Myeloid-derived suppressor cells (MDSCs) have emerged as a heterogeneic immunoregulatory population that can expand in response to inflammatory signals. Predominantly studied in cancer, MDSCs suppress T cells utilizing various mechanisms. In allogeneic hematopoietic stem cell transplantation (allo-HSCT) therapy-related toxicity and alloreactivity increase inflammatory cytokines that might favor an MDSC accumulation. To address this question, circulating CD14 þ HLA-DR low/neg cells were studied retrospectively in 51 allo-HSCT patients. These cells represent one of the few well-described human MDSC subsets under physiological and pathological conditions. The frequency of CD14 þ HLA-DR low/neg cells was significantly increased after allo-HSCT, especially in patients with acute graft-versus-host disease. Compared to healthy donor cells they were pSTAT1 low (phosphorylated signal transducer and activator of transcription) and indoleamine 2,3-dioxygenase (IDO) high. Serum levels of granulocyte colony-stimulating factor and interleukin-6, which both have been linked to MDSC induction, correlated positively with the frequency of CD14 þ HLA-DR low/neg cells. In vitro dysfunction of patient T cells, such as reduced proliferative capacity or CD3z-chain expression, was rescued by blocking the IDO activity of CD14 þ HLA-DR low/neg cells. Overall, we identified a T-cell-suppressive monocytic population that expands after allo-HSCT. The mechanisms responsible for such accumulation remain to be elucidated. It will be of great interest to prospectively investigate the influence of these cells on the graft-versus-tumor and-host reaction.

IL-33 is a new member of the IL-1 family, which plays a crucial role in inflammatory response, enhancing the differentiation of dendritic cells and alternatively activated macrophages (AAM). Based on the evidence of IL-33 expression in bone, we hypothesized that IL-33 may shift the balance from osteoclast to AAM differentiation and protect from inflammatory bone loss. Using transgenic mice overexpressing human TNF, which develop spontaneous joint inflammation and cartilage destruction, we show that administration of IL-33 or an IL-33R (ST2L) agonistic Ab inhibited cartilage destruction, systemic bone loss, and osteoclast differentiation. Reconstitution of irradiated hTNFtg mice with ST2−/− bone marrow led to more bone loss compared with the chimeras with ST2+/+ bone marrow, demonstrating an important endogenous role of the IL-33/ST2L pathway in bone turnover. The protective effect of IL-33 on bone was accompanied by a significant increase of antiosteoclastogenic cytokines (GM-CSF, IL-4, and IFN-γ) in the serum. In vitro IL-33 directly inhibits mouse and human M-CSF/receptor activator for NF-κB ligand-driven osteoclast differentiation. IL-33 acts directly on murine osteoclast precursors, shifting their differentiation toward CD206+ AAMs via GM-CSF in an autocrine fashion. Thus, we show in this study that IL-33 is an important bone-protecting cytokine and may be of therapeutic benefit in treating bone resorption.

Regulatory T cells (Tregs) play an important role in the maintenance of immune tolerance to self-antigens and are involved in modulating immune responses in autoimmunity, transplant rejection, and tumor immunity. Recently, a novel subset of TCR-αβ(+) CD4(-) CD8(-) (double negative, DN) T cells has been described to specifically suppress T-cell responses in mice. Here, we demonstrate that human DN T cells are highly potent suppressors of both CD4(+) and CD8(+) T-cell responses. In contrast to naturally occurring CD4(+) CD25(+) Tregs, DN T cells have to be activated by antigen-presenting cells (APCs) to induce their regulatory potential. The suppressive activity of DN T cells is neither mediated indirectly by modulation of APCs nor by competition for T-cell growth factors. Furthermore, DN T-cell-mediated suppression toward responder T cells is TCR dependent and requires novel protein synthesis. In contrast to murine DN T cells, which eliminate effector T cells via Fas/FasL or perforin/granzyme, human DN T cells suppress proliferation of responder T cells by cell contact-dependent mechanisms. Taken together, our data indicate that human DN T cells exert strong immunosuppressive effects on both CD4(+) and CD8(+) T cells and may serve as a new therapeutic approach to treat autoimmunity and transplant rejection.

Upon specific interaction with APCs, T cells capture membrane fragments and surface molecules in a process termed trogocytosis. In this study, we demonstrate that human Ag-specific CD8+ T cells acquire the coinhibitory molecule programmed death ligand 1 (PD-L1) from mature dendritic cells (mDC) and tumor cells in an Ag-specific manner. Immature dendritic cells were less effective in transferring surface molecules onto CD8+ T cells than mDCs. Interestingly, trogocytosis of PD-L1 requires cell–cell contact and cannot be induced by uptake of soluble proteins obtained from mDC lysates. The transfer process is impaired by inhibition of vacuolar ATPases in T cells as well as by fixation of dendritic cells. Of importance, CD8+ T cells that acquired PD-L1 complexes were able to induce apoptosis of neighboring programmed death 1–expressing CD8+ T cells. In summary, our data demonstrate that human CD8+ T cells take up functionally active PD-L1 from APCs in an Ag-specific fashion, leading to fratricide of programmed death 1–expressing, neighboring T cells. The transfer of functionally active coinhibitory molecules from APCs onto human CD8+ T cells could have a regulatory role in immune responses.

Soluble extracellular proteins usually do not enter the endogenous human leukocyte antigen (HLA) I–dependent presentation pathway of antigen-presenting cells, strictly impeding their applicability for the re-stimulation of protein-specific CD8+ cytotoxic T lymphocytes (CTL). Here we present for the Epstein-Barr virus (EBV) BZLF1 a novel strategy that facilitates protein translocation into antigen-presenting cells by its solubilisation in high molar urea and subsequent pulsing of cells in presence of low molar urea. Stimulation of PBMC from HLA-matched EBV-seropositive individuals with urea-treated BZLF1 but not untreated BZLF1 induces an efficient reactivation of BZLF1-specific CTL. Urea-treated BZLF1 (uBZLF1) enters antigen-presenting cells in a temperature-dependent manner by clathrin-mediated endocytosis and is processed by the proteasome into peptides that are bound to nascent HLA I molecules. Dendritic cells and monocytes but also B cells can cross-present uBZLF1 in vitro. The strategy described here has potential for use in the development of improved technologies for the monitoring of protein-specific CTL.

Delayed reconstitution of the T cell compartment in recipients of allogeneic stem cell grafts is associated with an increase of reactivation of latent viruses. Thereby, the transplanted T cell repertoire appears to be one of the factors that affect T cell reconstitution. Therefore, we studied the T cell receptor beta (TCRβ) gene rearrangements of flow cytometry–sorted CD4+ and CD8+ T cells from the peripheral blood of 23 allogeneic donors before G-CSF administration and on the day of apheresis. For this purpose, TCRβ rearrangements were amplified by multiplex PCR followed by high-throughput amplicon sequencing. Overall, CD4+ T cells displayed a significantly higher TCRβ diversity compared to CD8+ T cells irrespective of G-CSF administration. In line, no significant impact of G-CSF treatment on the TCR Vβ repertoire usage was found. However, correlation of the donor T cell repertoire with clinical outcomes of the recipient revealed that a higher CD4+ TCRβ diversity after G-CSF treatment is associated with lower reactivation of cytomegalovirus and Epstein–Barr virus. By contrast, no protecting correlation was observed for CD8+ T cells. In essence, our deep TCRβ analysis identifies the importance of the CD4+ T cell compartment for the control of latent viruses after allogeneic stem cell transplantation.

BackgroundA major complication after allogeneic hematopoietic stem cell transplantation (aSCT) is the reactivation of herpesviruses such as cytomegalovirus (CMV) and Epstein–Barr virus (EBV). Both viruses cause significant mortality and compromise quality of life after aSCT. Preventive transfer of virus-specific T cells can suppress reactivation by re-establishing functional antiviral immune responses in immunocompromised hosts.MethodsWe have developed a good manufacturing practice protocol to generate CMV/EBV-peptide-stimulated T cells from leukapheresis products of G-CSF mobilized and non-mobilized donors. Our procedure selectively expands virus-specific CD8+ und CD4+ T cells over 9 days using a generic pool of 34 CMV and EBV peptides that represent well-defined dominant T-cell epitopes with various HLA restrictions. For HLA class I, this set of peptides covers at least 80% of the European population.ResultsCMV/EBV-specific T cells were successfully expanded from leukapheresis material of both G-CSF mobilized and non-mobilized donors. The protocol allows administration shortly after stem cell transplantation (d30+), storage over liquid nitrogen for iterated applications, and protection of the stem cell donor by avoiding a second leukapheresis.ConclusionOur protocol allows for rapid and cost-efficient production of T cells for early transfusion after aSCT as a preventive approach. It is currently evaluated in a phase I/IIa clinical trial.Electronic supplementary materialThe online version of this article (10.1186/s12967-018-1498-3) contains supplementary material, which is available to authorized users.

In the version of this article initially published, the middle name of Katharina Marie Habenicht was misspelled (Maria), and has been amended in the HTML and PDF versions of the article.