Functional characterization of myeloid‐derived suppressor cell subpopulations during the development of experimental arthritis

Recent evidence indicates the existence of subpopulations of myeloid‐derived suppressor cells (MDSCs) with distinct phenotypes and functions. Here, we characterized the role of MDSC subpopulations in the pathogenesis of autoimmune arthritis in a collagen‐induced arthritis (CIA) mouse model. The splenic CD11b⁺Gr‐1⁺ MDSC population expanded in CIA mice, and these cells could be subdivided into polymorphonuclear (PMN) and mononuclear (MO) MDSC subpopulations based on Ly6C and Ly6G expression. During CIA, the proportion of splenic MO‐MDSCs was increased in association with the severity of joint inflammation, while PMN‐MDSCs were decreased. MO‐MDSCs expressed higher levels of surface CD40 and CD86 protein, but lower levels of Il10, Tgfb1, Ccr5, and Cxcr2 mRNA. PMN‐MDSCs exhibited a more potent capacity to suppress polyclonal T‐cell proliferation in vitro, compared with MO‐MDSCs. Moreover, the adoptive transfer of PMN‐MDSCs, but not MO‐MDSCs, decreased joint inflammation, accompanied by reduced levels of serum cytokine secretion and the frequencies of Th1 and Th17 cells in draining lymph nodes. These results suggest that there could be a shift from potently suppressive PMN‐MDSCs to poorly suppressive MO‐MDSCs during the development of experimental arthritis, which might reflect the failure of expanded MDSCs to suppress autoimmune arthritis.     

Emergence of a distinct HIV‐specific IL‐10‐producing CD8+ T‐cell subset with immunomodulatory functions during chronic HIV‐1 infection

Interleukin‐10 (IL‐10) plays a key role in regulating proinflammatory immune responses to infection but can interfere with pathogen clearance. Although IL‐10 is upregulated throughout HIV‐1 infection in multiple cell subsets, whether this is a viral immune evasion strategy or an appropriate response to immune activation is unresolved. Analysis of IL‐10 production at the single cell level in 51 chronically infected subjects (31 antiretroviral (ART) naïve and 20 ART treated) showed that a subset of CD8⁺ T cells with a CD25^neg FoxP3^neg phenotype contributes substantially to IL‐10 production in response to HIV‐1 gag stimulation. The frequencies of gag‐specific IL‐10‐ and IFN‐γ‐producing T cells in ART‐naïve subjects were strongly correlated and the majority of these IL‐10⁺ CD8⁺ T cells co‐produced IFN‐γ; however, patients with a predominant IL‐10⁺/IFN‐γ^neg profile showed better control of viraemia. Depletion of HIV‐specific CD8⁺ IL‐10⁺ cells from PBMCs led to upregulation of CD38 on CD14⁺ monocytes together with increased IL‐6 production, in response to gag stimulation. Increased CD38 expression was positively correlated with the frequency of the IL‐10⁺ population and was also induced by exposure of monocytes to HIV‐1 in vitro. Production of IL‐10 by HIV‐specific CD8⁺ T cells may represent an adaptive regulatory response to monocyte activation during chronic infection.     

Endoplasmic reticulum stress induced LOX‐1+ CD15+ polymorphonuclear myeloid‐derived suppressor cells in hepatocellular carcinoma

A recent study indicated that Lectin‐type oxidized LDL receptor‐1 (LOX‐1) was a distinct surface marker for human polymorphisms myeloid‐derived suppressor cells (PMN‐MDSC). The present study was aimed to investigate the existence LOX‐1 PMN‐MDSC in hepatocellular carcinoma (HCC) patients. One hundred and twenty‐seven HCC patients, 10 patients with mild active chronic hepatitis B, 10 liver cirrhosis due to hepatitis B, 10 liver dysplastic node with hepatitis B and 50 health control were included. LOX‐1⁺ CD15⁺ PMN‐MDSC were significantly elevated in HCC patients compared with healthy control and patients with benign diseases. LOX‐1⁺ CD15⁺ PMN‐MDSC in circulation were positively associated with those in HCC tissues. LOX‐1⁺ CD15⁺ PMN‐MDSCs significantly reduced proliferation and IFN‐γ production of T cells with a dosage dependent manner with LOX‐1^? CD15⁺ PMNs reached negative results. The suppression on T cell proliferation and IFN‐γ production was reversed by ROS inhibitor and Arginase inhibitor. ROS level and activity of arginase of LOX‐1 ⁺CD15⁺ PMN were higher in LOX‐1⁺ CD15⁺ PMN‐MDSCs than LOX‐1^? CD15⁺ PMNs, as well as the expression of the NADPH oxidase NOX2 and arginase I. RNA sequence revealed that LOX‐1⁺ CD15⁺ PMN‐MDSCs displayed significantly higher expression of spliced X‐box ‐binding protein 1 (sXBP1), an endoplasmic reticulum (ER) stress marker. ER stress inducer induced LOX‐1 expression and suppressive function for CD15⁺ PMN from health donor. For HCC patients, LOX‐1⁺ CD15⁺ PMN‐MDSCs were positively related to overall survival. Above all, LOX‐1⁺ CD15⁺ PMN‐MDSC were elevated in HCC patients and suppressed T cell proliferation through ROS/Arg I pathway induced by ER stress. They presented positive association with the prognosis of HCC patients.     

TRAF2 regulates peripheral CD8+ T‐cell and NKT‐cell homeostasis by modulating sensitivity to IL‐15

In this study, a critical and novel role for TNF receptor (TNFR) associated factor 2 (TRAF2) is elucidated for peripheral CD8⁺ T‐cell and NKT‐cell homeostasis. Mice deficient in TRAF2 only in their T cells (TRAF2TKO) show ∼40% reduction in effector memory and ∼50% reduction in naïve CD8⁺ T‐cell subsets. IL‐15‐dependent populations were reduced further, as TRAF2TKO mice displayed a marked ∼70% reduction in central memory CD8⁺CD44^hiCD122⁺ T cells and ∼80% decrease in NKT cells. TRAF2TKO CD8⁺CD44^hi T cells exhibited impaired dose‐dependent proliferation to exogenous IL‐15. In contrast, TRAF2TKO CD8⁺ T cells proliferated normally to anti‐CD3 and TRAF2TKO CD8⁺CD44^hi T cells exhibited normal proliferation to exogenous IL‐2. TRAF2TKO CD8⁺ T cells expressed normal levels of IL‐15‐associated receptors and possessed functional IL‐15‐mediated STAT5 phosphorylation, however TRAF2 deletion caused increased AKT activation. Loss of CD8⁺CD44^hiCD122⁺ and NKT cells was mechanistically linked to an inability to respond to IL‐15. The reduced CD8⁺CD44^hiCD122⁺ T‐cell and NKT‐cell populations in TRAF2TKO mice were rescued in the presence of high dose IL‐15 by IL‐15/IL‐15Rα complex administration. These studies demonstrate a critical role for TRAF2 in the maintenance of peripheral CD8⁺ CD44^hiCD122⁺ T‐cell and NKT‐cell homeostasis by modulating sensitivity to T‐cell intrinsic growth factors such as IL‐15.     

IFN‐γ regulates survival and function of tumor‐induced CD11b+Gr‐1high myeloid derived suppressor cells by modulating the anti‐apoptotic molecule Bcl2a1

Myeloid derived suppressor cells (MDSCs) play a critical role in suppression of immune responses in cancer and inflammation. Here, we describe how regulation of Bcl2a1 by cytokines controls the suppressor function of CD11b⁺Gr‐1^high granulocytic MDSCs. Coculture of CD11b⁺Gr‐1^high granulocytic MDSCs with antigen‐stimulated T cells and simultaneous blockade of IFN‐γ by the use of anti‐IFN‐γ blocking antibody, IFN‐γ^?/? effector T cells, IFN‐γR^?/? MDSCs or STAT1^?/? MDSCs led to upregulation of Bcl2a1 in CD11b⁺Gr‐1^high cells, improved survival, and enhanced their suppressor function. Molecular studies revealed that GM‐CSF released by antigen‐stimulated CD8⁺ T cells induced Bcl2a1 upregulation, which was repressed in the presence of IFN‐γ by a direct interaction of phosphorylated STAT‐1 with the Bcl2a1 promotor. Bcl2a1 overexpressing granulocytic MDSCs demonstrated prolonged survival and enhanced suppressor function in vitro. Our data suggest that IFN‐γ/ STAT1‐dependent regulation of Bcl2a1 regulates survival and thereby suppressor function of granulocytic MDSCs.     

Interrupting CD28 costimulation before antigen rechallenge affects CD8+ T‐cell expansion and effector functions during secondary response in mice

The role of CD28‐mediated costimulation in secondary CD8⁺ T‐cell responses remains controversial. Here, we have used two tools — blocking mouse anti‐mouse CD28‐specific antibodies and inducible CD28‐deleting mice — to obtain definitive answers in mice infected with ovalbumin‐secreting Listeria monocytogenes. We report that both blockade and global deletion of CD28 reveal its requirement for full clonal expansion and effector functions such as degranulation and IFN‐γ production during the secondary immune response. In contrast, cell‐intrinsic deletion of CD28 in transferred TCR‐transgenic CD8⁺ T cells before primary infection leads to impaired clonal expansion but an increase in cells able to express effector functions in both primary and secondary responses. We suggest that the proliferation‐impaired CD8⁺ T cells respond to CD28‐dependent help from their environment by enhanced functional differentiation. Finally, we report that cell‐intrinsic deletion of CD28 after the peak of the primary response does not affect the establishment, maintenance, or recall of long‐term memory. Thus, if given sufficient time, the progeny of primed CD8⁺ T cells adapt to the absence of this costimulator.     

IL‐12 and IL‐4 activate a CD39‐dependent intrinsic peripheral tolerance mechanism in CD8+ T cells

Immune responses to protein antigens involve CD4⁺ and CD8⁺ T cells, which follow distinct programs of differentiation. Naïve CD8 T cells rapidly develop cytotoxic T‐cell (CTL) activity after T‐cell receptor stimulation, and we have previously shown that this is accompanied by suppressive activity in the presence of specific cytokines, i.e. IL‐12 and IL‐4. Cytokine‐induced CD8⁺ regulatory T (Treg) cells are one of several Treg‐cell phenotypes and are Foxp3^? IL‐10⁺ with contact‐dependent suppressive capacity. Here, we show they also express high level CD39, an ecto‐nucleotidase that degrades extracellular ATP, and this contributes to their suppressive activity. CD39 expression was found to be upregulated on CD8⁺ T cells during peripheral tolerance induction in vivo, accompanied by release of IL‐12 and IL‐10. CD39 was also upregulated during respiratory tolerance induction to inhaled allergen and on tumor‐infiltrating CD8⁺ T cells. Production of IL‐10 and expression of CD39 by CD8⁺ T cells was independently regulated, being respectively blocked by extracellular ATP and enhanced by an A2A adenosine receptor agonist. Our results suggest that any CTL can develop suppressive activity when exposed to specific cytokines in the absence of alarmins. Thus negative feedback controls CTL expansion under regulation from both nucleotide and cytokine environment within tissues.     

T‐cell help dependence of memory CD8+ T‐cell expansion upon vaccinia virus challenge relies on CD40 signaling

Due to their capacity to differentiate into long‐lived memory cells, CD8⁺ T cells are able to resolve subsequent infections faster than during the primary response. Among other factors, CD4⁺ T cells play a crucial role during primary and secondary CD8⁺ T‐cell responses. However, the timing and mechanisms by which they influence CD8⁺ T cells may differ in primary and secondary responses. Here, we demonstrate that during both primary and secondary vaccinia virus infection, CD4⁺ T cells are necessary to promote CD8⁺ T‐cell responses. While CD4⁺ T cells contributed to memory CD8⁺ T‐cell development, they were even more important during memory recall responses during challenge, as absence of CD4⁺ T cells during challenge resulted in markedly decreased proliferation and increased apoptosis. T‐cell help during primary and secondary responses was mediated via CD40 signaling, with DCs being an integral part of that pathway. As opposed to primary CD8⁺ T‐cell responses where only a combination of agonistic CD40 signaling and provision of IL‐2 could substitute for T‐cell help, agonistic CD40 triggering alone was sufficient to rescue memory CD8⁺ T‐cell responses in absence of T‐cell help in the context of vaccinia virus infection.     

Myeloid‐derived suppressor cell activation by combined LPS and IFN‐γ treatment impairs DC development

Myeloid‐derived suppressor cells (MDSC) and DC are major controllers of immune responses against tumors or infections. However, it remains unclear how DC development and MDSC suppressor activity both generated from myeloid precursor cells are regulated. Here, we show that the combined treatment of BM‐derived MDSC with LPS plus IFN‐γ inhibited the DC development but enhanced MDSC functions, such as NO release and T‐cell suppression. This was not observed by the single treatments in vitro. In the spleens of healthy mice, we identified two Gr‐1^lowCD11b^highLy‐6C^highSSC^lowMo‐MDSC and Gr‐1^highCD11b^lowPMN‐MDSC populations with suppressive potential, whereas Gr‐1^highCD11b^high neutrophils and Gr‐1^lowCD11b^highSSC^low eosinophils were not suppressive. Injections of LPS plus IFN‐γ expanded these populations within the spleen but not LN leading to the block of the proliferation of CD8⁺ T cells. At the same time, their capacity to develop into DC was impaired. Together, our data suggest that spleens of healthy mice contain two subsets of MDSC with suppressive potential. A two‐signal‐program through combined LPS and IFN‐γ treatment expands and fully activates MDSC in vitro and in vivo.     

IL‐27 enhances the survival of tumor antigen‐specific CD8+ T cells and programs them into IL‐10‐producing,memory precursor‐like effector cells

IL‐27 is a member of the IL‐12 family of cytokines that is comprised of an IL‐12 p40‐related protein subunit, EBV‐induced gene 3, and a p35‐related subunit, p28. IL‐27 functions through IL‐27R and has been shown to have potent antitumor activity via activation of a variety of cellular components, including antitumor CD8⁺ T‐cell responses. However, the exact mechanisms of how IL‐27 enhances antitumor CD8⁺ T‐cell responses remain unclear. Here we show that IL‐27 significantly enhances the survival of activated tumor antigen‐specific CD8⁺ T cells in vitro and in vivo, and programs tumor antigen‐specific CD8⁺ T cells into memory precursor‐like effector cells, characterized by upregulation of Bcl‐6, SOCS3, Sca‐1, and IL‐10. While STAT3 activation and the CTL survival‐enhancing effects can be independent of CTL IL‐10 production, we show here that IL‐27‐induced CTL IL‐10 production contributes to memory precursor cell phenotype induction, CTL memory, and tumor rejection. Thus, IL‐27 enhances antitumor CTL responses via programming tumor antigen‐specific CD8⁺ T cells into a unique memory precursor type of effector cells characterized by a greater survival advantage. Our results have important implications for designing immunotherapy against human cancer.     

Activation‐induced CD137 is a fast assay for identification and multi‐parameter flow cytometric analysis of alloreactive T cells

Detection and isolation of viable alloreactive T cells at the single‐cell level requires a cell surface marker induced specifically upon T cell receptor activation. In this study, a member of the tumour necrosis factor receptor (TNFR)‐family, CD137 (4‐1BB) was investigated for its potential to identify the total pool of circulating alloreactive T cells. Optimal conditions for sensitive and specific detection of allogeneic‐induced CD137 expression on circulating T cells were established. Thereafter, CD137⁺ alloreactive T cells were phenotypically and functionally characterized by multi‐parameter flow cytometry. Alloantigen‐induced CD137 expression identified both alloreactive CD8⁺ T cells (mean ± standard error of the mean: 0·21 ± 0·07%) and alloreactive CD4⁺ T cells (0·21 ± 0·05%). CD137⁺ alloreactive T cells were detected in different T cell subsets, including naive T cells, but were found preferentially in CD28⁺ T cells and not in the terminally differentiated T cell subset. Upon allogeneic (re‐)stimulation, the cytokine‐producing as well as proliferative capacity of T cells resided mainly within the CD137‐expressing fraction. About 10% of the CD137⁺ alloreactive T cells produced any combination of interferon (IFN)‐γ, interleukin (IL)‐2 and TNF‐α. Polyfunctional alloreactive T cells, defined by multiple cytokine expression, were observed infrequently. In conclusion, activation‐induced CD137 expression is a fast assay allowing for detection and functional analysis of the total alloreactive T cell compartment at the single‐cell level by multi‐parameter flow cytometry.     

CD70 and IFN‐1 selectively induce eomesodermin or T‐bet and synergize to promote CD8+ T‐cell responses

CD70‐mediated stimulation of CD27 is an important cofactor of CD4⁺ T‐cell licensed dendritic cells (DCs). However, it is unclear how CD70‐mediated stimulation of T cells is integrated with signals that emanate from signal 3 pathways, such as type‐1 interferon (IFN‐1) and IL‐12. We find that while stimulation of CD27 in isolation drives weak Eomesodermin^hiT‐bet^lo CD8⁺ T‐cell responses to OVA immunization, profound synergistic expansion is achieved by cotargeting TLR. This cooperativity can substantially boost antiviral CD8⁺ T‐cell responses during acute infection. Concomitant stimulation of TLR significantly increases per cell IFN‐γ production and the proportion of the population with characteristics of short‐lived effector cells, yet also promotes the ability to form long‐lived memory. Notably, while IFN‐1 contributes to the expression of CD70 on DCs, the synergy between CD27 and TLR stimulation is dependent upon IFN‐1's effect directly on CD8⁺ T cells, and is associated with the increased expression of T‐bet in T cells. Surprisingly, we find that IL‐12 fails to synergize with CD27 stimulation to promote CD8⁺ T‐cell expansion, despite its capacity to drive effector CD8⁺ T‐cell differentiation. Together, these data identify complex interactions between signal 3 and costimulatory pathways, and identify opportunities to influence the differentiation of CD8⁺ T‐cell responses.     

Lack of transglutaminase 2 diminished T‐cell responses in mice

Transglutaminase 2 (TG2) has been reported to play a role in dendritic cell activation and B‐cell differentiation after immunization. Its presence and role in T cells, however, has not been explored. In the present study, we determined the expression of TG2 on mouse T cells, and evaluated its role by comparing the behaviours of wild‐type and TG2^?/? T cells after activation. In our results, naive T cells minimally expressed TG2, expression of which was increased after activation. T‐cell proliferation, expression of activation markers such as CD69 and CD25, and secretions of interleukin‐2 and interferon‐γ were suppressed in the absence of TG2, presumably due, in part, to diminished nuclear factor‐κB activation. These effects on T cells seemed to be reflected in the in vivo immune response, the contact hypersensitivity reaction elicited by 2,4‐dinitro‐1‐fluorobenzene, with lowered peak responses in the TG2^?/? mice. When splenic T cells from mice immunized with tumour lysate‐loaded wild‐type dendritic cells were re‐challenged ex vivo with the same antigen, the profile of surface markers including CD44, CD62L, and CD127 strongly indicated lesser generation of memory CD8⁺ T cells in TG2^?/? mice. In the TG2^?/? CD8⁺ T cells, moreover, Eomes expression was markedly decreased. These results indicate possible roles of TG2 in CD8⁺ T‐cell activation and CD8⁺ memory T‐cell generation.     

Monocytic myeloid‐derived suppressor cells regulate T‐cell responses against vaccinia virus

Vaccinia virus (VV) can potently activate NK‐ and T‐cell responses, leading to efficient viral control and generation of long‐lasting protective immunity. However, immune responses against viral infections are often tightly controlled to avoid collateral damage and systemic inflammation. We have previously shown that granulocytic myeloid‐derived suppressor cells (g‐MDSCs) can suppress the NK‐cell response to VV infection. It remains unknown what regulates T‐cell responses to VV infection in vivo. In this study, we first showed that monocytic MDSCs (m‐MDSCs), but not g‐MDSCs, from VV‐infected mice could directly suppress CD4⁺ and CD8⁺ T‐cell activation in vitro. We then demonstrated that defective recruitment of m‐MDSCs to the site of VV infection in CCR2^?/? mice enhanced VV‐specific CD8⁺ T‐cell response and that adoptive transfer of m‐MDSCs into VV‐infected mice suppressed VV‐specific CD8⁺ T‐cell activation, leading to a delay in viral clearance. Mechanistically, we further showed that T‐cell suppression by m‐MDSCs is mediated by indication of iNOS and production of NO upon VV infection, and that IFN‐γ is required for activation of m‐MDSCs. Collectively, our results highlight a critical role for m‐MDSCs in regulating T‐cell responses against VV infection and may suggest potential strategies using m‐MDSCs to modulate T‐cell responses during viral infections.     

Sympathetic neural signaling via the β2‐adrenergic receptor suppresses T‐cell receptor‐mediated human and mouse CD8+ T‐cell effector function

Postganglionic sympathetic neurons innervate secondary lymphoid organs and secrete norepinephrine (NE) as the primary neurotransmitter. NE binds and signals through five distinct members of the adrenergic receptor family. In this study, we show elevated expression of the β2‐adrenergic receptor (ADRB2) on primary human CD8⁺ effector memory T cells. Treatment of both human and murine CD8⁺ T cells with NE decreased IFN‐γ and TNF‐α secretion and suppressed their cytolytic capacity in response to T‐cell receptor (TCR) activation. The effects of NE were specifically reversed by β 2‐specific antagonists. Adrb2^?/? CD8⁺ T cells were completely resistant to the effects of NE. Further, the ADRB2‐specific pharmacological ligand, albuterol, significantly suppressed effector functions in both human and mouse CD8⁺ T cells. While both TCR activation and stimulation with IL‐12 + IL‐18 were able to induce inflammatory cytokine secretion, NE failed to suppress IFN‐γ secretion in response to IL‐12 + IL18. Finally, the long‐acting ADRB2‐specific agonist, salmeterol, markedly reduced the cytokine secretion capacity of CD8⁺ T cells in response to infection with vesicular stomatitis virus. This study reveals a novel intrinsic role for ADRB2 signaling in CD8⁺ T‐cell function and underscores the novel role this pathway plays in adaptive T‐cell responses to infection.     

Kinetics of cytomegalovirus (CMV) pp65 and IE‐1‐specific IFNγ CD8+ and CD4+ T cells during episodes of viral DNAemia in allogeneic stem cell transplant recipients: Potential implications for the management of active CMV infection

The dynamics of CMV pp65 and IE‐1‐specific IFNγ‐producing CD8⁺ (IFNγ CD8⁺) and CD4⁺ (IFNγ CD4⁺) T cells and CMV DNAemia were assessed in 19 pre‐emptively treated episodes of active CMV infection. Peripheral counts of IFNγ CD8⁺ and IFNγ CD4⁺ T cells inversely correlated with CMV DNAemia levels (P = <0.001 and P = 0.003, respectively). A threshold value of 1.3 cells/µl predicting CMV DNAemia clearance was established for IFNγ CD8⁺ T cells (PPV, 100%; NPV, 93%) and for IFNγ CD4⁺ T cells (PPV, 100%; NPV, 75%). Undetectable T‐cell responses were usually observed at the time of initiation of pre‐emptive therapy. Either a rapid (within 7 days) or a delayed (median 31 days) expansion of both T‐cell populations concomitant with CMV DNAemia clearance was observed in 5 and 8 episodes, respectively. An inconsistent or a lack of expansion of both T‐cell subsets was related to a persistent CMV DNAemia. Robust and maintained CMV‐specific T‐cell responses after CMV DNAemia clearance and cessation of antiviral therapy were associated with a null incidence of relapsing infections at least during the following month. Data obtained in the present study may be helpful in the design of therapeutic strategies for the management of active CMV infections in the allo‐SCT recipient. J. Med. Virol. 82: 1208–1215, 2010. © 2010 Wiley‐Liss, Inc.     

CD4+ CD25+ Treg regulate the contribution of CD8+ T-cell subsets in repopulation of the lymphopenic environment

Peripheral T‐cell expansion is of major relevance for immune function after lymphopenia. In order to promote regeneration, the process should result in a peripheral T‐cell pool with a similar subpopulation structure as before lymphopenia. We investigated the repopulation of the CD8⁺ central‐memory T cells (T_CM) and effector‐memory T cells (T_EM) pools after adoptive transfer of sorted CD8⁺ T cells from naïve, T_CM and T_EM subsets into T‐cell‐deficient hosts. We show that the initial kinetics of expansion are distinct for each subset and that the contribution to the repopulation of the CD8⁺ T‐cell pool by the progeny of each subset is not a mere function of its initial expansion. We demonstrate that CD4⁺CD25⁺ Treg play a major role in the repopulation of the CD8⁺ T‐cell pool and that CD8⁺ T‐cell subsets impact on each other. In the absence of CD4⁺CD25⁺ Treg, a small fraction of naïve CD8⁺ T cells strongly proliferates, correlating with further expansion and differentiation of co‐expanding CD8⁺ T cells. CD4⁺CD25⁺ Treg suppress these responses and lead to controlled repopulation, contributing decisively to the maintenance of recovered T_CM and T_EM fractions, and leading to repopulation of each pool with progeny of its own kind.     

Concomitant type I IFN receptor‐triggering of T cells and of DC is required to promote maximal modified vaccinia virus Ankara‐induced T‐cell expansion

Virus‐induced expansion of CD8⁺ T cells may be promoted by type I IFN receptor (IFNAR)‐triggering of T cells, depending on the pathogen tested. We studied modified vaccinia virus Ankara (MVA), a promising vaccine vector candidate, which was derived from conventional vaccinia virus (VACV) by more than 570 consecutive in vitro passages. In adoptive transfer experiments, we verified that VACV expressing the gp33 epitope of lymphocytic choriomeningitis virus (VACV_gp33) induced largely IFNAR‐independent expansion of gp33‐specific T cells. On the contrary, MVA_gp33‐induced T‐cell expansion was IFNAR dependent. Interestingly, under the latter conditions, T‐cell activation was IFNAR independent, whereas T‐cell apoptosis was enhanced in the absence of IFNAR. To address whether MVA‐induced T‐cell expansion was solely affected by IFNAR‐triggering of T cells, expansion of endogenous T cells was studied in conditional mice with a T‐cell‐ or DC‐specific IFNAR deletion. Interestingly, both mouse strains showed moderately reduced T‐cell expansion, whereas mice with a combined T‐cell‐ and DC‐specific IFNAR ablation showed massively reduced T‐cell expansion similar to that of IFNAR^?/? mice. These results are compatible with the model that IFN‐inducing viruses such as MVA confer virus‐specific CD8⁺ T‐cell expansion by concomitant IFNAR‐triggering of DC and of T cells.