Human γδ T-Cells: From Surface Receptors to the Therapy of High-Risk Leukemias

The complex existence of γδ T cells following transplantation: the good, the bad and the simply confusing

Gamma delta (γδ) T cells are a highly heterogeneous population of lymphocytes that exhibit innate and adaptive immune properties. Despite comprising the majority of residing lymphocytes in many organs, the role of γδ T cells in transplantation outcomes is under‐researched. γδ T cells can recognise a diverse array of ligands and exert disparate effector functions. As such, they may potentially contribute to both allograft acceptance and rejection, as well as impacting on infection and post‐transplant malignancy. Here, we review the current literature on the role and function of γδ T cells following solid organ and hematopoietic stem cell transplantation.

Improved Overall Survival, Relapse-Free-Survival, and Less Graft-vs.-Host-Disease in Patients With High Immune Reconstitution of TCR Gamma Delta Cells 2 Months After Allogeneic Stem Cell Transplantation

T-cell receptor (TCR) γδ cells are perceived as innate-like effector cells with the possibility of mediating graft-vs. -tumor (GVT) without causing graft-vs.-host disease (GVHD) in the setting of hematopoietic allogeneic stem cell transplantation (HSCT). We conducted a prospective study to assess the clinical impact of TCR γδ cell immune reconstitution on overall survival, relapse-free-survival, relapse and GVHD. The impact of CD3, CD4, and CD8 T cells together with NK cells including subtypes were analyzed in parallel. A total of 108 patients with hematological malignancies transplanted with HLA-matched, T cell replete stem cell grafts were included for analyses of absolute concentrations of CD3, CD4, and CD8 positive T cells and NK cells together with a multi-color flow cytometry panel with staining for TCRαβ, TCRγδ, Vδ1, Vδ2, CD3, CD4, CD8, HLA-DR, CD196, CD45RO, CD45RA, CD16, CD56, CD337, and CD314 at 28, 56, 91, 180, and 365 days after transplantation. Immune reconstitution data including subsets and differentiation markers of T and NK cells during the first year after transplantation was provided. Patients with TCR γδ cell concentrations above the median value of 21 (0–416) × 10⁶ cells/L 56 days after transplantation had significantly improved overall survival (p = 0.001) and relapse-free survival (p = 0.007) compared to patients with concentrations below this value. When day 56 cell subset concentrations were included as continuous variables, TCR γδ cells were the only T cell subsets with a significant impact on OS and RFS; the impact of TCR γδ cells remained statistically significant in multivariate analyses adjusted for pre-transplant risk factors. The risk of death from relapse was significantly decreased in patients with high concentrations of TCR γδ cells 56 days after transplantation (p = 0.003). Also, the risk of acute GVHD was significantly lower in patients with day 28 TCR γδ cell concentrations above the median of 18 × 10⁶ cells/L compared to patients with low concentrations (p = 0.01). These results suggest a protective role of TCR γδ cells in relapse and GVHD and encourage further research in developing adaptive TCR γδ cell therapy for improving outcomes after HSCT.

Graft Engineering and Adoptive Immunotherapy: New Approaches to Promote Immune Tolerance After Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative therapeutic option for a wide range of immune and hematologic malignant and non-malignant disorders. Once transplanted, allogeneic cells have to support myeloid repopulation and immunological reconstitution, but also need to become tolerant to the host via central or peripheral mechanisms to achieve the desired therapeutic effect. Peripheral tolerance after allogeneic HSCT may be achieved by several mechanisms, though blocking alloreactivity to the host human leukocyte antigens while preserving immune responses to pathogens and tumor antigens remains a challenge. Recently uncovered evidence on the mechanisms of post-HSCT immune reconstitution and tolerance in transplanted patients has allowed for the development of novel cell-based therapeutic approaches. These therapies are aimed at inducing long-term peripheral tolerance and reducing the risk of graft-vs-host disease (GvHD), while sparing the graft-vs-leukemia (GvL) effect. Thus, ensuring effective long term remission in hematologic malignancies. Today, haploidentical stem cell transplants have become a widely used treatment for patients with hematological malignancies. A myriad of ex vivo and in vivo T-cell depletion strategies have been adopted, with the goal of preventing GvHD while preserving GvL in the context of immunogenetic disparity. αβ T-cell/CD19 B-cell depletion techniques, in particular, has gained significant momentum, because of the high rate of leukemia-free survival and the low risk of severe GvHD. Despite progress, better treatments are still needed in a portion of patients to further reduce the incidence of relapse and achieve long-term tolerance. Current post-HSCT cell therapy approaches designed to induce tolerance and minimizing GvHD occurrence include the use of (i) γδ T cells, (ii) regulatory Type 1 T (Tr1) cells, and (iii) engineered FOXP3+ regulatory T cells. Future protocols may include post-HSCT infusion of allogeneic effector or regulatory T cells engineered with a chimeric antigen receptor (CAR). In the present review, we describe the most recent advances in graft engineering and post-HSCT adoptive immunotherapy.

Human NK cells: surface receptors, inhibitory checkpoints, and translational applications

NK cells play important roles in innate defenses against viruses and in the control of tumor growth and metastasis. The regulation/induction of NK cell function is mediated by an array of activating or inhibitory surface receptors. In humans, major activating receptors involved in target cell killing are the natural cytotoxicity receptors (NCRs) and NKG2D. Activating receptors recognize ligands that are overexpressed or expressed de novo upon cell stress, viral infection, or tumor transformation. The HLA-class I-specific inhibitory receptors, including KIRs recognizing HLA-class I allotypic determinants and CD94/NKG2A recognizing the class-Ib HLA-E, constitute a fail-safe mechanism to avoid unwanted NK-mediated damage to healthy cells. Other receptors such as PD-1, primarily expressed by activated T lymphocytes, are important inhibitory checkpoints of immune responses that ensure T-cell tolerance. PD-1 also may be expressed by NK cells in cancer patients. Since PD-1 ligand (PD-L1) may be expressed by different tumors, PD-1/PD-L1 interactions inactivate both T and NK cells. Thus, the reliable evaluation of PD-L1 expression in tumors has become a major issue to select patients who may benefit from therapy with mAbs disrupting PD-1/PD-L1 interactions. Recently, NKG2A was revealed to be an important checkpoint controlling both NK and T-cell activation. Since most tumors express HLA-E, mAbs targeting NKG2A has been used alone or in combination with other therapeutic mAbs targeting PD-1 or tumor antigens (e.g., EGFR), with encouraging results. The translational value of NK cells and their receptors is evidenced by the extraordinary therapeutic success of haploidentical HSCT to cure otherwise fatal high-risk leukemias.

Diminished cytolytic activity of γδ T cells with reduced DNAM-1 expression in neuroblastoma patients

Neuroblastoma is one of the children's malignant tumors with poor prognosis, as well as high recurrence and metastasis rates after surgical removal and chemotherapy. γδ T-cell based immunotherapy receives increasing attention thanks to the strong cytolytic activity to tumor cells. Our previous data revealed a significant increase in circulating γδ T-cell frequency in NB patients. In the present study, we found that beside a reduction of IFN-γ in serum of NB patients, DNAM-1 expression decreased in both circulating and PAM-expanded NB γδ T cells. Upon PAM stimulation, NB γδ T cells showed a reduced level of cell proliferation. In addition, the cytolytic activity of NB γδ T cells to NB cell lines was proved to be attenuated in a co-culture system. The fact that DNAM-1 neutralizing antibody abolished the tumor cell killing accentuates the indispensable role of DNAM-1 molecule in γδ T-cell cytolytic function.

Human natural killer cells and other innate lymphoid cells in cancer: Friends or foes?

Innate lymphoid cells (ILC) including NK cells (cytotoxic) and the recently identified "helper" ILC1, ILC2 and ILC3, play an important role in innate defenses against pathogens. Notably, they mirror analogous T cell subsets, regarding the pattern of cytokine produced, while the timing of their intervention is few hours vs days required for T cell-mediated adaptive responses. On the other hand, the effectiveness of ILC in anti-tumor defenses is controversial. The relevance of NK cells in the control of tumor growth and metastasis has been well documented and they have been exploited in the therapy of high risk leukemia in the haploidentical hematopoietic stem cell transplantation setting. In contrast, the actual involvement of helper ILCs remains contradictory. Thus, while certain functional capabilities of ILC1 and ILC3 may favor anti-tumor responses, other functions could rather favor tumor growth, neo-angiogenesis, epithelial-mesenchymal transition and metastasis. In addition, ILC2, by secreting type-2 cytokines, are thought to induce a prevalent pro-tumorigenic effect. Finally, the function of both NK cells and helper ILCs may be inhibited by the tumor microenvironment, thus adding further complexity to the interplay between ILC and tumors.