Strength of stimulus and clonal competition impact the rate of memory CD8 T cell differentiation

The developmental pathways of long-lived memory CD8 T cells and the lineage relationship between memory T cell subsets remain controversial. Although some studies indicate the two major memory T cell subsets, central memory T (T(CM)) and effector memory T (T(EM)), are related lineages, others suggest that these subsets arise and are maintained independently of one another. In this study, we have investigated this issue and examined the differentiation of memory CD8 T cell subsets by tracking the lineage relationships of both endogenous and TCR transgenic CD8 T cell responses after acute infection. Our data indicate that TCR transgenic as well as nontransgenic T(EM) differentiate into T(CM) in the absence of Ag. Moreover, the rate of memory CD8 T cell differentiation from T(EM) into the self-renewing and long-lived pool of T(CM) is influenced by signals received during priming, including Ag levels, clonal competition, and/or the duration of infection. Although some T(EM) appear to not progress to T(CM), the vast majority of T(CM) are derived from T(EM). Thus, long-lasting, Ag-independent CD8 T cell memory results from progressive differentiation of memory CD8 T cells, and the rate of memory T cell differentiation is governed by events occurring early during T cell priming.     

Bcl6 acts as an amplifier for the generation and proliferative capacity of central memory CD8+ T cells

Central memory CD8(+) T cells (T(CM)) are considered to be more efficient than effector ones (T(EM)) for mediating protective immunity. The molecular mechanism involved in the generation of these cells remains elusive. Because Bcl6 plays a role in the generation and maintenance of memory CD8(+) T cells, we further examined this role in the process in relation to T(CM) and T(EM) subsets. In this study, we show that T(CM) and T(EM) were functionally identified in CD62L(+) and CD62L(-) memory (CD44(+)Ly6C(+)) CD8(+) T cell subsets, respectively. Although T(CM) produced similar amounts of IFN-gamma and IL-2 to T(EM) after anti-CD3 stimulation, the cell proliferation capacity after stimulation and tissue distribution profiles of T(CM) differed from those of T(EM). Numbers of T(CM) were greatly reduced and elevated in spleens of Bcl6-deficient and lck-Bcl6 transgenic mice, respectively, and those of T(EM) were constant in nonlymphoid organs of these same mice. The majority of Ag-specific memory CD8(+) T cells in spleens of these mice 10 wk after immunization were T(CM), and the number correlated with Bcl6 expression in T cells. The proliferation of Ag-specific memory CD8(+) T cells upon secondary stimulation was dramatically up-regulated in lck-Bcl6 transgenic mice, and the adoptive transfer experiments with Ag-specific naive CD8(+) T cells demonstrated that some of the up-regulation was due to the intrinsic effect of Bcl6 in the T cells. Thus, Bcl6 is apparently a crucial factor for the generation and secondary expansion of T(CM).     

Persistence and function of central and effector memory CD4+ T cells following infection with a gastrointestinal helminth

Immunity in the gastrointestinal tract is important for resistance to many pathogens, but the memory T cells that mediate such immunity are poorly characterized. In this study, we show that following sterile cure of a primary infection with the gastrointestinal parasite Trichuris muris, memory CD4+ T cells persist in the draining mesenteric lymph node and protect mice against reinfection. The memory CD4+ T cells that developed were a heterogeneous population, consisting of both CD62L(high) central memory T cells (T(CM)) and CD62L(low) effector memory T cells (T(EM)) that were competent to produce the Th type 2 effector cytokine, IL-4. Unlike memory T cells that develop following exposure to several other pathogens, both CD4+ T(CM) and T(EM) populations persisted in the absence of chronic infection, and, critically, both populations were able to transfer protective immunity to naive recipients. CD62L(high)CD4+ T(CM) were not apparent early after infection, but emerged following clearance of primary infection, suggesting that they may be derived from CD4+ T(EM). Consistent with this theory, transfer of CD62L(low)CD4+ T(EM) into naive recipients resulted in the development of a population of protective CD62L(high)CD4+ T(CM). Taken together, these studies show that distinct subsets of memory CD4+ T cells develop after infection with Trichuris, persist in the GALT, and mediate protective immunity to rechallenge.     

The basis of distinctive IL-2- and IL-15-dependent signaling: weak CD122-dependent signaling favors CD8+ T central-memory cell survival but not T effector-memory cell development

Recent work suggests that IL-2 and IL-15 induce distinctive levels of signaling through common receptor subunits and that such varied signaling directs the fate of Ag-activated CD8(+) T cells. In this study, we directly examined proximal signaling by IL-2 and IL-15 and CD8(+) T cell primary and memory responses as a consequence of varied CD122-dependent signaling. Initially, IL-2 and IL-15 induced similar p-STAT5 and p-S6 activation, but these activities were only sustained by IL-2. Transient IL-15-dependent signaling is due to limited expression of IL-15Rα. To investigate the outcome of varied CD122 signaling for CD8(+) T cell responses in vivo, OT-I T cells were used from mouse models where CD122 signals were attenuated by mutations within the cytoplasmic tail of CD122 or intrinsic survival function was provided in the absence of CD122 expression by transgenic Bcl-2. In the absence of CD122 signaling, generally normal primary response occurred, but the primed CD8(+) T cells were not maintained. In marked contrast, weak CD122 signaling supported development and survival of T central-memory (T(CM)) but not T effector-memory (T(EM)) cells. Transgenic expression of Bcl-2 in CD122(-/-) CD8(+) T cells also supported the survival and persistence of T(CM) cells but did not rescue T(EM) development. These data indicate that weak CD122 signals readily support T(CM) development largely through providing survival signals. However, stronger signals, independent of Bcl-2, are required for T(EM) development. Our findings are consistent with a model whereby low, intermediate, and high CD122 signaling support T(CM) memory survival, T(EM) programming, and terminal T effector cell differentiation, respectively.     

Innate memory phenotype CD4+ T cells play a role in early protection against infection by Listeria monocytogenes in a CD30L-dependent manner

CD30 ligand (CD30L, CD153) is a type II membrane-associated glycoprotein belonging to the tumor necrosis factor family. It is shown here that CD30L knock out (KO) mice are highly susceptible to primary infection with Listeria monocytogenes as assessed by the survival rate. There were significantly more bacteria on day 3 after infection in the peritoneal cavity, spleen and liver of CD30LKO mice than in wild type (WT) mice. The innate function of memory phenotype (MP) CD44+ CD4+ T cells for interferon-gamma production was significantly lower in CD30LKO mice than in WT mice in response to interleukin (IL)-12 and IL-15 in vitro. Depletion of CD4+ T cells by in vivo administration of anti-CD4 mAb at an early stage after infection hampered protection against Listeria. Furthermore, in vivo administration of agonistic anti-CD30 mAb restored protection against Listeria in CD30LKO mice, whereas treatment with soluble mCD30-Ig hampered protection in WT mice. Taken together, it appears that CD30L/CD30 signaling plays an important role in innate MPCD4+ T cell-mediated protection against infection with L. monocytogenes.     

A novel role of CD30L/CD30 signaling by T-T cell interaction in Th1 response against mycobacterial infection

A CD30 ligand (CD30L, CD153) is a type II membrane-associated glycoprotein belonging to the TNF family. To illustrate the potential role of CD30L in CD4(+) Th1 cell responses, we investigated the fate of Ag-specific CD4(+) T cells in CD30L-deficient (CD30L(-/-)) mice after Mycobacterium bovis bacillus Calmette-Guérin (BCG) infection. The number of bacteria was significantly higher in organs of CD30L(-/-) mice than in wild-type (WT) mice 4 wk postinfection. The numbers of purified protein derivative- or Ag85B-specific-IFN-gamma-producing-CD4(+) T cells in spleen, lung, or peritoneal exudate cells were significantly fewer in CD30L(-/-) mice than in WT mice. During the infection, CD30L was expressed mainly by CD44(+)CD3(+)CD4(+) T cells but not by CD3(+)CD8(+) T cells, B cells, dendritic cells, or macrophages. Costimulation with agonistic anti-CD30 mAb or coculturing with CD30L-transfected P815 cells restored IFN-gamma production by CD4(+) T cells from BCG-infected CD30L(-/-) mice. Coculturing with CD30L(+/+)CD4(+) T cells from BCG-infected WT mice also restored the number of IFN-gamma(+)CD30L(-/-)CD4(+) T cells. When transferred into the CD30L(+/+) mice, Ag-specific donor CD30L(-/-) CD4(+) T cells capable of producing IFN-gamma were restored to the compared level seen in CD30L(+/+) CD4(+) T cells on day 10 after BCG infection. When naive CD30L(+/+) T cells were transferred into CD30L(-/-) mice, IFN-gamma-producing-CD4(+) Th1 cells of donor origin were normally generated following BCG infection, and IFN-gamma-producing-CD30L(-/-)CD4(+) Th1 cells of host origin were partly restored. These results suggest that CD30L/CD30 signaling executed by CD30(+) T-CD30L(+) T cell interaction partly play a critical role in augmentation of Th1 response capable of producing IFN-gamma against BCG infection.     

CD30/CD30 ligand (CD153) interaction regulates CD4+ T cell-mediated graft-versus-host disease

CD30, a TNFR family member, is expressed on activated CD4(+) and CD8(+) T cells and B cells and is a marker of Hodgkin's lymphoma; its ligand, CD30L (CD153) is expressed by activated CD4(+) and CD8(+) T cells, B cells, and macrophages. Signaling via CD30 can lead to proliferation or cell death. CD30-deficient (-/-) mice have impaired thymic negative selection and increased autoreactivity. Although human alloreactive T cells preferentially reside within the CD30(+) T cell subset, implicating CD30 as a regulator of T cell immune responses, the role of CD30/CD153 in regulating graft-vs-host disease (GVHD) has not been reported. We used a neutralizing anti-CD153 mAb, CD30(-/-) donor mice, and generated CD153(-/-) recipient mice to analyze the effect of CD30/CD153 interaction on GVHD induction. Our data indicate that the CD30/CD153 pathway is a potent regulator of CD4(+), but not CD8(+), T cell-mediated GVHD. Although blocking CD30/CD153 interactions in vivo did not affect alloreactive CD4(+) T cell proliferation or apoptosis, a substantial reduction in donor CD4(+) T cell migration into the gastrointestinal tract was readily observed with lesser effects in other GVHD target organs. Blockade of the CD30/CD153 pathway represents a new approach for preventing CD4(+) T cell-mediated GVHD.     

Molecular signatures distinguish human central memory from effector memory CD8 T cell subsets

Memory T cells are heterogeneous in terms of their phenotype and functional properties. We investigated the molecular profiles of human CD8 naive central memory (T(CM)), effector memory (T(EM)), and effector memory RA (T(EMRA)) T cells using gene expression microarrays and phospho-protein-specific intracellular flow cytometry. We demonstrate that T(CM) have a gene expression and cytokine signaling signature that lies between that of naive and T(EM) or T(EMRA) cells, whereas T(EM) and T(EMRA) are closely related. Our data define the molecular basis for the different functional properties of central and effector memory subsets. We show that T(EM) and T(EMRA) cells strongly express genes with known importance in CD8 T cell effector function. In contrast, T(CM) are characterized by high basal and cytokine-induced STAT5 phosphorylation, reflecting their capacity for self-renewal. Altogether, our results distinguish T(CM) and T(EM)/T(EMRA) at the molecular level and are consistent with the concept that T(CM) represent memory stem cells.     

Human effector memory CD4+ T cells directly recognize allogeneic endothelial cells in vitro and in vivo

The frequency of circulating alloreactive human memory T cells correlates with allograft rejection. Memory T cells may be divided into effector memory (T(EM)) and central memory (T(CM)) cell subsets, but their specific roles in allograft rejection are unknown. We report that CD4+ T(EM) (CD45RO+ CCR7- CD62L-) can be adoptively transferred readily into C.B-17 SCID/bg mice and mediate the destruction of human endothelial cells (EC) in vascularized human skin grafts allogeneic to the T cell donor. In contrast, CD4+ T(CM) (CD45RO+ CCR7+ CD62L+) are inefficiently transferred and do not mediate EC injury. In vitro, CD4+ T(EM) secrete more IFN-gamma within 48 h in response to allogeneic ECs than do T(CM). In contrast, T(EM) and T(CM) secrete comparable amounts of IFN-gamma in response to allogeneic monocytes (Mo). In the same cultures, both T(EM) and T(CM) produce IL-2 and proliferate in response to IFN-gamma-treated allogeneic human EC or Mo, but T(CM) respond more vigorously in both assays. Blockade of LFA-3 strongly inhibits both IL-2 and IFN-gamma secretion by CD4+ T(EM) cultured with allogeneic EC but only minimally inhibits responses to allogeneic Mo. Blockade of CD80 and CD86 strongly inhibits IL-2 but not IFN-gamma production by in response to allogeneic EC or Mo. Transduction of EC to express B7-2 enhances allogeneic T(EM) production of IL-2 but not IFN-gamma. We conclude that human CD4+ T(EM) directly recognize and respond to allogeneic EC in vitro by secreting IFN-gamma and that this response depends on CD2 but not CD28. Consistent with EC activation of effector functions, human CD4+ T(EM) can mediate allogeneic EC injury in vivo.     

Antigen-specific central memory CD4+ T lymphocytes produce multiple cytokines and proliferate in vivo in humans

The function of Ag-specific central (T(CM)) and effector (T(EM)) memory CD4+ T lymphocytes remains poorly characterized in vivo in humans. Using CD154 as a marker of Ag-specific CD4+T cells, we studied the differentiation of memory subsets following anti-hepatitis B immunization. Hepatitis B surface Ag (HBs)-specific memory CD4+T cells were heterogeneous and included T(CM) (CCR7+CD27+) and T(EM) (CCR7(-)CD27(+/-)). HBs-specific T(CM) and T(EM) shared the capacity to produce multiple cytokines, including IL-2 and IFN-gamma. Several years postimmunization, approximately 10% of HBs-specific memory CD4+ T cells were in cycle (Ki67+) and the proliferating cells were CCR7+. These results suggest that the model of functional specialization of T(CM) and T(EM) cannot be applied to protein vaccine Ags and support the concept that T(CM) are capable of self-renewal and contribute to maintain the pool of memory cells.     

Reversible senescence in human CD4+CD45RA+CD27- memory T cells

Persistent viral infections and inflammatory syndromes induce the accumulation of T cells with characteristics of terminal differentiation or senescence. However, the mechanism that regulates the end-stage differentiation of these cells is unclear. Human CD4(+) effector memory (EM) T cells (CD27(-)CD45RA(-)) and also EM T cells that re-express CD45RA (CD27(-)CD45RA(+); EMRA) have many characteristics of end-stage differentiation. These include the expression of surface KLRG1 and CD57, reduced replicative capacity, decreased survival, and high expression of nuclear γH2AX after TCR activation. A paradoxical observation was that although CD4(+) EMRA T cells exhibit defective telomerase activity after activation, they have significantly longer telomeres than central memory (CM)-like (CD27(+)CD45RA(-)) and EM (CD27(-)CD45RA(-)) CD4(+) T cells. This suggested that telomerase activity was actively inhibited in this population. Because proinflammatory cytokines such as TNF-α inhibited telomerase activity in T cells via a p38 MAPK pathway, we investigated the involvement of p38 signaling in CD4(+) EMRA T cells. We found that the expression of both total and phosphorylated p38 was highest in the EM and EMRA compared with that of other CD4(+) T cell subsets. Furthermore, the inhibition of p38 signaling, especially in CD4(+) EMRA T cells, significantly enhanced their telomerase activity and survival after TCR activation. Thus, activation of the p38 MAPK pathway is directly involved in certain senescence characteristics of highly differentiated CD4(+) T cells. In particular, CD4(+) EMRA T cells have features of telomere-independent senescence that are regulated by active cell signaling pathways that are reversible.     

Dynamic differentiation of activated human peripheral blood CD8+ and CD4+ effector memory T cells

Two functionally different memory T cell subsets were originally defined based on their different CCR7 expression profile, but the lineage relationship between these subsets referred to as central memory T cells (T(CM)) and effector memory T cells (T(EM)), is not resolved. A prevalent model proposes a linear progressive differentiation from T(CM) to T(EM). Our results demonstrate that on activation, human CCR7-CD62L- peripheral blood CD8+ and CD4+ T(EM) cells exhibit a dynamic differentiation, involving transient as well as stable changes to T(CM) phenotype and properties. Whereas the larger fraction of T(EM) cells increases expression of effector molecules, such as perforin or IFN-gamma, a smaller fraction first acquires CCR7 expression. We demonstrate that this acquisition of lymph node homing potential is associated with strong proliferation similar to that of activated T(CM) cells. After proliferation, most of these cells lose CCR7 expression again and acquire effector functions (e.g., perforin production). A small proportion (approximately 6%), however, maintain phenotypic and functional T(CM) properties over a long time interval. These results suggest that T(EM) cells provide immediate effector function by a fraction of cells as well as self-renewal by others through up-regulation of CCR7 followed by either secondary peripheral effector function or long term maintenance of T(CM)-like properties.     

Differential requirements for OX40 signals on generation of effector and central memory CD4+ T cells

Memory T cells can be divided into effector memory (T(EM)) and central memory (T(CM)) subsets based on their effector function and homing characteristics. Although previous studies have demonstrated that TCR and cytokine signals mediate the generation of the two memory subsets of CD8(+) T cells, the mechanisms for generation of the CD4(+) T(EM) and T(CM) cell subsets are unknown. We found that OX40-deficient mice showed a marked reduction in the number of CD4(+) T(EM) cells, whereas the number of CD4(+) T(CM) cells was normal. Adoptive transfer experiments using Ag-specific CD4(+) T cells revealed that OX40 signals during the priming phase were indispensable for the optimal generation of the CD4(+) T(EM), but not the CD4(+) T(CM) population. In a different transfer experiment with in vitro established CD4(+)CD44(high)CD62L(low) (T(EM) precursor) and CD4(+)CD44(high)CD62L(high) (T(CM) precursor) subpopulations, OX40-KO T(EM) precursor cells could not survive in the recipient mice, whereas wild-type T(EM) precursor cells differentiated into both T(EM) and T(CM) cells. In contrast, T(CM) precursor cells mainly produced T(CM) cells regardless of OX40 signals, implying the dispensability of OX40 for generation of T(CM) cells. Nevertheless, survival of OX40-KO T(EM) cells was partially rescued in lymphopenic mice. During in vitro recall responses, the OX40-KO T(EM) cells that were generated in lymphopenic recipient mice showed impaired cytokine production, suggesting an essential role for OX40 not only on generation but also on effector function of CD4(+) T(EM) cells. Collectively, the present results indicate differential requirements for OX40 signals on generation of CD4(+) T(EM) and T(CM) cells.     

Suppression of memory CD8 T cell generation and function by tryptophan catabolism

Dendritic cell-derived indoleamine 2,3-dioxygenase (IDO) suppresses naive T cell proliferation and induces their apoptosis by catalyzing tryptophan, and hence is essential for the maintenance of peripheral tolerance. However, it is not known whether memory T cells are subject to the regulation by IDO-mediated tryptophan catabolism, as memory T cells respond more rapidly and vigorously than their naive counterparts and are resistant to conventional costimulatory blockade. In this study, we present the evidence that memory CD8+ T cells are susceptible to tryptophan catabolism mediated by IDO. We found that overexpression of IDO in vivo attenuated the generation of both central memory CD8+ T cells (T(CM)) and effector memory CD8+ T cells (T(EM)) while suppressing IDO activity promoted their generation. Moreover, IDO overexpression suppressed the effector function of T(CM) cells or T(CM) cell-mediated allograft rejection as well as their proliferation in vivo. Interestingly, T(CM) cells were resistant to apoptosis induced by tryptophan catabolism. However, IDO overexpression did not suppress the effector function of T(EM) cells or T(EM) cell-mediated allograft rejection, suggesting that T(EM) cells, unlike T(CM) cells, do not require tryptophan for their effector function once they are generated. This study provides insight into the mechanisms underlying the differential regulation of memory T cell responsiveness and has clinical implications for vaccination or tolerance induction.     

Central memory CD8+ T cells appear to have a shorter lifespan and reduced abundance as a function of HIV disease progression

Progressive HIV disease has been associated with loss of memory T cell responses to Ag. To better characterize and quantify long-lived memory T cells in vivo, we have refined an in vivo labeling technique to study the kinetics of phenotypically distinct, low-frequency CD8(+) T cell subpopulations in humans. HIV-negative subjects and antiretroviral-untreated HIV-infected subjects in varying stages of HIV disease were studied. After labeling the DNA of dividing cells with deuterated water ((2)H(2)O), (2)H-label incorporation and die-away kinetics were quantified using a highly sensitive FACS/mass spectrometric method. Two different populations of long-lived memory CD8(+) T cells were identified in HIV-negative subjects: CD8(+)CD45RA(-)CCR7(+)CD28(+) central memory (T(CM)) cells expressing IL-7Ralpha and CD8(+)CD45RA(+)CCR7(-)CD28(-) RA effector memory (T(EMRA)) cells expressing CD57. In pilot studies in HIV-infected subjects, T(CM) cells appeared to have a shorter half-life and reduced abundance, particularly in those with high viral loads; T(EMRA) cells, by contrast, retained a long half-life and accumulated in the face of progressive HIV disease. These data are consistent with the hypothesis that IL-7Ralpha(+) T(CM) cells represent true memory CD8(+) T cells, the loss of which may be responsible in part for the progressive loss of T cell memory function during progressive HIV infection.     

Protective effector memory CD4 T cells depend on ICOS for survival

Memory CD4 T cells play a vital role in protection against re-infection by pathogens as diverse as helminthes or influenza viruses. Inducible costimulator (ICOS) is highly expressed on memory CD4 T cells and has been shown to augment proliferation and survival of activated CD4 T cells. However, the role of ICOS costimulation on the development and maintenance of memory CD4 T cells remains controversial. Herein, we describe a significant defect in the number of effector memory (EM) phenotype cells in ICOS(-/-) and ICOSL(-/-) mice that becomes progressively more dramatic as the mice age. This decrease was not due to a defect in the homeostatic proliferation of EM phenotype CD4 T cells in ICOS(-/-) or ICOSL(-/-) mice. To determine whether ICOS regulated the development or survival of EM CD4 T cells, we utilized an adoptive transfer model. We found no defect in development of EM CD4 T cells, but long-term survival of ICOS(-/-) EM CD4 T cells was significantly compromised compared to wild-type cells. The defect in survival was specific to EM cells as the central memory (CM) ICOS(-/-) CD4 T cells persisted as well as wild type cells. To determine the physiological consequences of a specific defect in EM CD4 T cells, wild-type and ICOS(-/-) mice were infected with influenza virus. ICOS(-/-) mice developed significantly fewer influenza-specific EM CD4 T cells and were more susceptible to re-infection than wild-type mice. Collectively, our findings demonstrate a role for ICOS costimulation in the maintenance of EM but not CM CD4 T cells.     

Lack of ICAM-1 on APCs during T cell priming leads to poor generation of central memory cells

ICAM-1/LFA-1 interactions are known to enhance T cell/APC interactions and to promote T cell activation and cytokine secretion. We have analyzed the consequences of ICAM-1-mediated signaling on the generation of memory T cell subsets. We report that lack of ICAM-1 on APCs, but not on T cells, leads to poor T cell activation and proliferation in vitro and in vivo, and that the defect can be compensated by Ag dose, exogenous IL-2, additional costimulation, and by increasing responder T cell density on APCs. ICAM-1-null mice do not respond to immunization with OVA peptide, but immunization with OVA or with Salmonella typhimurium leads to good T cell proliferation 7-10 days later, and clearance of a challenge infection is equivalent to that of wild-type mice. However, when followed over time, recall proliferation and antibacterial immunity decay rapidly in ICAM-1-null mice, while recall cytokine responses are unaffected. The decline in immunity is not related to poor survival of T cells activated on ICAM-1-null APCs, or to poor generation of effectors in ICAM-1-null mice. Phenotypic analysis of T cells stimulated on ICAM-1-null APCs reveals preferential generation of CD44(high) CD62L(low) effector memory cells (T(EM)) over CD44(high) CD62L(high) central memory cells (T(CM)). Further, while the proportion of naive:memory T cells is similar in unmanipulated wild-type and ICAM-1-null mice, there is an accumulation of T(EM) cells, and a high T(EM):T(CM) ratio in aging ICAM-1-null mice. Together, the data indicate that signaling through LFA-1 during T cell activation may be involved in commitment to a proliferation-competent memory pool.     

IL-15 transpresentation augments CD8+ T cell activation and is required for optimal recall responses by central memory CD8+ T cells

Although the adaptive immune system has a remarkable ability to mount rapid recall responses to previously encountered pathogens, the cellular and molecular signals necessary for memory CD8(+) T cell reactivation are poorly defined. IL-15 plays a critical role in memory CD8(+) T cell survival; however, whether IL-15 is also involved in memory CD8(+) T cell reactivation is presently unclear. Using artificial Ag-presenting surfaces prepared on cell-sized microspheres, we specifically addressed the role of IL-15 transpresentation on mouse CD8(+) T cell activation in the complete absence of additional stimulatory signals. In this study we demonstrate that transpresented IL-15 is significantly more effective than soluble IL-15 in augmenting anti-CD3epsilon-induced proliferation and effector molecule expression by CD8(+) T cells. Importantly, IL-15 transpresentation and TCR ligation by anti-CD3epsilon or peptide MHC complexes exhibited synergism in stimulating CD8(+) T cell responses. In agreement with previous studies, we found that transpresented IL-15 preferentially stimulated memory phenotype CD8(+) T cells; however, in pursuing this further, we found that central memory (T(CM)) and effector memory (T(EM)) CD8(+) T cells responded differentially to transpresented IL-15. T(CM) CD8(+) T cells undergo Ag-independent proliferation in response to transpresented IL-15 alone, whereas T(EM) CD8(+) T cells are relatively unresponsive to transpresented IL-15. Furthermore, upon Ag-specific stimulation, T(CM) CD8(+) T cell responses are enhanced by IL-15 transpresentation, whereas T(EM) CD8(+) T cell responses are only slightly affected, both in vitro and in vivo. Thus, our findings distinguish the role of IL-15 transpresentation in the stimulation of distinct memory CD8(+) T cell subsets, and they also have implications for ex vivo reactivation and expansion of Ag-experienced CD8(+) T cells for immunotherapeutic approaches.     

Tyk2 signaling in host environment plays an important role in contraction of antigen-specific CD8+ T cells following a microbial infection

Tyrosine kinase 2 (Tyk2), a member of JAK signal transducer family contributes to the signals triggered by IL-12 for IFN-gamma production. To elucidate potential roles of Tyk2 in generation and maintenance of Ag-specific CD8+ T cells, we followed the fate of OVA-specific CD8+ T cells in Tyk2-deficient (-/-) mice after infection with recombinant Listeria monocytogenes expressing OVA (rLM-OVA). Results showed that the numbers of OVA(257-264)/K(b) tetramer-positive CD8+ T cells in Tyk2(-/-) mice were almost the same as those in Tyk2(+/+) mice at the expansion phase on day 7 but were significantly larger in Tyk2(-/-) mice than those in Tyk2(+/+) mice at the contraction phase on day 10 and at the memory phase on day 60 after infection. The intracellular expression level of active caspase-3 was significantly decreased in the OVA-specific CD8+ T cells of Tyk2(-/-) mice on day 7 compared with those of Tyk2(+/+) mice. Adaptive transfer experiments revealed that Tyk2 signaling in other factors rather than CD8+ T cells played a regulatory role in CD8+ T cell contraction following infection. Administration of exogenous IFN-gamma from day 6 to day 9 restored the CD8+ T cell contraction in Tyk2(-/-) mice after infection with rLM-OVA. These results suggest that Tyk2 signaling for IFN-gamma production in host environment plays an important role in contraction of effector CD8+ T cells following a microbial infection.     

Stable CD8+ T cell memory during persistent Trypanosoma cruzi infection

CD8(+) T cell responses to persistent infections caused by intracellular pathogens are dominated by resting T effectors and T effector memory cells, with little evidence suggesting that a T central memory (T(CM)) population is generated. Using a model of Trypanosoma cruzi infection, we demonstrate that in contrast to the T effector/T effector memory phenotype of the majority of T. cruzi-specific CD8(+) T cells, a population of cells displaying hallmark characteristics of T(CM) cells is also present during long-term persistent infection. This population expressed the T(CM) marker CD127 and a subset expressed one or more of three other T(CM) markers: CD62L, CCR7, and CD122. Additionally, the majority of CD127(high) cells were KLRG1(low), indicating that they have not been repetitively activated through TCR stimulation. These CD127(high) cells were better maintained than their CD127(low) counterparts following transfer into naive mice, consistent with their observed surface expression of CD127 and CD122, which confer the ability to self-renew in response to IL-7 and IL-15. CD127(high) cells were capable of IFN-gamma production upon peptide restimulation and expanded in response to challenge infection, indicating that these cells are functionally responsive upon Ag re-encounter. These results are in contrast to what is typically observed during many persistent infections and indicate that a stable population of parasite-specific CD8(+) T cells capable of Ag-independent survival is maintained in mice despite the presence of persistent Ag.