**Emerging Role of MicroRNAs in the Pathophysiology of Immune System**

Marco Rossi, Pierosandro Tagliaferri and Pierfrancesco Tassone

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51543

#### **1. Introduction**

206 Immunodeficiency

1993;70:283.

[60] Wood P, Mayne A, Joyce H, et al. A mutation in Bruton's tyrosine kinase as a cause of

[61] Zhang Q, Finn A. Mucosal immunology of vaccines against pathogenic nasopharyngeal

[62] Zielen S, Buhring I, Strnad N, et al. Immunogenicity and tolerance of a 7-valent pneumococcal conjugate vaccine in nonresponders to the 23-valent pneumococcal

[63] Zora J, Silk H, Tinkelman D. Evaluation of postimmunization pneumococcal titers in children with recurrent infections and normal levels of immunoglobulin. Ann Allergy

selective anti-polysaccharide antibody deficiency. J Pediatr 2001;139:148.

bacteria. J Clin Pathol 2004;57:1015-21.

vaccine. Infect Immun 2000;68:1435.

#### **1.1. Overview of MiRNA biology**

MicroRNAs are small (22 nucleotides), noncoding, double-stranded RNA molecules, that can regulate gene expression primarily by reducing the abilities of specific mRNAs to be transduced to their encoded proteins. The first recognized miRNA found in 1993 is lin-4, that controls the cell fate at larval stages in Caenorhabditis elegans [1, 2]. Bioinformatic approaches suggested that the mammalian miRNA repertoire is involved in regulation of 30% of all protein-encoding genes [3].

Human miRNAs are encoded within introns of coding genes and introns and exons of noncoding transcripts [4]. Generation of mature miRNAs is due to a series of endonucleolytic steps starting from long primary transcripts (pri-miRNAs). The primiRNAs are cleaved in the nucleus to a\_70 nt intermediate with the typical stem-loop hairpin structure, precursor miRNAs (pre-miRNAs) by the Drosha- DGCR8 microprocessor complex [5, 6]. The pre-miRNAs are further processed into\_22 nt double-stranded miRNA duplex by the cytoplasmic RNase III enzyme Dicer [7]. One strand of this miRNA duplex (the guide strand) incorporates into a large protein complex, RNA-induced silencing complex (RISC), formed by Dicer, TRBP (a dsRNA-binding domain protein) and Ago2 (the Argonaute protein 2), and finally becomes the mature miRNA. The other strand, the socalled passenger strand, is degraded.

Each mature miRNA interacts with a specific mRNA in the mRNA's 3´-untranslated region (3´UTR), leading to translational repression or mRNA degradation. Besides, some evidences have shown that miRNA can increase translation [8].

© 2012 Tassone et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **2. The role of MiRNAs in hematopoietic cell development**

The development of hematopoietic and immune system requires an integrated network of survival, proliferation and apoptotic signals that are finely tuned along differentiation. miRNAs represent efficient modulators of such a system as they can affect the expression of multiple genes at different stages. The identification of putative miRNA involved in hematopoietic ontogeny has been one of primary topic of miRNA studies since their discovery. To clarify the role of specific miRNAs, Chen et al. [9] first cloned about 100 previously identified miRNAs and analyzed only those expressed in hematopoietic cells (miR181, 142 and 223). miR181 was found in lineage negative (Lin -) mouse bone marrow undifferentiated cells and strongly upregulated in mature B cells and within the thymus; miR142 was more ubiquitously expressed, while 223 was mostly confined to myeloid lineage. miR181 was then cloned into a GFP gene carrying retroviral vector and ectopically expressed in Lin- bone marrow cells. Infected cells were then followed in vitro to check their lineage commitment. A preferential development of B cells was observed. When Lin- miR181+ cells were transplanted in irradiated mice, lymphoid repopulation showed a prevalence of B cell population as compared to control (80% vs 32%, respectively). This study was of primary relevance as it has shown the effects of a single miRNA in lymphopoiesis and addressed a method to study next candidate miRNAs. De Yebenes and colleagues [10] observed that miR181b is involved also in immunoglobulin class switch at activated B cell level. Hence, miR 181 family is involved in early (switch from pro B cell to pre B cell) and late (from centroblasts to activated B cells) stages of B cell development.

Emerging Role of MicroRNAs in the Pathophysiology of Immune System 209

of granulocyte precursors and hyper mature circulating granulocytes. miR223 targets ELF-1 like factor (mef) 2c, a transcription factor that promotes myeloid differentiation and IGFR1, thus affecting expansion of myeloid precursors committed to granulocytic differentiation

Granulocytic differentiation is further regulated through a critical transcription factor, GFI1 (growth factor independent-1). GFI1 expression depends upon miR21 as demonstrated in a

Innate responses imply the final differentiation and interaction of intervening cells to the site where inflammatory stimuli were generated. Several miRNAs have been described as implicated in a complex network, that controls the on and off phases of the

miR146a is upregulated upon LPS stimulation in monocytes and is likely to be responsible of the phenomenon known as hyporesponsiveness to prolonged LPS exposure. Indeed, miR146a acts as a negative regulator of LPS induced responses. LPS-induced NF Kb promotes miR146a upregulation, which in turns suppresses TRAF6 (TNF receptor associated factor 6), IRAK1/2 (Interleukin 1 receptor associated kinase 1). These genes encode key adaptor molecules along TLR related pathways and are involved in innate responses through TNF activation and production of IL-1 dependent molecules such as IL-8 and RANTES [20-22]. Therefore, monocytes become hyporesponsive to further stimulation with LPS and relevant pro inflammatory molecules are reduced in the

miR155 seems to act on the same pathways but with opposite effects to miR146a. Indeed, engagement of several TLRs (3,4 and 9) promotes miR155 transcription through AP1 and NFkb. miR155 main targets are SOCS1 (suppressor of cytokine signaling 1) and SHIP1(Src Homology-2 domain-containing inositol-5'-phosphatase 1) that lead to release of proinflammatory cytokines in the microenvironment such as TNF alpha and IFN gamma

miR223 can regulate also monocyte-macrophage differentiation by targeting IKK-alpha (IKb kinase) and leaving NFKb to promote inflammatory genes transcription. The final result is

The emerging data suggest that miRNA regulation of inflammatory and innate responses is timely tuned according to microenviromental stimuli. Indeed, TLR stimulation evokes miR155 upregulation within 2hr , while other miRNAs, such as miR21 are produced according to a delayed time frame. These observations are likely related to a differential role

**3. MiRNAs at the cross-roads between innate and adaptive immune** 

[18].

knockout model [19].

**3.1. Innate immune responses** 

**responses** 

response.

microenvironment.

the transition of monocyte to macrophage [27].

[23-26].

The expression of miR181 in mouse thymus prompted investigators to evaluate their role in T cell selection. Interestingly, miR181 is expressed at higher levels in early T cell differentiation as its expression drops from double negative/double positive cells to single positive CD4/CD8 cells [11] MiR17-92 cluster (miR17, 18a, 19a, 20a, 19b and 92) has been implicated in B cell lymphopoiesis (transition pro B to pre B cells) by Ventura et al. [12]. Interestingly, in mice this cluster is homologous to the miR-106a-63 (except miR18 and 19), although only mice lacking miR17-92 show a relevant phenotype, including B cell differentation arrest. It is likely that miR17-92 cluster controls apoptotic signals through suppression of Bim and PTEN [13] miR-150 has been involved in the transition pro B to pre B cells through suppression of c-Myb [14-16], a transcription factor that leads this phase. miR-150 is strongly upregulated along T cell development beginning from double positive stage and modulates expression of NOTCH3 [17]. NOTCH3 gene is known to be involved in T cell differentiation and leukemogenesis.

Overall, these data indicate that miR181/miR17-92/miR150 are among the main regulators of early T and B lymphopoiesis from the common lymphoid precursor.

An analogue role is played by miR 223 in myeloid lineage. Chen et al [9] observed that miR223 is highly expressed in mouse bone marrow. Indeed, miR223 tunes granulocytic differentation both at an early and late phase [18]. MiR223 knock out mice show expansion of granulocyte precursors and hyper mature circulating granulocytes. miR223 targets ELF-1 like factor (mef) 2c, a transcription factor that promotes myeloid differentiation and IGFR1, thus affecting expansion of myeloid precursors committed to granulocytic differentiation [18].

Granulocytic differentiation is further regulated through a critical transcription factor, GFI1 (growth factor independent-1). GFI1 expression depends upon miR21 as demonstrated in a knockout model [19].
