BK and JC polyomavirus co-infection resulting in polyomavirus nephropathy and progressive multifocal leukoencephalopathy at the same time, a case report.

JC and BK polyomavirus-like particles as targets of innate and adaptive humoral immunity

Polyomavirus (PV) nephropathy has been attributed to reactivation of BK virus (BKV) or more rarely JC virus (JCV). The simian virus (SV) 40 is PV that was likely introduced into the human population through contaminated vaccines. The purpose of this study was to identify and characterize the PV that is associated with PV nephropathy. The clinical diagnosis of PV nephropathy (PVN) was made in patients with acute deterioration in renal function whose renal biopsies showed typical viral cytopathic changes in tubular epithelial cells and staining for PV T antigen. Polymerase chain reaction (PCR) amplification of DNA from peripheral blood mononuclear cells (PBMC), urinary cells, and renal biopsy tissue was performed using specific primers for the transcription control regions of BKV, JCV, and SV40, respectively. Six cases of PV nephropathy were identified in 91 renal transplant recipients (7%). Immunosuppressive therapy was modified in all patients. Renal function stabilized or improved in four patients and deteriorated in two patients, and one patient has lost his allograft, after follow-up from 2 to 25 months. PCR detection demonstrated BKV genome in three of five PBMC samples, six of six urinary cell samples, and two of four renal biopsies. SV40 genome was detected in two of five PBMC samples, one of six urinary cell samples, and two of four renal biopsies. Infectious SV40 and BKV was demonstrated in CV-1 co-cultures using urine from one patient. JCV was not detected in any PVN sample. Co-infection with BKV and SV40 was found in two PVN patients. Urine samples obtained 12 months after transplant from 26 transplant recipients without PVN on simultaneous protocol renal biopsy were analyzed by PCR; BKV genome was demonstrated in 5 of 25 samples, JCV genome was demonstrated in 3 of 25 samples, and SV40 genome was demonstrated in 0 of 25 samples. The authors report molecular evidence that co-infection with BKV and SV40 occurs in renal transplant patients with PVN, suggesting that SV40 may contribute to PVN after renal transplant.

Progressive multifocal leukoencephalopathy (PML) is a rapidly progressive degenerative demyelinating disease that results from infection with JC virus. PML usually occurs in an immunocompromised individual, putatively attributable to reactivation of a latent JC virus. In childhood PML has been reported in individuals with acquired immunodeficiency syndrome (AIDS),1–3 Wiskott–Aldrich syndrome,4 and inherited immunodeficiencies.5,,6We report here a 15-year-old boy with generalized morphea, a form of limited cutaneous sclerodema, and secondary amyloidosis who developed PML. The rapid onset of his neurologic manifestations and the initial neuroimaging studies suggested acute disseminated encephalomyelitis (ADEM), and a brain biopsy was needed for the diagnosis of PML. A 15-year-old boy with scleroderma and amyloidosis was admitted for progressive ataxia, dysarthria, and weakness. His medical history was notable for generalized morphea confirmed by a skin biopsy at 3 years of age. He had severe cutaneous involvement and finger and toe contractures attributable to skin and soft tissue involvement. He was treated with prednisone andD-penicillamine for several years. All autoantibodies were negative, and immunologic work-up revealed normal complement and immunoglobulin levels but a low absolute T-cell count of 533 cells/μL. There was no evidence of active arthritis or recurrent infections. Two years before admission, he was diagnosed with secondary amyloidosis with serum amyloid A protein by liver biopsy. A concomitant skin biopsy showed no evidence of amyloid deposition and continued to be consistent with morphea. Proteinuria was noted. Prednisone was stopped, and chlorambucil (4 mg per day) was started. He responded well to the chlorambucil over the next several months with significant regression of amyloidosis-related clinical findings and marked dermatologic improvement. He was maintained on chlorambucil only. However, 2 months before admission the dosage had to be decreased from 4 mg to 3 mg daily because of leukopenia with a white …

Polyomavirus nephropathy (PVN) is one cause of renal allograft loss. Disease incidence in renal allograft recipients was estimated to be 4% in 2010 with an overall graft failure rate of 34%, reaching up to 90% in advanced PVN disease subgroup C characterized by marked sclerosis (unpublished data collected by the "Banff working group" of PVN with nine participating U.S. and European centers). Because therapeutic strategies with anti-PV drug regimens are limited, the overall outcome is improved by proper risk assessment and an early diagnosis. Although all patients with disease or PVN show signs of PV activation with viruria and viremia (1–10), only a small minority of all viruric or viremic patients actually develop disease, that is biopsy-confirmed definitive PVN (3, 5, 7, 10). This phenomenon is well known from other DNA viruses that establish latency in humans after primary infection, such as Epstein-Barr virus or cytomegalovirus. Thus, how can screening for PV activation and overall risk assessment for PVN in renal allograft recipients be cost-efficient and cost-effective? Urine cytology and the search for "decoy cells" can help. We read the article by Chakera et al. (11) "Detection of polyomavirus BK reactivation after renal transplantation using an intensive decoy-cell surveillance program is cost-effective" with great interest. For more than a decade, we have successfully used urinary decoy-cell screening in the management of kidney transplant recipients (2, 4, 7, 8, 12, 13), and we would like to contribute to the discussion by sharing some of our observations. Decoy cells found in voided urine samples contain intranuclear PV (mainly BK virus) inclusions that appear as homogenous and glassy viral material on standard light microscopy. Decoy cells can easily be identified and counted by cytopathologists in standard Papanicolaou-stained urine cytology specimen using ThinPrep or cytospin preparation. The analysis can be performed in any pathology laboratory. The cutoff for clinically significant PV activation and decoy-cell shedding ("decoy positivity") is the detection of 10 or more decoy cells per slide (1, 7) with an overall positive predictive value (PPV) for biopsy-proven definitive PVN in renal allograft recipients of approximately 25% and a negative predictive value of 100%. The PPV can be increased by incorporating additional findings into the decision-making process, such as prolonged decoy-cell shedding exceeding 4 weeks (see in the next paragraph) or concurrent allograft dysfunction. The predictive values of decoy cells are very similar to those of quantitative urine polymerase chain reaction (PCR) assays for the detection of BK-virus DNA at threshold levels of 1×107 or more BK-virus copies per milliliter of urine (7). Since 2009, we have followed 212 renal allograft recipients prospectively including decoy-cell testing. We also have detailed longitudinal information on a cohort of 32 patients with biopsy-confirmed definitive PVN (Table 1). In our population of 212 patients, 51 (24%) were decoy-cell positive. Of these 51 patients, 8 (16%) presented with multiple individual flares of decoy positivity, whereas most (43/51, 84%) only experienced a single episode of decoy positivity of various durations. Of 51 "decoy-positive" patients, 26 (51%) experienced extended, continuous decoy-cell shedding for 1 month or longer; it is especially this patient group that is at very high risk for the development of PVN (see in the next paragraph). Of 51 decoy-cell positive patients followed prospectively, 14 (27%) developed biopsy-proven definitive PVN. PVN was not diagnosed in "decoy-negative" patients.TABLE 1: Decoy-cell sheddinga in selected adult kidney transplant recipients at The University of North Carolina at Chapel HillAt The University of North Carolina at Chapel Hill (UNC), we have detailed information on a cohort of 32 patients with PVN (including 14 from the prospectively followed group of 212 patients): all of them were decoy-cell positive at the time of initial biopsy diagnosis, and 28 (88%) were with prolonged continuous decoy-cell shedding for many weeks. In patients with PVN, decoy-cell positivity preceded the histologic biopsy diagnosis on average by 4 weeks, thereby serving as an initial "alarm signal" during the prodromal disease stage. Note that, during this prodromal time window, two of our patients showed negative renal biopsy results. In these two patients, typical signs of disease were only found in subsequent repeat biopsies, only then allowing for a definitive histologic diagnosis of PVN. At the other end of the spectrum, PVN disease resolution is also reflected by decoy-cell analyses that turn from decoy positive to decoy negative on repeat testing (generally conducted every 4 weeks in patients with PVN), thereby marking viral clearance and "healing." At UNC, all patients are tested at regular intervals for the shedding of decoy cells during the first 24 months after grafting. Positive decoy-cell readings trigger tighter surveillance at 4-week intervals, and patients are additionally tested with quantitative plasma PCR assays to detect BK virus, thereby following general recommendations for patient management with slight modifications (10). Decoy-positivity and concurrent plasma PCR test results of more than 250 BK virus copies per milliliter trigger a diagnostic kidney biopsy regardless of allograft function. Thus, decoy-cell testing serves as an initial mass-screening tool followed by a second line of targeted assays with higher predictive values for PVN (PPV for quantitative plasma PCR with a threshold of ≥1×104 BK virus copies per milliliter, approximately 75% [7]). This screening protocol resulted in the definitive diagnosis of PVN in a nonsclerotic disease stage in all of our 14 prospectively followed patients; 50% (7/14) were diagnosed in early PVN stage A, and none, in disease stage C (disease staging was according to the guidelines under discussion by the Banff PV working group). No graft was lost because of PVN, avoiding high costs associated with return to dialysis. At UNC, decoy-cell testing currently costs U.S. $286 compared with U.S. $409 for plasma or urine quantitative PCR assays for BK virus. Of all prospectively followed renal allograft recipients in our cohort, 76% (161/212) never activated PV defined by negative decoy-cell test results (Table 1); PCR assays were not ordered according to our screening guidelines, and these patients never developed PVN. Assuming that other laboratory tests including urinalysis and blood draws are routinely conducted during patient visits, our testing protocol does not incur any additional specimen collection fees. During the 24 months after transplantation, multiple routine surveillance tests monitoring for the activation of PV are performed per patient following general recommendations (10). In our prospectively followed cohort of 161 patients without PV activation, 1500 negative decoy-cell screening assays were conducted between January 2009 and February 2012 with a total cost for decoy-cell analysis of U.S. $429,400. Conversely, if screening had primarily been based on quantitative PCR assays, then the costs would have been U.S. $613,000. Using decoy-cell screening resulted in a total saving of more than U.S. $184,000 (U.S. $1140 per patient) for a period of 38 months at UNC. This conservative calculation does not consider additional savings in 37 patients in our prospectively followed cohort, which were decoy-cell positive at certain time points during follow-up (Table 1) but never developed PVN, and in whom quantitative PCR tests were limited to few selected time points. In conclusion, we share the encouraging observations made by Chakera and colleagues from Oxford University (11). Routine urine cytology including the quantitation of decoy-cell shedding is a well-established and cost-effective primary mass-screening tool to search for PV activation in renal allograft recipients and to identify patients at an increased risk for the development of PVN. Decoy positivity, particularly if detected for more than 1 month, should trigger second-line testing with additional assays, such as quantitative plasma PCR analyses to detect BK virus and renal biopsy in high-risk patients. In addition, we find decoy-cell testing intriguing because it also allows for targeted and highly predictive PV "Haufen testing" on the same urine sample (7, 14), thereby streamlining patient management and the clinical decision-making process. Volker Nickeleit 1,2 Karin True3 Randal Detwiler3 Tomasz Kozlowski4 Harsharan Singh1,2 1 Department of Pathology Division of Nephropathology The University of North Carolina at Chapel Hill Chapel Hill, NC 2 Department of Pathology and Laboratory Medicine Division of Nephropathology The University of North Carolina at Chapel Hill Chapel Hill, NC 3 Department of Internal Medicine Division of Nephrology The University of North Carolina at Chapel Hill Chapel Hill, NC 4 Department of Surgery The University of North Carolina at Chapel Hill Chapel Hill, NC

Progressive multifocal leukoencephalopathy (PML) is a rare demyelinating disease of the white matter central nervous system occurring in immunocompromised patients particularly those with T cell deficiency such as in HIV, haematological and solid organ malignancies and those taking immunomodulatory medications. PML is caused by JC virus however in rare cases BK virus has been isolated in the cerebral spinal fluid of patients presenting with PML.In this case we describe a 49 year old man who presented to the emergency department with a 2 week history of progressive right sided weakness and dysarthria. His background history included HIV diagnosed in 2005, he had not engaged with care in the past 2 years and had not been taking anti-retroviral therapy (ART). Other past medical history included untreated hepatitis C. His CD4 count was 90 (11%) cells/mm3 on admission and his HIV viral load VL) was 141,000 copies/ml. Magnetic resonance imaging(MRI) showed a hypointense lesion on T1, hyperintense on T2 and FLAIR without diffusion restriction and without mass effect. A lumbar puncture was performed which confirmed JC virus was positive (PCR <50 copies/ml) and also revealed BK virus was positive (PCR 46,511 copies/ml). The patient was commenced on tenofovir alafenamide fumarate/emtricitabine/darunavir/cobicistat in combination with dolutegravir 50mg twice daily. On day 40 post commencement of ART the patient was readmitted with worsening of his right arm weakness and dysarthria. A repeat MRI was performed which showed the hyperdense lesion on T2 and FLAIR appeared slightly larger with some slight enhancement with gadolinium contrast but no other features suggesting PML immune reconstitution inflammatory syndrome (IRIS). The CD4 count had increased to 141(17%) and HIV VL had decreased to 85 copies/ml. A clinical diagnosis of PML IRIS was made and the patient was commenced on prednisolone 30mg BD which lead to an initial improvement in symptoms.Interestingly in this case, both JC virus and BK virus were detected in the CSF of this patient with the level of JC virus being too low to quantify. BK virus was not detectable on peripheral serum sampling suggesting that BK virus is replicating in the CNS independent of other body sites. There have been 5 case reports in the literature of BK virus as the cause of PML.Testing for BK virus should be considered in patients presenting with signs and symptoms of PML and encephalitis particularly when no other cause is found.

To develop a sensitive and specific polymerase chain reaction (PCR) based system for detecting genomic variation in JC virus. To apply this system to formalin fixed, paraffin wax embedded brain tissue from patients with and without progressive multifocal leucoencephalopathy (PML). A pair of primers (JC1 and JC2) were designed to be complementary to the early and late regions of JC and BK polyomaviruses, respectively. A third primer (JC3), internal to JC1 and JC2, was designed to be specific for JC virus. The specificity of JC3 was investigated by amplifying plasmids with BK or JC virus genomes. Sensitivity was estimated by titration of a plasmid containing JC virus genome. Seven brains from patients with PML (PMLB) and 30 from patients without PML (non-PMLB) were amplified using JC1 and JC2, followed by JC1 and JC3. Amplification of the beta globin gene was used as an amplification control. Amplification with JC1 and JC2 was common for JC and BK viruses, but with JC1 and JC3 it was specific for JC virus. The sensitivity of the system was 25 copies of JC plasmid per 10 microliters of digested tissue. Five out of seven PMLB and 28 of the 30 non-PMLB amplified for beta globin, but only the PMLB gave a signal with polyoma primers. Hypervariation of the length of the regulatory region of the JC isolates in the PML tissues was consistent with the presence of multiple strains of JC. Variation in the regulatory region of JC virus can be specifically and sensitively detected from routinely processed, paraffin wax embedded brain tissue. Variation in the regulatory region is common in PML derived JC strains, but JC virus was not detectable in non-PMLB tissue.

Progressive multifocal leukoencephalopathy (PML) is a demyelinating disease of the central nervous system, which is thought to be a result of the reactivation of JC virus (JCV), a human polyomavirus. The disease occurs in individuals with immunosuppression and in recent years there has been an increase in PML cases due to AIDS. A nested polymerase chain reaction (n-PCR) was employed to detect JCV and BK virus (BKV) DNA in brain tissue collected postmortem from 28 AIDS patients with PML and from 13 patients without PML, but with other diagnoses, including solid tumors, Alzheimer's disease, thromboembolism, myocardial infarction and acute cerebrovascular diseases. All 28 brain specimens from the patients with PML were positive for JCV DNA when tested by n-PCR and three of the latter were also positive for BKV DNA. These results were confirmed by an enzyme restriction analysis and a DNA hybridization assay. Interestingly, in this study, JCV DNA was also found in 6 brain tissue specimens from 4 subjects with diseases unrelated to PML or AIDS. All the brain specimens from the control group were negative for BKV DNA. The results confirm that the n-PCR is a useful tool for PML diagnosis. The presence of JCV DNA in the brain tissue of patients without PML is particularly important since it indicates that JCV could be latent in the brains of immunocompetent individuals. Moreover, detection of simultaneous presence of JCV and BKV in the brain tissue of the patients with PML demonstrates that BKV may also infect the human brain without causing any apparent neurological disease.

JC virus (JCV) viruria is more common than BK virus (BKV) viruria in healthy individuals but in kidney transplants (KT), polyomavirus nephropathy (PVAN) is primarily caused by BKV. Few cases of PVAN have been attributed to JCV. Systematic studies on JCV replication in KT are lacking. Out of a cohort of KT patients screened with urine cytology, patients shedding decoy cells were studied (n=103). Molecular studies demonstrated BKV, JCV, or BKV+JCV shedding in 58 (56.3%), 28 (27.2%), and 17 (16.5%), respectively. Biopsy was performed when decoy cells persisted 2 months or serum creatinine increased >20%. BKV viruria was strongly associated with BKV viremia (93%), PVAN (48%, P=0.01) and graft loss (P=0.03). Higher BKV viremia correlated with graft dysfunction (P=0.01), more advanced histological pattern of PVAN (P<0.0001), and more infected cells in biopsy (P=0.0001). BKV viremia of > or =10,000 copies/mL was significantly associated with histologically confirmed PVAN (P=0.0001). Reduction of immunosuppression lead to disappearance of decoy cells in patients shedding BK (>93%). JCV viruria, was more often asymptomatic (P=0.002) and affected older patients (P=0.02). JCV PVAN was less common (21.4%) and was characterized by sparse cytopathic changes but significant inflammation and fibrosis. JCV viremia was rare (14.2%), transient, and low (mean 2.0E+03/mL). After reduction of immunosuppression decoy cells persisted in >50% of patients with JCV (P=0.0001), but no graft loss occurred. During the period of the current study, the incidence of BKV-PVAN was 5.5% and the incidence of JCV-PVAN was 0.9%. The data point to significant differences of BKV and JCV biology regarding replication and disease in KT patients, with important implications for screening and management.

Use of Expanded Allogeneic Third Party BK Virus Specific Cytotoxic T Cells to Target Progressive Multifocal Leukoencephalopathy

Progressive multifocal leukoencephalopathy (PML) after transplantation

Research on polyomaviruses began in 1953 when Ludwik Gross isolated a filterable agent that induced tumours in newborn mice. This agent became the archetypal member of the Polyomaviridae family. In 1971, JC virus and BK virus were reported and named after the patients’ initials. More than 30 years passed before the Karolinska Institute virus and Washington University virus were identified from respiratory samples in paediatric patients. The Merkel cell polyomavirus was identified in 2008 for its association with the aggressive Merkel cell carcinoma skin cancer. Recently, polyomaviruses 6, 7, 9 and the trichodysplasia spinulosa‐associated polyomavirus have been discovered. Along with simian virus 40, which has been associated with human disease, there are now 10 polyomaviruses relevant to humans. The range of diseases associated with these human polyomaviruses shows that they are significant causes of human infections. The accumulated knowledge gained from the study of each individual virus helps to understand each in their particular niche. Key Concepts: Polyomaviruses are DNA viruses. Polyomaviruses cause significant human infections, especially in the immunocompromised population. Human infection most likely occurs during childhood. Haemorrhagic cystitis and polyomavirus‐associated nephropathy are significant clinical entities caused by the BK virus in solid organ transplant recipients. Progressive multifocal encephalopathy is a fatal demyelinating disease caused by the JC virus. Merkel cell virus and SV40 are potential oncogenic agents. Treatment is supportive and an effective antiviral agent has not been identified so far.

Pathologic quiz case. Intranuclear inclusions in allograft kidney. Pathologic diagnosis: human polymavirus-associated interstitial nephritis in the allograft kidney.

Few studies have looked for the polyoma viruses JC or BK virus in the central nervous system (CNS) of patients without neurological symptoms or with neurological symptoms other than progressive multifocal leukoencephalopathy (PML). PCR-microplate hybridization method was employed for the detection of BKV-DNA or JCV-DNA in cerebrospinal fluid (CSF) specimens from patients with suspected meningitis or encephalitis. A total of 181 CSF specimens from 151 patients with suspected meningitis or encephalitis was examined for BKV or JCV using PCR-microplate hybridization method. None of the patients had (clinically diagnosed) PML. A control group consisting of 20 CSF specimens from normal subject was also included. BKV DNA was found in five out of 131 (3.8%) and JCV DNA in two out of 131 (1.5%) of the patients with suspected meningitis or encephalitis by PCR ELISA. BKV or JCV DNA was not detected in CSF samples of any of 19 HIV positive patients. BKV and JCV DNAs were detected respectively in two CSF samples in which Mycobacterium tuberculosis (TB) PCR was also positive. Another patient who was positive for JCV PCR died with a diagnosis of cerebral lymphoma. Among the BK virus infected patients there was a patient with a previous history of hemolytic uremia and acute renal failure. Neither BKV nor JCV DNA was found in any of the 20 CSF samples from normal patients undergoing lumbar puncture for myelography as a part of an investigation of lower back pain. These results suggest that BK virus may be associated with neurological diseases either in immunocompetent or immunocompromised patients. Detection of BKV and JCV DNA in the CSF of the patients suspected to have either meningitis or encephalitis suggests that these viruses may have an etiological role. Thus, diagnostic tests for BK and JC viruses should be included in the investigative program for meningitis or encephalitis patients.

PV infection is frequent in renal transplant patients. The BKV genotype in urine and serum is significantly related to a high frequency and high number of decoy cells. PVN occurs only in patients with BKV viremia and a high number and frequency of decoy cell excretion in urine. In the absence of decoy cells, PVN can be excluded. Cytologic analysis of urine is an important diagnostic tool for screening renal transplant patients at risk of PVN.

Allogeneic BK Virus-specific T Cells for Progressive Multifocal Leukoencephalopathy Muftuoglu M, Olson A, Marin D, et al. N Engl J Med. 2018;379:1443-1451. Over a dozen polyoma viruses have been identified in humans, with approximately 75% of the adult population being latently infected with BK virus (BKV) in the urothelium. Although immunocompetent patients are generally asymptomatic, in immunosuppressed transplant recipients of solid organ transplants, the infection may result in nephropathy due to the reactivation of the virus in the graft; hematopoietic stem cell (HSC) transplant recipients may develop a hemorrhagic cystitis.1 Treatment of BKV is complicated because it requires a reduction in immunosuppression, which is not possible in HSC transplant recipients who are inherently immunosuppressed as part of their preconditioning regimen. To address this unmet clinical need, there has been increasing interest in the use of virus-specific T cells for the treatment of BK virus hemorrhagic cystitis, with one group reporting encouraging results in 16 HSC transplant recipients.2 Another latent polyomavirus, JC virus (JCV), can result in a frequently fatal demyelinating progressive multifocal leukoencephalopathy (PML) in a very small number of transplant recipients. There are no satisfactory effective treatments for PML. Importantly, BKV and JCV share a significant degree of structural homology, meaning that T cells specific for BKV antigens can also target JCV. Capitalizing on this feature, Muftuoglu and colleagues have reported on the use of third party–derived ex vivo–expanded BK virus-specific cytotoxic T lymphocyte (CTL) cellular therapy in 2 patients who developed PML after an HSC transplant.3 In the full report published in the New England Journal of Medicine, the investigators report on their experience using this CTL product to treat 3 patients with PML.4 BKV-specific CTL products were generated from 27 healthy donors and cryopreserved for later use. The most closely HLA-matched T cell line was then selected for each treated patient and cells administered at a dose of 2 × 105/kg. An advantage of the partial HLA mismatch of the cell product was the ability to track it in vivo by flow cytometry using HLA-specific antibodies. This approach was assessed in 1 patient with approximately 20% of T cells in the cerebrospinal fluid (CSF) being of donor cellular therapy origin. Most importantly, after infusion, all 3 patients demonstrated a reduction in JCV load in the CSF with a marked clinical improvement. After further infusions of CTL therapy in the first patient, there was a complete clearance of JCV in the CSF and a resolution of clinical findings over a 2-year follow-up. Although a reduction in JCV load was observed in the second patient after a further CTL infusion, the symptoms of PML did not improve and the patient died 8 months after commencing CTL treatment. In the third patient, additional CTL infusions led to a complete clearance of CSF JCV with the patient regaining independent mobility. This study demonstrates the promise of cellular therapy in cases where current treatment modalities are unsatisfactory. There are inherent practical advantages to the use of third party–derived cellular therapies where the product may be banked for future use and HLA matching as required. Whether this is also possible for patients receiving solid organ transplants in whom the host immune system is not as profoundly suppressed will be of interest. REFERENCES Lamarche C, Orio J, Collette S, et al. BK polyomavirus and the transplanted kidney: immunopathology and therapeutic approaches. Transplantation. 2016;100:2276–2287. Tzannou I, Papadopoulou A, Naik S, et al. Off-the-shelf virus-specific Tcells to treat BK virus, human herpesvirus 6, cytomegalovirus, Epstein-Barr virus, and adenovirus infections after allogeneic hematopoietic stem-cell transplantation. J Clin Oncol. 2017;35:3547–3557. Muftuoglu M, Ahmed S, Olson A, et al. Use of expanded allogeneic third party BK virus specific cytotoxic t cells to target progressive multifocal Leukoencephalopathy (Abstract from the 56th ASH meeting, 2016). Blood. 2016;128:3365. MuftuogluM, Olson A,Marin D, et al. Allogeneic BK virus-specific Tcells for progressive multifocal leukoencephalopathy. N Engl J Med. 2018;379: 1443–1451. Cell Surface Polysaccharides of Bifidobacterium bifidum Induce the Generation of Foxp3+ Regulatory T Cells Verma R, Lee C, Jeun EJ, et al. Sci Immunol. 2018;3(28). The term “microbiome” refers to the combination of commensal organisms (ie, microbiota) together with their byproducts. Several factors may impact the makeup of the microbiome after transplantation and result in dysbiosis, including the use of immunosuppressants, antibiotics, antivirals, and chemotherapy. Although present on a number of mucosal and epithelial surfaces, the term “micriobiome” is commonly associated with the gastrointestinal tract where there is a high diversity and number of microbiota. These organisms contribute to several homeostatic processes with the ability to modulate both local and systemic immune responses. The microbiome has a role in T cell development and maturation, for example, through the promotion of specific T helper subset differentiation. Of interest in transplantation are the observations that the microbiome may alter the outcome of transplant alloresponses.1 However, the microbiome is a complex ecosystem with different microbial strains having distinct influences on immunity.2 There are previous reports that some commensals have a positive effect on regulatory T (Treg) cell differentiation and function, although the precise mechanisms underlying this are not clear.3 These reports offer a therapeutic opportunity to promote certain microbiota through the introduction of specific probiotics. In the study from Verma and coworkers, a collection of probiotic strains were screened ex vivo for their Treg cell-promoting abilities.4 Here, the authors show that monocolonization of germ-free mice with a strain of Bifidobacterium bifidum (Bb) promotes the development of Treg cells. Mechanisms involved include the generation of regulatory dendritic cells by Bb cell surface β-glucan/galactan polysaccharides. The Treg cells that develop in response have a broad specificity for both dietary antigens, commensal microbiota in addition to Bb and can suppress intestinal inflammation. These data are of interest given previous reports of germ-free or antibiotic-treated mice displaying prolonged allograft survival through a mechanism that includes reduced-type I IFN signaling in antigen-presenting cells.5 Understanding how to harness these potentially beneficial effects in transplant recipients is expected to be an important area for future investigation. REFERENCES Tabibian JH, Kenderian SS. The microbiome and immune regulation after transplantation. Transplantation. 2017;101:56–62. Yang Y, Torchinsky MB, Gobert M, et al. Focused specificity of intestinal TH17 cells toward commensal bacterial antigens. Nature. 2014;510: 152–156. Atarashi K, Tanoue T, Oshima K, et al. Treg induction by a rationally selected mixture of Clostridia strains from the human microbiota. Nature. 2013;500: 232–236. Verma R, Lee C, Jeun EJ, et al. Cell surface polysaccharides of Bifidobacterium bifidum induce the generation of Foxp3+ regulatory T cells. Sci Immunol. 2018;3. Lei YM, Chen L, Wang Y, et al. The composition of the microbiota modulates allograft rejection. J Clin Invest. 2016;126:2736–2744.