Pathways of memory CD8+ T‐cell development

CD8⁺ T‐cell responses must have at least two components, a replicative cell type that proliferates in the secondary lymphoid tissue and that is responsible for clonal expansion, and cytotoxic cells with effector functions that mediate the resolution of the infection in the peripheral tissues. To confer memory, the response must also generate replication‐competent T cells that persist in the absence of antigen after the primary infection is cleared. The current models of memory differentiation differ in regards to whether or not memory CD8⁺ T cells acquire effector functions during their development. In this review we discuss the existing models for memory development and the consequences that the recent finding that memory CD8⁺ T cells may express granzyme B during their development has for them. We propose that memory CD8⁺ T cells represent a self‐renewing population of T cells that may acquire effector functions but that do not lose the naïve‐like attributes of lymphoid homing, antigen‐independent persistence or the capacity for self‐renewal.     

CD8+ CD122+ regulatory T cells contain clonally expanded cells with identical CDR3 sequences of the T‐cell receptor β‐chain

We identified CD8⁺ CD122⁺ regulatory T cells (CD8⁺ CD122⁺ Treg cells) and reported their importance in maintaining immune homeostasis. The absence of CD8⁺ CD122⁺ Treg cells has been shown to lead to severe systemic autoimmunity in several mouse models, including inflammatory bowel diseases and experimental autoimmune encephalomyelitis. The T‐cell receptors (TCRs) expressed on CD8⁺ CD122⁺ Treg cells recognize the target cells to be regulated. To aid in the identification of the target antigen(s) recognized by TCRs of CD8⁺ CD122⁺ Treg cells, we compared the TCR diversity of CD8⁺ CD122⁺ T cells with that of conventional, naive T cells in mice. We analysed the use of TCR‐Vβ in the interleukin 10‐producing population of CD8⁺ CD122⁺ T cells marked by high levels of CD49d expression, and found the significantly increased use of Vβ13 in these cells. Immunoscope analysis of the complementarity‐determining region 3 (CDR3) of the TCR β‐chain revealed remarkable skewing in a pair of Vβ regions, suggesting the existence of clonally expanded cells in CD8⁺ CD122⁺ T cells. Clonal expansion in Vβ13⁺ cells was confirmed by determining the DNA sequences of the CDR3s. The characteristic TCR found in this study is an important building block for further studies to identify the target antigen recognized by CD8⁺ CD122⁺ Treg cells.     

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.     

GDF11在体外扩增CD8+记忆干性T细胞的作用

 目的: 探讨生长分化因子11(GDF11)在体外诱导扩增具有干细胞特性的记忆性T细胞(memory stem T cells,Tscm)中的作用,从而进一步提高免疫过继治疗的疗效。方法: 通过密度梯度离心的方法分离健康人的外周血单个核细胞,然后用磁珠分选获得CD8⁺T细胞;随后将所得的细胞分为实验组和对照组,实验组中加入等体积、不同浓度的GDF11,对照组中加入等体积的PBS缓冲液,最后分别在不同时点通过流式细胞术检测2组细胞中Tscm的扩增情况。结果: 在CD8⁺T细胞的体外扩增实验中,GDF11可以显著扩增CD8⁺T细胞;在CD8⁺T细胞的体外培养实验中,GDF11可以显著提高Tscm在CD8⁺T细胞中的比例。结论: GDF11可以有效地在体外扩增CD8⁺Tscm,为提高免疫过继治疗的效率提供了新的方法。     

维生素C对CD4+效应记忆性T细胞体外扩增的影响

 目的：探讨小分子药物维生素C（VC）对CD4+效应记忆性T细胞（TEM）体外扩增的影响从而改善过继免疫治疗的效果。方法：体外分离正常人的外周血CD4+ T淋巴细胞,将细胞分为2组（实验组和对照组）进行细胞培养。实验组加入VC后,通过细胞计数仪和流式细胞仪对2组细胞中TEM的扩增情况进行检测。结果：(1) 在CD4+ T细胞体外扩增实验中,VC对CD4+ T细胞的总数无显著影响。(2) VC使TEM在CD4+ T细胞中的比例显著升高,100 mg/L为扩增的最佳浓度。(3) 在CD4+ T细胞扩增的第10天检测其中TEM的细胞数量,对照组TEM的数量为(1.22±0.15)×106,实验组TEM的数量为(3.56±0.35)×106 ,两者的差异有统计学意义（P<0.01）。结论: VC可有效地促进CD4+ TEM的体外扩增,为过继免疫治疗提供简单、安全及有效的体外扩增方法。     

AMPK: A metabolic switch for CD8+ T‐cell memory

Adenosine monophosphate‐activated protein kinase (AMPK) is a serine/threonine kinase and is crucial for cellular energy homeostasis. The exact role of AMPK during memory CD8⁺ T‐cell differentiation, a process that changes from the metabolically active state of effector T cells to one of quiescence in memory cells is not well understood; however, a report by Cantrell and colleagues [Eur. J. Immunol. 2013. 43: 889‐896] in this issue of the European Journal of Immunology shows that AMPK, by sensing glucose stress, is an important upstream molecule of mammalian target of rapamycin (mTOR) complex 1 for memory CD8⁺ T‐cell differentiation. This study provides new insights into how AMPK monitors energy stress to control effector and memory CD8⁺ T‐cell formation as discussed in this Commentary.     

人类CD8+记忆T细胞体外扩增方法的研究

 目的： 研究体外扩增人类CD8+记忆T细胞的新方法,为抗病毒与抗肿瘤的过继性免疫治疗提供新的手段。方法：将anti-CD3抗体、anti-CD28抗体、CD70、白细胞介素（IL）-2、IL-7和IL-15进行排列组合,设计出63种刺激方式,对体外分离得到的正常人外周血CD8+ T细胞进行体外扩增;培养14 d后进行细胞计数,并检测CD8+ T细胞的纯度以及CD8+中枢记忆T细胞（TCM）和CD8+效应记忆T细胞（TEM）所占的比例,进而计算出CD8+ T细胞、CD8+ TCM和CD8+ TEM的体外扩增倍数,从而确定理想的刺激方法。结果：体外扩增CD8+ T细胞、CD8+ TCM和CD8+ TEM的理想刺激方式均为anti-CD3抗体、IL-2和IL-7三者的组合;该刺激方式使3种细胞在培养14 d后分别扩增了13.19、13.28和15.27倍。结论：Anti-CD3抗体、IL-2和IL-7三者的组合,是刺激人类CD8+记忆T细胞体外扩增的相对理想方法。     

Modelling pathways of CD8+ T‐cell differentiation

In an immune response to infection, naïve T lymphocytes proliferate and give rise to a heterogeneous population of effector and memory cells. How is this diversity generated, and how can it be manipulated? Answering these questions requires an understanding of the lineage relationships between different effector and memory‐cell subsets, but these relationships remain to be identified definitively. In this issue of the European Journal of Immunology, a study moves us closer to this goal by combining a mathematical model and data from influenza infections in mice to support the hypothesis that CD8⁺ T‐cell differentiation is strongly coupled to cell division.     

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.     

Differential requirements of CD4+ T‐cell signals for effector cytotoxic T‐lymphocyte (CTL) priming and functional memory CTL development at higher CD8+ T‐cell precursor frequency

Increased CD8⁺ T‐cell precursor frequency (PF) precludes the requirement of CD4⁺ helper T (Th) cells for primary CD8⁺ cytotoxic T‐lymphocyte (CTL) responses. However, the key questions of whether unhelped CTLs generated at higher PF are functional effectors, and whether unhelped CTLs can differentiate into functional memory cells at higher PF are unclear. In this study, ovalbumin (OVA) ‐pulsed dendritic cells (DC_OVA) derived from C57BL/6, CD40 knockout (CD40^?/?) or CD40 ligand knockout (CD40L^?/?) mice were used to immunize C57BL/6, Ia^b?/?, CD40^?/? or CD40L^?/? mice, whose PF was previously increased with transfer of 1 × 10⁶ CD8⁺ T cells derived from OVA‐specific T‐cell receptor (TCR) transgenic OTI, OTI(CD40^?/?) or OTI(CD40L^?/?) mice. All the immunized mice were then assessed for effector and memory CTL responses. Following DC immunization, relatively comparable CTL priming occurred without CD4⁺ T‐cell help and Th‐provided CD40/CD40L signalling. In addition, the unhelped CTLs were functional effectors capable of inducing therapeutic immunity against established OVA‐expressing tumours. In contrast, the functional memory development of CTLs was severely impaired in the absence of CD4⁺ T‐cell help and CD40/CD40L signalling. Finally, unhelped memory CTLs failed to protect mice against lethal tumour challenge. Taken together, these results demonstrate that CD4⁺ T‐cell help at higher PF, is not required for effector CTL priming, but is required for functional memory CTL development against cancer. Our data may impact the development of novel preventive and therapeutic approaches in cancer patients with compromised CD4⁺ T‐cell functions.     

CD45RA expression on HCMV-specific effector memory CD8+ T cells is associated with the duration and intensity of HCMV replication after transplantation

In this cross-sectional study on 42 solid organ transplant recipients, the association of kinetics of human cytomegalovirus (HCMV) replication and EMRA HCMV-specific CD8+ T cells was investigated. Correlation was observed between the duration of HCMV replication after transplantation and CD45RA+CD27− (r = 0.609; p = 0.004), CD45RA+ CD28− (r = 0.579; p = 0.008) or CD45RA+CCR7− (r = 0.488; p = 0.029) HCMV-specific CD8+ T cells percentages. In the multivariate regression analyses, CD45RA+CD27−, CD45RA+CD28− or CD45RA+CCR7− HCMV-specific CD8+ T cells percentages increased 5.58% (p = 0.001), 5.35% (p = 0.001) or 4.49% (p = 0.012), respectively, with every 10-day increase in the duration of HCMV replication. Moreover, CD45RA+CD27− or CD45RA+CD28− frequencies increased 4.16% (p = 0.024) or 3.58% (p = 0.049), respectively, with every unity increase in log₁₀ genomes/mL. These observations support the major association between the frequency of EMRA HCMV-specific CD8+ T cells and the duration of post-transplant HCMV replication episodes in solid organ transplantation recipients.     

Tumor‐specific CD4+ T cells maintain effector and memory tumor‐specific CD8+ T cells

Immunotherapies that augment antitumor T cells have had recent success for treating patients with cancer. Here we examined whether tumor‐specific CD4⁺ T cells enhance CD8⁺ T‐cell adoptive immunotherapy in a lymphopenic environment. Our model employed physiological doses of tyrosinase‐related protein 1‐specific CD4⁺ transgenic T cells‐CD4⁺ T cells and pmel‐CD8⁺ T cells that when transferred individually were subtherapeutic; however, when transferred together provided significant (p ≤ 0.001) therapeutic efficacy. Therapeutic efficacy correlated with increased numbers of effector and memory CD8⁺ T cells with tumor‐specific cytokine expression. When combined with CD4⁺ T cells, transfer of total (naïve and effector) or effector CD8⁺ T cells were highly effective, suggesting CD4⁺ T cells can help mediate therapeutic effects by maintaining function of activated CD8⁺ T cells. In addition, CD4⁺ T cells had a pronounced effect in the early posttransfer period, as their elimination within the first 3 days significantly (p < 0.001) reduced therapeutic efficacy. The CD8⁺ T cells recovered from mice treated with both CD8⁺ and CD4⁺ T cells had decreased expression of PD‐1 and PD‐1‐blockade enhanced the therapeutic efficacy of pmel‐CD8 alone, suggesting that CD4⁺ T cells help reduce CD8⁺ T‐cell exhaustion. These data support combining immunotherapies that elicit both tumor‐specific CD4⁺ and CD8⁺ T cells for treatment of patients with cancer.     

Circulating memory CD8+ T cells are limited in forming CD103+ tissue-resident memory T cells at mucosal sites after reinfection

Tissue-resident memory CD8⁺ T cells (T_RM) localize to barrier tissues and mediate local protection against reinvading pathogens. Circulating central memory (T_CM) and effector memory CD8⁺ T cells (T_EM) also contribute to tissue recall responses, but their potential to form mucosal T_RM remains unclear. Here, we employed adoptive transfer and lymphocytic choriomeningitis virus reinfection models to specifically assess secondary responses of T_CM and T_EM at mucosal sites. Donor T_CM and T_EM exhibited robust systemic recall responses, but only limited accumulation in the small intestine, consistent with reduced expression of tissue-homing and -retention molecules. Murine and human circulating memory T cells also exhibited limited CD103 upregulation following TGF-β stimulation. Upon pathogen clearance, T_CM and T_EM readily gave rise to secondary T_EM. T_CM also formed secondary central memory in lymphoid tissues and T_RM in internal tissues, for example, the liver. Both T_CM and T_EM failed to substantially contribute to resident mucosal memory in the small intestine, while activated intestinal T_RM, but not liver T_RM, efficiently reformed CD103⁺ T_RM. Our findings demonstrate that circulating T_CM and T_EM are limited in generating mucosal T_RM upon reinfection. This may pose important implications on cell therapy and vaccination strategies employing memory CD8⁺ T cells for protection at mucosal sites.     

CD28 days later: Resurrecting costimulation for CD8+ memory T cells

Rapid activation and proliferative expansion of specific CD8⁺ memory T (CD8⁺T_M) cells upon antigen re‐encounter is a critical component of the adaptive immune response that confers enhanced immune protection. In this context, however, the requirements for costimulation in general, and CD28 signaling in particular, remain incompletely defined. In the current issue of the European Journal of Immunology, Fröhlich et al. [Eur. J. Immunol. 2016. 46: 1644‐1655] provide definitive evidence that optimal elaboration of CD8⁺T_M‐cell recall responses is indeed contingent on CD28 expressed by these cells. Here, we discuss the “CD28 costimulation paradigm” in its historical context and highlight some of the unresolved complexities pertaining to CD28‐dependent interactions that shape CD8⁺ T‐cell phenotypes, functionalities, and recall reactivity.     

Division‐linked differentiation can account for CD8+ T‐cell phenotype in vivo

The CD8⁺ T‐cell response to infection involves a large initial expansion in the numbers of responding cells, accompanied by differentiation of these cells. Expression of the adhesion molecule CD62L is high on naïve cells and rapidly downregulated on the surface of the majority (～90%) of cells during the ‘effector’ phase of acute infection. Adoptive transfer studies have been used to study differentiation in this system; however, relatively little work has investigated the phenotype of cells in the endogenous repertoire. We demonstrate that the extent of CD62L down‐regulation is positively correlated with clone size in vivo, consistent with division‐linked differentiation of responding cells. Other features of the endogenous CD62L^hi and CD62L^lo repertoire are that the CD62L^lo repertoire is less diverse than the CD62L^hi repertoire and represents a subset of clonotypes found in the CD62L^hi repertoire. To test whether these observations are compatible with a mechanism of division‐linked differentiation, we developed a mathematical model, where there is a probability of CD62L down‐regulation associated with cell division. Comparison of model results with experimental data suggests that division‐linked differentiation provides a simple mechanism to explain the relationship between clone size and phenotype of CD8⁺ T cells during acute infection.     

TRAF2 regulates T cell immunity by maintaining a Tpl2-ERK survival signaling axis in effector and memory CD8 T cells

Generation and maintenance of antigen-specific effector and memory T cells are central events in immune responses against infections. We show that TNF receptor-associated factor 2 (TRAF2) maintains a survival signaling axis in effector and memory CD8 T cells required for immune responses against infections. This signaling axis involves activation of Tpl2 and its downstream kinase ERK by NF-κB-inducing kinase (NIK) and degradation of the proapoptotic factor Bim. NIK mediates Tpl2 activation by stimulating the phosphorylation and degradation of the Tpl2 inhibitor p105. Interestingly, while NIK is required for Tpl2-ERK signaling under normal conditions, uncontrolled NIK activation due to loss of its negative regulator, TRAF2, causes constitutive degradation of p105 and Tpl2, leading to severe defects in ERK activation and effector/memory CD8 T cell survival. Thus, TRAF2 controls a previously unappreciated signaling axis mediating effector/memory CD8 T cell survival and protective immunity.     

Mesenchymal stem cells control alloreactive CD8+CD28− T cells

CD28/B7 co-stimulation blockade with belatacept prevents alloreactivity in kidney transplant patients. However, cells lacking CD28 are not susceptible to belatacept treatment. As CD8⁺CD28⁻ T-cells have cytotoxic and pathogenic properties, we investigated whether mesenchymal stem cells (MSC) are effective in controlling these cells. In mixed lymphocyte reactions (MLR), MSC and belatacept inhibited peripheral blood mononuclear cell (PBMC) proliferation in a dose-dependent manner. MSC at MSC/effector cell ratios of 1:160 and 1:2·5 reduced proliferation by 38·8 and 92·2%, respectively. Belatacept concentrations of 0·1 μg/ml and 10 μg/ml suppressed proliferation by 20·7 and 80·6%, respectively. Both treatments in combination did not inhibit each other''s function. Allostimulated CD8⁺CD28⁻ T cells were able to proliferate and expressed the cytolytic and cytotoxic effector molecules granzyme B, interferon (IFN)-γ and tumour necrosis factor (TNF)-α. While belatacept did not affect the proliferation of CD8⁺CD28⁻ T cells, MSC reduced the percentage of CD28⁻ T cells in the proliferating CD8⁺ T cell fraction by 45·9% (P = 0·009). CD8⁺CD28⁻ T cells as effector cells in MLR in the presence of CD4⁺ T cell help gained CD28 expression, an effect independent of MSC. In contrast, allostimulated CD28⁺ T cells did not lose CD28 expression in MLR–MSC co-culture, suggesting that MSC control pre-existing CD28⁻ T cells and not newly induced CD28⁻ T cells. In conclusion, alloreactive CD8⁺CD28⁻ T cells that remain unaffected by belatacept treatment are inhibited by MSC. This study indicates the potential of an MSC–belatacept combination therapy to control alloreactivity.