Dendritic cells pulsed with polyomavirus BK antigen induce ex vivo polyoma BK virus-specific cytotoxic T-cell lines in seropositive healthy individuals and renal transplant recipients

Cellular Immunity Against BK Polyomavirus in Kidney Transplant Recipients: A Comprehensive Review

BK polyomavirus (BKPyV) is an important opportunistic viral infection that complicates kidney transplantation. Uncontrolled viral replication may result in BKPyV-associated nephropathy (BKPyVAN), a major cause of premature allograft damage and failure. In the continued absence of proven treatments, management relies on the empirical reduction of immunosuppression to facilitate an effective host immune response to clear the virus. This may be complicated by the risk of allograft rejection. There is compelling evidence that cellular immune responses are key to establishing control after viral reactivation. Measurable peripheral BKPyV-specific T cell responses temporally correlate with declining viral loads and subsequent clearance. Conversely, these responses are delayed or absent in BKPyVAN. How these peripheral findings correspond to the intragraft response, and whether BKPyV-specific T cells contribute to the immunopathology of BKPyVAN, remains poorly understood. Molecular techniques have provided some insights; however, these have been unable to fully discriminate BKPyVAN from cellular rejection to date. Furthermore, the contributions of components of innate cellular immunity, such as natural killer cells, are not known. Herein, we review the role of cellular immunity in BKPyV infection in kidney transplant recipients. We discuss advances in the understanding of how the development, phenotype, and functionality of these responses may determine the balance between viral control and immunopathology, and how this knowledge is being translated into tools to prognosticate and guide individualized immunosuppression reduction. Lastly, we consider how further elucidation of these responses may inform the design of therapies that would revolutionize how BKPyV is managed after transplantation.

Control of BKPyV-DNAemia by a Tailored Viro-Immunologic Approach Does Not Lead to BKPyV-Nephropathy Progression and Development of Donor-Specific Antibodies in Pediatric Kidney Transplantation

Polyomavirus BK (BKPyV)-associated nephropathy (BKPyV-nephropathy) remains a significant cause of premature kidney allograft failure. In the absence of effective antiviral treatments, current therapeutic approaches rely on immunosuppression (IS) reduction, possibly at the risk of inducing alloimmunity. Therefore, we sought to explore the long-term effects of a tailored viro-immunologic surveillance and treatment program for BKPyV on the development of alloimmunity and kidney graft outcome. Forty-five pediatric kidney transplant recipients were longitudinally monitored for BKPyV replication, virus-specific immunity, and donor-specific HLA antibodies (DSAs). DNAemia developed in 15 patients who were treated with stepwise IS reduction. Among the other 30 patients, 17 developed DNAuria without DNAemia and 13 always resulted as BKPyV-negative. All patients with DNAemia cleared BKPyV after having mounted a virus-specific cellular immune response, and no biopsy-proven BKPyV-nephropathy was observed. The presence of cytotoxic populations directed to the BKPyV Large-T (LT) antigen early after transplantation protected kidney recipients from developing BKPyV replication, and the appearance of LT-specific T cells in viruric patients prevented the development of BKPyV-DNAemia. In our cohort, no significant correlation was observed between BKPyV-DNAemia and the development of DSA and antibody-mediated rejection. However, patients who experienced and cleared BKPyV-DNAemia had a worse allograft survival at a median follow-up of 18.9 years (p = 0.048). These data need to be confirmed in larger cohorts.

Pretransplant BKV-IgG serostatus and BKV-specific ELISPOT assays to predict BKV infection after kidney transplantation

Introduction

Polyomavirus (BKV) infection can lead to major complications and damage to the graft in kidney transplant recipients (KTRs). We investigated whether pretransplant BK serostatus and BK-specific cell-mediated immunity (CMI) predicts post-transplant BK infection.

Methods

A total of 93 donor-recipient pairs who underwent kidney transplantation (KT) and 44 healthy controls were examined. Assessment of donor and recipient BKV serostatus and BKV-CMI in recipients was performed prior to transplantation using BKV-IgG ELISA and BKV-specific IFN-g ELISPOT assays against five BK viral antigens (LT, St, VP1, VP2, and VP3). BK viremia was diagnosed when blood BKV-DNA of 104 copies/mL or more was detected during follow-up periods.

Results

Anti-BKV IgG antibody was detected in 74 (79.6%) of 93 KTRs and in 68 (73.1%) of 93 KT donors. A greater percentage of KTRs who received allograft from donors with high levels of anti-BKV IgG had posttransplant BK viremia (+) than KTRs from donors with low anti-BKV IgG (25.5% [12/47] vs. 4.3% [2/46], respectively; P = 0.007). Pretransplant total BKV-ELISPOT results were lower in BK viremia (+) patients than in patients without viremia (-) 20.5 [range 9.9-63.6] vs. 72.0 [43.2 - 110.8]; P = 0. 027). The sensitivity and specificity of the total BKV-ELISPOT assay (cut-off ≤ 53 spots/3×105 cells) for prediction of posttransplant BK viremia were 71.4 (95% CI: 41.9-91.6) and 54.4 (42.8-65.7), respectively. The combination of high donor BKV-IgG, low recipient BKV-IgG, and low total BKV-ELISPOT results improved specificity to 91.1%.

Discussion

Our study highlights the importance of pretransplant BKV-IgG serostatus and BKV-specific CMI in predicting posttransplant BKV infection in KTRs. The combination of high donor BKV-IgG, low recipient BKV-IgG, and low total BKV-ELISPOT results predicted BK viremia after KT. Pretransplant identification of patients at highrisk for BK viremia could enable timely interventions and improve clinical outcomes of KTRs.

Lessons from Polyomavirus Immunofluorescence Staining of Urinary Decoy Cells

Decoy cells that can be detected in the urine sediment of immunosuppressed patients are often caused by the uncontrolled replication of polyomaviruses, such as BK-Virus (BKV) and John Cunningham (JC)-Virus (JCV), within the upper urinary tract. Due to the wide availability of highly sensitive BKV and JCV PCR, the diagnostic utility of screening for decoy cells in urine as an indicator of polyomavirus-associated nephropathy (PyVAN) has been questioned by some institutions. We hypothesize that specific staining of different infection time-dependent BKV-specific antigens in urine sediment could allow cell-specific mapping of antigen expression during decoy cell development. Urine sediment cells from six kidney transplant recipients (five males, one female) were stained for the presence of the early BKV gene transcript lTag and the major viral capsid protein VP1 using monospecific antibodies, monoclonal antibodies and confocal microscopy. For this purpose, cyto-preparations were prepared and the BK polyoma genotype was determined by sequencing the PCR-amplified coding region of the VP1 protein. lTag staining began at specific sites in the nucleus and spread across the nucleus in a cobweb-like pattern as the size of the nucleus increased. It spread into the cytosol as soon as the nuclear membrane was fragmented or dissolved, as in apoptosis or in the metaphase of the cell cycle. In comparison, we observed that VP1 staining started in the nuclear region and accumulated at the nuclear edge in 6-32% of VP1⁺ cells. The staining traveled through the cytosol of the proximal tubule cell and reached high intensities at the cytosol before spreading to the surrounding area in the form of exosome-like particles. The spreading virus-containing particles adhered to surrounding cells, including erythrocytes. VP1-positive proximal tubule cells contain apoptotic bodies, with 68-94% of them losing parts of their DNA and exhibiting membrane damage, appearing as "ghost cells" but still VP1⁺. Specific polyoma staining of urine sediment cells can help determine and enumerate exfoliation of BKV-positive cells based on VP1 staining, which exceeds single-face decoy staining in terms of accuracy. Furthermore, our staining approaches might serve as an early readout in primary diagnostics and for the evaluation of treatment responses in the setting of reduced immunosuppression.

In Vitro Generation of BK polyomavirus-specific T cells for adoptive cell therapy in refractory cystitis hemorrhagic patients after hematopoietic stem cell transplantation

INTRODUCTION

BKPyV associated hemorrhagic cystitis (BKPyV-HC) is a major and prevalent outcome of hematopoietic stem cell transplantation (HSCT) with no standard treatment option. Adoptive T cell therapy (ACT) against transplant-associated viruses has shown promising potential. We sought to produce virus-specific T cells (VSTs) against BKPyV with the aim of treating refractory HSCT-associated HC.

METHODS

Peripheral blood mononuclear cells (PBMC) from healthy donors were isolated by Ficoll-Hypaque density gradient centrifugation. BKPyV-pulsed, monocyte-derived dendritic cells (mo-DCs) and T cells were co-cultured and expanded over 2-3 weeks with the addition of IL-2. The T cells were examined for various functional assays.

RESULTS

Comparison analysis of Carboxyfluorescein diacetate succinimidyl ester (CFSE) indicated that the percentage of proliferated cells were significantly higher in donors (49.62 ± 7.09%) than controls (7.96 ± 4.55%). Furthermore, expanded T cells exhibited specificity to BKPyV antigens by IFN-γ ELISPOT assay. The expanded cells showed cytotoxic function versus human lymphoblastoid cell line (LCL). Final VST products mainly comprised of CD8/CD69 double-positive T cells, which were significantly higher in donors (46.8 ± 7.1%) than controls (16.91 ± 3.40%).

CONCLUSION

In this study we demonstrated the feasibility of producing functional BKPyV-specific T cells in healthy donors using BKPyV PepMixes. These functional cells were able to proliferate and produce IFN-γ cytokine in response to BKPyV PepMixes. In addition, these T cells had cytotoxic ability against BKPyV antigen-expressing target cells.

Innate Immunity Response to BK Virus Infection in Polyomavirus-Associated Nephropathy in Kidney Transplant Recipients

BK polyomavirus (BKV) mainly causes infection in uroepithelial and renal tubular epithelial cells of either immunocompetent or immunocompromised hosts. Despite asymptomatic or mild clinical features in immunocompetent hosts with BK infection, serious complications are frequently found in immunocompromised patients, especially patients with kidney transplantation. Accordingly, BKV-associated nephropathy (BKVN) demonstrates a wide range of clinical manifestations, including ureteric stenosis and hemorrhagic cystitis. In addition, BKV re-infection in post-kidney transplantation is also a main cause of kidney allograft dysfunction and graft loss. Since the direct anti-BKV is unavailable, immune response against BKV infection is the main mechanism for organism control and might be a novel strategy to treat or suppress BKV. As such, the innate immunity, consisting of immune cells and soluble molecules, does not only suppress BKV but also enhances the subsequent adaptive immunity to eradicate the virus. Furthermore, the re-activation of BKV in BKVN of kidney-transplanted recipients seems to be related to the status of innate immunity. Therefore, this review aims to collate the most recent knowledge of innate immune response against BKV and the association between the innate immunity status of kidney-transplanted recipients and BKV re-activation.

Non-permissive human conventional CD1c+ dendritic cells enable trans-infection of human primary renal tubular epithelial cells and protect BK polyomavirus from neutralization

The BK polyomavirus (BKPyV) is a ubiquitous human virus that persists in the renourinary epithelium. Immunosuppression can lead to BKPyV reactivation in the first year post-transplantation in kidney transplant recipients (KTRs) and hematopoietic stem cell transplant recipients. In KTRs, persistent DNAemia has been correlated to the occurrence of polyomavirus-associated nephropathy (PVAN) that can lead to graft loss if not properly controlled. Based on recent observations that conventional dendritic cells (cDCs) specifically infiltrate PVAN lesions, we hypothesized that those cells could play a role in BKPyV infection. We first demonstrated that monocyte-derived dendritic cells (MDDCs), an in vitro model for mDCs, captured BKPyV particles through an unconventional GRAF-1 endocytic pathway. Neither BKPyV particles nor BKPyV-infected cells were shown to activate MDDCs. Endocytosed virions were efficiently transmitted to permissive cells and protected from the antibody-mediated neutralization. Finally, we demonstrated that freshly isolated CD1c⁺ mDCs from the blood and kidney parenchyma behaved similarly to MDDCs thus extending our results to cells of clinical relevance. This study sheds light on a potential unprecedented CD1c⁺ mDC involvement in the BKPyV infection as a promoter of viral spreading.

Dr Sylvia Gardner first discovered the BK polyomavirus (BKPyV) in the urine of a kidney-transplant recipient in 1970. In the 1990’s, the widespread use of potent immunosuppressive drugs such as tacrolimus, sirolimus or mycophenolate mofetil led to the emergence of BKPyV nephropathy. Recently, various studies reported a specific influx of myeloid dendritic cells (mDCs) in the renal tissue of kidney-transplant patients who were diagnosed with a BKPyV nephropathy. MDCs are immune cells both residing in tissues and migrating to other organs or compartments like the blood when changes in their environment occur. Their main functions are the detection of danger signals such as pathogens or tumors and the processing of antigens to prime naïve specific effectors of the adaptive immune response. Although anti-BKPyV cellular immune responses have been investigated in post-transplant recipients as well as healthy individuals, supporting an active role of mDCs little is known about how mDCs and BKPyV interact with each other. Our study provides the basis to understand the role played by mDCs in virus capture through an unprecedented endocytic mechanism and possibly in viral protection from neutralization by specific antibodies. Moreover, we showed that mDCs are unable to sense BKPyV particles or BKPyV-infected dying cells as a danger signal, supporting the view that other DC subsets might act as the true antigen presenting cells that promote the adaptive immune response against BKPyV infection.

Adoptive Transfer of JC Virus-Specific T Lymphocytes for the Treatment of Progressive Multifocal Leukoencephalopathy

OBJECTIVE

Progressive multifocal leukoencephalopathy (PML) is still burdened by high mortality in a subset of patients, such as those affected by hematological malignancies. The aim of this study was to analyze the safety and carry out preliminary evaluation of the efficacy of polyomavirus JC (JCPyV)-specific T cell therapy in a cohort of hematological patients with PML.

METHODS

Between 2014 and 2019, 9 patients with a diagnosis of "definite PML" according to the 2013 consensus who were showing progressive clinical deterioration received JCPyV-specific T cells. Cell lines were expanded from autologous or allogenic peripheral blood mononuclear cells by stimulation with JCPyV antigen-derived peptides.

RESULTS

None of the patients experienced treatment-related adverse events. In the evaluable patients, an increase in the frequency of circulating JCPyV-specific lymphocytes was observed, with a decrease or clearance of JCPyV viral load in cerebrospinal fluid. In responsive patients, transient appearance of punctate areas of contrast enhancement within, or close to, PML lesions was observed, which was interpreted as a sign of immune control and which regressed spontaneously without the need for steroid treatment. Six of 9 patients achieved PML control, with 5 alive and in good clinical condition at their last follow-up.

INTERPRETATION

Among other novel treatments, T cell therapy is emerging as a viable treatment option in patients with PML, particularly for those not amenable to restoration of specific immunity. Neurologists should be encouraged to refer PML patients to specialized centers to allow access to this treatment strategy. ANN NEUROL 2021;89:769-779.

Air pollution as a contributor to the inflammatory activity of multiple sclerosis

OBJECTIVE

Air pollution has been recently identified as a risk factor for multiple sclerosis. Aim of this study was to investigate the immunological mechanism underlying the clinical association between air pollution, namely exposure to particulate matter 10 (PM10), and inflammatory activity of multiple sclerosis (MS) METHODS: Daily recording of PM10 was obtained by monitors depending on the residence of subjects. Expression of molecules involved in activation, adhesion, and migration of T lymphocytes were tested by flow cytometry in 57 MS patients and 19 healthy controls. We next assessed in vitro the effect of PM10 on expression of C-C chemokine receptors 6 (CCR6) by peripheral blood mononuclear cells (PBMCs), on cytokine production by monocyte-derived dendritic cells (mdDC), and on T cell polarization in PBMC/mdDC mixed cultures.

RESULTS

We identified a significant correlation between mean PM10 levels and expression of CCR6 CD4+ T circulating cells in MS patients. This was paralleled by the observation in vitro of a higher level of CCR6 expression on PBMC following treatment with increased doses of particulate matter. Moreover, in mdDC cultures, particulate matter induced the secretion by mdDC of Th17 polarizing IL1 beta, IL6, and IL23 and, in mdDC/PBMC mixed cultures, enhanced generation of IL17-producing T cells.

CONCLUSIONS

Ex vivo and in vitro studies support the pro-inflammatory role of PM in MS, by upregulating expression of CCR6 on circulating CD4+ T cells and inducing in innate immune cells the production of Th17 polarizing cytokines. Therefore, we speculate that in MS respiratory exposure to PM10 may induce the production in the lung of autoreactive Th17 lymphocytes and boost their migratory properties through the blood-brain barrier.

BK Polyomavirus-Specific CD8 T-Cell Expansion In Vitro Using 27mer Peptide Antigens for Developing Adoptive T-Cell Transfer and Vaccination

BACKGROUND

BK polyomavirus (BKPyV) remains a significant cause of premature kidney transplant failure. In the absence of effective antivirals, current treatments rely on reducing immunosuppression to regain immune control over BKPyV replication. Increasing BKPyV-specific CD8 T cells correlate with clearance of BKPyV DNAemia in kidney transplant patients. We characterized a novel approach for expanding BKPyV-specific CD8 T cells in vitro using 27mer-long synthetic BKPyV peptides, different types of antigen-presenting cells, and CD4 T cells.

METHODS

Langerhans cells and immature or mature monocyte-derived dendritic cells (Mo-DCs) were generated from peripheral blood mononuclear cells of healthy blood donors, pulsed with synthetic peptide pools consisting of 36 overlapping 27mers (27mP) or 180 15mers (15mP). BKPyV-specific CD8 T-cell responses were assessed by cytokine release assays using 15mP or immunodominant 9mers.

RESULTS

BKPyV-specific CD8 T cells expanded using 27mP and required mature Mo-DCs (P = .0312) and CD4 T cells (P = .0156) for highest responses. The resulting BKPyV-specific CD8 T cells proliferated, secreted multiple cytokines including interferon γ and tumor necrosis factor α, and were functional (CD107a+/PD1-) and cytotoxic.

CONCLUSIONS

Synthetic 27mP permit expanding BKPyV-specific CD8 T-cell responses when pulsing mature Mo-DCs in presence of CD4 T cells, suggesting novel and safe approaches to vaccination and adoptive T-cell therapies for patients before and after kidney transplantation.

BK polyomavirus-specific antibody and T-cell responses in kidney transplantation: update

PURPOSE OF REVIEW

BK polyomavirus (BKPyV) has emerged as a significant cause of premature graft failure after kidney transplantation. Without effective antiviral drugs, treatment is based on reducing immunosuppression to regain immune control over BKPyV replication. The paradigm of high-level viruria/decoy cells, BKPyV-DNAemia, and proven nephropathy permits early interventions. Here, we review recent findings about BKPyV-specific antibody and T-cell responses and their potential role in risk stratification, immune monitoring, and therapy.

RECENT FINDING

Kidney transplant recipients having low or undetectable BKPyV-specific IgG immunoglobulin G (IgG) are higher risk for developing BKPyV-DNAemia if the donor has high BKPyV-specific IgG. This observation has been extended to neutralizing antibodies. Immunosuppression, impaired activation, proliferation, and exhaustion of BKPyV-specific T cells may increase the risk of developing BKPyV-DNAemia and nephropathy. Clearance of BKPyV-DNAemia was correlated with high CD8 T cell responses to human leukocyte antigen (HLA)-types presenting BKPyV-encoded immunodominant 9mers. For clinical translation, these data need to be assessed in appropriately designed clinical studies, as outlined in recent guidelines on BKPyV in kidney transplantation.

SUMMARY

Evaluation of BKPyV-specific immune responses in recipient and donor may help to stratify the risk of BKPyV-DNAemia, nephropathy, and graft loss. Future efforts need to evaluate clinical translation, vaccines, and immunotherapy to control BKPyV replication.

BK polyomavirus in solid organ transplantation-Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice

The present AST-IDCOP guidelines update information on BK polyomavirus (BKPyV) infection, replication, and disease, which impact kidney transplantation (KT), but rarely non-kidney solid organ transplantation (SOT). As pretransplant risk factors in KT donors and recipients presently do not translate into clinically validated measures regarding organ allocation, antiviral prophylaxis, or screening, all KT recipients should be screened for BKPyV-DNAemia monthly until month 9, and then every 3 months until 2 years posttransplant. Extended screening after 2 years may be considered in pediatric KT. Stepwise immunosuppression reduction is recommended for KT patients with plasma BKPyV-DNAemia of >1000 copies/mL sustained for 3 weeks or increasing to >10 000 copies/mL reflecting probable and presumptive BKPyV-associated nephropathy, respectively. Reducing immunosuppression is also the primary intervention for biopsy-proven BKPyV-associated nephropathy. Hence, allograft biopsy is not required for treating BKPyV-DNAemic patients with baseline renal function. Despite virological rationales, proper randomized clinical trials are lacking to generally recommend treatment by switching from tacrolimus to cyclosporine-A, from mycophenolate to mTOR inhibitors or leflunomide or by the adjunct use of intravenous immunoglobulins, leflunomide, or cidofovir. Fluoroquinolones are not recommended for prophylaxis or therapy. Retransplantation after allograft loss due to BKPyV nephropathy can be successful if BKPyV-DNAemia is definitively cleared, independent of failed allograft nephrectomy.

Tailoring cells for clinical needs: Meeting report from the Advanced Therapy in Healthcare symposium (October 28-29 2017, Doha, Qatar)

New technologies and therapies designed to facilitate development of personalized treatments are rapidly emerging in the field of biomedicine. Strikingly, the goal of personalized medicine refined the concept of therapy by developing cell-based therapies, the so-called “living drugs”. Breakthrough advancements were achieved in this regard in the fields of gene therapy, cell therapy, tissue-engineered products and advanced therapeutic techniques. The Advanced Therapies in Healthcare symposium, organized by the Clinical Research Center Department of Sidra Medicine, in Doha, Qatar (October 2017), brought together world-renowned experts from the fields of oncology, hematology, immunology, inflammation, autoimmune disorders, and stem cells to offer a comprehensive picture of the status of worldwide advanced therapies in both pre-clinical and clinical development, providing insights to the research phase, clinical data and regulatory aspects of these therapies. Highlights of the meeting are provided in this meeting report.

BK Polyomavirus Immune Response With Stress on BK-Specific T Cells

Polyomavirus-associated nephropathy is a pertinent cause of poor renal allograft survival. Absence of defensive immunity toward BK polyomavirus may favor the occurrence of BK polyomavirus-active infection and influence the progression to polyomavirus-associated nephropathy. Humoral immune responses may offer incomplete protection. In this review, available data on both humoral and cellular immunity were examined, with a concentration on BK polyomavirus-specific T cells; in addition, their roles in BK polyomavirus cellular immune response and immunotherapy were discussed. This traditional narrative review used PubMed and Medline searches for English language reports on BK polyomavirus immune response and BK-specific T cells published between January 1990 and November 2017. The search included the key words BK virus, BK polyomavirus, immune and response, and specific T cells. Monitoring BK polyomavirus-specific T cells has both therapeutic and prognostic value. Innovative cellular immunotherapy approaches, including development of vaccinations and infectious recombinant BK polyomavirus, could further contribute to the prevention of BK polyomavirus infection and related diseases.

Strategies to prevent BK virus infection in kidney transplant recipients

PURPOSE OF REVIEW

Despite improvements in posttransplant care, BK virus (BKV) remains one of the most challenging posttransplant infections in kidney transplant recipients with high rates of allograft failure. In the absence of well tolerated and efficacious viral specific therapeutics, treatment is primarily focused on reduction of immunosuppression, which poses a risk of rejection and fails to lead to viral clearance in a number of patients.

RECENT FINDINGS

Recent work has turned toward preventive therapies analogous to those used for other infections like cytomegalovirus. These efforts have focused on the use of quinolone antibiotic prophylaxis to prevent BKV infection and pretransplant vaccination to boost humoral and cellular immunity.

SUMMARY

Despite promising in-vitro and observational data, quinolone antibiotic prophylaxis has not been effective in preventing BKV infection in prospective studies. However, prophylaxis with newer less toxic viral specific agents such as brincidofovir - the lipid oral formulation of cidofovir - may yet prove effective. Strategies focused on eliciting a humoral immune response to recombinant virus-like particles or using adoptive transfer of BKV-specific T cells have also shown significant potential to prevent BKV infection in organ transplant recipients.

BK Polyomavirus and the Transplanted Kidney: Immunopathology and Therapeutic Approaches

BK polyomavirus is ubiquitous, with a seropositivity rate of over 75% in the adult population. Primary infection is thought to occur in the respiratory tract, but asymptomatic BK virus latency is established in the urothelium. In immunocompromised host, the virus can reactivate but rarely compromises kidney function except in renal grafts, where it causes a tubulointerstitial inflammatory response similar to acute rejection. Restoring host immunity against the virus is the cornerstone of treatment. This review covers the virus-intrinsic features, the posttransplant microenvironment as well as the host immune factors that underlie the pathophysiology of polyomavirus-associated nephropathy. Current and promising therapeutic approaches to treat or prevent this complication are discussed in relation to the complex immunopathology of this condition.

Innate Immunity and BK Virus: Prospective Strategies

Recent information demonstrated that BK virus reactivation is a dominant complication after kidney transplantation, which occurs because of immunosuppression. BK virus reactivation is the main reason of transplanted kidney losing. Immune response against BK virus is the major inhibitor of the virus reactivation. Therefore, improving our knowledge regarding the main parameters that fight against BK viruses can shed light on to direct new treatment strategies to suppress BK infection. Innate immunity consists of numerous cell systems and also soluble molecules, which not only suppress virus replication, but also activate adaptive immunity to eradicate the infection. Additionally, it appears that immune responses against reactivated BK virus are the main reasons for induction of BK virus-associated nephropathy (BKAN). Thus, improving our knowledge regarding the parameters and detailed mechanisms of innate immunity and also the status of innate immunity of the patients with BK virus reactivation and its complications can introduce new prospective strategies to either prevent or as therapy of the complication. Therefore, this review was aimed to collate the most recent data regarding the roles played by innate immunity against BK virus and also the status of innate immunity in the patients with reactivation BK virus and BKAN.

The Loss of BKV-specific Immunity From Pretransplantation to Posttransplantation Identifies Kidney Transplant Recipients at Increased Risk of BKV Replication

Quantification of BKV-load and BKV-specific immunity have been evaluated to monitor BKV-replication and outcomes in kidney transplant recipients (KTRs) with BKV-infection. However, it remains crucial to better understand how immune markers can predict the risk for later infection. We studied all KTRs between 2008 and 2011. Twenty-four KTRs were diagnosed with BKV-replication and a control group of 127 KTRs was used for comparison. Samples were collected before at +1, +2, and +3 months posttransplantation. BKV-specific and alloreactive T cells were measured using an interferon-γ Elispot assay. The extent of immunosuppression was quantified by lymphocyte subpopulations and interferon-gamma levels. KTRs with a loss of BKV-specific T cells directed to Large T-antigen from pretransplantation to posttransplantation were at increased risk of BKV-replication (p < 0.001). In contrast, KTRs with stable/rising BKV-specific T cells were more likely not to develop BKV-replication (p < 0.05). KTRs developing BKV-replication showed significantly lower CD3+, CD4+, CD8+ T cells and interferon-γ levels posttransplantation, but significantly higher alloreactive T cells (p < 0.05). Monitoring pretransplant and posttransplant BKV-specific T cells is suggested a sensitive marker to identify KTRs at increased risk of BKV-replication. Increased susceptibility to immunosuppression predisposes KTRs to a loss of protective BKV-specific immunity that results in impaired virus control and BKV-replication.

Polyomavirus-Specific Cellular Immunity: From BK-Virus-Specific Cellular Immunity to BK-Virus-Associated Nephropathy?

In renal transplantation, BK-virus (BKV)-associated nephropathy has emerged as a major complication, with a prevalence of 1-10% and graft loss in >50% of cases. BKV is a member of the polyomavirus family and rarely induces apparent clinical disease in the general population. However, replication of polyomaviruses, associated with significant organ disease, is observed in patients with acquired immunosuppression. Monitoring of specific immunity combined with viral load could be used to individually assess the risk of viral reactivation and virus control. We review the current knowledge on BKV-specific cellular immunity and, more specifically, in immunocompromised patients. In the future, immune-based therapies could allow us to treat and prevent BKV-associated nephropathy.

Quantitative analysis of BKV-specific CD4+ T cells before and after kidney transplantation

BACKGROUND

BK virus (BKV) is the main infectious cause of renal allograft dysfunction. Although recent studies showed an inverse correlation between BKV-specific T-cell responses and viral load after transplantation, the importance of pre-transplant response in the process of virus reactivation has only been studied once. In this study, we aimed to determine whether pre-transplant CD4+ T-cell response can be used for prediction of BKV reactivation and BKV nephropathy (BKVN), by a method that can practically be used in routine patient monitoring.

METHODS

BKV-specific CD4+ T-cell responses of 31 kidney recipients (all from live donors) were measured by an IFN-γ-enzyme-linked-immunospot (ELISPOT) method using mixture of peptides, at day 0 and +1, +3, and +6 months posttransplant. Additionally, seven other reactivation patients as another group were also analyzed. BKV viral loads in plasma were measured by real-time polymerase chain reaction (PCR). Responses of 10 healthy people were also included as controls in the analysis.

RESULTS

All but one patient and all of the controls had detectable CD4+ T-cell responses. Reactivation occurred in 8 out of 31 patients. There was no significant association between pretransplant BKV-specific CD4+ T-cell responses and BKV reactivation and between BKV DNA levels and CD4+ T-cell responses. In the additional group consisting of reactivation patients, four patients who had BKVN showed negative correlation between BKV-DNA levels and BKV-specific CD4+ T-cell responses (p<0.05). One patient who developed BKVN, however, was not able to mount a similar CD4+ T-cell response to viral reactivation despite immunosuppressive reduction.

CONCLUSION

Even though our cohort is small, our results may suggest that pre-transplant measurement of BKV specific CD4+ T-cell response may not be necessary, and that post-transplant monitoring, particularly during reactivation, may be more helpful in the management of the infection.

The role of CD4(+) T cells in BKV-specific T cell immunity

Reactivation of polyomavirus BK (BKV) infection represents a severe complication in kidney transplant (KTX) patients. We previously reported an association between a declining BK viral load and the reconstitution of CD4(+) T cell BKV-specific immunity in patients following kidney transplantation. However, the specific contribution of CD4(+) T cells in the regulation of BKV-replication is unknown. Nevertheless, in vitro enrichment of BKV-specific T cells and subsequent adoptive T cell transfer may improve the restoration of immune competence in KTX patients with BKV infection. To date, strategies to capture human BKV-specific T cells with the ensuing expansion to clinically useful numbers are lacking. Here, we demonstrated a comprehensive flow cytometric analysis of the BKV-specific T cell response that permits access to the majority of T cells specific for immunodominant BKV antigens. A full-spectrum evaluation of the BKV-specific T cell response was performed by stimulating peripheral blood mononuclear cells (PBMC) with a mixture of BKV immunodominant peptide pools at varying concentrations and measuring activation marker expression and cytokine secretion. We also examined the effects of co-stimulation and PBMC resting time prior to activation. We defined the narrow range of stimulation conditions that permit the capture and expansion of functional BKV-specific T cell lines. The generated BKV-specific T cell lines showed the highest specificity and functionality when the T cells were captured according to IFNγ-secretion. This study highlights the multifunctional and cytolytic BKV-specific CD4(+) T cells as a dominant population within the generated T cell product. This method offers a novel approach for the generation of BKV-specific T cell lines for adoptive immunotherapy and underscores the critical role of CD4(+) T cells in the clearance of BKV.

Genetically engineered fixed K562 cells: potent "off-the-shelf" antigen-presenting cells for generating virus-specific T cells

BACKGROUND AIMS

The human leukemia cell line K562 represents an attractive platform for creating artificial antigen-presenting cells (aAPC). It is readily expandable, does not express human leukocyte antigen (HLA) class I and II and can be stably transduced with various genes.

METHODS

In order to generate cytomegalovirus (CMV) antigen-specific T cells for adoptive immunotherapy, we transduced K562 with HLA-A∗0201 in combination with co-stimulatory molecules.

RESULTS

In preliminary experiments, irradiated K562 expressing HLA-A∗0201 and 4-1BBL pulsed with CMV pp65 and IE-1 peptide libraries failed to elicit antigen-specific CD8⁺ T cells in HLA-A∗0201⁺ peripheral blood mononuclear cells (PBMC) or isolated T cells. Both wild-type K562 and aAPC strongly inhibited T cell proliferation to the bacterial superantigen staphylococcal enterotoxin B (SEB) and OKT3 and in mixed lymphocyte reaction (MLR). Transwell experiments suggested that suppression was mediated by a soluble factor; however, MLR inhibition was not reversed using transforming growth factor-β blocking antibody or prostaglandin E2 inhibitors. Full abrogation of the suppressive activity of K562 on MLR, SEB and OKT3 stimulation was only achieved by brief fixation with 0.1% formaldehyde. Fixed, pp65 and IE-1 peptide-loaded aAPC induced robust expansion of CMV-specific T cells.

CONCLUSIONS

Fixed gene-modified K562 can serve as effective aAPC to expand CMV-specific cytotoxic T lymphocytes for therapeutic use in patients after stem cell transplantation. Our findings have implications for broader understanding of the immune evasion mechanisms used by leukemia and other tumors.

HLA-A2, HLA-B44 and HLA-DR15 are associated with lower risk of BK viremia

BACKGROUND

Human leucocyte antigens (HLAs) modulate immunity to polyomavirus BK (BKV). Identification of HLAs that alter the course of infection will facilitate risk stratification, and customization of pre-emptive intervention strategies.

METHODS

We performed a retrospective cohort study with 998 kidney transplant patients with BKV infection status confirmed by polymerase chain reaction (PCR). Clinical parameters and donor-recipient matching for specific HLAs were examined in relation to occurrence of viremia. An emphasis was placed on donor-recipient matching rather than the actual frequency of specific HLA-alleles, since a successful immune response requires sharing of HLAs between a virus-infected target cell and the anti-viral effector cell.

RESULTS

Using multivariate statistics, low risk of BK viremia was associated with matching of HLA-A2 [hazard ratio (HR) 0.51, 95% confidence interval (CI) 0.28-0.85], HLA-B44 (HR 0.31, 95% CI 0.076-0.85) and HLA-DR15 (HR 0.35, 95% CI 0.084-0.93) (P < 0.05), whereas high risk of viremia was associated with male gender (HR 2.38, 95% CI 1.46-4.09, P < 0.001).

CONCLUSIONS

HLAs that associated with a lower predisposition to the development of BK viremia have been identified. Evaluation of donor-recipient mismatching for these HLAs could potentially be used to (i) fine tune virus screening strategies for BKV in individual patients and (ii) facilitate discovery of major histocompatibility complex (MHC) class I and II binding peptides that can elicit clinically meaningful BKV-specific immunity.

HLA-A2, HLA-B44 and HLA-DR15 are associated with lower risk of BK viremia

BACKGROUND

Human leucocyte antigens (HLAs) modulate immunity to polyomavirus BK (BKV). Identification of HLAs that alter the course of infection will facilitate risk stratification, and customization of pre-emptive intervention strategies.

METHODS

We performed a retrospective cohort study with 998 kidney transplant patients with BKV infection status confirmed by polymerase chain reaction (PCR). Clinical parameters and donor-recipient matching for specific HLAs were examined in relation to occurrence of viremia. An emphasis was placed on donor-recipient matching rather than the actual frequency of specific HLA-alleles, since a successful immune response requires sharing of HLAs between a virus-infected target cell and the anti-viral effector cell.

RESULTS

Using multivariate statistics, low risk of BK viremia was associated with matching of HLA-A2 [hazard ratio (HR) 0.51, 95% confidence interval (CI) 0.28-0.85], HLA-B44 (HR 0.31, 95% CI 0.076-0.85) and HLA-DR15 (HR 0.35, 95% CI 0.084-0.93) (P < 0.05), whereas high risk of viremia was associated with male gender (HR 2.38, 95% CI 1.46-4.09, P < 0.001).

CONCLUSIONS

HLAs that associated with a lower predisposition to the development of BK viremia have been identified. Evaluation of donor-recipient mismatching for these HLAs could potentially be used to (i) fine tune virus screening strategies for BKV in individual patients and (ii) facilitate discovery of major histocompatibility complex (MHC) class I and II binding peptides that can elicit clinically meaningful BKV-specific immunity.

Immunity to Polyomavirus BK Infection: Immune Monitoring to Regulate the Balance between Risk of BKV Nephropathy and Induction of Alloimmunity

Polyomavirus BK-associated nephropathy (PyVAN) is the main infectious cause of allograft damage after kidney transplantation. A number of studies revealed an association between the presence of BKV-specific cellular immunity and BK viral clearance, with patients failing to recover specific T cells progressing to PyVAN. Evolution to allograft dysfunction can be prevented by restoration of BKV-specific immunity through a stepwise reduction of maintenance immunosuppressive drugs. Prospective monitoring of BK viral load and specific immunity, together with B-cell alloimmune surveillance, may allow a targeted modification/reduction of immunosuppression, with the aim of obtaining viral clearance while preventing graft injury due to deposition of de novo donor-specific HLA antibodies and late/chronic antibody-mediated allograft injury. Innovative, immune-based therapies may further contribute to BKV infection prevention and control.

Immunity to Polyomavirus BK Infection: Immune Monitoring to Regulate the Balance between Risk of BKV Nephropathy and Induction of Alloimmunity

Polyomavirus BK-associated nephropathy (PyVAN) is the main infectious cause of allograft damage after kidney transplantation. A number of studies revealed an association between the presence of BKV-specific cellular immunity and BK viral clearance, with patients failing to recover specific T cells progressing to PyVAN. Evolution to allograft dysfunction can be prevented by restoration of BKV-specific immunity through a stepwise reduction of maintenance immunosuppressive drugs. Prospective monitoring of BK viral load and specific immunity, together with B-cell alloimmune surveillance, may allow a targeted modification/reduction of immunosuppression, with the aim of obtaining viral clearance while preventing graft injury due to deposition of de novo donor-specific HLA antibodies and late/chronic antibody-mediated allograft injury. Innovative, immune-based therapies may further contribute to BKV infection prevention and control.

Polyomavirus-associated nephropathy

Polyomaviruses BK and JC are ubiquitous viruses with high seroprevalence rates in general population. Following primary infection, polyomaviruses BK and JC persist latently in different sites, particularly in the reno-urinary tract. Reactivation from latency may occur in normal subjects with asymptomatic viruria, while it can be associated to nephropathy (PVAN) in kidney transplantat recipients. PVAN may occur in 1%-10% of renal transplant patients with loss of the transplanted organ in 30% up to 80% of the cases. Etiology of PVAN is mainly attributable to BK virus, although approximately 5% of the cases may be due to JC. Pathogenesis of PVAN is still unknown, although viral replication and the lack of immune control play a major role. Immunosuppression represents the condicio sine qua non for the development of PVAN and the modulation of anti-rejection treatment represents the first line of intervention, given the lack of specific antiviral agents. At moment, an appropriate immunemodulation can only be accomplished by early identification of viral reactivacation by evaluation of polyomavirus load on serum and/or urine specimens, particularly in the first year post-trasplantation. Viro-immunological monitoring of specific cellular immune response could be useful to identify patients unable to recover cellular immunity posttransplantation, that are at higher risk of viral reactivation with development of PVAN. Herein, the main features of polyomaviruses BK and JC, biological properties, clinical characteristics, etiopathogenesis, monitoring and diagnosing of PVAN will be described and discussed, with an extended citation of related relevant literature data.

BK virus-specific T cells for use in cellular therapy show specificity to multiple antigens and polyfunctional cytokine responses

BACKGROUND

BK virus (BKV) infection causes hemorrhagic cystitis posthemopoietic stem-cell transplant and graft loss in renal transplant recipients. Reactivation occurs in up to 60% of patients in both groups. Treatment-related cellular immunosuppression is a major contributor to the development of BKV-related disease, but the targets of the immune response are not well characterized. Immunotherapy by adoptive transfer of cellular effectors has been shown to be effective in controlling and preventing some virus-related diseases in transplant recipients, particularly Epstein-Barr virus and cytomegalovirus. Infusion of BKV-specific T cells may potentially reconstitute functional BKV immunity and reduce clinical complications of BKV infection.

METHODS

BKV-specific T cells for clinical use in adoptive immunotherapy were generated using monocyte-derived dendritic cells pulsed with overlapping peptide mixes spanning the five BKV proteins VP1, VP2, VP3, large T antigen, and small T antigen. Phenotypic and functional characteristics of the cells were investigated as well as their antigen specificity.

RESULTS

Expanded CD4(+) and CD8(+) cells responded to restimulation with BKV peptides principally from VP1, large T, or small T antigens; produced multiple cytokines; and showed cytotoxic activity against antigen-coated targets.

CONCLUSIONS

Possible clinical uses for BKV-specific T cells generated using this method include immune reconstitution posthemopoietic stem-cell transplantation or prophylaxis and treatment of immune deficiency in renal transplant recipients, fulfilling the need for effective therapy for BKV-related hemorrhagic cystitis and renal dysfunction.

BK virus-specific immunity kinetics: a predictor of recovery from polyomavirus BK-associated nephropathy

Impaired BKV-specific immunity is associated with development of BKV-associated nephropathy. Suitable immunological parameters to identify patients at risk, however, are still debated. We monitored 18 kidney-transplant recipients through the course of self-limited BKV-reactivation (n = 11) and BKV-associated nephropathy (n = 7). BKV-specific cellular immunity directed to nonstructural small and Large T-antigen, and structural VP1-3 was analyzed in an interferon-γ Elispot assay. BKV-specific IgM and IgG were measured using an enzyme-linked immunosorbent assay simultaneously. BKV-specific cellular immunity directed to five BKV-proteins increased significantly from diagnosis to resolution of BKV-reactivation (p < 0.001). Patients with self-limited BKV-reactivation developed BKV-specific T cells without therapeutic interventions, and cleared BKV-reactivation within a median period of 1 month. Patients with BKV-associated nephropathy, however, showed BKV-specific T cells after a median period of 5 months after therapeutic interventions only, and cleared BKV-reactivation after a median period of 8 months. Anti-structural T cells were detected earlier than anti-nonstructural T cells, which coincided with BKV-clearance. Patients with BKV-associated nephropathy showed the highest frequencies of BKV-specific T cells at recovery, the highest increase in BKV-specific IgG and persistence of increased IgM levels (p < 0.05). Our results suggest prognostic values of BKV-specific immune monitoring to identify those patients at risk of BKV-associated nephropathy and to aid in the management of therapeutic interventions.

Polyomavirus JC-targeted T-cell therapy for progressive multiple leukoencephalopathy in a hematopoietic cell transplantation recipient

Progressive multifocal leukoencephalopathy (PML) associated with polyomavirus JC (JCV) infection has been reported to be usually fatal in allogeneic hematopoietic SCT (HSCT) recipients. We present the case of a 19-year-old HSCT patient diagnosed with JCV-associated PML after prolonged immunosuppression for severe GVHD. No short-term neurological improvement was observed after antiviral treatment and discontinuation of immunosuppressive therapy. Donor-derived JCV Ag-specific CTLs were generated in vitro after stimulation with 15-mer peptides derived from VP1 and large T viral proteins. After adoptive CTL infusion, virus-specific cytotoxic cells were shown in the peripheral blood, JCV-DNA was cleared in the cerebrospinal fluid and the patient showed remarkable improvement. Adoptive T-lymphocyte therapy with JCV-specific CTLs was feasible and had no side effects. This case suggests that adoptive transfer of JCV-targeted CTLs may contribute to restore JCV-specific immune competence and control PML in transplanted patients.

Dendritic cell deficiency associated with development of BK viremia and nephropathy in renal transplant recipients

BACKGROUND

BK virus nephropathy (BKVN) is a significant cause of renal allograft loss. Although overall intensity of immunosuppression is the greatest risk factor, recipient immune factors likely also play a role in the pathogenesis. Dendritic cells (DC) are potent antigen-presenting cells important for the induction of anti-viral cytotoxic T-cell responses. In a previous univariate analysis, we demonstrated a peripheral blood DC (PBDC) deficiency in patients with biopsy-proven BKVN, raising the possibility that reduction in DC predisposed to BK reactivation.

METHODS

In this study, we refined our previous analysis by comparing random posttransplant PBDC levels between an expanded group of patients with BKVN and controls without viremia using a multivariate analysis that accounted for factors known to influence PBDC levels. Next, we compared pretransplant PBDC levels between patients stratified by the presence or absence of posttransplant viremia. Finally, we assessed the predictive value of pretransplant PBDC levels for the development of posttransplant viremia.

RESULTS

Analyses revealed a PBDC level deficiency not only posttransplant in patients with BKVN but also pretransplant in patients who subsequently developed posttransplant BK viremia. Furthermore, we identified a pretransplant PBDC level that is a reasonable predictor for the development of posttransplant viremia.

CONCLUSIONS

Our results identify PBDC deficiency as a previously unrecognized risk factor for BKV reactivation after renal transplantation. Pretransplant PBDC monitoring may prove to be a useful clinical tool in the assessment of patient vulnerability to BKVN posttransplant, which may allow more focused screening.

Inhibition of polyomavirus BK-specific T-Cell responses by immunosuppressive drugs

BACKGROUND

Reducing immunosuppression is the treatment of choice for polyomavirus-associated nephropathy in kidney transplant (KT) patients, but strategies and targets are uncertain.

METHODS

Using interferon-gamma ELISpot assays, we investigated immunosuppressive drug levels and polyomavirus BK (BKV) large T-antigen-specific T-cell responses in KT patients in vivo and in healthy donors after titrating immunosuppression in vitro.

RESULTS

In KT patients, BKV-specific T-cell responses were inversely correlated with tacrolimus trough levels (R=0.28, P<0.002), but not with mycophenolate levels, prednisone, or overall immunosuppressive dosing. In vitro tacrolimus concentrations above 6 ng/mL inhibited BKV- and cytomegalovirus-specific T-cells more than 50%, whereas less than 30% inhibition was observed below 3 ng/mL. Inhibition by cyclosporine A was more than 50% at concentrations of 1920 ng/mL and less than 30% below 960 ng/mL, corresponding to clinical C0 trough levels of 200 and 100 ng/mL, respectively. However, mycophenolate up to 8 microg/mL, leflunomide 50 microg/mL, or sirolimus concentrations 64 ng/mL did not inhibit BKV-specific interferon-gamma production, but antigen-dependent T-cell expansion.

CONCLUSIONS

Calcineurin-inhibitor concentrations are critical for BKV-specific T-cell activation. Reducing calcineurin inhibitors should be considered as first step, whereas conversion to mTOR inhibitors may be an attractive alternative or second step that should be validated in clinical BKV intervention trials.

Cytotoxic T lymphocyte therapy with donor T cells prevents and treats adenovirus and Epstein-Barr virus infections after haploidentical and matched unrelated stem cell transplantation

Viral infection or reactivation remains a major cause of morbidity and mortality after allogeneic stem cell transplantation. We now show that infusions of single cytotoxic T lymphocyte (CTL) lines (5 x 10(6)-1.35 x 10(8) cells/m(2)) with specificity for 2 commonly detected viruses, Epstein-Barr virus (EBV) and adenovirus, can be safely administered to pediatric transplantation recipients receiving partially human leukocyte antigen-matched and haploidentical stem cell grafts (n = 13), without inducing graft-versus-host disease. The EBV-specific component of the CTLs expanded in vivo and persisted for more than 12 weeks, but the adenovirus-specific component only expanded in vivo in the presence of concomitant adenoviral infection. Nevertheless, adenovirus-specific T cells could be detected for at least 8 weeks in peripheral blood, even in CTL recipients without viral infection, provided the adenovirus-specific component of their circulating lymphocytes was first expanded by exposure to adenoviral antigens ex vivo. After infusion, none of these 13 high-risk recipients developed EBV-associated lymphoproliferative disease, while 2 of the subjects had resolution of their adenoviral disease. Hence, bispecific CTLs containing both EBV- and adenovirus-specific T cells can safely reconstitute an antigen responsive "memory" population of CTLs after human leukocyte antigen-mismatched stem cell transplantation and may provide antiviral activity. This trial was registered at www.clinicaltrials.gov as #NCT00590083.

Cellular immune responses to BK virus

PURPOSE OF REVIEW

Defects in cellular immunity to persistent viral infections are associated with an increased frequency and severity of viral diseases after transplantation. Polyomavirus BK (BKV) infection has emerged as an important cause of virus-related nephropathy after kidney allograft. Cell-mediated immunity seems to have a central role in preserving BKV latency. However, characterization of BKV-specific immunity has only recently begun.

RECENT FINDINGS

Immune responses to BKV are not fully understood, but pioneer work points to cell-mediated immunity as a critical factor for the control of viral replication and recovery from BKV disease.

SUMMARY

Advances in immunological techniques will provide further insight into the specificity and patterns of cellular response to BKV, which should assist translation into improved patient management and development of immunotherapeutic approaches.

Generation of BKV-Specific T Cells for Adoptive Therapy against BKV Nephropathy

Nephropathy associated with BK virus has emerged as an important cause of allograft failure in renal transplant recipients. Here we exploited a recently developed novel monocyte based solid phase T cell selection system, in which monocytes are immobilized on solid support, for antigen-specific T cell purification. The underlying hypothesis of this new method is that antigen-specific T cells recognize, bind their cognate antigens faster than non-specific T cells and are concentrated on the surface after removing the non-adherent cells by washing. Moreover, activated antigen-specific T cells proliferate more rapidly than non-specific T cells, further increasing the frequency and purity of antigen-specific T cells. Optimal selection times for BK virus-specific T cells are studied. Our data demonstrated that T cell selection can usually increase the frequency of antigen-specific T cells by > 10 fold, whereas T cell expansion following the selection boost the frequency of antigen-specific T cells by another ~10 fold. This new T cell selection system is superior to traditional stimulation method (i.e. simply mixing antigen presenting cells and lymphocytes together) in generating antigen-specific T cells. This inexpensive and simple T cell selection system can produce large quantity of highly purified BK virus-specific T cells within 1–2 weeks after initial T cell activation.

Human dendritic cells and transplant outcome

Early interest in dendritic cells (DC) in transplantation centered on the role of graft interstitial DC in the instigation of rejection. Much information has subsequently accumulated concerning the phenotypic and functional diversity of these rare, migratory, bone marrow-derived antigen-presenting cells, and their role in the induction and regulation of immunity. Detailed insights have emerged from studies of freshly isolated or in vitro-propagated DC, and from analyses of their function in experimental animal models. The functional plasticity of these uniquely well-equipped antigen-presenting cells is reflected in their ability not only to induce alloimmune responses, but also to serve as potential targets and therapeutic agents for the long-term improvement of transplant outcome. Notably, however, a great deal remains to be understood about the immunobiology of DC populations in relation to human transplant outcome. Herein, we briefly review aspects of human DC biology in organ and bone marrow transplantation, the potential of these cells for monitoring outcome, and the role of DC in development of vaccines to protect against infectious disease or to promote allograft tolerance.

Functional characterization of BK virus-specific CD4+ T cells with cytotoxic potential in seropositive adults

BK polyomavirus (BKV) reactivation is associated with a failure of T cell immunity in kidney transplant patients, and may lead to BKV-associated nephropathy (BKVN) and loss of the allograft. BKV reactivation in hematopoietic stem cell transplant recipients is associated with hemorrhagic cystitis. We have investigated T cell responses to overlapping peptide mixtures corresponding to the whole BKV major T antigen (TAg) and major capsid protein (VP1) in peripheral blood mononuclear cell samples from a cohort of healthy BKV-seropositive subjects. The majority of these individuals possessed populations of both CD8(+) and CD4(+) T cells specific for these BKV antigens. After expansion in culture, the majority of the BKV-specific CD4(+) T cells, in addition to expressing CD40L (CD154), secreted both interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha, contained both granzyme A and granzyme B, and degranulated/mobilized CD107 in response to antigen-specific stimulation. These T cells thus represent potentially functional BKV-specific cytotoxic CD4(+) T lymphocytes. Secretion of both TNF-alpha and IFN-gamma by CD154(+)CD4(+) T cells on BKV-specific stimulation was associated with higher levels of granzyme B and a higher proportion of degranulating cells compared with CD154(+)CD4(+) T cells producing only IFN-gamma or neither cytokine. These healthy subjects also harbored populations of functional CD8(+) T cells specific for one or more of three newly defined HLA-A 02-restricted cytotoxic T lymphocyte epitopes within the BKV TAg as well as two HLA-A 02-restricted epitopes within the BKV VP1 we have previously described. The BKV-specific CD4(+) T cells characterized in this study may play a part in maintaining persistent memory T cell responses to the virus and thus contribute to the immune control of BKV in healthy individuals.

Polyomavirus-Associated Nephropathy in Renal Transplantation

Polyomavirus-associated nephropathy (PVAN) is an emerging disease in renal transplant patients with variable prevalence of 1-10% and graft loss up to 80%. BK virus (BKV) is the primary etiologic agent, but JC virus (JCV) and possibly simian virus SV40 may account for some cases. Intense immunosuppression is viewed as the most important risk factor. However, the preferential manifestation in renal transplants as compared to other allografts or to autologous kidneys of other organ transplants suggests that organ determinants and immunologic factors synergize: Renal tubular epithelial cells and their compensatory proliferation to restore tubular integrity after immunologic, ischemic or toxic injury may provide the critical cellular milieu supporting polyomavirus replication while immune control is impaired due to maintenance immunosuppression, anti-rejection treatment and HLA-mismatches. Patient determinants (older age, male gender, seronegative recipient), and viral factors (genotype, serotype) may have a contributory role. The definitive diagnosis of PVAN requires allograft biopsy which is, however, challenged by (i) limited sensitivity due to (multi-)focal involvement (sampling errors); (ii) varying presentations with cytopathic-inflammatory and/or fibrotic/scarring patterns; (iii) coexisting acute rejection which is difficult to differentiate, but impacts on intervention strategies. Screening for polyomavirus replication in the urine and in the plasma complements allograft biopsy by high sensitivity and allows for noninvasive monitoring. Thus, we suggest a terminology similar to invasive fungal diseases where viruria ("decoy cells") defines patients at risk ("possible PVAN") who should be evaluated for plasma viral load. Increasing BK viremia (>10,000 copies/mL) or urine VP-1 mRNA (>6.5x10(5) copies/ng total RNA) load defines "presumptive PVAN" for which an intervention of reducing immunosuppression should be considered even if the diagnosis could not be confirmed by allograft biopsy ("definitive PVAN"). The response to intervention should be monitored using plasma DNA or urine mRNA load.

Polyomavirus BK-specific cellular immune response to VP1 and large T-antigen in kidney transplant recipients

Polyomavirus BK (BKV) is the primary cause of polyomavirus-associated nephropathy (PVAN) in kidney transplant (KT) recipients. Using ELISpot assays, we compared the frequency of interferon-gamma (IFN-gamma) secreting peripheral blood mononuclear cells (PBMC) after stimulation with overlapping peptide pools covering BKV large T-antigen (LT) and VP1 capsid proteins (VP1). In 10 healthy donors, LT and VP1 responses were low with median 24 (range 15-95) and 25 (7-113) spot-forming units/10(6) PBMC (SFU), respectively. In 42 KT patients with current or recent plasma BKV loads, median LT and VP1 responses of 29 (0-524) and 114 (0-1432) SFU were detected, respectively. In KT patients with decreasing or past plasma BKV loads, significantly higher median BKV-specific IFN-gamma responses were detected compared to KT patients with increasing or persisting BKV loads [LT: 78 (8-524) vs. 22 (0-120) SFU, p=0.003; VP1: 285 (45-1432) vs. 53 (0-423) SFU, p=0.001, respectively]. VP1-specific IFN-gamma responses were higher and more likely to involve CD4(+) T cells, while CD8(+) T cells were more frequently directed against LT. Stimulation with JCV-specific VP1 and LT peptides indicated only low-level cross-recognition. The data suggest that control of BKV replication is correlated with differentiated expansion of BKV-specific cellular immune responses.

Immune responses to BK and JC viruses in immunocompromised patients

BK virus (BKV) and JC virus (JCV) are ubiquitous human polyomaviruses that establish persistent asymptomatic infections in immunocompetent individuals, but in a minority of immunocompromised patients, reactivate and cause clinical disease. BKV is associated with BKV nephropathy (BKVN) in kidney transplant recipients and JCV is the etiologic agent of progressive multifocal leukoencephalopathy (PML) in AIDS patients. Humoral responses do not appear adequate to protect against reactivation or disease, and both BKVN and PML appear to be due to a failure of cellular immune responses to control the virus. Among the goals of current research is the identification of the functional correlates of cellular immune protection against these viruses in immunocompetent individuals. Such insights may help identify the small subset of patients at risk of BKV and JCV reactivation, aid clinical management and permit the development of immunotherapeutic approaches.

Quantification and identification of polyomavirus DNA in blood and urine of renal transplant recipients

A cohort of 201 kidney transplant recipients (KTR) including 7 patients with evidence of renal function deterioration (as defined by creatinine levels >20% over baseline values) was analyzed for polyomavirus DNA in blood and urine samples by a new quantitative polymerase chain reaction method. Of 201 patients, 14 (6.9%) were positive for polyomavirus DNA in blood (median level, 500 copies per milliliter of blood) including all 7 patients with renal function deterioration. Polyomavirus DNA detection in blood for diagnosis of renal function deterioration in KTR showed a sensitivity of 100% and a specificity of 96%, whereas positive and negative predictive values were 50% and 100%, respectively. Diagnostic value of decoy cells detection and polyomavirus DNA quantification in urine samples was negligible.