3.1.4. Others

In recent years, nutritional status in kidney disease has attracted increasing attention and patients with CKD are facing protein energy wasting (PEW) complications [59]. Since proinflammatory cytokines a ffect the brain and cause anorexia, persistent inflammation is associated with PEW [65]. CKD is a very complicated disease, and the etiologies of inflammation observed in patients with CKD are multifactorial. Although any form of inflammation can cause PEW, periodontal disease is a potential etiology for persistent inflammation [66]. Periodontal pathogens that are swallowed and reach the intestine cause changes in the intestinal microbiota, resulting in a situation resembling metabolic bacteremia such as that found in obesity. Although the direct e ffect of periodontal pathogens on the gu<sup>t</sup> microbiota is controversial, oral microbiota could also alter the gu<sup>t</sup> microbiota [67]. Recent studies have revealed that not only diabetes and obesity but also CKD is closely associated with the intestinal flora [68]. Dysbiosis of the gu<sup>t</sup> microbiota is believed to be associated with periodontal disease, and administration of *P. gingivalis* was shown to alter the gu<sup>t</sup> microbiota and a ffect multiple organs in an animal model [69]. Thus, CKD should be considered a multifactorial disease; it is extremely di fficult to identify specific etiological factors, but it should be taken into consideration that periodontal disease indirectly causes renal dysfunction.

A scheme illustrating the complex mechanisms involved in periodontal disease and CKD is shown in Figure 1.

**Figure 1.** Impact of periodontal disease in the pathogenesis in chronic kidney disease.

### *3.2. Periodontal Therapy and CKD Patients with Periodontal Disease*

There is a bidirectional relationship between periodontal disease and CKD, and between periodontal disease and diabetes [70]. The results of this study still remain unclear, although if CKD proceeds and renal function deteriorates, lymphocyte function and monocyte/macrophage function will be impaired and the immune system will aggregate; consequently, the risk of infection will increase compared to the healthy population [4]. When it comes to CKD-MBD, hyperphosphatemia increases phosphate levels in saliva, which is associated with a higher risk of inflammation of the periodontium. Furthermore, higher levels of phosphorus itself are linked to systemic inflammation [71]. This will enhance the association of periodontal disease and CKD. Although periodontal disease is a crucial risk factor for the onset of kidney disease and progression of renal failure, it is a treatable and modifiable risk factor. eGFR might improve in patients with CKD, however patients with ESRD cannot improve their eGFR.

The treatment of periodontal disease attenuates systemic inflammation and improves surrogate markers of endothelial function [72]. Through the reduction of inflammatory cytokines, renal function was shown to improve after treatment of periodontal disease [73]. A systematic review also showed a favorable effect of periodontal treatment on eGFR [74]. Other reports have indicated that treatment for periodontal disease decreases asymmetric dimethylarginine [75] and 4-hydroxy-2-nonenal, surrogate markers of systemic inflammation [44] and endothelial function [9], respectively.

In a discussion on periodontal therapy in patients with CKD, we must also take into consideration the influence of immunosuppressive agents. In short, a variety of CKD, such as lupus nephritis and IgA nephropathy, is commonly treated with immunosuppressive agents [76,77]. Undoubtedly, the immune response is closely associated with infection control in almost organs and tissues including oral and periodontal tissues. Therefore, in CKD patients treated with immunosuppressive agents, a treatment strategy for periodontal disease must be planned according to its severity, renal function, and response to anti-bacterial agents. In addition, in some patients, decreasing exposure to immunosuppressive agents

should also be discussed. Unfortunately, however, there is little information regarding appropriate dosage in these patients. We emphasize the importance of further studies on appropriate dosage and types of immunosuppressive agents to be used in CKD patients with severe periodontal disease, especially in non-responders to anti-bacterial agents.

### **4. Periodontal Disease and Kidney Transplantation**

As mentioned above, KT is one of the major renal replacement approaches to maintain QoL in patients with ESRD, and KT might be the choice of renal replacement therapy to achieve normalized kidney function, which undeniably leads to both social and physical advantages for the patient [14,15]. However, immunosuppressive therapy is essential for maintenance of kidney allograft function to prevent rejection and renal dysfunction, although it may be the cause of several infectious diseases including periodontitis. In addition, there is a general agreemen<sup>t</sup> that immunosuppressive conditions sometimes lead to the progression of local infections to general bacteremia. In Japan a total of 1742 KT including 1544 from living donors (LD) and 198 from deceased donors (DD) were performed in the proportion of patients with ABO blood type, incompatible with KT and requiring desensitization for antibody removal before KT, was as large as 27.7% of KT from LD, while the dialysis period before KT was as long as 15.1 years from DD in a Japanese survey [78]. ABO incompatibility and exposure of long-term dialysis therapy lead to susceptibility to immunosuppression, which leads to the risk of developing infectious diseases including periodontal disease. It is important to understand the underlying biological mechanisms of periodontal disease and the immunological status of KT recipients. Moreover, it is important to identify an approach to treat periodontal diseases for the managemen<sup>t</sup> of KT.

### *4.1. The Screening of Periodontal Disease Before and After KT*

Screening for infectious diseases is important in the preparation of KT. It is well-known that poor oral health is common among CKD patients [79]. KT candidates are considered to have a high prevalence of periodontal disease due to the status of end-stage of renal insu fficiency, which is called stage-five CKD. There has been a recent increase in the number of preemptive kidney transplantation (PEKT) procedures, which are defined as transplant before initiation of maintenance dialysis therapy, for the avoidance of complication by dialysis therapy [80]. PEKT is often performed at the progression of renal deterioration in the pre-dialysis stage. Moreover, the progressed renal insu fficiency may occasionally fall into a severe immunocompromised state. Oral infections should be treated completely before exposure to immunosuppression for KT candidates, since recent potent immunosuppressive therapy regimens might lead to unexpected onset of infection including periodontitis. Oral health has been reported to be better during the KT period than at the pre-dialysis stage, and it thus it is important to treat oral infectious foci at the pre-dialysis stage in order to prevent adverse outcomes after KT [81]. However, lower salivary flow rates and higher numbers of drugs at the KT stage might influence the clinical outcome [81]. Patients with diabetic nephropathy associate with worse periodontal health and higher oral inflammatory conditions at the pre-dialysis stage to the same degree as the pre-transplant stage [82]. Due to the high prevalence of cardiovascular disease and requirements of extracorporeal circulation for hemodialysis, anti-coagulant therapy is used in many KT candidates. The surgical treatment of oral diseases in this condition may lead to active bleeding and cause general bacteremia or fatal sepsis. Preconditioning and necessary treatment are important for the KT recipient before and after exposure to immunosuppressive therapy. Finally, it should be considered that appropriate timing of treatment must be provided to KT recipients depending on the patient's general status, CKD stage, and original cause of renal insu fficiency [81,82].

### *4.2. Periodontal Condition and Clinical Outcome*

Several investigators have examined the e ffects of periodontal conditions in relation to clinical outcome following KT. The studies reported a higher [83,84], similar [85], or lower [86] incidence of periodontitis in transplant recipients than in healthy controls. A limitation to the study results is that these di ffer depending on the definition of occurrence of periodontitis used. Ioannidou et al. [87] reported that none of the continuous periodontal variables were significantly associated with deterioration of allograft function due to the presence of strict criteria such as HLA-matching or history of acute rejection. A systematic review described the associations between periodontal status and clinical outcomes in KT recipients. A patient's periodontal status might be associated with a larger left ventricular mass, greater carotid thickness, graft rejection, lower graft survival, and higher mortality rate in early KT periods among KT recipients [88]. These studies sugges<sup>t</sup> that periodontal status may affect clinical outcomes of patient survival and graft survival due to the occurrence of cardiovascular disease and abnormal immunological responses with essential immunosuppressive therapy [86–88].

### *4.3. Influence of Immunosuppressive Drugs*

Recently, potent strong immunosuppressive therapy is becoming more commonly used to prevent acute rejection, which has improved outcomes of KT in the modern era [89]. A protocol using everolimus (ERL) is often performed in addition to a standard protocol consisting of mycophenolate mofetil (MMF), tacrolimus (Tac), and corticosteroid (CS), which has allowed further improvements in treatment outcomes of KT [90,91]. However, protocols using multiple agents can lead to a condition of excessive immunosuppression, which may worsen oral status. There are currently di fferent immunosuppressive agents available and it is clinically important to understand di fferences in each drug relative to the possible occurrence of periodontal disease. Calcineurin inhibitors (CNI) such as Cyclosporin A (CsA) and Tac have played a central role in immunosuppressive therapy of KT since the 1980s. CsA is characterized by a major side e ffect of gingival overgrowth (GO), which may lead to occasionally worsening of oral health. CsA-induced gingival enlargement has been reported to vary from 7–80% in di fferent transplant centers [92]. Conversely, the use of ERL frequently results in adverse events of stomatitis and compromised status, and so it may be important to properly manage the oral environment during treatment [93]. However, few reports have examined the risk of developing periodontitis and other oral diseases in patients using ERL. Pereira-Lopes et al. compared the oral health status of KT recipients receiving Tac or ERL as immunosuppressants. The study showed that KT recipients receiving the ERL protocol presented reduced periodontal inflammation in comparison with patients receiving Tac [94]. In the study, patients receiving ERL were older and they experienced more limited periodontal inflammation. There might be a bias in the patient selection in the study. ERL is used for CNI minimization or corticosteroid elimination due to protection from CNI nephrotoxicity or various side e ffects of steroid therapy, which might a ffect long-term graft survival or patient survival [95–97]. Tac has similar adverse e ffects as CsA because both drugs share the same pharmacological mechanism of calcineurin inhibition. Several investigators have examined the occurrence of GO with the use of Tac, which has been less frequent than the use of CsA [98,99]. In a clinical study comparing exposure to CsA and Tac, it was reported that GO occurred later in the group using Tac, with the severity of GO in the Tac group being lower than that in the CA group [100]. Another study including a post-hoc analysis revealed that the prevalence of GO was 60.0% for CsA, 28.9% for Tac, and 15.6% for sirolimus, which was used as an mTOR inhibitor for the same purpose of ERL [101]. Tac may be an alternative agen<sup>t</sup> to CsA in attempting to avoid adverse e ffects of GO [102,103]. Tac seems advantageous with regard to periodontal e ffects when using CNI as a standard immunosuppression protocol.

### *4.4. Immunosuppressant-Induced Gingival Overgrowth and Periodontal Condition*

Gingival enlargement is commonly observed as a side e ffect of several di fferent types of drugs, including anticonvulsants, calcium channel blockers, and immunosuppressants [92]. As previously mentioned, GO is common adverse e ffect of CsA usage, which may lead to the worsening of periodontal conditions in KT recipients. A multitude of factors may a ffect clinical and histopathological manifestations of immunosuppressant-induced GO. Several investigators have examined the causes of CsA-induced GO, which might associate with periodontal disease. The results of several studies

examining specific factors found in the gingival crevicular fluid (GCF) were indicative of CsA-induced GO. Increased LL-37 peptide levels in the GCF is observed at the CsA-induced GO positive site with neutrophil infiltration and extended inflammation. LL-37 is an antimicrobial peptide and an important defense molecule of the host immune response [104]. Gürkan et al. [105] examined the association of many types of cytokine families with CsA-induced GO by the examination of the GCF. TGF-β1, whose levels are associated with clinical periodontal parameters, might be an exclusive mediator of CsA- or Tac-induced GO. Increased IL-6 and oncostain M under CsA usage have been reported to be involved in regulating the severity of inflammation and presence of GO, but cytokines of the IL-6 family might not be directly involved in the biological mechanisms associated with CsA-induced GO [106]. A study examining transgultaminase (TGM)-2, which had been shown to play a role in fibrosis by extracellular matrix accumulation, showed that TGM-2 might contribute to CsA-induced GO by modifying the GCF and plasma levels of oxidative stress markers [107].

The investigation of molecules contained in the GCF may provide researchers with a variety of insights associated with medication-induced GO. Conversely, several study groups have attempted to elucidate the mechanism of GO by investigating gene polymorphisms. IL-10 gene polymorphism might be associated with CsA-induced GO in KT recipients. A special genotype and allele might indicate an association with susceptibility to GO [108]. Alpha-2 integrin gene polymorphisms were not associated with CsA-induced GO but were associated with CsA-independent GO in KT recipients [109]. Interestingly, the length of the CAG repeat of the androgen receptor gene might link to CsA-induced GO via the analysis of polymorphisms [110]. Thus, many studies have been attempted to elucidate the mechanisms involved in drug-induced GO [104–110], but the biological mechanism of the gingival tissue response involved in immunosuppressant-induced GO is still not fully understood. Since gingival enlargement is directly associated with QoL, it is necessary to elucidate the causes of drug-induced GO by further scientific research and the challenge is to modify present immunosuppressive therapy regimens to avoid GO.

### *4.5. Inflammatory Markers of Periodontitis in KT*

Inflammation may be associated with the deterioration of solid-organ function in transplantation recipients. Systemic inflammation occasionally originates from periodontal inflammation. There is clinical evidence that periodontal inflammation is linked to the occurrence of di fferent systemic diseases including ESRD [111]. Moreover, the chronic inflammation of kidney allograft is often modulated by alloimmune-dependent mechanisms. The main cause of inflammation in solid-organ transplantation is dependent on human leukocyte antigen (HLA) mismatches or panel reactive antibody (PRA) scores, which may lead to acute humoral rejection [112,113]. Several inflammatory markers such as IL-6 and C-reactive protein (CRP) may be predictors of renal allograft survival associated with alloimmune-dependent acute rejection [114,115]. Moreover, specific cells associated with periodontal disease may produce significantly higher levels of IL-6, and serum CRP was reported to be elevated in patients with periodontal inflammation [116]. Shaqman et al. [117] compared periodontal disease and systemic inflammatory status of transplant recipients and age-matched controls, which led to the conclusion of the absence of any significant predictors of systemic inflammation in the population. In another study Blach et al. [118] examined several inflammatory markers such as TNFα, IL-6, and high-sensitive CRP (hs-CRP) in KT recipients. The study demonstrated that severe chronic periodontitis was associated with increased serum hs-CRP, but not with any significant elevation of TNFα or CRP. Elevated hs-CRP levels appeared to influence mortality after KT [118]; thus, it is important to monitor the levels of inflammatory markers such as hs-CRP, which may lead to the early detection and early treatment of periodontal disease. Furthermore, it would be more useful if an inflammatory marker specific to KT recipients undergoing immunosuppressive therapy were identified in real world studies.

### **5. Periodontal Pathogens in Chronic Kidney Disease and Kidney Transplantation**

Several investigators have reported an increased frequency of periodontal pathogens in CKD and KT patients. There is general agreemen<sup>t</sup> that periodontal disease occurred by mixed infection, but not by a single pathogen alone [119,120]. With regard to CKD, Bastos et al. [121] reported that the frequencies of *P. gingivalis* and *Candida albicans* in pre-dialysis CKD patients showed a higher trend than the control group (94.7 versus 72.2% and 52.0% versus 26.3%, respectively); however, such difference did not reach statistical significance (*p* = 0.078 and *p* = 0.079, respectively). Conversely, the frequencies of *P. gingivalis* and *Treponema denticola* (*T. denticola*) were significantly associated with clinical detectable levels (*p* = 0.008 and *p* = 0.013, respectively) in CKD patients with periodontitis [121]. Conversely, other investigators have shown that *T. denticola*, *Tanerella forsythia* (*T. forsythia*), and *Parvimonas micra* (*P. micra*) were significantly associated with periodontal disease in patients with CKD [122]. In addition, the authors also found that *T. forsythia* was independently associated with periodontal disease in a multivariate analysis model including other significant pathogens, age, and estimated glomerular filtration rate (*p* = 0.008) [122].

Regarding KT patients, the frequency of *Streptococcus constellatus* in plaque samples form subjects with alveolar bone loss (36.4%) was significantly higher (*p* = 0.019) than that in samples without alveolar bone loss (3.7%) [83]. Conversely, a report also indicated that total counts of micro-organisms were increased between day 0 and day 90 after renal transplantation and between day 30 and day 90 after surgery [123]. In addition, the same study also showed that the frequency of β-hemolytic Streptococcus on day 90 after surgery (28.6%) was significantly lower (*p* = 0.031) than that on day 30 (44.4%) in KT patients with GO, but not in patients without GO [123]. Thus, in KT patients, quantitative and qualitative changes of microorganisms in the subgingival plaque might occur 90 days after surgery, and GO had an effect on expression of these microorganisms [117]. Unfortunately, there is limited information on the pathological significance of pathogens in patients with KT.

Conversely, a recent study evaluated the relationships between immunosuppressive agents and periodontal pathogenic bacteria in patients following solid organ transplantation [124]. Although it should be noted that the study population included three different types of organs (kidney, liver, and lung), the study demonstrated that changes in the levels of periodontal pathogenic bacteria dependent on immunosuppressive agents. In short, the prevalence of *P. micra*, was associated with immunosuppression exclusively with glucocorticoids; however, *P. gingivalis* was associated with combined immunosuppression of glucocorticoids, MMF, and Tac [124]. A summary is shown in Table 1.


**Table 1.** Pathological significance and levels of pathogens according to kidney status.

CKD—chronic kidney disease, KT—kidney transplantation, SOT—solid organ transplantation, and ref—reference.

### **6. Care of Periodontal Conditions in Chronic Kidney Disease and Kidney Transplantation**

Many investigators pay special attention to the prevalence and severity of periodontal disease. In addition, the importance of periodontal treatment has been reported in CKD and KT patients because they may exhibit periodontal conditions worse than that of the healthy general population [124,125]. Currently, several pilot studies support this opinion in these patient groups [75,126]. Unfortunately, CRF or KT patients do not have much interest in screening and/or treatment of periodontal disease. Although we have no data in CRF and KT, there is a report that approximately 70% of hemodialysis facilities have no associated dental clinic [127]. There is an opinion that immunosuppressive therapy is not associated with the necessity of dental and periodontal treatment in patients with transplantation [124]. However, there is no general consensus for KT patients. Conversely, other investigators have suggested the importance of appropriate oral health in KT patients because oral hygiene status was closely associated with the development and degree of GO [128]. Finally, we also emphasize that periodic oral care and appropriate periodontal treatment with dentists are important to maintain QoL, inhibit the complications, and prolong the survival periods in CKD and KT patients. In recent years, the usefulness of various treatment strategies for improvement of oral health has been analyzed [129,130]. In addition, there is the opinion that clinical conditions, such as lipidemia in obesity, a ffect preventive and treatment strategies of chronic periodontitis [131]. Based on these facts, further detailed studies with larger study populations, longer observation periods, and analyses including broader periodontal disease-related factors are necessary to be able to reach definitive conclusions.
