The Regulatory Cross-Talk between microRNAs and Novel Members of the B7 Family in Human Diseases: A Scoping Review

The members of the B7 family, as immune checkpoint molecules, can substantially regulate immune responses. Since microRNAs (miRs) can regulate gene expression post-transcriptionally, we conducted a scoping review to summarize and discuss the regulatory cross-talk between miRs and new B7 family immune checkpoint molecules, i.e., B7-H3, B7-H4, B7-H5, butyrophilin like 2 (BTNL2), B7-H6, B7-H7, and immunoglobulin like domain containing receptor 2 (ILDR2). The current study was performed using a six-stage methodology structure and Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. PubMed, Embase, Scopus, Cochrane, ProQuest, and Google Scholar were systematically searched to obtain the relevant records to 5 November 2020. Two authors independently reviewed the obtained records and extracted the desired data. After quantitative and qualitative analyses, we used bioinformatics approaches to extend our knowledge about the regulatory cross-talk between miRs and the abovementioned B7 family members. Twenty-seven articles were identified that fulfilled the inclusion criteria. Studies with different designs reported gene–miR regulatory axes in various cancer and non-cancer diseases. The regulatory cross-talk between the aforementioned B7 family molecules and miRs might provide valuable insights into the pathogenesis of various human diseases.


Introduction
Immune checkpoints can considerably regulate immune responses [1]. These molecules are critical for maintaining self-tolerance and preventing the stimulation of immune responses against normal peripheral tissues. Indeed, suppressing inhibitory axes, e.g., the immune checkpoint axis of cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed death-ligand 1 (PD-L1), has revolutionized cancer immunotherapy [2].
The B7 family is a group of immune checkpoints commonly expressed in different immune cells, such as antigen-presenting cells, T cells, B cells, natural killer cells, and various

Methods
The methodology for the current scoping review was according to the framework recommended by Arksey and O'Malley [21] and later enhanced by Levac et al. [22]. It consists of five distinctive phases: (1) identifying the research question; (2) identifying relevant studies; (3) study selection; (4) charting the data; and (5) collating, summarizing, and reporting results. An optional sixth phase, consultation, was not a part of our scoping review. This study was also carefully guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA-ScR) Checklist [23].

Identifying the Research Question
This study aimed to map the current literature on new B7 family members and miRs' regulatory axis. To address this aim, we sought to answer the upcoming question: precisely what is known from existing literature about the regulation of new B7 family molecules in human diseases by miRs?

Identifying Relevant Studies
PubMed, Embase, Scopus, Cochrane, ProQuest, and Google Scholar were searched based on the specific search tips of each database without any restriction, in order to recognize all potentially eligible publications. MeSH and Emtree terms were used if available. The search strategy for PubMed and Embase databases are described in Supplementary Materials. The last search was conducted on 5 November 2020. We also included articles identified from hand-searching or reference lists of the selected studies.

Selecting Studies
The included studies fulfilled the following criteria: (1) explicitly discussing the regulatory axes between the new B7 family members and miRs, (2) evaluating human specimens or cell lines, and (3) written in English. The exclusion criteria were (1) studies on animals and (2) studies that were not primary/original research. First, publication titles and abstracts were independently screened by two investigators (N.K.A. and H.S.) for eligibility, according to the above-mentioned criteria. In the second phase, the full-text assessment of the selected studies was conducted, and studies that met the eligibility criteria were included in the final data analysis. Any disagreements were solved by involving a third reviewer (B.B.), if required.

Charting the Data
Two authors (N.K.A. and H.S.) independently extracted data into a predefined charting form in Microsoft Excel. It provided information about regulatory axes, samples, methods, diseases, and key findings of each study.

Collating, Summarizing, and Reporting the Results
Both quantitative and qualitative analyses were carried out. We presented a descriptive numerical summary of the chosen studies' features for the quantitative portion. We prepared a narrative review, addressing our abovementioned research question for the qualitative assessment, considering the importance of findings in the broader context proposed by Levac et al. [22].

Bioinformatics Analysis
The miRDB database (accessed on 15 February 2021) [24] was used to predict other possible new B7 family members and miRs' interaction. Only functional human miRs with a target score >80 were included. According to miRDB, a predicted target with a score >80 is most likely to be real. Also, miRs with >2000 predicted targets in the genome were excluded. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis was performed by miRPathDB v2.0 to obtain insights on the pathways that are targeted by predicted miRs [25]. Specifically, the focus was on the "Immune system" in the KEGG database. Selection of data based on strong experimental evidence and with a minimum of two significant miRs per pathway was used as criteria for the miRPathDB query.

Results
The flowchart of literature identification, inclusion, and exclusion is demonstrated in Figure 1. A total of 199 articles were identified through database searching and hand-searching, of which 96 were duplicates. Seventy-four articles were excluded during the title and abstract review, based on irrelevance to the inclusion criteria. The full-text assessment of the remaining 29 records was conducted, and two more records were excluded because they were not human studies [26,27]. Finally, a total of 27 eligible papers remained . The characteristics of the included studies are summarized in Table 1.

Clear cell carcinoma
The bioinformatic view showed that basic leucine zipper ATF-like transcription factor (BATF) in B lymphocyte and SMAD in monocytes might be responsible for the dysregulation of B7-H7 in kidney renal clear cell carcinoma (KIRC). miR-3116 and miR-6870-5p may have a role in the regulation of B7-H7.

ILDR2
There were no studies investigating the interaction of miR with this gene.

BTNL2
There were no papers investigating the interaction of miR with this gene.

B7-H3
B7-H3, also referred to as CD276, can regulate the stimulation and inhibition of T cells [55,56]. A variety of cells, e.g., natural killer cells, activated T-cells, dendritic cells, macrophages, and non-hematopoietic cells, can express B7-H3 [57]. Preliminary findings reported that B7-H3 could promote CD4+ and CD8+ T cell proliferation by T cell receptor (TCR) stimulation using immobilized Ig fusion protein [55]. However, it is wellestablished that B7-H3 can suppress the activation of CD4+ T-cell and the release of effector cytokines [58,59]. This suppression might facilitate the function of transcription factors like nuclear factor of activated T cells (NF-AT), nuclear factor kappa B (NF-κB), and activator protein 1 (AP-1), playing significant roles in T cell function [58]. Moreover, B7-H3 overexpression has been identified in various cancers, e.g., breast [60], lung [61], kidney, prostate [62], and ovarian cancer [63]. Furthermore, the inhibition of B7-H3 has decreased angiogenesis in medulloblastoma, indicating its essential role in tumor angiogenesis [37]. As an overexpressed oncogene in various cancers, MYC has a critical role in cancer development, e.g., angiogenesis, apoptosis, proliferation [64,65]. Since MYC inhibition has been associated with the suppressed expression of B7-H3 in medulloblastoma cells, the MYC-B7-H3 regulatory axis can play an essential role in regulating angiogenesis [37]. It has been indicated that B7-H3 knockdown can repress the PI3K/Akt pathway, resulting in decreased STAT3 activity. Since STAT3 can promote the expression of matrix metalloproteinase 2 (MMP2) and matrix metalloproteinase 9 (MMP9), B7-H3 can regulate the expression of MMP2 and MMP9 [66]. Moreover, B7-H3 can be involved in inflammatory conditions, e.g., sepsis and bacterial meningitis [51]. Since the mRNA expression of B7-H3 is not as remarkable as its protein expression, the post-transcriptional regulating process might have a considerable effect [57].
Transforming growth factor-beta 1 (TGF-β1) can upregulate the B7-H3 expression and pave the way for immune evasion of colorectal cancer cells via the miR-155/miR-143 axis. Indeed, miR-143 can inhibit B7-H3 expression in colorectal cancer cells [53]. The downregulation of miR-124 can increase the expression of B7-H3 in osteosarcoma. It has been reported that miR-124 might decrease proliferating cell nuclear antigen (PCNA) and cyclin D1, and elevate apoptotic protein poly-ADP ribose polymerase (PARP). Also, miR-124 mimics might increase the therapeutic efficacy of monoclonal antibodies targeting B7-H3 [42]. Furthermore, miR-1253 can inhibit cell proliferation and promotes the apoptosis of tumor cells in medulloblastoma. Indeed, miR-1253 restoration and B7-H3 silencing can substantially decrease the migration of medulloblastoma cells [32]. Zhou et al. found that the low expression of miR-192, miR-378, and miR-145 can result in B7-H3 overexpression and immune evasion in colorectal cancer [53]. Consistent with this, the downregulation of miR-145 has been associated with increased B7-H3 expression, facilitating lymph node metastasis in lung cancer patients [31]. In cervical cancer, miR-199a, which targets B7-H3, can inhibit cell proliferation, invasion, and migration [50].
In 2017, 62 hsa-miRs were identified as regulating B7-H4 in pancreatic cancer [38]. These miRs were mentioned above in the Results section.

B7-H5
B7-H5, also known as VISTA, C10orf54, Dies1, and PD-1H, is a type-I membrane protein that can stimulate terminal differentiation of embryonic stem cells (ESCs) into cardiomyocytes/neurons via the bone morphogenetic protein (BMP) signaling pathway [87]. It has been reported that miR-125a-5p can directly repress the transcription of B7-H5 and inhibit ESC differentiation [26]. B7-H5 also plays a pivotal regulatory function in adipocyte differentiation independently from BMP signaling. In particular, the elevated level of B7-H5 has been shown exclusively in differentiated fat cells and blocked adipocyte differentiation [88]. In B7-H5 knockout mice, the elevation of inflammatory cytokines can result in chronic multi-organ inflammation, indicating the critical role of B7-H5 in suppressing inflammation [89]. In Crohn's disease, there is a negative association between B7-H5 expression and hsa-miR-16-1 [47]. B7-H5 can serve as a ligand and receptor on T cells, suppressing the activation of naïve and memory T cells [90,91]. The presence of two PKC binding sites in the cytoplasmic region of B7-H5 might indicate that B7-H5 is a receptor [92,93]. B7-H5 can be overexpressed in cancer-associated/cancer-adjacent gastric myofibroblasts. However, B7-H5 expression is generally downregulated in epithelial gastric cancer cells. This can be explained by B7-H5 promoter methylation, the overexpression of miR-125a-5p, or a combination of both, and even the existence of mutant p53 [36]. Indeed, the downregulation of B7-H5 has been associated with de-differentiation and triggered epithelial-mesenchymal transition (EMT) in epithelial cells.

B7-H7
B7-H7, which has previously been referred to as B7-H5, is known as the human endogenous retro virus-H long repeat-associating 2 (HHLA2) [99,100]. Its receptors can be found on various immune cells, e.g., monocytes, T cells, B cells, and dendritic cells. TMIGD2, which is referred to as CD28 homolog, is one of the identified B7-H7 receptors [101]. In antigen-presenting cells, B7-H7 co-stimulates the proliferation of naïve T cell and cytokine production across TMIGD2 by serine-threonine kinase AKT phosphorylation. However, the second B7-H7 receptor on activated T cells can exert a coinhibitory role, because activated T cells do not express TMIGD2. The identification of the second receptor might clarify the role of B7-H7 in T cell activation and the tumor microenvironment [102]. It has been reported that B7-H7 is upregulated in lung cancer, osteosarcoma, and breast cancer, and its elevated expression is correlated with a poor prognosis in affected patients [103]. BATF in B lymphocytes and SMAD in monocytes might be involved in the dysregulation of B7-H7 in kidney clear-cell carcinoma. It has been indicated that hsa-miR-6870-5p and hsa-miR-3116 might have a role in this modulatory mechanism [40].

Bioinformatics Analysis
Based on our results, four potential new interactions between B7 family members and miRs have been identified: (1) the hsa-miR-29b-3p/B7-H3 axis, (2) the hsa-miR-29a-3p/B7-H3 axis, (3) the hsa-miR-125a-5p/B7-H4 axis, and (4) the hsa-miR-486-5p/B7-H4 axis. Of these four interactions, the association of different isoforms of miR-29 with B7-H3 has been investigated in previous studies (see above). As mentioned earlier, hsa-miR-125a-5p regulates B7-H5 expression in gastric cancer, but its association with B7-H4 has not been studied. Recent findings have shown that miR-125a-5p plays a pivotal role in suppressing the classical activation of macrophages (M1-type) induced by lipopolysaccharide (LPS) stimulation, while promoting IL-4-induced expression of the alternative M2 macrophages by targeting KLF13, a transcriptional factor that is involved in T lymphocyte activation and inflammation [104]. In addition, miR-486-5p is an immunomodulatory tumor suppressor miR that has been reported to have key roles in various oncological and non-oncological disorders [105]. Although our knowledge about the role of miR-125a-5p/B7-H4 and miR-486-5p/B7-H4 axes in the immune pathways and the pathogenesis of various diseases is still preliminary, our in silico analysis can pave the way for further investigations.

Data Availability Statement:
The data presented in this study are available in this review.