Significance Statement: The gut microbiota and its metabolites, in particular short-chain fatty acids derived from gut microbes’ fermentation of fiber, are emerging therapeutic targets for systemic inflammatory and metabolic diseases, including diabetic nephropathy. The authors report that high-fiber diets or supplementation with short-chain fatty acids (acetate, butyrate, or propionate) afforded protection against development of kidney disease in diabetic mice. Dietary fiber restored gut microbial ecology, corrected “dysbiotic” changes, and increased production of short-chain fatty acids. Mice deficient in the metabolite-sensing G protein–coupled receptors GPR43 or GPR109A were not protected by short-chain fatty acids, suggesting that protection was mediated by downstream binding to these receptors. Tapping into the metabolic potential of the gut microbiota through diet may offer a novel approach to address diabetic nephropathy. Background: Studies have reported “dysbiotic” changes to gut microbiota, such as depletion of gut bacteria that produce short-chain fatty acids (SCFAs) through gut fermentation of fiber, in CKD and diabetes. Dietary fiber is associated with decreased inflammation and mortality in CKD, and SCFAs have been proposed to mediate this effect. Methods: To explore dietary fiber’s effect on development of experimental diabetic nephropathy, we used streptozotocin to induce diabetes in wild-type C57BL/6 and knockout mice lacking the genes encoding G protein–coupled receptors GPR43 or GPR109A. Diabetic mice were randomized to high-fiber, normal chow, or zero-fiber diets, or SCFAs in drinking water. We used proton nuclear magnetic resonance spectroscopy for metabolic profiling and 16S ribosomal RNA sequencing to assess the gut microbiome. Results: Diabetic mice fed a high-fiber diet were significantly less likely to develop diabetic nephropathy, exhibiting less albuminuria, glomerular hypertrophy, podocyte injury, and interstitial fibrosis compared with diabetic controls fed normal chow or a zero-fiber diet. Fiber beneficially reshaped gut microbial ecology and improved dysbiosis, promoting expansion of SCFA-producing bacteria of the genera Prevotella and Bifidobacterium, which increased fecal and systemic SCFA concentrations. Fiber reduced expression of genes encoding inflammatory cytokines, chemokines, and fibrosis-promoting proteins in diabetic kidneys. SCFA-treated diabetic mice were protected from nephropathy, but not in the absence of GPR43 or GPR109A. In vitro, SCFAs modulated inflammation in renal tubular cells and podocytes under hyperglycemic conditions. Conclusions: Dietary fiber protects against diabetic nephropathy through modulation of the gut microbiota, enrichment of SCFA-producing bacteria, and increased SCFA production. GPR43 and GPR109A are critical to SCFA-mediated protection against this condition. Interventions targeting the gut microbiota warrant further investigation as a novel renoprotective therapy in diabetic nephropathy.

Significance Statement: The gut microbiome is known to affect immune responses in autoimmunity and cancer, but little is known about its role in transplant immunity. In a mouse model, the authors observed dysbiosis after kidney transplantation in the absence of antibiotics or other drugs. A high-fiber diet prevented dysbiosis and afforded protection against allograft rejection, as did supplementation with the short-chain fatty acids sodium acetate or sodium butyrate, microbial metabolites produced by gut fermentation of dietary fiber. This protection was dependent on the G protein–coupled receptor GPR43 and T regulatory cells. These findings show how the microbiome can be modified to retard alloimmunity in a mouse model of kidney transplantation, and provide a rationale to explore this strategy in humans as a means to facilitate transplant acceptance. Background: Short-chain fatty acids derived from gut microbial fermentation of dietary fiber have been shown to suppress autoimmunity through mechanisms that include enhanced regulation by T regulatory cells (Tregs). Methods: Using a murine kidney transplantation model, we examined the effects on alloimmunity of a high-fiber diet or supplementation with the short-chain fatty acid acetate. Kidney transplants were performed from BALB/c(H2^d) to B6(H2^b) mice as allografts in wild-type and recipient mice lacking the G protein–coupled receptor GPR43 (the metabolite-sensing receptor of acetate). Allograft mice received normal chow, a high-fiber diet, or normal chow supplemented with sodium acetate. We assessed rejection at days 14 (acute) and 100 (chronic), and used 16S rRNA sequencing to determine gut microbiota composition pretransplantation and post-transplantation. Results: Wild-type mice fed normal chow exhibited dysbiosis after receiving a kidney allograft but not an isograft, despite the avoidance of antibiotics and immunosuppression for the latter. A high-fiber diet prevented dysbiosis in allograft recipients, who demonstrated prolonged survival and reduced evidence of rejection compared with mice fed normal chow. Allograft mice receiving supplemental sodium acetate exhibited similar protection from rejection, and subsequently demonstrated donor-specific tolerance. Depletion of CD25⁺ Tregs or absence of the short-chain fatty acid receptor GPR43 abolished this survival advantage. Conclusions: Manipulation of the microbiome by a high-fiber diet or supplementation with sodium acetate modified alloimmunity in a kidney transplant model, generating tolerance dependent on Tregs and GPR43. Diet-based therapy to induce changes in the gut microbiome can alter systemic alloimmunity in mice, in part through the production of short-chain fatty acids leading to Treg cell development, and merits study as a potential clinical strategy to facilitate transplant acceptance.

Introduction Kidney transplant recipients (KTRs) are at an increased risk of hospitalisation and death from COVID-19. Vaccination against SARS-CoV-2 is our primary risk mitigation strategy, yet vaccine effectiveness in KTRs is suboptimal. Strategies to enhance vaccine efficacy are therefore required. Current evidence supports the role of the gut microbiota in shaping the immune response to vaccination. Gut dysbiosis is common in KTRs and is a potential contributor to impaired COVID-19 vaccine responses. We hypothesise that dietary fibre supplementation will attenuate gut dysbiosis and promote vaccine responsiveness in KTRs. Methods and analysis Rapamycin and inulin for third-dose vaccine response stimulation-inulin is a multicentre, randomised, prospective, double-blinded, placebo-controlled pilot trial examining the effect of dietary inulin supplementation prior to a third dose of COVID-19 vaccine in KTRs who have failed to develop protective immunity following a 2-dose COVID-19 vaccine schedule. Participants will be randomised 1:1 to inulin (active) or maltodextrin (placebo control), administered as 20 g/day of powdered supplement dissolved in water, for 4 weeks prior to and following vaccination. The primary outcome is the proportion of participants in each trial arm that achieve in vitro neutralisation of live SARS-CoV-2 virus at 4 weeks following a third dose of COVID-19 vaccine. Secondary outcomes include the safety and tolerability of dietary inulin, the diversity and differential abundance of gut microbiota, and vaccine-specific immune cell populations and responses. Ethics and dissemination Ethics approval was obtained from the Central Adelaide Local Health Network (CALHN) Human Research Ethics Committee (HREC) (approval number: 2021/HRE00354) and the Sydney Local Health District (SHLD) HREC (approval numbers: X21-0411 and 2021/STE04280). Results of this trial will be published following peer-review and presented at scientific meetings and congresses. Trial registration number ACTRN12621001465842.

Kidney transplant recipients are at an increased risk of hospitalisation and death from SARS-CoV-2 infection, and standard two-dose vaccination schedules are typically inadequate to generate protective immunity. Gut dysbiosis, which is common among kidney transplant recipients and known to effect systemic immunity, may be a contributing factor to a lack of vaccine immunogenicity in this at-risk cohort. The gut microbiota modulates vaccine responses, with the production of immunomodulatory short-chain fatty acids by bacteria such as Bifidobacterium associated with heightened vaccine responses in both observational and experimental studies. As SCFA-producing populations in the gut microbiota are enhanced by diets rich in non-digestible fibre, dietary supplementation with prebiotic fibre emerges as a potential adjuvant strategy to correct dysbiosis and improve vaccine-induced immunity. In a randomised, double-bind, placebo-controlled trial of 72 kidney transplant recipients, we found dietary supplementation with prebiotic inulin for 4 weeks before and after a third SARS-CoV2 mRNA vaccine to be feasible, tolerable, and safe. Inulin supplementation resulted in an increase in gut Bifidobacterium, as determined by 16S RNA sequencing, but did not increase in vitro neutralisation of live SARS-CoV-2 virus at 4 weeks following a third vaccination. Dietary fibre supplementation is a feasible strategy with the potential to enhance vaccine-induced immunity and warrants further investigation.

Key Points: Mechanistic target of rapamycin (mTOR) inhibitor–based immunosuppression was associated with an improved T-cell response to vaccination in kidney transplant recipients. Mice treated with an mTOR inhibitor exhibited improved T-cell responses to booster vaccination. Switching low and nonresponder kidney transplant recipients to an mTOR inhibitor did not improve T-cell response to a booster vaccination. Background: Failure to develop protective immunity in response to vaccination is common among kidney transplant recipients, rendering them susceptible to severe infection. Novel strategies are required. Here, we investigated the potential of mechanistic target of rapamycin (mTOR) inhibitors to improve vaccine responses. Methods: Humoral and cellular responses to primary coronavirus disease 2019 (COVID-19) vaccination (ChAdOx1 or BNT162b2) were assessed for kidney transplant recipients receiving mTOR inhibitor–based (mTOR inhibitor, mycophenolate, prednisolone, n =15) and standard-of-care (tacrolimus, mycophenolate, prednisolone, n =40) immunosuppression, and healthy cohabitants ( n =71), in a prospective observational study. Findings were validated and mechanisms explored in mice. Low/nonresponding kidney transplant recipients receiving standard-of-care immunosuppression ( N =54) were then randomized 1:1 to switch from mycophenolate to sirolimus or remain on standard of care for 4 weeks before receiving COVID-19 booster vaccination. Augmentation of immunity to COVID-19 was assessed as the primary outcome measure. Results: A 12-fold greater IFN γ T-cell response to primary vaccination was observed in kidney transplant recipients receiving mTOR inhibitor–based versus standard-of-care immunosuppression (520 versus 43 spot-forming units/10 ⁶ cells, P < 0.001). A greater frequency of functional memory T cells in the mTOR inhibitor group was observed for both the CD4 ⁺ (0.20% versus 0.05%, P < 0.001) and CD8 ⁺ (0.35% versus 0.07%, P = 0.006) compartments by flow cytometry, and kidney transplant recipients receiving mTOR inhibitor–based immunosuppression produced greater frequencies of severe acute respiratory syndrome coronavirus 2–specific CD4 ⁺ T cells than healthy cohabitants (1.17% versus 0.48%, P = 0.03). In mice, sirolimus treatment enhanced both recall and de novo T-cell responses to homologous and Omicron-specific booster vaccines. Switch from mycophenolate to sirolimus was well tolerated; however, no significant difference was observed in the proportion of kidney transplant recipients in the intervention and control arms that achieved protective virus neutralization (10/25 [40%] versus 9/21 [43%], respectively, P = 0.85) nor in T-cell response to vaccination ( P = 0.89). Conclusions: mTOR inhibition was associated with improved T-cell memory formation in kidney transplant recipients; however, this effect was not reproduced by a short-term mycophenolate to sirolimus switch strategy. Clinical Trial registry name and registration number: Australian New Zealand Clinical Trials Registry, ACTRN12621001412820.

Introduction Kidney transplantation remains the best treatment for end-stage kidney disease, however the requirement for indefinite immunosuppression increases the risk of cardiovascular disease, cancer and infection, leading to a reduction in long-term patient and graft survival. The gut microbiome is a critical determinant of health and modulates host immunity and metabolism through a number of recognised pathways, including through the production of immunomodulatory short-chain fatty acids (SCFA). Dietary supplementation with non-digestible fibre can augment the microbial production of SCFA and lead to favourable immune and metabolic outcomes, although this has yet to be shown in human kidney transplant recipients. Methods and analysis Dietary inulin for gut health in solid-organ transplantation (DIGEST) is a single-centre, unblinded, pilot parallel-arm randomised controlled trial designed to assess the feasibility and adherence of dietary inulin, a naturally occurring dietary fibre, in the early post-transplant period in kidney transplant recipients. Participants will be randomised at day 28 post-transplant to a 4-week period of dietary inulin (10–20 g/day) in addition to standard care, or standard care alone, and followed-up until week 12 post-transplant. The primary outcomes of the study are: (i) the feasibility of participant recruitment, randomisation and retention; (ii) adherence to the intervention (inulin) and (iii) the tolerability of inulin determined by changes in gastrointestinal symptoms as scored on the Gastrointestinal Symptom Rating Scale. Secondary outcomes include: (1) glycaemic variability determined by continuous glucose monitoring; (2) abundance of SCFA-producing microbiota, as determined by 16s rRNA sequencing of the faecal metagenome; (3) serum SCFA concentrations; (4) peripheral blood immune cell populations; (5) recipient inflammatory and metabolic profiles and (6) the incidence of biopsy-proven acute rejection and kidney function determined by estimated glomerular filtration rate. Ethics and dissemination All study visits, clinical and laboratory assessments will be integrated into usual post-transplant care, creating no additional healthcare encounters or procedures. The risks associated with this study are minor. Inulin has been shown to be well tolerated across a variety of cohorts, with the occurrence of short-term adverse gastrointestinal symptoms self-limiting. However, with gastrointestinal adverse events common following kidney transplantation, the tolerability of inulin in this cohort remains unknown. The results of DIGEST will be published in peer-reviewed journals and presented at academic conferences. This study has been approved by the Sydney Local Health District’s Ethics Committee (Royal Prince Alfred Hospital Zone). Trial registration number ACTRN12620000623998.