Alzheimer’s disease (AD) is a major cause of dementia in humans, and about 27 million people suffer from this disorder (Wimo et al., 2006; Rubio-Perez and Morillas-Ruiz, 2012). Neuroinflammation, a pathological hallmark of AD, occurs in susceptible regions in the AD brain (Griffin and Mrak, 2002; Cacquevel et al., 2004; Finch and Morgan, 2007; Di Bona et al., 2008, 2009; Rubio-Perez and Morillas-Ruiz, 2012) and plays an important role in AD progression (Ke et al., 2006). During AD, neuroinflammation increases the concentrations of pro-inflammatory cytokines (Bauer et al., 1991; Strauss et al., 1992; Remarque et al., 2001) and percentage of activated immune cells (Lombardi et al., 1999; Speciale et al., 2007; Saresella et al., 2011). Furthermore, it regulates accumulation of inflammatory molecules and activated glial cells in the surroundings of amyloid plaques in the brain of patients with AD and animal models (Bauer et al., 1991; Fillit et al., 1991; Cagnin et al., 2001). However, the functions of both the inflammatory and immune components need to be further investigated in AD pathogenesis (Lal and Forster, 1988; McGeer and McGeer, 2002; Steinman, 2006). Adaptive immune cells such as T and B lymphocytes play important roles in inflammatory responses in the AD brain. Several studies report that differentiation of cluster of differentiation (CD) 3+ T-cells in AD hippocampal parenchyma is increased compared with controls (Togo et al., 2002) and that T-cells are activated and infiltrate into the AD brain. In addition, subsets of T-cells in blood circulation as well as in the brain parenchyma are altered in AD (Town et al., 2005). During infiltration, T cells produce interferon gamma (IFN-γ) that leads to the deposition of amyloid beta peptides (Aβ) and subsequently, cognitive dysfunction (Browne et al., 2013). MicroRNAs (miRNAs) are single-stranded, ~22 nucleotide long non-coding RNAs that regulate gene expression (Kim and Nam, 2006). Several miRNAs are expressed in the brain and are involved in inflammation and microglia activation (Faraoni et al., 2009; Junker et al., 2009; Buck et al., 2010), cell cycle regulation (Johnson et al., 2007; Schultz et al., 2008) and in apoptosis (Chhabra et al., 2009). Recent studies report that miRNAs are also associated with the T cell functions, such as T cell activation and development (Gatto et al., 2008; Rusca et al., 2012; Yang et al., 2012). Among a number of miRNAs, miR-155 reportedly regulates inflammatory and immune responses via modulation of suppressor of cytokine signaling 1 (SOCS1; Dudda et al., 2013), activator protein 1 (Yin et al., 2008), and signal transducers and activators of transcription 5 (STAT5; Kopp et al., 2013). It is observed to be associated with multiple processes, such as regulation of IFN-γ signaling and thus, CD8+ T-cell activation (Gracias et al., 2013), T cell development (Kohlhaas et al., 2009; O’Connell et al., 2009), cell-cell interactions (Martin et al., 2006), and macrophage activation (O’Connell et al., 2007). Recent research demonstrates that the expression of several miRNAs change in AD (Nelson and Keller, 2007; Nelson et al., 2008; Barbato et al., 2009; Kocerha et al., 2009); including in the brain tissue and cerebrospinal fluid (Cogswell et al., 2008). Accordingly, miR-155 expression has been observed to be altered in brain tissue from patients with AD (Culpan et al., 2011). It enhances neuroinflammation in AD progression in a triple transgenic mouse model of AD (Guedes et al., 2014). In this review, we present a new perspective regarding the regulatory role of miR-155 in T-cell functions and thus, AD progression.

In AD, interaction between the central nervous system and immune system is facilitated by lymphocytes in the blood and by immune mediators (Britschgi and Wyss-Coray, 2007). During an inflammatory response, immune cells in the blood migrate and infiltrate the brain. However, the level of T-cells in the brain is significantly lower in AD than in other neurodegenerative diseases, such a multiple sclerosis or Parkinson’s disease (Lafaille, 1998; Nagelkerken, 1998). In normal, unaffected patients, there are few T-cells in the brain; however, due to disruption of the blood brain barrier (BBB), this number increases in the AD brain, specifically in the hippocampus and temporal cortex (Sardi et al., 2011). T-cells are derived from lymphoid stem cells in the bone marrow and mature in the thymus (Starr et al., 2003). Based on the expression of surface molecules such as CD3, CD4, and CD8, the development of T-cells in the thymus has been divided into three stages: initial, intermediate, and final (Starr et al., 2003). Mature T-cells are classified into naïve, effector, and memory T-cells. Each subset expresses specific surface molecules, such as the C-C chemokine receptor type 7 (CCR7), CD45RA, CD70, and CD27 (Romero et al., 2007; Salaun et al., 2011). Based on cytokine profiles, T helper (Th) cells are divided into Th1, Th2, Th9, and Th17 cells depending on the activity of other immune cells and based upon their ability to produce various cytokines (Harrington et al., 2005; Baumjohann and Ansel, 2013). A study of immune parameters in AD reports a decrease in the percentage of naïve T-cells, an increase in the number of memory T-cells and CD4+ T-cells, and a reduction of regulatory T-cells (Tregs) compared with the control group (Larbi et al., 2009). Furthermore, a clinical study of AD reports a significant reduction of naïve CD4+ T-cells in these patients and an increase in number of late-differentiated memory T-cells (Pellicano et al., 2012). Xue and colleagues report a significant reduction of CD3+ T-cells, but marginal changes in CD4+ and CD8+ T-cell subsets in AD (Xue et al., 2009). Richartz-Salzburger and colleagues confirmed the decrease of CD3+ and CD8+ T-cell number, but showed a minor increase in CD4+ cells in AD (Richartz-Salzburger et al., 2007). Several studies report that CD45RO+ T-cell expression increases in the brains of patients with AD (Togo et al., 2002; Lombardi et al., 2004). Further, Lombardi and colleagues (Lombardi et al., 1999) showed an increase in the CD4+ Th and CD25+ Treg subsets in patients with AD. Other studies report that CD45RO+ T-cell expression increases in the amyloid-beta peptide (Aβ), a marker of AD, has been reported to stimulate the macrophage inflammatory protein (MIP)-1α expression in peripheral T-cells and its receptor C-C chemokine receptor type 5 (CCR5) expression in brain endothelial cells. These alterations in signaling help T-cells cross the BBB (Man et al., 2007). In addition, accumulation of Aβ in AD stimulates microglia, which secrete granulocyte macrophage-colony stimulating factor (GM-CSF) to regulate antigen presentation (Tarkowski et al., 2001). Furthermore, circulating Aβ-reactive T-cells are observed in patients with AD (Monsonego et al., 2003). Interestingly, animal studies using APP/PS1 mice demonstrate that Aβ-reactive Th1 cells stimulate microglial activation (Browne et al., 2013) and decrease Aβ pathology (Butovsky et al., 2006; Ethell et al., 2006; Monsonego et al., 2006; Fisher et al., 2010). By secreting Th2-type cytokines (downregulate proinflammatory responses), Aβ-reactive T-cells reduce development of AD symptoms (Weiner et al., 2000; Tarkowski et al., 2001). Additionally, astrocytes secrete transforming growth factor–beta (TGF-β) that promotes Th2 responses and thus, alleviates Aβ pathology in an AD animal model (Wyss-Coray et al., 2001). Interestingly, a co-culture (T-cell and microglia) study demonstrates that Th1 cells up-regulate expression of major histocompatibility complex (MHC) class II and CD40, markers of antigen-presenting cells in microglia (Aloisi et al., 2000). Aβ-reactive Th1 cells increase the secretion of inflammatory cytokines such as interleukin (IL)-1β, IL-6, and tumor necrosis factor–alpha (TNF-α) and promote the expression of MHCII and CD86 in microglia (McQuillan et al., 2010). Th1 and Th17 cells increase microglial production of inflammatory cytokines and expression of MHCII, CD80, and CD86 (Murphy et al., 2010). In addition, hyperpermeability of the BBB in AD increases the infiltration of circulating immune cells, such as T-cells (Togo et al., 2002; Schindowski et al., 2007). In patients with AD, T-cell migration into the brain is followed by enhanced expression of MHC I and II in activated microglia (Mattila et al., 1994; Vugler et al., 2007). In AD, T-cells also participate in various activities, such as expression of neurotrophic factors (Aharoni et al., 2005; Butovsky et al., 2006; Hohlfeld et al., 2006) and neurogenesis (Butovsky et al., 2006; Baron et al., 2008; Mastrangelo et al., 2009; Wolf et al., 2009). Taken together, we propose that T-cells are one of the key regulators of pathological processes in AD. Thus, control of these cells may provide an effective treatment strategy for alleviating the pathogenesis of AD.

miRNAs are short, approximately 22 nucleotide-long, non-coding RNAs (Bartel, 2004) that regulate gene expression by stimulating either mRNA degradation or their translational repression by binding to the 3′-untranslated region of target mRNAs (Bartel et al., 2004; Bagga et al., 2005; Filipowicz et al., 2005; Chen et al., 2006). Similar to pre-mRNAs, a pri-miRNA sequence contains a CAP structure and ploy-A tail. Pri-miRNAs are transcribed by both RNA polymerase I and II (Lee et al., 2004). miRNAs play important roles in diverse mechanisms including cell proliferation, development, and differentiation (Gregory and Shiekhattar, 2005). They are not restricted to the cytoplasm, and are also functional in the nucleus (Foldes-Papp et al., 2009; Park et al., 2010). In humans, over 2500 miRNAs have been identified (Acunzo et al., 2014), and most are located at chromosomal regions exhibiting amplification, deletion, or translocation in various diseases, including cancer (Calin et al., 2004; Lu et al., 2005; Volinia et al., 2006), leukemia (Calin et al., 2004; Lawrie et al., 2007; Xu and Li, 2007), diabetes (Yu et al., 2014), cardiovascular disease (Maegdefessel, 2014), and AD (Cacabelos and Torrellas, 2014; Galimberti et al., 2014). Interestingly, miRNAs are also involved in the regulation of T-cell development, maturation, differentiation, and function (Neilson et al., 2007; Jindra et al., 2010). T-cells play an important role in the adaptive immune response. miR-155 is involved in multiple processes (Gatto et al., 2008; O’Connell et al., 2009), including inflammation (Tili et al., 2007), immunity (Kohlhaas et al., 2009; Sonkoly et al., 2010; Gracias et al., 2013; Kopp et al., 2013) and regulatory mechanisms in numerous diseases. The present review therefore emphasizes the role of miR-155 in T-cell alterations during AD pathology.

Several studies report that the expression of miR-155, mediated by Toll-like receptors, increases in monocytic cell lines during lipopolysaccharide (LPS)-induced inflammation (Taganov et al., 2006; O’Connell et al., 2007). miR was shown to regulate acute inflammation after pathogen recognition by Toll-like receptors on monocytes or macrophages; thus, it was involved in innate immunity (Taganov et al., 2006; O’Connell et al., 2007). In addition, inflammatory cytokines such as IFN-α, γ, and TNF-α also strongly stimulate miR-155 expression. These findings indicate that miR-155 is a component of the innate immune response that depends on functions of numerous inflammatory mediators (O’Connell et al., 2007). Interestingly, miR-155-null mice exhibit reduced IL-2 and IFN-γ production, indicating that it is necessary for T-cell responses (Rodriguez et al., 2007). In recent in vivo studies, elevated levels of miR-155 were observed following T-cell stimulation through the T-cell receptor (TCR; Thai et al., 2007; Dudda et al., 2013; Gracias et al., 2013). miR-155 is also required for development and generation of T cells after TCR activation in vivo (Georgantas et al., 2007), and also for T-cell response, such as dendritic cell-T-cell interactions (Tili et al., 2007; O’Connell et al., 2010). miR-155-deficient mice exhibit impaired antigen-presentation by dendritic cells as wells as defective dendritic cell-T-cell interactions (Rodriguez et al., 2007). Consequently, miR-155-null mice lack adequately activated T-cells (Rodriguez et al., 2007).

Further, miR-155 regulates BBB permeability in central nervous system neuroinflammatory disorders by regulating cell-cell interaction molecules in mouse brains (Lopez-Ramirez et al., 2014). As discussed above, miR-155 is associated with T-cell functions by regulating the TCR and inflammatory cytokine production. These evidence suggest that miR-155 is involved in T-cell immune functions and thus, in the inflammation during AD. Therefore, we summarize the multiple roles of miR-155 in functions of different T-cell types.

Recent in vitro studies report that the expression of miR-155 is up-regulated in activated T-cells (Tam, 2001; Cobb et al., 2006). Thai et al. observed that miR-155-deficient mice have reduced germinal center function, T-cell dependent immune responses, and cytokine production (Thai et al., 2007). In addition, the immune responses in miR-155-deficient mice are diverted toward a Th2 pattern, with a significant increase of IL-10, which mediates immunosuppressive effects against cell-mediated responses (Thai et al., 2007). In addition, T-cells from miR-155-null mice show an increased tendency to differentiate into Th2 type cells; they enhanced Th2-type cytokine production when cultured in vitro (Rodriguez et al., 2007). On the other hand, elevated levels of miR-155 in activated CD4+ T-cells induce Th1 cell differentiation by targeting the IFN-γ receptor alpha chain (Banerjee et al., 2010), and miR-155 deficient CD4+ T-cells are more likely to polarize toward Th2 cells (Rodriguez et al., 2007; Banerjee et al., 2010). miR-155 specifically targets c-Maf, affecting activation of Th2 specific cytokine IL-4 (Rodriguez et al., 2007). A reduced number of IFN-γ-producing cells lacking miR-155 results in T-cell dysfunction and antigen-presentation defects (O’Connell et al., 2009). Phosphatidylinositol 3, 4, 5-trisphosphate 5-phosphatase 1 (SHIP1) has also been suggested as a functional target of miR-155 in CD4+ T cells e.g., macrophages (O’Connell et al., 2009) and dendritic cells (O’Connell et al., 2010). The levels of SHIP1 are reduced in miR-155^−/− mice. SHIP1 suppresses Th1 responses (Tarasenko et al., 2007) and T-cells by modulating IFN-γ production (Huffaker et al., 2012). In human CD4+ T-cells, miR-155 targets the IFN-γ receptor alpha subunit and regulates proliferation of the Th1 and Th2 subsets (Banerjee et al., 2010). Th17 cells are a newly defined subset of CD4+ T-cells that modulate autoimmunity by producing pro-inflammatory cytokines, including IL-17, IL-21, and IL-22 (Langrish et al., 2005; Korn et al., 2007; Miossec et al., 2009). miR-155-deficient mice are characterized by reduced numbers of Th17 cells, and thus, suggest that miR-155 is required for Th17 differentiation (O’Connell et al., 2010; Figures 1, 2). Taken together, miR-155 appears to regulate the differentiation, proliferation, and activation of Th1, Th2, and Th17 cells in the inflammatory state.

Treg cells play an important role in regulating the immune response and preventing autoimmunity (Tang and Bluestone, 2006). Both mouse and human Treg cells express a set of miRNAs (Cobb et al., 2006; Rouas et al., 2009; Smigielska-Czepiel et al., 2014). miR-155 has been reported to regulate the development of Treg cells by inducing forkhead box P3 (Foxp3), which regulates Treg cell survival in vivo (Kohlhaas et al., 2009; Lu et al., 2009). In line with this finding, miR-155 knock-out mice are observed to have reduced Treg cell numbers. Consequently, they had reduced STAT5 phosphorylation and IL-2 receptor signaling due to insufficient SOCS1 suppression (Lu et al., 2009). Other studies postulate that miR-155 deficiency results in reduced numbers of Treg cells due to decreased proliferation and increased apoptosis (Lu et al., 2010; Skinner et al., 2014; Figures 1, 2). Nevertheless, miR-155 appears to modulate the activation and proliferation of Treg cells during inflammation. The evidence suggests that miR-155 also regulates the Treg cell-mediated inflammation during AD.

Differentiation of naïve CD8+ T-cells into effector or memory cytotoxic T-cells (CTLs) depends upon activation following interaction with antigen-presenting cells (Zhang and Bevan, 2010). A deficiency of miR-155 decreases CD8+ T-cell responses, whereas miR-155 overexpression increases CD8+ T-cell responses during inflammation (Dudda et al., 2013; Gracias et al., 2013; Lind et al., 2013). Antigen-specific CD8+ T cells lacking miR-155 show increased phosphorylation of STAT1 in response to Type I interferon signaling (Gracias et al., 2013). Inhibition of STAT1 and interferon regulatory factor 7 (IRF7) partially ameliorates the immune dysfunction of miR-155 deficient CD8+ T-cells in vivo (Gracias et al., 2013). Dudda et al. report that miR-155 deficient CD8+ T-cells exhibit improved immune systems following SOCS1 overexpression (Dudda et al., 2013). Taken together, miR-155 appears to affect the activation of CD8+ T-cells, which are involved in the expression of STAT1, IRF7, and SOCS1 during inflammation.

Inflammatory and immune responses play a crucial role in AD pathogenesis. Thus, an appropriate regulation of diverse T-cell types may alleviate AD related severe pathologies. miR-155 controls characteristics such as survival, differentiation, proliferation, and activation of Th1, Th2, Th17, Treg, and CD8+ T-cells during inflammation. Admittedly, miR-155 is not easy to identify the absolute beneficial function or the absolute negative function on inflammation caused in AD through T cell regulation, suggesting that it is associated with the various T cell type responses and the complicated T cell signaling. However, this review suggests promising approaches for AD treatment, involving control of miR-155. Although findings from clinical studies are still in the preliminary stages, further studies involving modulation of miR-155 levels could enable development of effective treatments for AD.

JS obtained the information and wrote the preliminary draft. JEL reviewed and revised the manuscript. JS and JEL revised details of the manuscript and provided overall supervision.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.