T and B memory cells

Vbeta8+ T lymphocytes are essential in the regulation of airway hyperresponsiveness and bronchoalveolar eosinophilia but not in allergen-specific IgE in a murine model of allergic asthma

BACKGROUND

There is increasing evidence that in allergic asthma the inflammatory process is regulated by T lymphocytes. In BALB/c mice the majority of ovalbumin responsive T lymphocytes express the Vbeta8.1+ and Vbeta8.2+ T-cell receptor.

OBJECTIVE

We analysed the contribution of Vbeta8+ T lymphocytes during the sensitization and challenge phase in the regulation of antigen-specific IgE, airway hyperresponsiveness and cellular infiltration in the airways in a murine model of allergic asthma.

METHODS

Mice strains genetically lacking (SJL/J and SJA/9) and expressing (BALB/c) the Vbeta8+ T cell receptor were used. In addition, prior to the sensitization and prior to the challenge BALB/c mice were treated with antibodies to Vbeta8. Mice were sensitized with ovalbumin, followed by repeated challenge with ovalbumin or saline aerosols.

RESULTS

In ovalbumin challenged BALB/c mice treated with control antibody a significant increase in eosinophils in the bronchoalveolar lavage, airway hyperresponsiveness and increased serum levels of ovalbumin-specific IgE were observed compared to control mice. Treatment of BALB/c mice with antibodies to Vbeta8 prior to the sensitization or prior to the challenge period completely inhibited the ovalbumin induced infiltration of eosinophils and airway hyperresponsiveness, while ovalbumin-specific IgE was slightly decreased. In SJA/9 and SJL/J mice ovalbumin challenge did not induce eosinophilic infiltration and airway hyperresponsiveness. In SJL/J mice ovalbumin challenge induced an upregulation of ovalbumin-specific IgE, however, in SJA/9 mice no upregulation was observed.

CONCLUSION

It is demonstrated that Vbeta8+ T lymphocytes are essential for infiltration of eosinophils in the airways and development of airway hyperresponsiveness in a murine model of allergic asthma. In contrast, although Vbeta8+ T lymphocytes seem to be important for the extent of IgE levels, no essential role for Vbeta8+ T lymphocytes in the induction of antigen-specific IgE was observed.

Differentiation of Naive CTL to Effector and Memory CTL: Correlation of Effector Function with Phenotype and Cell Division

Phenotypically and functionally, the early steps of T cell differentiation are not well characterized. In addition, the effector T cell stage shares several phenotypic characteristics with memory T cells, which has made the analysis of T cell memory difficult. In this study, we have investigated in vitro and in vivo the differentiation of naive CTL into effector and memory CTL as a function of cell division using lymphocytic choriomeningitis virus-specific TCR-transgenic spleen cells labeled with the vital dye carboxyfluorescein diacetate, succinimidyl ester. The following major points emerged. 1) During the first nine cell divisions, the investigated cell surface markers were strongly modulated. 2) The TCR was stepwise down-regulated during viral infection. 3) Cytotoxic effector function was acquired within one cell division and was retained during the next four to five divisions. 4) In vitro, CTL reached a CD44highCD62L+ memory phenotype after 6–10 cell divisions and required restimulation to exert effector function. 5) Lymphocytic choriomeningitis virus memory mice contained two distinct memory populations: a CD44highCD62L− population, predominately located in the spleen and exerting rapid effector function, and a CD44highCD62L+ population found in the spleen and the lymph nodes, which had lost immediate effector function. This finding suggests that two types of memory CTL exist. The correlation between CD62L expression, effector function, and Ag persistence is discussed.

The Memory B Cell Subset Responsible for the Secretory IgA Response and Protective Humoral Immunity to Rotavirus Expresses the Intestinal Homing Receptor, α4β7

Infection of mice with murine rotaviruses induces life-long immunity, characterized by high levels of IgA in the intestine and large numbers of rotavirus (RV)-specific Ab-secreting cells in gut-associated lymphoid tissues. Lymphocyte trafficking into gut-associated lymphoid tissues is mediated by interaction of the α4β7 integrin on lymphocytes with the vascular mucosal addressin cell adhesion molecule-1. To determine whether B cell memory for RV correlates with α4β7 expression, we transferred sorted B220+ phenotypically defined memory (IgD−α4β7high and IgD− α4β7−) and naive (IgD+α4β7+) splenocytes into recombination-activating gene-2 knockout mice (B and T cell-deficient) that were chronically infected with RV. Only mice receiving α4β7high memory (IgD−) B cells produced RV-specific IgA in the stool, cleared the virus, and were immune to reinfection. α4β7high (but not α4β7−) memory B cells from donors boosted as much as 7 mo previously also cleared the virus, indicating that α4β7high memory B cells maintain long term functional immunity to RV. Although only α4β7high memory cells provided mucosal immunity, α4β7− cells from recently boosted donor animals could generate RV-specific serum IgG, but, like naive (IgD+) B cells, were unable to induce viral clearance even 60 days after cell transfer. These data indicate that protective immunity for an intestinal pathogen, RV, resides in memory phenotype B cells expressing the intestinal homing receptor, α4β7.

Tolerance induction in mature T lymphocytes

T lymphocytes with self-destructive capacity are often found in healthy individuals, indicating efficient control mechanisms that prevent autoimmunity. Recently, we were able to demonstrate the existence of peripheral tolerance in double-transgenic mice expressing the foreign histocompatibility antigen H-2Kb exclusively outside the thymus and a T cell receptor (Des.TCR) directed against the Kb molecule. In mice expressing Kb only on keratinocytes anti-Kb T cells were still present but failed to reject Kb-positive tissue grafts. This observation would imply a continuous migration of naïve T cells exported from the thymus into non-lymphoid tissues where these fresh thymic emigrants would need to be tolerized. However, this is in contrast to the view that migration to peripheral tissues is restricted to activated T cells. To investigate whether there is a continuous process of tolerization of naïve T cells in adult DES.TCR x 2.4Ker-Kb mice, 2.4Ker-Kb mice were crossed with Rag-2-deficient mice and reconstituted with bone marrow cells of Des.TCR transgenic mice (Des.TCR x 2.4Ker-Kb.Rag-2-). Tolerance was not observed in these chimeric mice. We conclude from these results that in contrast to the neonate the adult physiological environment does not allow tolerance induction to antigens expressed on keratinocytes in T cells newly exported from the thymus. Furthermore, we have to postulate regulatory events responsible for the maintenance of peripheral tolerance in the adult Des.TCR x 2.4Ker-Kb animals.

Bacille Calmette-Guérin Vaccination Enhances Human γδ T Cell Responsiveness to Mycobacteria Suggestive of a Memory-Like Phenotype

Bacille Calmette-Guérin (BCG) immunity can be studied as one experimental model for mycobacterial protective immunity. We have used flow cytometry to investigate human T cell subsets induced by BCG vaccination. PBMC harvested from BCG-vaccinated individuals and controls were stimulated with mycobacterial Ags, and the T cell subsets present after 7 days of in vitro expansion were characterized. The most dramatic expansions induced by mycobacterial Ags were detected in γδ T cells. The γδ T cell expansions measured after in vitro stimulation with mycobacterial Ags were significantly greater in BCG responders compared with nonsensitized controls, indicating that BCG vaccination induced γδ T cell activation associated with enhanced secondary responses. The majority of γδ T cells induced by BCG vaccination were γ9+δ2+ T cells reactive with isoprenyl pyrophosphates. Coculture with CD4+ T cells induced optimal γδ T cell expansion, although IL-2 alone could provide this helper function in the absence of CD4+ T cells. γδ T cells were found to provide helper functions for mycobacterial specific CD4+ and CD8+ T cells as well, demonstrating reciprocal stimulatory interactions between γδ T cells and other T cell subsets. Finally, prominent mycobacterial specific γδ T cell expansions were detected in a subset of unvaccinated controls with evidence for prior sensitization to mycobacterial lysates (elevated mycobacterial specific lymphoproliferative responses). These latter findings are consistent with the hypothesis that exposure to atypical mycobacteria or related environmental Ags may induce γδ T cells cross-reactive with Ags present in the Mycobacterium tuberculosis complex. Our results suggest that γδ T cells may be capable of developing a memory immune-like phenotype, and therefore might be important targets for new vaccines.

Resistance to Apoptosis and Elevated Expression of Bcl-2 in Clonally Expanded CD4+CD28− T Cells from Rheumatoid Arthritis Patients

Patients with rheumatoid arthritis have a subset of CD4+ T lymphocytes that are characterized by a defect in CD28 expression. CD4+CD28− T cells frequently undergo clonal expansion in vivo. These clonotypes include autoreactive cells and persist over many years. The clonogenic potential and longevity of these T cells could be related to an altered response to apoptosis-inducing signals. To explore this possibility, CD4+CD28− T cell lines and clones were examined for their response pattern to stimuli inducing physiologic cell death. CD4+CD28− T cells were found to be resistant to apoptosis upon withdrawal of the growth factor, IL-2. To examine whether the altered sensitivity to this apoptotic signal was correlated with the expression of proteins of the bcl-2 family, the expression of bcl-2, bcl-x, and bax proteins was determined. CD28+ and CD28−CD4+ T cells could not be distinguished by the levels of bax or bcl-xL protein; however, CD4+CD28− T cells expressed higher amounts of bcl-2 protein than did CD4+CD28+ T cells. The increased bcl-2 expression in CD4+CD28− T cells was relatively independent of signals provided by exogenous IL-2. In CD28-deficient CD4+ T cells, bcl-2 was not significantly up-regulated by the addition of exogenous IL-2 and was maintained despite IL-2 withdrawal, as opposed to CD28-expressing CD4+ T cells. We propose that CD4+CD28− T cells are characterized by a dysregulation of the survival protein, bcl-2, which may favor the clonal outgrowth of autoreactive T cells and thus contribute to the pathogenesis of rheumatoid arthritis.

T-Cell response to woodchuck hepatitis virus (WHV) antigens during acute self-limited WHV infection and convalescence and after viral challenge

The infection of woodchucks with woodchuck hepatitis virus (WHV) provides an experimental model to study early immune responses during hepadnavirus infection that cannot be tested in patients. The T-cell response of experimentally WHV-infected woodchucks to WHsAg, rWHcAg, and WHcAg peptides was monitored by observing 5-bromo-2'-deoxyuridine and [2-3H]adenine incorporation. The first T-cell responses were directed against WHsAg 3 weeks after infection; these were followed by responses to rWHcAg including the immunodominant T-cell epitope of WHcAg (amino acids 97 to 110). Maximal proliferative responses were detected when the animals seroconvered to anti-WHs and anti-WHc (week 6). A decrease in the T-cell response to viral antigens coincided with clearance of viral DNA. Polyclonal rWHcAg-specific T-cell lines were established 6, 12, 18, and 24 weeks postinfection, and their responses to WHcAg peptides were assessed. Five to seven peptides including the immunodominant epitope were recognized throughout the observation period (6 months). At 12 months after infection, T-cell responses to antigens and peptides were not detected. Reactivation of T-cell responses to viral antigens and peptides occurred within 7 days after challenge of animals with WHV. These results demonstrate that a fast and vigorous T-cell response to WHsAg, rWHcAg, and amino acids 97 to 110 of the WHcAg occurs within 3 weeks after WHV infection. The peak of this response was associated with viral clearance and may be crucial for recovery from infection. One year after infection, no proliferation of T cells in response to antigens was observed; however, the WHV-specific T-cell response was reactivated after challenge of woodchucks with WHV and may be responsible for protection against WHV reinfection.

Thymic anti-tumor effectors in mice cured of lymphoma by cyclophosphamide plus TNF-alpha therapy: phenotypic and functional characterization up to 20 months after initial tumor inoculation

As reported previously, cyclophosphamide plus tumor necrosis factor-alpha treatment of C57BL/6 mice bearing advanced EL4 lymphoma induced approx. 60% long-term (i.e., >60 days) survivors. These mice developed protective immunity, as evidenced by 1) rejection (100% survival) of EL4 tumor re-implanted on day 60 (day 0 = initial tumor implantation); and 2) development of significant levels of specific EL4 tumor cell killing activity by both splenocytes and thymocytes. Using this model, age-related changes in functionally and phenotypically definable thymocyte subsets were assessed. In thymocytes from 90 to 308 day survivors, specific immune memory was long term; both CD4+ and CD8+ cells were required for the ex vivo stimulation of lytic activity, but the specific anti-EL4 cytotoxic effector was CD4-CD8+. On day 520, the surviving mice were randomized into 2 groups. One group received a second re-challenge with EL4 tumor cells and all survived. The other group was sacrificed on day 520. Their thymocytes, exposed to X-irradiated EL4, developed anti-EL4 lytic activity and, in comparison with thymocytes of young and age-matched control mice, were markedly enriched in CD4-CD8+CD44+ cells. On day 625, thymocytes from the survivors of the day 520 re-challenge were evaluated and were found to have developed specific anti-EL4 lytic activity. Phenotypically, they had returned toward the pattern seen in age-matched control mice although CD4-CD8+CD44+ cells remained increased. These mice were > or = 2 years old, the median life span of C57BL/6 mice. Thus, mice cured of tumor by an immuno-modulating regimen rejected re-implanted primary tumor and maintained specific thymic anti-tumor immune memory for life.

Bystander stimulation of T cells in vivo by cytokines

Immune responses to infectious agents, especially viruses, are often associated with extensive proliferation of T cells and transient enlargement of the lymphoid tissues. Since the precursor frequency of T cells for specific antigen is low, the bulk of the T cells proliferating in the primary response are presumably stimulated via non-antigen-specific mechanisms, e.g. via cytokines elicited by the infectious agent concerned. Such 'bystander' stimulation of T cells occurs in mice injected with agents that elicit production of type I interferon (IFN I). Induction of IFN I in vivo causes marked stimulation of the CD44hi subset of CD8+ T cells and is prominent after injection of live viruses or products of bacteria such as lipopolysaccharide. Cytokines elicited by infectious agents may act as adjuvants during the primary response and could serve to boost the survival of long-lived memory cells.

Differential migration of Th1 and Th2 cells--implications for vaccine and infection studies

Most lymphocytes migrate continuously between the blood and lymphatic system. This migration does not occur randomly and shows some bias for specific tissue compartments. In particular, CD4+ memory T cells have been shown to preferentially migrate to either peripheral or mucosal lymph nodes depending on their site of origin. The selective migration of lymphocytes into lymph nodes is facilitated by the differential expression of adhesion molecules on the lymphocyte surface interacting with their respective ligands on endothelial cells lining the capillary vessels. The acquisition of these 'mucosal' or 'peripheral' homing receptors was thought to be dictated by the particular tissue site in which lymphocyte were activated. A large amount of recent experimentation has shown that memory T cells generated against infectious agents can have different functional phenotypes as determined by their cytokine secretion patterns. Two of these distinct functional phenotypes. Th1 and Th2 cells, are differentially induced in peripheral and mucosal lymph nodes and recent data has suggested that the observed tissue-specific migration of memory T cells may be determined by this functional phenotype rather than the site of activation. Data in support of this new hypothesis are presented in this paper. In addition, as both the functional and surface phenotype of lymphocytes is dependent on local hormonal and cytokine environments, lymphocyte migration patterns may be manipulated by vaccination and infection.

Immunogenicity and protective efficacy in mice of influenza B virus vaccines grown in mammalian cells or embryonated chicken eggs

The immunogenicity and protective efficacy of formalin-inactivated influenza B/Memphis/1/93 virus vaccines propagated exclusively in Vero cells, MDCK cells, or embryonated chicken eggs (hereafter referred to as eggs) were investigated. Mammalian cell-grown viruses differ from the egg-grown variant at amino acid position 198 (Pro/Thr) in the hemagglutinin gene. The level of neuraminidase activity was highest in egg-grown virus, while MDCK and Vero cell-derived viruses possessed 70 and 90% less activity, respectively. After boosting, each of the vaccines induced high levels of hemagglutinin-inhibiting, neuraminidase-inhibiting, and neutralizing antibodies that provided complete protection from MDCK-grown virus challenge. Mammalian cell-derived virus vaccines induced serum antibodies that were more cross-reactive, while those induced by egg-grown virus vaccines were more specific to the homologous antigen. Enzyme-linked immunospot analysis indicated that cell-grown virus vaccines induced high frequencies of immunoglobulin G (IgG)-producing cells directed against both cell- and egg-grown virus antigens, whereas egg-grown virus vaccine induced higher frequencies of IgG- and IgM-producing cells reacting with homologous antigen and low levels of IgG-producing cells reactive with cell-grown viruses. These studies indicate that influenza B virus variants selected in different host systems can elicit different immune responses, but these alterations had no detectable influence on the protective efficacy of the vaccines with the immunization protocol used in this study.

Allospecific helper T-lymphocyte repertoire in monozygotic twins

In the course of maturation in the thymus there is a selection of T lymphocytes based on the avidity between their T-cell receptors and HLA/peptide complexes expressed on stromal thymic cells. The repertoire of mature T lymphocytes is further modulated by encounters with foreign antigens. Thus, the antigen specific repertoire of the peripheral T-lymphocyte pool is determined by genetic and environmental factors. We recently reported that pairs of monozygotic twins often display significant differences in their allospecific cytotoxic T-cell repertoire, suggesting an important role of confrontation with foreign antigens on the CD8+ T-cell repertoire. We have now performed similar studies on the repertoire of allospecific CD4+ T lymphocytes. Using positively selected CD4+ T cells in limiting dilution analyses we compared the differences in the allospecific helper T-lymphocyte precursor frequencies (HTLpf) between pairs of genetically identical monozygotic twins and pairs of unrelated, HLA disparate individuals. We found that all monozygotic twin pairs and most unrelated pairs had similar HTLpf to the same stimulator, i.e. the 95% confidence intervals were overlapping. However, when studied in greater detail, the differences in HTLpf within monozygotic twin pairs were found to be significantly smaller than the differences within pairs of unrelated responders. Thus, we find evidence of an influence by environmental antigens also on the repertoire of allospecific CD4+ T cells, but polymorphic genetic factors seem to be more important here than for the repertoire of allospecific CD8+ T cells.

Conformational stability and antibody response to the 18kDa heat-shock protein formulated into different vehicles

Protein stability is one of the most important obstacles for successful formulation in the development of new-generation vaccines. Here, the 18kDa heat-shock protein (18kDa-hsp) was chemically modified though conjugation with bovine serum albumin or by esterification with N-hydroxysuccinimide ester of palmitic acid. The biologically active conformation of the protein was preserved after chemical modification. The immune responses to the recombinant 18kDa-hsp from Mycobacterium leprae were studied in different presentations: free, copolymerized with bovine serum albumin in aggregates (18kDa-hsp-BSA), and either surface linked to liposomes or entrapped into liposomes. Measuring the antibody production of immunized genetically selected mice has compared the adjuvant effects of liposomes and proteic copolymer. Among the two liposome preparations, the strongest response was obtained with the surface-exposed antigen-liposomes. The copolymer 18kDa-hsp-BSA conferred a high titer of antibody in injected mice, and persisted 70 d after immunization. This approach should prove very useful for designing more effective vaccines by using 18kDa-hsp as carrier protein.

Induction of mucosal B-cell memory by intramuscular inoculation of mice with rotavirus

We investigated the capacity of intramuscular (i.m.) immunization with heterologous-host rotavirus (simian strain RRV) to induce mucosal virus-specific memory B cells in mice. We found that prior i.m. immunization enhanced the magnitude of mucosal virus-specific immunoglobulin A (IgA) production but did not alter the site and timing of induction of virus-specific IgA responses after challenge.

Lymphocyte trafficking: CD4 T cells with a 'memory' phenotype (CD45RC-) freely cross lymph node high endothelial venules in vivo

Antigen encounter not only induces a change in surface expression of CD45RC isoforms in the rat from a high (CD45RC+) to a low molecular weight molecule (CD45RC-), but also stimulates changes in expression of adhesion molecules that regulate CD4 T-cell migration. T cells with an activated or 'memory' phenotype (CD45RC-) are thought to enter lymph nodes almost exclusively via afferent lymphatics whereas T cells in a resting state (CD45RC+) migrate across high endothelial venules (HEV). The present study monitored the rapid recirculation from blood to lymph of allotype-marked CD45RC T-cell subsets. Surprisingly, we found that CD45RC- CD4 T cells entered the thoracic duct slightly faster and reached peak numbers 3 hr earlier (18 hr) than did the CD45RC+ subset. To determine whether the entrance of CD45RC+ and RC- subsets was restricted to HEV and afferent lymphatics, respectively, recirculation of CD4 T cells was monitored in mesenteric lymphadenectomized (MLNx) rats (on healing the intestinal afferent lymphatics are joined directly to the thoracic duct), or in recipients that had had the mesenteric lymph node (MLN) acutely (2-3 hr) deafferentized (entry would be restricted to HEV). In these studies CD45RC- CD4 T cells entered the MLN across HEV on an equal basis with T cells expressing a CD45RC+ phenotype. Contrary to currently held dogma the results showed that, in vivo, CD4 T cells with a memory phenotype freely enter lymph nodes (LN) across HEV as well as via afferent lymphatics.

Distinct Homing Pathways Direct T Lymphocytes to the Genital and Intestinal Mucosae in Chlamydia-Infected Mice

Immunity to genital tract infection with Chlamydia trachomatis is mediated by type 1 CD4+ T lymphocytes. To define the signals that govern lymphocyte trafficking to the genital mucosa, integrins expressed by infiltrating T cells and endothelial addressins displayed on local vasculature were characterized during the course of infection. All T cells expressed the αLβ2 heterodimer that binds vascular ICAM-1, and most displayed enhanced levels of the α4β1 integrin that interacts with VCAM-1. αE and β7low integrin chains were detected on approximately 15 and 30% of infiltrating T cells, respectively. Lymphocytes derived from the spleen or draining lymph nodes expressed this same integrin profile, suggesting that cells are recruited to the genital mucosa from the systemic circulation without significant selection pressure for these markers. Immunofluorescent staining for the corresponding vascular addressins revealed intense expression of VCAM-1 on small vessels within Chlamydia-infected genital tracts and up-regulation of ICAM-1 on endothelial, stromal, and epithelial cells. Mucosal addressin cell adhesion molecule-1 was not detected within genital tissues. These results indicate that T lymphocyte homing to the genital mucosa requires the interaction of αLβ2 and α4β1 with endothelial ICAM-1 and VCAM-1, respectively, which is the same pathway that directs lymphocytes to systemic sites of inflammation. Homing pathways defined for the intestinal mucosa and assumed to be relevant to all mucosal sites are not well represented in the genital tract. The identification of T lymphocyte trafficking pathways shared between systemic and mucosal tissues should facilitate vaccine strategies aimed at maximizing immune responses against Chlamydia and other pathogens of the urogenital tract.

The CD4+ T-cell response to protein immunization is independent of accompanying IFN-gamma-producing CD8+ T cells

By virtue of their strong bias towards production of interferon-gamma (IFN-gamma), CD8+ T cells have the potential to promote the development of type 1 immune responses. We have previously shown that the CD4+ T-cell response to immunization with the protein antigen keyhole limpet haemocyanin (KLH) has a mixed interleukin-4 (IL-4)/IFN-gamma production profile. Here we show that this immunization regimen also stimulates accumulation in the draining lymph nodes of CD8+ T cells, which preferentially contain IFN-gamma mRNA ex vivo and secrete IFN-gamma protein in vitro. This provides a model to test whether CD8+ cell-derived IFN-gamma participates in the normal control of the immune response to a non-viable exogenous antigen. To investigate regulation of the anti-KLH response by the CD8+ population or IFN-gamma produced by this or other cell types, mice were administered depleting antibodies. Depletion of CD8+ cells had no effect on the frequency of clonogenic KLH-specific CD4+ T cells, the IL-4/IFN-gamma profiles of their progeny, or the isotype profiles of the serum antibody response to KLH. In contrast, IFN-gamma neutralization diminished cell accumulation in the lymph nodes and reduced both the frequency of KLH-specific CD4+ T cells that gave rise to IFN-gamma-producing clones and serum titres of KLH-specific IgG2a and IgG3. Therefore, despite the potential for cross-regulation, the CD4+ T-cell response to this immunogen is independent of the IFN-gamma-skewed CD8+ response.

Immune recognition of influenza hemagglutinin as a viral and a neo-self-antigen

To analyze mechanisms governing tolerance and autoimmunity to self-antigens, we have generated lineages of transgenic mice that express the influenza virus PR8 hemagglutinin (HA) as a neo-self-antigen. By comparing the HA-specific T and B cell responses that can be induced in HA Tg mice with those that are induced in non-Tg (BALB/c) mice, the specificity and genetic basis with which tolerance is induced to the HA has been examined. This article summarizes studies using lineages of HA Tg mice that express different forms and amounts of the HA under the control of the SV40 promoter/enhancer. Our studies have revealed that specific subsets of HA-specific T and B cells are negatively selected from the primary repertoires of HA Tg mice. However, substantial populations of HA-specific T and B cells evade negative selection and can be activated by virus immunization. Understanding the capacity of these autoreactive lymphocytes to differentiate and participate in antigen-specific immune responses will provide important insights into mechanisms by which autoimmunity might be induced by viruses bearing structural similarities with self-antigens.

Pre-Exposure to Oxidative Stress Decreases the Nuclear Factor-κB-Dependent Transcription in T Lymphocytes

Reactive oxygen species (ROS) are used as signaling molecules in T cell activation. One of the main targets of ROS is the transcription factor nuclear factor-κB (NF-κB). NF-κB-dependent transcription is inhibited by antioxidants, and the activation is induced or potentiated by ROS. However, chronic oxidative stress is known to reduce the activation of T cells and NF-κB. To analyze these phenomena in more detail, we have exposed Jurkat T cells in vitro to oxidative stress (H2O2) at various times before or simultaneously with signals known to activate NF-κB (phorbol dibutyrate (PDBu) and TNF). Simultaneously applied H2O2 strongly potentiated the PDBu- or TNF-induced transcriptional activity of NF-κB. In contrast to this, H2O2 given 3 to 20 h before the activating signal reduced NF-κB-dependent transcriptional activity. This was not due to the oxidation-induced modification of NF-κB; cytoplasmic NF-κB was able to bind to DNA after dissociation from IκBα by detergent treatment. H2O2 pre-exposure effectively inhibited the PDBu- or TNF-induced phosphorylation and degradation of IκBα, but H2O2 given simultaneously with PDBu or TNF enhanced the degradation. Oxidative stress was also followed by a strongly decreased ability to form intracellular ROS. Taken together, these data indicate that IκBα phosphorylation is the target of action of ROS, and as the ROS-forming capacity is weaker after chronic oxidative stress, IκBα is not effectively phosphorylated and degraded, thus leading to decreased NF-κB-dependent transcription.

Phenotypical characterization of cells in the thoracic duct of patients with and without systemic inflammatory response syndrome and multiple organ failure

The subset composition and recirculation properties of the migrating lymphocyte pool in humans is largely unknown. The present study was conducted in order to phenotypically characterize cells in human thoracic duct lymph of patients under non-inflammatory and inflammatory conditions. These data were compared with data from peripheral blood, with special emphasis on those cells homing to the gut. Thoracic duct lymph and peripheral blood contained comparable proportions of B and T lymphocytes and CD8+ cells. Thoracic duct lymph contained proportionally more CD4+ cells, more CD4+CD45RO+ that express alpha 4 beta 7 cells and more CD8+CD45RO+ that express alpha 4 beta 7, as compared to peripheral blood. These data suggest an equal recirculation rate of B and T lymphocytes; a more active recirculation of CD4+ cells compared to CD8+ cells; and a more active recirculation of memory cells to the gut as compared to other extra-lymphoid sites in patients under non-inflammatory conditions. Data were also obtained in patients with the system inflammatory response syndrome and multiple organ failure. Although it is generally assumed that granulocytes and monocytes do not recirculate, lymph of multiple organ failure patients contained significantly more granulocytes than monocytes, indicating that in severe generalized inflammatory states these cells re-enter the circulation through the thoracic duct. Furthermore, no increased activation of cells homing to the gut was found in these patients.

Recruitment, activation and proliferation of CD8+ memory T cells in an immunoprivileged site

The capacity of a memory cytotoxic T lymphocyte (CTL) population to protect against viral infections is well established, but the processes underlying this protection are less well understood. We have used heterotypic intranasal immunization with influenza A/X31 (H3N2) to protect against a subsequent infection with the neurovirulent influenza A/WSN (H1N1) in either the cerebrospinal fluid or the immunoprivileged brain parenchyma. Viral clearance from both sites was associated with a local infiltration and proliferation of A/WSN-specific CD8+ T cells. Infection in the cerebrospinal fluid elicited a proliferative response in the draining lymph nodes, an anti-H1N1 serum antibody response and an increase in the extracerebral A/WSN-specific CTL precursor frequency. In contrast, infection in the brain parenchyma elicited no lymph node proliferative response or serum antibody response and caused a transient decrease in the extracerebral CTL precursor frequency. Thus the memory CTL population protected against an intracerebral viral infection independent of any immune response occurring in systemic lymphoid tissue.

T cell differentiation and cytokine expression in late life

Elderly humans are at significant risk with regard to the incidence and severity of many infectious diseases and cancers. Current theory holds that these late-life vulnerabilities arise, in part, through age-related changes in immune function, particularly in the T lymphocyte lineage. Herein, we discuss how such factors as thymic involution and ongoing T cell differentiation in the peripheral tissues contribute to progressive and irreversible shifts in the state of differentiation of the mature T cell pool. We propose that, by late life, these processes yield a T cell compartment with a suboptimal balance of naive and memory T cell subsets, each with altered, subset-specific programs for cytokine gene expression. As such, the T cell compartment in late life may be more prone to immune deficiency or cytokine-mediated dysregulation in response to new or previously encountered pathogens.

Requirement for CD44 in activated T cell extravasation into an inflammatory site

Leukocytes extravasate from the blood into inflammatory sites through complementary ligand interactions between leukocytes and endothelial cells. Activation of T cells increases their binding to hyaluronate (HA) and enables CD44-mediated primary adhesion (rolling). This rolling could be induced in vivo in murine Vbeta8(+) T cells in response to specific superantigen stimulation; it was initially found in lymph nodes, then in peripheral blood, and finally within the peritoneum, the original inflamed site. The migration of Vbeta8(+) cells into the peritoneal cavity was dependent on CD44 and HA, as shown by inhibition studies. Thus, CD44-HA interactions can target lymphocytes to specific extralymphoid effector sites.

Rapid effector function in CD8+ memory T cells

The nature of the CD8+ T cells that underlie antiviral protective immunological memory in vivo is unclear. We have characterized peptide-specific CD8+ T lymphocytes directly ex vivo from peripheral blood in humans with past exposure to influenza virus, using single cell interferon gamma (IFN-gamma) release as a measure of effector function. In individuals in the memory state with respect to influenza virus infection, unrestimulated antigen-specific CD8+ T cells displayed IFN-gamma release within 6 h of antigen contact, identifying a population of memory CD8+ T cells that exhibit effector function without needing to divide and differentiate over several days. We have quantified circulating CD8+ effector T cells specific for six different MHC class I-restricted influenza virus epitopes. Enumeration of these CD8+ T cells gives frequencies of peptide-specific T cells that correlate with, but are in general severalfold higher than, CTL precursor frequencies derived from limiting dilution analysis, indicating that this novel population of memory CD8+ T cells has hitherto been undetected by standard means. The phenotype of these cells, which persist at a low frequency long after recovery from an acute viral infection, suggests that they play a role in protective immunological memory.

Memory B cells are biased towards terminal differentiation: a strategy that may prevent repertoire freezing

Isolation of large numbers of surface IgD+CD38- naive and surface IgD-CD38- memory B cells allowed us to study the intrinsic differences between these two populations. Upon in vitro culture with IL-2 and IL-10, human CD40-activated memory B cells undergo terminal differentiation into plasma cells more readily than do naive B cells, as they give rise to five- to eightfold more plasma cells and three- to fourfold more secreted immunoglobulins. By contrast, naive B cells give rise to a larger number of nondifferentiated B blasts. Saturating concentrations of CD40 ligand, which fully inhibit naive B cell differentiation, only partially affect that of memory B cells. The propensity of memory B cells to undergo terminal plasma cell differentiation may explain the extensive extra follicular plasma cell reaction and the limited germinal center reaction observed in vivo after secondary immunizations, which contrast with primary responses in carrier-primed animals. This unique feature of memory B cells may confer two important capacities to the immune system: (a) the rapid generation of a large number of effector cells to efficiently eliminate the pathogens; and (b) the prevention of the overexpansion and chronic accumulation of one particular memory B cell clone that would freeze the available peripheral repertoire.

Antigen-specific CD8+ T cell subset distribution in lymph nodes draining the site of herpes simplex virus infection

Inoculation with replicating virus leads to an increase in T cell numbers within lymph nodes that drain the site of infection. This increase has been associated with a nonspecific proliferation of bystander cells, with only a minority thought to be directed to the infectious agent. Such an assumption is largely based on precursor cytotoxic T lymphocyte (CTL) estimations using limiting dilution analysis. Recently, studies using more advanced molecular approaches have suggested that such functionally derived precursor frequencies considerably underestimate the proportion of T cells specific for the antigen under investigation. We have defined T cell receptor sequences characteristic of CTL populations directed to a dominant determinant of the herpes simplex virus (HSV) glycoprotein B (gB). In this investigation, we used this receptor signature as a probe to directly monitor changes occurring within lymph nodes draining the sites of active infection with HSV. We found that although lymph node CD8+ T cell numbers increase as a consequence of HSV infection, the majority of these cells are small resting cells that are not enriched for gB-specific receptors. In contrast, a significant proportion of activated T cells are highly enriched for CTL bearing gB-specific receptors. Our results are therefore consistent with a nonspecific migration of CTL precursors into the lymph nodes draining the site of infection, followed by the activation and proliferation of the antigen-specific subset that normally makes up a small proportion of the naive T cell repertoire.

B cell subpopulations separated by CD27 and crucial collaboration of CD27+ B cells and helper T cells in immunoglobulin production

B cell immunoglobulin production is regulated by helper T cells through direct interaction and secreted cytokines. In the present study, we functionally analyzed CD27 in cord and peripheral blood B cells. Adult peripheral blood B cells were separated into CD27+ and CD27- cells, which differed in their morphology. Cord blood B cells did not express CD27, and CD27 expression on peripheral blood B cells increased with age. Only CD27+ B cells had the ability to produce immunoglobulin, which was increased by contact with a tumor necrosis factor-related transmembrane ligand, CD70. Adult peripheral blood CD27+ B cells can be further subdivided into two discrete subtypes: IgD- CD27+ and IgD+ CD27+ B cells. IgD- CD27+ B cells produce IgG, IgM and IgA, whereas IgD+ CD27+ B cells predominantly produce IgM. The addition of activated CD4+ CD45RO T cells expressing CD70 caused down-regulation of CD27 expression on activated B cells, and this down-modulation was completely blocked by anti-CD70 monoclonal antibody, indicating direct T-B cell contact via CD27/CD70. The triggering via CD27 and CD40 additively increased the immunoglobulin production under Staphylococcus aureus Cowan strain plus interleukin-2 stimulation. Taken together, our findings demonstrate that peripheral blood B cells are separated into subpopulations by CD27 and IgD expression and that CD27+ B cells produce large amounts of immunoglobulin by interaction with the CD70 molecule.

Marginal zone B-cell lymphomas including mucosa-associated lymphoid tissue type lymphoma (MALT), monocytoid B-cell lymphoma and splenic marginal zone cell lymphoma and their relation to the reactive marginal zone

The marginal zone of the B follicle represents a well-defined compartment of the B area. Its cellular composition is distinct from that of the follicle centre, from which it also differs in its functional role in the immune response. Several newly identified lymphoma entities, e.g. extranodal MALT type lymphoma, nodal monocytoid B-cell lymphoma and splenic marginal zone B-cell lymphoma, display in common a very peculiar organoid growth pattern reminiscent of the marginal zone. Moreover, their neoplastic components share morphologic and phenotypic similarities to the cellular components of the marginal zone. The clinical characteristics of these various marginal zone cell lymphomas may differ depending of the organ which is involved. Nevertheless, they all share common cytogenetic abnormalities suggesting a common pathogenesis.

A single peripheral CD8+ T cell can give rise to progeny expressing type 1 and/or type 2 cytokine genes and can retain its multipotentiality through many cell divisions

The lineage relationships between murine CD8(+) T cells with different cytokine profiles were investigated by paired-daughter analysis in the presence and absence of the type 2 cytokine-inducing stimulus, interleukin 4 (IL-4). Single CD8(+) CD44(low) lymph node T cells were activated to divide at high frequency with IL-2 and immobilized antibodies to CD3, CD8, and LFA-1. When these parent cells were subcloned by transferring their daughter or granddaughter cells into secondary cultures with or without IL-4, the subclones expressed diverse combinations of the mRNAs for the type 1 cytokines, interferon gamma (IFN-gamma), and IL-2, and the type 2 cytokines, IL-4, IL-5, IL-6, and IL-10. Frequencies of subclones that expressed IL-4, IL-6, and, to a lesser extent, IL-2, IL-5, and IL-10 were higher among those grown with IL-4, but a significant proportion of those grown without exogenous IL-4 also expressed one or more type 2 cytokines. Subclones within 89% of families displayed different cytokine profiles, indicating that their parent cells were multipotential for this function. Because 98% of parent cells yielded subclones that produced type 1 cytokines and 77% yielded type 2 cytokine producers, we conclude that type 1 and type 2 cytokine-producing CD8(+) T cells can be derived from a common precursor. Similar analyses performed by subcloning after >/=7 or >/=13 cell divisions without IL-4 showed that many CD8(+) T cells retained the potential to shift toward a type 2 cytokine profile in response to IL-4, even after prolonged expansion under conditions that favored type 1 cytokine expression. CD8(+) T cells that express type 1 and/or type 2 cytokines therefore are derived from the same peripheral T cell lineage whose multipotentiality can persist through many cell divisions.

Similar ligand densities required for restimulation and effector function of cytotoxic T cells

This study compared ligand densities on antigen-presenting cells (APCs) needed for in vitro restimulation of in vivo primed T cells and for in vitro assessed T cell effector function. Spleen cells of lymphocytic choriomeningitis virus (LCMV)-primed mice were restimulated in vitro with graded amounts of virus-derived peptides using macrophages or a cloned dendritic cell line as APCs. To test for effector function of these cytotoxic T cells, the same APCs pulsed with graded amounts of the peptides were used as target cells in an in vitro 51Cr release assay. The same peptide concentration that rendered an APC restimulatory for primed cytotoxic T lymphocytes (CTLs) also rendered it susceptible for lysis by the same CTLs. In addition, the same peptide concentrations that made macrophages susceptible for CTL-mediated lysis induced proliferative responses in vitro of in vivo primed memory CTLs. Thus, restimulation of in vivo primed T cells--measured by either proliferation or cytotoxic effector function--or sensibilization of target cells for lysis requires similar ligand densities on APCs and is therefore, contrary to expectations, governed by similar overall avidity thresholds. These results have implications for CTL memory.

Protein tyrosine phosphatase roles in the regulation of lymphocyte signaling

Tyrosine phosphorylation-based signaling cascades represent an integral component of the signaling circuitry connecting extracellular stimuli to cell response. As the molecular elements which drive such cascades have become increasingly well-characterized, appreciation has grown for the critical roles played by protein tyrosine phosphatases (PTPs) in intracellular signal relay and for the capacity of PTPs to act not only as a counterbalance for protein kinase activities, but also as pivotal enzymes in directing and modulating signal relay and the translation of given stimuli to cell behaviour. PTP function has been particularly well studied in relation to lymphocyte antigen receptor signaling and the results of these studies have provided many novel and significant insights into the biochemical mechanisms whereby PTPs participate in the integration and interpretation of the complex transmembrane stimulatory signals driving cell function and development.

Immunological memory

Most of the antigen-specific T and B cells participating in the primary immune response are rapidly eliminated, but some of the cells survive and become long-lived memory cells. There have been a number of recent developments on the features and functions of memory cells.

Pathogenesis of an infectious mononucleosis-like disease induced by a murine gamma-herpesvirus: role for a viral superantigen?

The murine gamma-herpesvirus 68 has many similarities to EBV, and induces a syndrome comparable to infectious mononucleosis (IM). The frequency of activated CD8+ T cells (CD62L(lo)) in the peripheral blood increased greater than fourfold by 21 d after infection of C57BL/6J (H-2(b)) mice, and remained high for at least a further month. The spectrum of T cell receptor usage was greatly skewed, with as many as 75% of the CD8+ T cells in the blood expressing a Vbeta4+ phenotype. Interestingly, the Vbeta4 dominance was also seen, to varying extents, in H-2(k), H-2(d), H-2(u), and H-2(q) strains of mice. In addition, although CD4 depletion from day 11 had no effect on the Vbeta4 bias of the T cells, the Vbeta4+CD8+ expansion was absent in H-2IA(b)-deficient congenic mice. However, the numbers of cycling cells in the CD4 antibody-depleted mice and mice that are CD4 deficient as a consequence of the deletion of MHC class II, were generally lower. The findings suggest that the IM-like disease is driven both by cytokines provided by CD4+ T cells and by a viral superantigen presented by MHC class II glycoproteins to Vbeta4+CD8+ T cells.

Functional differentiation of T cells in the intestine of T cell receptor transgenic mice

The intestinal lamina propria (LP) is a major effector site of the mucosal immune system where antigen-specific and antigen-nonspecific factors shape the functional responses of CD4+ T helper cells. To study the functional differentiation of LP T helper cells we utilized DO11.10 T cell receptor (TCR) transgenic (Tg) mice that expressed a clonotypic TCR specific for a class II major histocompatibility complex-restricted peptide of chicken ovalbumin. The majority of cells expressing Tg TCR (Tg+) in peripheral lymphoid tissue expressed naive surface phenotypes whereas nearly all Tg+ T cells in the intestinal LP expressed an activated/ memory-like phenotype. Flow cytometric analysis of Tg+ T cell populations revealed that a small proportion of cells in peripheral lymphoid tissue but nearly all cells in the LP expressed dual (Tg plus non-Tg) TCRs. In Tg x recombinase-activating-gene-1-deficient (Tg x RAG-1(-/-)) mice, splenic and LP T cells expressed naive surface phenotypes and produced cytokines equivalent to naive splenic cells from Tg x RAG-1(+/+) mice. In contrast, Tg LP cells from Tg x RAG-1(+/+) mice produced 35-fold greater levels of interferon-gamma and 5-fold greater levels of interleukin 4 compared with naive splenic cells. These findings suggested that activation of Tg+ T cells through endogenous non-Tg TCR had promoted the localization and differentiation of memory-like effector T helper cells in the intestine.

Factors controlling the turnover of T memory cells

Most of the T cells participating in the primary immune response are rapidly eliminated, but small numbers of these cells survive and differentiate into long-lived memory cells. Information on the life history of memory cells can be obtained by studying the component of memory-phenotype T cells found in normal animals; these cells are presumed to represent memory cells specific for various environmental antigens. For CD8+ cells, in vivo exposure to viruses and certain other infectious agents causes a large proportion of memory-phenotype (CD44hi) cells to enter the cell cycle. In this situation, stimulation of CD44hi CD8+ cells does not seem to require T-cell receptor ligation and appears to reflect release of various cytokines, especially type I interferon. The capacity of infectious agents to induce non-antigen-specific stimulation of T cells may play a role in boosting the survival of memory cells and perhaps also in providing an adjuvant function during the primary response.

Upregulation of Intracellular Glutathione by Fibroblast-Derived Factor(s): Enhanced Survival of Activated T Cells in the Presence of Low Bcl-2

Activated interleukin-2 (IL-2)–dependent T cells express high levels of Bcl-2 protein. On cytokine withdrawal, Bcl-2 expression decreases and the cells die rapidly by apoptosis. We have previously shown that the survival of IL-2–deprived T cells can be promoted by factor(s) secreted by fibroblasts. Here we report that reduced glutathione (GSH), but not its oxidized counterpart GSSG, also enhances the in vitro survival of these cells. Exogenous GSH mediates its effect intracellularly, as (1) endogenous glutathione concentrations are increased up to fivefold in the presence of GSH, and (2) acivicin, an inhibitor of transmembrane GSH transport, abrogates GSH-dependent survival. The GSH-rescued T cells do not proliferate and express only low levels of Bcl-2, resembling WI38 fibroblast-rescued T cells. We, therefore, investigated a role for GSH in fibroblast-promoted T-cell survival. We show that WI38-promoted survival results in elevated GSH levels in surviving T cells and is abrogated by buthionine sulfoximine (BSO), an inhibitor of GSH synthesis. Furthermore, both WI38-promoted T-cell survival and GSH upregulation are associated with large molecular weight molecules (<30 kD). Thus, the upregulation of GSH by WI38 fibroblasts appears to be crucial in their ability to enhance the survival of cytokine-deprived activated T cells in vitro.

Germinal center development

Using a set of surface markers including IgD and CD38, human tonsillar B cells were classified into discrete subpopulations. Molecular and functional analysis allowed us to identify: i) two sets of naive B cells (Bm1 and Bm2); ii) germinal center founder cells (Bm2'); iii) an obscure population of germinal center B cells, displaying a high load of somatic mutations in IgV genes, C mu to C delta switch and preferential Ig lambda light chain usage: these cells may represent the precursors of normal and malignant IgD-secreting plasma cells; iv) the centroblasts (Bm3) in which somatic mutation machinery is activated; v) the centrocytes (Bm4) in which isotype switch occurs; vi) the memory B cells. The characterization of these subpopulations showed that: i) programmed cell death is set before somatic mutations, possibly providing an efficient way for affinity maturation; ii) only high affinity centrocytes are allowed to switch isotype; iii) CD40-ligation inhibits plasmacytic differentiation of mature B lymphocytes; iv) memory B cells preferentially differentiate into plasma cells; v) IgD isotype switch occurs in normal B cells; vi) receptor editing may be induced by somatic mutations in germinal centers. We also characterized two types of antigen-presenting cells in germinal centers: follicular dendritic cells that select high affinity B cells, and a new subset of germinal center dendritic cells that activate germinal center T cells.

T-cell receptor biases and clonal proliferations in blood and pleural effusions of patients with lung cancer

We sought evidence that pulmonary carcinomas mediate a cellular immunologic response by analyzing T-cell antigen receptor beta-chain variable gene (TCRBV) repertoires of lymphocytes from peripheral blood (PBL) and malignant pleural effusions (PEL) of five lung cancer patients. Expression levels of 27 TCRBV were quantitated by multiprobe RNase protection assay (RPA), and clonal expansions were identified by sequence enrichment nuclease assay (SENA) and junctional region sequencing. Abnormal TCRBV expansions were identified in all subjects by RPA (mean 6.9 +/- 1.7/patient), and their number closely correlated with elapsed time since initial diagnosis (r = 0.97). SENA, performed in specimens from three patients, confirmed the presence of mono or oligoclonality in 48% of abnormal RPA expansions, and further identified T-cell clones among TCRBV with normal expression levels. The majority of clonal expansions were among PEL, and were nearly equally divided between CD4 and CD8. These data show that T-cell repertoires of lung cancer patients are characterized by marked abnormalities and frequent clonal expansions, most likely representing responses to unique, tumor-specific antigens (TSA). Moreover, this process appears exaggerated among PEL, further suggesting that malignant effusions include local proliferations of tumor reactive T cells. These findings imply the presence of lung cancer TSA capable of eliciting cellular immune responses and raise the possibility that selective immunotherapies can ultimately be developed.

Distinct costimulatory molecules are required for the induction of effector and memory cytotoxic T lymphocytes

A successful T cell immune response has two major products: effector T cells which directly or indirectly remove the antigens, and memory T cells, which allow a faster and more efficient recall response when challenged by related antigens. An important issue is whether costimulatory molecules on the antigen-presenting cells are involved in determining whether T cells will differentiate into effector or memory cells after antigenic stimulation. To address this issue, we have produced mice with targeted mutations of either the heat-stable antigen (HSA), or both HSA and CD28. We show that CD28/B7 and HSA provide two alternative costimulatory pathways for induction of immunological memory to influenza virus. Furthermore, our results revealed that B7 is essential for the generation of effector T cells from either naive or memory T cells, while HSA is not necessary for the generation of effector T cells. Our results demonstrate that the induction of memory T cells and effector T cells can utilize distinct costimulatory molecules. These results have important implications on lineage relationship between effector and memory T cells.

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.

Bcl-x and the regulation of survival in the immune system

A variety of experimental models indicate that programmed cell death, or apoptosis, of lymphocytes is a key mechanism in the homeostatic regulation of immunity. Apoptosis is important in early B- and T-cell development to delete cells with nonfunctional antigen receptors, and is also critical for censoring self-reactive cells at the immature lymphocyte stage and at various stages after lymphocytes reach maturity. In this article we focus on the role of the apoptosis regulatory gene bcl-x in controlling survival during lymphocyte development and following B- and T-cell activation. Interesting parallels are observed for bcl-x expression between the B- and T-lineages. The available data also indicate that bcl-x and bcl-2 are expressed in reciprocal patterns during the lifespan of a lymphocyte, suggesting unique regulatory roles for these two survival proteins.

Immunology and immunity studied with viruses

Immunity to viruses is used to define important biological parameters of immunology. Specificity, tolerance and T and B cell memory were analysed with murine model infections. The key parameters of antigen kinetics, localization and patterns of T and B cell response induction in maintaining memory and in causing deletion of reactive lymphocytes were compared for self and for viral foreign antigens. Evidence is reviewed that suggests that B cells essentially recognize antigen patterns, whereas T cells react against antigens newly brought into lymphoid tissues; antigens outside lymphoid tissues are ignored, and antigens always present in, or spreading too fast throughout, lymphoid tissues exhaust and delete T cell responses. Finally, effector mechanisms of antiviral immunity are summarized, as they vary with different viruses. On this basis immunological T and B cell memory against viruses is reviewed. Memory studies suggest that increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody: B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a non-cytopathic virus, cytotoxic T cells have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is yet available of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of protective immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

Long-lived and transferable tumor immunity in mice after targeted interleukin-2 therapy

A major goal of tumor immunotherapy is the induction of tumor-specific T cell responses that are effective in eradicating disseminated tumor, as well as mounting a persistent tumor-protective immunity. We demonstrate here that a genetically engineered fusion protein consisting of human/mouse chimeric anti-ganglioside GD2 antibody and human interleukin-2 is able to induce eradication of established B78-D14 melanoma metastases in immunocompetent syngeneic C57BL/6J mice. This therapeutic effect is mediated by host immune cells, particularly CD8+ T cells and is associated with the induction of a long-lived immunity preventing tumor growth in the majority of animals when challenged up to four months later with B78-D14 cells. This effect was tumor-specific, since no cross-protection against syngeneic, ganglioside GD2+ EL-4 thymoma cells was observed. Furthermore, this tumor-specific protection can be transmitted horizontally to naive, syngeneic SCID mice by passive transfer of CD8+ T lymphocytes derived from immune animals. These results suggest that antibody-targeted delivery of cytokines provides a means to elicit effective immune responses against established tumors in the immunotherapy of neoplastic disease.

Epitope focusing in the primary cytotoxic T cell response to Epstein-Barr virus and its relationship to T cell memory

The relationship between primary and memory cytotoxic T lymphocyte (CTL) responses, and the factors influencing entry into memory, are poorly understood. Here we address this in the context of Epstein-Barr virus (EBV), a persistent human herpesvirus in which memory CTL responses in long-term virus carriers are highly focused on epitopes preferentially drawn from just three of the eight available virus latent proteins, EBNAs 3A, 3B, and 3C. To determine whether this unusual level of focusing is a consequence of long-term virus challenge, we carried out a detailed analysis of EBV antigen/epitope specificities in the primary virus-induced CTL response in 10 infectious mononucleosis (IM) patients of different HLA types. Primary effectors, studied in ex vivo assays and by limiting dilution cloning in vitro, were again highly skewed toward a small number of viral epitopes, almost all derived from the EBNA3 proteins, with CTL to the immunodominant epitope accounting for at least 1% of the circulating CD8+ IM T cell pool. This is the first unequivocal demonstration of an EBV-specific CD8+ CTL response in IM. Prospective studies on individual patients showed that, whereas all of the EBV reactivities found in CTL memory had been detectable earlier during primary infection, the memory population was not simply a scaled down version of the primary response. In particular (a) differences in the relative frequencies of CTL to immunodominant versus subdominant epitopes appeared to be much less marked in memory than in primary populations, and (b) we found at least one clear example in which a significant virus-specific reactivity within the primary response was never detectable in memory.

Fibroblasts prevent apoptosis of IL-2-deprived T cells without inducing proliferation: a selective effect on Bcl-XL expression

The apoptosis of human cytokine-deprived activated T cells can be prevented by a soluble mediator secreted by fibroblasts, epithelial and endothelial cells, and this rescue occurs with fibroblasts from different species. Fractionation of W138 fibroblast-conditioned medium indicated that the survival-promoting agent(s) were > 30,000 MW. The continuous presence of the survival factor was required for prevention of apoptosis, which did not involve the induction of proliferation. Nevertheless, the co-cultured T cells remained in a primed state. The expression of the apoptosis-inducing proteins Bax and CD95 (Fas/Apo-1) was either unchanged or slightly increased in fibroblast-rescued T cells, suggesting that constraints on survival still existed after co-culture. A fundamental observation in the present study was that although Bcl-2 was reduced, the levels of Bcl-XL was maintained in cytokine-deprived T cells by fibroblast co-culture. This suggests that fibroblasts and/or other stromal cells may promote activated T-cell survival by a selective effect on Bcl-XL expression, which is consistent with histological examination of activated T cells within lymphoid tissue in vivo. The rescued T cell could be re-activated by CD3 antibody, but only in the presence of CD28 co-stimulation, which induced both Bcl-2 and Bcl-XL expression and also proliferation. Thus, survival signals from stromal cells in tissue microenvironments may enable activated T-cell persistence in a primed but quiescent state, and our data suggest that the regulation of Bcl-XL expression may be central in this process. The further characterization of this process is essential to clarify how signals from stromal cells can influence the resolution and/or chronicity of immune responses in different tissues in vivo.