T-Cell Manipulation Strategies to Prevent Graft-Versus-Host Disease in Haploidentical Stem Cell Transplantation

Abstract

Allogeneic haematopoietic stem cell transplantation (HSCT) from an human leukocyte antigen (HLA)-identical donor can be curative for eligible patients with non-malignant and malignant haematological disorders. HSCT from alternative donor sources, such as HLA-mismatched haploidentical donors, is increasingly considered as a viable therapeutic option for patients lacking HLA-matched donors. Initial attempts at haploidentical HSCT were associated with vigorous bidirectional alloreactivity, leading to unacceptably high rates of graft rejection and graft-versus-host disease (GVHD). More recently, new approaches for mitigating harmful T-cell alloreactivity that mediates GVHD, while preserving the function of tumour-reactive natural killer (NK) cells and γδ T cells, have led to markedly improved clinical outcomes, and are successfully being implemented in the clinic. This article will provide an update on in vitro strategies and in vivo approaches aimed at preventing GVHD by selectively manipulating key components of the adaptive immune response, such as T-cell receptor (TCR)-αβ T cells and CD45RA-expressing naive T cells.

Keywords: dendritic cells; graft-versus-host disease; haploidentical haematopoietic stem cell transplantation; immunomagnetic T-cell depletion; photodynamic purging; regulatory T cells.

Figure 1

Overview of graft-versus-host disease (GVHD) pathogenesis. As reviewed elsewhere [3,7,8], the pre-transplant conditioning regimen (chemotherapy with or without radiotherapy) contributes to GVHD induction via tissue destruction, bacterial translocation across gut mucosal cells, and release of pro-inflammatory cytokines such as tumour necrosis factor (TNF)-α, interleukin (IL)-1, and IL-7. Lipopolysaccharide (LPS) and other components of the bacterial cell wall leaking through the damaged intestinal mucosa stimulate mononuclear cells, amplify the production of inflammatory cytokines, and contribute to apoptosis. Innate immune cells partake in tissue damage and cytokine production, a phenomenon referred to as “cytokine storm”. Both host and donor antigen-presenting cells (APCs) initiate GVH responses through the release of IL-12 and IL-23. Activated T cells, natural killer (NK) cells, and macrophages mediate end-organ damage through cytokine production and direct cytotoxic effects on target tissues (release of cytolytic granules and expression of the CD95 ligand). Acute GVHD develops in parenchymal targets containing highly proliferating cells (bone marrow, skin, liver, gut, and lungs). CD4⁺ T cells are also activated by macrophages and dendritic cells (DCs) to produce pro-inflammatory mediators. CTL = cytotoxic T lymphocyte; IFN = interferon; PAMP = pathogen-associated molecular pattern; DAMP = damage-associated molecular pattern; TCR = T-cell receptor; BM = bone marrow; GI = gastrointestinal.

Figure 2

T-cell manipulation strategies to control GVHD and improve post-haematopoietic stem cell transplantation (HSCT) immune reconstitution. Ex vivo T-cell depletion techniques have evolved and currently include CD3/CD19 depletion, TCR-αβ/CD19 depletion, and infusion of CD45RA-depleted grafts [14,15,16]. To boost immune reconstitution after T-cell depleted haploidentical HSCT, genetically modified T cells and pathogen-specific T cells are increasingly used in the clinic [17,18]. Treg = regulatory T cell; GVL = graft-versus-leukaemia; mTOR = mechanistic target of rapamycin; NOTCH = neurogenic locus notch homolog protein; JAK-STAT = Janus kinase/signal transducer and activator of transcription.

Figure 3

Flow cytometry-based enumeration of residual T and B cells after TCR-αβ/CD19 depletion. Cells were gated on low side scatter/low forward scatter events (P1), followed by gating on CD3⁺ T cells (P2) and on CD45⁺ leukocytes (P3), as already published [22]. Residual TCR-αβ⁺ and CD20⁺ B cells were enumerated as shown in this representative procedure. Anti-CD20 monoclonal antibodies were used because of strong internalization of CD19 after the incubation of cells with CD19 reagent or steric hindrance by the beads, as suggested in [14].