown function for GAPLINC as a negative regulator of inflammation and uncover a key role for this lncRNA in modulating endotoxic shock.

High-Throughput CRISPR Screening Identifies Genes Involved in Macrophage Viability and Inflammatory Pathways.

Macrophages are critical effector cells of the immune system, and understanding genes involved in their viability and function is essential for gaining insights into immune system dysregulation during disease. We use a high-throughput, pooled-based CRISPR-Cas screening approach to identify essential genes required for macrophage viability. In addition, we target 3' UTRs to gain insights into previously unidentified cis-regulatory regions that control these essential genes. Next, using our recently generated nuclear factor κB (NF-κB) reporter line, we perform a fluorescence-activated cell sorting (FACS)-based high-throughput genetic screen and discover a number of previously unidentified positive and negative regulators of the NF-κB pathway. We unravel complexities of the TNF signaling cascade, showing that it can function in an autocrine manner in macrophages to negatively regulate the pathway. Utilizing a single complex library design, we are capable of interrogating various aspects of macrophage biology, thus generating a resource for future studies.

CRISPR/Cas-based screening of long non-coding RNAs (lncRNAs) in macrophages with an NF-κB reporter.

The innate immune system protects against infections by initiating an inducible inflammatory response. NF-κB is one of the critical transcription factors controlling this complex response, but some aspects of its regulation remain unclear. For example, although long non-coding RNAs (lncRNAs) have been shown to critically regulate gene expression, only a fraction of these have been functionally characterized, and the extent to which lncRNAs control NF-κB expression is unknown. Here, we describe the generation of a GFP-based NF-κB reporter system in immortalized murine bone marrow-derived macrophages (iBMDM). Activation of this reporter, using Toll-like receptor ligands, resulted in GFP expression, which could be monitored by flow cytometry. We also established a CRISPR/Cas9 gene deletion system in this NF-κB reporter line, enabling us to screen for genes that regulate NF-κB signaling. Our deletion-based approach identified two long intergenic non-coding(linc)RNAs, lincRNA-Cox2 and lincRNA-AK170409, that control NF-κB signaling. We demonstrate a potential novel role for lincRNA-Cox2 in promoting IκBα degradation in the cytoplasm. For lincRNA-AK170409, we provide evidence that this nuclearly-localized lincRNA regulates a number of inflammation-related genes. In conclusion, we have established an NF-κB-GFP iBMDM reporter cell line and a line that stably expresses Cas9. Our approach enabled the identification of lincRNA-Cox2 and lincRNA-AK170409 as NF-κB regulators, and this tool will be useful for identifying additional genes involved in regulating this transcription factor critical for immune function.