Cytomegalovirus drives Vδ1+ γδ T cell expansion and clonality in common variable immunodeficiency.

The function and phenotype of γδ T cells in the context of common variable immunodeficiency (CVID) has not been explored. CVID is a primary immunodeficiency disorder characterized by impaired antibody responses resulting in increased susceptibility to infections. γδ T cells are a subset of unconventional T cells that play crucial roles in host defence against infections. In this study, we aim to determine the roles and functions of γδ T cells in CVID. We observe a higher frequency of Vδ1+ γδ T cells compared to healthy controls, particularly in older patients. We also find a higher proportion of effector-memory Vδ1+ γδ T cells and a more clonal T cell receptor (TCR) repertoire in CVID. The most significant driver of the Vδ1+ γδ T cell expansion and phenotype in CVID patients is persistent cytomegalovirus (CMV) viremia. These findings provide valuable insights into γδ T cell biology and their contribution to immune defence in CVID.

A single-center experience of COVID-19 infection in patients with primary immunodeficiency.

Background: Reported outcomes in patients with primary immunodeficiency (PID) infected by coronavirus disease 2019 (COVID-19) have been variable owing to a combination of viral strain heterogeneity, differences in patient populations and health systems, and local availability of vaccination and specific COVID-19 therapies. There are few reports on the experience of Australian patients with PID during the pandemic. Objectives: In this retrospective study, we describe the baseline characteristics and short-term outcomes of patients with PID who were infected by COVID-19 and known to the Royal Melbourne Hospital, a major tertiary center in Victoria, Australia. Methods: Between April 2021 and April 2022, a total of 31 of 138 patients with PID were affected by COVID-19. More than half of them had 3 vaccine doses at the time of infection (which at the time was considered being fully vaccinated) and received COVID-19-targeted treatment. Results: All of the infected patients had ambulatory disease, with no cases of morbidity or mortality. In line with the current literature, the PID subtypes described did not appear to independently predict worse outcomes. Conclusions: Some protective factors include this cohort's relatively younger average age and its high uptake of vaccination and COVID-19 therapies.

Non-viral pathogens of infectious diarrhoea post-allogeneic stem cell transplantation are associated with graft-versus-host disease.

Infectious diarrhoea is common post-allogeneic haematopoietic stem-cell transplantation (alloHSCT). While the epidemiology of Clostridioides difficile infection (CDI) post-alloHSCT has been described, the impact of other diarrhoeal pathogens is uncertain. We reviewed all alloHSCT between 2017 and 2022 at a single large transplant centre; 374 patients were identified and included. The 1-year incidence of infectious diarrhoea was 23%, divided into viral (13/374, 3%), CDI (65/374, 17%) and other bacterial infections (16/374, 4%). There was a significant association between infectious diarrhoea within 1 year post-transplant and the occurrence of severe acute lower gastrointestinal graft-versus-host disease (GVHD, OR = 4.64, 95% CI 2.57-8.38, p < 0.001) and inferior GVHD-free, relapse-free survival on analysis adjusted for age, donor type, stem cell source and T-cell depletion (aHR = 1.64, 95% CI = 1.18-2.27, p = 0.003). When the classes of infectious diarrhoea were compared to no infection, bacterial (OR = 6.38, 95% CI 1.90-21.40, p = 0.003), CDI (OR = 3.80, 95% CI 1.91-7.53, p < 0.001) and multiple infections (OR = 11.16, 95% CI 2.84-43.92, p < 0.001) were all independently associated with a higher risk of severe GI GVHD. Conversely, viral infections were not (OR = 2.98, 95% CI 0.57-15.43, p = 0.20). Non-viral infectious diarrhoea is significantly associated with the development of GVHD. Research to examine whether the prevention of infectious diarrhoea via infection control measures or modulation of the microbiome reduces the incidence of GVHD is needed.

Evolution of Humoral and Cellular Immunity Post-Breakthrough Coronavirus Disease 2019 in Vaccinated Patients With Hematologic Malignancy Receiving Tixagevimab-Cilgavimab.

Background: In-depth immunogenicity studies of tixagevimab-cilgavimab (T-C) are lacking, including following breakthrough coronavirus disease 2019 (COVID-19) in vaccinated patients with hematologic malignancy (HM) receiving T-C as pre-exposure prophylaxis. Methods: We performed a prospective, observational cohort study and detailed immunological analyses of 93 patients with HM who received T-C from May 2022, with and without breakthrough infection, during a follow-up period of 6 months and dominant Omicron BA.5 variant. Results: In 93 patients who received T-C, there was an increase in Omicron BA.4/5 receptor-binding domain (RBD) immunoglobulin G (IgG) antibody titers that persisted for 6 months and was equivalent to 3-dose-vaccinated uninfected healthy controls at 1 month postinjection. Omicron BA.4/5 neutralizing antibody was lower in patients receiving B-cell-depleting therapy within 12 months despite receipt of T-C. COVID-19 vaccination during T-C treatment did not incrementally improve RBD or neutralizing antibody levels. In 16 patients with predominantly mild breakthrough infection, no change in serum neutralization of Omicron BA.4/5 postinfection was detected. Activation-induced marker assay revealed an increase in CD4+ (but not CD8+) T cells post infection, comparable to previously infected healthy controls. Conclusions: Our study provides proof-of-principle for a pre-exposure prophylaxis strategy and highlights the importance of humoral and cellular immunity post-breakthrough COVID-19 in vaccinated patients with HM.

High Rates of Seroprotection and Seroconversion to Vaccine-Preventable Infections in the Early Post-Autologous Stem Cell Transplant Period.

In patients early post-autologous stem cell transplant, seroprotection rates were high for Hemophilus influenzae type B and tetanus toxoid (70%-90%) but lower for Streptococcus pneumoniae (30%-50%) including after revaccination. There were high rates of seropositivity (67%-86%) to measles, mumps, and rubella and varicella zoster virus. Durability of protection requires assessment.

CMV prevention strategies in allogeneic hematopoietic cell transplantation; the role of prophylaxis and pre-emptive monitoring in the era of letermovir.

The preferred strategy for preventing CMV in at-risk populations in alloHCT has undergone a significant practice shift in recent years where the pendulum has swung from a pre-emptive approach to now offering letermovir prophylaxis to all CMV seropositive recipients. Letermovir prophylaxis has resulted in significant reductions in post-transplant clinically significant CMV infection (csCMVi) as well as other important outcomes such as CMV disease, resistant, and refractory CMV infections and nonrelapse mortality. However, prophylactic strategies are not without some limitations, namely delayed onset CMV infections, delayed CMV-specific T cell immune reconstitution, increased drug costs and limited data within pediatric populations. Thus, this review aims to provide an overview of prophylaxis and pre-emptive CMV preventative strategies, and how they are applicable in the current era of letermovir prophylaxis.

Invasive aspergillosis in adult patients in Australia and New Zealand: 2017-2020.

Background: New and emerging risks for invasive aspergillosis (IA) bring the need for contemporary analyses of the epidemiology and outcomes of IA, in order to improve clinical practice. Methods: The study was a retrospective, multicenter, cohort design of proven and probable IA in adults from 10 Australasian tertiary centres (January 2017-December 2020). Descriptive analyses were used to report patients' demographics, predisposing factors, mycological characteristics, diagnosis and management. Accelerated failure-time model was employed to determine factor(s) associated with 90-day all-cause mortality (ACM). Findings: Of 382 IA episodes, 221 (in 221 patients) fulfilled inclusion criteria - 53 proven and 168 probable IA. Median patient age was 61 years (IQR 51-69). Patients with haematologic malignancies (HM) comprised 49.8% of cases. Fifteen patients (6.8%) had no pre-specified immunosuppression and eleven patients (5.0%) had no documented comorbidity. Only 30% of patients had neutropenia. Of 170 isolates identified, 40 (23.5%) were identified as non-Aspergillus fumigatus species complex. Azole-resistance was present in 3/46 (6.5%) of A. fumigatus sensu stricto isolates. Ninety-day ACM was 30.3%. HM (HR 1.90; 95% CI 1.04-3.46, p = 0.036) and ICU admission (HR 4.89; 95% CI 2.93-8.17, p < 0.001) but not neutropenia (HR 1.45; 95% CI 0.88-2.39, p = 0.135) were associated with mortality. Chronic kidney disease was also a significant predictor of death in the HM subgroup (HR 3.94; 95% CI 1.15-13.44, p = 0.028). Interpretation: IA is identified in high number of patients with mild/no immunosuppression in our study. The relatively high proportion of non-A. fumigatus species complex isolates and 6.5% azole-resistance rate amongst A. fumigatus sensu stricto necessitates accurate species identification and susceptibility testing for optimal patient outcomes. Funding: This work is unfunded. All authors' financial disclosures are listed in detail at the end of the manuscript.

Evidence supporting the use of therapeutic drug monitoring of ganciclovir in transplantation.

PURPOSE OF REVIEW: This review describes current knowledge of ganciclovir (GCV) and valganciclovir (ValGCV) pharmacokinetic/pharmacodynamic characteristics, highlighting the likely contribution from host genetic factors to interpatient variability. The evidence and challenges surrounding optimization of drug dosing through therapeutic drug monitoring (TDM) are examined, with recommendations made. RECENT FINDINGS: Pharmacokinetic studies of current dosing guidelines have shown high interindividual and intraindividual variability of GCV concentrations. This is sometimes associated with a slow decline in cytomegalovirus (CMV) viral load in some transplant recipients. A high incidence of GCV-associated myelosuppression has limited the use of this drug in the transplant setting. Patient groups identified to benefit from GCV TDM include pediatric patients, cystic fibrosis with lung transplantation, obese with kidney transplantation, and patients with fluctuating renal function or on hemodialysis. The emergence of refractory resistant CMV, particularly in immune compromised patients, highlights the importance of appropriate dosing of these antivirals. Host genetic factors need to be considered where recently, two host genes were shown to account for interpatient variation during ganciclovir therapy. Therapeutic Drug Monitoring has been shown to improve target antiviral-level attainment. The use of TDM may guide concentration-based dose adjustment, potentially improving virological and clinical outcomes. However, evidence supporting the use of TDM in clinical practice remains limited and further study is needed in the transplant cohort. SUMMARY: Further studies examining novel biomarkers are needed to guide target concentrations in prophylaxis and treatment. The use of TDM in transplant recipients is likely to improve the clinical efficacy of current antivirals and optimize outcomes in transplant recipients.

COVID-19 infection among patients with cancer in Australia from 2020 to 2022: a national multicentre cohort study.

Background: The global COVID-19 pandemic disproportionately affected certain populations and its management differed between countries. This national study describes characteristics and outcomes of COVID-19 in patients with cancer in Australia. Methods: We performed a multicentre cohort study of patients with cancer and COVID-19 from March 2020 to April 2022. Data were analysed to determine varying characteristics between cancer types and changes in outcomes over time. Multivariable analysis was performed to determine risk factors associated with oxygen requirement. Findings: 620 patients with cancer from 15 hospitals had confirmed COVID-19. There were 314/620 (50.6%) male patients, median age 63.5 years (IQR 50-72) and majority had solid organ tumours (392/620, 63.2%). The rate of COVID-19 vaccination (≥1 dose) was 73.4% (455/620). Time from symptom onset to diagnosis was median 1 day (IQR 0-3), patients with haematological malignancy had a longer duration of test positivity. Over the study period, there was a significant decline in COVID-19 severity. Risk factors associated with oxygen requirement included male sex (OR 2.34, 95% CI 1.30-4.20, p = 0.004), age (OR 1.03, 95% CI 1.01-1.06, p = 0.005); not receiving early outpatient therapy (OR 2.78, 95% CI 1.41-5.50, p = 0.003). Diagnosis during the omicron wave was associated with lower odds of oxygen requirement (OR 0.24, 95% CI 0.13-0.43, p < 0.0001). Interpretation: Outcomes from COVID-19 in patients with cancer in Australia over the pandemic have improved, potentially related to changing viral strain and outpatient therapies. Funding: This study was supported by research funding from MSD.

Effective CMV prophylaxis with high-dose valaciclovir in allogeneic hematopoietic stem-cell recipients at a high risk of CMV infection.

BACKGROUND: Cytomegalovirus (CMV) infection increases mortality and morbidity following allogeneic hematopoietic stem-cell transplantation (alloHSCT). Universal antiviral prophylaxis with letermovir is effective but unsubsidized in Australia. Valaciclovir demonstrates anti-CMV activity in high doses, but few current real-world studies explore its use as primary prophylaxis in high-risk patients post-alloHSCT. METHODS: We performed a retrospective analysis of alloHSCT recipients at high risk of clinically significant CMV infection (cs-CMVi), defined as a plasma CMV DNA viral load of >400 IU/ml requiring preemptive therapy, or CMV disease. High-risk recipients were CMV seropositive and underwent T-cell depleted, haploidentical or umbilical cord stem-cell transplants. Consecutive patients transplanted from July 2018 to January 2020, treated with valaciclovir 2 g TDS from day +7 to +100 (HD-VALA), were compared to a historical cohort (July 2017-June 2018) who only received preemptive CMV therapy, and standard valaciclovir (SD-VALA) for varicella/herpes prophylaxis. We compared incidence of and time to cs-CMVi. RESULTS: In the SD-VALA cohort (n = 27, median CMV follow-up duration 259 days), 23/27 (85%) developed cs-CMVi at a median of 39 days. For the HD-VALA cohort (n = 35, median CMV follow-up duration 216 days), 19/35 (54%) developed cs-CMVi, at a median of 68 days. Time to cs-CMVi was significantly longer in HD-VALA cohort (p < .0001). On multivariate analysis, HD VALA reduced the risk of cs-CMVi (HR 0.32, p = .0005). CONCLUSIONS: In alloHSCT recipients at high risk for cs-CMVi, HD-VALA resulted in lower cumulative reactivation, and delayed reactivation, reducing requirement for preemptive CMV therapy in the early post-engraftment period.

Risk factors and outcome of concurrent and sequential multiviral cytomegalovirus, Epstein-Barr virus, BK virus, adenovirus and other viral reactivations in transplantation.

PURPOSE OF REVIEW: Reactivation of viral infections occurs frequently in immunosuppressed populations, particularly in solid organ (SOT) or allogeneic haematopoietic cell (HCT) transplant patients. Concurrent and sequential multivirus infections are common, yet risk factors and outcomes remain unclear. This review aims to identify the patients vulnerable to multivirus infections and characterize the impact of increased viral burden to formulate prevention and treatment strategies. RECENT FINDINGS: Incidences of up to 89% in SOT and 36% in HCT have been reported for two viruses, and 32% in SOT and 28% in HCT for at least three viruses. Risk factors appear related to an increased burden of immunosuppression, with most viral coinfections occurring within 12 months of transplantation. Direct viral complications such as cytomegalovirus disease are more frequent in coinfected patients, with documented prolonged duration of viraemia, higher viral load and increased end-organ disease. Graft dysfunction, acute rejection and graft-vs.-host disease (GVHD) have also been associated. Increased mortality is reported in the HCT population. SUMMARY: Multivirus infections occur in a significant proportion of transplant patients and is linked to immunosuppressive burden. There is increasing evidence that this leads to worse graft and patient outcomes. Further prospective studies are required to further comprehensively characterise viral epidemiology, mechanisms and treatment strategies to ameliorate this risk.

Cytomegalovirus DNAemia and disease: current-era epidemiology, clinical characteristics and outcomes in cancer patients other than allogeneic haemopoietic transplantation.

BACKGROUND: High-intensity chemotherapy and advances in novel immunotherapies have seen the emergence of cytomegalovirus (CMV) infections in cancer patients other than allogeneic haemopoietic cell transplantation (HCT). Aim To evaluate the epidemiology, clinical characteristics and outcomes of CMV infection in this population. METHODS: A retrospective review of cancer patients other than allogeneic HCT who had CMV DNAemia and/or disease from July 2013 till May 2020 at a quaternary cancer centre was performed. RESULTS: Of 11 485 cancer patients who underwent treatment during this period, 953 patients had CMV DNA testing performed and 238 of them had CMV DNAemia. After excluding patients with allogeneic HCT, 62 patients with CMV DNAemia were identified, of whom 10 had concurrent CMV disease. The most frequent underlying malignancies were B-cell lymphoproliferative disease (LPD) (31%; 19/62), T-cell LPD (21%; 13/62), chronic lymphocytic leukaemia (11%; 7/62) and multiple myeloma (10%; 6/62). Most patients had lymphopenia (77%; 48/62), multiple cancer therapies (63%; 39/62 received ≥2 previous therapies), co-infection (56%; 35/62 had ≥1 co-infection) and corticosteroid therapy (48%; 30/62) within 1 month before CMV diagnosis. CMV DNAemia and disease were observed in patients receiving novel immunotherapies, including bispecific antibody therapy, chimeric-antigen receptor T-cell therapy and immune checkpoint inhibitors. CONCLUSION: Patients with haematological malignancy, particularly B-cell LPD, T-cell LPD, chronic lymphocytic leukaemia and multiple myeloma, were frequently identified to have CMV DNAemia and disease. Lymphopenia, multiple cancer therapies, co-infection and recent receipt of systemic corticosteroids were also commonly observed. Future studies are necessary to determine optimal identification and management of CMV in these patients.

Consensus guidelines for the diagnosis and management of invasive aspergillosis, 2021.

Invasive aspergillosis (IA) in haematology/oncology patients presents as primary infection or breakthrough infection, which can become refractory to antifungal treatment and has a high associated mortality. Other emerging patient risk groups include patients in the intensive care setting with severe respiratory viral infections, including COVID-19. These guidelines present key diagnostic and treatment recommendations in light of advances in knowledge since the previous guidelines in 2014. Culture and histological-based methods remain central to the diagnosis of IA. There is increasing evidence for the utility of non-culture methods employing fungal biomarkers in pre-emptive screening for infection, as well as for IA diagnosis when used in combination. Although azole resistance appears to be uncommon in Australia, susceptibility testing of clinical Aspergillus fumigatus complex isolates is recommended. Voriconazole remains the preferred first-line antifungal agent for treating primary IA, including for extrapulmonary disease. Recommendations for paediatric treatment broadly follow those for adults. For breakthrough and refractory IA, a change in class of antifungal agent is strongly recommended, and agents under clinical trial may need to be considered. Newer immunological-based imaging modalities warrant further study, while surveillance for IA and antifungal resistance remain essential to informing the relevance of current treatment recommendations.

Beyond antivirals: virus-specific T-cell immunotherapy for BK virus haemorrhagic cystitis and JC virus progressive multifocal leukoencephalopathy.

PURPOSE OF REVIEW: The clinical manifestations of the polyomaviruses BK and JC in immunocompromised patients include BK virus (BKV) induced haemorrhagic cystitis and nephropathy, and JC virus (JCV) associated progressive multifocal leukoencephalopathy (PML) and are typically a consequence of impaired adaptive immunity in the host. To date, little clinical success has been achieved with antiviral agents or other drug therapies to treat these conditions. Here we review the methods and outcomes of the most recent clinical studies utilising adoptive immunotherapy with BK and/or JC virus-specific T-cells (VST) as either prophylaxis or treatment alternatives. RECENT FINDINGS: In the last 12-18 months, several clinical trials have been published in the post-haemopoietic stem cell transplant (HSCT) setting showing good clinical success with the use of VST for treatment of BK viremia ± haemorrhagic cystitis. Between 82 and 100% clinical response has been observed in haemorrhagic cystitis using either third-party or donor-derived VST. The therapy was well tolerated with few cases of graft versus host disease in HSCT recipients, but immune mediated renal allograft loss was observed in one renal transplant recipient. Studies using BKV/JCV VST to treat PML are hindered by few patients who are sufficiently stable to receive VST. In a condition that otherwise carries such poor prognosis, VST were associated with clearance of JC virus, clinical and radiological improvement in some patients. Immune reconstitution inflammatory syndrome was a noted adverse event. SUMMARY: Restoration of BK and JC virus immunity using VST immunotherapy has shown good clinical outcomes in BKV associated infections. Further evaluation with the administration of VST earlier in the course of disease is warranted for the treatment of BKV associated nephropathy in renal allograft and in JCV PML. In both indications, larger cohorts and standardisation of dosing and outcome measures would be of benefit.

Cytomegalovirus in primary immunodeficiency.

PURPOSE OF REVIEW: Cytomegalovirus (CMV) infection and disease are well described in the setting of secondary immunodeficiency. Less is known about CMV in the context of primary immunodeficiencies (PIDs), where inborn errors in one or more arms of the immune system result in variable degrees of CMV susceptibility. RECENT FINDINGS: PID presents unique challenges in the diagnosis and management of CMV disease. The clinical presentation of CMV in PID is often severe, accelerated by underlying immune dysregulation and iatrogenic immunosuppression. Here we describe the clinical significance of CMV infection in PID, the key components of immune defence against CMV and how these are affected in specific PIDs. CMV disease is under-recognized as a complication of common variable immunodeficiency (CVID). High rates of CMV end-organ disease, mortality, development of CMV resistance and prolonged antiviral use have been observed in individuals with CVID. SUMMARY: We recommend that clinicians tailor their approach to the individual based on their underlying immune deficit and maintain a high index of suspicion and low threshold for treatment. More research is required to improve stratification of CMV risk in PID, develop new diagnostic tools and manage end-organ disease in this cohort.

American Society for Transplantation and Cellular Therapy Series: #4 - Cytomegalovirus treatment and management of resistant or refractory infections after hematopoietic cell transplantation.

The Practice Guidelines Committee of the American Society of Transplantation and Cellular Therapy (ASTCT) partnered with its Transpl. Infect. Dis. Special Interest Group (TID-SIG) to update its 2009 compendium-style infectious disease guidelines for hematopoietic cell transplantation (HCT). A new approach was employed with the goal of better serving clinical providers by publishing each standalone topic in the infectious diseases series as a concise format of frequently asked questions (FAQ), tables, and figures. Adult and pediatric infectious diseases and HCT content experts developed and answered FAQs. Topics were finalized with harmonized recommendations that were made by assigning an A through E strength of recommendation paired with a level of supporting evidence graded I through III. The fourth topic in the series focuses on the management and treatment of cytomegalovirus (CMV) resistant and refractory infections. The diagnosis, definitions of resistant and refractory CMV, risk factors, virological genotypes and treatment algorithms are reviewed.

Dynamics of Epstein-Barr virus on post-transplant lymphoproliferative disorders after antithymocyte globulin-conditioned allogeneic hematopoietic cell transplant.

BACKGROUND: The use of antithymocyte globulin (ATG) in allogeneic hematopoietic cell transplant (HCT) is associated with an increased risk of Epstein-Barr virus (EBV) reactivation and post-transplant lymphoproliferative disorders (PTLD). The dynamics and outcomes of EBV-DNAemia are not well described in this population. METHODS: We retrospectively assessed the kinetics of EBV-DNAemia after ATG conditioning of HCT recipients. Receiver operating characteristic (ROC) curves were used to assess EBV-DNAemia to predict EBV-PTLD in this group. RESULTS: A total of 174/405 (43%) consecutive HCT recipients from two centers met inclusion criteria of ATG conditioned, non-B-cell lymphoma patients. Of these with EBV-DNA measured using standardized IU/ml, 78.6% (92/117) developed EBV-DNAemia: 62% spontaneously resolved; 19% cleared after preemptive rituximab, and 13% developed EBV-PTLD. ROC curve analysis using maximum pre-EBV-PTLD EBV-DNAemia, demonstrated an AUC of 0.912 with EBV-DNAemia of 9782 IU/ml, associated with 82.6% sensitivity and 94.4% specificity for development of EBV-PTLD. Median time for EBV-DNAemia to increase from initial detection to >1000 IU/ml was 7 days; to >10 000 IU/ml, 12 days; and to >100 000 IU/ml, 18 days. Median EBV-DNAemia level prior to administration of rituximab was significantly lower in patients with successful preemptive treatment, compared with those who developed EBV-PTLD (3.41 log10  IU/ml [3.30-3.67] vs. 4.34 log10  IU/ml [3.85-5.13], p = .002; i.e., 2628 IU/ml vs. 21 965 IU/ml, respectively). CONCLUSIONS: EBV-DNAemia >10 000 IU/ml was the strongest predictor of the development of EBV-PTLD, and progression to this level was rapid in ATG-conditioned HCT recipients. This information may guide EBV-PTLD management strategies in these high-risk patients.

Safety and efficacy of intravenously administered cidofovir in adult haematopoietic cell transplant recipients: a retrospective multicentre cohort study.

OBJECTIVES: To evaluate the safety and efficacy of cidofovir for the treatment of double-stranded DNA (dsDNA) viral infections following allogeneic haematopoietic cell transplant (HCT). METHODS: This was a retrospective multicentre cohort study including adult HCT recipients who received ≥1 dose of IV-administered cidofovir for any dsDNA viral infection from 2006 to 2019. The objectives were to describe the rate of and risk factors for nephrotoxicity and virological response by the end of treatment (EOT). RESULTS: We included 165 patients from nine centres. Cidofovir was administered at 5 mg/kg/week (N = 115; 69.7%), 1 mg/kg/week (18; 10.9%), 3 mg/kg/week (12; 7.3%) or 1 mg/kg three times/week (11; 6.7%). Cidofovir was administered for adenovirus, cytomegalovirus (CMV) and BK virus infection in 75 (45.5%), 64 (38.8%) and 51 (30.9%) patients, respectively. Among 158 patients with renal function data at baseline and EOT, 40 (25.3%) developed nephrotoxicity. In multivariable analyses, age (OR 1.04; P = 0.05), weight (OR 1.05; P = 0.01), CMV infection (OR 3.6; P = 0.02), liposomal amphotericin B (OR 8.06; P = 0.05) and IV voriconazole/posaconazole (OR 13.0; P = 0.003) were predictors of nephrotoxicity. Creatinine concentration was significantly higher at EOT (1.16 ±â€Š0.95 mg/dL) compared with baseline (0.91 ±â€Š0.39 mg/dL; P < 0.001), but improved by 2 weeks (0.91 ±â€Š0.84 mg/dL; P = 0.007) and 4 weeks (0.96 ±â€Š0.89 mg/dL; P = 0.03) post-EOT. Median viral load significantly declined for patients with adenovirus DNAaemia by EOT (P < 0.0001) and for patients with CMV DNAaemia by EOT + 4 weeks (P = 0.003), but not for patients with BK virus DNAaemia. CONCLUSIONS: One in four HCT recipients treated with IV cidofovir developed largely reversible nephrotoxicity. Careful selection of patients and close follow-up of renal function may minimize toxicity.