Human allo-reactive CD4+ T cells as strong mediators of anti-tumor immunity in NOD/scid mice engrafted with human acute lymphoblastic leukemia

Human leukocyte antigen evolutionary divergence influences outcomes of paediatric patients and young adults affected by malignant disorders given allogeneic haematopoietic stem cell transplantation from unrelated donors

High genetic heterogeneity in the human leukocyte antigen (HLA) increases the likelihood of efficient immune response to pathogens and tumours. As measure of HLA diversity, HLA evolutionary divergence (HED) has been shown to predict the response of tumours to immunotherapy and haematopoietic stem cell transplantation (HSCT) in adults. We retrospectively investigated the association of HED with outcomes of 153 paediatric/young adults patients, treated for malignant disorders with HSCT from 9-10/10 HLA-matched unrelated donors. HED was calculated as pairwise genetic distance between alleles in patient HLA-A, -B, -C, -DRB1, -DQB1 and -DPB1, using the locus median to stratify patients with 'high' or 'low' HED. Patients with high HED-B and -DRB1 showed significantly improved disease-free survival (DFS), especially when combined (70.8% vs 53.7% p = 0.008). High HED-B + -DRB1 was also associated with improved overall survival (OS) (82.1 vs 66.4% p = 0.014), and concomitant reduction of non-relapse-mortality (5.1% vs 21.1% p = 0.006). The impact on OS and DFS of combined HED-B + -DRB1 was confirmed in multivariate analysis [hazard ratio (HR) 0.39, p = 0.009; and HR 0.45, p = 0.007 respectively]. Only high HED scores for HLA-DPB1 were associated, in univariate analysis, with reduced incidence of relapse (15.9% vs 31.1%, p = 0.03). These results support HED as prognostic marker in allogeneic HSCT and, if confirmed in larger cohorts, would allow its use to inform clinical risk and potentially influence clinical practice.

Immune landscape after allo-HSCT: TIGIT- and CD161-expressing CD4 T cells are associated with subsequent leukemia relapse

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the most effective treatment for selected patients with acute myeloid leukemia (AML) and relies on a "graft-versus-leukemia" effect (GVL) where donor T lymphocytes mediate control of malignant cell growth. However, relapse remains the major cause of death after allo-HSCT. In various malignancies, several immunoregulatory mechanisms have been shown to restrain antitumor immunity, including ligand-mediated engagement of inhibitory receptors (IRs) on effector cells, and induction of immunosuppressive cell subsets, such as regulatory T cells (Tregs) or myeloid-derived suppressor cells (MDSCs). Relapse after HSCT remains a major therapeutic challenge, but immunoregulatory mechanisms involved in restraining the GVL effect must be better deciphered in humans. We used mass cytometry to comprehensively characterize circulating leukocytes in 2 cohorts of patients after allo-HSCT. We first longitudinally assessed various immunoregulatory parameters highlighting specific trends, such as opposite dynamics between MDSCs and Tregs. More generally, the immune landscape was stable from months 3 to 6, whereas many variations occurred from months 6 to 12 after HSCT. Comparison with healthy individuals revealed that profound alterations in the immune equilibrium persisted 1 year after HSCT. Importantly, we found that high levels of TIGIT and CD161 expression on CD4 T cells at month 3 after HSCT were distinct features significantly associated with subsequent AML relapse in a second cross-sectional cohort. Altogether, these data provide global insights into the reconstitution of the immunoregulatory landscape after HSCT and highlight non-canonical IRs associated with relapse, which could open the path to new prognostic tools or therapeutic targets to restore subverted anti-AML immunity.

High preharvest donor Foxp3 mRNA level predicts late relapse of acute lymphoblastic leukaemia after haematopoietic stem cell transplantation

Objectives

The curative effect of allogeneic haematopoietic stem cell transplantation (HSCT) for acute leukaemia is due in part to the donor T cell–mediated graft‐versus‐leukaemia immune reaction (GvL). Several studies have suggested that donor CD25+CD4+Foxp3+regulator T cells (Tregs) may decrease graft‐versus‐host disease (GvHD) without abrogating GVL. This notion may need modification in acute lymphoblastic leukaemia (ALL).

Methods

Foxp3 mRNA level was measured by qPCR in preharvest donor blood CD4+ T cells. The study comprised 45 patients with ALL in 1st or 2nd CR who received myeloablative HSCT using T‐replete bone marrow grafts.

Results

Relapse occurred in 17 patients median 363 days after HSCT. The relapse risk was estimated by Cox univariate and multivariate proportional hazard regression. The proportionality assumption was met by analysing the preharvest donor Foxp3 mRNA level as a time‐dependent covariate. Early relapse was not modified by the Foxp3 mRNA level. However, a higher Foxp3 mRNA level was associated with a significantly increased relapse risk after day 363 after transplantation, compatible with inhibition of GvL. In contrast, a higher preharvest donor CD4+ T‐cell concentration was associated with reduced relapse risk.

Conclusion

A higher preharvest donor Foxp3 mRNA level may be predictive of late ALL relapse after HSCT.

HLA Evolutionary Divergence as a Prognostic Marker for AML Patients Undergoing Allogeneic Stem Cell Transplantation

The diversity of human leukocyte antigens (HLAs) enables the presentation of immense repertoires of peptides, including tumor-associated antigens (TAAs). As a surrogate for immunopeptidome diversity, the HLA evolutionary divergence (HED) between individual HLA alleles might directly define the ability to present TAAs, a prerequisite for graft-versus-leukemia effects. We therefore analyzed the impact of HED on survival within a cohort of 171 acute myeloid leukemia (AML) patients after matched donor allogeneic hematopoietic stem cell transplantation (HSCT). Low HED (<25th percentile) of HLA class I (HED^{class I}) or HLA-DR antigens (HED^DR) was a strong determinant for adverse overall survival after allogeneic HSCT (OS), with a hazard ratio for death of 1.9 (95% CI 1.2–3.2) and 2.1 (95% CI 1.3–3.4), respectively. Defining a cutoff value for the combined HED^total (HED^{class I} and HED^DR), the respective 5 year OS was 29.7% and 64.9% in patients with low and high HED^total (p < 0.001), respectively. Furthermore, the risk of relapse was significantly higher in patients with low HED^total (hazard ratio (HR) 2.2, 95% CI 1.3–3.6) and event-free survival (EFS) was significantly reduced (5 year EFS 25.7% versus 54.4%, p < 0.001). We here introduce HED, a fundamental metric of immunopeptidome diversity, as a novel prognostic factor for AML patients undergoing allogeneic HSCT.

HLA-DPB1 Reactive T Cell Receptors for Adoptive Immunotherapy in Allogeneic Stem Cell Transplantation

HLA-DPB1 antigens are mismatched in about 80% of allogeneic hematopoietic stem cell transplantations from HLA 10/10 matched unrelated donors and were shown to be associated with a decreased risk of leukemia relapse. We recently developed a reliable in vitro method to generate HLA-DPB1 mismatch-reactive CD4 T-cell clones from allogeneic donors. Here, we isolated HLA-DPB1 specific T cell receptors (TCR DP) and used them either as wild-type or genetically optimized receptors to analyze in detail the reactivity of transduced CD4 and CD8 T cells toward primary AML blasts. While both CD4 and CD8 T cells showed strong AML reactivity in vitro, only CD4 T cells were able to effectively eliminate leukemia blasts in AML engrafted NOD/SCID/IL2Rγc^−/− (NSG) mice. Further analysis showed that optimized TCR DP and under some conditions wild-type TCR DP also mediated reactivity to non-hematopoietic cells like fibroblasts or tumor cell lines after HLA-DP upregulation. In conclusion, T cells engineered with selected allo-HLA-DPB1 specific TCRs might be powerful off-the-shelf reagents in allogeneic T-cell therapy of leukemia. However, because of frequent (common) cross-reactivity to non-hematopoietic cells with optimized TCR DP T cells, safety mechanisms are mandatory.

S1PR5 regulates NK cell responses in preventing graft-versus-host disease while preserving graft-versus-tumour activity in a murine allogeneic haematopoietic stem cell transplantation model

Graft-versus-host disease (GVHD) remains a major complication following allogeneic haematopoietic stem cell transplantation (allo-HSCT) leading to high transplant-related mortality. Natural killer (NK) cells have been found to mitigate GVHD without attenuating the graft-versus-tumour (GVT) activity in the murine model of haematopoietic stem cell transplantation. Sphingosine-1-phosphate receptor 5 (S1PR5) is a very important chemokine receptor on NK cells that governs NK cell distribution in vivo and trafficking at lesion sites. Our preliminary studies showed that the incidence of GVHD was negatively correlated with S1PR5 expression in the NK cells of patients after allo-HSCT. In the present study, we found that S1PR5 deficiency in murine NK cells blocked the migration of NK cells from the bone marrow to the GVHD target organs and attenuated the inhibitory effects on the alloreactive T cells, especially CD3⁺ CD8⁺ T cells, which may be the reason why the loss of S1PR5 in NK cells could aggravate GVHD in recipient mice. Furthermore, we also demonstrated that the absence of S1PR5 expression in NK cells did not interfere with the antitumour effects of NK cells and T cells in vivo. Taken together, our data indicate that S1PR5 plays an essential role in balancing GVHD and GVT activity.

Graft versus tumor effects and why people relapse

Graft-versus-tumor (GVT) reactivity mediated by donor T cells in the context of allogeneic stem cell transplantation (alloSCT) is one of the most potent forms of cellular immunotherapy. The antitumor effect against hematologic malignancies is mediated by a polyclonal T-cell response targeting polymorphic antigens expressed on hematopoietic tissues of the recipient, leaving donor hematopoiesis in the patient after transplantation unharmed. Fortunately, hematopoietic tissues (including malignant hematopoietic cell populations) are relatively susceptible to T-cell recognition. If, however, nonhematopoietic tissues of the recipient are targeted as well, graft-versus-host disease (GVHD) will occur. The balance between GVT and GVHD is influenced by the genetic disparity between donor and recipient, the number and origin of professional antigen-presenting cells provoking the immune response, the target antigen specificity, magnitude and diversity of the response, and the in vivo inflammatory environment, whereas inhibitory factors may silence the immune response. Manipulation of each of these factors will determine the balance between GVT and GVHD.

In-vitro blockade of the CD4 receptor co-signal in antigen-specific T-cell stimulation cultures induces the outgrowth of potent CD4 independent T-cell effectors

T-cell receptor (TCR) redirected T cells are promising tools for adoptive cancer immunotherapy. Since not only CD8 but also CD4 T cells are key players for efficient antitumor responses, the targeted redirection of both subsets with the same antigen-specific TCR comes more and more into focus. Although rapidly evolving technologies enable the reliable genetic re-programming of T cells, the limited availability of TCRs that induce T-cell activation in both T-cell subsets without CD4/CD8 co-receptor contribution hampers the broad application of this approach. We developed a novel stimulation approach, which drives the activation and proliferation of CD4 T-cell populations capable of inducing effector functions in a CD4-independent manner. Naive-enriched CD4 T cells were stimulated against dendritic cells (DC) expressing allogeneic HLA-DP antigens upon RNA transfection and CD4/HLA interactions were blocked by the addition of CD4 binding antibody. Evolving CD4 T-cell populations were specifically activated independent of the CD4 co-signal and induced strong TCR-mediated IFN-γ secretion as well as cytolysis upon recognition of leukemia cells expressing HLA-DP antigen. Our novel stimulation approach may facilitate the generation of CD4 T cells as source for co-receptor independent TCRs for future immunotherapies.

Graft predominance after double umbilical cord blood transplantation: a review

Several parameters are involved in graft predominance after double umbilical cord blood transplantation (dUCBT), of which T-cell alloreactivity between the grafts is now considered to be the major denominator. We recently showed that alloreactive CD4+ T-cells originating from the predominant CBU recognize HLA-class II allele mismatches and can readily be detected in the majority of patients. In addition, it was shown that HLA-class II allele-specific CD4+ T-cells were able to recognize primary leukemic cells when the mismatched HLA-class II allele was shared between the rejected CBU and the patient. These results further underscored the role of alloreactive T-cells, notably class II specific CD4+ T-cells, in graft-versus-graft reactions and in graft-versus-leukemia after dUCBT.

CD4+ T-cell alloreactivity toward mismatched HLA class II alleles early after double umbilical cord blood transplantation

Although double umbilical cord blood transplantation (dUCBT) in adult patients may be associated with less graft failure compared with single UCBT, hematopoietic recovery generally originates from a single cord blood unit (CBU). CBU predominance is still incompletely understood. We recently showed that blood CD4⁺ T-cell numbers rapidly increase after dUCBT, and early CD4⁺ T-cell chimerism predicts for graft predominance. Given the frequent HLA class II allele mismatches between CBUs in dUCBT, we hypothesized that alloreactive HLA class II-specific CD4⁺ T cells from the "winning" CBU may contribute to rejection of the "loser" CBU. We evaluated whether CD4⁺ T cells originating from the predominant (PD)-CBU would recognize HLA class II allele mismatches, expressed by the nonengrafting (NE)-CBU. Alloreactive effector CD4⁺ T cells toward 1 or more mismatched HLA class II alleles of the NE-CBU were detected in 11 of 11 patients, with reactivity toward 29 of 33 (88%) tested mismatches, and the strongest reactivity toward DR and DQ alleles early after dUCBT. Mismatched HLA class II allele-specific CD4⁺ T cells recognized primary leukemic cells when the mismatched HLA class II allele was shared between NE-CBU and patient. Our results suggest that cytotoxicity exerted by CD4⁺ T cells from the PD-CBU drives the rapid rejection of the NE-CBU, whose alloreactive effect might also contribute to graft-versus-leukemia.

Patient-individualized CD8⁺ cytolytic T-cell therapy effectively combats minimal residual leukemia in immunodeficient mice

Adoptive transfer of donor-derived cytolytic T-lymphocytes (CTL) has evolved as a promising strategy to improve graft-versus-leukemia (GvL) effects in allogeneic hematopoietic stem-cell transplantation. However, durable clinical responses are often hampered by limited capability of transferred T cells to establish effective and sustained antitumor immunity in vivo. We therefore analyzed GvL responses of acute myeloid leukemia (AML)-reactive CD8(+) CTL with central and effector memory phenotype in a new allogeneic donor-patient specific humanized mouse model. CTL lines and clones obtained upon stimulation of naive CD45RA(+) donor CD8(+) T cells with either single HLA antigen-mismatched or HLA-matched primary AML blasts, respectively, elicited strong leukemia reactivity during cytokine-optimized short to intermediate (i.e., 2-8 weeks) culture periods. Single doses of CTL were intravenously infused into NOD/scidIL2Rcg(null) mice when engraftment with patient AML reached bone marrow infiltration of 1-5%, clinically defining minimal residual disease status. This treatment resulted in complete regression of HLA-mismatched and strong reduction of HLA-matched AML infiltration, respectively. Most importantly, mice receiving AML-reactive CTL showed significantly prolonged survival. Transferred CTL were detectable in murine bone marrow and spleen and demonstrated sustained AML-reactivity ex vivo. Moreover, injections with human IL-15 clearly promoted CTL persistence. In summary, we show that naive donor-derived CD8(+) CTL effectively combat patient AML blasts in immunodeficient mice. The donor-patient specific humanized mouse model appears suitable to evaluate therapeutic efficacy of AML-reactive CTL before adoptive transfer into patients. It may further help to identify powerful leukemia rejection antigens and T-cell receptors for redirecting immunity to leukemias even in a patient-individualized manner.

Naive tumour-specific CD4+ T cells were efficiently primed in acute lymphoblastic leukaemia

The recognition and neutralization of tumour cells is one of the big challenges in immunity. The immune system has to recognize syngeneic tumour cells and has to be primed and respond in an adequate manner. Priming of a leukaemia-specific immune response is a crucial step in tumour immunology that can mislead to tumour tolerance either by T cell ignorance, deletion or Treg induction. To resemble the situation of acute lymphoblastic leukaemia (ALL) in patients, we used the murine BALB/c model with syngeneic BM185 tumour cells. We established a tumour cell line that expresses the neo-antigen ovalbumin (BM185-OVA/GFP) to allow the application of T cell receptor transgenic, antigen-specific CD4(+) T cells. Here, we demonstrate that effective anti-ALL immunity can be established by in vivo priming of CD4(+) T cells that is sufficient to differentiate into effector cells. Yet they failed to control tumour alone, but initiated a Th1 response. An efficient tumour clearance was dependent on both antigen-specific CD4(+) T cells and CD8(+) effector T cells from the endogenous repertoire. The tolerogeneic milieu was characterized by increased Tregs numbers and elevated IL-10 level. Tregs hamper effective antitumour immune response, but their depletion did not result in reduced tumour growth. In contrast, neutralization of IL-10 improved median mouse survival. Future therapies should focus on establishing a strong CD4+ T cells response, either by adjuvant or by adoptive transfer.

Rapid induction of single donor chimerism after double umbilical cord blood transplantation preceded by reduced intensity conditioning: results of the HOVON 106 phase II study

Double umbilical cord blood transplantation is increasingly applied in the treatment of adult patients with high-risk hematological malignancies and has been associated with improved engraftment as compared to that provided by single unit cord blood transplantation. The mechanism of improved engraftment is, however, still incompletely understood as only one unit survives. In this multicenter phase II study we evaluated engraftment, early chimerism, recovery of different cell lineages and transplant outcome in 53 patients who underwent double cord blood transplantation preceded by a reduced intensity conditioning regimen. Primary graft failure occurred in one patient. Engraftment was observed in 92% of patients with a median time to neutrophil recovery of 36 days (range, 15-102). Ultimate single donor chimerism was established in 94% of patients. Unit predominance occurred by day 11 after transplantation and early CD4(+) T-cell chimerism predicted for unit survival. Total nucleated cell viability was also associated with unit survival. With a median follow up of 35 months (range, 10-51), the cumulative incidence of relapse and non-relapse mortality rate at 2 years were 39% and 19%, respectively. Progressionfree survival and overall survival rates at 2 years were 42% (95% confidence interval, 28-56) and 57% (95% confidence interval, 43-70), respectively. Double umbilical cord blood transplantation preceded by a reduced intensity conditioning regimen using cyclophosphamide/fludarabine/4 Gy total body irradiation results in a high engraftment rate with low non-relapse mortality. Moreover, prediction of unit survival by early CD4(+) lymphocyte chimerism might suggest a role for CD4(+) lymphocyte mediated unit-versus-unit alloreactivity. www.trialregister.nl NTR1573.

Allogeneic transplantation as anticancer immunotherapy

Allogeneic stem cell transplantation (AlloSCT) utilizes HLA-matched donor bone marrow or peripheral blood stem cell grafts to reconstitute haematopoiesis and immunity in patients with bone marrow failure or hematological malignancies. It is now clear that much of the anti-cancer effect of AlloSCT is due to the ability of engrafting donor derived lymphocyte populations to eradicate residual malignant clones, through a phenomenon known as the graft-versus-tumor (GVT) effect. Recognition of the importance of GVT in the long-term control of cancer has allowed substantial reductions in the pre-transplant conditioning intensity, leading to the development of reduced-intensity or even non-myeloablative transplant regimens in some patient groups. These reduced intensity regimens still allow donor cell engraftment and GVT, whilst reducing the morbidity and mortality associated with traditional myeloablative conditioning. Through clinical observations and experimental models of AlloSCT substantial insights have been provided into the mechanisms of immunological control of malignancy even outside the setting of AlloSCT, providing an opportunity to duplicate these anti-cancer mechanisms via non-allogeneic immunotherapies.

Collateral damage of nonhematopoietic tissue by hematopoiesis-specific T cells results in graft-versus-host disease during an ongoing profound graft-versus-leukemia reaction

After allogeneic stem cell transplantation (allo-SCT), donor T cells may recognize minor histocompatibility antigens (MiHA) specifically expressed on cells of the recipient. It has been hypothesized that T cells recognizing hematopoiesis-restricted MiHA mediate specific graft-versus-leukemia (GVL) activity without inducing graft-versus-host disease (GVHD), whereas T cells recognizing ubiquitously expressed MiHA induce both GVL and GVHD reactivity. It also has been hypothesized that alloreactive CD4 T cells are capable of mediating specific GVL reactivity due to the hematopoiesis-restricted expression of HLA class II. However, clinical observations suggest that an overt GVL response, associated with expansion of T cells specific for hematopoiesis-restricted antigens, is often associated with GVHD reactivity. Therefore, we developed in vitro models to investigate whether alloreactive T cells recognizing hematopoiesis-restricted antigens induce collateral damage to surrounding nonhematopoietic tissues. We found that collateral damage to MiHA-negative fibroblasts was induced by misdirection of cytotoxic granules released from MiHA-specific T cells activated by MiHA-positive hematopoietic cells, resulting in granzyme-B-mediated activation of apoptosis in the surrounding fibroblasts. We demonstrated that direct contact between the activated T cell and the fibroblast is a prerequisite for this collateral damage to occur. Our data suggest that hematopoiesis-restricted T cells actively participate in an overt GVL response and may contribute to GVHD via induction of collateral damage to nonhematopoietic targets.

CD4(+)and CD8(+)T-cell reactions against leukemia-associated- or minor-histocompatibility-antigens in AML-patients after allogeneic SCT

T-cells play an important role in the remission-maintenance in AML-patients (pts) after SCT, however the role of LAA- (WT1, PR1, PRAME) or minor-histocompatibility (mHag, HA1) antigen-specific CD4(+) and CD8(+)T-cells is not defined. A LAA/HA1-peptide/protein stimulation, cloning and monitoring strategy for specific CD8(+)/CD4(+)T-cells in AML-pts after SCT is given. Our results show that (1) LAA-peptide-specific CD8+T-cells are detectable in every AML-pt after SCT. CD8(+)T-cells, recognizing two different antigens detectable in 5 of 7 cases correlate with long-lasting remissions. Clonal TCR-Vβ-restriction exemplarily proven by spectratyping in PRAME-specific CD8(+)T-cells; high PRAME-peptide-reactivity was CD4(+)-associated, as shown by IFN-γ-release. (2) Two types of antigen-presenting cells (APCs) were tested for presentation of LAA/HA1-proteins to CD4(+)T-cells: miniEBV-transduced lymphoblastoid cells (B-cell-source) and CD4-depleted MNC (source for B-cell/monocyte/DC). We provide a refined cloning-system for proliferating, CD40L(+)CD4(+)T-cells after LAA/HA1-stimulation. CD4(+)T-cells produced cytokines (GM-CSF, IFN-γ) upon exposure to LAA/HA1-stimulation until after at least 7 restimulations and demonstrated cytotoxic activity against naive blasts, but not fibroblasts. Antileukemic activity of unstimulated, stimulated or cloned CD4(+)T-cells correlated with defined T-cell-subtypes and the clinical course of the disease. In conclusion we provide immunological tools to enrich and monitor LAA/HA1-CD4(+)- and CD8(+)T-cells in AML-pts after SCT and generate data with relevant prognostic value. We were able to demonstrate the presence of LAA-peptide-specific CD8(+)T-cell clones in AML-pts after SCT. In addition, we were also able to enrich specific antileukemic reactive CD4(+)T-cells without GvH-reactivity upon repeated LAA/HA1-protein stimulation and limiting dilution cloning.

Exchange of cytosolic content between T cells and tumor cells activates CD4 T cells and impedes cancer growth

BACKGROUND

T cells are known to participate in the response to tumor cells and react with cytotoxicity and cytokine release. At the same time tumors established versatile mechanisms for silencing the immune responses. The interplay is far from being completely understood. In this study we show contacts between tumor cells and lymphocytes revealing novel characteristics in the interaction of T cells and cancer cells in a way not previously described.

METHODS/ FINDINGS

Experiments are based on the usage of a hydrophilic fluorescent dye that occurs free in the cytosol and thus transfer of fluorescent cytosol from one cell to the other can be observed using flow cytometry. Tumor cells from cell lines of different origin or primary hepatocellular carcinoma (HCC) cells were incubated with lymphocytes from human and mice. This exposure provoked a contact dependent uptake of tumor derived cytosol by lymphocytes--even in CD4⁺ T cells and murine B cells--which could not be detected after incubation of lymphocytes with healthy cells. The interaction was a direct one, not requiring the presence of accessory cells, but independent of cytotoxicity and TCR engagement. Electron microscopy disclosed 100-200 nm large gaps in the cell membranes of connected cells which separated viable and revealed astonishing outcome. While the lymphocytes were induced to proliferate in a long term fashion, the tumor cells underwent a temporary break in cell division. The in vitro results were confirmed in vivo using a murine acute lymphoblastic leukemia (ALL) model. The arrest of tumor proliferation resulted in a significant prolonged survival of challenged mice.

CONCLUSIONS

The reported cell-cell contacts reveal new characteristics i.e. the enabling of cytosol flow between the cells including biological active proteins that influence the cell cycle and biological behaviour of the recipient cells. This adds a completely new aspect in tumor induced immunology.

HLA class II upregulation during viral infection leads to HLA-DP-directed graft-versus-host disease after CD4+ donor lymphocyte infusion

CD8+ T cell-depleted (TCD) donor lymphocyte infusion (DLI) after TCD allogeneic hematopoietic stem cell transplantation (alloSCT) has been associated with a reduced risk of graft-versus-host disease (GVHD) while preserving conversion to donor hematopoiesis and antitumor immunity, providing a rationale for exploring CD4+ T cell-based immunotherapy for hematologic malignancies. Here, we analyzed the clinical course and specificity of T cell immune responses in 2 patients with acute myeloid leukemia (AML) who converted to full-donor chimerism but developed severe acute GVHD after prophylactic CD4+ DLI after 10/10-HLA-matched, but HLA-DPB1-mismatched TCD-alloSCT. Clonal analysis of activated T cells isolated during GVHD demonstrated allo-reactivity exerted by CD4+ T cells directed against patient-mismatched HLA-DPB1 molecules on hematopoietic cells and skin-derived fibroblasts only when cultured under inflammatory conditions. At the time of CD4+ DLI, both patients contained residual patient-derived T cells, including cytomegalovirus (CMV)-specific T cells as a result of CMV reactivations. Once activated by CMV antigens, these CMV-specific T cells could stimulate HLA-DPB1-specific CD4+ T cells, which in turn could target nonhematopoietic tissues in GVHD. In conclusion, our data demonstrate that GVHD after HLA-DPB1-mismatched CD4+ DLI can be mediated by allo-reactive HLA-DPB1-directed CD4+ T cells and that ongoing viral infections inducing HLA class II expression on nonhematopoietic cells may increase the likelihood of GVHD development. This trial is registered at http://www.controlled-trials.com/ISRCTN51398568/LUMC as #51398568.

Zebrafish xenografts as a tool for in vivo studies on human cancer

The zebrafish has become a powerful vertebrate model for genetic studies of embryonic development and organogenesis and increasingly for studies in cancer biology. Zebrafish facilitate the performance of reverse and forward genetic approaches, including mutagenesis and small molecule screens. Moreover, several studies report the feasibility of xenotransplanting human cells into zebrafish embryos and adult fish. This model provides a unique opportunity to monitor tumor-induced angiogenesis, invasiveness, and response to a range of treatments in vivo and in real time. Despite the high conservation of gene function between fish and humans, concern remains that potential differences in zebrafish tissue niches and/or missing microenvironmental cues could limit the relevance and translational utility of data obtained from zebrafish human cancer cell xenograft models. Here, we summarize current data on xenotransplantation of human cells into zebrafish, highlighting the advantages and limitations of this model in comparison to classical murine models of xenotransplantation.

Endogenous HLA class II epitopes that are immunogenic in vivo show distinct behavior toward HLA-DM and its natural inhibitor HLA-DO

CD4+ T cells play a central role in adaptive immunity. The acknowledgment of their cytolytic effector function and the finding that endogenous antigens can enter the HLA class II processing pathway make CD4+ T cells promising tools for immunotherapy. Expression of HLA class II and endogenous antigen, however, does not always correlate with T-cell recognition. We therefore investigated processing and presentation of endogenous HLA class II epitopes that induced CD4+ T cells during in vivo immune responses. We demonstrate that the peptide editor HLA-DM allowed antigen presentation of some (DM-resistant antigens) but abolished surface expression of other natural HLA class II epitopes (DM-sensitive antigens). DM sensitivity was shown to be epitope specific, mediated via interaction between HLA-DM and the HLA-DR restriction molecule, and reversible by HLA-DO. Because of the restricted expression of HLA-DO, presentation of DM-sensitive antigens was limited to professional antigen-presenting cells, whereas DM-resistant epitopes were expressed on all HLA class II–expressing cells. In conclusion, our data provide novel insights into the presentation of endogenous HLA class II epitopes and identify intracellular antigen processing and presentation as a critical factor for CD4+ T-cell recognition. This opens perspectives to exploit selective processing capacities as a new approach for targeted immunotherapy.

Patient HLA-DP-specific CD4+ T cells from HLA-DPB1-mismatched donor lymphocyte infusion can induce graft-versus-leukemia reactivity in the presence or absence of graft-versus-host disease

Clinical studies have demonstrated that HLA-DPB1-mismatched allogeneic stem cell transplantation (allo-SCT) is associated with a decreased risk of disease relapse and an increased risk of graft-versus-host disease (GVHD) compared with HLA-DPB1-matched SCT. In T cell-depleted allo-SCT, mismatching of HLA-DPB1 was not associated with an increased risk of severe GVHD, but a significant decreased risk of disease relapse was still observed. To investigate whether patient HLA-DP-specific CD4(+) T cell responses were frequently induced after T cell-depleted HLA-DPB1-mismatched allo-SCT and donor lymphocyte infusion (DLI), we developed a method to screen for the presence of HLA-DP-specific CD4(+) T cells using CD137 as an activation marker and analyzed 24 patient-donor combinations. The patients suffered from various B cell malignancies, multiple myeloma, and myeloid leukemias. Patient HLA-DP-specific CD4(+) T cells were detected after DLI in 13 of 18 patients who exhibited a clinical response to DLI, compared with only 1 of 6 patients without a clinical response to DLI. Eight patients developed significant GVHD. These data show that patient HLA-DP-specific CD4(+) T cells frequently occur after HLA-DPB1-mismatched T cell-depleted allo-SCT and DLI, and are associated with graft-versus-leukemia reactivity both in the presence and absence of GVHD.