scholarly journals The differential immune responses to COVID-19 in peripheral and lung revealed by single-cell RNA sequencing

2020 ◽  
Author(s):  
Gang Xu ◽  
Furong Qi ◽  
Hanjie Li ◽  
Qianting Yang ◽  
Haiyan Wang ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Single Cell ◽  
Mononuclear Cells ◽  
Immune Cell ◽  
Close Association ◽  
Suppressor Cells ◽  
T Cell Subsets ◽  
Peripheral Blood Mononuclear ◽  
Peripheral T Cells

Understanding the mechanism that leads to immune dysfunction induced by SARS-CoV2 virus is crucial to develop treatment for severe COVID-19. Here, using single cell RNA-seq, we characterized the peripheral blood mononuclear cells (PBMC) from uninfected controls and COVID-19 patients, and cells in paired broncho-alveolar lavage fluid (BALF). We found a close association of decreased dendritic cells (DC) and increased monocytes resembling myeloid-derived suppressor cells (MDSC) which correlated with lymphopenia and inflammation in the blood of severe COVID-19 patients. Those MDSC-like monocytes were immune-paralyzed. In contrast, monocyte-macrophages in BALFs of COVID-19 patients produced massive amounts of cytokines and chemokines, but secreted little interferons. The frequencies of peripheral T cells and NK cells were significantly decreased in severe COVID-19 patients, especially for innate-like T and various CD8+ T cell subsets, compared to health controls. In contrast, the proportions of various activated CD4+ T cell subsets, including Th1, Th2 and Th17-like cells were increased and more clonally expanded in severe COVID-19 patients. Patients' peripheral T cells showed no sign of exhaustion or augmented cell death, whereas T cells in BALFs produced higher levels of IFNG, TNF, CCL4 and CCL5 etc. Paired TCR tracking indicated abundant recruitment of peripheral T cells to the patients' lung. Together, this study comprehensively depicts how the immune cell landscape is perturbed in severe COVID-19.

scholarly journals The differential immune responses to COVID-19 in peripheral and lung revealed by single-cell RNA sequencing

2020 ◽  
Vol 6 (1) ◽  
Author(s):  
Gang Xu ◽  
Furong Qi ◽  
Hanjie Li ◽  
Qianting Yang ◽  
Haiyan Wang ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Single Cell ◽  
Rna Sequencing ◽  
Mononuclear Cells ◽  
Immune Cell ◽  
Close Association ◽  
T Cell Subsets ◽  
Peripheral T Cells ◽  
Single Cell Rna Sequencing

Abstract Understanding the mechanism that leads to immune dysfunction in severe coronavirus disease 2019 (COVID-19) is crucial for the development of effective treatment. Here, using single-cell RNA sequencing, we characterized the peripheral blood mononuclear cells (PBMCs) from uninfected controls and COVID-19 patients and cells in paired broncho-alveolar lavage fluid (BALF). We found a close association of decreased dendritic cells (DCs) and increased monocytes resembling myeloid-derived suppressor cells (MDSCs), which correlated with lymphopenia and inflammation in the blood of severe COVID-19 patients. Those MDSC-like monocytes were immune-paralyzed. In contrast, monocyte-macrophages in BALFs of COVID-19 patients produced massive amounts of cytokines and chemokines, but secreted little interferons. The frequencies of peripheral T cells and NK cells were significantly decreased in severe COVID-19 patients, especially for innate-like T and various CD8+ T cell subsets, compared to healthy controls. In contrast, the proportions of various activated CD4+ T cell subsets among the T cell compartment, including Th1, Th2, and Th17-like cells were increased and more clonally expanded in severe COVID-19 patients. Patients’ peripheral T cells showed no sign of exhaustion or augmented cell death, whereas T cells in BALFs produced higher levels of IFNG, TNF, CCL4, CCL5, etc. Paired TCR tracking indicated abundant recruitment of peripheral T cells to the severe patients’ lung. Together, this study comprehensively depicts how the immune cell landscape is perturbed in severe COVID-19.

scholarly journals Global immune characterization of HBV/HCV-related hepatocellular carcinoma identifies macrophage and T-cell subsets associated with disease progression

2020 ◽  
Vol 6 (1) ◽  
Author(s):  
Guohe Song ◽  
Yang Shi ◽  
Meiying Zhang ◽  
Shyamal Goswami ◽  
Saifullah Afridi ◽  
...  
Keyword(s):  
Hepatocellular Carcinoma ◽  
T Cells ◽  
T Cell ◽  
Single Cell ◽  
Immune Cells ◽  
Immune Cell ◽  
T Cell Subsets ◽  
M2 Macrophage ◽  
Advanced Hcc ◽  
Immune Cell Subsets

AbstractDiverse immune cells in the tumor microenvironment form a complex ecosystem, but our knowledge of their heterogeneity and dynamics within hepatocellular carcinoma (HCC) still remains limited. To assess the plasticity and phenotypes of immune cells within HBV/HCV-related HCC microenvironment at single-cell level, we performed single-cell RNA sequencing on 41,698 immune cells from seven pairs of HBV/HCV-related HCC tumors and non-tumor liver tissues. We combined bio-informatic analyses, flow cytometry, and multiplex immunohistochemistry to assess the heterogeneity of different immune cell subsets in functional characteristics, transcriptional regulation, phenotypic switching, and interactions. We identified 29 immune cell subsets of myeloid cells, NK cells, and lymphocytes with unique transcriptomic profiles in HCC. A highly complex immunological network was shaped by diverse immune cell subsets that can transit among different states and mutually interact. Notably, we identified a subset of M2 macrophage with high expression of CCL18 and transcription factor CREM that was enriched in advanced HCC patients, and potentially participated in tumor progression. We also detected a new subset of activated CD8+ T cells highly expressing XCL1 that correlated with better patient survival rates. Meanwhile, distinct transcriptomic signatures, cytotoxic phenotypes, and evolution trajectory of effector CD8+ T cells from early-stage to advanced HCC were also identified. Our study provides insight into the immune microenvironment in HBV/HCV-related HCC and highlights novel macrophage and T-cell subsets that could be further exploited in future immunotherapy.

scholarly journals Functional biomarker signatures of circulating T-cells and its association with distinct clinical status of leprosy patients and their respective household contacts

2020 ◽  
Author(s):  
Pedro Henrique Ferreira Marçal ◽  
Rafael Silva Gama ◽  
Lorena Bruna de Oliveira Pereira ◽  
Olindo Assis Martins Filho ◽  
Roberta Olmo Pinheiro ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Mononuclear Cells ◽  
Clinical Status ◽  
T Cell Subsets ◽  
Diagnostic Tools ◽  
Peripheral Blood Mononuclear ◽  
Household Contacts ◽  
Leprosy Patients

Abstract Background: Leprosy is a chronic infectious disease classified into two subgroups for therapeutic purposes: paucibacillary (PB) and multibacillary (MB), closely related to the host immune responses. In this context it is noteworthy looking for immunological biomarkers applicable as complementary diagnostic tools as well as a laboratorial strategy to detect subclinical leprosy in household contacts. Methods: The main goal of the present study was to characterize the global cytokine signatures of CD4+ and CD8+ T-cells from leprosy patients with distinct clinical forms and their respective household contacts (HHC) upon in vitro antigen-specific stimuli. Short-term culture of peripheral blood mononuclear cells was done. After incubation, cells were harvested and prepared for surface and intracytoplasmic cytokine staining Results: The cytokine signature analysis demonstrated that leprosy patients presented a polyfunctional profile of T-cells subsets, with increased frequency of IFN-g+ T-cell subsets along with IL-10+ and IL-4+ from CD4+ T-cells. Moreover, L(PB) displayed a polyfunctional profile characterized by enhanced percentage of IFN-g+, IL-10+ and IL-4+ produced by most T-cell subsets, as compared to L(MB) that presented a more restricted cytokine functional profile mediated by IL-10+ and IL-4+ T-cells with minor contribution of IFN-g produced by CD4+ T-cells. Noteworthy was that HHC(MB) exhibited enhanced frequency of IFN-g+ T-cells, contrasting with HHC(PB) that presented a cytokine profile limited to IL-10 and IL-4. Conclusions: Together, our findings provide additional immunological features associated with leprosy and household contacts. These data provide evidence that biomarkers of immune response can be useful complementary diagnostic/prognostic tools as well as insights that household contacts may present subclinical infection.

Immunological correlates of response to immune checkpoint inhibitors (ICI) in metastatic urothelial carcinoma (mUC) patients (pts).

2018 ◽  
Vol 36 (6_suppl) ◽  
pp. 454-454
Author(s):  
Alice Tzeng ◽  
C. Marcela Diaz-Montero ◽  
Patricia A. Rayman ◽  
Jin Sub Kim ◽  
Paul G Pavicic ◽  
...  
Keyword(s):  
T Cells ◽  
Peripheral Blood ◽  
Mixed Model ◽  
Mononuclear Cells ◽  
Immune Cell ◽  
Checkpoint Inhibitors ◽  
Suppressor Cells ◽  
Peripheral Blood Mononuclear ◽  
Blood Samples ◽  
Never Smokers

454 Background: Identification of biomarkers predictive of response to ICI could help guide treatment (tx) decisions. We assessed the correlation between PD1/PDL1 expression in key immunomodulatory subsets (myeloid-derived suppressor cells [MDSC]; CD8+ T cells) and tx response in mUC pts treated with ICI. Methods: Serial peripheral blood samples were collected from mUC pts treated with ICI. Flow cytometry was used to quantify PD1/PDL1 expression in MDSC (CD33+HLADR−) and CD8+ T cells (CD8+CD4−) from live peripheral blood mononuclear cells. MDSC were subdivided into monocytic (M)-MDSC (CD14+CD15−), polymorphonuclear (PMN)-MDSC (CD14− CD15+), and immature (I)-MDSC (CD14− CD15−). Mixed-model regression and Wilcoxon rank-sum tests were performed to assess post-ICI changes in immune marker expression and identify correlations between PD1/PDL1 expression and best overall response (BOR) to ICI. Results: Of 36 ICI-treated pts with ≥2 blood samples, 24 received anti-PDL1 (22 atezolizumab/2 avelumab; [A]) and 12 received anti-PD1 (pembrolizumab [P]). 78% were men, median age 69 (46–81), 28% never smokers, 19% had prior intravesical BCG, 39% prior neoadjuvant chemotherapy, and 64% prior cystectomy. BOR to ICI included 3 PR/14 SD/7 PD (A) and 1 CR/2 PR/6 SD/3 PD (P). Successive doses of A correlated with decreased %PDL1+ M-MDSC (mean change −5.26/dose; p = 0.009), while those of P correlated with decreased %PD1+ M- and I- MDSC (mean change −1.55 and −1.14/dose; p = 0.04 and 0.02, respectively). Though pre-tx %PD1+ CD8+ T cells did not predict BOR, greater PD1 expression by CD8+ T cells within 12 weeks after ICI initiation correlated with BOR (Table). Conclusions: ICI tx correlated with distinct changes in PD1/PDL1 expression by specific peripheral immune cell subsets. Responders to ICI had higher % of PD1+ CD8+ T cells after ICI than non-responders, though pre-tx % were comparable between groups. Further validation of these and other potential blood/tissue biomarkers is ongoing. [Table: see text]

scholarly journals CD4+T cells in ageing-associated interstitial lung abnormalities show evidence of pro-inflammatory phenotypic and functional profile

Thorax ◽  
2020 ◽  
pp. thoraxjnl-2020-215520
Author(s):  
Carlos Machahua ◽  
Ivette Buendia-Roldan ◽  
Ranferi Ocaña-Guzman ◽  
María Molina-Molina ◽  
Annie Pardo ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Cd4 T Cells ◽  
Mononuclear Cells ◽  
Killer Cell ◽  
T Cell Subsets ◽  
Effector Memory ◽  
Peripheral Blood Mononuclear ◽  
Lung Abnormalities ◽  
Immune Deficiencies

BackgroundInterstitial lung abnormalities (ILA) occur in around 10% of subjects over 60 years, and are associated with a higher rate of all-cause mortality. The pathogenic mechanisms are unclear, and the putative contribution of alterations in the immune response has not been explored. Normal ageing is associated with immune deficiencies, including Naïve T-cell decrease and greater expression of the proliferative-limiting, co-inhibitory receptor killer-cell lectin-like receptor G1 (KLRG1).ObjectiveTo evaluate the frequency and activation state of different T-cell subpopulations in ILA subjects.MethodsPeripheral blood mononuclear cells were obtained from 15 individuals with ILA, 21 age-matched controls and 28 healthy young subjects. T-cells phenotype was characterised by flow cytometry, and proliferation and activation by stimulation with anti-CD3/anti-CD28 or phorbol myristate acetate/ionomycin; KLRG1 isoforms were evaluated by western blot and cytokines were quantified by ELISA and Multiplex.ResultsA significant increase of Naïve CD4+T cells together with a decrease of central and effector memory CD4+T cells was observed in ILA compared with age-matched controls. CD4+T cells from ILA subjects exhibited greater basal proliferation, which raised after anti-CD3/anti-CD28 stimulation. Additionally, a significant increase in the levels of interleukin-6 and interferon gamma was observed in isolated CD4+T cells and plasma of ILA subjects. They also displayed fewer KLRG1+/CD4+T cells with an increase of circulating E-cadherin, the ligand of KLRG1+. No changes were observed with CD8+T cell subsets.ConclusionCD4+T cells from ILA subjects are highly proliferative and show an excessive functional activity, likely related to the loss of KLRG1 expression, which may contribute to an inflammatory state and the development of ILA.

scholarly journals Deoxynivalenol Affects Proliferation and Expression of Activation-Related Molecules in Major Porcine T-Cell Subsets

Toxins ◽  
2019 ◽  
Vol 11 (11) ◽  
pp. 644 ◽  
Author(s):  
Vatzia ◽  
Pierron ◽  
Saalmüller ◽  
Mayer ◽  
Gerner
Keyword(s):  
T Cells ◽  
T Cell ◽  
T Cell Activation ◽  
Cd8 T Cells ◽  
Cell Activation ◽  
Mononuclear Cells ◽  
Animal Feed ◽  
Γδ T Cells ◽  
T Cell Subsets ◽  
Peripheral Blood Mononuclear

The Fusarium mycotoxin deoxynivalenol (DON) contaminates animal feed worldwide. In vivo, DON modifies the cellular protein synthesis, thereby also affecting the immune system. However, the functional consequences of this are still ill-defined. In this study, peripheral blood mononuclear cells from healthy pigs were incubated with different DON concentrations in the presence of Concanavalin A (ConA), a plant-derived polyclonal T-cell stimulant. T-cell subsets were investigated for proliferation and expression of CD8α, CD27, and CD28, which are involved in activation and costimulation of porcine T cells. A clear decrease in proliferation of all ConA-stimulated major T-cell subsets (CD4+, CD8+, and γδ T cells) was observed in DON concentrations higher than 0.4 µM. This applied in particular to naïve CD4+ and CD8+ T cells. From 0.8 μM onwards, DON induced a reduction of CD8α (CD4+) and CD27 expression (CD4+ and CD8+ T cells). CD28 expression was diminished in CD4+ and CD8+ T cells at a concentration of 1.6 µM DON. None of these effects were observed with the DON-derivative deepoxy-deoxynivalenol (DOM-1) at 16 µM. These results indicate that DON reduces T-cell proliferation and the expression of molecules involved in T-cell activation, providing a molecular basis for some of the described immunosuppressive effects of DON.

scholarly journals Individual Immune-Modulatory Capabilities of MSC-Derived Extracellular Vesicle (EV) Preparations and Recipient-Dependent Responsiveness

2019 ◽  
Vol 20 (7) ◽  
pp. 1642 ◽  
Author(s):  
Lambros Kordelas ◽  
Esther Schwich ◽  
Robin Dittrich ◽  
Peter Horn ◽  
Dietrich Beelen ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Mononuclear Cells ◽  
Therapeutic Option ◽  
T Cell Response ◽  
Cytokine Profile ◽  
T Cell Subsets ◽  
Effector Memory ◽  
Peripheral Blood Mononuclear ◽  
Vitro Assay

Treatment with extracellular vesicles (EVs) derived from mesenchymal stem/stromal cells (MSCs) have been suggested as novel therapeutic option in acute inflammation-associated disorders due to their immune-modulatory capacities. As we have previously observed differences in the cytokine profile of independent MSC-EV preparations, functional differences of MSC-EV preparations have to be considered. To evaluate the immune-modulatory capabilities of specific MSC-EV preparations, reliable assays are required to characterize the functionality of MSC-EV preparations prior to administration to a patient. To this end, we established an in vitro assay evaluating the immune-modulatory capacities of MSC-EV preparations. Here, we compared the efficacy of four independent MSC-EV preparations to modulate the induction of T cell differentiation and cytokine production after phorbol 12-myristate 13-acetate (PMA)/Ionomycin stimulation of peripheral blood mononuclear cells (PBMC) derived from six healthy donors. Flow cytometric analyses revealed that the four MSC-EV preparations differentially modulate the expression of surface markers, such as CD45RA, on CD4+ and CD8+ T cells, resulting in shifts in the frequencies of effector and effector memory T cells. Moreover, cytokine profile in T cell subsets was affected in a MSC-EV-specific manner exclusively in CD8+ naïve T cells. Strikingly, hierarchical clustering revealed that the T cell response towards the MSC-EV preparations largely varied among the different PBMC donors. Thus, besides defining functional activity of MSC-EV preparations, it will be crucial to test whether patients intended for treatment with MSC-EV preparations are in principal competent to respond to the envisioned MSC-EV therapy.

scholarly journals Characterization of CD4+ and CD8+ T Cell Subsets and Interferon Regulatory Factor 4 (IRF4) in MS Patients Treated with Fingolimod (FTY-720): A Follow-up Study

2018 ◽  
Author(s):  
Bahareh Laribi ◽  
Mohammad Ali Sahraian ◽  
Mehdi Shekarabi ◽  
Mohsen Marzban ◽  
Shokufeh Sadaghiani ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Peripheral Blood ◽  
Mononuclear Cells ◽  
Cd8 T Cell ◽  
T Cell Subsets ◽  
Peripheral Blood Mononuclear ◽  
Before And After ◽  
Ifn Γ ◽  
Interferon Regulatory Factor 4

Fingolimod is a novel immunomodulatory drug used in patients with relapsing multiple sclerosis (MS) which reversibly inhibits egress of lymphocytes from lymph nodes. In this longitudinal study, the frequency of Interferon- gamma (IFN-γ)+, IL4+, IL17+ and IL10+ CD4+ and CD8+ T cell subsets were measured in Fingolimod treated patients before and after 12 months’(12M) therapy using flow cytometry and compared to those of naive, Betaferon treated MS patients and healthy individuals. Additionally, the level of transcription factor IRF4 and IL-6, IL-23, TGF-β1 cytokines, required for differentiation of IL-17+ T cells, were assessed by RT-PCR and ELISA, respectively. In Fingolimod treated MS patients, we observed a significant decrease in the percentage of IFN-γ+/IL17+ CD4+ and CD8+ T cell subsets. In contrast, Fingolimod increased IL10+ CD4+ T cells. We also showed that IFN-γ+IL17+ co-producing CD8+ T cells were reduced in patients under fingolimod therapy. furthermore, Fingolimod could reduce the expression level of IRF4 in patients while IL6 was increased in the supernatant of cultured peripheral blood mononuclear cells. Our data showed that Fingolimod treatment alters CD4+ and CD8+ T cell subsets and reduces expression of IRF-4, which affects the proportion of pathogenic memory T cells in peripheral blood.

scholarly journals Identification of Human Immune Cell Subtypes Most Vulnerable to IL-1β-induced Inflammatory Signaling Using Mass Cytometry

2020 ◽  
Author(s):  
Hema Kothari ◽  
Corey M. Williams ◽  
Chantel McSkimming ◽  
Mythili Vigneshwar ◽  
Eli R. Zunder ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Mononuclear Cells ◽  
Immune Cell ◽  
T Cell Subsets ◽  
Effector Memory ◽  
High Morbidity ◽  
Cytokine Storm ◽  
Mass Cytometry ◽  
Immune Cell Subsets

ABSTRACTIL-1β has emerged as a key mediator of the cytokine storm linked to high morbidity and mortality from COVID-19 and blockade of the IL-1 receptor (IL-1R) with Anakinra has entered clinical trials in COVID-19 subjects. Yet, knowledge of the specific immune cell subsets targeted by IL-1β and IL-1β-induced signaling pathways in humans is limited. Utilizing mass cytometry (CyTOF) of human peripheral blood mononuclear cells, we identified effector memory CD4 T cells and CD4−CD8low/-CD161+ T cells as the circulating immune subtypes with the greatest expression of p-NF-κB in response to IL-1β stimulation. Notably, CCR6 distinctly identified T cells most responsive to IL-1β. Other subsets including CD11c myeloid dendritic cells (mDCs), classical monocytes (CM), two subsets of natural killer cells (CD16−CD56brightCD161− and CD16−CD56dimCD161+) and a population of lineage−(Lin-) cells expressing CD161 and CD25 also showed IL-1β-induced expression of p-NF-kB. The IL-1R antagonist, Anakinra significantly inhibited IL-1β-induced p-NF-kB in the CCR6+ T cells and CD11c mDCs with a trending inhibition in CD14 monocytes and Lin−CD161+CD25+ cells. IL-1β also induced a rapid but much less robust increase in p-p38 expression as compared to p-NF-kB in the majority of these same immune cell subsets. Prolonged IL-1β stimulation greatly increased p-STAT3 and to a much lesser extent p-STAT1 and p-STAT5 in T cell subsets, monocytes, DCs and the Lin−CD161+CD25+ cells suggesting IL-1β-induced production of downstream STAT-activating cytokines, consistent with its role in cytokine storm. Interindividual heterogeneity and inhibition of this activation by Anakinra raises the intriguing possibility that assays to measure IL-1β-induced p-NF-kB in CCR6+ T cell subtypes could identify those at higher risk of cytokine storm and those most likely to benefit from Anakinra therapy.

Both Naïve and Memory T-Cell Subsets Participate in Alloresponses to HLA-Mismatched Targets – Implications for Adoptive Transfer of Viral Antigen-Specific T Cells.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 2439-2439
Author(s):  
J. Joseph Melenhorst ◽  
Phillip Scheinberg ◽  
Ann Williams ◽  
Keyvan Keyvanfar ◽  
Melody Smith ◽  
...  
Keyword(s):  
T Cells ◽  
T Cell ◽  
Viral Antigen ◽  
Mononuclear Cells ◽  
Memory T Cells ◽  
T Cell Subsets ◽  
Dna Virus ◽  
Activated T Cells ◽  
Peripheral Blood Mononuclear ◽  
Memory T Cell

Abstract Abstract 2439 Poster Board II-416 It is generally assumed that T cell responses to HLA disparate targets arise from the naïve pool. However, the risk of graft-versus-host disease (GvHD) also increases if the donor has circulating T cells recognizing multiple persistent DNA viruses, suggesting that memory T cells may contribute to the overall alloresponse. We used a flow cytometric CFSE-based proliferation & activation (CD38 expression acquisition) assay to systematically assess the contribution of naïve and memory T cells to alloreactivity. In some experiments donor peripheral blood mononuclear cells (PBMC) were primed for 8 days with allogeneic, HLA mismatched PBMC (alloPBMC), after which the intracellular cytokine production (ICS) was examined upon rechallenge with autologous or allogeneic PBMC (alloPBMC). CFSE-labeled umbilical cord mononuclear cells (UCMC) from five donors were stimulated with an irradiated pool of HLA-disparate donor peripheral blood mononuclear cells (alloPBMC). By day 8, over 80% of the CD4+ and CD8+ T cells were CFSE[dim], i.e. had proliferated, and expressed the activation marker CD38, indicating that naïve T cells can mount an alloresponse. Next, PBMC from adult donors were separated into naïve (CD57-CD45RO-), memory (CD57-CD45RA- and CD57-CD62L- T cells), and effector (CD27-CD45RO-) T cells. We found that adult donor naïve T cells can also mount an alloresponse; importantly, both memory T cell subsets – and to a lesser extent, effector cells – were CFSE[dim]CD38+ at least to the same extent as naïve T cells, indicating their potential to respond to alloantigens in addition to their priming foreign antigen. These data were confirmed using a different approach: By first priming responder T cells with HLA-disparate donor PBMC for 8 days, followed by the enumeration of alloreactivity by ICS after stimulation with the original stimulator PBMC. To exclude reactivity of responder T cells with viral antigens present in PBMC (EBV; CMV), the experiment was repeated using activated T cells (T-APC) as antigen presenting cells, and yielded identical results. The direct ex vivo alloreactivity of T cell subsets was next tested by stimulating donor PBMC with HLA mismatched donor PBMC or activated T cells as APC in an 18 hour ICS. This experiment confirmed the rapid kinetics of alloreactivity and dominance of memory T cells in the alloresponse. Since a prominent role of memory T cells was apparent from our experiments we next tested T cells specific for DNA virus-derived antigens. EBV- and CMV-specific T cells, tested against a panel of 30 T-APC with a broad coverage of the most prominent HLA, displayed exquisite specificity for certain mismatched HLA alleles, indicating that DNA virus antigen-specific T cells may indeed cross-react with cellular antigens presented in the context of mismatched HLA class I and II proteins. Collectively our data demonstrate that both naïve and memory T cells mount an alloresponse, but that memory T cells are more rapidly and strongly recruited by alloantigen stimulation. These findings indicate that in man alloresponses are not confined to particular subsets of post-thymic T cells and that donor-derived viral antigen-specific T cells in an unrelated recipient may cross-react with peptide-MHC complexes against which they were not negatively selected. Disclosures: No relevant conflicts of interest to declare.

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