Table 1.

Clues suggesting a contribution of oral–gingival microbiota to the contribution of rheumatoid arthritis.

Clinical and In Vitro Studies
Author, Year	Objective	Patients (Mean Age)	Study Design	Study Group	Main Results
Rotsein, I. and Katz, J., 2021 [10]	To assess the prevalence of periapical abscesses in patients with RA, and to evaluate the effects of commonly used antirheumatic medications on such prevalence	NR	A cross-sectional study	Patients with RA were included and data of diagnosis of RA antirheumatic medications, and the presence of periapical abscess were recorded.	Statistically higher prevalence of periapical abscesses in patients with RA than in the general patient population (OR: 2.60) (p < 0.0001). Treatment with TNF-α inhibitors may lower the prevalence of periapical abscesses in such patients
Xiao, F. et al., 2021 [11]	To explore the correlation between RA and PD	631 (40.93)	A cross-sectional comparative study	307 patients with RA (RA group) and 324 healthy individuals (control group) were selected, the incidence of PD in both groups was analyzed, and ELISA was used to detect the IL-1β and TNF-α levels in the (GCF) of both groups	The prevalence of PD in the RA group was significantly higher than that in the control group and increased with age and disease duration in patients with RA. The presence of RA can increase risk of PD occurrence and is positively correlated with levels of IL-1β and TNF-α in the GCF.
González, D.A. et al., 2021 [12]	To identify clinical and/or serological variables in patients with RA that are associated with their periodontal severity	110 (48.5)	A cross-sectional study	Subjects with RA and chronic periodontitis were included. RA clinical parameters and rheumatoid factor, presence of bone erosions, and rheumatic nodules) and corticosteroid therapy were considered. Periodontitis was evaluated according to the American Academy of Periodontology (1999).	Periodontitis severity is associated with higher RA disease activity (DAS-284.1; OR: 51.4; 95%, CI 9.4–281.5), longer disease duration, and rheumatoid nodules (OR: 6.; 95% CI: 1.3–31.6).
Moentadj, R. et al., 2021 [13]	To study the oral microbiota in a prospective cohort of patients with RA, FDR, and HC, then to genomically and functionally characterize streptococcal species from each group to understand their potential contribution to RA development.	222 (45.7–53.5)	A cross-sectional comparative study	116 patients with RA, 63 FDR, and 43 HC were included; salivary samples were taken by tongue swabbing for analysis of microbial dysbiosis by using 16S rRNA gene sequencing technology	Dysbiosis-associated periodontal inflammation and barrier dysfunction may permit arthritogenic insoluble pro-inflammatory pathogen-associated molecules, such as streptococcal cell walls, to reach synovial tissue. Streptococcus species can also be enriched in the oral cavity of some RA and their FDRs, and are the source of peptidoglycan-polysaccharide polymers that can induce arthritis in mice
Lehenaff, R. et al., 2021 [14]	To analyze the subgingival microbiome of both shallow (health-associated) and deep (disease-associated) subgingival sites in patients with RA, and then compare them with non-RA household controls.	18 (53.2–55.1)	A case-control study	RA (n = 8) ok and non-RA (n = 10) subjects were recruited and subgingival plaque samples from both shallow (periodontal health-associated, probing depth ≤3 mm) and deep subgingival sites (periodontal disease-associated, probing depth ≥4 mm) were collected. RA subjects also had rheumatological evaluation. Plaque community profiles were analyzed using 16 S rRNA sequencing.	Lack of consistency of gingival microbiomes in RA may relate to numerous explanations: differences in RA subtypes, timing of sampling (RA onset or flares, versus RA in remission), treatments given, and how subgingival plaques are sampled: samples were collected from both shallow (periodontal health-associated, probing depth ≤3 mm) and deep subgingival sites (periodontal disease-associated, probing depth ≥4 mm). In RA, the microbiomes of deep and shallow sites in patients with RA were more similar to each other.
Arévalo-Caro, C. et al., 2021 [15]	To determine the relationship between titres of anti-Porphyromonas gingivalis (P. gingivalis) antibody andthe RA, HLA-DRB1 susceptibility region associated with SE using the Gregersen’s and de Vries’s classification methods.	100	A case-control study	Results of IgG1 and IgG2 anti-P. gingivalis antibodies, ACPA, diagnosis for RA, and PD, and a genetic study of the HLA-DRB1 region were obtained from 50 patients with RA and 50 control individuals.	Although no association was found between SE and anti-P. gingivalis antibodies; according to the de Vries’s classification, an association existed between HLA-DRB1 neutral alleles, and high titres of IgG anti-P.gingivalis antibodies for RA, focusing on novel associations between P.gingivalis and RA. NETosis or intracellular infections by gut pathobionts may also contribute to citrullination. This could explain why anti-Pg antibodies were not clearly associated with SE or ACPAs in patients with RA.
Manoil, D. et al., 2021 [16]	To examine potential correlations between detached subgingival bacteria collected in GCF and RA parameters.	149 (34.95–48.04)	A case-control study	Patients with RA (n = 52) and patients with BD, (n = 40) as another systemic inflammatory disease, were studied along with a systemically healthy control group (HC; n = 57). Full mouth periodontal parameters were recorded. RA activity was assessed using the 28-joint DAS-28. RFs-IgM and -IgA were measured using ELISA. GCF samples were investigated by using fluorescent in situ hybridization for 10 different bacterial taxa.	Altogether, although RA and ACPAs could also be driven by other triggers such as gut bacteria, the PPAD of Pg might contribute to ACPAs in a subset of patients with RA. Pg displayed significant correlations with not only plaque scores and bleeding on probing, but also with RF-IgA. Patients with RA displaying RF-IgA levels >75 IU/mL exhibited fivefold more abundant Pg levels than those with levels below this threshold did. This association with RF-IgA levels appeared even more pronounced, by sixfold more Pg levels (p = 0.025), in patients with a DAS-28 score >3.2, indicative of being moderately/very active.
Peng, H.Y. et al., 2020 [17]	To determine the role of Pg in RA and to identify novel therapeutic targets for auto-inflammatory diseases.	238	A case-control study	Serum samples were obtained from patients with RA (n = 155), PD subjects (n = 48), and HCs (n = 35). The profile of antibody response to gingipain RgpA-specific domains, a cysteine protease produced by Pg, was determined in all included patients’ sera, and the potential protective effects of RgpA domains in an experimental arthritis model were also tested.	The pathobiont Pg is the bacteria detected most often in patients with both PD and RA, especially in smokers; smoking also strongly worsens PD and RA. Several lines of evidence suggest that Pg might contribute to some PD and RA pathogenesis: Pg produces cysteine proteases, such as gingipain RgpA, endowed with the potential to induce significant bone loss in animal model systems and in patients in either alveolar or subchondral bone.
Frid, P. et al. 2020 [18]	To characterize the salivary oral microbiome associated with JIA, and correlate it with the disease activity, including gingival inflammation	93 (12.6)	A case-control study	Patients with JIA (n = 59) and healthy controls (HC; n = 34) were recruited, and microbiome profiling of saliva samples was performed by sequencing the V1 V3 region of the 16S rRNA gene.	No differences were found in alpha and beta diversity of oral bacteria among children with JIA and HC. Several taxa associated with chronic inflammation were found to be associated with JIA and disease activity.
Lopez-Oliva, I. et al., 2018 [19]	To characterize the periodontal microbiome in periodontally healthy individuals with and without RA, using next-generation sequencing	41 (36–60)	A case-control study	Patients with RA (n = 22) and non-RA controls (n = 19) were recruted. All participants were periodontally healthy. Subgingival plaque samples were collected and analyzed using 16S rDNA sequencing.	In periodontally healthy individuals with RA, the oral microbiome is enriched for pro inflammatory organisms, and those capable of producing substantial amounts of citrulline (pro-antigenic).
Dong, X.H. et al., 2018 [20]	To present evidence showing that P. gingivalis OMVs promote the mucosal transmission of HIV-1	NA	In vitro	The host cells were co-incubated with HIVNL4.3 alone, HIV/P. gingivalis 33,277 vesicle complexes, or HIV/KDP 128 vesicle complexes for 30 min. Cell-free complexes were removed by washing the cells with PBS, and the cells were then fixed, permeabilized, immunostained, and analyzed using confocal microscopy.	RANK overexpression and an increase in the ratio of RANK-L to osteoprotegerin was observed in both PD and RA, with a high level of RANK-L expression on gingival B cells, most notably those capable of recognizing Pg. Infection with the virus worsens both periodontitis and RA, which act by promoting the growth of organisms such as Pg. Reciprocally, Pg could foster infection of oral keratinocytes by viruses such as EBV and CMV, as Pg OMVs promote the mucosal transmission of viruses
Stobernack, T. et al., 2018 [21]	To identify possible functions of PPAD in the periodontal environment.	NA	In vitro	Human paraffin-embedded gingival tissues were collected from patients with Pg-colonized periodontitis. Deparaffinization of 5 m sections was performed by several xylene, ethanol, and water washes. Endogenous peroxidase activity was inhibited by the addition of hydrogen peroxide in methanol, followed by blocking of nonspecific antibody binding with 1% bovine serum albumin and 1% normal goat serum in PBS.	PPAD induces the citrullination of various autoantigens. The rationale for this Pg-induced citrullination may be linked to the ability of PPAD to citrullinate the histone H3, thereby facilitating bacterial escape from NETs, and immune evasion of Pg. The PAD enzyme, detectable in the gingiva of patients with PD, neutralizes human innate immune defenses at three other distinct levels: bacterial phagocytosis, capture in NETs, and killing by the lysozyme-derived cationic antimicrobial peptide LP9
Zhang, X. et al., 2015 [2]	To characterize the oral and gut microbiomes in patients with RA compared to heathly controls.	191 (18–65)	A case-control study	A metagenomic shotgun sequencing and a metagenome-wide association study of fecal, dental, and salivary samples from treatment-naive individuals with RA (n = 94), and HC (n = 97) was conducted.	Compared to heathly controls, dysbiosis was detected in the gut and oral microbiomes of patients with RA, but it was partially resolved after RA treatment. Alterations in the gut, dental, or saliva microbiome distinguished individuals with RA from HC.
Scher, J.U. et al., 2013 [22]	To determine if particular intestinal bacteria are associated with RA	114 (42.4–50)	A cross-sectional comparative study	Subjects with NORA (n = 44), chronic, treated RA (n = 26), PsA (n = 16), and HCs (n = 28) were included, and 16S sequencing (of the 16S gene (regions V1–V2, 454 platform) on 114 stool samples from patients with RA and controls, and shotgun sequencing on a subset of 44 such samples were performed.	Many cases of RA may later be induced by pathobionts from gut microbiota, such as Prevotella Copri, indicating a potential role of this bacterium in the pathogenesis of RA.
Review articles
Alghamdi, M.A., 2021 [23]	To highlight the effect of both gut and oral microbiota dysbiosis on the development of RA, as well as to discuss how the alteration in microbiota composition leads to the overgrowth of some bacterial species entangled in RA pathogenicity.	NA	Review	NA	Pg-induced citrullination may contribute to the occurrence of antibodies to citrullinated peptides (ACPA), the most specific signature of RA, especially in patients with the SE HLA-DRB1-04/01
Nik-Azis, N.M. et al., 2021 [24]	To discuss how RA and specifically ACPA-positive RA link to PD, and to appraise the epidemiologicalevidenceonthe relationship between ACPA-positive RA and PD	NA	Review	Articles were searched following the PRISMA guidelines across the Medline, Web of Science, Scopus, and Cochrane Library databases	The severity manifestations of periodontal disease did not differ much in patients with RA with and without ACPA
Chu, X.J. et al., 2021 [25]	To describe a possible relationship between RA and the microbiome of the oral cavity and gut.	NA	A systematic review	A bibliographic search was performed in three databases (EMBASE, Cochrane Library, and PubMed) from inception to 7 June 2020 to identify case control studies that compared the oral and gut microbiomes in adult patients with RA to those of controls	Of 26 articles reviewed, ≥3 articles reported decreased Streptococcus and Haemophilus contrasting with increased Prevotella in the oral cavity of patients with RA compared with heathy controls. In addition, some Prevotella species, including P. histicola and P. oulorum, showed increased trends in oral cavites of patients with RA, compared with HCs.
Gonzalez-Febles, J. et al., 2021 [26]	To update existing information on the epidemiological association between RA and PD and the biological mechanisms linking these two diseases. To determine whether treatment of PD could influence the initiation and progression of RA.	NA	Review	PubMed, Medline, and Cochrane Library	There was a clear association between PD and RA, with ORs ranging from 1.82 to 20.57 and patients with RA having a high prevalence of PD and tooth loss. The presence of both periodontal inflammation and high numbers of periodontopathic bacteria (Pg and Aa) have been associated with the onset of RA and increased RA disease activity. Nonsurgical periodontal therapy seems to play a role in the control of RA disease activity.
Zorba, M. et al. 2020 [27]	To review current literature on the possible role of the oral microbiome in the pathogenesis of autoimmune diseases.	NA	Review	PubMed, Medline, Research Gate, and Google Scholar	Oral dysbiosis has also been reported in other adult disorders sometimes overlapping with RA (Sjögren’s syndrome, systemic lupus erythematosus, RA, BD, Crohn’s disease, and psoriasis).
Berthelot, J.M and Le Goff, B., 2010 [28]	To assess the prevalence of PD in patients with RA.	NA	Review	PubMed, Medline, and EMBASE	Modest but increased prevalence of PD among patients with RA compared to the general population, unrelated to secondary Sjögren’s syndrome. Indeed, the prevalence of the SE HLA-DRB1-04 is increased in both RA and PD, and exacerbated T-cell responsiveness with high tissue levels of IL-17 and exaggerated B-cell/plasma cells responses are found in both the synovium and gingival tissue affected with PD.

Abbreviations: ACPA: anticitrullinated peptides antibodies; BD: Behcet’s disease; CMV: cytomegalovirus; CI: confidence interval; DAS; Disease Activity Score; EBV: Epstein–Barr virus; FDR: first-degree relative; GCF: gingival crevicular fluid; HC: healthy controls; IgG1: immunoglobulin G1; IgG2: immunoglobulin G2; IL: interleukin; JIA: juvenile idiopathic arthritis; NORA: new-onset rheumatoid arthritis; OMVs: outer-membrane vesicles; OR: odds ratio; PBS: phosphate-buffered saline; PD: periodontal disease; Pg: Porphyromonas gingivalis; PPAD: porphyromonas peptidylarginine deiminase; PsA: psoriatic arthritis; RA: rheumatoid arthritis; RANK: receptor activator of nuclear factor κ B; RANK-L: receptor activator of nuclear factor κ B ligand; RFs: rheumatoid factors; rRNA: ribosomal ribonucleic acid; SE: shared epitope; TNF: tumor necrosis factor. However, another look at the contribution of oral bacteria to RA is welcome, as the hypothesis that some oral bacteria such as Pg could elicit autoimmunity just because of cross-reactivity between citrullinated peptides in the synovium and other peptides hypercitrullinated in gingiva by the PAD of Pg does not fit well with numerous observations made by rheumatologists.