HLA-C KIR-Ligands Determine the Impact of Anti-Thymocyte Globulin (ATG) on Graft versus Host and Graft versus Leukemia Effects Following Hematopoietic Stem Cell Transplantation

Posttransplant cyclophosphamide beyond haploidentical transplantation

Posttransplant cyclophosphamide (PTCy) has practically revolutionized haploidentical (Haplo) hematopoietic cell transplantation (HCT). Comparisons between Haplo with PTCy and unrelated donor (URD) with conventional graft-versus-host disease (GVHD) prophylaxis have shown comparable overall survival with lower incidences of GVHD with Haplo/PTCy and led to the following question: is it PTCy so good that can be successfully incorporated into matched related donor (MRD) and URD HCT? In this review, we discuss other ways of doing PTCy, PTCy in peripheral blood haploidentical transplants, PTCy in the context of matched related and matched unrelated donors, PTCy with mismatched unrelated donors, and PTCy following checkpoint inhibitor treatment. PTCy is emerging as a new standard GVHD prophylaxis in haploidentical, HLA-matched, and -mismatched HCT.

Impact of the Recipient's Pre-Treatment Blood Lymphocyte Count on Intended and Unintended Effects of Anti-T-Lymphocyte Globulin in Allogeneic Hematopoietic Stem Cell Transplantation

Background: In allogeneic hematopoietic stem cell transplantation (HSCT), Anti-T-Lymphocyte Globulin (ATLG) may be used for the prevention of severe graft-versus-host disease (GVHD). ATLG targets both the recipient's lymphocytes and those transferred with the graft. Assuming an inverse relation between the recipient's absolute lymphocyte count (ALC) and exposure of remaining ATLG to the graft, we aim to evaluate the impact of the recipient's ALC before the first ATLG administration on the benefits (prevention of GVHD and GVHD-associated mortality) and potential risks (increased relapse incidence) associated with ATLG. Methods: In recipients of HLA-matched, ATLG-based HSCT (n = 311), we assessed the incidence of acute GVHD, GVHD-related mortality and relapse, as well as other transplant-related outcomes, in relation to the respective ALC (divided into tertiles) before ATLG. Results: The top-tertile ALC group had a significantly increased risk of aGVHD (subhazard ratio (sHR) 1.81; [CI 95%; 1.14-2.88]; p = 0.01) and aGVHD-associated mortality (sHR 1.81; [CI 95%; 1.03-3.19]; p = 0.04). At the highest ATLG dose level (≥45 mg/kg), recipients with lowest-tertile ALC had a trend towards increased relapse incidence (sHR 4.19; [CI 95%; 0.99-17.7]; p = 0.05, n = 32). Conclusions: ATLG dosing based on the recipient's ALC may be required for an optimal balance between GVHD suppression and relapse prevention.

Comparison of Benefits and Risks Associated with Anti-T-Lymphocyte Globulin (ATLG) Serotherapy in Methotrexate (MTX)- versus Mycophenolate Mofetil (MMF)-Based Hematopoietic Stem Cell Transplantation

Background: Serotherapy with anti-T lymphocyte globulin (ATLG, Grafalon, formerly ATG-Fresenius) is established for the prevention of severe graft-versus-host disease (GVHD) after hematopoietic stem cell transplantation (HSCT). The evidence from prospective studies is predominantly derived from a setting where methotrexate (MTX) and a calcineurin inhibitor (CNI) are used as the backbone of GVHD prophylaxis. The efficacy of ATLG in combination with CNI and mycophenolate mofetil (MMF) has not been investigated as much, particularly in terms of a direct comparison with its effects when combined with CNI/MTX. A total of 401 HSCTs from two Austrian transplant centers were retrospectively evaluated. We included peripheral blood transplants from early- or intermediate-stage (excluding advanced/refractory) hematological diseases from matched siblings or 10/10 or 9/10 matched unrelated donors with CNI/MTX or CNI/MMF prophylaxis, either without (n = 219) or with ATLG (n = 182). Overall, ATLG significantly reduced the risk for all-cause mortality by multivariate Cox analysis (HR 0.53; p = 0.002). Stratification by postgrafting prophylaxis type revealed a significant survival advantage for ATLG in the CNI/MMF cohort (HR 0.49; p = 0.001; n = 193), while its effect on survival in the CNI/MTX cohort was not significant (HR 0.87; p = 0.56; n = 208). In unrelated HSCT with CNI/MMF prophylaxis, ATLG exhibited its greatest survival benefit (HR 0.34; p = 0.001; n = 104). In the context of CNI/MMF, ATLG may provide even greater benefits than in the setting of CNI/MTX for post-grafting immunosuppression. Future prospective studies on ATLG should therefore focus on CNI/MMF-based transplants, which are widely performed in elderly or comorbid patients not expected to tolerate a standard course of MTX.

HSCT with Mismatched Unrelated Donors (MMUD): A Comparison of Different Platforms for GvHD Prophylaxis

HSCT from an unrelated HLA-mismatched donor (MMUD) is one of the alternatives where an HLA-matched donor is not found. The aim of this study was to compare GvHD prophylaxis with anti-thymocyte globulin (ATG) vs. post-transplant cyclophosphamide (PT-Cy). Thirty-nine adult patients were uniformly treated with rabbit ATG-Cy-A-MTX and peripheral blood stem cell (PBSC) and 40 adult patients with PT-Cy-MMF-tacrolimus and PBSC. This retrospective study was registered at ClinicalTrials.gov NCT04598789. Three-year overall survival was 42% vs. 64% for ATG and PT-Cy (p < 0.0005), three-year treatment-related mortality (TRM) was 36% vs. 8% (p = 0.0033) and the three-year relapse incidence (RI) was 15% vs. 28% (p = NS), respectively. The incidences of day-100 GvHD graded II–IV and III–IV were 39% vs. 7% (p = 0.0006) and 11% vs. 0% (p = 0.04), respectively, whereas the three-year cGvHD incidences were 48% vs. 13% (p = 0.0005), respectively. We were able to show how PT-Cy can reduce the incidence of GvHDs and TRM in adults, but relapse remains an issue.

Increased donor inhibitory KIR with known HLA interactions provide protection from relapse following HLA matched unrelated donor HCT for AML

Killer immunoglobulin-like receptor (KIR) and KIR-ligand (KIRL) interactions play an important role in natural killer cell-mediated graft versus leukemia effect (GVL) after hematopoietic cell transplant (HCT) for AML. Accounting for known KIR-KIRL interactions may identify donors with optimal NK cell-mediated alloreactivity and GVL. A retrospective study of 2359 donor-recipient pairs (DRP) who underwent unrelated donor (URD) HCT for AML was performed. KIR-KIRL combinations were determined and associations with clinical outcomes examined. Relapse risk was reduced in DRP with both higher inhibitory KIR-KIRL (iKIR) and missing KIRL (mKIR) scores, with HR 0.86 (P = 0.01) & HR 0.84 (P = 0.02) respectively. The iKIR and mKIR score components were summed to give a maximal inhibitory KIR ligand (IM-KIR) score for each donor, which if it was 5, as opposed to <5, was also associated with a lower relapse risk, SHR 0.8 (P = 0.004). All IM = 5 donors possess KIR Haplotype B/x. Transplant-related mortality was increased among those with IM-KIR = 5, HR, 1.32 (P = 0.01). In a subset analysis of those transplanted with 8/8 HLA-matched DRP, anti-thymocyte globulin recipients with IM-KIR = 5, had a lower relapse rate HR, 0.61 (p = 0.001). This study demonstrates that HLA-matched unrelated donors with the highest inhibitory KIR content confer relapse protection, albeit with increased TRM. These donors all have KIR haplotype B. Clinical trials utilizing donors with a higher iKIR content in conjunction with novel strategies to reduce TRM should be considered for URD HCT in recipients with AML to optimize clinical outcomes.

Major Histocompatibility Complex and Hematopoietic Stem Cell Transplantation: Beyond the Classical HLA Polymorphism

Allogeneic hematopoietic stem cell transplantation (HSCT) represents a curative treatment for many patients with hematological malignant or non-malignant disorders. Evaluation of potential donors for HSCT includes a rigorous assessment of the human leukocyte antigens (HLA) match status of family members, and the identification of suitable unrelated donors. Genes encoding transplantation antigens are placed both within and outside the major histocompatibility complex (MHC). The human MHC is located on the short arm of chromosome 6 and contains a series of genes encoding two distinct types of highly polymorphic cell surface glycoproteins. Donors for HSCT are routinely selected based on the level of matching for HLA-A, -B, -C, -DRB1, and -DQB1 loci. However, disease relapse, graft-versus-host-disease, and infection remain significant risk factors of morbidity and mortality. In the same breath, in high-risk patients, graft-versus-leukemia effects inherent in HLA mismatching play a substantial immunological role to limit the recurrence of post-transplant disease. The definition of a suitable donor is ever changing, shaped not only by current typing technology, but also by the specific transplant procedure. Indeed, a more complete understanding of permissible HLA mismatches and the role of Killer Immunoglobulin-like receptors' genes increases the availability of HLA-haploidentical and unrelated donors.

Homozygous HLA-C1 is Associated with Reduced Risk of Relapse after HLA-Matched Transplantation in Patients with Myeloid Leukemia

Natural killer (NK) cells assume graft-versus-leukemia alloreactivity after hematopoietic stem cell transplantation (HSCT) through their inhibitory killer cell immunoglobulin-like receptors (KIRs). KIR2D family members recognize HLA-C alleles with Asn80 (HLA-C1) or Lys80 (HLA-C2). The predominance of HLA-C1 over HLA-C2 and the frequent presence of KIR2DL1 are characteristic of Japanese people. We compared clinical outcomes among homozygous HLA-C1 (HLA-C1/C1) patients and heterozygous HLA-C1/C2 patients who underwent HLA-matched HSCT for hematologic malignancies by assessing the data of 10,638 patients from the Japanese national registry. HLA-C1/C1 recipients had a lower rate of relapse than HLA-C1/C2 recipients after transplantation for acute myelogenous leukemia (AML) (hazard ratio [HR], .79; P = .006) and chronic myelogenous leukemia (CML) (HR, .48; P = .025), but not for acute lymphoblastic leukemia (HR, 1.36), lymphoma (HR, .97), or low-grade myelodysplastic syndrome (HR, 1.40). We then grouped AML and CML patients together and divided them into several subgroups. Advantages of HLA-C1/C1 recipients over HLA-C1/C2 recipients regarding relapse were observed irrespective of donor relation (related: HR, .79, P = .069; unrelated: HR, .77, P = .022), preparative regimen (myeloablative: HR, .79, P = .014; reduced intensity: HR, .73, P = .084), and occurrence of acute graft-versus-host disease (yes: HR, .70, P = .122; no, HR .71, P = .026) or cytomegalovirus reactivation (reactivated: HR .67,P = .054; nonreactivated: HR .71, P = .033); however, these advantages were not observed in recipients with a delay in achieving complete chimerism (HR, 1.06). The advantage of decreasing relapse and extending relapse-free survival of C1/1 over C1/2 KIR-ligand status was most pronounced in T cell-depleted HSCT (HR, .27; P < .001 and HR, .30; P = .002, respectively) and in children age <15 years (HR, .29; P < .001 and HR .31; P < .001, respectively). Our findings represent an important mechanism responsible for the immunity against HLA-C2-negative myeloid leukemia cells after HLA-matched transplantation.

Role of αβ T Cell Depletion in Prevention of Graft versus Host Disease

Graft versus host disease (GVHD) represents a major complication of allogeneic hematopoietic stem cell transplantation (allo HCT). Graft cellular manipulation has been used to mitigate the risk of GVHD. The αβ T cells are considered the primary culprit for causing GVHD therefore depletion of this T cell subset emerged as a promising cellular manipulation strategy to overcome the human leukocyte antigen (HLA) barrier of haploidentical (haplo) HCT. This approach is also being investigated in HLA-matched HCT. In several studies, αβ T cell depletion HCT has been performed without pharmacologic GVHD prophylaxis, thus unleashing favorable effect of donor’s natural killer cells (NK) and γδ T cells. This article will discuss the evolution of this method in clinical practice and the clinical outcome as described in different clinical trials.