The real unmet need: A multifactorial approach for identifying sensitized kidney candidates with low access to transplant.

BACKGROUND: At the start of 2020, the kidney waiting list consisted of 2526 candidates with a calculated panel reactive antibody (CPRA) of 99.9% or greater, a cohort demonstrated in published research to have meaningfully lower than average access to transplantation even under the revised kidney allocation system (KAS). METHODS: This was a retrospective analysis of US kidney registrations using data from the OPTN [Reference (https://optn.transplant.hrsa.gov/data/about-data/)]. The period-prevalent study cohort consisted of US kidney-alone registrations who waited at least 1 day between April 1, 2016, when HLA DQ-Alpha and DP-Beta unacceptable antigen data became available in OPTN data collection, to December 31, 2019. Poisson rate regression was used to model deceased donor kidney transplant rates per active year waiting and using an offset term to account for differential at-risk periods. Median time to transplant was estimated for each IRR group using the Kaplan-Meier method. Sensitivity analyses were included to address geographic variation in supply-to-demand ratios and differences in dialysis time or waiting time. RESULTS: In this study, we found 1597 additional sensitized (CPRA 50-<99.9%) candidates with meaningfully lower than average access to transplant when simultaneously taking into account CPRA and other factors. In combination with CPRA, candidate blood type, Estimated Post-Transplant Survival Score (EPTS), and presence of other antibody specificities beyond those in the current, 5-locus CPRA were found to influence the likelihood of transplant. CONCLUSION: In total, this suggests approximately 4100 sensitized candidates are on the waiting list who represent a community of disadvantaged patients who may benefit from progressive therapies and interventions to facilitate incompatible transplantation. Though associated with higher risks, such interventions may nevertheless be more attractive than remaining on dialysis with the associated accumulation of mortality risk over time.

Outcomes at 3 years posttransplant in imlifidase-desensitized kidney transplant patients.

Imlifidase is a cysteine proteinase which specifically cleaves IgG, inhibiting Fc-mediated effector function within hours of administration. Imlifidase converts a positive crossmatch to a potential donor (T cell, B cell, or both), to negative, enabling transplantation to occur between previously HLA incompatible donor-recipient pairs. To date, 39 crossmatch positive patients received imlifidase prior to a kidney transplant in four single-arm, open-label, phase 2 studies. At 3 years, for patients who were AMR+ compared to AMR-, death-censored allograft survival was 93% vs 77%, patient survival was 85% vs 94%, and mean eGFR was 49 ml/min/1.73 m2 vs 61 ml/min/1.73 m2 , respectively. The incidence of AMR was 38% with most episodes occurring within the first month post-transplantation. Sub-analysis of patients deemed highly sensitized with cPRA ≥ 99.9%, and unlikely to be transplanted who received crossmatch-positive, deceased donor transplants had similar rates of patient survival, graft survival, and eGFR but a higher rate of AMR. These data demonstrate that outcomes and safety up to 3 years in recipients of imlifidase-enabled allografts is comparable to outcomes in other highly sensitized patients undergoing HLA-incompatible transplantation. Thus, imlifidase is a potent option to facilitate transplantation among patients who have a significant immunologic barrier to successful kidney transplantation. Clinical Trial: ClinicalTrials.gov (NCT02790437), EudraCT Number: 2016-002064-13.

Complement Activation Occurs at the Surface of Platelets Activated by Streptococcal M1 Protein and This Results in Phagocytosis of Platelets.

Platelets circulate the bloodstream and principally maintain hemostasis. Disturbed hemostasis, a dysregulated inflammatory state, and a decreased platelet count are all hallmarks of severe invasive Streptococcus pyogenes infection, sepsis. We have previously demonstrated that the released M1 protein from S. pyogenes activates platelets, and this activation is dependent on the binding of M1 protein, fibrinogen, and M1-specific IgG to platelets in susceptible donors. In this study, we characterize the M1-associated protein interactions in human plasma and investigate the acquisition of proteins to the surface of activated platelets and the consequences for platelet immune function. Using quantitative mass spectrometry, M1 protein was determined to form a protein complex in plasma with statistically significant enrichment of fibrinogen, IgG3, and complement components, especially C1q. Using flow cytometry, these plasma proteins were also confirmed to be acquired to the platelet surface, resulting in complement activation on M1-activated human platelets. Furthermore, we demonstrated an increased phagocytosis of M1-activated platelets by monocytes, which was not observed with other physiological platelet agonists. This reveals a novel mechanism of complement activation during streptococcal sepsis, which contributes to the platelet consumption that occurs in sepsis.

Targeted Proteomics and Absolute Protein Quantification for the Construction of a Stoichiometric Host-Pathogen Surface Density Model.

Sepsis is a systemic immune response responsible for considerable morbidity and mortality. Molecular modeling of host-pathogen interactions in the disease state represents a promising strategy to define molecular events of importance for the transition from superficial to invasive infectious diseases. Here we used the Gram-positive bacterium Streptococcus pyogenes as a model system to establish a mass spectrometry based workflow for the construction of a stoichiometric surface density model between the S. pyogenes surface, the surface virulence factor M-protein, and adhered human blood plasma proteins. The workflow relies on stable isotope labeled reference peptides and selected reaction monitoring mass spectrometry analysis of a wild-type strain and an M-protein deficient mutant strain, to generate absolutely quantified protein stoichiometry ratios between S. pyogenes and interacting plasma proteins. The stoichiometry ratios in combination with a novel targeted mass spectrometry method to measure cell numbers enabled the construction of a stoichiometric surface density model using protein structures available from the protein data bank. The model outlines the topology and density of the host-pathogen protein interaction network on the S. pyogenes bacterial surface, revealing a dense and highly organized protein interaction network. Removal of the M-protein from S. pyogenes introduces a drastic change in the network topology, validated by electron microscopy. We propose that the stoichiometric surface density model of S. pyogenes in human blood plasma represents a scalable framework that can continuously be refined with the emergence of new results. Future integration of new results will improve the understanding of protein-protein interactions and their importance for bacterial virulence. Furthermore, we anticipate that the general properties of the developed workflow will facilitate the production of stoichiometric surface density models for other types of host-pathogen interactions.

Development of Phage-Based Antibody Fragment Reagents for Affinity Enrichment of Bacterial Immunoglobulin G Binding Proteins.

Disease and death caused by bacterial infections are global health problems. Effective bacterial strategies are required to promote survival and proliferation within a human host, and it is important to explore how this adaption occurs. However, the detection and quantification of bacterial virulence factors in complex biological samples are technically demanding challenges. These can be addressed by combining targeted affinity enrichment of antibodies with the sensitivity of liquid chromatography-selected reaction monitoring mass spectrometry (LC-SRM MS). However, many virulence factors have evolved properties that make specific detection by conventional antibodies difficult. We here present an antibody format that is particularly well suited for detection and analysis of immunoglobulin G (IgG)-binding virulence factors. As proof of concept, we have generated single chain fragment variable (scFv) antibodies that specifically target the IgG-binding surface proteins M1 and H of Streptococcus pyogenes. The binding ability of the developed scFv is demonstrated against both recombinant soluble protein M1 and H as well as the intact surface proteins on a wild-type S. pyogenes strain. Additionally, the capacity of the developed scFv antibodies to enrich their target proteins from both simple and complex backgrounds, thereby allowing for detection and quantification with LC-SRM MS, was demonstrated. We have established a workflow that allows for affinity enrichment of bacterial virulence factors.

IgM single antigen bead HLA-assay is affected by imlifidase through the cleavage of IgG but not IgM.

AIM: The aim of this study was to investigate if human IgM is a cleavable substrate for imlifidase and to explain an observed effect in anti-HLA IgM single antigen bead (SAB) assays in sensitized patients. METHODS: Serum samples collected pre- and 24 h post-imlifidase administration from sensitized patients enrolled in a phase II trial were investigated for anti-HLA IgG and IgM using SAB assays, with and without in vitro IgG depletion using a CaptureSelect™ affinity matrix. In addition, pre-dose samples and purified human IgM samples were treated with imlifidase in vitro and evaluated by SDS-PAGE, Western blot (PE-conjugated anti-human IgM) and SAB (IgG, IgM) assays. RESULTS: By comparing the mean fluorescence intensity (MFI) of HLA-beads, pre- and post-imlifidase administration, three IgM-related patterns were observed; IgM-specific HLA-SABs with an increased MFI post-imlifidase, IgM-specific HLA-SABs with a decreased MFI post-imlifidase, and IgM-specific HLA-SABs with a marginal MFI difference between the pre- and post-imlifidase administration. These IgM signal patterns were observed despite neither purified IgM nor serum IgM could be cleaved by imlifidase. After removing IgG, the effects observed on anti-HLA IgM was largely eliminated with the biggest differences seen in patients with very high anti-HLA IgG in pre-dose samples. CONCLUSION: We demonstrate that imlifidase does not cleave human IgM, including HLA-specific IgM antibodies from highly sensitized subjects. Observed decreases of SAB-HLA IgM signals after imlifidase treatment may result from the cleavage of IgG-IgM complexes which are bound to SAB-HLA. Serum analysis of patients with high levels of anti-HLA IgG will result in a more accurate SAB-HLA IgM reading after IgG depletion.

Imlifidase Desensitization in Crossmatch-positive, Highly Sensitized Kidney Transplant Recipients: Results of an International Phase 2 Trial (Highdes).

BACKGROUND: Highly HLA sensitized patients have limited access to life-saving kidney transplantation because of a paucity of immunologically suitable donors. Imlifidase is a cysteine protease that cleaves IgG leading to a rapid decrease in antibody level and inhibition of IgG-mediated injury. This study investigates the efficacy and safety of imlifidase in converting a positive crossmatch test to negative, allowing highly sensitized patients to be transplanted with a living or deceased donor kidney. METHODS: This open-label, single-arm, phase 2 trial conducted at 5 transplant centers, evaluated the ability of imlifidase to create a negative crossmatch test within 24 h. Secondary endpoints included postimlifidase donor-specific antibody levels compared with predose levels, renal function, and pharmacokinetic/pharmacodynamic profiles. Safety endpoints included adverse events and immunogenicity profile. RESULTS: Of the transplanted patients, 89.5% demonstrated conversion of baseline positive crossmatch to negative within 24 h after imlifidase treatment. Donor-specific antibodies most often rebounded 3-14 d postimlifidase dose, with substantial interpatient variability. Patient survival was 100% with graft survival of 88.9% at 6 mo. With this, 38.9% had early biopsy proven antibody-mediated rejection with onset 2-19 d posttransplantation. Serum IgG levels began to normalize after ~3-7 d posttransplantation. Antidrug antibody levels were consistent with previous studies. Seven adverse events in 6 patients were classified as possibly or probably related to treatment and were mild-moderate in severity. CONCLUSIONS: Imlifidase was well tolerated, converted positive crossmatches to negative, and enabled patients with a median calculated panel-reactive antibody of 99.83% to undergo kidney transplantation resulting in good kidney function and graft survival at 6 mo.

Comprehensive Proteomic Characterization of Ontogenic Changes in Hematopoietic Stem and Progenitor Cells.

Hematopoietic stem and progenitor cells (HSPCs) in the fetus and adult possess distinct molecular landscapes that regulate cell fate and change their susceptibility to initiation and progression of hematopoietic malignancies. Here, we applied in-depth quantitative proteomics to comprehensively describe and compare the proteome of fetal and adult HSPCs. Our data uncover a striking difference in complexity of the cellular proteomes, with more diverse adult-specific HSPC proteomic signatures. The differential protein content in fetal and adult HSPCs indicate distinct metabolic profiles and protein complex stoichiometries. Additionally, adult characteristics include an arsenal of proteins linked to viral and bacterial defense, as well as protection against ROS-induced protein oxidation. Further analyses show that interferon α, as well as Neutrophil elastase, has distinct functional effects in fetal and adult HSPCs. This study provides a rich resource aimed toward an enhanced mechanistic understanding of normal and malignant hematopoiesis during fetal and adult life.

A comprehensive analysis of the Streptococcus pyogenes and human plasma protein interaction network.

Streptococcus pyogenes is a major human bacterial pathogen responsible for severe and invasive disease associated with high mortality rates. The bacterium interacts with several human blood plasma proteins and clarifying these interactions and their biological consequences will help to explain the progression from mild to severe infections. In this study, we used a combination of mass spectrometry (MS) based techniques to comprehensively quantify the components of the S. pyogenes-plasma protein interaction network. From an initial list of 181 interacting human plasma proteins defined using liquid chromatography (LC)-MS/MS analysis we further subdivided the interacting protein list using selected reaction monitoring (SRM) depending on the level of enrichment and protein concentration on the bacterial surface. The combination of MS methods revealed several previously characterized interactions between the S. pyogenes surface and human plasma along with many more, so far uncharacterised, possible plasma protein interactions with S. pyogenes. In follow-up experiments, the combination of MS techniques was applied to study differences in protein binding to a S. pyogenes wild type strain and an isogenic mutant lacking several important virulence factors, and a unique pair of invasive and non-invasive S. pyogenes isolates from the same patient. Comparing the plasma protein-binding properties of the wild type and the mutant and the invasive and non-invasive S. pyogenes bacteria revealed considerable differences, underlining the significance of these protein interactions. The results also demonstrate the power of the developed mass spectrometry method to investigate host-microbial relationships with a large proteomics depth and high quantitative accuracy.