How Immunocompromised Hosts Were Left Behind in the Quest to Control the Covid-19 Pandemic.

The immunocompromised population was disproportionately affected by the SARS-CoV-2 pandemic. However, these individuals were largely excluded from clinical trials of vaccines, monoclonal antibodies, and small molecule antivirals. While the community of scientists, clinical researchers, and funding agencies have proven that these therapeutics can be made and tested in record time, extending this progress to vulnerable and medically complex individuals from the start has been a missed opportunity. Here we advocate that it is paramount to plan for future pandemics by investing in specific clinical trial infrastructure for the immunocompromised population to be prepared when the need arises.

Human herpesvirus-6, HHV-8 and parvovirus B19 after allogeneic hematopoietic cell transplant: the lesser-known viral complications.

PURPOSE OF REVIEW: Viral infections continue to burden allogeneic hematopoietic cell transplant (HCT) recipients. We review the epidemiology, diagnosis, and management of human herpesvirus (HHV)-6, HHV-8 and parvovirus B19 following HCT. RECENT FINDINGS: Advances in HCT practices significantly improved outcomes but impact viral epidemiology: post-transplant cyclophosphamide for graft-versus-host disease prevention increases HHV-6 reactivation risk while the impact of letermovir for CMV prophylaxis - and resulting decrease in broad-spectrum antivirals - is more complex. Beyond the well established HHV-6 encephalitis, recent evidence implicates HHV-6 in pneumonitis. Novel less toxic therapeutic approaches (brincidofovir, virus-specific T-cells) may enable preventive strategies in the future. HHV-8 is the causal agent of Kaposi's sarcoma, which is only sporadically reported after HCT, but other manifestations are possible and not well elucidated. Parvovirus B19 can cause severe disease post-HCT, frequently manifesting with anemia, but can also be easily overlooked due to lack of routine screening and ambiguity of manifestations. SUMMARY: Studies should establish the contemporary epidemiology of HHV-6, and other more insidious viruses, such as HHV-8 and parvovirus B19 following HCT and should encompass novel cellular therapies. Standardized and readily available diagnostic methods are key to elucidate epidemiology and optimize preventive and therapeutic strategies to mitigate the burden of infection.

Unveiling the proteome-wide autoreactome enables enhanced evaluation of emerging CAR-T therapies in autoimmunity.

Given the global surge in autoimmune diseases, it is critical to evaluate emerging therapeutic interventions. Despite numerous new targeted immunomodulatory therapies, comprehensive approaches to apply and evaluate the effects of these treatments longitudinally are lacking. Here, we leveraged advances in programmable-phage immunoprecipitation (PhIP-Seq) methodology to explore the modulation, or lack thereof, of autoantibody profiles, proteome-wide, in both health and disease. Using a custom set of over 730,000 human derived peptides, we demonstrated that each individual, regardless of disease state, possesses a distinct and complex constellation of autoreactive antibodies. For each individual, the set of resulting autoreactivites constituted a unique immunological fingerprint, or "autoreactome," that was remarkably stable over years. Using the autoreactome as a primary output, we evaluated the relative effectiveness of various immunomodulatory therapies in altering autoantibody repertoires. We found that therapies targeting B-Cell Maturation Antigen (BCMA) profoundly altered an individual's autoreactome, while anti-CD19 and CD20 therapies had minimal effects. These data both confirm that the autoreactome is comprised of autoantibodies secreted by plasma cells, and strongly suggest that BCMA or other plasma cell targeting therapies may be highly effective in treating currently refractory autoantibody mediated diseases.

SARS-CoV-2 vaccination in the first year after hematopoietic cell transplant or chimeric antigen receptor T cell therapy: A prospective, multicenter, observational study.

BACKGROUND: The optimal timing of vaccination with SARS-CoV-2 vaccines after cellular therapy is incompletely understood. The objectives of this study are to determine whether humoral and cellular responses after SARS-CoV-2 vaccination differ if initiated <4 months versus 4-12 months after cellular therapy. METHODS: We conducted a multicenter prospective observational study at 30 cancer centers in the United States. SARS-CoV-2 vaccination was administered as part of routine care. We obtained blood prior to and after vaccinations at up to five time points and tested for SARS-CoV-2 spike (anti-S) IgG in all participants and neutralizing antibodies for Wuhan D614G, Delta B.1.617.2, and Omicron B.1.1.529 strains, as well as SARS-CoV-2-specific T cell receptors (TCRs), in a subgroup. RESULTS: We enrolled 466 allogeneic hematopoietic cell transplant (HCT; n=231), autologous HCT (n=170), and chimeric antigen receptor T cell (CAR-T cell) therapy (n=65) recipients between April 2021 and June 2022. Humoral and cellular responses did not significantly differ among participants initiating vaccinations <4 months vs 4-12 months after cellular therapy. Anti-S IgG ≥2,500 U/mL was correlated with high neutralizing antibody titers and attained by the last time point in 70%, 69%, and 34% of allogeneic HCT, autologous HCT, and CAR-T cell recipients, respectively. SARS-CoV-2-specific T cell responses were attained in 57%, 83%, and 58%, respectively. Pre-cellular therapy SARS-CoV-2 infection or vaccination were key predictors of post-cellular therapy immunity. CONCLUSIONS: These data support mRNA SARS-CoV-2 vaccination prior to, and reinitiation three to four months after, cellular therapies with allogeneic HCT, autologous HCT, and CAR-T cell therapy.

Final Outcomes from a Phase 2 Trial of Posoleucel in Allogeneic Hematopoietic Cell Transplant Recipients.

Allogeneic hematopoietic cell transplantation (allo-HCT) recipients are susceptible to viral infections. We conducted a phase 2 trial evaluating the safety and rate of clinically significant infections (CSIs; viremia requiring treatment or end-organ disease) following infusion of posoleucel, a partially HLA-matched, allogeneic, off-the-shelf, multivirus-specific T cell investigational product for preventing CSIs with adenovirus, BK virus, cytomegalovirus, Epstein-Barr virus, human herpesvirus-6, or JC virus. This open-label trial enrolled high-risk allo-HCT recipients based on receiving grafts from umbilical cord blood, haploidentical, mismatched, or matched unrelated donors; post-HCT lymphocytes <180/mm3; or use of T cell depletion. Posoleucel dosing was initiated within 15-49 days of allo-HCT and subsequently every 14 days for up to seven doses. The primary endpoint was the number of CSIs due to the six target viruses by week 14. Of the 26 patients enrolled just three (12%) had a CSI by week 14, each with a single target virus. In vivo expansion of functional virus-specific T cells detected via interferon-γ ELISpot assay was associated with viral control. Persistence of posoleucel-derived T cell clones for up to 14 weeks after the last infusion was confirmed by T cell receptor deep-sequencing. Five patients (19%) had acute GVHD grade II-IV. No patient experienced cytokine release syndrome. All six deaths were due to relapse or disease progression. High-risk allo-HCT patients who received posoleucel had low rates of CSIs from six targeted viruses. Repeat posoleucel dosing was generally safe and well tolerated and associated with functional immune reconstitution. www.clinicaltrials.gov NCT04693637.

Human Herpesvirus-6 Reactivation and Disease Are Infrequent in Chimeric Antigen Receptor T-cell Therapy Recipients.

Human herpesvirus-6B (HHV-6B) reactivation and disease are increasingly reported after CAR-T-cell therapy (CARTx). HHV-6 reactivation in the CAR-T-cell product was recently reported, raising questions about product and patient management. Due to overlapping manifestations with immune effector cell-associated neurotoxicity syndrome, diagnosing HHV-6B encephalitis is challenging. We provide two lines of evidence assessing the incidence and outcomes of HHV-6B after CARTx. First, in a prospective study with weekly HHV-6B testing for up to 12 weeks post-infusion, HHV-6B reactivation occurred in eight of 89 participants; three had chromosomally integrated HHV-6 and were excluded, resulting in a cumulative incidence of HHV-6B reactivation of 6% (95% confidence interval (CI), 2.2-12.5%). HHV-6B detection was low level (median peak, 435 copies/mL; IQR, 164-979) and did not require therapy. Second, we retrospectively analyzed HHV-6B detection in blood and/or cerebrospinal fluid (CSF) within 12 weeks post-infusion in CARTx recipients. Of 626 patients, 24 had symptom-driven plasma testing with detection in one. Among 34 patients with CSF HHV-6 testing, one patient had possible HHV-6 encephalitis for a cumulative incidence of 0.17% (95% CI, 0.02-0.94%), although symptoms improved without treatment. Our data demonstrate that HHV-6B reactivation and disease are infrequent after CARTx. Routine HHV-6 monitoring is not warranted.

Human Herpes Virus-6 (HHV-6) Reactivation after Hematopoietic Cell Transplant and Chimeric Antigen Receptor (CAR)- T Cell Therapy: A Shifting Landscape.

HHV-6B reactivation affects approximately half of all allogeneic hematopoietic cell transplant (HCT) recipients. HHV-6B is the most frequent infectious cause of encephalitis following HCT and is associated with pleiotropic manifestations in this setting, including graft-versus-host disease, myelosuppression, pneumonitis, and CMV reactivation, although the causal link is not always clear. When the virus inserts its genome in chromosomes of germ cells, the chromosomally integrated form (ciHHV6) is inherited by offspring. The condition of ciHHV6 is characterized by the persistent detection of HHV-6 DNA, often confounding diagnosis of reactivation and disease-this has also been associated with adverse outcomes. Recent changes in clinical practice in the field of cellular therapies, including a wider use of post-HCT cyclophosphamide, the advent of letermovir for CMV prophylaxis, and the rapid expansion of novel cellular therapies require contemporary epidemiological studies to determine the pathogenic role and spectrum of disease of HHV-6B in the current era. Research into the epidemiology and clinical significance of HHV-6B in chimeric antigen receptor T cell (CAR-T cell) therapy recipients is in its infancy. No controlled trials have determined the optimal treatment for HHV-6B. Treatment is reserved for end-organ disease, and the choice of antiviral agent is influenced by expected toxicities. Virus-specific T cells may provide a novel, less toxic therapeutic modality but is more logistically challenging. Preventive strategies are hindered by the high toxicity of current antivirals. Ongoing study is needed to keep up with the evolving epidemiology and impact of HHV-6 in diverse and expanding immunocompromised patient populations.

Defining and Grading Infections in Clinical Trials Involving Hematopoietic Cell Transplantation: A Report From the BMT CTN Infectious Disease Technical Committee.

The Blood and Marrow Transplant Clinical Trials Network (BMT-CTN) was established in 2001 to conduct large multi-institutional clinical trials addressing important issues towards improving the outcomes of HCT and other cellular therapies. Trials conducted by the network investigating new advances in HCT and cellular therapy not only assess efficacy but require careful capturing and severity assessment of adverse events and toxicities. Adverse infectious events in cancer clinical trials are typically graded according to the National Cancer Institute's Common Terminology Criteria for Adverse Events (CTCAE). However, there are limitations to this framework as it relates to HCT given the associated immunodeficiency and delayed immune reconstitution. The BMT-CTN Infection Grading System is a monitoring tool developed by the BMT CTN to capture and monitor infectious complications and differs from the CTCAE by its classification of infections based on their potential impact on morbidity and mortality for HCT recipients. Here we offer a report from the BMT CTN Infectious Disease Technical Committee regarding the rationale, development, and revising of BMT-CTN Infection Grading System and future directions as it applies to future clinical trials involving HCT and cellular therapy recipients.

Outcomes in Hematopoietic Cell Transplant and Chimeric Antigen Receptor T Cell Therapy Recipients with Pre-Cellular Therapy SARS-CoV-2 Infection.

BACKGROUND: Hematopoietic cell transplant (HCT) or chimeric antigen receptor T cell (CAR-T) therapy recipients have high morbidity from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. There are limited data on outcomes from SARS-CoV-2 infection shortly before cellular therapy and uncertainty whether to delay therapy. METHODS: We conducted a retrospective cohort study of patients with SARS-CoV-2 infection within 90 days prior to HCT or CAR-T therapy between January 2020 and November 2022. We characterized the kinetics of SARS-CoV-2 detection, clinical outcomes following cellular therapy, and impact on delays in cellular therapy. RESULTS: We identified 37 patients (n=15 allogeneic HCT, n=11 autologous HCT, n=11 CAR-T therapy) with SARS-CoV-2 infections within 90 days of cellular therapy. Most infections (73%) occurred between March and November 2022, when Omicron strains were prevalent. Most patients had asymptomatic (27%) or mild (68%) coronavirus disease 2019 (COVID-19). SARS-CoV-2 positivity lasted a median of 20.0 days [IQR, 12.5-26.25]. The median time from first positive SARS-CoV-2 test to cellular therapy was 45 days [IQR, 37.75-70]; one patient tested positive on the day of infusion. After cellular therapy, no patients had recrudescent SARS-CoV-2 infection or COVID-19-related complications. Cellular therapy delays related to SARS-CoV-2 infection occurred in 70% of patients for a median of 37 days. Delays were more common after allogeneic (73%) and autologous (91%) HCT compared to CAR-T cell therapy (45%). CONCLUSIONS: Patients with asymptomatic or mild COVID-19 may not require prolonged delays in cellular therapy in the context of contemporary circulating variants and availability of antiviral therapies.

SARS-CoV-2 vaccination in the first year after hematopoietic cell transplant or chimeric antigen receptor T cell therapy: A prospective, multicenter, observational study (BMT CTN 2101).

Background: The optimal timing of vaccination with SARS-CoV-2 vaccines after cellular therapy is incompletely understood. Objective: To describe humoral and cellular responses after SARS-CoV-2 vaccination initiated <4 months versus 4-12 months after cellular therapy. Design: Multicenter prospective observational study. Setting: 34 centers in the United States. Participants: 466 allogeneic hematopoietic cell transplant (HCT; n=231), autologous HCT (n=170), or chimeric antigen receptor T cell (CAR-T cell) therapy (n=65) recipients enrolled between April 2021 and June 2022. Interventions: SARS-CoV-2 vaccination as part of routine care. Measurements: We obtained blood prior to and after vaccinations at up to five time points and tested for SARS-CoV-2 spike (anti-S) IgG in all participants and neutralizing antibodies for Wuhan D614G, Delta B.1.617.2, and Omicron B.1.1.529 strains, as well as SARS-CoV-2-specific T cell receptors (TCRs), in a subgroup. Results: Anti-S IgG and neutralizing antibody responses increased with vaccination in HCT recipients irrespective of vaccine initiation timing but were unchanged in CAR-T cell recipients initiating vaccines within 4 months. Anti-S IgG ≥2,500 U/mL was correlated with high neutralizing antibody titers and attained by the last time point in 70%, 69%, and 34% of allogeneic HCT, autologous HCT, and CAR-T cell recipients, respectively. SARS-CoV-2-specific T cell responses were attained in 57%, 83%, and 58%, respectively. Humoral and cellular responses did not significantly differ among participants initiating vaccinations <4 months vs 4-12 months after cellular therapy. Pre-cellular therapy SARS-CoV-2 infection or vaccination were key predictors of post-cellular therapy anti-S IgG levels. Limitations: The majority of participants were adults and received mRNA vaccines. Conclusions: These data support starting mRNA SARS-CoV-2 vaccination three to four months after allogeneic HCT, autologous HCT, and CAR-T cell therapy. Funding: National Marrow Donor Program, Leukemia and Lymphoma Society, Multiple Myeloma Research Foundation, Novartis, LabCorp, American Society for Transplantation and Cellular Therapy, Adaptive Biotechnologies, and the National Institutes of Health.

A Systematic Literature Review to Identify Diagnostic Gaps in Managing Immunocompromised Patients With Cancer and Suspected Infection.

Patients with cancer are increasingly vulnerable to infections, which may be more severe than in the general population. Improvements in rapid and timely diagnosis to optimize management are needed. We conducted a systematic literature review to determine the unmet need in diagnosing acute infections in immunocompromised patients with cancer and identified 50 eligible studies from 5188 records between 1 January 2012 and 23 June 2022. There was considerable heterogeneity in study designs and parameters, laboratory methods and definitions, and assessed outcomes, with limited evaluation of diagnostic impact on clinical outcomes. Culture remains the primary diagnostic strategy. Fewer studies employing molecular technologies exist, but emerging literature suggests that pathogen-agnostic molecular tests may add to the diagnostic armamentarium. Well-designed clinical studies using standardized methodologies are needed to better evaluate performance characteristics and clinical and economic impacts of emerging diagnostic techniques to improve patient outcomes.

HHV-6B detection and host gene expression implicate HHV-6B as pulmonary pathogen after hematopoietic cell transplant.

Limited understanding of the immunopathogenesis of human herpesvirus 6B (HHV-6B) has prevented its acceptance as a pulmonary pathogen after hematopoietic cell transplant (HCT). In this prospective multicenter study of patients undergoing bronchoalveolar lavage (BAL) for pneumonia after allogeneic HCT, we test blood and BAL fluid (BALF) for HHV-6B DNA and mRNA transcripts associated with lytic infection and perform RNA-seq on paired blood. Among 116 participants, HHV-6B DNA is detected in 37% of BALs, 49% of which also have HHV-6B mRNA detection. We establish HHV-6B DNA viral load thresholds in BALF that are highly predictive of HHV-6B mRNA detection and associated with increased risk for overall mortality and death from respiratory failure. Participants with HHV-6B DNA in BALF exhibit distinct host gene expression signatures, notable for enriched interferon signaling pathways in participants clinically diagnosed with idiopathic pneumonia. These data implicate HHV-6B as a pulmonary pathogen after allogeneic HCT.

Serial Quantitation of Plasma Microbial Cell-Free DNA Before and After Diagnosis of Pulmonary Invasive Mold Infections After Hematopoietic Cell Transplant.

BACKGROUND: Plasma microbial cell-free DNA sequencing (mcfDNA-Seq) is a noninvasive test for microbial diagnosis of invasive mold infection (IMI). The utility of mcfDNA-Seq for predicting IMI onset and the clinical implications of mcfDNA concentrations are unknown. METHODS: We retrospectively tested plasma from hematopoietic cell transplant (HCT) recipients with pulmonary IMI and ≥1 mold identified by mcfDNA-Seq in plasma collected within 14 days of clinical diagnosis. Samples collected from up to 4 weeks before and 4 weeks after IMI diagnosis were evaluated using mcfDNA-Seq. RESULTS: Thirty-five HCT recipients with 39 IMIs (16 Aspergillus and 23 non-Aspergillus infections) were included. Pathogenic molds were detected in 38%, 26%, 11%, and 0% of samples collected during the first, second, third, and fourth week before clinical diagnosis, respectively. In non-Aspergillus infections, median mcfDNA concentrations in samples collected within 3 days of clinical diagnosis were higher in infections with versus without extrapulmonary spread (4.3 vs 3.3 log10 molecules per microliter [mpm], P = .02), and all patients (8/8) with mcfDNA concentrations >4.0 log10 mpm died within 42 days after clinical diagnosis. CONCLUSIONS: Plasma mcfDNA-Seq can identify pathogenic molds up to 3 weeks before clinical diagnosis of pulmonary IMI. Plasma mcfDNA concentrations may correlate with extrapulmonary spread and mortality in non-Aspergillus IMI.

Cytomegalovirus (CMV) Reactivation and CMV-Specific Cell-Mediated Immunity After Chimeric Antigen Receptor T-Cell Therapy.

BACKGROUND: The epidemiology of cytomegalovirus (CMV) after chimeric antigen receptor-modified T-cell immunotherapy (CARTx) is poorly understood owing to a lack of routine surveillance. METHODS: We prospectively enrolled 72 adult CMV-seropositive CD19-, CD20-, or BCMA-targeted CARTx recipients and tested plasma samples for CMV before and weekly up to 12 weeks after CARTx. We assessed CMV-specific cell-mediated immunity (CMV-CMI) before and 2 and 4 weeks after CARTx, using an interferon γ release assay to quantify T-cell responses to IE-1 and pp65. We tested pre-CARTx samples to calculate a risk score for cytopenias and infection (CAR-HEMATOTOX). We used Cox regression to evaluate CMV risk factors and evaluated the predictive performance of CMV-CMI for CMV reactivation in receiver operator characteristic curves. RESULTS: CMV was detected in 1 patient (1.4%) before and in 18 (25%) after CARTx, for a cumulative incidence of 27% (95% confidence interval, 16.8-38.2). The median CMV viral load (interquartile range) was 127 (interquartile range, 61-276) IU/mL, with no end-organ disease observed; 5 patients received preemptive therapy based on clinical results. CMV-CMI values reached a nadir 2 weeks after infusion and recovered to baseline levels by week 4. In adjusted models, BCMA-CARTx (vs CD19/CD20) and corticosteroid use for >3 days were significantly associated with CMV reactivation, and possible associations were detected for lower week 2 CMV-CMI and more prior antitumor regimens. The cumulative incidence of CMV reactivation almost doubled when stratified by BCMA-CARTx target and use of corticosteroids for >3 days (46% and 49%, respectively). CONCLUSIONS: CMV testing could be considered between 2 and 6 weeks in high-risk CARTx recipients.

Incidence and Impact of Fungal Infections in Post-Transplantation Cyclophosphamide-Based Graft-versus-Host Disease Prophylaxis and Haploidentical Hematopoietic Cell Transplantation: A Center for International Blood and Marrow Transplant Research Analysis.

Fungal infection (FI) after allogeneic hematopoietic cell transplantation (HCT) is associated with increased morbidity and mortality. Neutropenia, HLA mismatch, graft-versus-host disease (GVHD), and viral infections are risk factors for FI. The objectives of this Center for International Blood and Marrow Transplant Research registry study were to compare the incidence and density of FI occurring within 180 days after HCT in matched sibling (Sib) transplants with either calcineurin inhibitor (CNI)-based or post-transplantation cyclophosphamide (PTCy)-based GVHD prophylaxis and related haploidentical transplants receiving PTCy, and to examine the impact of FI by day 180 on transplantation outcomes. METHODS: Patients who underwent their first HCT between 2012 and 2017 for acute myeloid leukemia, acute lymphoblastic leukemia, and myelodysplastic syndrome and received a related haploidentical transplant with PTCy (HaploCy; n = 757) or a Sib transplant with PTCy (SibCy; n = 403) or CNI (SibCNI; n = 1605) were analyzed. The incidence of FI by day 180 post-HCT was calculated as cumulative incidence with death as the competing risk. The associations of FI with overall survival, transplant-related mortality, chronic GVHD, and relapse at 2 years post-HCT were examined in Cox proportional hazards regression models. Factors significantly associated with the outcome variable at a 1% level were kept in the final model. RESULTS: By day 180 post-HCT, 56 (7%) HaploCy, 24 (6%), SibCy, and 59 (4%) SibCNI recipients developed ≥1 FI (P < .001). The cumulative incidence of yeast FI was 5.2% (99% confidence interval [CI], 3.3% to 7.3%) for HaploCy, 2.2% (99% CI, .7% to 4.5%) for SibCy, and 1.9% (99% CI, 1.1% to 2.9%) for SibCNI (P = .001), and that of mold FI was 2.9% (99% CI, 1.5% to 4.7%), 3.7% (99% CI, 91.7% to 6.6%), and 1.7% (99% CI, 1.0% to 2.6%), respectively (P = .040). FI was associated with an increased risk of death, with an adjusted hazard ratio (HR) of 4.06 (99% CI, 2.2 to 7.6) for HaploCy, 4.7 (99% CI, 2.0 to 11.0) for SibCy, and 3.4 (99% CI, 1.8 to 6.4) for SibCNI compared with SibCNI without FI (P < .0001 for all). Similar associations were noted for transplantation-related mortality. FI did not impact rates of relapse or chronic GVHD. CONCLUSIONS: Rates of FI by day 180 ranged between 1.9% and 5.2% for yeast FI and from 1.7% to 3.7% for mold FI across the 3 cohorts. The use of PTCy was associated with higher rates of yeast FI only in HaploHCT and with mold FI in both HaploHCT and SibHCT. The presence of FI by day 180 was associated with increased risk for overall mortality and transplant-related mortality at 2 years regardless of donor type or PTCy use. Although rates of FI were low with PTCy, FI is associated with an increased risk of death, underscoring the need for improved management strategies.

Post-transplantation cyclophosphamide is associated with increased bacterial infections.

Post-transplant cyclophosphamide (PTCy) is increasingly used to reduce graft-versus-host disease after hematopoietic cell transplantation (HCT); however, it might be associated with more infections. All patients who were ≥2 years old, receiving haploidentical or matched sibling donor (Sib) HCT for acute leukemias or myelodysplastic syndrome, and either calcineurin inhibitor (CNI)- or PTCy-based GVHD prophylaxis [Haploidentical HCT with PTCy (HaploCy), 757; Sibling with PTCy (SibCy), 403; Sibling with CNI-based (SibCNI), 1605] were included. Most bacterial infections occurred within the first 100 days; 953 patients (34.5%) had at least 1 infection and 352 patients (13%) had ≥2 infections. Patients receiving PTCy had a greater incidence of bacterial infections by day 180 [HaploCy 46%; SibCy 48%; SibCNI 35%; p < 0.001]. Compared with the SibCNI without infection cohort, 1.99-fold, 3.33-fold, 2.78-fold, and 2.53-fold increased TRM was seen for the HaploCy cohort without infection and HaploCy, SibCy, and SibCNI cohorts with infection, respectively. Bacterial infections increased mortality [HaploCy (HR1.84, 99% CI: 1.45-2.33, p < 0.0001), SibCy cohort (HR,1.68, 99% CI: 1.30-2.19, p < 0.0001), and SibCNI cohort (HR,1.76, 99% CI: 1.43-2.16, p < 0.0001). PTCy was associated with increased bacterial infections regardless of donor, and bacterial infections were associated with increased mortality irrespective of GVHD prophylaxis. Patients receiving PTCy should be monitored carefully for bacterial infections following PTCy.