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2021 ◽  
Vol 12 ◽  
Author(s):  
Soumya Mukherjee ◽  
Alvaro Sanchez-Bernabeu ◽  
Laura C. Demmers ◽  
Wei Wu ◽  
Albert J. R. Heck

Mass-spectrometry based immunopeptidomics has provided unprecedented insights into antigen presentation, not only charting an enormous ligandome of self-antigens, but also cancer neoantigens and peptide antigens harbouring post-translational modifications. Here we concentrate on the latter, focusing on the small subset of HLA Class I peptides (less than 1%) that has been observed to be post-translationally modified (PTM) by a O-linked N-acetylglucosamine (GlcNAc). Just like neoantigens these modified antigens may have specific immunomodulatory functions. Here we compiled from literature, and a new dataset originating from the JY B cell lymphoblastoid cell line, a concise albeit comprehensive list of O-GlcNAcylated HLA class I peptides. This cumulative list of O-GlcNAcylated HLA peptides were derived from normal and cancerous origin, as well as tissue specimen. Remarkably, the overlap in detected O-GlcNAcylated HLA peptides as well as their source proteins is strikingly high. Most of the O-GlcNAcylated HLA peptides originate from nuclear proteins, notably transcription factors. From this list, we extract that O-GlcNAcylated HLA Class I peptides are preferentially presented by the HLA-B*07:02 allele. This allele loads peptides with a Proline residue anchor at position 2, and features a binding groove that can accommodate well the recently proposed consensus sequence for O-GlcNAcylation, P(V/A/T/S)g(S/T), essentially explaining why HLA-B*07:02 is a favoured binding allele. The observations drawn from the compiled list, may assist in the prediction of novel O-GlcNAcylated HLA antigens, which will be best presented by patients harbouring HLA-B*07:02 or related alleles that use Proline as anchoring residue.


2021 ◽  
Vol 12 ◽  
Author(s):  
Silvia D’Amico ◽  
Valerio D’Alicandro ◽  
Mirco Compagnone ◽  
Patrizia Tempora ◽  
Giusy Guida ◽  
...  

The endoplasmic reticulum aminopeptidase ERAP1 regulates innate and adaptive immune responses by trimming peptides for presentation by major histocompatibility complex (MHC) class I molecules. Previously, we have shown that genetic or pharmacological inhibition of ERAP1 on murine and human tumor cell lines perturbs the engagement of NK cell inhibitory receptors Ly49C/I and Killer-cell Immunoglobulin-like receptors (KIRs), respectively, by their specific ligands (MHC class I molecules), thus leading to NK cell killing. However, the effect of ERAP1 inhibition in tumor cells was highly variable, suggesting that its efficacy may depend on several factors, including MHC class I typing. To identify MHC class I alleles and KIRs that are more sensitive to ERAP1 depletion, we stably silenced ERAP1 expression in human HLA class I-negative B lymphoblastoid cell line 721.221 (referred to as 221) transfected with a panel of KIR ligands (i.e. HLA-B*51:01, -Cw3, -Cw4 and -Cw7), or HLA-A2 which does not bind any KIR, and tested their ability to induce NK cell degranulation and cytotoxicity. No change in HLA class I surface expression was detected in all 221 transfectant cells after ERAP1 depletion. In contrast, CD107a expression levels were significantly increased on NK cells stimulated with 221-B*51:01 cells lacking ERAP1, particularly in the KIR3DL1-positive NK cell subset. Consistently, genetic or pharmacological inhibition of ERAP1 impaired the recognition of HLA-B*51:01 by the YTS NK cell overexpressing KIR3DL1*001, suggesting that ERAP1 inhibition renders HLA-B*51:01 molecules less eligible for binding to KIR3DL1. Overall, these results identify HLA-B*51:01/KIR3DL1 as one of the most susceptible combinations for ERAP1 inhibition, suggesting that individuals carrying HLA-B*51:01-like antigens may be candidates for immunotherapy based on pharmacological inhibition of ERAP1.


2021 ◽  
Vol 12 ◽  
Author(s):  
Antonio J. Martín-Galiano ◽  
Francisco Díez-Fuertes ◽  
Michael J. McConnell ◽  
Daniel López

The effect of emerging SARS-CoV-2 variants on vaccine efficacy is of critical importance. In this study, the potential impact of mutations that facilitate escape from the cytotoxic cellular immune response in these new virus variants for the 551 most abundant HLA class I alleles was analyzed. Computational prediction showed that most of these alleles, that cover >90% of the population, contain enough epitopes without escape mutations in the principal SARS-CoV-2 variants. These data suggest that the cytotoxic cellular immune protection elicited by vaccination is not greatly affected by emerging SARS-CoV-2 variants.


Author(s):  
Aleksandr S. Golota

This review provides updated information on HLA class I and II antigens in cancer. The expression of HLA antigens in normal and tumor tissues, the physiological organization of the components of HLA antigen-processing machinery, the expression patterns of HLA antigens associated with the molecular and regulatory defects identified to date, as well as their functional and clinical significance, are described. This review summarizes clinical and experimental data on the complexity of immune escape mechanisms used by tumour cells to avoid T and natural killer cell responses. The variety of class I HLA phenotypes that can be produced by tumor cells during this process is presented. We also discuss here the potential capacity of metastatic lesions to recover MHC/HLA class I expression after immunotherapy, which depends on the reversible/ soft or irreversible/hard nature of the molecular mechanism responsible for the altered HLA class I phenotypes, and which determines the progression or regression of metastatic lesions in response to treatment. HLA сlass II genes play key roles in connecting innate and adaptive immunity in tumor rejection and when the escape route via HLA-I is already established. Antigens сlass II HLA expression in tumor cells and gives tumor cells the ability to present antigens, becoming less aggressive, and improves prognosis. Malignant tumors, as a genetic disease, are caused by structural alterations of the genome which can give rise to the expression of tumor-associated antigens in the form of either structurally altered molecules or of overexpressed normal molecules. Tumor associated antigens recognized by the immune system and induce a T-cell-mediated immune response. Outgrowing cancers use different strategies to evade destruction by the immune system. Immune evasion mechanisms affecting the expression and/or function of HLA-antigens are of special interest to tumor immunologists, since these molecules play a crucial role in the interaction of malignant cells with immune cells. This review describes the potential role of immunity control points in immunosuppression and therapeutic strategies for restoring the cytotoxicity of immune cells.


Cells ◽  
2021 ◽  
Vol 10 (11) ◽  
pp. 3108
Author(s):  
Sarah Vollmers ◽  
Annabelle Lobermeyer ◽  
Christian Körner

The human leukocyte antigen system (HLA) is a cluster of highly polymorphic genes essential for the proper function of the immune system, and it has been associated with a wide range of diseases. HLA class I molecules present intracellular host- and pathogen-derived peptides to effector cells of the immune system, inducing immune tolerance in healthy conditions or triggering effective immune responses in pathological situations. HLA-C is the most recently evolved HLA class I molecule, only present in humans and great apes. Differentiating from its older siblings, HLA-A and HLA-B, HLA-C exhibits distinctive features in its expression and interaction partners. HLA-C serves as a natural ligand for multiple members of the killer-cell immunoglobulin-like receptor (KIR) family, which are predominately expressed by natural killer (NK) cells. NK cells are crucial for the early control of viral infections and accumulating evidence indicates that interactions between HLA-C and its respective KIR receptors determine the outcome and progression of viral infections. In this review, we focus on the unique role of HLA-C in regulating NK cell functions and its consequences in the setting of viral infections.


Blood ◽  
2021 ◽  
Vol 138 (Supplement 1) ◽  
pp. 3990-3990
Author(s):  
Benjamin Peton ◽  
Melissa Valerio ◽  
Michiko Taniguchi ◽  
Ivan Rodriguez ◽  
Ebtsesam Nafie ◽  
...  

Abstract Note: BP, MV and LG, KG contributed equally Background Relapsed acute myeloid leukemia (AML) remains the most common reason for allogeneic hematopoietic cell transplant (HCT) failure. Thus, understanding AML immune escape mechanism is important for improving the odds of curing HCT patients with AML. Downregulation of HLA Class I and II expression by AML is one of the potential immune escape mechanisms. Therefore, treatment to restore HLA surface expression is crucial to prevent and treat relapse. Endogenous cytokines, such as IFN-γ, have been shown to stimulate HLA expression but are poorly tolerated by patients. However, two hypomethylating agents (HMA), decitabine (Dec) and azacitadine (Aza), that are routinely used in AML treatment are known to augment HLA expression. For AML, HMAs are often combined with venetoclax (Ven), a drug that blocks the anti-apoptotic B-cell lymphoma-2 (Bcl-2) protein. Thus, while HMAs have been reported to increase HLA expression, what is unknown is whether these agents impact individual HLA loci differently and whether Ven has any impact on HLA expression. To address these questions, we treated the THP-1 cell line with Dec, Aza or Ven and measured changes in cell-surface expression of HLA proteins by flow cytometry using locus-specific HLA mAbs. Methods THP-1 cells were incubated with IFN-γ (500 U/mL), Aza (2µM), Dec (5µM), or Ven (30nM) for 48 hours (drug concentrations were determined by earlier titration experiments). THP-1 cells are a monocytic cell line, derived from the peripheral blood of a childhood case of acute monocytic leukemia (M5 subtype), that express HLA Class I and HLA-DR but not HLA-DQ or -DP under basal conditions, although they are inducible by IFN-γ. Thus, the induction of HLA Class II expression by IFN-γ serves as a positive control. Isotype controls were included to measure background. Data is presented as the difference in MFI (delta MFI) between cells treated with a drug and those treated with diluent only. Results Treatment of THP-1 cells with either IFN-γ or Dec led to increases in Class I HLA-A, -B & -C (Figure 1) compared to untreated cells (a mean fold increase of 1.4 and 1.2, respectively). Notably, Aza did not stimulate additional HLA-C expression and induced less of an increase in HLA-A & -B expression (an increase of 1.1-fold) than IFN-γ or Dec. Treatment of THP-1 cells by Ven did not induce a change in HLA Class I expression. For Class II, IFN-γ or Dec increased HLA-DR, -DQ and -DP expression in comparison to untreated cells (Figure 1). IFN-γ induced greater HLA-DR expression compared to Dec (an increase of 2.3-fold and 1.5-fold, respectively), and both stimulated similar increases in HLA-DQ (increases of 1.5-fold and 1.4-fold, respectively) & -DP (increases of 1.9-fold and 1.5-fold, respectively). However, treatment of cells with either Aza or Ven did not lead to changes in HLA Class II expression. Discussion Previous studies have illustrated the ability of IFN-γ to induce HLA Class II expression in THP-1 cells, however, data for Dec to induce HLA Class II expression was unconfirmed. We report differences in the degree to which IFN-γ and Dec are capable of stimulating HLA-DR with IFN-γ being more potent. The inability of Aza to induce HLA Class II expression in THP-1 cells may be related to the differing drug activating pathways of the two HMAs. Indeed, there are conflicting reports as to whether Aza can stimulate HLA Class II expression. Though Ven treatment of THP-1 cells did not impact HLA expression, because it is given with HMAs, it remains to be seen what effect these drugs may have on HLA expression when administered together. Additional studies to confirm these observations in patient-derived AML blasts are ongoing. Conclusion We report that HMAs increased expression of HLA-A, -B, & -C loci and Dec but not Aza stimulated HLA-DR, -DQ, and -DP expression in THP-1 cells. Given these data, Dec may be superior in increasing HLA Class II expression post-HCT. Figure 1 Figure 1. Disclosures Marcucci: Abbvie: Other: Speaker and advisory scientific board meetings; Agios: Other: Speaker and advisory scientific board meetings; Novartis: Other: Speaker and advisory scientific board meetings. Al Malki: Neximmune: Consultancy; CareDx: Consultancy; Jazz Pharmaceuticals, Inc.: Consultancy; Rigel Pharma: Consultancy; Hansa Biopharma: Consultancy.


Blood ◽  
2021 ◽  
Vol 138 (Supplement 1) ◽  
pp. 3917-3917
Author(s):  
Jun Zou ◽  
Tao Wang ◽  
Yung-Tsi Bolon ◽  
Shahinaz M. Gadalla ◽  
Steven G.E. Marsh ◽  
...  

Abstract ABSTRACT BACKGROUND The number of haploidentical hematopoietic stem cell transplantations (haplo-HSCT) being performed has substantially increased in recent years. Single-center studies have previously used in silico algorithms to quantitively measure HLA disparity and shown an association of the number of HLA molecular mismatches with relapse protection and/or increased risk of acute graft-versus-host disease (GVHD) in haplo-HSCT. However, inconsistent results from small studies have made it difficult to understand the full clinical impact of molecular mismatch in haplo-HSCT. OBJECTIVE In the current study, we investigated the potential of the HLA class I and II mismatched eplet (ME) score measured by HLAMatchmaker, as well as ME load at a specific locus to predict outcomes in a registry-based cohort of haplo-HSCT recipients. STUDY DESIGN We analyzed data from patients (n= 1,287) who underwent their first haplo-HSCT for acute lymphoblastic leukemia, acute myeloid leukemia, or myelodysplastic syndrome between 2013 and 2017, as reported to the Center for International Blood and Marrow Transplant Research database. ME load at each HLA locus and total Class-I and -II were scored using the HLAMatchmaker module incorporated in HLA Fusion software v4.3, which identifies predicted eplets based on the crystalized HLA molecule models and identifies ME by comparing donor and recipient eplets. RESULTS In the cohort studied, ME scores derived from total HLA Class I or Class II loci or individual HLA loci were not associated with overall survival, disease-free survival, non-relapse mortality, relapse, acute or chronic GVHD (P< .01). An unexpected strong association was identified between total class II ME load in the GVH direction and slower neutrophil engraftment (HR 0.82; 95% CI, 0.75 - 0.91; P < .0001) and platelet engraftment (HR 0.80; 95% CI, 0.72 - 0.88; P < .0001). This was likely attributable to ME load at the HLA-DRB1 locus, which was similarly associated with slower neutrophil engraftment (HR 0.82; 95% CI, 0.73 - 0.92; P = .001) and slower platelet engraftment (HR 0.76; 95% CI, 0.70 - 0.84; P < .0001). Additional analyses suggested that this effect is attributable to matched vs. mismatched in the GVH direction and not to ME load, as there was no dose effect identified. CONCLUSION These findings contradict those of prior relatively small studies reporting that ME load, as quantified by HLAMatchmaker, was associated with haplo-HSCT outcomes. As the study failed to demonstrate the predictive value of ME from HLA molecules for major clinical outcomes, other molecular mismatch algorithms in haplo-HSCT settings should be tested. Disclosures Lee: Pfizer: Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; National Marrow Donor Program: Membership on an entity's Board of Directors or advisory committees; Incyte: Research Funding; Janssen: Other; Takeda: Research Funding; Syndax: Research Funding; AstraZeneca: Research Funding; Kadmon: Research Funding; Amgen: Research Funding.


Blood ◽  
2021 ◽  
Vol 138 (Supplement 1) ◽  
pp. 4303-4303
Author(s):  
Carmelo Gurnari ◽  
Simona Pagliuca ◽  
Tariq Kewan ◽  
Waled Bahaj ◽  
Ishani Nautiyal ◽  
...  

Abstract Paroxysmal nocturnal hemoglobinuria (PNH) is considered to be curable only through the means of allogeneic HSCT. One of the many fascinating and scientifically instructive aspects of the pathogenesis of this disease is the rare possibility of its spontaneous remission with disappearance of PNH clone and abatement of clinical symptoms, which has always captivated the research community. Due to the orphan nature of the condition, no clinical predictors have been identified so far as harbingers of this phenomenon. In a classical scenario, exhaustion of PNH clone may be associated with reappearance of aplastic anemia (AA), in which PNH clone reflects a semi-maladaptive attempt of recovery. Consequently, one could stipulate that the retraction of PNH clone(s) would have to be associated with a compensatory re-expansion of normal hematopoiesis should normal counts be maintained. The recent insights into the AA/PNH pathobiology shed light onto molecular underpinnings of polyclonal vs oligoclonal hematopoiesis and their dynamics. Here, through application of NGS we attempted to better discern the mechanism of PNH spontaneous remission taking advantage of our internal cohort of PNH patients. Among 92 patients with a diagnosis of hemolytic PNH (M:F ratio 0.88, median age 38 years, range 9-84) 41% were primary PNH (pPNH) while 59% were secondary to AA (sPNH). Overall, patients were clinically followed-up for a median time of 68 months (2-339). Median granulocyte clone size was 73% (22-99) with the majority of cases being classified as having a type III dominant red blood cells (RBCs) clone (80%) while 20% type II. Within this cohort, a total of 3 patients underwent spontaneous remission. UPN1 was a 69-year-old male diagnosed with pPNH at the age of 46 after an episode of deep vein thrombosis. He had been managed with prednisone, transfusions and anti-coagulation because of recurrent thrombotic episodes. Once available, he was started on eculizumab and later continued on ravulizumab. His initial flow cytometry study revealed the presence of a type III RBCs clone of 40% and a granulocyte clone of 89%. After 9 years of anti-complement therapy, the patient's clone started a slow decrease and the most recent study revealed a granulocyte clone of 0.02%. Molecular analysis performed at the time of eculizumab start showed a co-dominant mutational configuration by variant allelic frequency (VAF) with PIGA deletion (p.94_95del; VAF 29%) and a BCOR nonsense (p.Y1446X; VAF 27%). No HLA class I/II mutations were found in two longitudinal samples collected 1 year before and after eculizumab start. However, at the last sequencing performed after the complete disappearance of the PNH clone, the patient developed ASXL1 (p.E635Rfs*, VAF 26%) and ZRSR2 (p.E120Gfs*, VAF 42%) mutations along with retraction of the previous PIGA lesion. No decrease in blood counts was noted. UPN2 was a 58-year-old male initially diagnosed with severe AA at the age of 48 and treated with ATG + CsA. At that time, he had a co-existing PNH granulocyte clone of 28%. After 1 year from IST his PNH clone dropped to 1% and since then has been consistently below 1%. Patient has never received anti-complement therapy. At the time of PNH clone retraction, no HLA class I/II or myeloid driver mutations were found and PIGA mutations were not detectable. However, longitudinal molecular studies performed after disappearance of PNH clone revealed the acquisition of ASXL1 p.Q512X mutation at an initial VAF of 23%, which doubled (45%) at last follow-up 5 years later while normal counts persisted. UPN3 was instead a 59-year-old lady diagnosed with pPNH at the age of 30. She had a granulocyte clone as high as 43% with a type II RBCs clone of 17% and a typical PIGA splice site c.981+1G>A mutation (VAF 15%). She was initially treated with transfusions and steroids and her course was complicated by a cerebral venous sinus thrombosis. Patient was eventually given eculizumab and her PNH clone started decreasing until it vanished (last 0.04%) after 8 years. Analysis of samples prior to and after PNH disappearance did not show any HLA class I/II nor myeloid driver gene mutations with absence of PIGA alterations at last sequencing. PNH spontaneous remissions are rare events. In addition to be replaced by polyclonal hematopoiesis, PIGA clones may be swept by CHIP lesions in myeloid genes (e.g. ASXL1) characterized by improved fitness advantage in a process of Darwinian selection. Figure 1 Figure 1. Disclosures Maciejewski: Regeneron: Consultancy; Novartis: Consultancy; Bristol Myers Squibb/Celgene: Consultancy; Alexion: Consultancy.


Blood ◽  
2021 ◽  
Vol 138 (Supplement 1) ◽  
pp. 1798-1798
Author(s):  
Rebeca Bailen ◽  
Jose Luis Vicario ◽  
Beatriz Herruzo ◽  
Luisa Maria Guerra ◽  
Ana Vallés ◽  
...  

Abstract Background. Donor specific antibodies (DSAs) are preformed IgG antibodies with specificity against HLA molecules not shared with the donor that can lead to graft failure (GF) in the setting of mismatched HSCT. The aim of this study is to report the experience of the Spanish Group of Hematopoietic Transplant (GETH-TC) in patients with DSAs undergoing haplo-HSCT. Methods. Patients undergoing haplo-HSCT in centers from the GETH-TC from 2013 to 2021 were included in the study. DSAs were analyzed with a solid-phase single-antigen immunoassay (Luminex®); monitoring was performed prior to desensitization, prior to infusion and after infusion. Desensitization strategies used depended on center experience, immunofluorescence intensity, complement fixation and type of antibodies. Results. 59 haplo-HSCT with DSAs were performed in 57 patients in 13 centers. Characteristics of the population are shown in Table 1. 53 (93%) patients were female (91% with prior pregnancies). All patients lacked a suitable alternative donor. 51 (89%) received peripheral blood as stem cell source. Conditioning was myeloablative in 58% and all patients received post-transplant cyclophosphamide based GVHD prophylaxis; 3 (5%) patients received also ATG. 28 (49%) patients presented anti-HLA class I DSAs 22 of them with >5000MFI), 14 (25%) presented anti-HLA class II (6 with >5000MFI) and 15 (26%) presented both anti-HLA class I and II DSAs (13 with >5000MFI). Five patients did not receive desensitization treatment, 4 of them with <5000MFI. Of 52 patients receiving desensitization treatment, 49 received at least two treatments as desensitization strategy and all but 3 (6%) experienced a decrease of MFI after desensitization (mean reduction 80%); 2 out of those 3 patients developed GF. Desensitization treatments used included RTX in 83% of patients, IVIG (65%), therapeutic plasma exchange (TPE) (60%), incompatible platelets (16%), MMF (42%), buffy coat (only in patients with class II DSAs, 23%), tacrolimus (21%), bortezomib (4%) and steroids (2%). Cumulative incidence of neutrophil engraftment at day 30 was 74% (Figure 1), in a median of 18 days (IQR, 15-20); five patients died before engraftment due to toxicity and 7 patients experienced primary GF despite desensitization in 6 of them. 4 of them received a 2 nd transplant, one was alive after day 100. 30 (53%) patients died during the study period: 6 due to GF, 7 due to relapse, 7 due to infection, 6 due to endothelial complications (SOS, TA-TMA and diffuse alveolar hemorrhage) and 4 because of GVHD. After a median follow-up of 24 months, 2-year OS and EFS were 52% and 42%, respectively. 2-year cumulative incidence of relapse at was 14% and NRM was 41%. Cumulative incidence of grade II-IV aGVHD at day 180 was 13% and chronic GVHD was 25%. Conclusions. The use of desensitization treatment guided by DSAs intensity kinetics constitute an effective approach with high rates of engraftment for patients with DSAs in need for an haplo-HSCT lacking an alternative suitable donor, including non-malignant disorders. Figure 1 Figure 1. Disclosures Bailen: Pfizer, Kite-Gilead, Gilead: Honoraria. Oarbeascoa: Gilead: Honoraria, Speakers Bureau. Kwon: Novartis, Celgene, Gilead, Pfizer: Consultancy, Honoraria.


Blood ◽  
2021 ◽  
Vol 138 (Supplement 1) ◽  
pp. 97-97
Author(s):  
Pietro Crivello ◽  
Esteban Arrieta-Bolanos ◽  
Meilun He ◽  
Tao Wang ◽  
Shahinaz M. Gadalla ◽  
...  

Abstract Introduction Unrelated donor (UD)-recipient disparity for human leukocyte antigen (HLA) class I adversely affects outcome of hematopoietic cell transplantation (UD-HCT). HLA polymorphisms in the peptide antigen binding groove can affect the repertoire of presented peptides. We have recently shown that the degree of peptide divergence between mismatched HLA-DP allotypes is related to T-cell alloreactivity and clinical permissiveness after UD-HCT (Meurer et al Blood 2021). Here, we hypothesized that the clinical tolerability also of HLA class I mismatches in UD-HCT might depend on the divergence of their respective peptide repertoires. Methods We studied 2,562 patients after 9/10 HLA-A, -B, -C, -DRB1, -DQB1-matched UD-HCT for acute myeloid or lymphocytic leukemia, or myelodysplastic syndrome, between 2008 and 2018, and 14,426 10/10 HLA-matched UD-HCT with similar characteristics. Peptide divergence of the mismatched HLA allotypes was predicted based on hierarchical clustering of experimentally determined peptide binding motifs (PBM) (Bassani-Sternberg et al Front Immunol 2018), with 21 different PBM groups identified in 122 HLA class I allotypes (44, 63 and 18 for HLA-A, -B and -C, respectively). The mismatched cohort was stratified into PBM-matches or PBM-mismatches, and within the latter into host-versus-graft (HvG), graft-versus-host (GvH) or bidirectional PBM-mismatches (Figure 1A). The primary study endpoint was overall survival (OS); secondary endpoints included treatment-related mortality (TRM), GVHD and relapse. P-value<0.01 was considered statistically significant. Results The available PBM data allowed us to classify 1,629/2,562 (63.6%) of our pairs. Of these, 386 (23.7%) were PBM-matched and 1,243 (76.3%) were PBM-mismatched, and in the latter, 254 (20.5%), 238 (19.1%) and 751 (60.4%) had HvG, GvH or bidirectional PBM-mismatches, respectively. Transplants were performed mainly with peripheral blood stem cells (78%), myeloablative conditioning (65%) and tacrolimus-based graft-versus-host disease (GvHD) prophylaxis (74%). About half of the 9/10 HLA-matched HCT were performed using in vivo T-cell depletion by anti-thymocyte globulin or Campath, and none used post-transplant cyclophosphamide. Multivariable analyses showed that 10/10 HLA-matched transplants had significantly higher OS, lower TRM and aGvHD 3-4 compared to 9/10 HLA-matched transplants but relapse was similar (Figure 1B,C). There were no significant differences between the PBM-matched and aggregate PBM-mismatched group (Figure 1B). In further analysis, pairs with a bidirectional or only GvH PBM-mismatch had significantly worse OS, compared to pairs in the PBM-matched group or with only a unidirectional HvG (hazards ratio [HR] 0.76, 95% confidence interval [CI] 0.63-0.92, P = 0.0036; Figure 1C). The hazards of TRM and aGvHD 3-4 were lower for the HvG or PBM-matched group compared to the reference (HR 0.78, 95% CI 0.65-0.95, P = 0.0135 and HR 0.79, 95% CI 0.65-0.95, P = 0.0126, respectively), although these were not statistically significant (Figure 1C). Conclusion We show that single HLA class I PBM-mismatches with high peptide divergence in the unidirectional or bidirectional GvH directions are significantly associated with worse survival after 9/10 HLA-matched UD-HCT compared to PBM-matched or unidirectional mismatching in the HvG direction. These data suggest that the mechanistic role of peptide-diversity for T-cell alloreactivity we previously observed for HLA-DPB1 disparity (Meurer et al., Blood 2021), is also a relevant to class I mismatches, providing a new rationale for selecting permissive donors in the setting of 9/10 HLA-matched UD-HCT. Avoiding class I PBM mismatches in the GvH direction is associated with better survival. Figure 1 Figure 1. Disclosures Paczesny: Medical University of South Carolina: Patents & Royalties: inventor on the ST2 bispecific antibody patent application. Lee: Amgen: Research Funding; AstraZeneca: Research Funding; Incyte: Research Funding; Janssen: Other; Kadmon: Research Funding; National Marrow Donor Program: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Research Funding; Syndax: Research Funding; Takeda: Research Funding.


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