Increased HERV-E clone 4–1 expression contributes to DNA hypomethylation and IL-17 release from CD4+ T cells via miR-302d/MBD2 in systemic lupus erythematosus

Background Increased human endogenous retroviruses E clone 4–1 (HERV-E clone 4–1) mRNA expression is observed in systemic lupus erythematosus (SLE) patients and associates with the disease activity. In this study, we want to further investigate the mechanism of HERV-E clone 4–1 mRNA upregulation and its roles in SLE progression. Methods CD4+ T cells were isolated from venous blood of SLE patients or healthy controls and qRT-PCR was used to detect HERV-E clone 4–1 mRNA expression. We then investigated the regulation of Nuclear factor of activated T cells 1 (NFAT1) and Estrogen receptor-α (ER-α) on HERV-E clone 4–1 transcription and the functions of HERV-E clone 4–1 3′ long terminal repeat (LTR) on DNA hypomethylation and IL-17 release. Results We found HERV-E clone 4–1 mRNA expression was upregulated in CD4+ T cells from SLE patients and positively correlated with SLE disease activity. This is associated with the activation of Ca2+/calcineurin (CaN)/NFAT1 and E2/ER-α signaling pathway and DNA hypomethylation of HERV-E clone 4–1 5’LTR. HERV-E clone 4–1 also takes part in disease pathogenesis of SLE through miR-302d/Methyl-CpG binding domain protein 2 (MBD2)/DNA hypomethylation and IL-17 signaling via its 3’LTR. Conclusions HERV-E clone 4–1 mRNA upregulation is due to the abnormal inflammation/immune/methylation status of SLE and it could act as a potential biomarker for diagnosis of SLE. HERV-E clone 4–1 also takes part in disease pathogenesis of SLE via its 3’LTR and the signaling pathways it involved in may be potential therapeutic targets of SLE. Electronic supplementary material The online version of this article (10.1186/s12964-019-0416-5) contains supplementary material, which is available to authorized users.


Background
Systemic lupus erythematosus (SLE) is an autoimmune disease in which autoreactive CD4 + T cells play an important role [1]. Genetic interactions with environmental factors, particularly ultraviolet light exposure, infection and hormonal factors, might initiate the disease, resulting in immune dysregulation at the level of cytokines, T cells, B cells and macrophages [2].
Human endogenous retroviruses (HERV) are descendants of occasional germline invasion by exogenous retroviruses which occupy as much as 8% of the human genome [3]. HERV-E clone 4-1 is inserted in the short arm of chromosome 19 at position 19p12 upstream of the ZNF66 gene locus and in the antisense orientation. This full-length HERV-E clone 4-1 is considered to be an LTR2C prototype containing 5′ and 3′ LTR elements that are 95.5% identical and encompass gag, pol and env genes (GenBank: M10976, Additional file 1: Figure S1) [4].
Enhanced expression of mRNA from HERV-E clone 4-1 was reported in SLE than healthy controls (HCs) [5,6], and our former study demonstrated that HERV-E clone 4-1 mRNA expression was increased in SLE patients, and the expression level of HERV-E clone 4-1 was associated with SLE disease activity index (SLEDAI) [7]. HERV-E clone 4-1 5'LTR/LTR2C was hypomethylated in CD4 + T cells from SLE patients [7][8][9] which might have close relationship with its expression.
In this study, we sought to further investigate the mechanism of HERV-E clone 4-1 mRNA upregulation and its roles in SLE progression, and to estimate the potential value of HERV-E clone 4-1 in acting as a biomarker and therapeutic target for SLE.

Ethics and selection of patients
This research was approved by the Institutional Research Ethics Committee of Shanghai General Hospital and abided by the ethical guidelines of the Declaration of Helsinki. All the patients involved in this study were adult and written informed consents were obtained from all the patients. All patients with SLE were diagnosed in accordance with the 1997 ACR revised criteria for classification of SLE. Disease activity was assessed using the SLE disease activity index (SLEDAI), and active disease was defined as an SLEDAI score ≥ 5. Age-and sexmatched healthy controls were recruited from the medical staff at Shanghai General Hospital.

Luciferase assay
An NFAT luciferase reporter plasmid (pNFAT-Luc) containing NFAT1 binding promoter elements was used to detect the NFAT1 transcriptional activity. CD4 + T cells were co-transfected with a mixture of 300 ng pNFAT-Luc reporter and 5 ng pRL-TK Renilla luciferase reporter. After different treatment, the luciferase activities were measured using the Dual Luciferase Reporter assay (Promega, Madison, WI, USA). pRL-TK Renilla luciferase reporter was used to normalize the transfection efficiency.
Full-length sequences of HERV-E clone 4-1 5′ LTR containing wild-type of NFAT1 or ER-α predicted binding site was inserted into PGL3-Basic luciferase reporter vector (Promega). Mutant reporter plasmids were prepared using Mutagenesis Kit (Stratagene, La Jolla, CA, USA). Cells were co-transfected with a mixture of 300 ng firefly luciferase reporter, 5 ng pRL-TK Renilla luciferase reporter, and NFAT1 or ER-α plasmids. After 48 h of incubation, the luciferase activities were quantified using the Dual Luciferase Assay System (Promega). The sequences of 3'LTR of HERV-E clone 4-1 mRNA or MBD2 3'UTR containing potential wild-type or mutant binding sites of miR-302d were constructed into pmirGLO vectors (Promega). The luciferase vectors and miR-302d mimics were transfected into CD4 + cells along with pRL-TK vector. The dual-luciferase Reporter assay system (Promega) was used to detect luciferase activity. pRL-TK Renilla luciferase reporter was used to normalize the transfection efficiency.

DNA extraction and global methylation analysis
Assays of DNA extraction and global methylation analysis was described in our previous study [11].

Enzyme-linked immunosorbent assay
The concentration of IL-17 in culture supernatants were measured by Human IL-17 ELISA Kit (ab119535, Abcam) according to the manufacturer's instructions. Optical density values were read at 450 nm using ELx800 Absorbance Microplate Reader (BioTek, VT, USA).

Statistical analysis
Statistical analysis was performed using the SPSS program (version 18.0; SPSS, Chicago, IL, USA). The statistical significance of differences between two groups was tested using Student's t test. Spearman's analysis was used to test correlation. P < 0.05 was considered as statistically significant.

HERV-E clone 4-1 mRNA expression was upregulated in CD4 + T cells from SLE patients
In our former study, we found that HERV-E clone 4-1 mRNA expression was higher in lupus CD4 + T cells than in cells from healthy controls and the HERV-E clone 4-1 mRNA expression level was positively correlated with SLE disease activity [7]. To continue our study, first, we used new samples (Additional file 1: Table S1) to prove HERV-E clone 4-1 mRNA expression was higher in SLE CD4 + T cells than in cells from healthy controls using Quantitative reverse transcription-PCR (qRT-PCR) (Fig. 1a). We also found that HERV-E clone 4-1 mRNA expression level was higher in active patients than that of inactive patients (Fig. 1b) and positively correlated with SLE disease activity (Fig. 1c). We also followed-up some patients who got oral prednisolone and hydroxychloroquine treatment and the activity of SLE changed from active to inactive. We found that the HERV-E clone 4-1 mRNA expressions decreased as the SLEDAI decreased ( Fig. 1d and e). But the HERV-E clone 4-1 mRNA expressions of the inactive patients were also higher than that of HCs (Fig. 1f). What's more, to assess the diagnostic value of HERV-E clone 4-1 mRNA for SLE, we performed Receiver Operating Characteristic (ROC) curve analysis to differentiate SLE from HC with the relative HERV-E clone 4-1 mRNA expressions of the SLE patients and healthy controls (Fig. 1f). The Area Under Curve (AUC) was 0.760, the 95% confidence interval (95% CI) was 0.622 to 0.897, and the best Youden's index is 0.5. This indicated that HERV-E clone 4-1 mRNA might have good diagnostic value for SLE and could act as a potential diagnostic biomarker for SLE.
NFAT1 activity was increased in SLE and associated with increased HERV-E clone 4-1 mRNA To explain why HERV-E clone 4-1 mRNA was upregulated in CD4 + T cells from SLE patients, we wondered if some transcription factors could promote the transcription of HERV-E clone 4-1 mRNA. Since the 5′ LTR contained the transcription factor binding sites [12], this region was used to predict the potential transcription factors. Using Trans-Fac and JASPAR database, we found some transcription factors that might regulate the expression of HERV-E clone 4-1 mRNA. NFAT1, which was proved to play critical roles in SLE [13] caught our attention. First, full length fragment of the human Human endogenous retroviral DNA (4-1) 5′ LTR with wild type (wt) or mutant (mut) predicted NFAT1 binding site was inserted into the luciferase reporter plasmid (Fig. 2a). Then, we use NFAT1 overexpression plasmids to overexpress NFAT1 and NFAT1 siRNA to knockdown NFAT1 (Fig. 2b-e). Luciferase reporter analysis showed that overexpression of NFAT1 led to an increase in luciferase activity of the wt HERV-E clone 4-1 5'LTR plasmid in CD4 + T cells, while mut NFAT1 binding site attenuated the increase of luciferase activity (Fig. 2f). In addition, Chromatin immunoprecipitation (ChIP) assay clearly showed that the predicted NFAT1-binding site in HERV-E clone 4-1 5′ LTR presented the ability to bind to NFAT1 protein (Fig. 2g). Moreover, qRT-PCR analysis showed that overexpression of NFAT1 could increase the expression of HERV-E clone 4-1 mRNA and knockdown of NFAT1 with siRNA could decrease the expression of HERV-E clone 4-1 mRNA ( Fig. 2h and i). Then, we collected CD4+ T cells of SLE patients and HCs to detected NFAT1 activity using NFAT luciferase reporter assay and HERV-E clone 4-1 mRNA expression. We found NFAT1 activity was upregulated in CD4+ T cells from SLE patients (Fig. 2j) and higher in active patients than that of inactive patients (Fig. 1k). What's more, the relative NFAT1 activity had strong correlation with HERV-E clone 4-1 mRNA expression (Fig. 2l). c Correlation between expression of HERV-E clone 4-1 mRNA and SLEDAI score analyzed with Spearman's analysis. d and e SLEDAI score and HERV-E clone 4-1 mRNA expression in patients who got oral prednisolone and hydroxychloroquine treatment (N = 9); data were compared using the paired Student's t test. f Relative expression of HERV-E clone 4-1 mRNA in CD4 + cells from inactive SLE patients (N = 11) and HCs (N = 21) compared using the unpaired Student's t test. g ROC curve of relative HERV-E clone 4-1 mRNA expression for differentiating SLE patients from HCs. Data were represented as mean ± SD. *P < 0.05, **P < 0.01, ***P < 0.001 So, these results all together suggested that NFAT1 could induce HERV-E clone 4-1 mRNA expression via binding to its 5′ LTR. We also detected the influence of some factors on NFAT1 activity and HERV-E clone 4-1 mRNA expression (Fig. 2m and n).
E2 could upregulate HERV-E clone 4-1 mRNA expression via ER-α in CD4 + T cells from SLE patients When selecting the potential transcript factors that might regulate the expression of HERV-E clone 4-1 mRNA, ER-α, which was the receptor of E2 drew our a Predicted wild-type (wt) binding sites and corresponding mutant (mut) sites of NFAT1 on HERV-E clone 4-1 5'LTR. b-e qRT-PCR and western-blot assays showing relative NFAT1 mRNA and protein expression in CD4 + cells from SLE patient with NFAT1 overexpression or knockdown. f Luciferase assays were performed in CD4 + T cells from SLE patient transfected with wt or mut luciferase reporter. Each luciferase activity was normalized to the value obtained in the cells transfected with vector (N = 3). g ChIP assay was used to assess NFAT1 binding site at HERV-E clone 4-1 5'LTR. h and i) Relative HERV-E clone 4-1 mRNA expression in CD4 + cells from SLE patient with NFAT1 overexpression or knockdown (N = 3). j and k Relative NFAT1 activity in CD4 + cells from SLE patients (N = 15) and HCs (N = 13) or active SLE patients (N = 12) and inactive SLE patients (N = 3) compared using the unpaired Student's t test. l Correlation between relative NFAT1 activity and relative HERV-E clone 4-1 mRNA expression analyzed with Spearman's analysis (N = 15). m and n CD4 + T cells from SLE patient were treated with TNF-α, IL-6, E2, LPS, UVB, HCQ or prednisolone in vitro. Relative NFAT1 activity (L) and relative HERV-E clone 4-1 mRNA expression (m) were detected. Data were represented as mean ± SD. *P < 0.05, **P < 0.01, ***P < 0.001 attention. This is because SLE has a predilection for females of child-bearing age who have relatively high estrogen level and estrogen is also a risk factor for SLE [14] and HERV-E was upregulated in breast cancer and ovarian cancer [15,16]. Then, we further explored the role of E2 and ER-α in SLE. Accordingly, full length fragment of the human endogenous retroviral DNA (4-1) 5′ LTR with wild type (wt) or mutant (mut) predicted ER-α binding site was inserted into the luciferase reporter plasmid (Fig. 3a). Then, we use ER-α overexpression plasmids to overexpress ER-α and ER-α siRNA to knockdown ER-α (Fig. 3b-e). Luciferase reporter analysis showed that overexpression of ER-α led to an increase in luciferase activity of the wt HERV-E clone 4-1 5'LTR plasmid in CD4 + T cells, while mut ER-α binding site attenuated the increase of luciferase activity (Fig. 3f). In addition, ChIP assay clearly showed that the predicted ER-α-binding site in HERV-E clone 4-1 5′ LTR presented the ability to bind to ER-α protein (Fig. 3g). Moreover, qRT-PCR analysis showed that ER-α plasmids could increase the expression of HERV-E clone 4-1 mRNA while ER-α antagonist AZD9496 maleate and ER-α siRNA could decrease the expression of HERV-E clone 4-1 mRNA (Fig. 3h-j). In addition, AZD9496 and ER-α siRNA could reverse the upregulated HERV-E clone 4-1 mRNA expression induced by E2 (Fig. 3k and l). So, these results all together suggested that E2 could also upregulate HERV-E clone 4-1 mRNA expression via ER-α in CD4 + T cells from SLE patients.

DNA hypomethylation of HERV-E clone 4-1 5'LTR contributed to the increase of HERV-E clone 4-1 mRNA
In our former study, we found the HERV-E clone 4-1 5'LTR was hypomethylated in CD4 + T cells from SLE patients and its methylation could be inhibited by 5-aza C [7]. Here, we investigated whether this DNA hypomethylation was involved in the NFAT1 or ER-α induced HERV-E clone 4-1 mRNA upregulation. We found that HERV-E clone 4-1 mRNA expressions were upregulated when NFAT1 or ER-α was overexpressed or 5-aza C was used in CD4 + T cells from SLE patients and HCs (Fig. 4a-d). In CD4 + T cells from SLE patients and HCs, HERV-E clone 4-1 mRNA expressions were higher when both NFAT1 was overexpressed and 5-aza C was used than that when NFAT1 was overexpressed or 5aza C was used ( Fig. 4a and b); accordingly, HERV-E clone 4-1 mRNA expressions were higher when both ER-α was overexpressed and 5-aza C was used than that when ER-α was overexpressed or 5-aza C was used ( Fig. 4c and d). Besides, the times of HERV-E clone 4-1 mRNA upregulation were higher in CD4 + T cells from SLE patients than that of HCs when NFAT1 or ER-α was overexpressed ( Fig. 4e and  f), and the times of HERV-E clone 4-1 mRNA upregulation were higher in CD4 + T cells from HCs than that of SLE patients when 5-aza C was used (Fig. 4g). These results together suggested that DNA hypomethylation contributed to the upregulation of HERV-E clone 4-1 mRNA induced by NFAT1 and ER-α.

HERV-E clone 4-1 3'LTR induced DNA hypomethylation and IL-17 release via miR-302d/MBD2
Since 3'UTRs of mRNAs were reported to act as natural miRNA sponges and could serve as competitive endogenous RNAs (ceRNAs) of other genes through sharing the common miRNAs [17][18][19][20]. We want to explore whether the 3'LTR of HERV-E clone 4-1 mRNA could act as a miRNA sponge and act ceRNAs of other genes. Through programs based on microRNA.org and Targetscan, we found that there was a potential binding site of miR-302d in the 3'LTR of HERV-E clone 4-1 mRNA (Fig. 5a). Then, we performed luciferase reporter assays to determine this interaction. Luciferase assay showed that miR-302d mimics could decrease the luciferase activity of reporter containing wt 3'LTR of HERV-E clone 4-1 while mut binding site attenuated the increase of luciferase activity (Fig. 5b). This suggested that 3'LTR of HERV-E clone 4-1 could bind to miR-302d and act as a sponge for miR-302d. We also found MBD2 was another potential target of miR-302d (Fig. 5a) and verified the interaction between MBD2 3'UTR and miR-302d using luciferase assay (Fig. 5c). Then, we found that overexpression of 3'LTR of HERV-E clone 4-1 (Fig. 5d) increased the protein levels of MBD2 and miR-302d mimics could rescue the increase of MBD2 protein by the 3'LTR (Fig. 5e). These results suggested that HERV-E clone 4-1 acts as a ceRNA of MBD2 to positively regulate MBD2 expression in 3'LTR and miR-302d dependent manners. We also detected the expression of IRF-9 which was a proved target of miR-302d in SLE [21] and found that overexpression of 3'LTR of HERV-E clone 4-1 increased the protein levels of IRF9 and miR-302d mimics could rescue the increase of IRF9 protein by the 3'LTR (Fig. 5e).
The mRNA levels of MBD2 in was increased in CD4 + T cells of SLE patients and inversely correlated with global DNA methylation and positively correlated with and SLE-DAI score [22,23]. What's more, MBD2 was found to stimulates Th17 cell differentiation and IL-17 release in other autoimmune diseases [24][25][26] and IL-17 play critical functions in the pathophysiology of SLE [27,28] So, MBD2 might play important roles in SLE progression. Then, we intended to further study the role of HERV-E clone 4-1, miR-302d and MBD2 in global DNA methylation and IL-17 expression in CD4 + T cells of SLE patients. CD4+ T cells were transfected with HERV-E clone 4-1 3'LTR expression plasmids, miR-302d mimics or MBD2 expression plasmids. Global DNA methylation levels, intracellular IL-17 level and IL-17 level in culture supernatants were subsequently measured. The results showed that global DNA methylation level decreased when CD4 + T cells of SLE were transfected with 3'LTR expression plasmids or MBD2 expression plasmids and increased when transfected with miR-302d mimics (Fig.  5f). Intracellular IL-17 level and IL-17 level in culture supernatants increased when CD4 + T cells of SLE were transfected with 3'LTR expression plasmids or MBD2 expression plasmids and decreased when transfected with miR-302d mimics (Fig. 5g-j). All together, these results suggested that HERV-E clone 4-1 3'LTR induce DNA hypomethylation and IL-17 release via miR-302d/MBD2 in CD4 + T cells of SLE.

Discussion
Some studies had proved that HERV-E clone 4-1 mRNA expression was increased in SLE patients, and the expression level of HERV-E clone 4-1 was associated with SLE disease activity [5][6][7], however, they didn't thoroughly investigate the function and mechanism of HERV-E clone 4-1 in SLE. In this study, we investigated the mechanism of HERV-E clone 4-1 mRNA upregulation in CD4 + T cells Fig. 3 E2 could upregulate HERV-E clone 4-1 mRNA expression via ER-α in CD4 + T cells from SLE patients. a Predicted wild-type (wt) binding sites and corresponding mutant (mut) sites of ER-α on HERV-E clone 4-1 5'LTR. b-e qRT-PCR and western-blot assays showing relative ER-α mRNA and protein expression in CD4+ cells from SLE patient with ER-α overexpression or knockdown. f Luciferase assays were performed in CD4 + T cells from SLE patient transfected with wt or mut luciferase reporter. Each luciferase activity was normalized to the value obtained in the cells transfected with vector. g ChIP assay was used to assess ER-α binding site at HERV-E clone 4-1 5'LTR. h Relative HERV-E clone 4-1 mRNA expression in CD4+ cells from SLE patient with ER-α overexpression compared using the paired Student's t test. i and j Relative HERV-E clone 4-1 mRNA expression in CD4 + cells from SLE patient with ER-α inhibition using siRNA or AZD9496 compared using the paired Student's t test. k and l Relative HERV-E clone 4-1 mRNA expression in CD4 + cells from SLE patient when ER-α siRNA or AZD9496 was used after E2 treatment compared using the paired Student's t test. Data were represented as mean ± SD, N = 3. *P < 0.05, **P < 0.01, ***P < 0.001 from SLE patients and its roles in SLE progression. First, we found NFAT1 could induce HERV-E clone 4-1 mRNA expression by binding to its 5′ LTR. NFAT1, which is a key factor of Ca 2+ / calcineurin (CaN)/NFAT signaling pathways, was verified to be activated in SLE [13]. We also demonstrated that NFAT1 activity was upregulated in SLE and positively correlated with HERV-E clone 4-1 mRNA expression. NFAT1 are phosphorylated and reside in the cytoplasm in resting cells; upon stimulation, they are dephosphorylated by calcineurin, translocate to the nucleus, and become transcriptionally active [29][30][31]. Then the activated NFAT1 can regulate transcription of some inflammatory cytokines such as IL-6, IL-8, TNF-α and interferon-γ (IFN-γ) [32][33][34][35]. Furthermore, we found TNF-α, IL-6, E2, LPS, UVB could upregulate NFAT1 activity and HERV-E clone 4-1 mRNA expression and these factors play critical roles in SLE [14,[36][37][38]. These results together may explain the roles of NFAT1 in HERV-E clone 4-1 mRNA expression in SLE.
Adreno cortico hormones are an important class of anti-inflammatory/immunosuppressive drugs. They can inhibit the expression of TNF-α and IL-6 and decrease the activity of SLE [39]. Ca 2+ /CaN/NFAT signaling is an important pathway in the T-cell activation of SLE and some calcineurin inhibitors such as cyclosporine A and tacrolimus have been used in the clinical treatment of SLE [40]. Hydroxychloroquine, which could block Ca 2+ /CaN/ NFAT signaling pathway through inhibiting the sustained Ca 2+ storage release from the endoplasmic reticulum [41], was found to repress NFAT1 activity and HERV-E clone 4-1 expression. Prednisolone and hydroxychloroquine are first-line drugs in the treatment of SLE and all the patients followed-up got oral prednisolone and hydroxychloroquine treatment. These reasons interpret it well why HERV-E clone 4-1 mRNA expressions decreased after prednisolone and hydroxychloroquine treatment. So, we hold that the upregulation of HERV-E clone 4-1 mRNA is mainly due to the abnormal inflammation / immune status of SLE which involving many inflammatory cytokines and other risk factors. We also found that E2 could upregulate HERV-E clone 4-1 mRNA expression via ER-α. ER-α is one of the estrogen receptors which can be activated by estrogen and regulate gene transcription in nucleus [42]. Interestingly, HERV-E was upregulated in breast cancer and ovarian cancer [15,16] and this probably also has close relationship with E2 and ER-α. ER-α antagonist is also a good CD4 + T cells from SLE patient or HC were treated with NFAT1 plasmids, ER-α plasmids, 5-aza C alone or in combination with 5-aza C in vitro. Relative HERV-E clone 4-1 mRNA expression (a-d) were detected. e and f Times of HERV-E clone 4-1 mRNA upregulation were compared by the Student's t test in CD4 + T cells from SLE patient and HC when NFAT1 or ER-α was overexpressed. g Times of HERV-E clone 4-1 mRNA upregulation were compared by the Student's t test in CD4 + T cells from SLE patient and HC when 5-aza C was used. Data were represented as mean ± SD, N = 3. *P < 0.05, **P < 0.01, ***P < 0.001 approach to restrain the expression of HERV-E clone 4-1. Taken together, we think these signaling pathways are good therapeutic targets for HERV-E clone 4-1.
Some studies found the HERV-E clone 4-1 5'LTR was hypomethylated in CD4 + T cells from SLE patients [7][8][9]. We found that DNA hypomethylation contributed to upregulation of HERV-E clone 4-1 mRNA induced by NFAT1 and ER-α. We think DNA hypomethylation of HERV-E clone 4-1 5'LTR is an indispensable factor that account for the upregulation of HERV-E clone 4-1 mRNA for that upregulation of HERV-E clone 4-1 mRNA mainly exists in SLE while not in some other diseases that involving NFAT1 and ER-α activation.
In this study, we found that HERV-E clone 4-1 3'LTR could act as natural miRNA sponges for miR-302d to restrain miR-302d activity. MiR-302d was proved to be downregulated in SLE patient monocytes and could inhibit the type I IFN pathway which was a major contributor to SLE pathogenesis via its target IRF-9 [21]. HERV-E clone 4-1 3'LTR could positively regulate MBD2 expression by acting as a ceRNA of MBD2 via miR-302d and HERV-E clone 4-1 3'LTR could induce DNA hypomethylation and IL-17 release via miR-302d/MBD2 in CD4 + T cells of SLE. DNA hypomethylation of immune cells in SLE is associated with immune dysfunction and play important roles in the initiation b and c Effects of miR-302d on the expression of luciferase reporter genes containing HERV-E clone 4-1 3'LTR or MBD2 3'UTR. Luciferase activity was normalized to the value obtained in cells transfected with NC oligonucleotides. d Relative expression of HERV-E clone 4-1 3'LTR mRNA when CD4 + T cells were transfected with HERV-E clone 4-1 3'LTR expression plasmids. e Western blot analysis of IRF9 and MBD2 proteins in CD4 + T cells from SLE patients transfected with HERV-E clone 4-1 3'LTR expression plasmids and/or miR-302d mimics and the corresponding negative controls. Relative global DNA methylation level (f), intracellular IL-17 level (g-i) and IL-17 level in culture supernatants (j) in CD4+ T cells from SLE patient transfected with HERV-E clone 4-1 3'LTR expression plasmids, miR-302d mimics or MBD2 expression plasmids and the corresponding negative controls and development of SLE [43,44]. IL-17 is a proinflammatory cytokine produced by activated T cells and plays a crucial role in disease pathogenesis and represent an attractive therapeutic target for SLE [27,28]. Thus, we hold that HERV-E clone 4-1 takes part in disease pathogenesis of SLE through miR-302d/MBD2/DNA hypomethylation and IL-17 signaling via its 3'LTR. So, HERV-E clone 4-1 3'LTR may be a potential therapeutic target of SLE. Taken together, we draw a network diagram hypothesis showing relationship between HERV-E clone 4-1 and SLE which shows the important roles of HERV-E clone 4-1 in SLE pathogenesis (Fig. 6).
However, we should admit that we didn't further investigate the role of HERV-E clone 4-1 proteins and this is a shortcoming of this study. This mainly because there is no specific antibody for these proteins.

Conclusions
In conclusion, we found that HERV-E clone 4-1 mRNA expression was upregulated in CD4 + T cells from SLE patients and could act as a good biomarker for diagnosis of SLE. This is associated with the activation of Ca 2+ /CaN/NFAT1 and E2/ER-α signaling pathway and DNA hypomethylation of HERV-E clone 4-1 5'LTR. HERV-E clone 4-1 also takes part in disease pathogenesis of SLE through miR-302d/MBD2/ DNA hypomethylation and IL-17 signaling via its 3'LTR. These signaling pathways may be potential therapeutic targets of SLE.

Additional file
Additional file 1: Figure S1. The structure of HERV-E clone 4-1. Table  S1. Clinical characteristics of SLE patients and healthy controls.