Unidirectional development of CD8+ central memory T cells into protective Listeria-specific effector memory T cells

Three distinct subsets of antigen-experienced CD8(+) T cells have been identified so far: short-living effector T cells (T(EC)) and two long-living subsets, described as central (T(CM)) and effector memory (T(EM)) T cells. The lineage relationships of these subpopulations as well as their involvement in protection have not yet been conclusively determined. We recently described a novel marker combination (CD127 and CD62L) to identify all three major CD8(+) T cell subsets in mice infected with Listeria monocytogenes (L.m.). Extensive lineage relationship analyses on highly purified subpopulations after in vitro and in vivo stimulation demonstrated that T(CM) can develop into T(EM) or T(EC), whereas T(EM) can only progress to T(EC) cells. Short-living T(EC) never regained a T(EM) or T(CM) phenotype. These data strongly suggest a hierarchical and unidirectional order of developmental stages. In vivo priming protocols that preferentially induced one of the different CD8(+) T cell subsets demonstrated that predominance of T(EM) (CD40 stimulation) correlated best with effective protection against L.m., whereas generation of neither T(CM) (by immunization with heat-killed L.m.) nor T(EC) (by systemic co-administration of CpG during primary infection) conferred substantial long-term protective immunity. These findings have important implications for the design of more effective T cell-based vaccines.     

AsialoGM1+CD8+ central memory-type T cells in unimmunized mice as novel immunomodulator of IFN-gamma-dependent type 1 immunity

In unimmunized specific pathogen-free mice, there are unique memory-type CD8(+) T cell populations expressing asialoGM1 (ASGM1). These cells were classified into central memory-type T cells (T(CMT)) judging from their expression profile of CD44, IL-2Rbeta, CD62L and CCR7 cell-surface molecules. Among CD44(high)CD8(+) so-called memory CD8(+) T cell population, ASGM1(+)CD44(high)CD8(+) T(CMT), but not ASGM1(-)CD44(high)CD8(+) memory T cells, produced IFN-gamma by stimulation with anti-CD3 mAb. The physiological significance of ASGM1(+)CD8(+) T(CMT) as early source of IFN-gamma was also demonstrated in vivo. Namely, intravenous injection of anti-CD3 mAb (2 microg) resulted in early activation of IFN-gamma-producing ASGM1(+)CD8(+) T(CMT) cells as well as NKT and NK cells. Unexpectedly, however, few IFN-gamma-producing CD4(+) T cells were detected until 4 h after anti-CD3 mAb administration. Thus, ASGM1(+)CD8(+) T(CMT) were demonstrated to be early IFN-gamma producer, which may be crucial for T(h)1-dependent cellular immunity. Indeed, co-culture of naive CD4(+) T cells with ASGM1(+)CD8(+) T(CMT) but not ASGM1(-)CD8(+) T cells caused a great acceleration of IFN-gamma-producing T(h)1 cells in vitro. Finally, we found that T(h)1-prone C57BL/6 mice possessed higher percentage (10%) of ASGM1(+)CD8(+) T(CMT) in CD8(+) T cells compared with that (3%) of T(h)2-prone BALB/c mice. Moreover, ASGM1(+)CD8(+) T(CMT) derived from C57BL/6 mice produced higher levels of IFN-gamma compared with those from BALB/c mice. Thus, ASGM1(+)CD8(+) T(CMT), whose differentiation in vivo is genetically controlled, appear to play a critical role in the control of type 1 immunity, which is essential for therapy of tumors and infectious diseases.     

Increased CD95/Fas-induced apoptosis of HIV-specific CD8(+) T cells.

Why HIV-specific CD8(+) T cells ultimately fail to clear or control HIV infection is not known. We show here that HIV-specific CD8(+) T cells exhibit increased sensitivity to CD95/Fas-induced apoptosis. This apoptosis is 3-fold higher compared to CMV-specific CD8(+) T cells from the same patients. HIV-specific CD8(+) T cells express the CD45RA(-)CD62L(-) but lack the CD45RA(+)CD62L(-) T cell effector memory (T(EM)) phenotype. This skewing is not found in CMV- and EBV-specific CD8(+) T cells in HIV-infected individuals. CD95/Fas-induced apoptosis is much higher in the CD45RA(-)CD62L(-) T(EM) cells. However, cytotoxicity and IFNgamma production by HIV-specific CD8(+) T cells is not impaired. Our data suggest that the survival and differentiation of HIV-specific CD8(+) T cells may be compromised by CD95/Fas apoptosis induced by FasL-expressing HIV-infected cells.     

Development of CD25(+) T cells secreting transforming growth factor-beta1 by altered peptide ligands expressed as self-antigens

This study demonstrates that CD4(+) T cells specific for an altered self-antigen differentiate to T cells secreting transforming growth factor (TGF)-beta1. In this study, we utilized mice expressing an altered peptide ligand containing a single amino acid substitution of moth cytochrome c 88-103 peptide. In these mice, antigen-specific T cells escaping thymic negative selection differentiated into T cells with an effector/memory phenotype, CD44(high), CD45RB(low), CD62L(-) and CD25(intermediate). The expression of CD25 and high levels of CD44 was initiated in the thymus during the development from CD4(+)CD8(+) to CD4(+); a large proportion of maturing CD4(+) thymocytes expressed both CD25 and high levels of CD44. Upon antigen stimulation, CD4(+) T cells derived from these mice did not proliferate or secrete IL-2, but secreted TGF-beta1. Neutralizing antibodies to TGF-beta1 reversed the impaired proliferative responses to the antigen, suggesting that TGF-beta1 secreted from these T cells negatively regulates T cell responses.     

Immune response to Moloney-murine leukemia virus-induced antigens in bone marrow

By exploring induction and persistence of virus-specific memory CD8(+) T cells in the BM of Moloney-murine sarcoma/leukemia virus-immune mice, we observed that the amount of activated CD8(+)CD62L(-) cells increased more rapidly and persisted for a longer period than in peripheral organs. Among the CD8(+)CD62L(-) subset, the few cells, specific for M-MuLV encoded antigens, expressing TCRVβ5 rearrangements increased in an explosive manner doubling the percentage of TCRVβ5(+) subset so that as a final result more than 10% of CD8(+) lymphocytes became potential virus-specific cytotoxic effectors. The numerical expansion of Vβ5(+) cells started and persisted in the same proportion among both CD8(+)CD62L(-) and CD8(+)CD62L(+) subsets. In these subsets the analysis of CD44 phenotype, to distinguish effector (TEM) and central (TCM) memory, evidenced a twofold increase of Vβ5(+) TEM percentage and fourfold increase of Vβ5(+) TCM. In parallel, the non virus-specific Vβ5(-) counterpart, also numerically increased due to the CD8(+) expansion, was partially reduced as TEM percentage and doubled as TCM percentage. We conclude that the immune response to M-MuLV encoded antigens in BM generate not only a large number of virus-specific memory cells but also the re-shaping of the entire memory T cell repertoire.     

Activating CD94:NKG2C and inhibitory CD94:NKG2A receptors are expressed by distinct subsets of committed CD8+ TCR alphabeta lymphocytes

A subset of CD8(+) T cells express the natural killer cell receptors CD94:NKG2A or CD94:NKG2C. We found that although many CD8(+) T cells transcribe CD94 and NKG2C, expression of a functional CD94:NKG2C receptor is restricted to highly differentiated effector cells. CD94:NKG2A is expressed by a different subset consisting of CCR7(+) memory cells and CCR7(-) effector cells. Since NKG2A can only be induced on naive CD8(+) T cells while CD94(-) memory cells are refractory, it is likely that commitment to the CD94:NKG2A(+) subset occurs during the first encounter with antigen. CCR7(+)CD94:NKG2A(+) T cells recirculate through lymph nodes where upon activation, they produce large quantities of IFN-gamma. These cells occur as a separate CD94:NKG2A(+) T cell lineage with a distinct TCR repertoire that differs from that of the other CD8(+)CD94(-) T cells activated in situ.     

Accumulation of memory T cells from childhood to old age: central and effector memory cells in CD4(+) versus effector memory and terminally differentiated memory cells in CD8(+) compartment

Memory T cells can be classified as central memory (T(CM), CD45RA(neg)CCR7(+)), effector memory (T(EM), CD45RA(neg)CCR7(neg)), and terminally differentiated cells (T(TD), CD45RA(+)CCR7(neg)) with different homing and effector capacities. In 101 healthy subjects aged from 5 to 96 years, distinct dynamics were evidenced between circulating CD4(+) and CD8(+) T cell populations. Naive CD4(+) and CD8(+) T cells decreased linearly with age, CD8(+) twice more rapidly. Memory cells outnumbered naive cells on average at 37.4 in the CD4(+) and 29.5 years of age in the CD8(+) pool. CD4(+) T(CM) and T(EM) cells were positively correlated and increased linearly at a similar rate with age, while CD4(+) T(TD) remained rare. CD8(+) T(EM) and T(TD) accumulated linearly with age, while T(CM) increased only slightly, and each memory subset was negatively correlated to the two others. Almost all CD8(+) T(TD) and some CD8(+) T(EM) had lost CD28 expression. Despite different dynamics, each individual CD4(+) naive and memory subset was correlated to the synonymous CD8(+) subset. Half of the subjects aged 65 years or older were characterized by extremely reduced CD8(+) naive and increased CD8(+) T(TD) cell counts, which could indicate an acceleration of the decay of the immune system from this age onward.     

Characterization of Effector Memory CD8+ T Cells in the Synovial Fluid of Rheumatoid Arthritis

Little is known about the cellular characteristics of CD8(+) T cells in rheumatoid arthritis (RA). We addressed this by investigating whether the frequency of the CD8(+) T cell subsets and their phenotypic characteristics are altered in the peripheral blood and synovial fluid (SF) from patients with RA. In this study, CD8(+) T cells, mainly CD45RA(-) effector memory (EM) CD8(+) T cells, were increased significantly in the SF, but not in the peripheral blood from RA patients, compared with healthy controls. The synovial EM CD8(+) T cells were activated phenotypes with high levels of CD80, CD86, and PD-1, and had a proliferating signature in vivo upon Ki-67 staining, whereas the Fas-positive cells were prone to apoptosis. In addition, EM CD8(+) T cells in the SF were less cytotoxic, as they expressed less perforin and granzyme B. In particular, the proportions of synovial fluid mononuclear cells that were CCR4(+)CD8(+) T cells and IL-4-producing CD8(+) T cells (i.e., Tc2 cells) were significantly higher than those in peripheral blood mononuclear cells of patients with RA and healthy controls. In addition, the number of IL-10-producing CD8(+) suppressor T (Ts) cells increased significantly in the SF of RA patients. Especially, CD8(+) T cells were inversely correlated with disease activity. These findings strongly suggest that EM CD8(+) T cells in the SF are increased, likely because of inflammation, and they may be involved in modulating inflammation, thereby affecting the development and progression of RA.     

Sialyl lewisx antigen-expressing human CD4+ T and CD8+ T cells as initial immune responders in memory phenotype subsets

Cytokine production by memory T cells in secondary immune responses has a critical role in host defenses. Previously, we had demonstrated that a unique antigen composed of sialyl lewis(x) (sLe(x)) was expressed on CD45RO(+) memory-phenotype subsets of human T cells. Here, we found that the sLe(x) antigen was up-regulated on CD45RA(+) na?ve human CD4(+) T and CD8(+) T cells by TCR stimulation. In addition, sLe(x) antigen-expressing CD4(+) T and CD8(+) T cells in human PBMCs were activated immediately by cytokine stimulations composed of IL-2 plus IL-12 or IL-15 in an antigen-independent manner. Moreover, the sLe(x)-positive human CD8(+) T cells significantly enhanced reverse antibody-dependent cellular cytotoxicity compared with a sLe(x)-negative population. These findings clearly indicate that sLe(x) antigen-expressing memory phenotype CD4(+) T and CD8(+) T cells contribute to early-stage immunity by providing a source of IFN-gamma and cytotoxicity, suggesting that they would be a key immunomodulator in host defenses.     

Characterization of the immune status of CD8+ T cells in oral lesions of human immunodeficiency virus-infected persons with oropharyngeal Candidiasis

Oropharyngeal candidiasis (OPC) remains the most common oral infection in human immunodeficiency virus (HIV) disease. In a high percentage of HIV(+) persons with reduced CD4(+) T cells, oral lesions with Candida present at the outer epithelium have an accumulation of CD8(+) T cells at the epithelium-lamina propria interface associated with reduced expression of the mucosal cell-trafficking adhesion molecule E-cadherin. The purpose of the present study was to characterize the immune status of these CD8(+) T cells. Immunohistochemical staining for phenotypic and activation and costimulation markers was performed on frozen biopsy tissue sections from HIV(+) OPC(+) persons with accumulated CD8(+) T cells. CD8(+) T cells consisted primarily of central memory cells by virtue of positive CD45RO (memory) and CD27 (central memory) expression. However, concomitant negative expression of CD62L and CCR7 (effector memory) was suggestive of a transitioning memory phenotype within the tissue. Despite this, the cells are considered to be activated on the basis of positive expression of CD69. The CD8(+) T cells are not considered to be NK T cells or anti-HIV CD8(+) T cells because of negative or low expression of CD161 and vascular cell adhesion molecule, respectively. These results suggest that the accumulated mucosal migratory-challenged CD8(+) T cells are otherwise normal memory T cells in an activated state.     

Functionally distinct CD8+ memory T cell subsets in persistent EBV infection are differentiated by migratory receptor expression

Human memory T lymphocytes have recently been re-defined as central or effector memory cells (Sallusto, F., Lenig, D., Forster, R., Lipp, M. and Lanzavecchia, A., Nature 1999. 401: 708-712). Effector memory cells (T(em)) are targeted to the peripheral tissues and show rapid effector function in response to antigenic stimulation. Central memory (T(cm)) cells are targeted to the lymph nodes and cannot be immediately activated. In this report HLA-A2-Epstein-Barr virus (EBV) peptide tetramers have been used to characterize the EBV-specific CD8+ T cell subsets in persistent EBV infection. In short-term activation studies two populations of tetramer-positive T cells were identified. One group resembled T(em) cells in that they rapidly produced IFN-gamma and lacked the lymph node homing receptor, CD62L, the second was similar to T(cm) cells since they were CD62L+ but could not be immediately induced to express IFN-gamma.     

Distinct compartmentalization of CD4+ T-cell effector function versus proliferative capacity during pulmonary cryptococcosis

The activation and expansion of T cells and their acquisition of effector function are key steps in the development of the adaptive immune response. Most infections are predominantly outside of the lymphoid tissues, and it is unclear at what point developmentally and anatomically T cells acquire effector function in vivo. In these studies, we compared the activation and polarization of T cells during murine pulmonary Cryptococcus neoformans infection in the secondary lymphoid tissues and at the site of primary infection. Few CD4(+) and CD8(+) T cells expressed an activated phenotype (CD44(hi,) CD25(+), CD69(+), CD62L(lo), CD45RB(lo)) at the sites of clonal expansion (lymph nodes, spleen, and blood). In contrast, a high percentage of T cells expressed activation markers at the site of primary infection, the lungs. Additionally, the polarization of CD4(+) T cells to interferon-gamma-producing effector cells occurred at the site of infection, the lungs. CD4(+) and CD8(+) T cells from secondary lymphoid organs responded to TCR restimulation by proliferating, whereas T cells from the lungs proliferated poorly. This report demonstrates for the first time that T-cell activation and effector function in secondary lymphoid tissues during fungal infection is characteristically different from that at the site of primary infection.     

Down-regulation of CXCR4 expression on human CD8+ T cells during peripheral differentiation

Multi-color flow cytometric analysis on human CD8(+) T cell subsets revealed that CXCR4 is predominantly expressed on CD8(+) T cells with the naive CD27(+)CD28(+)CD45RA(+) phenotype, and is down-regulated during differentiation into those with an effector phenotype. The down-regulation of CXCR4 expression during peripheral differentiation was supported by the fact that the expression of CXCR4 on CD8(+) T cells was negatively correlated with that of perforin. The analysis of CCR5, CCR7, and CXCR4 co-expression further showed that CD8(+) T cells expressing a high level of CXCR4 are CCR7(+)CCR5(-) naive or central memory subsets, and those expressing a low level of CXCR4 were included in the CCR7(-)CCR5(+/-) memory/effector and effector subsets. Epstein Barr virus-specific CD8(+) T cells, which mostly express the memory phenotype, expressed CXCR4, while human cytomegalovirus-specific CD8(+) T cells, which mostly express the effector phenotype, partially expressed this receptor, showing that the expression of CXCR4 is also down-regulated during differentiation of viral antigen-specific CD8(+) T cells. The classification of human CD8(+) T cells based on the expression of these chemokine receptors should prove useful for studies that clarify the differentiation of human CD8(+) T cells.     

A clonal model for human CD8+ regulatory T cells: unrestricted contact-dependent killing of activated CD4+ T cells

Previous studies in murine systems have demonstrated that CD8(+) Treg cells down-regulate immune responses in vivo through suppressing activated CD4(+) T cells. Here we describe novel regulatory CD8(+) T-cell clones isolated from healthy human peripheral blood following in vitro stimulation with autologous Epstein-Barr virus (EBV)-specific CD4(+) T cells. TCR activation of CD4(+) target T cells was required for CD8(+) Treg cells to exert suppressive activity, which was mediated through lysis of CD4(+) targets in a cell contact-dependent manner. Suppression was independent of Foxp3 expression in CD8(+) Treg cells, HLA compatibility between CD8(+) Treg cells and CD4(+) target cells and antigen-specificity of CD4(+) target T cells. CD8(+) Treg clones expressed CD3 and a variety of TCR V(β) chains as well as CD56, CD69, CD62L and CD95 but did not express CD16, CD161, CXCR4 and CCR7. When used together, antibodies specific for CD11a/CD18 and CD8 inhibited suppressive activity of CD8(+) Treg clones. The ability to establish clonal CD8(+) T cells that maintain regulatory function in vitro will facilitate further studies to define this population in vivo and to identify the mechanisms used for recognition and suppression of activated target cells.     

Enhanced antigen-specific primary CD4+ and CD8+ responses by codelivery of ovalbumin and toll-like receptor ligand monophosphoryl lipid A in poly(D,L-lactic-co-glycolic acid) nanoparticles

The purpose of this research was to investigate the use of biodegradable poly(D,L-lactic-co-glycolic acid) nanoparticles (PLGA-NP) as a vaccine delivery system to codeliver antigen, ovalbumin (OVA) along with monophosphoryl lipid A (MPLA) as adjuvant for induction of potent CD4(+) and CD8(+) T cell responses. The primary CD4(+) T responses to OVA/MPLA NP were investigated using OVA-specific T cells from DO11.10 transgenic mice. Following adoptive transfer of these cells, mice were immunized s.c. by NP formulations. For assessing the CD8(+) responses, bone marrow derived dendritic cells (DCs) were pulsed with different OVA formulations, then, cocultured with CD8(+) T cells from OT-1 mice. T cell proliferation/activation and IFN-gamma secretion profile have been examined. Particulate delivery of OVA and MPLA to the DCs lead to markedly increase in in vitro CD8(+) T cell T cell proliferative responses (stimulation index >3000) and >13-folds increase in in vivo clonal expanded CD4(+) T cells. The expanded T cells were capable of cytokine secretion and expressed an activation and memory surface phenotype (CD62L(lo), CD11a(hi), and CD44(hi)). Codelivery of antigen and MPLA in PLGA-NP offers an effective method for induction of potent antigen specific CD4(+) and CD8(+) T cell responses.     

Virus-induced non-specific signals cause cell cycle progression of primed CD8(+) T cells but do not induce cell differentiation.

In this report the significance of virus-induced non-specific T cell activation was re-evaluated using transgenic mice in which about half of the CD8(+) T cells expressed a TCR specific for amino acids 33-41 of lymphocytic choriomeningitis virus glycoprotein I. This allowed tracing of cells with known specificity and priming history in an environment also containing a normal heterogeneous CD8(+) population which served as an intrinsic control. Three parameters of T cell activation were analyzed: cell cycle progression, phenotypic conversion and cytolytic activity. Following injection of the IFN inducer poly(I:C), proliferation of memory (CD44(hi)) CD8(+) T cells but no phenotypic or functional activation was observed. Following injection of an unrelated virus [vesicular stomatitis virus (VSV)], naive TCR transgenic cells did not become significantly activated with respect to any of the parameters investigated. In contrast, memory TCR transgenic cells were found to proliferate extensively early after VSV infection (day 0-3), whereas limited proliferation was observed later (day 3-6) when proliferation of non-transgenic CD8(+) T cells is maximal. This aborted response did not result from anergy to TCR stimulation, as memory TCR transgenic cells proliferated vigorously upon stimulation with their nominal peptide. Despite the massive proliferation of memory cells observed early after VSV infection, no phenotypic or functional activation was observed. Together these findings indicate that both non-specific and antigen-specific signals contribute to the initial virus-induced proliferation of CD8(+) T cells, but for further proliferation and differentiation to take place, TCR-ligand interaction is required. The implications for maintenance of T cell memory is discussed.     

Evidence that human CD8+CD45RA+CD27- cells are induced by antigen and evolve through extensive rounds of division.

We recently showed that circulating human CD8(+) effector cells have a CD45RA+CD27(-) membrane phenotype. In itself this phenotype appeared to pose a paradox: CD45RA, a marker expressed by unprimed cells, combined with absence of CD27, characteristic for chronically stimulated T cells. To investigate whether differentiation towards the CD45RA+CD27(-) phenotype is dependent on antigenic stimulation and involves cellular division, TCR Vbeta usage and telomeric restriction fragment (TRF) length were analyzed within distinct peripheral blood CD8(+) subsets. FACS analysis showed that the TCR Vbeta repertoire of CD8(+)CD45RA+CD27(-) cells differed significantly from that of unprimed CD8(+)CD45RA+CD27(+) cells. Moreover, in two out of six individuals large expansions of particular Vbeta families were observed in the CD8(+)CD45RA+CD27(-) subset. CDR3 spectrotyping and single-strand confirmation analysis revealed that within the CD8(+)CD45RA+CD27(-) population most of the 22 tested Vbeta families were dominated by oligoclonal expansions. The mean TRF length was found to be 2.3+/-1.0 kb shorter in the CD8(+)CD45RA+CD27(-) subset compared with the unprimed CD8(+)CD45RA+CD27(+) population, but did not differ substantially from that of memory type, CD8(+)CD45RA-CD27(+) T cells. These findings indicate that the CD8(+)CD45RA+CD27(-) cytotoxic effector population consists of antigen-induced, clonally expanded cells and confirm that the expression of CD45RA is not a strict marker of antigen non-experienced T cells.     

Differential requirements for antigen or homeostatic cytokines for proliferation and differentiation of human Vgamma9Vdelta2 naive, memory and effector T cell subsets

We have compared four human subsets of Vgamma9Vdelta2 T cells, naive (T(naive), CD45RA(+)CD27(+)), central memory (T(CM), CD45RA(-)CD27(+)), effector memory (T(EM), CD45RA(-)CD27(-)) and terminally differentiated (T(EMRA), CD45RA(+)CD27(-)), for their capacity to proliferate and differentiate in response to antigen or homeostatic cytokines. Cytokine responsiveness and IL-15R expression were low in T(naive) cells and progressively increased from T(CM) to T(EM) and T(EMRA) cells. In contrast, the capacity to expand in response to antigen or cytokine stimulation showed a reciprocal pattern and was associated with resistance to cell death and Bcl-2 expression. Whereas antigen-stimulated cells acquired a T(CM) or T(EM) phenotype, IL-15-stimulated cells maintained their phenotype, with the exception of T(CM) cells, which expressed CD27 and CD45RA in various combinations. These results, together with ex vivo bromodeoxyuridine incorporation experiments, show that human Vgamma9Vdelta2 memory T cells have different proliferation and differentiation potentials in vitro and in vivo and that T(EMRA) cells are generated from the T(CM) subset upon homeostatic proliferation in the absence of antigen.     

Active Crohn's Disease Patients Show a Distinctive Expansion of Circulating Memory CD4+CD45RO+CD28null T Cells

In a previous study we found an expansion of circulating memory (CD45RO(+)) CD4(+) T cells in patients with Crohn's disease (CD). The aim of this work was to investigate the phenotypic and functional characteristics of this T-cell subset in CD. We analyzed in peripheral blood CD4(+)CD45RO(+) T cells from CD patients the expression of surface markers associated to immune activation, costimulation, and apoptosis. In sorted CD4(+)CD45RO(+) T cells apoptosis was quantified by fluorescent annexin V binding. Healthy subjects and patients with ulcerative colitis and acute bacterial enterocolitis served as control groups. An increased percentage of memory CD4(+)CD45RO(+) T cells lacking the expression of costimulatory receptor CD28 was detected in patients with active CD when compared to the other groups evaluated. This expanded CD4(+)CD45RO(+)CD28(null) T-cell subset expressed mostly the effector-cell marker CD57(+). Both CD28 downregulation and CD57 expression correlated to CDAI and surrogate markers of disease activity. These phenotypic changes observed on CD4(+)CD45RO(+) T cells from active CD returned to values similar to healthy controls after clinical remission. Moreover, this memory CD28(null) T-cell subset might express more intracytoplasmic TNF and IFN-gamma than their CD28(+) counterpart. Significantly lower frequencies of memory CD4(+)CD45RO(+) T cells expressing CD95 apoptosis receptor were found in patients with active CD. Moreover, sorted CD4(+)CD45RO(+)and CD4(+)CD45RO(+) CD28(null) T cells from patients with active CD exhibited a lower apoptotic rate than that found in healthy controls and inactive CD patients. According to our data, circulating T lymphocytes from active CD patients show distinctive phenotypic and functional changes, characterized by an expansion of memory CD4(+)CD45RO(+)CD28(null) T cells expressing effector-associated cell surface molecules and displaying enhanced resistance to apoptosis.     

IL-15 enhances the function and inhibits CD95/Fas-induced apoptosis of human CD4+ and CD8+ effector-memory T cells

Although the effect of IL-15 has been described on murine cells in vitro and in vivo, its effect on human memory CD8(+) T cells is not well characterized. We show here that IL-15 preferentially enhances the activation and effector function of human effector-memory CD45RA(-)CD62L(-) and CD45RA(+)CD62L(-) CD4(+) and CD8(+) T cells in both healthy and HIV-infected individuals. We find that IL-15 increases 2- to 5-fold both the activation and secretion of the effector cytokines IFN-gamma and tumor necrosis factor (TNF)-alpha by anti-CD3-stimulated purified CD4(+) and CD8(+) T cells and peripheral blood mononuclear cells from healthy and HIV-infected individuals. Furthermore, IL-15 potently inhibits CD95/Fas-induced apoptosis of the effector-memory CD4(+) and CD8(+) T cells from HIV-infected individuals. These findings suggest that in addition to being a growth and survival factor for memory CD8(+) T cells, IL-15 is also a potent activator of human effector-memory CD8(+) T cells both in healthy and in HIV-infected individuals.