Enhanced responsiveness of human memory T cells to CD2 and CD3 receptor-mediated activation

T-cell receptor-engineered T cells for cancer treatment: current status and future directions

T-cell receptor (TCR)-engineered T cells are a novel option for adoptive cell therapy used for the treatment of several advanced forms of cancer. Work using TCR-engineered T cells began more than two decades ago, with numerous preclinical studies showing that such cells could mediate tumor lysis and eradication. The success of these trials provided the foundation for clinical trials, including recent clinical successes using TCR-engineered T cells to target New York esophageal squamous cell carcinoma (NY-ESO-1). These successes demonstrate the potential of this approach to treat cancer. In this review, we provide a perspective on the current and future applications of TCR-engineered T cells for the treatment of cancer. Our summary focuses on TCR activation and both pre-clinical and clinical applications of TCR-engineered T cells. We also discuss how to enhance the function of TCR-engineered T cells and prolong their longevity in the tumor microenvironment.

Evolving models of the immunopathogenesis of T cell-mediated drug allergy: The role of host, pathogens, and drug response

Immune-mediated (IM) adverse drug reactions (ADRs) are an underrecognized source of preventable morbidity, mortality, and cost. Increasingly, genetic variation in the HLA loci is associated with risk of severe reactions, highlighting the importance of T-cell immune responses in the mechanisms of both B cell-mediated and primary T cell-mediated IM-ADRs. In this review we summarize the role of host genetics, microbes, and drugs in IM-ADR development; expand on the existing models of IM-ADR pathogenesis to address multiple unexplained observations; discuss the implications of this work in clinical practice today; and describe future applications for preclinical drug toxicity screening, drug design, and development.

Immunity to Pathogens Taught by Specialized Human Dendritic Cell Subsets

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) that have a key role in immune responses because they bridge the innate and adaptive arms of the immune system. They mature upon recognition of pathogens and upregulate MHC molecules and costimulatory receptors to activate antigen-specific CD4⁺ and CD8⁺ T cells. It is now well established that DCs are not a homogeneous population but are composed of different subsets with specialized functions in immune responses to specific pathogens. Upon viral infections, plasmacytoid DCs (pDCs) rapidly produce large amounts of IFN-α, which has potent antiviral functions and activates several other immune cells. However, pDCs are not particularly potent APCs and induce the tolerogenic cytokine IL-10 in CD4⁺ T cells. In contrast, myeloid DCs (mDCs) are very potent APCs and possess the unique capacity to prime naive T cells and consequently to initiate a primary adaptive immune response. Different subsets of mDCs with specialized functions have been identified. In mice, CD8α⁺ mDCs capture antigenic material from necrotic cells, secrete high levels of IL-12, and prime Th1 and cytotoxic T-cell responses to control intracellular pathogens. Conversely, CD8α⁻ mDCs preferentially prime CD4⁺ T cells and promote Th2 or Th17 differentiation. BDCA-3⁺ mDC2 are the human homologue of CD8α⁺ mDCs, since they share the expression of several key molecules, the capacity to cross-present antigens to CD8⁺ T-cells and to produce IFN-λ. However, although several features of the DC network are conserved between humans and mice, the expression of several toll-like receptors as well as the production of cytokines that regulate T-cell differentiation are different. Intriguingly, recent data suggest specific roles for human DC subsets in immune responses against individual pathogens. The biology of human DC subsets holds the promise to be exploitable in translational medicine, in particular for the development of vaccines against persistent infections or cancer.

Mechanisms behind functional avidity maturation in T cells

During an immune response antigen-primed B-cells increase their antigen responsiveness by affinity maturation mediated by somatic hypermutation of the genes encoding the antigen-specific B-cell receptor (BCR) and by selection of higher-affinity B cell clones. Unlike the BCR, the T-cell receptor (TCR) cannot undergo affinity maturation. Nevertheless, antigen-primed T cells significantly increase their antigen responsiveness compared to antigen-inexperienced (naïve) T cells in a process called functional avidity maturation. This paper covers studies that describe differences in T-cell antigen responsiveness during T-cell differentiation along with examples of the mechanisms behind functional avidity maturation in T cells.

Vitamin D: emerging roles in infection and immunity

In the preantibiotic era, TB of the skin was treated successfully with UV light. By the 1920s, pulmonary TB was being treated with regular sun exposure. During the last decade, basic laboratory research into the antimicrobial actions of vitamin D has provided new insights into these historical observations. Vitamin D has a critical role in the innate immune system through the production of antimicrobial peptides - particularly cathelicidin. Vitamin D would appear to have an important role in respiratory tract, skin and potentially gut health. A number of autoimmune diseases, including multiple sclerosis, Type I diabetes, systemic lupus erythematosus and rheumatoid arthritis, are associated with vitamin D deficiency. Vitamin D could have an important role in the prevention and possible treatment of these conditions; however, much of the current evidence relates to basic science and epidemiological research. In many situations, appropriate double-blind, randomized controlled trial data to guide clinicians treating infectious and autoimmune disease is still lacking.

Vitamin D controls T cell antigen receptor signaling and activation of human T cells

Phospholipase C (PLC) isozymes are key signaling proteins downstream of many extracellular stimuli. Here we show that naive human T cells had very low expression of PLC-gamma1 and that this correlated with low T cell antigen receptor (TCR) responsiveness in naive T cells. However, TCR triggering led to an upregulation of approximately 75-fold in PLC-gamma1 expression, which correlated with greater TCR responsiveness. Induction of PLC-gamma1 was dependent on vitamin D and expression of the vitamin D receptor (VDR). Naive T cells did not express VDR, but VDR expression was induced by TCR signaling via the alternative mitogen-activated protein kinase p38 pathway. Thus, initial TCR signaling via p38 leads to successive induction of VDR and PLC-gamma1, which are required for subsequent classical TCR signaling and T cell activation.

New tools to monitor the impact of viral infection on the alloreactive T-cell repertoire

Accumulating evidence suggests that alloreactive memory T-cells may be generated as a result of viral infection. So far, a suitable tool to define the individual human leukocyte antigen (HLA) cross-reactivity of virus-specific memory T-cells is not available. We therefore aimed to develop a novel system for the detection of cross-reactive alloresponses using single HLA antigen expressing cell lines (SALs) as stimulator. Herein, we generated Epstein-Barr Virus (EBV) EBNA3A specific CD8 memory T-cell clones (HLA-B*0801/FLRGRAYGL peptide restricted) and assayed for alloreactivity against a panel of SALs using interferon-gamma Elispot as readout. Generation of the T-cell clones was performed by single cell sorting based on staining with viral peptide/major histocompatibility complex-specific tetramer. Monoclonality of the T-cell clones was confirmed by T-cell receptor (TCR) polymerase chain reaction analysis. First, we confirmed the previously described alloreactivity of the EBV EBNA3A-specific T-cell clones against SAL-expressing HLA-B*4402. Further screening against the entire panel of SALs also showed additional cross-reactivity against SAL-expressing HLA-B*5501. Functionality of the cross-reactive T-cell clones was confirmed by chromium release assay using phytohemagglutinin blasts as targets. SALs are an effective tool to detect cross-reactivity of viral-specific CD8 memory T-cell clones against individual class I HLA molecules. This technique may have important implications for donor selection and monitoring of transplant recipients.

Intracellular calcium signalling patterns reflect the differentiation status of human T cells

Stimulation of T lymphocytes results in the calcium-dependent activation and repression of a large number of genes. However, the functional response made by different T cell subsets is heterogeneous, as their differentiation results in alterations in their sensitivity to activation and in the secretion of cytokines. Here we have investigated the patterns of calcium responses in CD4 and CD8 T cell subsets to help explain their different responses to activation. CD4(+) CD45RA(+) T cells isolated freshly from human blood gave a sustained calcium signal after stimulation, but this was smaller than elicited in CD4(+) CD45RO(+) cells. On in vitro differentiation of CD4(+) CD45RA(+) cells to CD45RO(+), the level of the cytoplasmic calcium response rose initially, but then declined steadily during further rounds of differentiation. The proportion producing an oscillatory calcium response or not responding was increased and differentiation was accompanied by a shift in the calcium between intracellular pools. CD8(+) T cells gave a smaller calcium response than paired CD4(+) T cells and showed a difference in the numbers of cells giving a transient, rather than sustained, calcium signal. The increase in oscillating cells in the CD4(+) CD45RO(+) population may reflect the heterogeneity of this population, particularly in terms of cytokine production. The changing patterns of calcium responses in T cells as they differentiate may explain variation in the cellular response to activation at different stages in their lifespan and emphasize the importance of the both the quantity and the quality of the calcium signal in determining the outcome of T cell activation.

Control of memory CD4 T cell recall by the CD28/B7 costimulatory pathway

The CD28/B7 costimulatory pathway is generally considered dispensable for memory T cell responses, largely based on in vitro studies demonstrating memory T cell activation in the absence of CD28 engagement by B7 ligands. However, the susceptibility of memory CD4 T cells, including central (CD62L(high)) and effector memory (T(EM); CD62L(low)) subsets, to inhibition of CD28-derived costimulation has not been closely examined. In this study, we demonstrate that inhibition of CD28/B7 costimulation with the B7-binding fusion molecule CTLA4Ig has profound and specific effects on secondary responses mediated by memory CD4 T cells generated by priming with Ag or infection with influenza virus. In vitro, CTLA4Ig substantially inhibits IL-2, but not IFN-gamma production from heterogeneous memory CD4 T cells specific for influenza hemagglutinin or OVA in response to peptide challenge. Moreover, IL-2 production from polyclonal influenza-specific memory CD4 T cells in response to virus challenge was completely abrogated by CTLA4Ig with IFN-gamma production partially inhibited. When administered in vivo, CTLA4Ig significantly blocks Ag-driven memory CD4 T cell proliferation and expansion, without affecting early recall and activation. Importantly, CTLA4Ig treatment in vivo induced a striking shift in the phenotype of the responding population from predominantly T(EM) in control-treated mice to predominantly central memory T cells in CTLA4Ig-treated mice, suggesting biased effects of CTLA4Ig on T(EM) responses. Our results identify a novel role for CD28/B7 as a regulator of memory T cell responses, and have important clinical implications for using CTLA4Ig to abrogate the pathologic consequences of T(EM) cells in autoimmunity and chronic disease.

Defective T cell receptor-mediated signal transduction in memory CD4 T lymphocytes exposed to superantigen or anti-T cell receptor antibodies

Lymphocytes must promote protective immune responses while still maintaining self-tolerance. Stimulation through the T cell receptor (TCR) can lead to distinct responses in naive and memory CD4 T cells. Whereas peptide antigen stimulates both naive and memory T cells, soluble anti-CD3 antibodies and bacterial superantigens stimulate only naive T cells to proliferate and secrete cytokines. Further, superantigens, like staphylococcal enterotoxin B (SEB), cause memory T cells to become anergic while soluble anti-CD3 does not. In the present report, we show that signal transduction through the TCR is impaired in memory cells exposed to either anti-CD3 or SEB. A block in signaling leads to impaired activation of the kinase ZAP-70 so that downstream signals and cell proliferation do not occur. We further show that the signaling defect is unique to each agent. In anti-CD3-treated memory T cells, the src kinase Lck is only transiently activated and does not phosphorylate and activate ZAP-70. In SEB-treated memory T cells, ZAP-70 does not interact with the TCR/CD3 complex to become accessible to Lck. Finally, we provide evidence that alternative signaling pathways are initiated in SEB-treated memory cells. Altered signaling, indicated by an elevation in activity of the src kinase Fyn, may be responsible for memory cell anergy caused by SEB. Thus, differentiation of naive T cells into memory cells is accompanied by alterations in TCR-mediated signaling that can promote heightened recall immunity or specific tolerance.

Augmentation in expression of activation-induced genes differentiates memory from naive CD4+ T cells and is a molecular mechanism for enhanced cellular response of memory CD4+ T cells

In an attempt to understand the molecular basis for the immunological memory response, we have used cDNA microarrays to measure gene expression of human memory and naive CD4+ T cells at rest and after activation. Our analysis of 54,768 cDNA clones provides the first glimpse into gene expression patterns of memory and naive CD4+ T cells at the genome-scale and reveals several novel findings. First, memory and naive CD4+ T cells expressed similar numbers of genes at rest and after activation. Second, we have identified 14 cDNA clones that expressed higher levels of transcripts in memory cells than in naive cells. Third, we have identified 135 (130 known genes and 5 expressed sequence tags) up-regulated and 68 (42 known genes and 26 expressed sequence tags) down-regulated cDNA clones in memory CD4+ T after in vitro stimulation with anti-CD3 plus anti-CD28. Interestingly, the increase in mRNA levels of up-regulated genes was greater in memory than in naive CD4+ T cells after in vitro stimulation and was higher with anti-CD3 plus anti-CD28 than with anti-CD3 alone in both memory and naive CD4+ T cells. Finally, the changes in expression of actin and cytokine genes identified by cDNA microarrays were confirmed by Northern and protein analyses. Together, we have identified approximately 200 cDNA clones whose expression levels changed after activation and suggest that the level of expression of up-regulated genes is a molecular mechanism that differentiates the response of memory from naive CD4+ T cells.

T cell memory: heterogeneity and mechanisms

Immunological memory is manifested by the body's ability to enjoy long-term protection against specific pathogens previously encountered through illness or vaccination. This memory response resides in the long-lived, previously activated memory T and B lymphocytes that are believed to exist in a quiescent state. Recent advances in studies on T cell memory have revealed heterogeneity in the T cells that mediate memory responses that may have implications for the generation and maintenance of these cells over time. This review will present these recent findings on memory T cells in the context of past research and current models for the generation and persistence of memory T cells.

Normalization of CD4+ cell numbers and reduced levels of memory CD8+ cells in blood and tonsillar tissue after highly active antiretroviral therapy in early HIV type-1 infection

Antiretroviral therapy increases the number of both CD4+ and CD8+ T cells in the blood of HIV-1-positive patients with advanced disease. In the present study, we have examined the kinetics of CD4+ and CD8+ T cell restoration in blood and lymphoid tissue in asymptomatic HIV-1-positive individuals with high CD4+ cell counts during highly active antiretroviral treatment. Tonsillar biopsies and blood samples were collected at baseline and at regular intervals during the following 48 weeks and from HIV-1-negative controls. Mononuclear cells from blood and tonsils were phenotyped and quantified by three-color flow cytometry. After 48 weeks of therapy, blood CD4+ cell counts in the HIV-1-infected group were comparable to those found in uninfected controls. Naive CD4+ T cells in blood increased during the initial 2 weeks in parallel with reduced plasma viremia. Both naive and memory CD4+ T cells in blood reached normal numbers by week 48, whereas the CD4+ naive/memory cell ratio in tonsils was within normal range throughout the study. The level of memory CD8+ T cells in blood declined during the first 8 weeks in parallel with a reduction in the tonsillar memory CD8+ T cells. Naive CD8+ T cells in the blood increased after 4 weeks, while the level of naive CD8+ T cells in tonsils remained unaltered. Our data indicate that in the early stages of HIV-1 infection antiretroviral therapy normalizes CD4+ cell counts and causes a decrease in the level of memory CD8+ cells in blood and lymphoid tissue, suggesting reduced CD8+ cell turnover in response to reduced viral replication.

Diminished secondary CTL response in draining lymph nodes on cutaneous challenge with herpes simplex virus

We have shown that C57BL/6-derived CD8(+) CTL specific for an immunodominant herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) determinant express a highly conserved Vbeta10/junctional sequence combination. This extreme T cell receptor beta-chain bias can be used to track the activation of gB-specific CTL in lymph nodes draining the site of HSV-1 infection. In this report we have examined the accumulation of gB-specific CTL in the primary and secondary or recall CTL responses to HSV-1 infection. We found that gB-specific cytolytic activity present within popliteal lymph nodes draining HSV-infected foot-pads peaked at day 5 post-infection during the primary response. As found previously, this correlates with the accumulation of Vbeta10(+)CD8(+) CTL in the activated T cell subset. Lymph node-derived cytotoxicity peaked between days 3 and 4 on secondary challenge with virus and, somewhat surprisingly, was considerably below that seen in the primary response. This reduced gB-specific cytolytic activity mirrored a near absence of Vbeta10(+)CD8(+) T cell enrichment found within the draining lymph nodes during this recall response, consistent with the overall diminution of gB-specific CTL accumulation in this site. Finally, there was a second wave of biased accumulation of Vbeta10(+)CD8(+) activated T cells within the popliteal lymph nodes well after the resolution of infection in both the primary and secondary responses. These results are discussed in terms of preferential activation of virus-specific memory T cells directly in infected tissues during a secondary CTL response at the expense of draining lymphoid organs.

A New Role for Platelet-Endothelial Cell Adhesion Molecule-1 (CD31): Inhibition of TCR-Mediated Signal Transduction

Platelet-endothelial cell adhesion molecule-1 (PECAM-1) is a 130-kDa transmembrane glycoprotein expressed by endothelial cells, platelets, monocytes, neutrophils, and certain T cell subsets. The PECAM-1 extracellular domain has six Ig-homology domains that share sequence similarity with cellular adhesion molecules. The PECAM-1 cytoplasmic domain contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) that, when appropriately engaged, becomes phosphorylated on tyrosine residues, creating docking sites for nontransmembrane, Src homology 2 domain-bearing protein tyrosine phosphatase (SHP)-1 and SHP-2. The purpose of the present study was to determine whether PECAM-1 inhibits protein tyrosine kinase (PTK)-dependent signal transduction mediated by the immunoreceptor tyrosine-based activation motif-containing TCR. Jurkat cells, which coexpress PECAM-1 and the TCR/CD3 complex, were INDO-1AM-labeled and then incubated with anti-CD3ε mAbs, anti-PECAM-1 mAbs, or both, and goat anti-mouse IgG was used to cross-link surface-bound mAbs. Calcium mobilization induced by CD3 cross-linking was found to be attenuated by coligation of PECAM-1 in a dose-dependent manner. PECAM-1-mediated inhibition of TCR signaling was attributable, at least in part, to inhibition of release of calcium from intracellular stores. These data provide evidence that PECAM-1 can dampen signals transduced by ITAM-containing receptors and support inclusion of PECAM-1 within the family of ITIM-containing inhibitors of PTK-dependent signal transduction.

Memory CD44(int) CD8 T cells show increased proliferative responses and IFN-gamma production following antigenic challenge in vitro

F5 TCR transgenic mice challenged in vivo with peptide generate long-lived primed CD8 T cells that hyper-proliferate in response to peptide in vitro. These primed CD8 T cells can be subdivided into three distinct populations on the basis of CD44 cell surface expression. In this report, we show that among primed CD8 T cells, those expressing intermediate levels of CD44 appear to be true memory T cells by the measurement of a variety of characteristics. Indeed, these cells hyper-proliferate in response to peptide re-stimulation in vitro, and produce IFN-gamma with faster kinetics and at higher levels than naive populations in vitro. We also show that CD8 T cells expressing high levels of CD44 express several activation markers and cycle in vivo in the absence of antigen. However, this population is unable to respond to peptide stimulation in vitro as measured by both proliferation and IFN-gamma secretion. The origin and specificity of these cells is unknown. These results provide evidence that memory CD8 T cells are functionally different from naive CD8 T cells both in terms of proliferation and cytokine secretion. They identify the CD8/CD44(int) T cells as the population responsible for hyper-reactivity in vitro.

Effect of didanosine, stavudine, and hydroxyurea therapy on apoptosis in CD45RA+ and CD45RO+ T lymphocyte subpopulations

The effect of aggressive antiretroviral therapy on spontaneous apoptosis (AP) in CD4+ and CD8+ lymphocytes expressing CD45RO (memory cells) and CD45RA (naive cells) and their relationship to cellular activation and viral load were examined. Ten patients receiving simultaneous treatment with d4T, ddI, and HU were evaluated. Flow cytometric analysis showed significant levels of AP (measured by TUNEL assay) among memory and naive T cells and an enhanced expression of CD38 and HLA-DR activation markers. The percentage of apoptotic CD4+CD45RO+ and CD4+CD45RA+ cells decreased, respectively, from 34 +/- 3.3 and 29 +/- 3.6 prior to treatment to 20.5 +/- 4 and 22 +/- 3.8 at week 8 into therapy. The percentage of apoptotic CD8+CD45RO+ and CD8+CD45RA+ cells similarly decreased, respectively, from 20 +/- 2.5 and 24 +/- 3 prior to treatment to 14.5 +/- 2.7 and 16 +/- 3 at week 8 into treatment. The percentage of CD4+ cells expressing the activation markers CD38 and HLA-DR decreased from 27 +/- 6 to 13 +/- 2 and from 26 +/- 4 to 13.5 +/- 3, respectively. The percentage of CD8+ cells expressing either CD38 or HLA-DR fell from 22 +/- 3 to 10 +/- 2 for the former and from 39 +/- 5 to 22 +/- 4 for the latter. This was associated with a significant decrease in viral load (mean, 1.4 log10), and a decline in circulating plasma TNF-alpha and sIL-2R levels from 50.5 +/- 10 to 21 +/- 6 and 92.5 +/- 11 to 68 +/- 9, respectively. These data indicate that short-term therapy with ddI, d4T, and HU in combination diminished AP, immune activation, and partially restored naive and memory T cell subpopulations.

Analysis of the cord blood T lymphocyte response to superantigen

We have characterized the T lymphocyte population of the human neonate in respect of the expression of phenotypic profiles for naive, memory and differentiated populations. We have examined the response of the neonate T cell to the superantigen Staphylococcus enterotoxin B (SEB) and compared the response to T cells from healthy adults. We found that the primary response to SEB is equivalent in neonates and adults but that the secondary response demonstrates hyporesponsiveness in the neonate that is more profound than in adults. This response was associated with increased expression of CD25; the alpha chain of the IL-2 receptor, equivalent to that seen in responding cells from adults. A modest increased expression of CD122 and CD132, the beta and gamma chains of the IL-2 receptor, was also observed. There was no increase in the IL-4 receptor (CD124). The hyporesponsive neonate T cells proliferated in response to exogenous IL-2 but the response was less than none SEB treated cells. The neonate cells did not respond to IL-4. We also examined the expression of MHC class II molecules on SEB stimulated cells and found that both neonate and adult T cells upregulate MHC class II to a similar degree. The difference in the hyporesponsive cells appears to result in part from a lower production of IL-2 and in part from a lower ability of cord cells to respond to IL-2. Since the stimulated cord cells expressed IL-2 receptor at the same levels as similarly treated adult cells; there may be differences in down stream signaling pathways.

Nuclear factor-kappaB induction in CD45RO+ and CD45RA+ T cell subsets during aging

An increase in the ratio of memory to naive T cells has been postulated to underlie immune hyporesponsiveness accompanying aging. Our analyses of the induction of nuclear factor-kappaB (NFkappaB) in activated memory (CD45RO+) and naive (CD45RA+) T cell subsets from young and elderly donors has demonstrated that, regardless of donor age, memory T cells are not significantly altered in their responsiveness to TNF-alpha-mediated induction of NFkappaB. Although treatment with TNF-alpha induced nuclear localization of NFkappaB in both memory and naive T cell subsets, irrespective of the age of the donor, the levels of induced NFkappaB were significantly lower in both subsets of T cells obtained from the elderly, when compared to those in young. Examination of IkappaB alpha regulation revealed that TNF-alpha-mediated degradation of IkappaB alpha in both memory and naive T cells from the elderly was severely impaired, thus contributing to the lowered induction of the observed NFkappaB. In addition, this age-related decrease in induction of nuclear NFkappaB correlated with decrease in intracellular IL-2 receptor expression and anti-CD3-induced proliferation of both memory and naive T cells subsets. Taken together, our results suggest that the age-related hyporesponsiveness cannot be attributed to a skewing of the T cell population towards a memory phenotype in the elderly.

CD8+ Memory T Cells (CD44high, Ly-6C+) Are More Sensitive than Naive Cells (CD44low, Ly-6C−) to TCR/CD8 Signaling in Response to Antigen

Memory CD8+ T cells from mice previously primed with alloantigen (alloAg) can respond in vitro to IL-2 and purified class I alloAg presented on microspheres, while no response can be detected using cells from naive mice. Similar results have been obtained using cells from OT-1 mice expressing a transgenic TCR that is specific for OVA257–264 (SIINFEKL) peptide bound to H-2Kb. A population of resting memory cells (defined on the basis of low forward scatter and CD44high, Ly-6C+, CD25−, CD69− surface phenotype) that is present in the OT-1 mice exhibits a substantially higher sensitivity to Ag-stimulation than do naive cells (CD44low, Ly-6C−) expressing the same TCR. CD44high cells respond vigorously to H-2Kb immobilized on microspheres and pulsed with peptide, while CD44low cells respond weakly and only at high class I density and peptide concentration. The Ag-presenting surface only has ligands for TCR and CD8 (class I and peptide), thus ruling out the possibility that differences are due to ligand binding by other adhesion or costimulatory receptors that are expressed at high levels on the memory cells. Experiments using anti-TCR mAb as the stimulus and coimmobilized non-Ag class I as a ligand for CD8 suggest that the difference between naive and memory cells may be at the level of stimulation through the TCR. Thus, in addition to expressing increased levels of adhesion receptors that may enhance responses to Ag on APCs, memory CD8+ T cells appear to be intrinsically more sensitive than naive cells to stimulation through the TCR/CD8 complex.

Phenotypic and functional separation of memory and effector human CD8+ T cells

Human CD8+ memory- and effector-type T cells are poorly defined. We show here that, next to a naive compartment, two discrete primed subpopulations can be found within the circulating human CD8+ T cell subset. First, CD45RA-CD45R0(+) cells are reminiscent of memory-type T cells in that they express elevated levels of CD95 (Fas) and the integrin family members CD11a, CD18, CD29, CD49d, and CD49e, compared to naive CD8+ T cells, and are able to secrete not only interleukin (IL) 2 but also interferon gamma, tumor necrosis factor alpha, and IL-4. This subset does not exert cytolytic activity without prior in vitro stimulation but does contain virus-specific cytotoxic T lymphocyte (CTL) precursors. A second primed population is characterized by CD45RA expression with concomitant absence of expression of the costimulatory molecules CD27 and CD28. The CD8+CD45RA+CD27- population contains T cells expressing high levels of CD11a, CD11b, CD18, and CD49d, whereas CD62L (L-selectin) is not expressed. These T cells do not secrete IL-2 or -4 but can produce IFN-gamma and TNF-alpha. In accordance with this finding, cells contained within this subpopulation depend for proliferation on exogenous growth factors such as IL-2 and -15. Interestingly, CD8+CD45RA+CD27- cells parallel effector CTLs, as they abundantly express Fas-ligand mRNA, contain perforin and granzyme B, and have high cytolytic activity without in vitro prestimulation. Based on both phenotypic and functional properties, we conclude that memory- and effector-type T cells can be separated as distinct entities within the human CD8+ T cell subset.

IFN-beta 1b treatment of relapsing multiple sclerosis has no effect on CD3-induced inflammatory or counterregulatory anti-inflammatory cytokine secretion ex vivo after nine months

Multiple sclerosis (MS) is presumed to be a T-cell mediated chronic inflammatory disease of the central nervous system. We have previously reported that IFN-beta 1b (Betaseron) decreases CD3-mediated TNF-alpha secretion but increases another inflammatory cytokine, IL-6 after three months of treatment. We have now examined cytokine secretion of peripheral blood mononuclear (PMNC) cells after stimulation with OKT3 (anti-CD3) monoclonal antibody (mAb) or Con A in subjects with clinically stable relapsing MS before and three, six and nine months after initiating IFN-beta 1b treatment. At nine months Con A-induced TNF-alpha secretion decreased significantly below baseline but IFN-gamma secretion increased above baseline. There were no significant changes in Con A-induced IL-4 over the six month period and no changes in IL-10 and IL-2 over the nine month period. After nine months on treatment the CD3-induced TNF-alpha and IFN-gamma secretion was not significantly different from the original baseline values. Increased CD3-mediated IL-6 secretion in on-treatment compared to pre-treatment samples at three months gradually declined to baseline values by nine months on-treatment. There was no significant changes from baseline compared to nine months on-treatment in CD3-mediated IL-2, IL-4, IL-10. IFN-beta 1b (Betaseron) treatment has no clear persistent effect on CD3-induced inflammatory or counterregulatory anti-inflammatory cytokine secretion.

Degenerate antigen recognition by CD4+ effector T cells in experimental autoimmune encephalomyelitis

Peptide-specific tolerance with PLP139-151 peptide analogs was used to compare the fine antigen-specificity requirements at both the inductive and effector phases of relapsing EAE (R-EAE). A PLP139-151 analog peptide containing a single substitution at the primary T cell receptor (TcR) contact residue (A144) did not induce proliferation in PLP139-151-primed CD4+ T cells. In addition, tolerance induced with ECDI-treated. A144-coupled splenocytes failed to prevent the inductive phase of PLP139-151-induced R-EAE or to inhibit the induction of peptide-specific DTH indicating that naive PLP139-151-specific T cells do not react with the A144 peptide analog. In contrast, A144-coupled splenocytes did prevent the expression of the effector phase of R-EAE and inhibited the elicitation of peptide-specific DTH responses upon administration to mice seven days after immunization with PLP139-151. The results provide in vivo evidence that "antigen-experienced' T cells recognize a broader repertoire of antigens than do naive T cells and have important implications for the regulation of immune responses and for advancing our understanding of the pathogenesis and treatment of autoimmune disease.

Loss of Inducible Virus in CD45RA Naive Cells After Human Immunodeficiency Virus-1 Entry Accounts for Preferential Viral Replication in CD45RO Memory Cells

Controversy exists concerning the preferential infection and replication of human immunodeficiency virus-1 (HIV-1) within naive (CD45RA+) and memory (CD45RO+) subsets of CD4+ lymphocytes. To explore the susceptibility of these subsets to HIV-1 infection, we purified CD45RA+/CD4+ (RA) and CD45RO+/CD4+ (RO) cells from normal donors and subjected them to a novel monokine activation culture scheme. Following HIV-1 infection and interleukin-2 (IL-2) induction, viral production measured on day 13 was 19-fold greater in RO cultures compared with RA cultures. IL-2–stimulated proliferation in uninfected control cultures was equivalent. To explore the mechanisms by which RA cells were reduced in viral production capacity, RA and RO cells were exposed to HIV-1 followed by treatment with trypsin, and then phytohemagglutinin antigen (PHA)-stimulated at days 4, 7, and 10 postinfection. HIV-1 production in day 4 postinfection RA and RO cultures was analogous, indicating that viral fusion and entry had occurred in both cell types. However, whereas similarly treated day 7 and 10 postinfection RO cultures produced virus, HIV-1 was markedly reduced or lost in the corresponding RA cultures. These results suggest that a temporally labile postfusion HIV-1 complex exists in unstimulated RA cells that requires cellular activation signals beyond that provided by IL-2 alone for productive infection.

Loss of Inducible Virus in CD45RA Naive Cells After Human Immunodeficiency Virus-1 Entry Accounts for Preferential Viral Replication in CD45RO Memory Cells

Controversy exists concerning the preferential infection and replication of human immunodeficiency virus-1 (HIV-1) within naive (CD45RA+) and memory (CD45RO+) subsets of CD4+ lymphocytes. To explore the susceptibility of these subsets to HIV-1 infection, we purified CD45RA+/CD4+ (RA) and CD45RO+/CD4+ (RO) cells from normal donors and subjected them to a novel monokine activation culture scheme. Following HIV-1 infection and interleukin-2 (IL-2) induction, viral production measured on day 13 was 19-fold greater in RO cultures compared with RA cultures. IL-2–stimulated proliferation in uninfected control cultures was equivalent. To explore the mechanisms by which RA cells were reduced in viral production capacity, RA and RO cells were exposed to HIV-1 followed by treatment with trypsin, and then phytohemagglutinin antigen (PHA)-stimulated at days 4, 7, and 10 postinfection. HIV-1 production in day 4 postinfection RA and RO cultures was analogous, indicating that viral fusion and entry had occurred in both cell types. However, whereas similarly treated day 7 and 10 postinfection RO cultures produced virus, HIV-1 was markedly reduced or lost in the corresponding RA cultures. These results suggest that a temporally labile postfusion HIV-1 complex exists in unstimulated RA cells that requires cellular activation signals beyond that provided by IL-2 alone for productive infection.

Enhancement of calcium signaling and proliferation responses in activated human T lymphocytes. Inhibitory effects of K+ channel block by charybdotoxin depend on the T cell activation state

T cell receptor (TCR) stimulation, leading to T cell activation and ultimately to cell proliferation and differentiation, evokes elevations of [Ca2+]i with a high variability between individual T lymphocytes. We have used Ca(2+)-imaging of Fura-2 loaded cells to study the origin of the variation in Ca2+ signals and its consequences for the final cellular response. We found that, compared to resting cells, the percentage of responding cells and the average amplitude of the Ca2+ signal upon TCR re-stimulation by PHA increases in the first 5 days of T cell activation and declines thereafter, with more pronounced [Ca2+]i oscillations in later stages. In parallel, an enhancement of T cell proliferation is observed. Stronger stimulation of the TCR/CD3 complex by co-crosslinking CD3 with CD4/CD8 molecules evokes oscillating Ca2+ responses irrespective of the activation state, indicating that the basic capacity for Ca2+ signaling is essentially the same in resting and activated cells. Nevertheless, also the amplitude of the CD3+CD4/8 response shows a transient additional increase during the first days of T cell activation. Experiments with the K+ channel blocker charybdotoxin (CTX) indicate that [Ca2+]i oscillations depend critically on K+ channel functioning, but suppression of these oscillations by CTX does not significantly affect the average amplitude of the Ca2+ signal nor PHA-induced proliferation. However, when applied during the first 4-5 days of activation, CTX reduces in addition the average level of the TCR evoked Ca2+ response and inhibits subsequent proliferation.

Shift toward T lymphocytes with a T helper 1 cytokine-secretion profile in the joints of patients with rheumatoid arthritis

OBJECTIVE

To investigate whether T cells in the inflamed joints of patients with rheumatoid arthritis (RA) preferentially produce the T helper 1 (Th1) cytokines, interferon-gamma (IFN gamma) and interleukin-2 (IL-2), or the Th2 cytokine, IL-4, when compared with corresponding peripheral blood-derived T cells.

METHODS

Synovial fluid mononuclear cells (SFMC) and corresponding peripheral blood mononuclear cells (PBMC) from 10 patients with RA were analyzed, either directly or after in vitro stimulation, for the intracellular presence of Th1 and Th2 cytokines. The amount of secreted cytokine in the cell culture supernatants was measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

IFN gamma-containing cells were detected in the unstimulated SFMC, but not in the PBMC, of 3 patients with RA. Cells positive for IL-2 or IL-4 were not detected in the unstimulated samples. Following stimulation, the mean percentage of cells containing Th1 cytokines was significantly increased in the SFMC compared with the PBMC; no differences were found in the mean percentage of IL-4-containing cells. A comparable shift toward Th1 cytokines was observed when the amount of secreted cytokine was determined by ELISA.

CONCLUSION

A shift toward T cells with a Th1 cytokine profile was observed in the joints of patients with RA. Since an imbalance between Th1 and Th2 cells is thought to be of pathogenic significance, this finding might have implications for the development of new therapies for RA.

Resting memory CD8+ T cells are hyperreactive to antigenic challenge in vitro

The characteristics of CD8+ T cells responsible for memory responses are still largely unknown. Particularly, it has not been determined whether different activation thresholds distinguish naive from memory CD8+ T cell populations. In most experimental systems, heterogeneous populations of primed CD8+ T cells can be identified in vivo after immunization. These cells differ in terms of cell cycle status, surface phenotype, and/or effector function. This heterogeneity has made it difficult to assess the activation threshold and the relative role of these subpopulations in memory responses. In this study we have used F5 T cell receptor transgenic mice to generate a homogeneous population of primed CD8+ T cells. In the F5 transgenic mice, peptide injection in vivo leads to activation of most peripheral CD8+ T cells. In vivo BrdU labeling has been used to follow primed T cells over time periods spanning several weeks after peptide immunization. Our results show that the majority of primed CD8+ T cells generated in this system are not cycling and express increased levels of CD44 and Ly6C. These cells remain responsive to secondary peptide challenge in vivo as evidenced by short term upregulation of activation markers such as CD69 and CD44. The activation thresholds of naive and primed CD8+ T cells were compared in vitro. We found that CD8+ T cells from primed mice are activated by peptide concentrations 10-50-fold lower than naive mice. In addition, the kinetics of interleukin 2R alpha chain upregulation by primed CD8+ T cells differ from naive CD8+ T cells. These primed hyperresponsive CD8+ T cells might play an important role in the memory response.

CD7-negative T cells represent a separate differentiation pathway in a subset of post-thymic helper T cells

The absence of CD7 protein and the corresponding mRNA is a stable feature in a subset of normal circulating CD4+ memory T cells. It is still unresolved whether the CD7- subset represents a specific T-cell lineage. Here we show that repeated stimulation of highly purified CD4+ CD45RA+ CD45RO- naive T cells in vitro leads to the development of a distinct memory subset that is defined by the expression versus non-expression of the CD7 antigen. Comparing different T-cell activation pathways (TCR/CD3, CD2), we observed that alternative signals were critically involved in the development of CD4+ CD7- T cells. Peak mean numbers of CD7- memory cells occurred after 3-5 cycles of restimulation in vitro. Naive T cells that had undergone repeated stimulations were harvested and sorted into CD7+ and CD7- subsets. The vast majority (> 97%) of CD7+ T cells retained their expression, whereas the CD7- population did not re-express the antigen during further propagation of separated T-cell subsets. In CD7- cells no CD7 mRNA was monitored, indicating transcriptional regulation of CD7 expression. Certain differentiation-related antigens, including the cutaneous lymphocyte antigen CLA, were preferentially expressed on CD7- T cells. We suggest that absence of CD7 expression in a subset of CD4+ memory cells reflects a separate and stable differentiation state occurring late in the immune response. These T cells may represent the physiological counterpart of malignant T cells in certain forms of cutaneous T-cell lymphoma.

Influence of CD26 and integrins on the antigen sensitivity of human memory T cells

The antigen sensitivity of class II MHC restricted human CD4 T-cell clones is demonstrated to increase gradually with time after restimulation. This is manifested in a requirement of less antigen in culture, as well as decreased numbers of peptide-MHC complexes per APC for T-cell activation, and in an increased resistance to inhibition by class II MHC blockade. The increase in antigen sensitivity is accompanied by increased cell-surface expression of CD26, LFA-1, and VLA-1, whereas the expression of TCR and a series of other cell-surface molecules remains unchanged. Using appropriate monoclonal antibodies, we have shown that CD26 and LFA-1 contribute directly to the increased antigen sensitivity of "late-stage" T-cell clones. The late-memory T-cell phenotype established in this study is shown to occur also among T cells activated in vivo. We suggest that increasing the antigen sensitivity via antigen-nonspecific molecules is a physiologic mechanism for maintaining T-cell memory in face of decreasing antigen concentration, and for ensuring preferential activation of memory T cells upon repeated encounter with antigen.

CD2-mediated stimulation of the naive CD4+ T-cell subset promotes the development of skin-associated cutaneous lymphocyte antigen-positive memory cells

Directed migration of lymphocytes from blood into lymph nodes and organ-associated lymphatic tissue, also referred to as homing, is initiated by T-cell adhesion to specialized high endothelial cells of postcapillary vessels. Here, we demonstrate that selective signal transduction pathways specifically modulate the expression of the cutaneous lymphocyte antigen (CLA), the putative skin-homing receptor, during naive to memory transition of CD4+ T cells in vitro. The results show that the expression of CLA is strongly induced by activation via CD2 [T11.1 + T11.2 monoclonal antibodies (mAb)]. Addition of transforming growth factor-beta 1 (TGF-beta 1), interleukin-6 (IL-6), and, to a lesser extent, IL-2 further enhanced the generation of CLA+ T cells, whereas the induction of this antigen was markedly inhibited by IL-4. Periodic restimulation via CD2 and long-term culture of activated cells in the presence of IL-2 and TGF-beta 1 resulted in stable expression of CLA during a culture period of more than 100 days. In contrast, activation of naive CD4+ T cells via CD3, CD28 or by mitogens induced a rapid naive to memory phenotype transition but a much lower percentage of CLA+ T cells showing only weak expression of the antigen. Furthermore, activation of purified CD4+ memory T cells by CD2 strongly induced expression of activation-related antigens CD25 and HLA-DR, but failed to up-regulate CLA expression. Our results show that primary stimulation conditions highly modulate the development of skin-associated T cells and indicate a new functional role for costimulatory adhesion pathways in regulating the expression of molecules associated with T-cell homing.

Trypanosoma cruzi-induced immunosuppression: selective triggering of CD4+ T-cell death by the T-cell receptor-CD3 pathway and not by the CD69 or Ly-6 activation pathway

In a model of experimental Chagas' disease induced with metacyclic forms of Trypanosoma cruzi, CD4+ but not CD8+ T cells undergo T-cell receptor (TCR)-CD3-mediated activation-induced cell death (AICD) in vitro. CD4+ T cells from T. cruzi-infected mice also developed unresponsiveness in proliferative responses to TCR-CD3-mediated stimulation. A linear correlation was found between extent of proliferative unresponsiveness and loss of CD4+ T-cell viability. CD4+ T-cell activation through the CD69 or Ly-6 A/E pathway, on the other hand, did not result in proliferative unresponsiveness compared with controls. Lack of suppression in proliferation assays correlated with lack of AICD by cells stimulated through the CD69 or Ly-6 A/E pathway. Concomitant stimulation through CD69, however, did not rescue CD4+ T cells from CD3-induced death. Flow cytometry study of cells stimulated in vitro showed no defect in interleukin-2 receptor expression by CD4+ T cells from infected donors, which escaped TCR-mediated AICD. In vivo injection of anti-CD3 into acutely infected mice, but not into control mice, led to splenocyte DNA fragmentation and failed to increase splenic CD4+ T-cell numbers. These results show that TCR-CD3-mediated AICD is involved in CD4+ T-cell unresponsiveness in vitro following infection with T. cruzi. In addition, successful activation of these cells through the CD69 and Ly-6 pathways is due to differences in the inability of these stimuli to trigger AICD. Since TCR-CD3-mediated AICD can be induced in vivo in infected mice, these findings may be relevant for the onset of immunological disturbances in the host.

Visualization, characterization, and turnover of CD8+ memory T cells in virus-infected hosts

The cellular basis of T cell memory is a controversial issue and progress has been hampered by the inability to induce and to trace long-term memory T cells specific for a defined antigen in vivo. By using the murine model of lymphocytic choriomeningitis virus (LCMV) infection and an adoptive transfer system with CD8+ T cells from transgenic mice expressing an LCMV-specific T cell receptor, a population of authentic memory T cells specific for LCMV was generated and analyzed in vivo. The transgenic T cells that have expanded (1,000-fold) and then decreased (10-fold) in LCMV-infected C57BL/6 recipient mice exhibited the following characteristics: they were (a) of larger average cell size than their naive counterparts but smaller than day 8 effector cells; (b) heterogeneous with respect to expression of cell surface "memory" markers; and (c) directly cytolytic when isolated from recipient spleens. The time-dependent proliferative activity of these LCMV-specific memory T cells was analyzed in the recipients by bromodeoxyuridine labeling experiments in vivo. The experiments revealed that LCMV-specific CD8+ memory T cells can persist in LCMV-immune mice for extended periods of time (>2 mo) in the absence of cell division; the memory population as a whole survived beyond 11 mo.

IL-2 gene therapy of solid tumors: an approach for the prevention of signal transduction defects in T cells

The identification of tumor-associated antigens has focused attention on the mechanisms that underlie the failure of T cells to destroy tumor cells. A deeper understanding of the process of signal transduction following the binding of ligand by the T cell receptor can help to identify underlying defects that may be involved. Gene therapy using tumor cells genetically modified to express cytokines or surface determinants is a promising technique for stimulating antitumor responses. A potential pitfall in its application to cancer, however, is that some patients' T cells are immune suppressed and may resist stimulation by such genetically engineered vaccines. Recent studies have demonstrated that T cells from tumor-bearing patients exhibit abnormalities in signal transduction events, possibly rendering them unable to respond to activation signals. Gene therapy with interleukin 2 secreting tumor cells in an animal model has been shown effective in preventing the onset of signaling defects. A more precise definition of the molecular mechanisms that enable cytokine-secreting tumor cells to stimulate specific antitumor responses may make it feasible to optimize immunotherapeutic approaches resulting in better clinical results.

Interleukin 4 (IL-4) and IL-6-producing memory T-cells in peripheral blood and gingival tissue in periodontitis patients with high serum antibody titers to Porphyromonas gingivalis

We assessed cytokine production and proliferation of memory T-cells that were isolated from peripheral blood of adult periodontitis patients with high anti-Porphyromonas gingivalis titer. Memory T-cells were stimulated with P. gingivalis lipopolysaccharide, sonicates and formalin-killed whole cells. Interleukin 4(IL-4)- and IL-6-producing cells were stained by immunocytochemistry on peripheral blood smears and compared with cryostat sections of autologous gingival biopsies. Memory T-cells in the peripheral blood of patients rated significantly higher than in healthy subjects (32.3 + or - 7.1 vs 25.3 + or - 3.0%). Stimulation of patient-derived memory T-cells with P. gingivalis whole cells induced higher IL-4 production than in healthy subjects (4.4 + or - 4.1% vs 0.7 + or - 0.6%). Induction of IL-4 producing memory T-cells by P. gingivalis lipopolysaccharide and whole cells was respectively 1.37 and 1.56 times that induced by medium alone. IL-6 production did not differ between the groups. Proliferation of memory T-cells in healthy subjects tended to be more inhibited by P. gingivalis antigens than that in patients. In some patients, induction of IL-4- and IL-6-producing memory T-cells in peripheral blood and in autologous gingival biopsies tended to coincide. Memory T-cells with functional characteristics of Th2 could be a crucial cell population capable of reflecting individual susceptibility to periodontitis.

CD50 (intercellular adhesion molecule-3) is expressed at higher levels on memory than on naive human T cells but induces a similar calcium mobilization on both subsets

CD50, the intercellular adhesion molecule-3 (ICAM-3), is expressed almost exclusively on hematopoietic cells. T lymphocytes display a bimodal distribution on CD50 expression levels. It was observed that CD45RO+ cells expressed higher levels of CD50 than CD45RA+ T lymphocytes. A similar situation was observed when CD4 and CD8 subpopulations were analyzed, with CD8+ cells expressing higher levels of CD50 than CD4+ cells. When adult T lymphocytes were analyzed by three-color flow cytometry in CD8+CD45RA+ cells both CD50low and CD50high expressing cells were detected, in accordance with several memory markers on T lymphocytes, whereas only cells with a low level of CD50 were observed in CD4+CD45RA+. The different level of CD50 expression was confirmed by analyzing purified CD45RA+ and CD45RO+ T cells. Moreover, after the comparison of CD50 expression level in thymocytes, cord blood and adult T lymphocytes, a progressive increase was observed. When T cells were sorted by their intensity of CD50 expression, only CD50high cells proliferated in response to tetanus toxoid. Therefore, the phenotypic and functional analysis of adult and cord blood T lymphocytes as well as thymocytes indicates that CD50 expression increases during the maturation process of T lymphocytes: from the lowest CD50 levels present on CD1+ thymocytes, to the highest levels of CD50 on human memory T cells. In addition, we have observed that after CD50 cross-linking on human T lymphocytes, a transient increase in intracellular calcium concentration ([Ca2+]i) is produced. When CD45RA+ and CD45RO+ T cells were analyzed, in spite of the level of CD50 expression, the stimulation through CD50 induced a similar level of Ca2+ mobilization in both subpopulations, contrasting with the higher rise in [Ca2+]i induced by CD3 stimulation on CD45RA+ versus CD45RO+. These data suggest that the signal transduction pathways activated after CD50 cross-linking are, at least partially, independent of those involved after CD3 stimulation.

The thymus atrophy-inducing organotin compound DBTC inhibits the binding of thymocytes to thymic epithelial cells

In this study, it is examined whether the organotin compound di-n-butyltindichloride (DBTC), which has been shown to inhibit immature thymocyte proliferation, is able to disturb the binding between thymocytes and thymic epithelial cells (TEC). To that end, an enzyme-linked binding assay was developed in which the amount of binding of Thy-1+ (mAb ER4)-thymocytes to the rat-derived TEC-line IT45R1 (IT45R1-TEC) could be detected. It was found that preincubation of thymocytes with 3-5 microM DBTC for 30 min inhibited the binding by 50-60% during a 1 h adhesion period. By extending the preincubation period to 1 h and the adhesion period to 22 h, 0.1 microM DBTC was already sufficient to reduce the binding with 60-80%. Further characterization of the binding revealed that splenic lymphocytes were unable to bind to the MHC class II-negative IT45R1-TEC. Since dextran sulfate inhibited the binding as well, sulfated polysaccharide-binding molecules such as Thy-1 and CD2 are likely to be involved in the binding. Electron microscopy showed filament-containing microvilli at the site of interaction. The results are discussed in relation to the mechanism of DBTC-induced thymus atrophy.

A subset of CD4+ memory T cells contains preformed CD40 ligand that is rapidly but transiently expressed on their surface after activation through the T cell receptor complex

Signaling through surface CD40 is essential for selecting B cells that have mutated their immunoglobulin variable region genes in germinal centers and is an important signal in the early stages of antibody responses to T cell-dependent antigens. It is shown that a subset of CD45RO+, CD4+ T cells isolated from human tonsil contains preformed 30-35-kD ligand for CD40. This is expressed on their surfaces within 5 min of their antigen-receptor complexes interacting with CD3 epsilon antibodies bound to ox erythrocytes. This surface expression does not require de novo protein synthesis and lasts for only 1-2 h. Preformed CD40 ligand (CD40L) was not detected in any CD4+ CD45RA+ T cells, but > 90% of all CD4+ T cells from the tonsil can be induced to express large amounts of CD40L on culture with phorbol myristate acetate and the calcium ionophore ionomycin. This expression of CD40L starts between 1 and 2 h, peaks at 6 h, and remains at a high level for > 20 h. It is totally prevented by adding a concentration of cycloheximide that inhibits CD25 synthesis by these activated cells. While CD3 epsilon antibody bound to ox red cells is a good inducer of surface expression of CD40L, it is a much less potent inducer of CD40L synthesis than phorbol myristate acetate with ionomycin. Immunohistological analysis of tonsil sections shows that cells containing CD40L are located mainly in the outer zone of germinal centers and the margins of the T zones that are rich in dendritic cells (interdigitating cells). The distribution of these cells is consistent with: (a) their interaction in T zones with B cells that have taken up and processed antigen and (b) their involvement in B cell selection in germinal centers.

Generation of CD4+ cytotoxic T lymphocytes stimulated by immobilized anti-CD3 monoclonal antibody and interleukin-2 in cancer patients

The proliferation of autologous tumor-reactive cytotoxic T lymphocytes (CTL), induced by autologous mixed lymphocyte tumor-cell culture, was remarkably enhanced by activation with immobilized anti-CD3 monoclonal antibody (MAb) and interleukin-2 (IL-2), as compared with IL-2 alone. The activated CTL exhibited high cytotoxicity against autologous tumor cells. Cytotoxicity against autologous tumor cells was inhibited by anti-HLA-DR MAb. In negative selection with immunomagnetic beads, cytotoxicity against autologous tumor cells was inhibited by the elimination of CD4+ cells. The major cell-surface antigens of the activated CTL were CD3+, CD4+, CD25+, CD45RO+ and CD45RA-, suggesting helper T cells, and the activated CTL produced IL-2. It is concluded that the CTL activated by immobilized anti-CD3 MAb and IL-2 were CD4 cells that had both killer and helper functions. Our findings indicate that adoptive immunotherapy using these activated CTL would be effective in cancer patients.

Control of memory CD4 T cell activation: MHC class II molecules on APCs and CD4 ligation inhibit memory but not naive CD4 T cells

Memory or antigen-experienced CD4 T cells differ from naive CD4 T cells both phenotypically by cell surface marker expression, and functionally by their dissimilar pattern of cytokine secretion and activation requirements through their T cell receptor (TCR). We show here that activation of memory CD4 T cells (CD45RBlo subset), but not naive CD4 T cells (CD45RBhi subset), is inhibited by MHC class II molecules on antigen-presenting cells and by CD4 ligation. We propose that the selective negative signal in memory cells is a direct result of the differences in signaling via CD4 and CD3, exemplified in the disparate pattern of tyrosine-phosphorylated proteins visible after activation of the two subsets. In vivo, this inhibitory signal may serve to prevent irrelevant interactions between memory CD4 T cells and bystander MHC class II+ cells, and may also be responsible for the defective functioning of memory CD4 T cells in AIDS.

Functional defects in peripheral blood T cells of multiple sclerosis patients. Diminished in vitro responsiveness in accessory cell dependent activation systems

Function and phenotype of peripheral blood (PB) T cells in multiple sclerosis (MS) patients were analyzed. In whole blood cultures, T cell proliferation of multiple sclerosis (MS) patients, using soluble CD3 mAb and CD2 mAb as stimulants, was reduced in comparison to healthy controls. A similar difference was seen when isolated PBMC were tested after stimulation with soluble CD3 mAb. However, in accessory cell-independent activation systems, i.e. after stimulation of PBMC with immobilized CD3 mAb or after co-stimulation with CD28 mAb, both patients and controls responded equally well. Phenotypical analysis of the circulating T cell population showed that there were no differences in the percentage of CD26+, 'memory' (CD45R0+) or 'effector' (CD4+CD45R0+CD27-) cells between MS patients and healthy controls. Finally, although MS patients did show an enhanced proportion of 'naive' (CD4+CD45RA+) T cells, this did not correlate with the observed functional defects.

Human CD45RA+ and CD45R0+ T cells exhibit similar CD3/T cell receptor-mediated transmembrane signaling capacities but differ in response to co-stimulatory signals

CD45RA+ cells have been described to be less responsive to CD3/T cell receptor (TcR)-mediated activation than CD45R0+ T cells. To analyze the underlying mechanism of the differential responses we compared CD3/TcR-triggered tyrosine phosphorylation in the two subsets and studied the role of co-stimulatory signals provided either by accessory cells or pharmacologic activation of protein kinase C by phorbol ester. Stimulation of purified CD45RA+ and CD45R0+ T cells with CD3/TcR antibodies induced similar patterns and intensities of tyrosine phosphorylation in the two subsets, but no proliferation. If accessory cells were used as the source of co-stimulatory signals, strong expression of the 55-kDa chain of the interleukin-2 (IL-2) receptor (CD25), significant IL-2 production and vigorous proliferation were observed in CD45R0+ cells, whereas CD45RA+ cells responded weakly. However, when CD3/TcR-mediated triggering was combined with activation of protein kinase C by phorbol ester, CD45RA+ cells responded strongly. These data indicate that the transmembrane signaling capacity of the T cell receptor expressed by CD45RA+ and CD45R0+ cells is similar and, therefore, is presumably not responsible for the differential reactivities of the two subsets. It is more likely that co-stimulatory signals determine whether CD3/TcR-initiated activation results in strong or weak responses.

Age-related increase in the fraction of CD27-CD4+ T cells and IL-4 production as a feature of CD4+ T cell differentiation in vivo

The influence of ageing on phenotype and function of CD4+ T cells was studied by comparing young (19-28 years of age) and aged (75-84 years of age) donors that were selected using the SENIEUR protocol to exclude underlying disease. An age-related increase was observed in the relative number of memory cells, not only on the basis of a decreased CD45RA and increased CD45RO expression, but also on the basis of a decrease in the fraction of CD27+CD4+ T cells. Our observation that the absolute number of CD45RO+CD4+ T cells was increased, while absolute numbers of CD27-CD4+ T cells remained unchanged in aged donors, indicates that the latter subset does not merely reflect the size of the CD45RO+CD4+ T cell pool. The increased fraction of memory cells in the aged was functionally reflected in an increased IL-4 production and T cell proliferation, when cells were activated with the combination of anti-CD2 and anti-CD28, whereas IL-2 production was comparable between both groups. No differences were observed with respect to proliferative T cell responses or IL-2 production using plate-bound anti-CD3 or phytohaemagglutinin (PHA). The observation that IL-4 production correlated with the fraction of memory cells in young donors but not in aged donors suggests different functional characteristics of this subset in aged donors.

Preparation and characterization of a new monoclonal antibody CAF7 specific for a leukocyte activation antigen

A new murine hybridoma, CAF7, is raised using as an antigen the T-leukemic cell line CEM. It produces a monoclonal antibody (mAb) specific for an activation antigen on human lymphocytes. CAF7 stains monocytes, very weakly resting lymphocytes, and granulocytes, part of the thrombocytes, but not erythrocytes. After activation with phytohemagglutinin (PHA), CAF7 expression on peripheral lymphocytes rises as early as 5 hr post stimulation and 72-hr blasts express it at a considerable level. The cell lines Jurkat, CEM, MOLT4, Raji, Reh, K562, and MONOMAC6 are positive for CAF7. CAF7 immunoprecipitated an antigen, which when subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under nonreducing conditions migrated as a single band with a molecular weight of 120-140 kD. Under reducing conditions, it appears as two bands at 90-100 and 40 kD. The pattern of expression and the biochemical characteristics of CAF7 do not match any of the clusters defined in the Leukocyte Typing Workshops, but do resemble that of a previously described antigen, 4F2.

Isoforms of the transmembrane tyrosine phosphatase CD45 differentially affect T cell recognition

Activation of T cells has been shown to require CD45. CD45 is expressed on T cells as distinct isoforms and these isoforms are expressed differentially on subsets of CD4 T cells. We have generated T cell lines expressing a T cell receptor (TCR) of known specificity, with or without CD4, and examined the effect of different CD45 isoforms on stimulation through the antigen receptor. We find that isoforms differ in their ability to participate in antigen recognition, with the null isoform that is predominantly found on memory CD4 T cells being the most effective. The ability of the CD4 T cells being the most effective. The ability of the CD45 ectodomain to differentially affect sensitivity to specific ligands represents a novel way of regulating the efficacy of signaling through a receptor without altering its specificity. It may play a crucial role both in immunological memory and during intrathymic maturation of T cells.

Stimulation with specific antigen can block superantigen-mediated deletion of T cells in vivo

The T-cell response to pigeon cytochrome c peptide, residues 88-104 (pcytC), in B10.BR mice is mediated largely by cells bearing both V beta 3 and V alpha 11 variable regions of the T-cell antigen receptor. These cells are, therefore, reactive with the superantigen staphylococcal enterotoxin A (SEA). Recent reports have shown that in vivo exposure to superantigen can lead to deletion of superantigen-reactive T cells from the pool of mature T cells in the periphery. Here we show that upon cotreatment of animals with both SEA and pcytC, bulk deletion of the population of SEA-reactive cells is maintained, while the subpopulation of SEA-reactive T cells that also responds to pcytC is not deleted but instead proliferates in response to pcytC. These results are discussed with regard to mechanisms regulating the balance between T-cell tolerance and T-cell activation in vivo.

Continuous in vivo activation and transient hyporesponsiveness to TcR/CD3 triggering of human gut lamina propria lymphocytes

Three-color immunofluorescence and flow cytometric analysis showed that the vast majority of normal human T-lamina propria lymphocytes (LPL) expressed high levels of the early activation antigen CD69, together with CD45R0, irrespective of their CD4, CD8 or gamma/delta-TcR phenotype, indicating that they are continuously stimulated in vivo. Importantly, measurement of cytoplasmic [Ca2+]i showed that T-LPL had significantly higher basal [Ca2+]i levels, compared to autologous peripheral blood lymphocytes (PBL). Both cytoplasmic [Ca2+]i elevation and inositol (1,4,5) trisphosphate generation following CD3 cross-linking by monoclonal antibodies in vitro were essentially abolished in T-LPL, as compared to autologous T-PBL. Moreover, freshly isolated LPL could be induced to proliferate by CD2- or CD28-mediated signals, but not by CD3-mediated signals. Surprisingly however, impairment in TcR/CD3-mediated early signaling and proliferation in T-LPL could be completely reversed by 24 h incubation of the cells at 37 degrees C in culture medium, a condition which allowed basal intracellular [Ca2+]i to return to levels comparable to peripheral T cells. Our data suggest that selective hyporesponsiveness to TcR/CD3-mediated signaling may represent a transient event during continuous in vivo activation of mucosal lymphocytes.