T and B memory cells

Unique patterns of surface receptors, cytokine secretion, and immune functions distinguish T cells in the bone marrow from those in the periphery: impact on allogeneic bone marrow transplantation

The "conventional" NK1.1(-) T cells from mouse blood and marrow were compared with regard to surface receptors, cytokine secretion, and function. Most blood NK1.1(-) CD4(+) and CD8(+) T cells expressed the naive CD44(int/lo)CD62L(hi)CD45RB(hi) T-cell phenotype typical of those in the peripheral lymphoid tissues. In contrast, most marrow NK1.1(-) CD4(+) and CD8(+) T cells expressed an unusual CD44(hi)CD62L(hi)CD45RB(hi) phenotype. The blood NK1.1(-) CD4(+) T cells had a naive T-helper cytokine profile and a potent capacity to induce lethal graft versus host (GVH) disease in a C57BL/6 donor to a BALB/c host bone marrow transplantation model. In contrast, the marrow NK1.1(-) CD4(+) T cells had a Th0 cytokine profile and failed to induce lethal GVH disease, even at 20-fold higher numbers than those from the blood. NK1.1(-) CD8(+) T cells from the blood but not the marrow induced lethal GVH disease. Nevertheless, the marrow NK1.1(-) CD8(+) T cells induced potent antitumor activity that was augmented by marrow NK1.1(-) CD4(+) T cells and facilitated hematopoietic progenitor engraftment. The inability of marrow CD4(+) and CD8(+) T cells to induce GVH disease was associated with their inability to expand in the blood and gut of allogeneic recipients. Because neither the purified marrow CD4(+) or CD8(+) T cells induced GVH disease, their unique features are desirable for inclusion in allogeneic bone marrow or hematopoietic progenitor transplants.

Burst-enhancing role of the IgG membrane tail as a molecular determinant of memory

The basis of immune memory leading to heightened secondary antibody responses is a longstanding unanswered issue. Here we show that a single irreversible molecular change in the B cell antigen receptor, which is brought about by immunoglobulin M (IgM) to IgG isotype switching, is sufficient to greatly increase the extrafollicular proliferative burst of antigen-specific B cells. The unique membrane-spanning regions of IgG do not alter the T cell-dependent activation and proliferation of antigen-specific B cells in vivo, but markedly increase the number of progeny cells and plasmablasts that accumulate. These results establish a key molecular determinant of immunological memory and define an unexpected cellular basis by which it enhances the magnitude of secondary antibody responses.

T cell clones from Schistosoma haematobium infected and exposed individuals lacking distinct cytokine profiles for Th1/Th2 polarisation

T cell clones were derived from peripheral blood mononuclear cells of Schistosoma haematobium infected and uninfected individuals living in an endemic area. The clones were stimulated with S. haematobium worm and egg antigens and purified protein derivative. Attempts were made to classify the T cell clones according to production of the cytokines IL-4, IL-5 and IFN-gamma. All the T cell clones derived were observed to produce cytokines used as markers for the classification of Th1/Th2 subsets. However, the 'signature' cytokines marking each subset were produced at different levels. The classification depended on the dominating cytokine type, which was having either Th0/1 or Th0/2 subsets. The results indicated that no distinct cytokine profiles for polarisation of Th1/Th2 subsets were detected in these S. haematobium infected humans. The balance in the profiles of cytokines marking each subset were related to infection and re-infection status after treatment with praziquantel. In the present study, as judged by the changes in infection status with time, the T cell responses appeared to be less stable and more dynamic, suggesting that small quantitative changes in the balance of the cytokines response could result in either susceptibility or resistant to S. haematobium infection.

CD4 ligation promotes the IL-4-independent development of IL-4-producing clones from naive CD4(+) T cells

The signals that trigger IL-4-independent IL-4 synthesis by conventional CD4(+) T cells are not yet defined. In this study, we show that coactivation with anti-CD4 mAb can stimulate single naive CD4(+) T cells to form IL-4-producing clones in the absence of APC and exogenous IL-4, independently of effects on proliferation. When single CD4(+) lymph node cells from C57BL/6 mice were cultured with immobilized anti-CD3epsilon mAb and IL-2, 65-85% formed clones over 12-14 days. Coimmobilization of mAb to CD4, CD11a, and/or CD28 increased the size of these clones but each exerted different effects on their cytokine profiles. Most clones produced IFN-gamma and/or IL-3 regardless of the coactivating mAb. However, whereas 0-6% of clones obtained with mAb to CD11a or CD28 produced IL-4, 10-40% of those coactivated with anti-CD4 mAb were IL-4 producers. A similar response was observed among CD4(+) cells from BALB/c mice. Most IL-4-producing clones were derived from CD4(+) cells of naive (CD44(low) or CD62L(high)) phenotype and the great majority coproduced IFN-gamma and IL-3. The effect of anti-CD4 mAb on IL-4 synthesis could be dissociated from effects on clone size since anti-CD4 and anti-CD11a mAb stimulated formation of clones of similar size which differed markedly in IL-4 production. Engagement of CD3 and CD4 in the presence of IL-2 is therefore sufficient to induce a substantial proportion of naive CD4(+) T cells to form IL-4-producing clones in the absence of other exogenous signals, including IL-4 itself.

Acute and long-term effects of booster immunisation on frequencies of antigen-specific memory B-lymphocytes

In search for a parameter that is predictive of long-term immunity, we analysed the influence of booster immunisations on frequencies of circulating memory B-lymphocytes. Specific IgG-secreting B-cells were determined by ELISPOTassay in 13 healthy adults, using diphtheria and tetanus toxoid as model antigens. Our results show that memory B-cells accumulate with every immunisation dose and remain elevated over several years. In addition, secondary B-cell responses were studied during the first 90 days after diphtheria re-immunisation. A significant indirect correlation was found between the number of previous boosters and the magnitude of specific B-cell expansion. In contrast, effects of booster immunisations did not correlate likewise with antigen-specific serology. Hence, this study illustrates that frequencies of antigen-specific B-lymphocytes can be used as an indirect measure for immunological memory. This parameter could be helpful to find scientifically based immunisation strategies for currently available and novel vaccines.

Migratory properties of naive, effector, and memory CD8(+) T cells

It has been proposed that two different antigen-experienced T cell subsets may be distinguishable by their preferential ability to home to lymphoid organs (central memory cells) or nonlymphoid tissues (effector memory/effector cells). We have shown recently that murine antigen-primed CD8(+) T cells cultured in interleukin (IL)-15 (CD8(IL-15)) resemble central memory cells in phenotype and function. In contrast, primed CD8(+) T cells cultured in IL-2 (CD8(IL-2)) become cytotoxic effector cells. Here, the migratory behavior of these two subsets was investigated. Naive, CD8(IL-15) cells and, to a lesser degree, CD8(IL-2) cells localized to T cell areas in the spleen, but only naive and CD8(IL-15) cells homed to lymph nodes (LNs) and Peyer's patches. Intravital microscopy of peripheral LNs revealed that CD8(IL-15) cells, but not CD8(IL-2) cells, rolled and arrested in high endothelial venules (HEVs). Migration of CD8(IL-15) cells to LNs depended on L-selectin and required chemokines that bind CC chemokine receptor (CCR)7. Both antigen-experienced populations, but not naive T cells, responded to inflammatory chemokines and accumulated at sites of inflammation. However, CD8(IL-2) cells were 12 times more efficient in migrating to inflamed peritoneum than CD8(IL-15) cells. Furthermore, CD8(IL-15) cells proliferated rapidly upon reencounter with antigen at sites of inflammation. Thus, central memory-like CD8(IL-15) cells home avidly to lymphoid organs and moderately to sites of inflammation, where they mediate rapid recall responses, whereas CD8(IL-2) effector T cells accumulate in inflamed tissues, but are excluded from most lymphoid organs.

Effector differentiation is not prerequisite for generation of memory cytotoxic T lymphocytes

The lineage relationship between short-lived effector T cells and long-lived memory cells is not fully understood. We have described T-GFP mice previously, in which naive and early activated T cells express GFP uniformly, whereas cells that have differentiated into effector cytotoxic T cells selectively lose GFP expression. Here we studied antigen-specific CD8 T cell differentiation using T-GFP mice crossed to the TCR transgenic (Tg) mice P14 (specific for the lymphocytic choriomeningitis virus glycoprotein peptide, gp33-41). After activation with antigenic peptide, P14XT-GFP CD8(+) T cells cultured in high-dose IL-2 developed into cells with effector phenotype and function: they were blastoid, lost GFP expression, expressed high levels of activation and effector markers, and were capable of immediate cytotoxic function. In contrast, cells cultured in IL-15 or low-dose IL-2 never developed into full-fledged effector cells. Rather, they resembled memory cells: they were smaller, were GFP(+), did not express effector markers, and were incapable of immediate cytotoxicity. However, they mediated rapid-recall responses in vitro. After adoptive transfer, they survived in vivo for at least 10 weeks and mounted a secondary immune response after antigen rechallenge that was as potent as endogenously generated memory cells. In addition to providing a simple means to generate memory cells in virtually unlimited numbers, our results suggest that effector differentiation is not a prerequisite for memory cell generation.

Distribution of rotavirus-specific memory B cells in gut-associated lymphoid tissue after primary immunization

We found previously that mice inoculated orally with simian rotavirus strain RRV developed virus-specific memory B cell responses 16 weeks after immunization that were greater than those found 6 weeks after immunization. Memory B cell responses were defined as the quantity of virus-specific IgA detected in small intestinal lamina propria (LP) fragment cultures of immunized mice at various intervals after challenge. Enhanced memory B cell responses correlated with enhanced protection against shedding. In order to understand better the delayed onset of rotavirus-specific memory B cell responses, a method was developed to determine the frequencies of rotavirus-specific memory B cells in gut-associated lymphoid tissues (GALT). We found that protection against rotavirus challenge was determined by the frequency of rotavirus-specific memory B cells in GALT LP.

Presence of a population of CD20+, CD38- B lymphocytes with defective proliferative responsiveness in the synovial compartment of patients with rheumatoid arthritis

OBJECTIVE

To provide a comprehensive understanding of the humoral immune response that takes place at the site of inflammation in rheumatoid arthritis (RA), we studied the functional properties of synovial B cells. In particular, the response to various modes of mitogen stimulation was investigated.

METHODS

Purified synovial fluid (SF) B cells were cultured in the presence of CD40 ligand (CD40L)-expressing fibroblasts and cytokines, activated T cells, or phorbol myristate acetate (PMA)/ionomycin. Proliferation was determined by 3H-thymidine incorporation. Release of intracellular calcium was studied by flow cytometry.

RESULTS

The inflamed joints of RA patients contained a population of CD20+,CD38- B cells with dramatically impaired mitogen responsiveness. Although the Ig-producing capacity was intact, these cells failed to proliferate in response to (a) CD40 in the presence of interleukin-2 (IL-2) and IL-10, (b) activated T cells, or (c) stimulation via the B cell receptor. Moreover, SF CD20+,CD38- B cells revealed a defective B cell receptor-induced Ca2+ influx, reminiscent of anergic B cells. Release of intracellular Ca2+ by ionomycin in the presence of the protein kinase C activator PMA did not restore the proliferative capacity. These findings indicate blockades in the proximal and distal intermediates involved in mitogen signaling.

CONCLUSION

SF CD20+,CD38- B cells have functionally impaired proliferative responsiveness. The capacity of these cells to respond to activation by the production of Ig supports the notion that these cells might serve as Ig-producing effector cells and, as such, play a role in the pathophysiology of RA.

Mechanism of hypergammaglobulinemia by HIV infection: circulating memory B-cell reduction with plasmacytosis

The mechanism of hypergammaglobulinemia in patients infected with HIV has remained unclear in spite of the identification of a reduction of CD4+ T cells. The amounts of CD27+ memory B cells were remarkably reduced in the peripheral blood and immunoglobulin (Ig) production was diminished in HIV-infected patients. Some of the freshly isolated patients' T cells expressed the CD70 (CD27 ligand) on the surface and the CD70 expression on both of the CD4+ and CD8+ T cells was greatly enhanced by various stimuli. It was also striking that plasmacytosis was observed in patients' bone marrow. Thus, our findings suggest that CD70 expressed spontaneously or by activation on T cells of HIV-infected patients stimulates memory B cells via CD27 and promotes their differentiation into plasma cells, resulting in the elevation of serum Ig levels and the elimination of circulating memory B cells in HIV-infected patients.

Burnet oration. T-cell survival and the role of cytokines

Typical T cells are long-lived resting cells. Despite their quiescent appearance, there is increasing evidence that T cells are subjected to continuous stimulation through contact with various stimuli, notably by self peptide/MHC complexes and cytokines. These stimuli keep T cells alive and also cause intermittent entry into cell cycle.

Short-term immunoglobulin A B-cell memory resides in intestinal lymphoid tissues but not in bone marrow of gnotobiotic pigs inoculated with Wa human rotavirus

Immunological memory is important for protecting the host from reinfection. To investigate the development and sites of residence of intestinal memory B cells, and their role in protective immunity to reinfection with an enteric virus, we assessed the association between memory B cell and antibody-secreting cell (ASC) responses and protection using a gnotobiotic pig model for human rotavirus (HRV) infection and diarrhoea. The isotypes, quantities and tissue distribution of rotavirus-specific memory B cells and ASC were evaluated prechallenge (28 and 83 postinoculation days [PID]) and postchallenge (7 postchallenge days [PCD]), using enzyme-linked immunospot (ELISPOT) assay, in gnotobiotic pigs inoculated once with virulent or three times with attenuated HRV and challenged at PID 28 with the corresponding virulent HRV. Complete protection against HRV shedding and diarrhoea was associated with significantly higher numbers of immunoglobulin A (IgA) and immunoglobulin G (IgG) memory B cells and ASC in the ileum of virulent HRV-inoculated pigs at challenge. In contrast, pigs inoculated with attenuated HRV had lower numbers of IgA and IgG memory B cells and ASC in intestinal lymphoid tissues, but higher numbers in the spleen. The bone marrow had the lowest mean numbers of IgA and IgG memory B cells and ASC prechallenge in both groups of HRV-inoculated pigs. Therefore, bone marrow was not a site for IgA and IgG rotavirus-specific antibody production or for memory B cells after inoculation with live rotavirus, from 28 PID up to at least 83 PID. The effect of in vitro antigen dose was examined and it was determined to play an important role in the development of ASC from memory B cells for the different tissues examined.

Both CD45R(low) and CD45R(high) "revertant" CD4 memory T cells provide help for memory B cells

During a primary response to thymus dependent antigens, B cells undergo a number of qualitative changes to become memory B cells - processes that require co-stimulatory signals and cytokine help from CD4 T cells. The question of whether distinct, antigen-experienced memory CD4 T cells are subsequently needed to program memory B cells into antibody synthesis has not been clearly resolved. Using an adoptive transfer model in which memory but not naive B cells were stimulated, we evaluated CD4 T cell help using lymphocytes obtained from primed or unprimed thymectomized donors and expressing a naive (CD45R(high)) or a memory (CD45R(low)) phenotype. Memory B cells, most of which were committed to the IgG1 (Th2) subclass, could be stimulated to produce antibody using help transferred by the CD45R(high) naive subset of unprimed donors (slow onset of response), the CD45R(low) subset of 7 day primed donors (large, rapid antibody response) or by both the CD45R(low) and the CD45R(high) "revertant" subsets of 6 month primed donors. We found that antigen primed CD45R(low) CD4 T cells reverted (defaulted) with time to a CD45R(high) resting state, a change that was prevented by persisting antigen. The evidence suggests that CD4 memory T cells are partitioned into two different functional states (CD45R(high) and CD45R(low)) and that these determine the characteristics of the memory B cell response in terms of speed, size and longevity.

Direct B/B-cell interactions in immunoglobulin synthesis

The principal roles of B/B-cell interactions in immune response have not yet been established. We therefore investigated B/B-cell interactions in immunoglobulin synthesis via direct cell-to-cell contact, particularly in the tumour necrosis factor receptor (TNFR)/tumour necrosis factor (TNF) family. We prepared highly purified peripheral blood B cells and stimulated them with Epstein--Barr virus (EBV)-transformed B lymphoblastoid cell lines (LCLs) as activated human B cells. The IgG production by B cells was increased by the addition of fixed LCLs in a dose-dependent manner in the presence of IL-10 plus IL-2. LCLs strongly expressed CD40 and CD70 on their surface, but marginal or no CD154, CD27, OX40 (CD134) and CD134 ligand. The enhancement of immunoglobulin production by LCLs was completely blocked by the initial addition of anti-CD70 blocking MoAb, but not by anti-CD154 or anti-CD134 ligand MoAb. The addition of LCLs also caused a reduction in CD27 expression on B cells, and this effect was completely blocked by anti-CD70 MoAb, indicating a direct B cell--LCL contact via CD27/CD70. LCLs markedly promoted B-cell differentiation into plasma cells in the presence of IL-10 plus IL-2. These findings demonstrate that direct interactions between B and B cells via CD27/CD70 induce immunoglobulin production by promoting the generation of plasma cells.

Extrafollicular plasmablast B cells play a key role in carrying retroviral infection to peripheral organs

B cells can either differentiate in germinal centers or in extrafollicular compartments of secondary lymphoid organs. Here we show the migration properties of B cells after differentiation in murine peripheral lymph node infected with mouse mammary tumor virus. Naive B cells become activated, infected, and carry integrated retroviral DNA sequences. After production of a retroviral superantigen, the infected B cells receive cognate T cell help and differentiate along the two main differentiation pathways analogous to classical Ag responses. The extrafollicular differentiation peaks on day 6 of mouse mammary tumor virus infection, and the follicular one becomes detectable after day 10. B cells participating in this immune response carry a retroviral DNA marker that can be detected by using semiquantitative PCR. We determined the migration patterns of B cells having taken part in the T cell-B cell interaction from the draining lymph node to different tissues. Waves of immigration and retention of infected cells in secondary lymphoid organs, mammary gland, salivary gland, skin, lung, and liver were observed correlating with the two peaks of B cell differentiation in the draining lymph node. Other organs revealed immigration of infected cells at later time points. The migration properties were correlated with a strong up-regulation of alpha(4)beta(1) integrin expression. These results show the migration properties of B cells during an immune response and demonstrate that a large proportion of extrafolliculary differentiating plasmablasts can escape local cell death and carry the retroviral infection to peripheral organs.

Cell surface CD28 levels define four CD4+ T cell subsets: abnormal expression in rheumatoid arthritis

CD28 is a costimulatory receptor expressed in most CD4(+) T cells. Despite the long-standing evidence for up- and downregulation of surface CD28 expression in vitro, and the key regulatory role assigned to the upregulation of CD28 counterreceptor [the CD152 (CTLA-4) molecule], in vivo CD28 induction has attracted little attention. We studied CD28 and CD152 expression and function in 33 rheumatoid arthritis (RA) patients, 20 clinically active and 13 inactive, and in 24 healthy donors. Four subsets of CD28(-), CD28(low), CD28(int), and CD28(high) peripheral blood human CD4(+) T cells were defined using three-color flow cytometry. The three CD28(+) subsets displayed a one-, two-, or threefold quantitative difference in their relative number of CD28 antibody binding sites, respectively (P < 0.01). RA patients, whether active or inactive, showed a distinct phenotype when compared to healthy donors: (i) the percentage of CD4(+)CD28(high) cells was increased twofold and the CD4(+)CD28(low) subset was reduced twofold (P < 0.01) and (ii) the CD4(+)CD28(high) cells from RA patients showed an in vivo activated phenotype, CD45RO(+)CD5(high)IL-2Ralpha(+) (P < 0.01). Active RA patients were different from inactive patients. They showed a twofold increase in mean CD28 expression (P < 0.05), whereas each of the CD28(+) subsets in the inactive RA patients showed reduced expression when compared to healthy donors. Notably, both active and inactive RA patients showed abnormal CD28 upregulation when T cells were activated in vitro with CD3 antibodies, but only inactive RA patients showed a hypoproliferative response to TCR/CD3 triggering when compared to healthy donors (P < 0.01). This defective proliferation was normalized by concurrent crosslinking with CD28 antibody. No differences were noted in the expression of CD152 or CD80, a CD28 and CD152 shared ligand. The disregulated in vivo expression of CD28 was related to the RA patients' disease activity and suggests that modulation of CD28 surface levels may be an additional mechanism to finely tune the delicate responsiveness/tolerance balance.

Migration and function of antigen-primed nonpolarized T lymphocytes in vivo

Upon antigenic stimulation, naive T lymphocytes proliferate and a fraction of the activated cells acquire a T helper cell type 1 (Th1) or Th2 phenotype as well as the capacity to migrate to inflamed tissues. However, the antigen-primed T cells that receive a short T cell receptor (TCR) stimulation do not acquire effector function and remain in a nonpolarized state. Using TCR transgenic CD4(+) T cells in an adoptive transfer system, we compared the in vivo migratory capacities of naive, nonpolarized, Th1 or Th2 cells. Although all cell types migrated to the spleen, only naive and nonpolarized T cells efficiently migrated to lymph nodes. In addition Th1, but not Th2, migrated to inflamed tissues. In the lymph nodes, nonpolarized T cells proliferated and acquired effector function in response to antigenic stimulation, displaying lower activation threshold and faster kinetics compared with naive T cells. These results suggest that nonpolarized T cells are in an intermediate state of differentiation characterized by lymph node homing capacity and increased responsiveness that allows them to mount a prompt and effective secondary response.

Interaction between photodynamic therapy and BCG immunotherapy responsible for the reduced recurrence of treated mouse tumors

Subcutaneous mouse EMT6 tumors were treated by individual or combined regimens of a single Bacillus Calmette-Guérin (BCG) vaccine administration and photodynamic therapy (PDT). Six clinically relevant photosensitizers characterized by different action mechanisms were used: Photofrin, benzoporphyrin derivative, tetra(m-hydroxyphenyl)chlorin (foscan), mono-L-aspartylchlorin e6, lutetium texaphyrin or zinc phthalocyanine. Irrespective of the type of photosensitizer used, the optimized BCG protocols improved the cure rate of PDT-treated tumors. This indicates that the interaction does not take place during the early phase of tumor ablation but at later events involved in preventing tumor recurrence. Beneficial effects on tumor cure were observed even when the BCG injection was delayed to 7 days after PDT. The accumulation of activated myeloid cells that markedly increases in tumors treated by Photofrin-based PDT was not additionally affected by BCG treatment. However, the incidence of immune memory T cells in tumor-draining lymph nodes that almost doubled at 6 days after Photofrin-PDT further increased close to three-fold with adjuvant BCG. This suggests that BCG immunotherapy amplifies the T-lymphocyte-mediated immune response against PDT-treated tumors. Since both these modalities are established for the treatment of superficial bladder carcinomas, use of their combination for this condition should be clinically tested.

Suicide induced by cytolytic activity controls the differentiation of memory CD8(+) T lymphocytes

Cytotoxic T lymphocytes (CTL) confer protection against intracellular pathogens, yet the mechanism by which some escape activation induced cell death (AICD) and give rise to long-lived memory cells is unclear. We studied the differentiation of transgenic TCR CD8(+) cells into CTL and memory cells using a novel system that allowed us to control cytolytic activity. The perforin/granzyme granules used to lyse targets induced the apoptosis of CTL in a fratricide-independent manner. After adoptive transfer to antigen-free mice, the ability of CTL to give generate memory cells was determined. We found that the extent of cytolysis by a common pool of CTL controlled the differentiation into memory cells, which were only generated under conditions of minimal cytolytic activity. Thus, the differentiation of naive CD8(+) cells into memory cells may not depend on the presence on a subset of committed CTL precursors, but rather is controlled by the extent of granule-mediated cytolysis.

Immune function

The innate and the adaptive immune systems have evolved to provide a rapid and specific means for protecting hosts against the many microbes experienced over a lifetime. These two immune responses interact cooperatively to enhance the host defense. Defects in either of these two pathways can have devastating consequences, as evidenced [figure: see text] by primary immune deficiencies, many of which are discussed in this issue of the Pediatric Clinics of North America. The immune system has a central role in the pathogenesis of many disorders that involve an inflammatory response, including allergic and autoimmune diseases. New and more effective therapies for these many disorders will develop as the understanding of the immune system and its many secreted mediators continues to increase.

Immunization with the C-terminal region of Trypanosoma cruzi ribosomal P1 and P2 proteins induces long-term duration cross-reactive antibodies with heart functional and structural alterations in young and aged mice

The R13 peptide sequence (EEEDDDMGFGLFD) that corresponds to the C-terminal region of Trypanosoma cruzi ribosomal P1 and P2 proteins differs from the eukariotic P concensus sequence EESDDDMGFGLFD (H13) only in a nonconservative amino acid substitution. The immunization of BALB/c mice with R13 synthetic peptide coupled to a carrier protein (OVA) induces specific (anti-R13) and autoreactive (anti-H13 and anti-heart) antibodies as well as heart functional alterations. Since aged human and experimental animals are impaired in their responses to most foreign antigens but they produce greater amounts of autoantibodies, in this work we used aged mice as an experimental model able to exaggerate the autoimmune component of the R13-induced response in case it was present. We studied whether these antibodies generated in the absence of the parasite would induce pathological changes in heart tissues. The levels of antibodies against R13 (foreign antigen) and H13 (autoantigen) studied comparatively in 2- and 12-month-old mice 10 days after the third immunization with R13 coupled to OVA were, as we expected for a foreign antigen, higher in almost all sera from 2-month-old mice tested than in sera from 12-month-old mice. Besides, these specific and cross-reactive antibody response remain elevated as long as 150 days post third immunization. In addition, the isotype pattern that recognizes R13 and the self-sequence H13 showed no differences between sera from young and aged mice. Moreover, when ECG traces were obtained from immunized mice, the heart functional alterations observed at 10 days continued at 80 and 150 days after the third immunization, showing an association with the levels of antibodies. In addition, despite the fact that the heart tissue morphology showed no alterations 10 days post third immunization, several abnormalities in the tissue architecture were revealed at 80 and 150 days post third immunization. This report demonstrates the biological relevance of R13-induced cross-reactive antibodies in some of the electrophysiologic and histological changes found in T. cruzi-infected mammalians.

Memory B-cell persistence is independent of persisting immunizing antigen

Immunological memory in the antibody system is generated in T-cell-dependent responses and carried by long-lived memory B cells that recognize antigen by high-affinity antibodies. But it remains controversial whether these B cells represent true 'memory' cells (that is, their maintenance is independent of the immunizing antigen), or whether they are a product of a chronic immune response driven by the immunizing antigen, which can be retained in the organism for extended time periods on the surface of specialized antigen-presenting cells (follicular dendritic cells). Cell transfer experiments provided evidence in favour of a role of the immunizing antigen; however, analysis of memory cells in intact animals, which showed that these cells are mostly resting and can persist in the absence of detectable T-cell help or follicular dendritic cells, argued against it. Here we show, by using a genetic switch mediated by Cre recombinase, that memory B cells switching their antibody specificity away from the immunizing antigen are indeed maintained in the animal over long periods of time, similar to cells retaining their original antigen-binding specificity.

Naturally anergic and suppressive CD25(+)CD4(+) T cells as a functionally and phenotypically distinct immunoregulatory T cell subpopulation

A CD4(+) T cell subpopulation defined by the expression levels of a particular cell surface molecule (e.g. CD5, CD45RB, CD25, CD62L or CD38) bears an autoimmune-preventive activity in various animal models. Here we show that the expression of CD25 is highly specific, when compared with other molecules, in delineating the autoimmune-preventive immunoregulatory CD4(+) T cell population. Furthermore, although CD25 is an activation marker for T cells, the following findings indicate that immunoregulatory CD25(+)CD4(+) T cells are functionally distinct from activated or anergy-induced T cells derived from CD25(-)CD4(+) T cells. First, the former are autoimmune-preventive in vivo, naturally unresponsive (anergic) to TCR stimulation in vitro and, upon TCR stimulation, able to suppress the activation/proliferation of other T cells, whereas the latter scarcely exhibit the in vivo autoimmune-preventive activity or the in vitro suppressive activity. Second, such activated or anergy-induced CD25(-) spleen cells produce various autoimmune diseases when transferred to syngeneic athymic nude mice, whereas similarly treated normal spleen cells, which include CD25(+)CD4(+) T cells, do not. Third, upon polyclonal T cell stimulation, CD25(+)CD4(+) T cells express CD25 at higher levels and more persistently than CD25(-)CD4(+) T cell-derived activated T cells; moreover, when the stimulation is ceased, the former revert to the original levels of CD25 expression, whereas the latter lose the expression. These results collectively indicate that naturally anergic and suppressive CD25(+)CD4(+) T cells present in normal naive mice are functionally and phenotypically stable, distinct from other T cells, and play a key role in maintaining immunologic self-tolerance.

Tumor biology of mucosa-associated lymphoid tissue lymphomas

Extranodal lymphomas arising at mucosal sites exhibit clinicopathological features that suggest a closer relationship of these tumors to the structure and function of mucosa-associated lymphoid tissue (MALT) than to lymph nodes. The factors that induce MALT in these tissues are operative in early MALT lymphoma development and the progressive independence on T-cell help defines late stages of MALT lymphoma genesis.

Human autoreactive CD4+ T cells from naive CD45RA+ and memory CD45RO+ subsets differ with respect to epitope specificity and functional antigen avidity

T cells with specificity for self-Ags are normally present in the peripheral blood, and, upon activation, may target tissue Ags and become involved in the pathogenesis of autoimmune processes. In multiple sclerosis, a demyelinating disease of the CNS, it is postulated that inflammatory damage is initiated by CD4+ T cells reactive to myelin Ags. To investigate the potential naive vs memory origin of circulating myelin-reactive cells, we have generated myelin basic protein (MBP)- and tetanus toxoid-specific T cell clones from CD45RA+/RO- and CD45RO+/RA- CD4+ T cell subsets from the peripheral blood of multiple sclerosis patients and controls. Our results show that 1) the response to MBP, different from that to TT, predominantly emerges from the CD45RA+ subset; 2) the reactivity to immunodominant MBP epitopes mostly resides in the CD45RA+ subset; 3) in each individual, the recognition of single MBP epitopes is skewed to either subset, with no overlap in the Ag fine specificity; and 4) in spite of a lower expression of costimulatory and adhesion molecules, CD45RA+ subset-derived clones recognize epitopes with higher functional Ag avidity. These findings point to a central role of the naive CD45RA+ T cell subset as the source for immunodominant, potentially pathogenic effector CD4+ T cell responses in humans.

Role of B cells in maintaining helper T-cell memory

The cellular interactions involved in maintaining CD4+ T-cell memory have hitherto not been identified. In this report, we have investigated the role played by B cells in this process. We show that that long-lasting helper T-cell memory depends on the presence of B cells, but that direct antigen presentation by B cells is not required. These findings provide new insights into the mechanisms which underlie helper T-cell memory. They also suggest that the efficacy of future vaccines will depend critically on the inclusion of B- as well as T-cell epitopes.

T-cell proliferation in vivo and the role of cytokines

Unlike typical naive T cells, T cells with an activated (CD44hi) memory phenotype show a rapid rate of proliferation in vivo. The turnover of memory-phenotype CD8+ T cells can be considerably augmented by injecting mice with various compounds, including polyinosinic polycytidylic acid, lipopolysaccharide and immunostimulatory DNA (CpG DNA). Certain cytokines, notably type I (alpha, beta) interferons (IFN-I), have a similar effect. These agents appear to induce proliferation of CD44hi CD8+ cells in vivo by an indirect process involving production of effector cytokines, possibly interleukin-15, by antigen-presenting cells. Although none of the agents tested induces proliferation of naive-phenotype T cells, IFN-I has the capacity to cause upregulation of surface markers on purified naive T cells. Depending upon the experimental conditions used, IFN-I can either inhibit or enhance primary responses of naive T cells to specific antigen.

Activation-induced inhibition of interleukin 6-mediated T cell survival and signal transducer and activator of transcription 1 signaling

The cytokines interleukin (IL)-2, IL-4, IL-6, IL-7, and IL-15 have all previously been shown to inhibit resting T cell death in vitro. We have found a difference in the response of T cells to IL-6, depending on the activation status of the cells. IL-6 inhibited the death of naive T cells, but had no effect on the death of either superantigen-activated T cells, or T cells bearing memory markers. This was true even when the resting and activated T cells were isolated from the same animal; thus, the determining factor for IL-6 insensitivity was the activation status or activation history of the cell, and not the milieu in the animal from which the cells were isolated. Activated T cells expressed lower levels of IL-6 receptors on their surfaces, yet there were sufficient levels of receptors for signaling, as we observed similar levels of signal transducer and activator of transcription (Stat)3 phosphorylation in resting and activated T cells treated with IL-6. However, there was profound inhibition of IL-6-induced Stat1 phosphorylation in activated T cells compared with resting T cells. These data suggest that there is activation-induced inhibition of IL-6 receptor signaling in T cells. This inhibition appears to be specific for some but not all of the IL-6-mediated signaling cascades in these cells.

Stimulation of memory T cells by cytokines

Mature T cells can be classified on the basis of cell surface markers into naïve- and memory-phenotype cells. These phenotypically-defined subsets exhibit distinct kinetic behaviour in vivo. Thus, naïve-phenotype T cells persist long-term in a non-dividing state, while memory-phenotype T cells include cycling cells and have a more rapid rate of turnover. We have investigated the possibility that the different kinetic behaviour of naïve- and memory-phenotype T cells reflects a differential responsiveness to cytokines. It was discovered that memory-, but not naïve-, phenotype T cells were stimulated to proliferate by a variety of infection-induced cytokines. These results suggest that cytokines contribute to the high background rate of turnover exhibited by memory T cells.

Activated and memory CD8+ T cells can be distinguished by their cytokine profiles and phenotypic markers

Dissecting the mechanisms of T cell-mediated immunity requires the identification of functional characteristics and surface markers that distinguish between activated and memory T lymphocytes. In this study, we compared the rates of cytokine production by virus-specific primary and memory CD8+ T cells directly ex vivo. Ag-specific IFN-gamma and TNF-alpha production by both primary and long-term memory T cells was observed in </=60 min after peptide stimulation. Although the on-rate kinetics of cytokine production were nearly identical, activated T cells produced more IFN-gamma, but less TNF-alpha, than memory T cells. Ag-specific cytokine synthesis was not a constitutive process and terminated immediately following disruption of contact with peptide-coated cells, demonstrating that continuous antigenic stimulation was required by both T cell populations to maintain steady-state cytokine production. Upon re-exposure to Ag, activated T cells resumed cytokine production whereas only a subpopulation of memory T cells reinitiated cytokine synthesis. Analysis of cytokine profiles and levels of CD8, LFA-1, and CTLA-4 together revealed a pattern of expression that clearly distinguished in vivo-activated T cells from memory T cells. Surprisingly, CTLA-4 expression was highest at the early stages of the immune response but fell to background levels soon after viral clearance. This study is the first to show that memory T cells have the same Ag-specific on/off regulation of cytokine production as activated T cells and demonstrates that memory T cells can be clearly discriminated from activated T cells directly ex vivo by their cytokine profiles and the differential expression of three well-characterized T cell markers.

Biologic functions of the IFN-gamma receptors

Interferon-gamma (IFN-gamma) is a cytokine that plays an important role in inducing and modulating an array of immune responses. Cellular responses to IFN-gamma are mediated by its heterodimeric cell-surface receptor (IFN-gammaR), which activates downstream signal transduction cascades, ultimately leading to the regulation of gene expression. In order to study the role of IFN-gamma in a number of immune responses and pathways, researchers have generated mice with altered patterns of IFN-gammaR gene expression. These studies, together with analyses of naturally occurring mutations of the IFN-gammaR in man, have been instrumental in elucidating the diverse functions of IFN-gamma, and are the subject of this review.

Proliferation and differentiation of alloselective NK cells after alloimmunization-evidence for an adaptive NK response

We have previously demonstrated the activation of NK cells exhibiting RT1 allele-specific cytolytic activity following intraperitoneal immunization of certain rat strains with allogeneic cells. In the present study, we show that the NK allocytolytic activity in BN (RT1(n)) rats immunized with WF (RT1(u)) cells was associated with an increased proportion of peritoneal, as well as splenic, NK cells. Furthermore, the proliferation of NK cells was substantially increased in BN (RT1(n)) rats immunized with WF (RT1(u)) cells when compared to that in naive BN rats. In addition, the NK subpopulation exhibiting the allocytolytic activity in alloimmunized rats exhibited a decreased expression of the NKR-P1 and L-selectin molecules, but an increased expression of the LFA-1 molecule when compared to NK cells from naive rats. Thus, we have shown that the existence of peritoneal NK cells, exhibiting selective allocytolytic activity in alloimmunized rats, is probably due to a combination of RT1 allele-selective recruitment, proliferation, and the differentiation of NK cells. Therefore, in the same way that we regard T cells as being capable of adapting the immune response, this study presents evidence for the hypothesis that the specific cytolytic response of alloreactive NK cells should also be regarded as adaptive.

Kinetic Analysis of Oral Tolerance: Memory Lymphocytes Are Refractory to Oral Tolerance

Oral administration of soluble Ag before immunization induces peripheral tolerance and is effective in suppressing animal models of autoimmune diseases. Although tolerance induction in primed animals is more clinically relevant, it is not well studied. Therefore, this study was designed to examine the feeding effects on different phases of the immune response. We observed that feeding a single high dose (250 mg) of OVA to OVA-primed BALB/c mice could induce OVA-specific suppression in the Ab production and T cell proliferation only at the naive and the activation phases of the immune response, whereas multiple high doses (100 mg/feed for 10 days) were effective at the effector phase. OVA-specific IL-4 production in culture supernatant was also suppressed in the tolerized groups. However, when the mice had resting memory lymphocytes, even multiple feeding regimens were not effective in tolerance induction, although multiple low doses (1 mg/feed for 10 days) partially suppressed Ab production. This phenomenon was confirmed by adoptive transfer study. Nevertheless, the reactivated memory response was suppressed partially by multiple high doses. Our findings have an important implication for understanding the mechanism of oral tolerance and for the therapeutic applications of oral tolerance to autoimmune diseases.

T Cell Memory Against Colon Carcinoma Is Long-Lived in the Absence of Antigen

Eradication of established colon carcinoma metastases is a major goal for adjuvant immunotherapy of this disease. This was accomplished in a murine model by targeting IL-2 to the tumor microenvironment with a recombinant Ab-IL-2 fusion protein (huKS1/4-IL-2). The generation of a long-lived protective immunity was demonstrated by a 10- to 14-fold increase in CTL precursor (pCTL) frequency and induction of genes encoding Th1 cytokines, followed by the generation of tumor-specific CD8+ T effector cells, some of which differentiated into long-lived T memory cells. The frequency of pCTL correlated with enhanced immune protection against tumor cell challenge, and long-lived T cell memory was maintained in syngeneic SCID mice in the absence of tumor Ag. Tumor cell challenge of these SCID mice, concomitant with a boost of two noncurative doses of huKS1/4-IL-2 fusion protein, resulted in the generation of primed CD8+ T effector cells with concurrent release of Th1 cytokines. These events culminated in the complete rejection of the tumor cell challenge and prevention of pulmonary metastases. Taken together, the data suggest that T cell memory against colon carcinoma can be maintained in the absence of Ag.

Primed T Cells Are More Resistant to Fas-Mediated Activation-Induced Cell Death than Naive T Cells

Memory T cells respond in several functionally different ways from naive T cells and thus function as efficient effector cells. In this study we showed that primed T cells were more resistant to Fas-mediated activation-induced cell death (AICD) than naive T cells using OVA-specific TCR transgenic DO10 mice and Fas-deficient DO10 lpr/lpr mice. We found that apoptosis was efficiently induced in activated naive T cells at 48 and 72 h after Ag restimulation (OVA peptide; 0.3 and 3 μM), whereas apoptosis was not significantly increased in activated primed T cells at 24–72 h after Ag restimulation. We further showed that the resistance to AICD in primed T cells was due to the decreased sensitivity to apoptosis induced by Fas-mediated signals, but TCR-mediated signaling equally activated both naive and primed T cells to induce Fas and Fas ligand expressions. Furthermore, we demonstrated that primed T cells expressed higher levels of Fas-associated death domain-like IL-1β-converting enzyme inhibitory protein (FLIP), an inhibitor of Fas-mediated apoptosis, at 24–48 h after Ag restimulation than naive T cells. In addition, Bcl-2 expression was equally observed between activated naive and primed T cells after Ag restimulation. Thus, these results indicate that naive T cells are sensitive to Fas-mediated AICD and are easily deleted by Ag restimulation, while primed/memory T cells express higher levels of FLIP after Ag restimulation, are resistant to Fas-mediated AICD, and thus function as efficient effector cells for a longer period.

Dichotomy between naïve and memory CD4(+) T cell responses to Fas engagement

Engagement of Fas (APO-1, CD95), a member of the tumor necrosis factor receptor superfamily, can induce apoptotic cell death. However, Fas engagement also can costimulate lymphocyte proliferation. The physiologic regulation of these two outcomes is poorly understood. Here, we have used two systems, the first in vitro and the second in vivo, to demonstrate that naïve and memory CD4(+) T cells display dichotomous responses to Fas ligation. Naïve CD4(+) T cells (CD44(lo), CD45RB+, CD62L+) die as a consequence of Fas ligation in the presence of anti-CD3 antibody, whereas memory T cells (CD44(hi), CD45RB-, CD62L-), freshly isolated from the same starting population and subjected to the same stimulation conditions, are costimulated to proliferate by Fas ligation. In vitro, we demonstrate that CD28-mediated signals or T helper 1 and T helper 2 differentiation cytokines alter the response of naïve T cells, but not of memory T cells, to Fas ligation. In vivo experiments in hen egg lysozyme (HEL) T cell receptor transgenic mice show that CD4(+) T cells from HEL-naïve mice are killed by Fas ligation, but CD4(+) T cells from long-term HEL-exposed mice are costimulated by Fas ligation. Thus, the physiological outcome of Fas ligation in CD4(+) T cells is determined primarily by the antigenic history of the T cell.

Thymus and Autoimmunity: Production of CD25+CD4+ Naturally Anergic and Suppressive T Cells as a Key Function of the Thymus in Maintaining Immunologic Self-Tolerance

This study shows that the normal thymus produces immunoregulatory CD25+4+8− thymocytes capable of controlling self-reactive T cells. Transfer of thymocyte suspensions depleted of CD25+4+8− thymocytes, which constitute ∼5% of steroid-resistant mature CD4+8− thymocytes in normal naive mice, produces various autoimmune diseases in syngeneic athymic nude mice. These CD25+4+8− thymocytes are nonproliferative (anergic) to TCR stimulation in vitro, but potently suppress the proliferation of other CD4+8− or CD4−8+ thymocytes; breakage of their anergic state in vitro by high doses of IL-2 or anti-CD28 Ab simultaneously abrogates their suppressive activity; and transfer of such suppression-abrogated thymocyte suspensions produces autoimmune disease in nude mice. These immunoregulatory CD25+4+8− thymocytes/T cells are functionally distinct from activated CD25+4+ T cells derived from CD25−4+ thymocytes/T cells in that the latter scarcely exhibits suppressive activity in vitro, although both CD25+4+ populations express a similar profile of cell surface markers. Furthermore, the CD25+4+8− thymocytes appear to acquire their anergic and suppressive property through the thymic selection process, since TCR transgenic mice develop similar anergic/suppressive CD25+4+8− thymocytes and CD25+4+ T cells that predominantly express TCRs utilizing endogenous α-chains, but RAG-2-deficient TCR transgenic mice do not. These results taken together indicate that anergic/suppressive CD25+4+8− thymocytes and peripheral T cells in normal naive mice may constitute a common T cell lineage functionally and developmentally distinct from other T cells, and that production of this unique immunoregulatory T cell population can be another key function of the thymus in maintaining immunologic self-tolerance.

The regulation of apoptosis by microbial pathogens

In the past few years, there has been remarkable progress unraveling the mechanism and significance of eukaryotic programmed cell death (PCD), or apoptosis. Not surprisingly, it has been discovered that numerous, unrelated microbial pathogens engage or circumvent the host's apoptotic program. In this chapter, we briefly summarize apoptosis, emphasizing those studies which assist the reader in understanding the subsequent discussion on PCD and pathogens. We then examine the relationship between virulent bacteria and apoptosis. This section is organized to reflect both common and diverse mechanisms employed by bacteria to induce PCD. A short discussion of parasites and fungi is followed by a detailed description of the interaction of viral pathogens with the apoptotic machinery. Throughout the review, apoptosis is considered within the broader contexts of pathogenesis, virulence, and host defense. Our goals are to update the reader on this rapidly expanding field and identify topics in the current literature which demand further investigation.

Characterization of CD4+ T cells in mouse bone marrow. I. Increased activated/memory phenotype and altered TCR Vbeta repertoire

A significant proportion of memory B cells home to bone marrow (BM) which is a major site of anamnestic antibody responses in mice. We hypothesized that memory T cells likewise accumulate in BM perhaps to provide help for antibody production, and that the compartment of CD4+ T cells in BM of unimmunized mice would be enriched for memory phenotype cells that might have been activated by environmental antigens. The phenotype of activated/memory CD4+ lymphocytes has been defined as CD44hi CD45RBlo CD62L-. Conversely, the phenotype of immunologically naive cells is CD44lo CD45RBhi CD62L+. Flow cytrometric analysis of tissue from normal, adult C57BL/6 mice identified 1-2 % CD3+CD4+ cells in BM. Up to 40 % of CD3+CD4+ cells in the BM expressed the activated/memory phenotype compared with < or = 10% in the spleen and lymph nodes. Analysis of TCR Vbeta repertoire revealed that expression of Vbeta3 and Vbeta7 genes was increased as much as fourfold in BM compared to the periphery; most of this increase was within the CD44hi T cells. The accumulation of activated/memory T cells and clonotypic expansion(s) was not seen in the BM of germ-free mice, indicating that it reflects the history of the animal's exposure to antigens. Finally, immunization of mice which express a transgenic T cell receptor specific for ovalbumin peptide resulted in appearance of antigen-specific T cells with activated/memory phenotype in the BM.

Functional Characteristics and Survival Requirements of Memory CD4+ T Lymphocytes In Vivo

The phenotypic and functional characteristics of Ag-specific memory CD4+ lymphocytes are poorly defined. To examine the properties and cytokine responsiveness of these cells, we have developed an adoptive transfer system using in vitro-activated T cells expressing the DO.11 transgenic TCR specific for OVA323–339+ I-Ad. In vitro-activated DO.11 CD4+ cells exhibit comparable survival patterns at 1, 6, and 10 wk after adoptive transfer, indicating that a stable population of memory cells has been generated. In the absence of Ag, previously activated T cells survive longer than their naive counterparts in vivo, rapidly revert to a partially naive phenotype, and maintain their effector cytokine profile. The DO.11 CD4+ memory cells are capable of proliferating in response to IL-2 and IL-4, while naive DO.11 CD4+ cells exhibit no such proliferative responses.

Presence of Pentoxifylline During T Cell Priming Increases Clonal Frequencies in Secondary Proliferative Responses and Inhibits Apoptosis

Naive T cells appear to be primed by specific Ag to differentiate into either effectors or memory cells. We have been analyzing the factors involved in this differential commitment in the priming of alloresponsive human T cells in vitro and have shown that the presence of a phosphodiesterase inhibitor, pentoxifylline (POX), during priming results in a decrease in the primary response and enhancement in the secondary proliferative response. We now show that the POX-mediated effect can be mimicked by dibutyryl cAMP. The secondary response enhancement is due to the effects of POX on the T cells rather than the APCs, because even fixed APCs can prime T cells in the presence of POX. POX affects T cells directly by increasing clonal frequency rather than the burst size of the secondary responders. The known inhibition of IL-2 production by POX is not responsible for this effect, because exogenous IL-2 supplementation does not block it. The presence of POX during priming alters the outcome of T cell activation, resulting in a lower frequency of cells expressing IL-2Rα (CD25) and a decrease in their subsequent apoptosis, and this anti-apoptotic effect is consistent with the enhanced commitment of T cells to secondary responsiveness by POX.

T Cell Responses to Heat-Shock Protein 60: Differential Responses by CD4+ T Cell Subsets According to Their Expression of CD45 Isotypes

We demonstrate that human T lymphocytes proliferate in vitro to highly purified human heat-shock protein 60 (Hu.hsp60). The response to this self Ag was confined to the CD45RA+RO− T cell subset, with minimal responses by adult CD45RA−RO+ T cells. Experiments using keyhole limpet hemocyanin as a prototypic novel Ag, or tetanus toxoid as a recall Ag, were consistent with the notion that CD45RA+RO− and CD45RA−RO+ T cell subsets can be designated as naive and memory cells, respectively; thus, responses to Hu.hsp60 were confined to the putative naive subset. In contrast, both CD45RA+RO− and CD45RA−RO+ T cell populations proliferated to bacterial hsp60 from Mycobacterium leprae, Escherichia coli, or Chlamydia trachomatis. However, only CD45RA−RO+ (memory) T cells responded to a mycobacterial hsp60-derived peptide previously defined as a major bacteria-specific epitope. Experiments with cord blood T cells, which are CD45RA+RO− and can be considered truly naive, showed that the peptide could elicit responses from naive T cells in vitro; cord blood cells also responded to Hu.hsp60. Since bacterial hsp60 Ags contain both conserved and nonconserved epitopes, we speculate that in vivo challenge with bacterial hsp60 will activate T cells capable of seeing either type of epitope, but only those that see nonconserved epitopes maintain the CD45RA−RO+ memory phenotype. However, T cells recognizing conserved epitopes, while not apparently being recruited to the memory pool, may nevertheless play a role in immunoregulation, particularly in the context of inflammation, when expression of Hu.hsp60 is increased.

A Single Specific Amino Acid Residue in Peptide Antigens Is Sufficient to Activate Memory CTL: Potential Role of Cross-Reactive Peptides in Memory T Cell Maintenance

In the present study, we examined the structural requirements of peptide Ags for productive interactions with the TCR of CTL. For this purpose, we used as a model a previously identified immunodominant epitope that represents the target of EBV-specific HLA-A11-restricted CTL responses. By the use of peptides having minimal sequence homology with the wild-type epitope, we demonstrated that it is possible to selectively expand and reactivate memory CTL precursors without triggering the lytic mechanisms of wild-type specific effectors. In fact, stimulation of PBL from EBV-seropositive donors by polyalanine analogues, sharing only the putative TCR contact residue with the natural epitope, exclusively induced clonal expansion and reactivation of EBV-specific memory CTL precursors. Interestingly, these polyalanine peptides failed to trigger the cytotoxic function of CTLs specific for the wild-type viral epitope. This clearly indicates that reactivation of memory CTL precursors and triggering of the cytotoxic function have different requirements. The same phenomenon was observed using as stimulators naturally occurring peptides carrying the appropriate TCR contact residue. These data strongly suggest that cross-reactive peptides may play an important role in the expansion and reactivation of CTL clones from the memory T cell pool, and may be involved in long-term maintenance of T cell memory.

Distinct kinetics of cytokine production and cytolysis in effector and memory T cells after viral infection

In the present study, naive T cells were compared with in vivo generated effector and memory T cells expressing the same TCR specific for lymphocytic choriomeningitis virus. Upon restimulation in vitro, the same minimal concentrations of the full agonist peptide p33 and also of weak and partial agonist peptides were required for proliferation of naive, effector and memory T cells, indicating no difference in threshold of activation. However, activation kinetics were distinct. While effector cytotoxic T cells exhibited immediate ex vivo lytic effector function, naive and memory T cells required 12 h and more exposure to antigen to develop lytic activity. However, both effector and memory T cells contained IFN-gamma mRNA in vivo and required less than 3 h for secretion of cytokines upon restimulation in vitro. In contrast, naive T cells did not contain IFN-gamma mRNA and required more than 12 h for cytokine secretion. Our results show that memory T cells exhibit a unique phenotype in that they produce cytokines and commit to proliferation as rapidly as effector cells, whereas they resemble naive T cells in the time requirement for development of cytolytic function.