MicroRNAs play a critical role in immune system by regulating/fine tuning inflammatory response

MicroRNAs (miRNAs) have proven to be an extremely important part of the gene expression regulation mechanism. Expression profiling and functional studies indicate that miRNAs participate in the regulation of almost every cellular process investigated and that changes in their expression have a profound effect on their gene targeting activities. While the role of miRNAs in cell development and tumorigenesis was recognized early on, the importance of these small RNAs on immune system development and function has only been touched on recently. Initial studies have indicated that miRNAs have a role in the regulation of immune response and inflammation. Although inflammation protects against pathogenic stimuli and promotes healing, it may actually cause more damage than the activating event if not strictly controlled by essential, internal negative regulators. Out of control immune response causes various forms of chronic inflammatory diseases.

A quartet of papers published recently (July-Aug 09) have shown that miRNAs can act as fine tuners to regulate inflammatory response to various pathogens/stimuli. Many of these are a first look at their respective areas of study and illustrate that miRNA profiling leads to better understanding of this fine tuning process by shedding light on the immunological signaling pathways and the mechanism of biogenesis of miRNAs.

Ting Chen and colleagues at Jiao Tong University School of Medicine, Shanghai, China used a combination of microarray and real time quantitative PCR to analyze miRNA expression profiles in oxidized low density lipoprotein (oxLDL)-stimulated human primary peripheral blood monocytes to study the possible roles of miRNAs in atherosclerotic processes in these cells1. Atherosclerosis is a chronic inflammatory disease of the arterial wall affecting an enormous number of people. Basically, macrophages uptake oxLDL, convert to foam cells, secrete proinflammatory factors, and undergo necrosis to form plaques. MiRNA microarray analysis showed an aberrant miRNA expression in oxLDL-stimulated cells and real-time quantitative PCR validated the results.

Specifically, they found that miRNA-125a-5p expression had increased more than 11-fold. Bioinformatics predicted its target to be the ORP9 gene that – not surprisingly – regulates lipid metabolism. Luciferase assay was used to confirm this finding. They next showed that inhibition of miRNA-125a-5p increased secretion of inflammatory cytokines and significantly induced lipid uptake indicating that miRNA-125a-5p may be an important negative regulator of the inflammatory response in oxLDL-stimulated monocytes/macrophages and may play a protective role against the development of excessive inflammation leading to atherosclerosis.

Viral invasion stimulates an innate immune response, but as discussed earlier, if left uncontrolled this immune response can cause inflammatory disorders. So it is necessary for immune cells to use various layers of negative regulators (including miRNA expression) to avoid excessive inflammation when encountering viral invasion. However, it is possible that these negative regulators, though critical to the host, may also in fact be used by viruses as a method for evading and subverting of the hosts immune responses, aiding their survival in host cells. Jin Hou and colleagues at Tsinghua University School of Medicine, Beijing, China studied the fine tuning effect that miRNA has on inflammatory response to viral infection and propose a new mechanism for the evasion of innate immune control by viruses2.

Upon recognition of viral components, host cells are activated to produce interferons and proinflammatory cytokines. The correct level of production of these proteins induces cellular resistance to the viral infection; however, excessive production may promote the development of immunopathological conditions or immune disorders. Therefore, production must be tightly controlled to strike a delicate balance.

MiRNA microarray data revealed, and real-time quantitative PCR confirmed, many up-regulated miRNAs in vesicular stomatitis virus (VSV) infected mouse macrophages. Specifically miR-146a expression was significantly up-regulated. Further studies revealed that miR-146a negatively regulated VSV-triggered interferon production (which is necessary to exert feedback control over inflammation) promoting VSV replication in macrophages.

Previous studies revealed that, in addition to the miRNAs encoded by viruses themselves, viral infection is able to up- or down-regulate expression of miRNAs in host cells. This suggests that the regulatory network of miRNA expression and immune response is complex to say the least. However, there lies still another level of complexity in the control of the immune response to stimuli/pathogens by miRNAs.

Tina Ruggiero and colleagues at The RNA Technology Center, Genoa, Italy studied how the regulation of biogenesis of a specific miRNA regulates inflammation3. They investigated the contribution of miRNAs to macrophage activation induced by lipopolysaccharide (LPS). MiRNA microarray analysis of certain macrophages revealed that LPS significantly induces the expression of a single miRNA, miR-155 and this induction depends on enhanced miR-155 maturation from its precursors which is promoted by the KH-type splicing regulatory protein (KSRP). Furthermore, they demonstrated that both inhibition of miR-155 and KSRP knockdown enhanced the LPS-induced expression of select inflammation mediators. The effect of KSRP knockdown is reverted by mature miR-155 indicating that miR-155 finely tunes the expression of these inflammation mediators in response to LPS.

Previous studies have demonstrated that Toll-like receptor (TLR) stimulation induces different, but overlapping T helper cytokine/chemokine profiles suggestive of a complex interplay of stimulatory and inhibitory signals which demands tight regulation. Manuela Del Corno and colleagues at Instituto Superiore di Sanita, Rome, Italy studied TLR signaling activation by pathogens and report in this study that CC chemokine ligand 2 (CCL2) secretion triggered by TLR4 or TLR8 engagement is strongly inhibited upon simultaneous activation of both TLRs in human monocyte-derived dendritic cells (MD-DCs)4. The authors used miRNA microarrays to show that this combination of stimulus also effected miRNA expression and wonder whether these regulators of gene expression could somehow contribute to the fine tuning of CCL2 expression in MD-DCs. They suggest that a complete understanding of the role of TLRs in host response to pathogens will require a complete decoding of these multiple receptor interactions and that their findings should serve as a stimulus for future studies of the role of miRNA in DC-driven immune responses.

It is well known now that miRNAs are important players for regulation of cellular activities and it makes sense that they would be involved in a system that requires fine tuned control to function properly such as the immune system. Expression profiling is often the first look at the role that miRNA can play in biological systems and is enabling new discoveries in this exciting field of study.

Ultimately, the identification of miRNAs and their respective targets, and a better understanding of their expression in response to various stimuli/pathogens may reveal that miRNAs offer multiple new therapeutic targets/strategies for fine tuning the immune response to treat and prevent of a number of inflammatory diseases.

  1. Ting Chen, Zhouqing Huang, Liansheng Wang, Yue Wang, Feizhen Wu, Shu Meng, Changqian Wang. (2009) MicroRNA-125a-5p partly regulates the inflammatory response, lipid uptake, and ORP9 expression inoxLDL-stimulated monocyte/macrophages.  Cardiovascular Research 83(1), 131-39. [abstract]
  2. Hou J, Wang P, Lin L, Liu X, Ma F, An H, Wang Z, Cao X. (2009) MicroRNA-146a feedback inhibits RIG-I-dependent Type I IFN production in macrophages by targeting TRAF6, IRAK1, and IRAK2. J Immunol 183(3), 2150-58. [abstract]
  3. Ruggiero T, Trabucchi M, De Santa F, Zupo S, Harfe BD, McManus MT, Rosenfeld MG, Briata P, Gherzi R. (2009) LPS induces KH-type splicing regulatory protein-dependent processing of microRNA-155 precursors in macrophages. FASEB J 23(9), 2898-908. [abstract]
  4. Del Cornò M, Michienzi A, Masotti A, Da Sacco L, Bottazzo GF, Belardelli F, Gessani S. (2009) CC chemokine ligand 2 down-modulation by selected Toll-like receptor agonist combinations contributes to T helper 1 polarization in human dendritic cells. Blood 114(4), 796-806. [abstract]

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