Occurrence of a silencer of the interleukin-2 gene in naive but not in memory resting T helper lymphocytes

Transcription Factor Ets-2 Acts as a Preinduction Repressor of Interleukin-2 (IL-2) Transcription in Naive T Helper Lymphocytes

IL-2 is the first cytokine produced when naive T helper (Th) cells are activated and differentiate into dividing pre-Th0 proliferating precursors. IL-2 expression is blocked in naive, but not activated or memory, Th cells by the transcription factor Ets-2 that binds to the antigen receptor response element (ARRE)-2 of the proximal IL-2 promoter. Ets-2 acts as an independent preinduction repressor in naive Th cells and does not interact physically with the transcription factor NFAT (nuclear factor of activated T-cells) that binds to the ARRE-2 in activated Th cells. In naive Th cells, Ets-2 mRNA expression, Ets-2 protein levels, and Ets-2 binding to ARRE-2 decrease upon cell activation followed by the concomitant expression of IL-2. Cyclosporine A stabilizes Ets-2 mRNA and protein when the cells are activated. Ets-2 silences directly constitutive or induced IL-2 expression through the ARRE-2. Conversely, Ets-2 silencing allows for constitutive IL-2 expression in unstimulated cells. Ets-2 binding to ARRE-2 in chromatin is stronger in naive compared with activated or memory Th cells; in the latter, Ets-2 participates in a change of the IL-2 promoter architecture, possibly to facilitate a quick response when the cells re-encounter antigen. We propose that Ets-2 expression and protein binding to the ARRE-2 of the IL-2 promoter are part of a strictly regulated process that results in a physiological transition of naive Th cells to Th0 cells upon antigenic stimulation. Malfunction of such a repression mechanism at the molecular level could lead to a disturbance of later events in Th cell plasticity, leading to autoimmune diseases or other pathological conditions.

Design and synthesis of a novel potent myelin basic protein epitope 87-99 cyclic analogue: enhanced stability and biological properties of mimics render them a potentially new class of immunomodulators

A cyclic analogue, [cyclo(87-99)MBP(87)(-)(99)], of the human immunodominant MBP(87)(-)(99) epitope, was designed based on ROESY/NMR distance information and modeling data for linear epitope 87-99, taking into account T-cell (Phe(89), Lys(91), Pro(96)) and HLA (His(88), Phe(90), Ile(93)) contact side-chain information. The cyclic analogue was found to induce experimental allergic encephalomyelitis (EAE), to bind HLA-DR4, and to increase CD4 T-cell line proliferation, like that of the conformationally related linear MBP(87)(-)(99) epitope peptide. The mutant cyclic peptides, the cyclo(91-99)[Ala(96)]MBP(87)(-)(99) and the cyclo(87-99)[Arg(91)Ala(96)]MBP(87)(-)(99), reported previously for suppressing, to a varying degree, autoimmune encephalomyelitis in a rat animal model, were found in this study to possess the following immunomodulatory properties: (i) they suppressed the proliferation of a CD4 T-cell line raised from a multiple sclerosis patient, (ii) they scored the best in vitro TH2/TH1 cytokine ratio in peripheral blood mononuclear cell cultures derived from 13 multiple sclerosis patients, inducing IL-10 selectively, and (iii) they bound to HLA-DR4, first to be reported for cyclic MBP peptides. In addition, cyclic peptides were found to be more stable to lysosomal enzymes and Cathepsin B, D, and H, compared to their linear counterparts. Taken together, these data render cyclic mimics as putative drugs for treating multiple sclerosis and potentially other Th1-mediated autoimmune diseases.

Mining microarray data to identify transcription factors expressed in naïve resting but not activated T lymphocytes

Transcriptional repressors controlling the expression of cytokine genes have been implicated in a variety of physiological and pathological phenomena. An unknown repressor that binds to the distal NFAT element of the interleukin-2 (IL-2) gene promoter in naive T-helper lymphocytes has been implicated in autoimmune phenomena and has emerged as a potentially important factor controlling the latency of HIV-1. The aim of this paper was the identification of this repressor. We resorted to public microarray databases looking for DNA-binding proteins that are present in naïve resting T cells but are downregulated when the cells are activated. A Bayesian data mining statistical analysis uncovered 25 candidate factors. Of the 25, NFAT4 and the oncogene ets-2 bind to the common motif AAGGAG found in the HIV-1 LTR and IL-2 probes. Ets-2 binding site contains the three G's that have been shown to be important for binding of the unknown factor; hence, we considered it the likeliest candidate. Electrophoretic mobility shift assays confirmed cross-reactivity between the unknown repressor and anti-ets-2 antibodies, and cotransfection experiments demonstrated the direct involvement of Ets-2 in silencing the IL-2 promoter. Designing experiments for transcription factor analysis using microarrays and Bayesian statistical methodologies provides a novel way toward elucidation of gene control networks.

Distinctive response of naïve lymphocytes from cord blood to primary activation via TCR

Umbilical cord blood (UCB) is now being considered an alternative to bone marrow for restoring hematopoiesis after myeloablative therapy. The lower risk of acute and chronic graft-versus-host disease in patients who received UCB cells seems related to the nature of UCB-T cells. Phenotypically, UCB-CD3+ cells are mostly naive (CD45RA+) and represent a transitional population between thymocytes and adult T cells. We examined the immune reactivity of highly purified, negatively selected CD4+CD45RA+ cells by mimicking activation via T cell receptor (TCR). All experiments included the extensively characterized adult peripheral blood (APB) cells as reference. On the contrary to APB, naive UCB-CD4+ cells were able to proliferate with anti-CD3 stimulation alone. With addition of interleukin (IL)-2 or costimulatory signal, both populations reached similar proliferation. Forty-eight hours after anti-CD3 stimulation, CD4+CD45RA+ from UCB, but not APB, showed characteristic blastic morphology and significant expression of CD25 on the surface. A low concentration of IL-2 was detected at 24 h by anti-CD3-stimulated UCB CD4+CD45RA+, which rapidly disappeared. By 72 h after activation, CD4+CD45RA+ UCB cells showed extensive apoptosis, whereas CD4+CD45RA+ APB cells showed low levels of apoptosis. Using RNase protection assay, we observed that CD95L levels were significantly higher in naive CD4+ cells from UCB than from APB after activation. However, neutralizing Fas-Fc protein was unable to inhibit anti-CD3-induced apoptosis, suggesting that this was a CD95-independent mechanism. These results indicate that UCB-CD4+CD45RA+ cells are able to start proliferating as a result of early IL-2 production after TCR engagement alone, but probably, as a result of the consumption of this IL-2, they undergo cell death.

Expression patterns of Th1 and Th2 cytokine genes in childhood idiopathic thrombocytopenic purpura (ITP) at presentation and their modulation by intravenous immunoglobulin G (IVIg) treatment: their role in prognosis

Childhood idiopathic thrombocytopenic purpura (ITP) resolves usually after the first episode, although it may recur, and in 10% to 20% of patients develops into a chronic disorder. Evidence of the immunoregulatory role of Th1/Th2 responses in autoimmune diseases prompted us to perform a prospective study of Th1/Th2 gene expression profiles and transforming growth factor β (TGF-β) plasma levels in 18 children (median age, 6.4 years) with acute ITP, before and after intravenous immunoglobulin G (IVIg) infusion, and during a follow-up period (0.5-5 years). Initially, 12 of 18 patients had either low Th0/Th1 plus interleukin 10 (IL-10) or no in vivo cytokine gene expression (0). At 24 hours after IVIg infusion this pattern became 0 or Th2 (9 of 12) or remained low Th0/Th1 (3 of 12). During follow-up these patients did not relapse and maintained 0 or Th2 pattern without IL-10. Of the remaining 6 patients, 4 presented with a Th1 or Th0/Th1 pattern plus IL-10 that persisted after IVIg treatment (although interferon γ [IFN-γ] expression diminished) and stabilized to Th1 plus IL-10 at follow-up, which was marked by infrequent episodes of ITP. Two patients presenting with a strict Th1 pattern characterized by high expression of IFN-γ, which remained unchanged after IVIg and at follow-up, can be characterized as chronic ITP. TGF-β plasma levels were low in patients with active disease and increased in remission. Overall, acute ITP presents with Th1, Th0/Th1, or 0 in vivo cytokine gene expression. Stable remission is associated with a 0 or Th2 pattern. A 0 or Th2 pattern after IVIg gave the best prognosis, whereas sustained high expression of IFN-γ and refractoriness to IVIg were the main indicators of poor prognosis.

Antagonistic effects of human cyclic MBP(87-99) altered peptide ligands in experimental allergic encephalomyelitis and human T-cell proliferation

The immunodominant myelin basic protein (MBP) peptide comprising residues 87-99 is a self-antigen in multiple sclerosis (MS). In Lewis rats this epitope induces experimental allergic encephalomyelitis (EAE), a demyelinating disease of the central nervous system, and is a model of MS. Structure-activity studies have shown that Lys(91) and Pro(96) residues are important for encephalitogenicity. Replacement of Lys and/or Pro residues with Arg and/or Ala, respectively, results in suppression of EAE. A potent linear altered peptide ligand of the immunodominant sequence MBP(83-99) has been selected for clinical trial (Nat. Med. 2000, 6, 1167, 1176). In the present report, two cyclic analogues, cyclo(91-99)[Ala(96)]MBP(87-99) and cyclo(87-99)[Arg(91), Ala(96)]MBP(87-99) were designed by NMR and molecular modeling data on human MBP(87-99) epitope (Val(87)-His-Phe-Phe-Lys-Asn-Ile-Val-Thr-Pro-Arg-Thr-Pro(99)) and its linear antagonist peptide analogue [Arg(91), Ala(96)]MBP(87-99). These analogues (altered peptide ligands) inhibited EAE in Lewis rats and decreased inflammation in the spinal cord. In addition, the analogue cyclo(87-99)[Arg(91), Ala(96)]MBP(87-99) induced proliferation of human peripheral blood T-cells. These cyclic MBP(87-99) peptide analogues may lead to the design of potent antagonist mimetics for treating MS.

Synthetic peptides as putative therapeutic agents in transplantation medicine: application of PEPSCAN to the identification of functional sequences in the extracellular domain of the interleukin-2 receptor beta chain (IL-2Rbeta)

A desired treatment strategy in transplantation medicine is the selective targeting of alloreactive T cells without impairing antileukemic and antiviral activities. One approach is the synthesis of peptides that interfere with the binding of interleukin-2 (IL-2) to its high affinity receptor (IL-2R). This blocks the activation and proliferation of the antigen-activated T cells and the secretion of IL-2. The latter binds to its receptor, via the extracellular domain of the IL-2Rbeta chain, while its cytoplasmic domain is required for intracellular signal transduction. In this study, the PEPSCAN method was applied in order to identify antigenic sequences (epitopes) in the extracellular domain of the IL-2Rbeta. Based on the primary amino acid (aa) sequence of the IL-2Rbeta, a total of 239 overlapping dodecapeptides, spanning the entire sequence of IL-2Rbeta, were synthesized by PEPSCAN and their immunoreactivity was tested by ELISA using monoclonal antibodies (mAbs) specific for IL-2Rbeta such as TU11, Mikbeta1, HuMikbeta1 and TU27. TU11 recognized a linear epitope located in the region 85R-Q(96). None of the 239 synthetic peptides was recognized by TU27. Mikbeta1 (and HuMikbeta1) recognized a discontinuous epitope formed by aa located in the IL-2Rbeta domains L(106) to P(148) and E(170) to A(202). Subsequently, synthetic peptides corresponding to the identified putative epitopic sequences were prepared by solid phase synthesis and their immunogenicity in vivo was assessed by raising polyclonal antibodies. Given that Mikbeta1 and HuMikbeta1 inhibit binding of IL-2 on the IL-2Rbeta, we addressed the question of whether the identified antigenic sequences serve as putative IL-2 binding domains. Synthetic peptides corresponding to these sequences were tested for their ability to compete with IL-2 for binding and, thereby, inhibit IL-2-induced proliferation of mitogen-stimulated human peripheral blood T cells. Sequences 107M-E(118) and 178Y-Q(199) probably represent functional IL-2 binding domains on IL-2Rbeta, since these synthetic peptides significantly inhibited the proliferation of activated T cells and secretion of IL-2.

Defective IL2 gene expression in newborn is accompanied with impaired tyrosine-phosphorylation in T cells

Here we confirmed that IL2 mRNA expression in CD3-stimulated T cells is defective at birth. Because protein-tyrosine phosphorylation is an important part of signaling through CD3 and plays a key role in IL2 transcription, we further investigated whether impaired IL2 response to CD3 in newborns would be accompanied with an alteration of tyrosine phosphorylation. In this purpose, CD3-induced tyrosine phosphorylation was evaluated comparatively in newborn and adult cells by immunoblotting of total cellular extract with an antiphosphotyrosine antibody. Results show that, in both peripheral lymphocytes or purified CD4 T cells from both cord and adult, CD3 stimulation could induce small even significant tyrosine-phosphorylation. Tyrosine phosphorylation occurs as soon as 2' following CD3 ligation and was still evident up to 15-20'. Yet, by using a highly sensitive method to analyze CD3-induced accumulation of phosphorylated substrates, which consisted in adding pervanadate, an inhibitor of phosphatases, during the last 2 min of CD3 stimulation, we showed that the intensity of tyrosine phosphorylation was clearly decreased in cord cells. From these results, it is tempting to speculate that suboptimal capacities of cord T cells to up-regulate tyrosine phosphorylation might contribute to defective IL2 production in neonates.

Characterisation of inducible DNase I hypersensitive sites flanking the human interleukin-5 gene

Interleukin-5 (IL-5) production is necessary for eosinophilia associated with allergic conditions and parasitic infection. IL-5 mRNA is transiently expressed by activated T-lymphocytes. In this report, we have analysed DNA regulatory regions associated with inducible IL-5 expression in the human HSB-2 T-cell line. Only low levels of transcriptional activity were induced in cells transfected with up to 1.2 kb of DNA upstream of the IL-5 gene. DNase I hypersensitivity analysis was employed to identify additional regulatory sequences located outside this region. Two hypersensitivity sites (HS) were identified, one 2.5 kb 5' and the other 1.6 kb 3' from the gene, that were induced on activation of HSB-2 cells by stimuli that induced IL-5 expression. The 5' site, but not the 3' site, was found in primary human T-cells. The presence of the 5' HS did not always coincide with IL-5 expression. Inclusion of the region encompassing the 5' HS in promoter studies mediated a moderate increase in transcriptional activity, suggesting that enhancer elements essential for induction of maximal IL-5 transcription reside at a greater distance from the IL-5 gene.

Transcriptional Repression of the IL-2 Gene in Th Cells by ZEB

Th1- and Th2-type cells mediate distinct effector functions via cytokine secretion in response to immunologic challenge. Precursor Th cells transcribe IFN-γ, IL-2, and IL-4 upon activation. Repeated stimulation of Th precursor cells in the presence of IL-4 leads to terminally differentiated Th2 cells that have lost the ability to transcribe the IL-2 gene. We provide evidence that repression of IL-2 gene expression in Th2 cells and partial repression in Th1 cells are mediated by ZEB, a zinc finger, E box-binding transcription factor. This factor binds to a negative regulatory element, NRE-A, in the IL-2 promoter, thereby acting as a potent repressor of IL-2 transcription.

Immune memory in CD4+ CD45RA+ T cells

This study addresses the question of whether human peripheral CD4+ CD45RA+ T cells possess antigen-specific immune memory. CD4+ CD45RA+ T cells were isolated by a combination of positive and negative selection. Putative CD4+ CD45RA+ cells expressed CD45RA (98.9%) and contained < 0.1% CD4+ CD45RO+ and < 0.5% CD4+ CD45RA+ CD45RO+ cells. Putative CD45RO+ cells expressed CD45RO (90%) and contained 9% CD45RA+ CD45RO+ and < 0.1% CD4+ CD45RA+ cells. The responder frequency of Dermatophagoides pteronyssinus-stimulated CD4+ CD45RA+ and CD4+ CD45RO+ T cells was determined in two atopic donors and found to be 1:11,314 and 1:8031 for CD4+ CD45RA+ and 1:1463 and 1:1408 for CD4+ CD45RO+ T cells. The responder frequencies of CD4+ CD45RA+ and CD4+ CD45RO+ T cells from two non-atopic, but exposed, donors were 1:78031 and 1:176,903 for CD4+ CD45RA+ and 1:9136 and 1:13,136 for CD4+ CD45RO+ T cells. T cells specific for D. pteronyssinus were cloned at limiting dilution following 10 days of bulk culture with D. pteronyssinus antigen. Sixty-eight clones were obtained from CD4+ CD45RO+ and 24 from CD4+ CD45RA+ T cells. All clones were CD3+ CD4+ CD45RO+ and proliferated in response to D. pteronyssinus antigens. Of 40 clones tested, none responded to Tubercule bacillus purified protein derivative (PPD). No difference was seen in the pattern of interleukin-4 (IL-4) or interferon-gamma (IFN-gamma) producing clones derived from CD4+ CD45RA+ and CD4+ CD45RO+ precursors, although freshly isolated and polyclonally activated CD4+ CD45RA+ T cells produced 20-30-fold lower levels of IL-4 and IFN-gamma than their CD4+ CD45RO+ counterparts. Sixty per cent of the clones used the same pool of V beta genes. These data support the hypothesis that immune memory resides in CD4+ CD45RA+ as well as CD4+ CD45RO+ T cells during the chronic immune response to inhaled antigen.

Quantitative alterations of the functionally distinct subsets of CD4 and CD8 T lymphocytes in asymptomatic HIV infection: changes in the expression of CD45RO, CD45RA, CD11b, CD38, HLA-DR, and CD25 antigens

We determined the representation in asymptomatic human immunodeficiency virus (HIV) infection of the CD45RO+ and CD45RO- CD45RA+ subsets of CD4+ and CD8+ T lymphocytes, CD11b+ and CD11b- subsets of CD8+ T cells, and activated populations of these subsets. Three-color flow cytometry was used to quantitate the different CD4+ and CD8+ T cell populations in 116 asymptomatic HIV+ individuals. In asymptomatic HIV+ infection there was a significant relative increase in the CD4+ CD45RO+ and CD8+ CD45RO+ T cell subsets, which express CD38 and DR antigens, that correlated strongly with the decline in total CD4+ T cells. In addition, we found a loss of CD4+ CD45RO- and CD8+ CD45RO- T cells associated with progression of HIV infection (as measured by the decline in total CD4+ T cells). Studies presented here also indicate that, with the progression of asymptomatic HIV infection, CD8+ CD11b- T lymphocytes showed a significant decrease, whereas CD8+ CD11b+ T cells were significantly increased. This study demonstrates that the progression of HIV infection in asymptomatic patients involves the increase in CD45RO+ subsets of CD4+ and CD8+ T cells, the increase in CD8+ CD11b+ T cells, the decrease in CD45RO- CD45RA+ subsets of CD4 and CD8 T cells, and the decline in CD8+ CD11b- T cells.

CD45RA+ and CD45RO+ T cells differ in susceptibility to cyclosporin A mediated inhibition of interleukin-2 production

Lymphocytes in different states of activation use different intracellular signalling pathways and may therefore differ in their susceptibility to immunosuppressive agents. In this study we examined the proliferation and production of interleukin-2 (IL-2) by unprimed/naive CD4+CD45RA+ T cells and previously activated/memory CD4+CD45RO+ T cells from human peripheral blood when stimulated in vitro in the presence of cyclosporin A (CsA). Further, the dependency of the IL-2 response on calcium (Ca2+) ions was analysed by the addition of the chelating agent EGTA. The CD4+CD45RO+ memory T cells were shown to be less susceptible to CsA and less dependent on the level of Ca+ ions than the naive CD4+CD45RA+ T cells. The subcellular mechanisms involved in this difference and the potential clinical implications are discussed.

Polyclonal hypergammaglobulinaemia in a case of B-cell chronic lymphocytic leukaemia: the result of IL-2 production by the proliferating monoclonal B cells?

SEQ DATA who developed polyclonal hypergammaglobulinaemia: 38.3 milligrams polyclonal IgG, 0.97 milligram IgA and 0.33 milligram IgM. Immunophenotyping showed a monoclonal lymphocytic population CD19+ CD5+ CD40+ CD23+, low sIg+ (95%), kappa type in the great majority (96%). RT-PCR of immunoglobulin genes gave evidence of monoclonal rearrangement of the IgM type. Our tests showed that IL-2 was produced when leukaemic B cells were stimulated with phorbol myristate acetate, ionomycin and lipopolysaccharide. In addition, transfections with the full IL-2 promoter or elements thereof revealed that IL-2 expression is inducible and mediated through the NF-kB-promoter element. Finally, the amount of IL-2 secreted by these cells is about 39 ng/ml/10(6) cells, which is remarkably high for non-T cells. These results suggest that the large amounts of polyclonal IgG seen in this case of B-CLL are secreted by normal B cells which are in turn stimulated by IL-2 produced by proliferating monoclonal (leukaemic) B cells. Under cyclosporin A treatment, immunoglobulin secretion and B cell count remained low.

Interleukin-2 promoter activity in Epstein-Barr virus-transformed B lymphocytes is controlled by nuclear factor-chi B

The regulation of interleukin (IL)-2 gene expression has been investigated mainly in T lymphocytes, the predominant producers of IL-2. However, B cells can also synthesize IL-2. In the present study we analyzed the control of IL-2 promoter activity in Epstein-Barr virus (EBV)-transformed B cell clones which are capable of secreting IL-2 at a low level after stimulation with phorbol 12-myristate 13-acetate and the Ca2+ ionophore ionomycin. Transient transfections using reporter constructs with multiples of transcription factor binding sites from the IL-2 promoter [distal nuclear factor (NF)-AT, proximal NF-AT, AP-1/Octamer (UPS) or NF-chi B (TCEd) sites] were performed. In EBV-transformed B clones, the chi B site exerted the strongest inducible activity; the NF-AT binding sites showed either no or only weak activity compared to Jurkat T cells. An IL-2 promoter bearing a defective NF-chi B site was completely inactive in EBV-transformed B cells, while it still had activity in Jurkat T cells. In seven EBV-B cell clones or lines differing in their capacity to secrete IL-2, the activity of the IL-2 promoter correlated well with the status of IL-2 secretion. Similarly, a human immunodeficiency virus promoter, whose activity is controlled through chi B factors, was found to be active in the IL-2 producing EBV-B cells, but inactive in the non-IL-2-producing cells. Electrophoretic mobility shift assays using protein extracts from EBV-B cells and the IL-2 NF-chi B probe revealed the constitutive generation of chi B complexes in IL-2-secreting cells consisting mainly of heterodimeric p50/p65 complexes. A weaker chi B complex formation and faster-migrating complexes were detected in non-IL-2-secreting cells. These results demonstrate that the IL-2 NF-chi B site is indispensable for the activity of the IL-2 promoter in EBV-transformed B cells, whereas other transcription factors appear to be less important for IL-2 expression in these cells.

Analysis of proteins that interact with the IL-2 regulatory region in patients with rheumatic diseases

In order to investigate transcriptional regulation of lymphokine genes in rheumatic diseases, peripheral blood mononuclear cells from patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and systemic sclerosis (SSc) were analysed for expression of DNA-binding proteins. Nuclear extracts prepared from unstimulated and mitogen-activated cells were studied for their ability to bind to 32P-labelled oligonucleotides containing the AP-1, NF-AT, NF-B and CD28RC sites of the IL-2 promoter. Using gel mobility-shift assay, detection of protein binding to the AP-1 site was reduced in SLE compared with controls. NF-AT binding activity was enhanced in all groups of patients, and was associated with measures of disease activity in RA. In addition, SSc patients showed increased NF-kappa B binding activity. Altered patterns of DNA-binding proteins suggest disturbed intracellular signalling which may contribute to abnormal lymphokine production in rheumatic diseases.

Differences in responsiveness to CD3 stimulation between naive and memory CD4+ T cells cannot be overcome by CD28 costimulation

Activation of naive CD4+ T cells is essential for the induction of primary immune responses. However, this subset is less responsive to signaling via T cell receptor/CD3 (TcR/CD3) complex than memory CD4+ cells. For mitogenic activation of T cells, in addition to triggering of the TcR/CD3 complex, costimulatory signals are required that can be generated by surface structures present on the antigen-presenting cells. We investigated here whether differences in responsiveness to TcR/CD3 stimulation of naive and memory cells can be overcome by the costimulatory pathway B7/CD28. Using a B7-dependent system we show that even in the presence of optimal CD28 costimulation, CD4+ naive cells still have more stringent TcR/CD3 activation requirements than memory cells. Furthermore, titration of the B7 signal revealed that for activation of naive CD4+ cells a higher level of cross-linking of CD28 molecules is required than for memory cells. Thus, our results show that at least two signals are required for activation of both CD4+ memory and naive cells, but that for activation of naive cells higher cross-linking of both CD3 and CD28 molecules is necessary.

Activation of naive, memory and effector T cells

An increased understanding of the types of T-cell subsets that exist in vivo, their relationships to one another, and how to identify and isolate them or effect their generation, has led to a comprehensive view of the antigen-presenting cells (APCs) which may be active and regulatory during the course of an immune response. Recent studies show that naive T cells only respond efficiently to dendritic cells and activated B cells whereas memory and effector cells respond to all APC types to some extent, including resting B cells. High level co-stimulatory molecule expression largely explains why APCs such as dendritic cells are far more effective stimulators than resting B cells. The available data, therefore, suggest that the requirement for co-stimulation, and hence capacity to respond to various APCs, is largely a function of the differentiation state of the T cell, and that previous encounter with antigen fundamentally increases the ability of T cells to subsequently respond to antigen rechallenge.