The association between rheumatoid arthritis and periodontal disease

Chronic, plaque-associated inflammation of the gingiva and the periodontium are among the most common oral diseases. Periodontitis (PD) is characterized by the inflammatory destruction of the periodontal attachment and alveolar bone, and its clinical appearance can be influenced by congenital as well as acquired factors. The existence of a rheumatic or other inflammatory systemic disease may promote PD in both its emergence and progress. However, there is evidence that PD maintains systemic diseases. Nevertheless, many mechanisms in the pathogenesis have not yet been examined sufficiently, so that a final explanatory model is still under discussion, and we hereby present arguments in favor of this. In this review, we also discuss in detail the fact that oral bacterial infections and inflammation seem to be linked directly to the etiopathogenesis of rheumatoid arthritis (RA). There are findings that support the hypothesis that oral infections play a role in RA pathogenesis. Of special importance are the impact of periodontal pathogens, such as Porphyromonas gingivalis on citrullination, and the association of PD in RA patients with seropositivity toward rheumatoid factor and the anti-cyclic citrullinated peptide antibody.


Introduction
Periodontitis (PD), the most common oral disease, is a destructive infl ammatory disease of the supporting tissues of the teeth and is caused by specifi c microorganisms [1]. As a rule, PD develops through gingivitis, an infl ammation of the marginal periodontium. However, not every gingivitis develops further into PD. Both the amount and virulence of the microorganisms and the resistance factors of the host (risk factors and immune status) are crucial for the progression of the periodontal destruction ( Figure 1). PD has been proposed as having an etiologic or modulating role in cardiovascular and cerebrovascular disease, diabetes, and respiratory disease and adverse pregnancy outcome, and several mechanisms have been proposed to explain or support such theories. Moreover, oral lesions are indicators of disease progression, and the oral cavity can be a window to overall health and body systems. In recent years, remarkable epidemiological and pathological relationships between periodontal diseases and rheumatic diseases, especially rheumatoid arthritis (RA), have been presented.

Pathogenesis of periodontal diseases
Chronic, plaque-associated infl ammation of the periodontium is among the most common oral diseases and has a prevalence of 80% to 90% [1], resulting in soft and hard periodontal tissue destruction and ultimately in tooth loss [2]. Both the amount and virulence of the microorganisms and the resistance factors of the host (risk factors and immune status) are crucial for the initiation and progression of the periodontal destruction [3]. Besides detailed concepts about microbiological, molecular, and cellular mechanisms, which determine the strength and balance of the cellular and humoral host response in tissues, it became apparent that a complex and primarily endogenous periodontal microfl ora is responsible for disease initiation and progression.

Bacterial oral infection
PD is to be understood as an opportunistic infection [4]. It results directly in tissue injury or provokes excessive, autodestructive infl ammatory responses, depending on the pathogenicity of the agents or the performance of the immune defense. In particular, Gram-negative anaerobic bacteria, which form a bacterial plaque biofi lm on the tooth surface, initiate this tissue-destroying process [5]. Among a complex and still largely unknown microfl ora, about 20 bacteria species, which live in the subgingival environment, have been identifi ed as periodontal pathogens and are linked to several forms of PD. Th e best analyzed of these bacteria are Porphyromonas gingivalis,

Abstract
Chronic, plaque-associated infl ammation of the gingiva and the periodontium are among the most common oral diseases. Periodontitis (PD) is characterized by the infl ammatory destruction of the periodontal attachment and alveolar bone, and its clinical appearance can be infl uenced by congenital as well as acquired factors. The existence of a rheumatic or other infl ammatory systemic disease may promote PD in both its emergence and progress. However, there is evidence that PD maintains systemic diseases. Nevertheless, many mechanisms in the pathogenesis have not yet been examined suffi ciently, so that a fi nal explanatory model is still under discussion, and we hereby present arguments in favor of this. In this review, we also discuss in detail the fact that oral bacterial infections and infl ammation seem to be linked directly to the etiopathogenesis of rheumatoid arthritis (RA). There are fi ndings that support the hypothesis that oral infections play a role in RA pathogenesis. Of special importance are the impact of periodontal pathogens, such as Porphyromonas gingivalis on citrullination, and the association of PD in RA patients with seropositivity toward rheumatoid factor and the anti-cyclic citrullinated peptide antibody.

Biofi lm infection
In the periodontal pocket of periodontal disease, there exists a condition in which periodontopathic bacteria form a fi lm-like colony by adherence and aggregation [7]. After a few hours, resident microorganisms -most of which are mostly Gram-positive -stick to the membranaceous layer of the smooth surface of the teeth (pellicle layer), which settles within minutes up to a few hours after thorough mechanical tooth cleaning. By means of fi mbriae, pili, and so-called glycocalyx, Gramnegative bacteria can attach to those microorganisms [8]. A complex and extremely resistant biofi lm with a decisive evolutionary advantage for the bacteria develops. Th ey can cooperate metabolically [8], and because of the complexity and subgingival location, the bacteria are protected from immunologic defense mechanisms of the host as well as from antibiotic agents. Th e enhancement of the bacterial pathogenicity is a result thereof [8]. Continuous stimulation by bacteria infl icts injury within the gingiva, destroys the local immune system, and activates osteoclasts in the tissue, so that the PD can progress [9].

Virulence factors
Besides the ability to form biofi lms, the synthesis of toxic substances is among the most important characteristics of dental pathogenic bacteria. Enzymes have a direct tissue-destroying eff ect (neutral phosphatases and collagenases). Leucotoxins and immunoglobulin-splitting substances elude defense mechanisms [10]. Moreover, osteoclast-activating alkaline and acid phosphatases lead indirectly to the loss of the periodontal attachment apparatus. Lipopolysaccharides (LPSs) and proteo gly cans, which are both part of the cell wall of Gram-negative bacteria, play a key role in activating the immune system with the release of diverse cytokines, subsequently causing alveolar bone resorption.

The two phases of the immune responsethe critical pathway
Th e pathogenetic model of the critical pathway assumes the simple cause-and-eff ect principle and permits a diff erentiated view of the processes of the marginal development of PD. According to this model, there are two phases [11].

First phase: initial immune response
Th rough the junctional epithelium, which adheres to the tooth surface, a constant immunologic interaction between periodontal-pathogenic bacteria and the cells of the immune system takes place [10]. Bacteria release metabolites and toxins and thereby activate the immune system. LPS is one of the strongest activators of immunologic infl ammatory processes [10,12]. Apart from the formation of integrin ICAM-1 (intercellular adhesion molecule-1), numerous proinfl ammatory and infl ammatory cytokines such as interleukin-1-beta (IL-1β) as well as IL-6, IL-8, IL-1, and tumor necrosis factor-alpha (TNF-α) [13] are involved in this process. In this way, the path of the neutrophil granulocytes via leucodiapedesis through the endothelium to the infl ammatory center is established [10]. Th ere, pathogenic bacteria, which are opsonized by complement factors (C3b and C4b) and immunoglobulins (IgG1 and IgG2), are phagocytosed [14]. Th is early infl ammatory response to the biofi lm is dominated by polymorph nuclear leucocytes (PMNLs), which have a modulatory eff ect on T and B lymphocytes and which increasingly penetrate the tissue [15]. An increased secretion of gingival sulcus fl uid leads to the washout of proinfl ammatory mediators and the recruitment of infl ammatory cells from the tissue. Together with immunoglobulins, PMNLs form an eff ective defense bank against pathogenic microorganisms [11]. If the microbial attack is successfully and suffi ciently embanked in this fi rst phase, the infl ammation is -according to the model of the critical pathway -restricted to the marginal gingiva (resulting in gingivitis) [11]. Th e second phase of the immune response is activated if the described defense mechanisms are insuffi cient.

Second phase of the immune reaction
Current data show that PD develops preferably in disposed persons with an abnormal infl ammatory immune reaction to microbial plaque. Monocytes or macrophages or both become active as an unspecifi c immune response [16]. Histologically, the longer-lasting infl ammation manifests itself in the conversion of the junctional epithelium into the so-called pocket epi thelium, which is characterized by its increased prolifera tion rate in the basal area and an increased permeability for larger molecules. Because of the stimulation of the CD14 receptors by LPS complex formation, macrophages secrete the prostaglandin E 2 (PgE 2 ) and the proinfl am matory cytokines IL-1β and TNF-α, which convey the periodontal bone absorption. Matrix metalloproteinases (MMPs) participate in the des truction of the extracellular matrix [12]. Furthermore, secondary infl ammatory media tors (for example, the platelet-activated factor and biogenic amines [bradykinin and histamin] [16]) support the adhesion of neutrophil granulocytes to endothelic cells by endothelial expression of adhesion molecules [12]. IL-1β and PgE 2 both have an inhibiting eff ect on the collagen synthesis [12]. PgE 2 leads to vasodilation, bone atrophy, and edema formation [12]. B and T cells and plasma cells are to be found in the infl ammatory infi ltrate and gingival sulcus fl uid, respectively. Most of the plasma cells form immunoglobulins of the type IgG (approxi mately 60% to 80%). Signifi cantly fewer of those formed are IgAs (approximately 10% to 40%), and only a few are IgMs [17]. Th e local antibody synthesis produces antibodies that are diverse in quantity and specifi city. Th ese antibodies can be more frequently local than systemic [17].

Rheumatic diseases such as rheumatoid arthritis and periodontitis
Many recent studies have identifi ed statistically significant associations between established PD and rheumatic diseases. In particular, RA as a chronic infl ammatory joint disease shows numerous characteristics and pathogenetic processes that have similarities to PD. RA and its relationship to PD are the best-studied topics, and there are numerous publications in this regard. Th erefore, this review focuses on this disease. Interesting and important results that describe the importance of microcirculation, osteoporosis, and other common risk factors in respect to the relationship between the rheumatic diseases such as RA and PD are expected to appear in the future.

Association studies in rheumatoid arthritis and periodontitis
It has been reported that patients with longstanding active RA have a signifi cantly increased incidence of PD when compared with healthy subjects [18][19][20] and that patients with PD have a higher prevalence of RA than patients without PD [21]. de Pablo and colleagues [22], using data from the Th ird National Health and Nutrition Examination Survey (NHANES III), show a signifi cant association between RA and PD in the US population. In patients with RA, a signifi cant correlation in teeth loss and alveolar bone loss was found, and this may well represent various aspects of periodontal health [23].

Role of oral infections and immune response
Successful treatment of RA with antibiotics against bacterial anaerobic infections points to the involvement of bacteria in the etiopathogenesis of RA [24]. Th e hypothesis that oral infections play a role in RA pathogenesis may be supported by the detection of bacterial DNA of anaerobes and high antibody titers against these bacteria in both the serum and the synovial fl uid of RA patients in the early and later stages of the disease [25]. Th e highly pathogenic bacteria of the oral fl ora can maintain a chronic bacteremia that may damage distant organs (joints and endocardium) [26]. Periodontal pathogens, as P. gingivalis, have the ability to impair epithelium integrity, invade human endothelium cells, and infl uence both transcription and protein synthesis [27]. By these means, periodontal pathogenes have a direct systemic access to the blood circulation [28]. Examinations of patients with RA show an increased number of specifi c antibodies and the DNA of these bacteria in the blood and synovial liquid [29]. Recently, it was shown that P. gingivalis is able to invade primary human chondrocytes that were isolated from knee joints and to induce cellular eff ects [30]. As a consequence of this invasion, P. gingivalis delayed cell cycle progression and increased cell apop tosis in these chondroncytes [30].

P. gingivalis and gingipains
P. gingivalis produces arginine-specifi c (gingipain R) and lysine-specifi c (gingipain K) cystein endopeptidase [24], which play a role in bacterial housekeeping and infection, including amino acid uptake from host proteins and fi mbriae maturation [31,32]. Gingipains are proteolytic enzymes [32] responsible for the expression of the virulence. Proteinases such as MMP-1, MMP-3, and MMP-9 are activated and extracellular matrix host proteins such as laminin, fi bronectin, and collagen are degraded by P. gingivalis gingipains [33]. Furthermore, gingipains are responsible for an increased vascular permeability and for the degradation of complement factors [33].

P. gingivalis and the enzyme peptidylarginine deiminase
P. gingivalis is currently the only known bacterium with expression of peptidyl arginine deiminase (PAD), which represents an important pathogenic factor of RA [20,34]. Th e enzymatic deimination of arginine residuals to citrulline through the enzyme PAD is a form of posttranslational protein modifi cation [35]. Th e consequence is a modifi cation of the structu re of the protein, by which its biochemical and antigen characteristics are changed. However, the PAD expressed by P. gingivalis is not entirely homologue to human PAD but leads to an irreversible, post-translational conversion of arginine to citrulline [20,35]. So far, citrullination has been found particularly in proteins of the cytoskeleton (for example, cytokeratin, vimentin, and fi laggrin) in the course of the apoptosis. Diseases like RA result in the (patho)physiological citrullination of structure proteins and in an increased accumulation of citrullinated proteins (for example, mutated citrullinated vimentin) [26]. Th e reduced immunotolerance of these patients to citrullinated proteins seems to be a key problem, so thatespecially for RAs of high severity -an increased formation of autoantibodies develops [26]. For P. gingivalis, the by-product ammoniac serves as a neutralizer of the acid milieu and thereby ensures the growth of the bacterium [34]. Interestingly, patients with PD show a high concentration of ammonia in the sulcus fl uid [34]. Possibly, periodontal infections with pathogens such as P. gingivalis in association with a genetic predisposition support infl ammatory diseases like RA or have an immuno regulating eff ect on the course of the disease or both [10,20]. In this context, the P. gingivalis titer in patients with RA correlated signifi cantly with the concentration of anti-citrullinated protein/peptide antibody (ACPA) [10,20]. It is thus hypothesized that in a genetically susceptible (shared epitope-positive) individual, such citrullinated peptides may interrupt tolerance to endogenous citrullinated antigens, resulting in the generation of an immune response to citrullinated self-antigens. Hitchon and colleagues [36] reported an association between immune responses to the oral pathogen P. gingivalis and the presence of ACPA in a population with a high background prevalence of RA-predisposing HLA-DRB1 alleles. Th is gene-environment interaction may result in breaking self-tolerance to citrullinated antigens or amplifi cation of these autoimmune responses or both and may ultimately lead to the development of RA [36].

P. gingivalis and the rheumatoid factor
Th e rheumatoid factor (RF) has been found in RA and in other chronic infl ammation diseases, including PD [20]. Th e RF could be verifi ed in the gingiva, in the subgingival plaque, and in the serum of patients with PD [34]. Seropositive patients with PD showed increased titers of IgG and IgM antibodies against oral microorganisms when compared with seronegative patients with PD [34]. Th e RF of seropositive patients shows a cross-reaction with oral bacterial epitopes [37]. Th e P. gingivalis proteinase is responsible for the epitope development in the RF-Fc region. Th e proteinases are regarded as important virulence factors since they make the growth of P. gingivalis possible and lead to the degradation of the host tissue [38]. Bonagura and colleagues [39] identifi ed the lysine and arginine amino acid sequences for these Fc regions of the IgG molecule. Since P. gingivalis decomposes lysine and arginine in particular and the IgG3 CH2 and CH3 domains are processed by the P. gingivalis proteinase, they take over a key function in the RF production of rheumatoid cells [40]. A current study (n = 69) of patients with RA evaluated the prevalence and severity of PD and their relationship to RA disease activity and severity [41]. Patients with osteoarthritis (OA) served as controls (n = 35) [41]. PD was more common and severe in patients with RA when compared with patients with OA [41]. Th ough apparently unrelated to disease activity, the presence itself of these autoantibodies does seem to be hi ghly relevant in association to disease pathogenesis in RA and the occurrence of poor outcomes.

Periodontitis, rheumatoid arthritis, and genetic factors
In 1987, a successful demonstration of the connection between HLA-DR4 and rapidly progressive periodontitis (RPP) by specifi c typing of the HLA gene loci HLA-A, HLA-B, HLA-C, and HLA-D was achieved. In that study, a DR4 frequency of 80% in patients with RPP as opposed to 38% in the control group was observed [42]. Another study [43] confi rmed these results by an examination of the DRB1*04 alleles that code HLA-DR4. In patients with RPP, a signifi cantly higher frequency (42%) of one of the DRB1 subtypes *0401, *0404, *0405, or *0408 could be detected, whereas the control group showed a frequency of these subtypes to be only 7%. Th ese DRB1 subtypes are part of the so-called shared epitope genotypes, which also play a role in other infl ammatory diseases like RA [26].

Superantigens and heat shock proteins
Heat-resistant, hydrophilic molecules with a molecular weight of 24 to 30 kDa are referred to as superantigens. Th ey are able to virtually glue together T-cell receptors (TCRs) and major histocompatibility complex II molecules [44], which trigger a permanent signal in T cells. Th e region V beta (VβV) has been identifi ed as the binding position for superantigens and is located in the variable part of the beta chain of the TCR. TCRs of the Vβ gene (Vβ 6, 8, 14, and 17) are more frequent in patients with RA than in the control groups [45]. Th ese superantigens of RA can be infl uenced by oral bacteria, although the P. intermedia stimulates the expression of Vβ 8 and Vβ 17 genes in CD4 (+) T cells, and both bacteria P. gingivalis and P. intermedia can also increase the expression of Vβ 6 and Vβ 8 [46].
Heat shock proteins (HSPs) protect the cell from stress by reversibly interacting with abnormal proteins and peptides and by participating in their backfolding and decomposition. Furthermore, HSPs ful fi ll a function in the hereditary and acquired immunity and are associated with the pathogenesis of RA [47].
In the serum of patients with RA, high levels of oral bacterial HSP were found. Seventy-kilodalton Prevotella melaninogenica HSP and P. intermedia HSP have also been identifi ed in periodontal disease [24]. However, HSP 70 antibodies are also found in the synovium of patients with RA and occur in the synovialis if the HSP 70 expression is triggered by specifi c stress factors (for example, heat, trauma, endotoxins, and anti-infl ammatory drugs) and by proinfl ammatory cytokines (TNF-α, IL-1, and IL-6) [24]. Th erefore, superantigens and HSP in patients with RA are not specifi c to oral bacteria [24].

Autoantigens
Th e citrullinated form of the α-enolase is an autoantigen that plays a role in the glycolysis. α-Enolase operates as a receptor and activator of plasminogen, as an HSP, and as a Myc-binding protein [26]. Th e citrullinated α-enolase has been detected together with other citrullinated antigens in the synovial tissue of patients with early RA [26]. However, the fi nding of a specifi city of 97.1% in this cohort is remarkable. Lundberg and colleagues [27] identifi ed an immunodominant epitope of the citrullinated α-enolase. Th e data on the sequence similarity and cross-reactivity let us assume that this immunodominant epitope of the citrulli nated α-enolase plays a role in the primary autoimmunity of a subgroup of RA patients with bacterial infections, especially P. gingivalis [20,27].
It is well known that immunoglobulins of the class IgG act as antigens. Interestingly, IgG is glycolized diff erently in patients with RA. In 60% of the patients but not in healthy control groups matched by age, the terminal galactose is missing in the carbohydrate groups of the Fc part. Anti-agalactosyl IgG antibodies (CARF) showed a slightly higher sensitivity of 73.9% but a specifi city as low as that for RF. Th is lack of terminal galactose is associated with a poor prognosis in the course of the disease [48]. Under these conditions, the saccharolytic bacterium P. melaninogenica is able to bind at the Fc region of the IgG molecule and to metabolize galactose with its enzyme [49]. Variations in the composition of the sugar moiety can infl uence antibody activity in autoimmune disorders. Furthermore, there are bacteria (Escherichia coli) that are inhibited by galactose (Gal) or N-acetylgalactosamines (GalNAc) and other carbohydrates [50].
Th e pathogenic periodontal bacteria produce enzymes (collagenases, hyaluronidases, neuraminidases, and others) that degenerate the intercellular matrix and the collagenous skeleton, thereby facilitating the infi ltration of further microorganisms into the tissue [38]. In patients with RA, the presence of autoantibodies against collagen II (CII), a main component of the hyaline cartilage, has been verifi ed [51]. P. gingivalis expresses a lysine-specifi c proteinase, shows collagenase activity, and reduces all collagen molecules except from those for CII [52]. Lysine in position 270 of CII 263 to 370 can be hydroxylized and further on glycolized to monosaccharides or disaccharides (for example, with a beta-D-galactopyranosyl or with an alpha-glycopyranosyl-(1,2)-beta-galactopyranosyl residue).

T helper 17 cells and interleukin-17
Th e role of the T helper 17 (Th 17) cells in the host defense is not completely known. It was able to be shown that IL-17 stimulates the generation and mobilization of neutrophils and plays an important role in the defense of extracellular bacteria [53]. Th 17 cells and IL-17 play an important role in the pathogenesis of RA. On the other hand, Th 17 cells are also present in chronic periodontal disease [54]. IL-17 can be found in periodontal lesions and potentially plays a role in the etiopathogenesis of periodontal disease. Th e P. gingivalis antigen stimulates the T cells to express IL-17 [54].

Metallomatrix proteinases
Under a clinically healthy gingival situation, the continuous cellular composition and decomposition processes in the periodontium are balanced, so that collagen decomposing MMP and tissue inhibitors of MMP (TIMPs), for example, are always to be found. In PD, TIMPs are overbalanced in favor of the MMPs, which consequently have an increased active concentration. A key enzyme for the tissue destruction in the context of PD, MMP-8 in its active form decomposes fi brillar collagen structures and also is associated with alveolar bone destruction [12,55]. Consequently, the detection of mediators such as MMP-8 in the gingival sulcus fl uid may be a method to monitor infl ammatory activities, and this adds to classical periodontal diagnostics (probing depths, clinical attachment level, and bleeding on probing). A recent study showed lower MMP-8 levels in the healthy control group than in the RA group with gingivitis, in the RA group with PD, or in the systemically healthy PD group (P <0.05) [55]. In contrast, MMP-13 levels were similar in all groups (P >0.05). RA patients with gingivitis or PD had similar MMP-8, MMP-13, and TIMP-1 levels as did the systemic healthy control group (P >0.05). Th is study indicates that the simultaneous appearance of RA and PD has no infl uence on the investigated parameters. Increased MMP-8 levels in the gingival sulcus fl uid can be found in periodontal infl ammation. Th e long-term application of gluco corticoids and nonsteroidal anti-infl ammatory drugs causes similar high MMP-8 and MMP-13 levels in RA patients and systemic healthy probands and possibly results in an overproduction of those enzymes.

Risk factors in periodontitis or rheumatoid arthritis or both
Th e clinical appearance of PD ( Figure 1) and RA can be infl uenced by congenital as well as acquired factors, which can increase the probability of the development and progression of the disease. From an etiologic and pathogenetic point of view, risk factors other than pathogenic bacteria and oral hygiene play a crucial role. Th ese risk factors can infl uence every sub-step of the pathogenesis, and this explains the individually diff erent manifestations of the disease [3]. Th e individual risk factors such as age, gender, body mass, and genetic factors (IL-1β polymorphism and HLA gene associations) are the focus of attention. Other exogenic risk factors such as nutritive factors, socioeconomic status, psychological factors (for example, stress), and lifestyle (cigarette smoking and alcohol consumption) as well as systemic diseases may infl uence the pathogenesis [56,57]. However, it has also been known for some years now that patients with PD not only suff er from local loss of the connective tissue and the hard tissue but also have an increased risk of developing systemic diseases [58]. Th is interrelation is referred to as 'periodontal medicine' .

Smoking
Th e best documented environmental factor that contributes to RA susceptibility is smoking. Importantly, smoking appears to contribute to disease susceptibility only in individuals who develop autoantibody-positive RA characterized by the presence of ACPAs. Th ere is a clustering of RA risk associated with smoking, presence of shared epitope alleles and the presence of ACPA [58]. Hitchon and colleagues [36] tried to determine whether oral hygiene and smoking are associated with RA, ACPA, and immune responses to P. gingivalis. It is known that smoking and poor oral health habits both increase the risk of PD [36]. Smoking is a risk factor for PD possibly through the eff ects of nicotine on infl ammatory cytokine profi les [59] and MMP-3 activity [60] or potentially even through direct eff ects on P. gingivalis gene expression. Hitchon and colleagues [36] did fi nd a high prevalence of both smoking and poor oral health habits in the study population on the basis of self-report questionnaire data; however, they could not defi ne a clear association between these factors and the presence of either RA or ACPA.

Conclusions
We conclude that there is some evidence for the relationship between the presence of PD and the development of RA. Th e existence of an infl ammatory systemic disease may promote PD in both its emergence and progress. Periodontal pathogens have direct systemic access to the blood circulation. Th erefore, treatment with antibiotics in patients with RA can be eff ective. Th e enzyme PAD represents an important pathogenetic factor for RA. P. gingivalis is currently the only known bacterium with expression of PAD and plays a role in the humoral immune response and in the presence of APCA in patients with RA. Oral hygiene and smoking represent environmental factors infl uencing the risk for the develop ment of RA. Other risk factors (for example, nutritive factors, stress, and lifestyle factors) should also be examined in the future with regard to their relation ship to PD and RA. Further basic and clinical studies are needed. In particular, longitudinal studies are needed in order to clarify the temporal relationship between PD and RA. Th ese studies will need to include high numbers of patients but a low number of confounding factors in order to derive fi rm conclusions. For clinical studies, it is necessary to create a network of cooperation with rheumatologists and dentists.