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Single-cell profiling reveals epithelial and immune responses in BK polyomavirus-infected human kidney biopsies.

INTRODUCTIONBK polyomavirus (BKV) infection is associated with injury and subsequent graft loss due to the extent of injury or rejection. However, the molecular mechanisms driving injury and subsequent adverse outcomes remain poorly understood.METHODSIn a cross-sectional study, single-cell RNA-seq from kidney allograft biopsies was used to assess cell type-specific responses between uninfected controls and 2 distinct phases of BKV infection: peaking (increasing viral blood titers) and resolving (decreasing viral titers following immunosuppression reduction).RESULTSGenes upregulated in BK viral nephropathy (BKVN) were enriched for polyomavirus infection hallmarks, including ribosome biogenesis, translation, and energy restructuring. Additionally, enriched pathways included wound healing, cellular stress, antigen presentation and immune signaling. Even without BKVN (peaking BK viremia alone), epithelial cells expressed signatures for wound healing, cellular stress, and extracellular matrix remodeling. In vivo tubular cell responses at single-cell resolution were validated against single cell transcriptomic data of BKV-infected cells in a cell culture model. Despite similarities, in vivo tubular cells underwent metabolic adaptation favoring fatty acid oxidation and proinflammatory responses not observed in culture models, likely due to an absent innate and adaptive immune system. Despite lymphopenia and immunosuppressive therapies, the proportion of recipient-derived intrarenal adaptive immune cells was increased in biopsies associated with peaking viremia alongside activation of innate immune responses. Adaptive immune cells exhibited persistent inflammatory signaling and remodeling of energy metabolism during the resolving phase of infection.CONCLUSIONThese not previously reported insights into BKV-associated injury may have implications for clinical management and improved allograft outcomes.

Humans

Immunological elimination of infected cells as the candidate mechanism for tumor protection in polyomavirus-infected mice.

The uniformly lethal development of mammary tumors in polyomavirus-infected adult female nude mice was prevented by adoptive cell transfer of polyomavirus-immune splenocytes or peritoneal cells. Transferred immune cells also lowered the growth rate of emerging tumors. The induction of other relatively less frequent tumors of the skin and bone was decreased as well. Using in situ hybridization of whole-body sections as well as hybridization of nucleic acids from the mammary glands, we show for the first time that transferred immune cells, but not normal cells, virtually eliminated virus signal in the whole mouse and in the mammary glands. Since infected and tumorous mammary glands produce very little infectious virus, it appears that a major mechanism mediating the prevention of polyomavirus oncogenesis involves the immunological elimination of nonproductively and persistently infected cells.

Animals

Influence of inactivated Sendai virus on early events in polyomavirus infection of permissive and nonpermissive host cells.

Quantitative radiochemical investigations have been performed regarding the influence of betapropiolactone-inactivated Sendai virus on the early events of interaction of polyomavirus with permissive and nonpermissive cells. These experiments have shown: (i) Sendai virus did not increase polyomavirus adsorption on permissive or nonpermissive cells; (ii) Sendai virus induced polyomavirus elution from permissive and nonpermissive cells during the first 40 min and 6 h postinfection, respectively; and (iii) Sendai virus promoted the penetration of polyomavirus into lysosomes only in nonpermissive cells. The maximal amounts of radioactively labeled virion DNA and viral coat proteins were found in lysosomes at 2--3 h postinfection.

Adsorption

Phosphorylation of the retinoblastoma protein is modulated in mouse kidney cells infected with polyomavirus.

Lytic infection with polyomavirus, an oncogenic DNA-containing virus, leads in G0-arrested primary baby mouse kidney (BMK) cell cultures to a mitotic host reaction. In the present work, we examined the expression of the retinoblastoma gene (RB) and of its product (Rb) in virus-infected BMK with the aim of correlating its modulation with the sequential activation of cellular processes leading to the induction of S phase by virus. In contrast to cell cycle-regulated genes whose expression is induced by viral infection, expression of RB is not altered during the transition from G0/G1 to S phase. In BMK cell cultures irreversibly arrested in the G0 phase of the cell cycle, an unphosphorylated species is the only detectable form of the RB protein (Rb). Time course analysis showed that in polyoma-infected cells induced to re-enter the S phase of the cell cycle the appearance of the phosphorylated forms of Rb coincided in time with the accumulation of large T antigen and preceded DNA synthesis. During the late phase of infection, the majority of Rb was present as phosphorylated forms. Ongoing DNA synthesis was not required for the cells to phosphorylate Rb, indicating that this post-translational modification takes place during the activation of the cellular DNA-synthesizing apparatus. Using hamster anti-polyoma tumor serum, it was observed that the underphosphorylated form of Rb co-precipitated with polyoma large T antigen extracted from infected cells late during infection. Our data add more evidence to the proposal that interactions between viral early proteins encoded by DNA tumor viruses and the product of RB may play a pivotal role in the mitogenic effect induced by viral infection.

Animals

Evidence of chronic persistent infections with polyomaviruses (BK type) in renal transplant recipients.

Ten renal transplant recipients showing a significant increase in human polyomavirus antibodies, indicative of an acute infection, were followed up serologically over periods ranging from two months to more than two years. Fifty-four serum specimens were available for the study and they were tested by both haemagglutination-inhibition and complement-fixation. Polyomavirus antigens were prepared from the BK and SV40-like strains of polyomaviruses, and from the SV40 virus. One strain of polyomavirus, related to the BK strain was isolated from the urine of one of these patients. Two other BK strains were recovered from the urine and kidney, respectively, of transplant recipients not included in this study. Sera of these two patients were not obtained until the transplantation was made; they were already highly positive for polyomavirus antibodies, precluding the demonstration of an increase in antibody titer. Serologic results have shown that HAI antibodies persist at high titers throughout the observation period. This persistence ranged from two to four months (four cases), seven to eleven months (three cases) and thirteen to twenty months (three cases). In none of the cases could a decrease of high titer be demonstrated. Moreover, density gradient studies have shown that specific IgM antibodies also tend to persist over many months. Similar serologic results were obtained in complement-fixation tests with a BK antigen. Titers were at least 1 in 30 in the study group, but were not observed among healthy blood donors. All sera were uniformly negative for SV40 and SV40-like antigens. One polyomavirus isolation was successful from urine obtained six months after initial serologic evidence for a polyomavirus infection. The other two viruses were isolated from materials taken four and seven months after first detection of polyomavirus antibodies at high titer. Both serologic evidence and viral isolations seem to indicate that polyomaviruses (BK type) might cause a chronic infection in humans.

Animals

Infection by polyomavirus of murine cells deficient in class I major histocompatibility complex expression.

Embryonic fibroblasts and kidney epithelial cells from beta 2-microglobulin-deficient mice were as infectible by polyomavirus as cells from normal littermates were, as judged by expression of nuclear viral capsid antigen, development of cytopathic effects, and yields of infectious virus. We conclude that expression of intact class I major histocompatibility complex molecules is not essential for polyomavirus infection.

Animals

Human polyomavirus (BK) infection and ureteric stenosis in renal allograft recipients.

Human polyomavirus (BK) was detected in two renal allograft recipients as a result of routine examination of Papanicolaou-stained smears of urinary sediment in the light microscope. Infection with this recently identified virus was confirmed by virus isolation and electron microscopy. The cytological, histological, and ultrastructural changes due to the virus are described, and virus excretion is correlated with the clinical progress of the patients and the pathological findings. The transplant ureters in both patients were found to be ulcerated and stenosed, and virus-infected cells were observed in the ureteric epithelium. We suggest that the administration of high-dose steroids in transplantation may permit active infection with human polyomavirus to occur in ureteric epithelium which has been damaged by ischaemia or inflammation.

Animals

Binding capacity of Intravenous immunoglobulin G to BK polyomavirus determines its anti-BK polyomavirus activity in infected cultures.

BK polyomavirus (BKPyV) causes disease in immunocompromised individuals. This study tested the hypothesis that the antiviral efficacy of intravenous immunoglobulin (IVIG) against BKPyV depends on the relationship between its virus-binding capacity and the viral burden. We first quantified the BKPyV-binding capacity of IVIG and then characterized BKPyV replication in kidney-derived HK-2 and HEK293&#x202f;cells and in HEL cells under IVIG treatment. Subsequently, we analyzed the efficacy of IVIG at 0.03-10&#x202f;mg/mL against low-multiplicity of infection (MOI) and high-MOI infection in relation to the BKPyV-binding capacity of IVIG. BKPyV productively infected all three cell lines, with 14-day replication cycles of 2.2, 4.0, and 7.0 in HK-2, HEK293, and HEL cells, respectively, indicating that HEL cells were the most permissive. IVIG bound approximately 108 copies of BKPyV DNA per milligram. In the low-MOI infection model, where the total viral load remained within this estimated binding capacity, IVIG showed clear neutralizing activity, significantly reducing viral spread, viral DNA levels, and the number of VP1-positive cells in a dose-dependent manner (p&#x202f;<&#x202f;0.001). In contrast, in the high-MOI infection model, the viral load appeared to be high relative to the estimated IVIG binding capacity even at 10&#x202f;mg/mL, and IVIG showed little or no neutralizing effect on viral production or spread of infected cells. These findings identify an experimental relationship between IVIG binding capacity, viral burden, and antiviral efficacy and suggest that the antiviral effect of IVIG is greatest when administered early, thereby supporting further clinical evaluation of early or preemptive IVIG administration.

Humans

Evidence of human polyomavirus BK and JC infection in normal brain tissue.

Infection with the polyomaviruses JC and BK is ubiquitous in the human population and JCV is the only virus associated with the central nervous system disease progressive multifocal leukoencephalopathy. In the attempt to analyze the pathogenesis of polyomavirus infections we asked whether human polyomaviruses invade the brain during persistence. Brain autopsy material from 67 individuals with disorders other than PML was examined for the presence of polyomavirus DNA. Southern blot analysis demonstrated JCV-specific full-length virus genomes in healthy brain tissue in about 20% of the patients. Type-specific analysis with polymerase chain reaction and sequencing confirmed these data. Additionally, the presence of BKV DNA sequences covering an early gene fragment and the control region with flanking early and late protein coding sequences was detected. Cloning of the complete BKV genome from two cases supported the assumption that not only full-length JCV DNA was present in those tissue specimens but also BKV genomes. The data obtained demonstrate that dual infection of the brain with the polyomaviruses JCV and BKV is a common event and give strong evidence that both viruses frequently establish a latent CNS infection.

BK Virus

Organ- and age-specific replication of polyomavirus in mice.

A novel organ- and age-specific pattern of polyomavirus DNA replication in mice is described. Two broadly defined classes of response to polyomavirus infection were observed: class I organs (mammary gland, bone, and skin) responded with high levels of replication in neonate mice and moderate levels in adults; class II organs (kidney, liver, and lung) responded with high levels in neonates and very low levels in adults. Thus, aging affected replication in all organs, and organ specificity was superimposed on this age-related decrease. We argue that the organ- and age-specific pattern likely reflects in part the activities of a multiplicity of general or tissue-specific, age-dependent transcription factors, which modulate viral replication or viral transcription or both. Interestingly, the majority of tumors in mice infected as neonates or as immunoincompetent adults originate in class I organs, suggesting that the ability to replicate in adult tissues is an important factor controlling polyomavirus oncogenesis. From the analysis of the infection process in adult mammary glands, a novel mode of polyomavirus infection emerged which contrasts with that derived from observations of tissue culture systems. A nonproductive infection was seen, characterized by very low levels of live virus (in the range of 10(-4) PFU per cell) and maintenance of the viral genome in an unintegrated, moderately replicating state. Maintenance of the viral genome was accomplished without integration into host cell DNA in all three tumor-prone organs, both prior to as well as beyond oncogenesis.

Aging

Detection of human polyomavirus DNA in Papanicolaou stained smears of urinary sediment by in situ hybridization.

Papanicolaou stained smears of urinary sediment containing inclusion bearing urothelial cells suggestive of human polyomavirus infection were destained and reprocessed for in situ hybridization using a biotinylated probe for human polyomavirus DNA. Seven slides were processed in this way. A hybridization signal for viral DNA was noted in each case, even in smears that had previously been stored for 11 years. This simple and rapid non-radioactive detection system is a valuable supplement to routine urinary cytology for the definitive diagnosis of this virus infection.

DNA, Viral

Characterization of the mammary hyperplasia, dysplasia and neoplasia induced in athymic female adult mice by polyomavirus.

We have characterized mammary oncogenesis induced after polyomavirus infection of adult female nude mice regarding histopathogenesis, viral replication and viral and cellular oncogene expression. A unique transient generalized epithelial hyperplasia was observed (starting at 2 weeks post infection), preceding the development of dysplasias (onset 6 weeks post infection) and multiple neoplasias (onset 6 weeks post infection) in all glands. The ductal epithelium was the target for neoplastic transformation, and the occurrence of numerous ductal dysplasias coincided with the appearance of frank tumors. Stromal abnormalities were also seen. Tumor growth was not dependent upon ovarian hormones, and new tumors continued to develop in ovariectomized mice. Viral replication, high although variable, preceded but did not correlate with oncogenesis. Most but not all tumors contained high levels of unintegrated viral DNA. Tumors produced very low levels of live virus. Viral gene expression was markedly increased in the tumors compared with the infected but morphologically normal glands. The expression of c-myc was moderately increased (fourfold); changes in c-int-2 and c-Ha-ras expression were slight and inconsistent, while expression of c-neu and c-int-1 was unchanged.

Animals

Genetic Diversity of BK Polyomavirus Among Renal Transplant Recipients in Yunnan, China.

BK polyomavirus (BKV) infection, a common complication following kidney transplantation, can lead to BKV-associated nephropathy (BKVN). Molecular genetic studies have classified BKV into four genotypes (I-IV); however, comprehensive molecular characterization of BKV strains circulating in China remains limited. This study aimed to elucidate the predominant subtypes and clinical infection characteristics of BKV strains among kidney transplant recipients in Yunnan, a province in southwestern China. PCR-amplified BKV DNA sequences from kidney transplant recipients were aligned with reference strains and subjected to phylogenetic analysis. The viral VP1 gene was successfully amplified from 180 participants, spanning 16 ethnic groups. Genotype I was the predominant viral strain (56.66%, 102/180), followed by genotype IV (43.33%, 78/180), while genotypes II and III were not detected. Among genotypic subtypes, IVc-1 was most prevalent (40.0%, 72/180), followed by Ic (38.3%, 69/180) and Ib-1 (18.3%, 33/180). IVa-1 and IVa-2 were rare, identified in only 0.6% (n&#x2009;=&#x2009;1) and 2.2% (n&#x2009;=&#x2009;4) of cases, respectively. No significant differences in sex, age, BKVN incidence, BK viremia, or viruria were observed between patients with BKV-I and BKV-IV infections. Among the five confirmed BKVN cases, two were genotyped as subtype Ic, one as Ib-1, and two as IVc-1. Clinical phenotypes were also comparable between patients with BKV-I and BKV-IV infections. This study represents the largest single-center sequencing analysis of BKV in kidney transplant recipients in China, offering a valuable genomic resource for future research.

Humans

Virus infections in renal transplant recipients.

534 serum samples from 73 renal transplant recipients, 41 haemodialysis patients, and 99 blood and organ donors were examined serologically for antibodies against Cytomegalo, H. simplex (types 1 and 2), Varicella-zoster, Epstein-Barr, Adeno, Influenza, Parainfluenza, Respiratory syncytial, Measles, Picorna- and human Polyoma- Viruses. Serum specimens were stored in the lyophilized state until examined thus enabling a simultaneous testing of all samples belonging to one patient. All antigens, complement, and control antisera were prepared, lyophilized, and standardized in this laboratory. This has enabled the use of single batches of any preparation throughout the study. Serologic results with antigens of the Herpesvirus group (CMV, HSV and VZV) compared favourably with previous results showing that infections with these agents, especially with CMV, can frequently be encountered among transplant recipients. Our results have indicated a moderately increased incidence of infections with some Herpesviruses in haemodialysis patients as well. Infections with VZV, for instance, were as frequently demonstrated in these patients, as in transplant patients although the former received no immunosuppressive therapy. Serologic results with non-Herpesvirus antigens indicated an increased incidence of infections with Polyomavirus, Myxoviruses (Influenza, Parainfluenza and RS) and Picornaviruses among transplant recipients. The incidence of acute infections with RS virus among adults was unusually high and there is no evidence so far to indicate such a high frequency of RS infections in any other group of adults. We were unable to demonstrate acute infections with non-Herpesviruses among haemodialysis patients, even though most of the patients were followed over a period of more than 2 years. Virus isolation attempts were performed with samples of urines and biopsy or autopsy samples. 23 out of 28 cytopathic agents recovered from urines, throat-swabs and/or from organs of transplant recipients were identified as CMV. Two HSV type 1, 1 HSV type 2, and 2 Coxsackie B type 3 viruses were also isolated. No viruses were isolated from a series of 31 kidneys randomly selected among autopsy cases.

Antibodies, Viral

Benchmarking urinary cell transcriptomes for noninvasive differentiation of BK polyomavirus-associated nephropathy from T cell-mediated rejection.

BK polyomavirus-associated nephropathy (BKVN) adversely impacts kidney allograft survival and often mimics acute T cell-mediated rejection (TCMR), confounding diagnosis and management. To address this conundrum, we performed unbiased RNA sequencing of urinary cells matched to biopsies classified as BKVN with intragraft inflammation (BKVN-P), BKVN without inflammation (BKVN-N), TCMR, or no rejection (NR). BKVN-N displayed dominant host DNA replication, cell cycle, and repair programs, while BKVN-P samples exhibited expansive innate immune activation, antigen presentation, chemokine upregulation, and epithelial injury. Both BKVN subtypes shared signatures of T cell exhaustion and mature and tolerogenic dendritic cell activation but differed in immune orientation - Th1 predominance in BKVN-N versus Treg and CD8 enrichment in BKVN-P. Compared with TCMR samples, BKVN-P lacked robust TCR/CD28 signaling and was enriched for viral and innate modules; BKVN-N lacked alloimmune activation. B cell exhaustion characterized BKVN-N, while BKVN-P displayed robust B cell activation with metabolic downregulation. A ratiometric urinary cell biomarker, CXCL10 mRNA/CD3E mRNA, distinguished both BKVN subtypes from TCMR with diagnostic accuracy, replicated by quantitative reverse transcription PCR for clinical translation, and confirmed in an independent cohort. These findings demonstrate the utility of urinary cell transcriptomics for resolving viral injury from alloimmunity, enabling precision diagnostics and targeted immunomodulation in kidney transplantation.

Humans

Single-section multiplex spatial proteomics of immune microenvironments in kidney transplantation.

Characterizing kidney disease is challenged by marked cellular heterogeneity and limited tissue availability from renal biopsies. Conventional diagnostic workflows rely on multiple serial sections for parallel staining, increasing tissue consumption, sampling bias, and loss of spatial information, thereby constraining molecular characterization within intact tissue architecture. High-plex spatial proteomics may overcome these limitations by enabling comprehensive molecular profiling on a single section. Here, we present and evaluate a high-plex cyclic immunofluorescence imaging workflow (MACSima&#x2122;, Miltenyi Biotec) applied to kidney transplant biopsies, including BK virus nephropathy (BKVN) and focal segmental glomerulosclerosis (FSGS), to characterize spatial immune organization with a focus on complement system components. Feasibility and subcellular resolution were first assessed in a lupus nephritis section, demonstrating compatibility with diagnostic immune panels and preservation of tissue morphology. A 48-marker multiplex panel interrogating immunity, oxidative stress, senescence, and fibrosis was then applied to BKVN samples, including paired pre- and post-treatment biopsies, revealing distinct proteomic patterns and dynamic changes following therapy. In FSGS, a glomerulus-focused panel identified spatially resolved innate and adaptive immune signatures, including complement-related patterns supporting exploratory analysis of glomerular immune architecture. Structural, nuclear, membrane, and phosphorylated signaling markers enabled precise delineation of renal compartments and assessment of cellular states such as proliferation, DNA damage, and pathway activation. The workflow also supported detection of extracellular vesicles in cultured renal cells, highlighting its versatility. Overall, this approach provides a robust, tissue-sparing platform for integrated spatial and molecular profiling of renal biopsies, reducing sampling bias while enabling discovery-level phenotyping from a single section. This unified strategy is particularly suited to kidney transplantation, where diagnosis, therapeutic decision-making, and longitudinal monitoring are closely interconnected.

Kidney Transplantation