Effect of Graft-versus-Host Disease Prophylaxis Regimens on T and B Cell Reconstitution after Allogeneic Hematopoietic Stem Cell Transplantation.

Lymphocyte reconstitution is pivotal for successful long-term outcome after allogeneic hematopoietic stem cell transplantation (HSCT), and conditioning regimen and post-transplantation immunosuppression are risk factors for prolonged immunodeficiency. Nevertheless, the effects of different immunosuppressive protocols on lymphocyte output and replicative capacity have not been investigated. Here we assessed T cell receptor excision circles (TREC), kappa-deleting recombination excision circles (KREC), and T cell telomere length (TL) as proxy markers for immune reconstitution in patients in a prospective randomized trial comparing graft-versus-host disease (GVHD) prophylaxis after transplantation (cyclosporine/methotrexate versus tacrolimus/sirolimus; n = 200). Results showed that medians of TREC, KREC, and TL were not significantly different between the prophylaxis groups at any assessment time point during follow-up (24 months), but the kinetics of TREC, KREC, and TL were significantly influenced by other transplantation-related factors. Older recipient age, the use of antithymocyte globulin before graft infusion, and use of peripheral blood stem cell grafts were associated with lower TREC levels, whereas acute GVHD transiently affected KREC levels. Patients with lymphocyte excision circle levels above the median at ≤6 months post-transplantation had reduced transplantation-related mortality and superior 5-year overall survival (P < .05). We noticed significant T cell telomere shortening in the patient population as a whole during follow-up. Our results suggest that lymphocyte reconstitution after transplantation is not altered by different immunosuppressive protocols. This study has been registered at ClinicalTrials.gov (identifier: NCT00993343).

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

Acute graft-versus-host disease (aGVHD) is 1 of the main major complications of post-hematopoietic stem cell transplantation (HSCT). Identifying patients at risk of severe aGVHD may lead to earlier intervention and treatment, resulting in increased survival and a better quality of life. We aimed to identify biomarkers in donor grafts and patient plasma around the time of transplantation that might be predictive of aGVHD development. We build on our previously published methods by using multiplex assays and multicolor flow cytometry. We identified 5 easily assessable cellular markers in donor grafts that combined could potentially be used to calculate risk for severe aGVHD development. Most noteworthy are the T cell subsets expressing IL-7 receptor-α (CD127) and PD-1. Additionally, we identified a potential role for elevated tumor necrosis factor-α levels in both graft and patient before HSCT in development of aGVHD.

Hypogammaglobulinemia in children after allogeneic hematopoietic stem cell transplantation: a cytokine mediated immunoglobulin isotype class switch arrest?

BACKGROUND: Hypogammaglobulinemia (hypo-IgG) is common early post-HSCT in children, occasionally necessitating long-term immunoglobulin (Ig) G replacement therapy. IgG replacement may not reduce mortality, although infectious complications are decreased PROCEDURE: Clinical data and samples from 86 children were analyzed retrospectively with the aim to identify risk factors for developing long-term hypo-IgG (i.e., requiring ≥ 3 months IgG replacement) post-HSCT and studying the underlying biology. Laboratory studies covered serum cytokines, IGHG2 genotyping and routine laboratory investigations. Results were analyzed statistically. RESULTS: Forty-eight percent of the children developed long-term hypo-IgG. These children were younger (<5 years) and had higher acute GvHD incidence, but had better overall survival (88% vs. 69%, P = 0.036). Significantly lower Ig levels post-HSCT but equal immune cell recovery were seen in patients with long-term hypo-IgG compared with those of transient or no hypo-IgG. Pre-HSCT IL-6 and -7 and post-HSCT BAFF and APRIL levels were significantly higher in the long-term hypo-IgG group. CONCLUSIONS: Findings suggests an unfavorable cytokine milieu for graft-derived immune recovery, possibly inducing Ig isotype class switch arrest. Younger age, acute GvHD, and higher pre-HSCT IL-6 levels were identified as significant risk factors for long-term hypo-IgG. Long-term hypo-IgG post-HSCT does not need to be unfavorable and could be an effect of deteriorated cytokine signaling.

Long-term complications, immunologic effects, and role of passage for outcome in mesenchymal stromal cell therapy.

Thirty-one patients treated with mesenchymal stromal cells (MSCs) for acute graft-versus-host disease (aGVHD) or hemorrhagic cystitis between 2002 and 2007 were followed to investigate predictors of outcome, immunologic effects in vivo, and long-term survival. There was no correlation between in vitro suppression by MSCs in mixed lymphocyte cultures and outcome. Soluble IL-2 receptors were measured in blood before and after MSC infusion and declined significantly during the first week after MSC infusion (P = .03). Levels of interleukin-6 and HLA-G were unaffected. Infectious complications occurred several years after recovery from aGVHD. Cytomegalovirus viral load was high, and cytomegalovirus disease was common. Among patients recovering from aGVHD, 54% died of late infections, between 4 months and 2 years after MSC treatment. No increase in leukemia relapse or graft rejection was found. Children had a better survival rate than adults (P = .005). In GVHD patients, 1-year survival was 75% in patients who received early-passage MSCs (from passages 1-2) in contrast to 21% using later passage MSCs (from passages 3-4) (P < .01). We conclude that treatment with early-passage MSCs improved survival in patients with therapy-resistant GVHD. Death from infection was common in MSC-treated patients, but there was no increase in leukemia relapse.

Mesenchymal stem cells exert differential effects on alloantigen and virus-specific T-cell responses.

Mesenchymal stem cells (MSCs) suppress alloantigen-induced T-cell functions in vitro and infusion of third-party MSCs seems to be a promising therapy for graft-versus-host disease (GVHD). Little is known about the specificity of immunosuppression by MSCs, in particular the effect on immunity to pathogens. We have studied how MSCs affect T-cell responses specific to Epstein-Barr virus (EBV) and cytomegalovirus (CMV). We found that EBV- and CMV-induced proliferation and interferon-gamma (IFN-gamma) production from peripheral blood mononuclear cells (PBMCs) was less affected by third-party MSCs than the response to alloantigen and that MSCs had no effect on expansion of EBV and CMV pentamer-specific T cells. Established EBV-specific cytotoxic T cells (CTL) or CMV-CTL cultured with MSCs retained the ability to proliferate and produce IFN-gamma in response to their cognate antigen and to kill virally infected targets. Finally, PBMCs from 2 patients who received MSCs for acute GVHD showed persistence of CMV-specific T cells and retained IFN-gamma response to CMV after MSC infusion. In summary, MSCs have little effect on T-cell responses to EBV and CMV, which contrasts to their strong immunosuppressive effects on alloreactive T cells. These data have major implications for immunotherapy of GVHD with MSCs and suggest that the effector functions of virus-specific T cells may be retained after MSC infusion.

Low serum levels of total rabbit-IgG is associated with acute graft-versus-host disease after unrelated donor hematopoietic stem cell transplantation: results from a prospective study.

In a prospective study we determined rabbit-IgG (r-IgG) levels in serum samples before (day 0) and after (days 1 and 7) unrelated donor hematopoietic stem cell transplantation (HSCT). Most patients suffered from a hematologic malignancy. All patients received rabbit antithymocyte globulin (ATG) as part of the conditioning for 2-4 days (2 mg/kg/day). We found a good correlation (r = 0.34, r = 0.42 and r = 0.46) between the dose of ATG and serum r-IgG levels at all 3 time points. The cumulative incidence of acute graft-versus-host disease (aGVHD) grades II-IV in patients given the 4, 6, and 8 mg/kg ATG dose was 50%, 34%, and 15% (P = .04), respectively. In patients with r-IgG < or = 70 microg/mL (n = 54) the cumulative incidence of grades II-IV aGVHD was 33% compared with only 6% in those with r-IgG >70 microg/mL (n = 18), P = .023. Low serum levels of r-IgG seem to be a strong predictor for aGVHD grades II-IV in patients treated with Thymoglobulin before unrelated donor HSCT.

Mesenchymal stem cells for treatment of steroid-resistant, severe, acute graft-versus-host disease: a phase II study.

BACKGROUND: Severe graft-versus-host disease (GVHD) is a life-threatening complication after allogeneic transplantation with haemopoietic stem cells. Mesenchymal stem cells modulate immune responses in vitro and in vivo. We aimed to assess whether mesenchymal stem cells could ameliorate GVHD after haemopoietic-stem-cell transplantation. METHODS: Patients with steroid-resistant, severe, acute GVHD were treated with mesenchymal stem cells, derived with the European Group for Blood and Marrow Transplantation ex-vivo expansion procedure, in a multicentre, phase II experimental study. We recorded response, transplantation-related deaths, and other adverse events for up to 60 months' follow-up from infusion of the cells. FINDINGS: Between October, 2001, and January, 2007, 55 patients were treated. The median dose of bone-marrow derived mesenchymal stem cells was 1.4x10(6) (min-max range 0.4-9x10(6)) cells per kg bodyweight. 27 patients received one dose, 22 received two doses, and six three to five doses of cells obtained from HLA-identical sibling donors (n=5), haploidentical donors (n=18), and third-party HLA-mismatched donors (n=69). 30 patients had a complete response and nine showed improvement. No patients had side-effects during or immediately after infusions of mesenchymal stem cells. Response rate was not related to donor HLA-match. Three patients had recurrent malignant disease and one developed de-novo acute myeloid leukaemia of recipient origin. Complete responders had lower transplantation-related mortality 1 year after infusion than did patients with partial or no response (11 [37%] of 30 vs 18 [72%] of 25; p=0.002) and higher overall survival 2 years after haemopoietic-stem-cell transplantation (16 [53%] of 30 vs four [16%] of 25; p=0.018). INTERPRETATION: Infusion of mesenchymal stem cells expanded in vitro, irrespective of the donor, might be an effective therapy for patients with steroid-resistant, acute GVHD.

Persistence of human parvovirus B19 in multipotent mesenchymal stromal cells expressing the erythrocyte P antigen: implications for transplantation.

Multipotent mesenchymal stromal cells (MSCs) are used to improve the outcome of hematopoietic stem cell transplantation (HCST) and in regenerative medicine. MSCs may harbor persistent viruses that may compromise their clinical benefit, however. Retrospectively screened, 1 of 20 MSCs from healthy donors contained parvovirus B19 (B19) DNA. MSCs express the B19 receptor (P antigen/globoside) and a co-receptor (Ku 80) and can transmit B19 to bone marrow cells in vitro, suggesting that the virus can persist in the marrow stroma of healthy individuals. Two patients undergoing HSCT received the B19-positive MSCs as treatment for graft-versus-host disease; neither developed viremia nor symptomatic B19 infection. These findings demonstrate for the first time that persistent B19 in MSCs can infect hematopoietic stem cells and underscore the importance of monitoring B19 transmission by MSC products.

Expansion of Gammadelta T Cells from Cord Blood: A Therapeutical Possibility.

Gammadelta (γδ) T cells are found in both blood and tissues and have antiviral and antitumor properties. The frequency of γδ T cells in umbilical cord blood (UCB) is low, and the majority express δ1, in contrast to blood, whereas the main subset is δ2γ9 T cells. UCB γδ T cells are functionally immature, which together with their scarcity complicates the development of UCB γδ T cell therapies. We aimed to develop an effective expansion protocol for UCB γδ T cells based on zoledronate and IL-2. We found that culture with 5 µM zoledronate and 200 IU IL-2/ml medium for 14 days promoted extensive proliferation. The majority of the cultured cells were γ9δ2 T cells. The fold expansion of this, originally infrequent, subset was impressive (median and maximum fold change 253 and 1085, resp.). After culture, the cells had a polyclonal γδ T cell repertoire and the main memory subset was central memory (CD45RO+ CD27+). The cells produced cytokines such as IL-1B, IL-2, and IL-8 and displayed significant tumor-killing capacity. These results show that development of in vitro expanded UCB γδ T cell therapies is feasible. It could prove a valuable treatment modality for patients after umbilical cord blood transplantation.

No alloantibodies against mesenchymal stromal cells, but presence of anti-fetal calf serum antibodies, after transplantation in allogeneic hematopoietic stem cell recipients.

BACKGROUND AND OBJECTIVES: Mesenchymal stromal cells (MSC) may be used in cellular therapy to treat graft-versus-host-disease and autoimmune disorders, and in regenerative medicine. Preliminary data suggest limited cellular allogeneic rejection, but less is known about humoral responses. The objective of this study was to investigate whether antibodies against MSC were present after hematopoietic stem cell transplantation (HSCT) including treatment with matched or mismatched allogeneic MSC. DESIGN AND METHODS: Twelve patients were evaluated using flow cytometric cross matches (FCXM) and enzyme-linked immunosorbent assays. Expression of blood group antigens, regarded as alloantigens giving rise to humoral alloimmunity, on MSC were explored using flow cytometry and immunofluorescence. RESULTS: Three of 12 patients exhibited late positivity in the FCXM. In absorption studies, antibodies directed against fetal calf serum (FCS), a component of the MSC culture medium, were identified. Healthy individuals expressed varying levels of anti-FCS antibodies and the same pattern was seen in immunosuppressed HSCT patients. MSC did not express blood group antigens. The patients with positive FCXM are alive and well. INTERPRETATION AND CONCLUSIONS: We have shown that immunosuppressed patients can exhibit anti-FCS antibodies, but no alloantibodies, that may bind to MSC. These antibodies seem clinically insignificant.

Characterization of infiltrating lymphocytes in human benign and malignant prostate tissue.

Immune checkpoint blockade has shown promising results in numerous cancer types. However, in prostate cancer (PC), absent or limited responses have been reported. To investigate further, we compared the phenotype of infiltrating T-cells isolated from prostate tissue from patients with PC (n = 5), benign prostatic hyperplasia (BPH) (n = 27), BPH with concurrent PC (n = 4) and controls (n = 7). The majority of T-cells were CD8+ and had a CCR7-CD45RO+ effector memory phenotype. However, the yield of T-cells isolated from PC lesions was on average 20-fold higher than that obtained from control prostates. Furthermore, there were differences between the prostate conditions regarding the percentage of T-cells expressing several activation markers and co-inhibitory receptors. In conclusion, many prostate-infiltrating T-cells express co-inhibitory receptors PD-1 and LAG-3, regardless of prostate condition. Despite the observed increase in counts and percentages of PD-1+ T-cells in PC, the concomitant demonstration of high percentage of PD-1+ T-cells in control prostates suggests that PD-1 may play a role in controlling the homeostasis of the prostate rather than in contributing to PC-associated immune-suppression. Thus, PD-1 may not be a good candidate for checkpoint blockade in PC and these data are relevant for evaluation of clinical trials and in designing future immunotherapeutic approaches of PC.

Combining Flow and Mass Cytometry in the Search for Biomarkers in Chronic Graft-versus-Host Disease.

Chronic graft-versus-host disease (cGVHD) is a debilitating complication arising in around half of all patients treated with an allogeneic hematopoietic stem cell transplantation. Even though treatment of severe cGVHD has improved during recent years, it remains one of the main causes of morbidity and mortality in affected patients. Biomarkers in blood that could aid in the diagnosis and classification of cGVHD severity are needed for the development of novel treatment strategies that can alleviate symptoms and reduce the need for painful and sometimes complicated tissue biopsies. Methods that comprehensively profile complex biological systems such as the immune system can reveal unanticipated markers when used with the appropriate methods of data analysis. Here, we used mass cytometry, flow cytometry, enzyme-linked immunosorbent assay, and multiplex assays to systematically profile immune cell populations in 68 patients with varying grades of cGVHD. We identified multiple subpopulations across T, B, and NK-cell lineages that distinguished patients with cGVHD from those without cGVHD and which were associated in varying ways with severity of cGVHD. Specifically, initial flow cytometry demonstrated that patients with more severe cGVHD had lower mucosal-associated T cell frequencies, with a concomitant higher level of CD38 expression on T cells. Mass cytometry could identify unique subpopulations specific for cGVHD severity albeit with some seemingly conflicting results. For instance, patients with severe cGVHD had an increased frequency of activated B cells compared to patients with moderate cGVHD while activated B cells were found at a reduced frequency in patients with mild cGVHD compared to patients without cGVHD. Moreover, results indicate it may be possible to validate mass cytometry results with clinically viable, smaller flow cytometry panels. Finally, no differences in levels of blood soluble markers could be identified, with the exception for the semi-soluble combined marker B-cell activating factor/B cell ratio, which was increased in patients with mild cGVHD compared to patients without cGVHD. These findings suggest that interdependencies between such perturbed subpopulations of cells play a role in cGVHD pathogenesis and can serve as future diagnostic and therapeutic targets.

Mesenchymal stem cells for treatment of therapy-resistant graft-versus-host disease.

BACKGROUND: Mesenchymal stem cells (MSC) have immunomodulatory effects. The aim was to study the effect of MSC infusion on graft-versus-host disease (GVHD). METHODS: We gave MSC to eight patients with steroid-refractory grades III-IV GVHD and one who had extensive chronic GVHD. The MSC dose was median 1.0 (range 0.7 to 9)x10(6)/kg. No acute side-effects occurred after the MSC infusions. Six patients were treated once and three patients twice. Two patients received MSC from HLA-identical siblings, six from haplo-identical family donors and four from unrelated mismatched donors. RESULTS: Acute GVHD disappeared completely in six of eight patients. One of these developed cytomegalovirus gastroenteritis. Complete resolution was seen in gut (6), liver (1) and skin (1). Two died soon after MSC treatment with no obvious response. One of them had MSC donor DNA in the colon and a lymph node. Five patients are still alive between 2 months and 3 years after the transplantation. Their survival rate was significantly better than that of 16 patients with steroid-resistant biopsy-proven gastrointestinal GVHD, not treated with MSC during the same period (P = 0.03). One patient treated for extensive chronic GVHD showed a transient response in the liver, but not in the skin and he died of Epstein-Barr virus lymphoma. CONCLUSION: MSC is a very promising treatment for severe steroid-resistant acute GVHD.

Long-Term Stable Mixed Chimerism after Hematopoietic Stem Cell Transplantation in Patients with Non-Malignant Disease, Shall We Be Tolerant?

Long-term stable mixed chimerism is a rare and poorly understood phenomenon post hematopoietic stem cell transplantation. This study aims to shed light on whether the two hematopoietic systems in patients with mixed chimerism remain functional. Additionally, we investigate possible immunologic differences in these individuals compared to patients with only donor derived immune cells. Patients with donor and mixed chimerism, at median 10 (5-16) years post-HSCT for non-malignant diseases, were assessed regarding clinical situation and immune system (phenotypical and functional). No difference in long-term outcome was seen in terms of general wellbeing, central phenotypic immune system features (e.g., differentiation status, CD4/CD8 ratio, B and NK-cell frequency) and antibody responses to immunizations. At a median of 10 years post transplantation, patients with mixed chimerism had significantly higher IgG3 and platelet levels. Additionally, these patients had higher NKT-cell levels (CD94+CD8+ and CD56+CD8+) than patients with donor chimerism. In depth phenotypic analysis of patients with mixed chimerism demonstrated recipient-derived fractions in most immune cell lineages (e.g., T-cell, B-cell and NK-cell subsets). Recipient cells were also capable of responding to mitogenic stimulation with production of several cytokines. In conclusion, long-term mixed chimerism did not negatively affect patient wellbeing and long-term outcome. Moreover, recipient-derived immunity may still be functional in these patients, suggesting an active state of tolerance and immunologic dependence on both hematopoietic systems.

Progression of benign prostatic hyperplasia is associated with pro-inflammatory mediators and chronic activation of prostate-infiltrating lymphocytes.

Benign prostatic hyperplasia (BPH) is a common chronic non-malignant condition whose prevalence substantially increases with age. Immune cell infiltration and pro-inflammatory mediators have been implicated in the pathogenesis. Here, we characterized 21 extracellular markers on prostate-infiltrating lymphocytes (PILs) and analyzed expression of 26 soluble proteins in prostate tissue obtained from BPH patients (n = 31). These data were correlated with clinical parameters and compared with peripheral blood mononuclear cells (PBMCs) (n = 10). Increased frequencies of T cells expressing co-inhibitory receptors LAG-3, PD-1, TIM-3 or CTLA-4, and co-stimulatory receptors CD28, OX40 or 4-1BB were observed in BPH tissue compared to PBMCs. These findings are consistent with chronic activation and possible functional exhaustion of PILs that may be further augmented by several identified pro-inflammatory factors, such as IL-8 and MCP-1, promoting inflammation and chemotaxis of immune cells to the prostate. Prostate size and plasma prostate-specific antigen levels positively correlated with IL-8 and MCP-1 concentrations, and frequencies of T cells expressing CTLA-4 and TIM-3. It remains to be established whether the link between inflammation and BPH progression supported by our findings reflects a progressive failure of the immune system leading to decreased immune surveillance and development of prostate cancer.

Treatment of severe acute graft-versus-host disease with third party haploidentical mesenchymal stem cells.

Adult bone-marrow-derived mesenchymal stem cells are immunosuppressive and prolong the rejection of mismatched skin grafts in animals. We transplanted haploidentical mesenchymal stem cells in a patient with severe treatment-resistant grade IV acute graft-versus-host disease of the gut and liver. Clinical response was striking. The patient is now well after 1 year. We postulate that mesenchymal stem cells have a potent immunosuppressive effect in vivo.

Decreased serum levels of clara cell secretory protein (CC16) are associated with bronchiolitis obliterans and may permit early diagnosis in patients after allogeneic stem-cell transplantation.

BACKGROUND: Bronchiolitis obliterans (BO) is a common complication and is associated with high mortality after allogeneic stem-cell transplantation (SCT). Early diagnosis of BO may improve outcome. Low levels of Clara cell secretory protein (CC16) have previously been associated with BO in lung transplant recipients. METHODS: Serum samples were collected from eight patients with BO, eight patients with chronic graft-versus-host disease (GVHD), and eight control patients with neither BO nor chronic GVHD in a matched patient analysis. Patients were matched for diagnosis, conditioning, donor match, and GVHD prophylaxis. Another seven patients with BO were also analyzed separately. CC16 was measured with an enzyme-linked immunosorbent assay method. RESULTS: In the matched analysis, eight patients were diagnosed with BO at a median of 11.5 months (range, 4-13 months) after SCT and in non-matched BO patients at a median of 12 months (range, 9-36 months). In the matched patient analysis, patients with BO had significantly lower (P=0.03) or decreasing (P=0.02) levels of CC16 compared with patients with only chronic GVHD or controls. In the matched patient analysis, measurement of CC16 showed a sensitivity of 88% and a specificity of 81%. With the criteria of low levels of CC16 or a decrease of more than 40% compared with the previous sample, BO was detected with analysis of CC16 in 13 of 15 patients. In 11 of the 13 patients, low or decreasing values of CC16 were detected at a median of 10 months (range, 1-30 months) before BO was diagnosed clinically. CONCLUSIONS: Low levels of CC16 are associated with BO after allogeneic SCT. Monitoring of CC16 in serum after SCT may have potential as an early marker for BO.

Granulocyte colony-stimulating factor affects serum levels of soluble interleukin-2 receptors after allogeneic stem cell transplantation.

Soluble interleukin-2 receptor (sIL-2R) levels were analyzed in 127 stem-cell transplant recipients. Granulocyte-colony stimulating factor (G-CSF) was given to 57 patients after transplantation. We found an association between G-CSF and increased sIL-2R levels. This indicates increased T-cell activation and may be one reason for the previously found increased incidence of acute graft-versus-host disease in G-CSF-treated patients.

Rabbit-immunoglobulin G levels in patients receiving thymoglobulin as part of conditioning before unrelated donor stem cell transplantation.

BACKGROUND AND OBJECTIVES: The role of serum concentrations of rabbit antithymoglobulin (ATG) in the development of acute graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (HSCT) with unrelated donors is unknown. DESIGN AND METHODS: We determined the serum concentration of rabbit immunoglobulin-G (IgG) using an enzyme linked immunosorbent assay in 61 patients after unrelated donor HSCT. The doses of ATG ranged between 4 and 10 mg/kg. The conditioning consisted mainly of cyclophosphamide and total body irradiation or busulfan. Most patients received GVHD prophylaxis with cyclosporine and methotrexate. RESULTS: The rabbit IgG levels varied widely in each dose group. The levels of rabbit IgG gradually declined and could still be detected up to five weeks after HSCT. We found a correlation between the grade of acute GVHD and the concentration of rabbit IgG in serum before the transplantation (p=0.017). Patients with serum levels of rabbit IgG >70 mg/mL before HSCT ran a very low risk of developing acute GVHD grades II-IV, as compared to those with levels <70 mg/mL (11% vs. 48%, p=0.006). INTERPRETATION AND CONCLUSIONS: The measurement of rabbit IgG levels in patients receiving ATG as prophylaxis against GVHD after HSCT may be of value in lowering the risk of severe GVHD.

Mesenchymal stem cells inhibit the formation of cytotoxic T lymphocytes, but not activated cytotoxic T lymphocytes or natural killer cells.

BACKGROUND: Mesenchymal stem cells (MSCs) can reduce the incidence of graft-versus-host disease because of their ability to inhibit T-lymphocyte proliferation. There are no publications on the effect that MSCs have on cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells, effector cells vital for the graft-versus-leukemia effect. METHODS: Cytotoxic T cells were primed in mixed lymphocyte culture (MLC) against irradiated stimulator lymphocytes, and irradiated third-party MSCs were added at different time points. The CTLs were collected, and their cytotoxic potential was analyzed in a chromium-release assay against the same stimulator cells as in the MLC. Purified NK cells were mixed with irradiated MSCs, and the lysis was measured in chromium-release assay against K562 target cells. RESULTS: We found that MSCs inhibited CTL-mediated lysis by 70% if added at the beginning of the 6-day MLC. The lysis was not affected on day 3 or in the cytotoxic phase. Furthermore, MSCs inhibited the formation of cytotoxic lymphocytes when the cells were separated in a transwell system, which indicates that the effect is mediated by a soluble factor. NK cell-mediated lysis of K562 cells was not inhibited by MSCs. MSCs did not induce proliferation of allogeneic lymphocytes, and they were not lysed by allogeneic CTLs or NK cells. CONCLUSION: Our findings indicate that MSCs escape recognition by CTLs and alloreactive NK cells, and inhibit the formation of cytotoxic T cells by secreting a soluble factor, but that they do not interfere with CTLs and NK cell lysis.