Risk Factors for Severe Acute Graft-versus-Host Disease in Donor Graft Composition

Biomarkers for early complications post hematopoietic cell transplantation: Insights and challenges

Hematopoietic cell transplantation is an established curative treatment option for various hematological malignant, and non-malignant diseases. However, the success of HCT is still limited by life-threatening early complications post-HCT, such as Graft Versus Host Disease (GVHD), Sinusoidal Obstruction Syndrome (SOS), and transplant-associated microangiopathy, to name a few. A decade of research in the discovery and validation of novel blood-based biomarkers aims to manage these early complications by using them for diagnosis or prognosis. Advances in this field have also led to predictive biomarkers to identify patients' likelihood of response to therapy. Although biomarkers have been extensively evaluated for different complications, these are yet to be used in routine clinical practice. This review provides a detailed summary of various biomarkers for individual early complications post-HCT, their discovery, validation, ongoing clinical trials, and their limitations. Furthermore, this review also provides insights into the biology of biomarkers and the challenge of obtaining a universal cut-off value for biomarkers.

Naive T Cells in Graft Versus Host Disease and Graft Versus Leukemia: Innocent or Guilty?

The outcome of allogeneic hematopoietic cell transplantation (allo-HCT) largely depends on the development and management of graft-versus-host disease (GvHD), infections, and the occurrence of relapse of malignancies. Recent studies showed a lower incidence of chronic GvHD and severe acute GvHD in patients receiving naive T cell depleted grafts compared to patients receiving complete T cell depleted grafts. On the other hand, the incidence of acute GvHD in patients receiving cord blood grafts containing only naive T cells is rather low, while potent graft-versus-leukemia (GvL) responses have been observed. These data suggest the significance of naive T cells as both drivers and regulators of allogeneic reactions. The naive T cell pool was previously thought to be a quiescent, homogenous pool of antigen-inexperienced cells. However, recent studies showed important differences in phenotype, differentiation status, location, and function within the naive T cell population. Therefore, the adequate recovery of these seemingly innocent T cells might be relevant in the imminent allogeneic reactions after allo-HCT. Here, an extensive review on naive T cells and their contribution to the development of GvHD and GvL responses after allo-HCT is provided. In addition, strategies specifically directed to stimulate adequate reconstitution of naive T cells while reducing the risk of GvHD are discussed. A better understanding of the relation between naive T cells and alloreactivity after allo-HCT could provide opportunities to improve GvHD prevention, while maintaining GvL effects to lower relapse risk.

CD69+ resident memory T cells are associated with graft-versus-host disease in intestinal transplantation

Graft-versus-host disease (GvHD) is a common, morbid complication after intestinal transplantation (ITx) with poorly understood pathophysiology. Resident memory T cells (T_RM ) are a recently described CD69+ memory T cell subset localizing to peripheral tissue. We observed that T effector memory cells (T_EM ) in the blood increase during GvHD and hypothesized that they derive from donor graft CD69+T_RM migrating into host blood and tissue. To probe this hypothesis, graft and blood lymphocytes from 10 ITx patients with overt GvHD and 34 without were longitudinally analyzed using flow cytometry. As hypothesized, CD4+ and CD8+CD69+T_RM were significantly increased in blood and grafts of GvHD patients, alongside higher cytokine and activation marker expression. The majority of CD69+T_RM were donor derived as determined by multiplex immunostaining. Notably, CD8/PD-1 was significantly elevated in blood prior to transplantation in patients who later had GvHD, and percentages of HLA-DR, CD57, PD-1, and naïve T cells differed significantly between GvHD patients who died vs. those who survived. Overall, we demonstrate that (1) there were significant increases in T_EM at the time of GvHD, possibly of donor derivation; (2) donor T_RM in the graft are a possible source; and (3) potential biomarkers for the development and prognosis of GvHD exist.

Peripheral blood stem cell grafts in allogeneic hematopoietic cell transplantation: It is not all about the CD34+ cell dose

Allogeneic Hematopoietic Cell Transplantation is a curative approach in various malignant and non-malignant disorders. The majority of adult transplants in the current era are performed using mobilized stem cells, harvested from the peripheral blood by leukapheresis. Peripheral blood stem cell (PBSC) collections are designed to target a dose of stem cells that will result in safe engraftment and hematopoietic recovery; however, 99 % of the cells contained in a PBSC graft are not stem cells and a growing number of studies attempt to characterize the associations between graft composition and transplant outcomes. A better understanding of the impact of the quantity and quality of various cell types in PBSC grafts may lead to development of novel collection strategies or improved donor selection algorithms. Here we review relevant findings from recent studies in this area.

Reconstitution of T Cell Subsets Following Allogeneic Hematopoietic Cell Transplantation

Allogeneic (allo) hematopoietic cell transplantation (HCT) is the only curative treatment option for patients suffering from chemotherapy-refractory or relapsed hematological malignancies. The occurrence of morbidity and mortality after allo-HCT is still high. This is partly correlated with the immunological recovery of the T cell subsets, of which the dynamics and relations to complications are still poorly understood. Detailed information on T cell subset recovery is crucial to provide tools for better prediction and modulation of adverse events. Here, we review the current knowledge regarding CD4+ and CD8+ T cells, γδ T cells, iNKT cells, Treg cells, MAIT cells and naive and memory T cell reconstitution, as well as their relations to outcome, considering different cell sources and immunosuppressive therapies. We conclude that the T cell subsets reconstitute in different ways and are associated with distinct adverse and beneficial events; however, adequate reconstitution of all the subsets is associated with better overall survival. Although the exact mechanisms involved in the reconstitution of each T cell subset and their associations with allo-HCT outcome need to be further elucidated, the data and suggestions presented here point towards the development of individualized approaches to improve their reconstitution. This includes the modulation of immunotherapeutic interventions based on more detailed immune monitoring, aiming to improve overall survival changes.

Association between P2X7 Polymorphisms and Post-Transplant Outcomes in Allogeneic Haematopoietic Stem Cell Transplantation

Allogeneic stem cell transplantation (alloSCT) is a highly effective treatment method for haematologic malignancies. However, infection of acute organ dysfunction and graft versus host disease (GVHD) impact negatively on patient outcomes. Pre-transplant conditioning regimes are associated with high levels of immunogenic cell death and the release of extracellular ATP, which binds to the P2X7 receptor. It has been proposed that signaling through the P2X7 receptor may lead to activation of downstream effectors that influence alloSCT outcome. In this study, we examined the effect of gain-of-function (GOF) or loss-of-function (LOF) P2X7 Single Nucleotide Polymorphisms (SNP) in 453 paired alloSCT donors and recipients and correlated their presence or absence to the major post-transplant outcomes of acute GVHD, relapse free survival and overall survival. The allelic frequency of P2X7 SNP in recipients and donors was not different from those SNP for which there is published population data. The LOF SNP Glu496Ala was overrepresented in recipients who did not develop severe acute GVHD and was associated with improved overall survival in rare homozygous recipients, whereas the LOF SNP Ile568Asn was more common in patients with grade 1–4 GVHD but lost statistical association in patients with grade 2–4 aGVHD, and was associated with reduced overall survival in heterozygotes due to an excess of infection-related deaths. The GOF variant haplotype (homozygous Gln460Arg-Ala348Thr) had no impact on post-alloSCT outcomes. Overall, our data indicate that allelic variations in recipients or donors occurs at the same frequency as the general population and may have a minor, but clinically nominal, impact on post-alloSCT outcomes.

Human placenta derived mesenchymal stromal cells alleviate GVHD by promoting the generation of GSH and GST in PD-1+T cells

AIMS

To investigate whether placenta-derived mesenchymal stromal cells (hPMSCs) have immunoregulatory effects on PD-1⁺ T cell generation by controlling ROS production and thus alleviating GVHD.

MAIN METHODS

Flow cytometry was used to analyze the percentage of PD-1⁺ T cells, as well as the generation of ROS, GSH and GST in PD-1⁺ T cells. The expression of GST in the spleen and liver was analyzed by western blotting.

KEY FINDINGS

The percentage of PD-1⁺ T cells was increased, but the ratio of GSH/GSSG was decreased in GVHD patients and the GVHD^high mouse model compared with that in the normal control group. hPMSCs downregulated the level of malondialdehyde (MDA) and upregulated the ratio of GSH/GSSG and the expression of glutathione S transferase (GST) in the plasma, spleen and liver of GVHD mice compared with those of PBS-treated GVHD mice. Further studies showed that the ROS level, as well as the expression of PD-1, in both CD3⁺ and CD4⁺ T cells from the spleen and liver of hPMSC-treated GVHD mice were decreased compared with those observed in PBS-treated mice.

SIGNIFICANCE

hPMSCs downregulated ROS generation by increasing GSH and GST levels and further reduced the expression of PD-1 on T cells, thereby alleviating inflammation in GVHD mice.

CD8+ γδ T Cells Are More Frequent in CMV Seropositive Bone Marrow Grafts and Display Phenotype of an Adaptive Immune Response

The role of gamma delta (γδ) T cells in human cytomegalovirus (HCMV) immune surveillance has been the focus of research interest for years. Recent reports have shown a substantial clonal proliferation of γδ T cells in response to HCMV, shedding light on the adaptive immune response of γδ T cells. Nevertheless, most efforts have focused on Vδ2^neg γδ T cell subset while less attention has been given to investigate other less common γδ T cell subsets. In this regard, a distinct subpopulation of γδ T cells that expresses the CD8 coreceptor (CD8⁺ γδ T cells) has not been thoroughly explored. Whether it is implicated in HCMV response and its ability to generate adaptive response has not been thoroughly investigated. In this study, we combined flow cytometry and immune sequencing of the TCR γ-chain (TRG) to analyze in-depth bone marrow (BM) graft γδ T cells from CMV seropositive (CMV+) and CMV seronegative (CMV-) donors. We showed that the frequency of CD8⁺ γδ T cells was significantly higher in CMV+ grafts compared to CMV- grafts (P < 0.001). Further characterization revealed that CD8⁺ γδ T cells from CMV+ grafts express Vγ9^- and preferentially differentiated from a naive to terminal effector memory phenotype (CD27^low/-CD45RO^-). In line with these findings, TRG immune sequencing revealed clonal focusing and reduced usage of the Vγ9/JP gene segment in a CMV+ graft. Furthermore, CD8⁺ γδ T cells showed an enhanced response to TCR/CD3 and cytokine stimulation in contrast to CD8^- γδ T cells. We conclude that γδ T cells in BM grafts are reshaped by donor CMV serostatus and highlight the potential adaptive role of CD8⁺ γδ T cells in HCMV immune response.

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.

The importance of graft cell composition in outcome after allogeneic stem cell transplantation in patients with malignant disease

BACKGROUND

Graft-versus-host disease (GVHD) and relapse remain majobstacles ftreatment success in allogeneic hematopoietic stem cell transplantation (HSCT). In the present study, we evaluated the immune cell profile of the graft to outcome after HSCT.

STUDY DESIGN AND METHOD

Flow cytometry data of graft cell subsets [CD34⁺ , CD3⁺ , CD19⁺ , CD4⁺ , CD8⁺ , CD3-CD56⁺ CD16⁺ , CD4⁺ CD127^low CD25^high ] from G-CSF primed peripheral blood stem cell (PBSC) donors was collected retrospectively from 299 patients with hematological malignancies undergoing HSCT between 2006 and 2013. The association to overall survival, transplant-related mortality (TRM), GVHD and probability of relapse was analyzed. Patients outcome from HLA-identical sibling (Sib) (n = 97) and unrelated donors (URD) (n = 202) were analyzed separately as all URD patients received anti-thymocyte globulin (ATG).

RESULTS

Five-year overall survival was similar in the two cohorts (68% (Sib) vs 65% (URD)). The relapse incidence was significantly lower in the Sib cohort (24% vs 35%, P = 0.04). Multivariate analysis in the URD group revealed an association between a higher CD8⁺ dose and less relapse (HR, 0.94; 95%CI, 0.90-0.98; P = 0.006) as well as an association between higher CD34⁺ dose and both higher TRM (HR, 1.09; 95%CI, 1.02-1.20; P = 0.02) and relapse (HR, 1.09; 95%CI, 1.01-1.17; P = 0.025). The Sib analysis showed an association between a higher graft CD19⁺ dose and more severe acute GVHD (HR, 1,09; 95%CI, 1.03-1.15; P = 0.003) and TRM (HR, 1.09; 95%CI, 1.01-1.17; P = 0.036). In addition, a higher CD4⁺ graft content was associated to an increased risk for chronic GVHD (HR, 1.02; 95%CI 1.00-1.04; P = 0.06).

CONCLUSION

These data indicate an importance of PBSC dongraft composition in patients with a hematological malignancy.

T-cell frequencies of CD8+ γδ and CD27+ γδ cells in the stem cell graft predict the outcome after allogeneic hematopoietic cell transplantation

The impact of intra-graft T cells on the clinical outcome after allogeneic hematopoietic cell transplantation has been investigated. Most previous studies have focused on the role of αβ cells while γδ cells have received less attention. It has been an open question whether γδ cells are beneficial or not for patient outcome, especially with regards to graft versus host disease. In this study, graft composition of γδ cell subsets was analyzed and correlated to clinical outcome in 105 recipients who underwent allogeneic hematopoietic cell transplantation between 2013 and 2016. We demonstrate for the first time that grafts containing higher T-cell proportions of CD8⁺γδ cells were associated with increased cumulative incidence of acute graft versus host disease grade II-III (50% vs 22.6%; P = 0.008). Additionally, graft T-cell frequency of CD27⁺γδ cells was inversely correlated with relapse (P = 0.006) and CMV reactivation (P = 0.05). We conclude that clinical outcome after allogeneic hematopoietic cell transplantation is influenced by the proportions of distinct γδ cell subsets in the stem cell graft. We also provide evidence that CD8⁺γδ cells are potentially alloreactive and may play a role in acute graft versus host disease. This study illustrates the importance of better understanding of the role of distinct subsets of γδ cells in allogeneic hematopoietic cell transplantation.