Potent and selective stimulation of memory-phenotype CD8+ T cells in vivo by IL-15

Lipopolysaccharide-stimulated or granulocyte-macrophage colony-stimulating factor-stimulated monocytes rapidly express biologically active IL-15 on their cell surface independent of new protein synthesis

Although IL-15 shares many of the biological activities of IL-2, IL-2 expression is primarily under transcriptional regulation, while the mechanisms involved in the regulation of IL-15 are complex and not completely understood. In the current study, we found that CD14(+) monocytes constitutively exhibit both IL-15 mRNA and protein. IL-15 protein was found stored intracellularly and stimulation of CD14(+) monocytes with either LPS or GM-CSF resulted in mobilization of IL-15 stores to the plasma membrane. This rapidly induced surface expression was the result of a translocation of preformed stores, confirming that posttranslational regulatory stages limit IL-15, because it was not accompanied by an increase in IL-15 mRNA and occurred independent of de novo protein synthesis. After fixation, activated monocytes, but not resting monocytes, were found to support T cell proliferation, and this effect was abrogated by the addition of an IL-15-neutralizing Ab. The presence of preformed IL-15 stores and the ability of stimulated monocytes to mobilize these stores to their surface in an active form is a novel mechanism of regulation for IL-15.

Type I interferon is required to mount an adaptive response to immunostimulatory DNA

Immunostimulatory DNA sequences (ISS, CpG motifs) potently stimulate Th1 and cytotoxic T lymphocyte (CTL) responses to antigens and have thus generated considerable interest due to their potential use in immunotherapeutics. An array of cytokines are produced in response to ISS exposure, but the relative importance of each of these mediators in the stimulation of innate and adaptive ISS-induced immunity has yet to be fully investigated. To address this issue, we measured immune responses in mice with targeted deletions of the ISS-induced genes encoding IL-12 (IL-12(-/-)), IFN-gamma (IFN-gamma(-/-)), the IFN-gamma receptor (IFN-gammaR(-/-)), and the IFN-alpha/beta receptor (IFN-alpha/betaR(-/-)) after immunization with ISS-containing oligodeoxynucleotides and model antigens. IL-12(-/-) and IFN-alpha/betaR(-/-) mice were compromised in their ability to develop a cross-primed CTL response, whereas IFN-gamma(-/-) and IFN-gammaR(-/-) mice were not. In addition, lymphocytes from immunized IFN-alpha/betaR(-/-) mice had defective IFN-gamma responses to antigen restimulation. Antigen nonspecific ISS-induced B cell proliferation was normal in the four deficient strains; however, innate IL-6 production was reduced in IFN-gamma(-/-) and IFN-gammaR(-/-) splenocytes and eliminated in IFN-alpha/betaR(-/-) cells. While IL-12 production was defective in only the IFN-gamma(-/-) splenocytes, innate natural killer cell IFN-gamma synthesis was virtually absent in the IL-12(-/-) and IFN-alpha/betaR(-/-) mice. Thus, while IFN-alpha/beta, IFN-gamma, and IL-12 each play important and distinct roles in the development of the innate and adaptive immune responses to ISS, IFN-alpha/beta is a particularly crucial and currently under-appreciated factor in this system.

Induction of cytotoxic CD8+CD56+ T cells from human thymocytes by interleukin-15

CD8(+) CD56(+) cells isolated from human peripheral blood lymphocytes have been shown recently to represent a population of cytotoxic active T cells. However, it is not known if these cells are intrathymically or extrathymically developed or how these cells are influenced by growth factors. In the present study, we investigated the effects of interleukin-2 (IL-2) and IL-15 on human thymocytes with respect to development of CD8(+) CD56(+) T cells. Freshly isolated thymocytes contain few CD8(+) CD56(+) cells, but the number of these cells increases significantly when thymocytes are grown in the presence of IL-15 or IL-2. However, IL-15 induced a significantly higher fraction of CD8(+) CD56(+) cells compared with IL-2. Thus, although IL-2 and IL-15 are known to have a number of redundant functions, we here demonstrate that IL-15 is superior to IL-2 in inducing CD8(+) CD56(+) T cells from cultures of thymocytes. The majority of the IL-15-grown CD8(+) CD56(+) cells were CD45R0(+), representing a memory phenotype, and showed high expression of the IL-15R-complex and high numbers of CD69(+) cells. Moreover, cytotoxic activity was confined to this cell population.

Selective loss of natural killer T cells by apoptosis following infection with lymphocytic choriomeningitis virus

Natural killer T (NKT) cells, a unique subpopulation of T cells, coexpress markers also present on NK cells and recognize the major histocompatibility complex class I-like CD1d1 molecule. We studied the effect of an acute virus infection on NKT cells. Mice were infected with the nonhepatotropic Armstrong strain of lymphocytic choriomeningitis virus (LCMV), and at various times postinfection, mononuclear cells from the liver, peritoneum, and spleen were isolated. It was found that within 2 to 3 days, there was a selective loss of NKT cells from the liver with an apparent rapid recovery within 8 to 14 days. There was no increase in peritoneal or splenic NKT cells, indicating that NKT cells did not traffic to these tissues. This loss of NKT cells was independent of gamma interferon (IFN-gamma) and interleukin 12 (IL-12) production, but did occur in mice treated with poly(I-C), a classical inducer of IFN-alpha/beta. The reduction in NKT cells was CD28 and fas/fasL independent and occurred via apoptosis. It was not observed in LCMV-infected DNA fragmentation factor 45-deficient mice, and an increase in active caspase 3-specific staining was found in liver NKT cells from LCMV-infected and poly(I-C)-treated mice compared to uninfected wild-type mice. Interestingly, it was also found that liver NKT cells from LCMV-infected mice were themselves infected. These results suggest that the loss of NKT cells following an acute LCMV infection could be due to the induction of IFN-alpha/beta resulting in NKT-cell apoptosis and is important for the host's immune response to LCMV.

T cell-independent interleukin 15Ralpha signals are required for bystander proliferation

Cytokine driven or "bystander" proliferation of T cells occurs in vivo independently of major histocompatibility complex-T cell receptor interactions. This process may be important for supporting T cell homeostasis and facilitating T cell responses to microbial antigens, and may involve the cytokine interleukin (IL)-15. In this study, we find that IL-15Ralpha-deficient (IL-15Ralpha(-/-)) mice fail to undergo poly I:C or IL-15 driven bystander proliferation of CD8(+) T cells. Surprisingly, IL-15Ralpha(-/-) CD8(+) T cells proliferate in response to poly I:C when adoptively transferred into normal mice, and normal CD8(+) T cells fail to proliferate in IL-15Ralpha(-/-) mice. Normal mice reconstituted with IL-15Ralpha(-/-) bone marrow cells also fail to exhibit bystander responses. Thus, CD8(+) T cell independent IL-15Ralpha signals from radiation sensitive hematopoietic cells are likely required for bystander responses. Moreover, normal CD8(+) T cells proliferate in IL-15Ralpha(-/-) mice after treatment with IL-15. Therefore, IL-15Ralpha signals may mediate a positive feedback loop involving the further physiological production of IL-15. These findings provide new insights into how IL-15Ralpha supports memory phenotype CD8(+) T cell proliferation, and suggest novel mechanisms by which memory CD8(+) T cells are maintained in vivo.

IL-2 during in vitro priming promotes subsequent engraftment and successful adoptive tumor immunotherapy by persistent memory phenotypic CD8(+) T cells

Adoptive T cell tumor immunotherapy potentially consists of two protective components by the transferred effector cells, the immediate immune response and the subsequent development of memory T cells. The extent by which adoptively transferred CD8(+) CTL are destined to become memory T cells is ambiguous as most studies focus on the acute effects on tumor shortly following adoptive transfer. In this study we show that a substantial fraction of the input CTL develop into memory cells that reject a s.c. tumor challenge. The use of exogenous IL-2 or a combination of IL-2 and IL-4, but not solely IL-4, during the ex vivo culture for the CTL inoculation was necessary for efficient development of CD8(+) memory T cells. Thus, an important component of adoptive immunotherapy using CTL is the production of CD8(+) Ag-specific memory cells which is primarily favored by IL-2 receptor signaling during ex vivo generation of the effector CTL.

Fatal leukemia in interleukin-15 transgenic mice

The role of inflammation in the early genesis of certain malignancies has recently been appreciated. Interleukin (IL)-15, a proinflammatory cytokine and growth factor, is required for lymphocyte homeostasis. Intriguingly, the expression of IL-15 protein is tightly controlled by multiple posttranscriptional mechanisms, suggesting that inappropriate expression of IL-15 may be detrimental to the host. We recently engineered a transgenic mouse in which the normal posttranscriptional control of IL-15 is eliminated, thereby overexpressing the murine IL-15 protein. IL-15 transgenic mice have early expansions in NK and CD8+ T lymphocytes and later develop fatal lymphocytic leukemia with a T-NK phenotype. This article recapitulates the phenotype of these IL-15 transgenic mice and discusses the utility of this model as a tool to further our understanding of leukemogenesis.

Interleukin-10 promotes the maintenance of antitumor CD8(+) T-cell effector function in situ

Interleukin-10 (IL-10) is a multifunctional cytokine that can exert suppressive and stimulatory effects on T cells. It was investigated whether IL-10 could serve as an immunostimulant for specific CD8(+) cytotoxic T cell (CTL) in vivo after vaccination and, if so, under what conditions. In tumor prevention models, administration of IL-10 before, or soon after, peptide-pulsed primary dendritic cell immunization resulted in immune suppression and enhanced tumor progression. Injection of IL-10, however, just after a booster vaccine significantly enhanced antitumor immunity and vaccine efficacy. Analysis of spleen cells derived from these latter animals 3 weeks after IL-10 treatment revealed that the number of CD8(+) CD44(hi) CD122(+) T cells had increased and that antigen-specific proliferation in vitro was enhanced. Although cytotoxicity assays did not support differences between the various treatment groups, 2 more sensitive assays measuring antigen-specific interferon-gamma production at the single-cell level demonstrated increases in the number of antigen-specific responder T cells in animals in the vaccine/IL-10 treatment group. Thus, IL-10 may maintain the number of antitumor CD8(+) T cells. In adoptive transfer studies, the ability of IL-10 to maintain CTL function could be enhanced by the depletion of CD4(+) T cells. This suggests that IL-10 mediates contrasting effects on both CD4(+) and CD8(+) T cells that result in either immune dampening or immune potentiation in situ, respectively. Appreciation of this dichotomy in IL-10 immunobiology may allow for the design of more effective cancer vaccines designed to activate and maintain specific CD8(+) T-cell effector function in situ.

Clonal biases of peripheral CD8 T cell repertoire directly reflect local inflammation in polymyositis

Polymyositis (PM) involves destruction of striated muscles by autoaggressive CD8 T cells, which accumulate and secrete cytotoxic effector molecules in the affected muscles. Previous studies of peripheral T cell repertoires from normal individuals and patients with viral infections have shown that primed CD8 T cells, unlike CD4 T cells, are prone to expand clonally and persist as large populations in the peripheral blood. These facts made us assume that autoaggressive myocytotoxic CD8 T cells would expand clonally in the peripheral blood from patients with PM. By clonal analyses of peripheral T cells from patients and age-matched controls, we show here that clonal expansion of CD8 T cells was more frequent in patients. This was not significant in CD4 T cells. In analogy to virus-specific T cells, the expanded T cells persisted as large populations over time. Analysis of the muscle biopsy specimens revealed that some of the expanded clones were infiltrating in the affected muscles from the same patients. These results provide the first evidence that local autoimmune reaction directly elicits significant biases in peripheral T cell repertoire. The expanded cells, which should be candidate autoaggressive T cells, were readily isolated from the peripheral blood for analysis of expressed genes including perforin. Thus, our findings should give us an immediate clue to analysis of the pathogenic T cells in PM.

Toll-like receptor expression reveals CpG DNA as a unique microbial stimulus for plasmacytoid dendritic cells which synergizes with CD40 ligand to induce high amounts of IL-12

Human plasmacytoid dendritic cells (DC) (PDC, CD123+) and myeloid DC (MDC, CD11c+) may be able to discriminate between distinct classes of microbial molecules based on a different pattern of Toll-like receptor (TLR) expression. TLR1-TLR9 were examined in purified PDC and MDC. TLR9, which is critically involved in the recognition of CpG motifs in mice, was present in PDC but not in MDC. TLR4, which is required for the response to LPS, was selectively expressed on MDC. Consistent with TLR expression, PDC were susceptible to stimulation by CpG oligodeoxynucleotide (ODN) but not by LPS, while MDC responded to LPS but not to CpG ODN. In PDC, CpG ODN supported survival, activation (CD80, CD86, CD40, MHC class II), chemokine production (IL-8, IP-10) and maturation (CD83). CD40 ligand (CD40L) and CpG ODN synergized to activate PDC and to stimulate the production of IFN-alpha and IL-12 including bioactive IL-12 p70. Previous incubation of PDC with IL-3 decreased the amount of CpG-induced IFN-alpha and shifted the cytokine response in favor of IL-12. CpG ODN-activated PDC showed an increased ability to stimulate proliferation of naive allogeneic CD4 T cells, butTh1 polarization of developing T cells required simultaneous activation of PDC by CD40 ligation and CpG ODN. CpG ODN-stimulated PDC expressed CCR7, which mediates homing to lymph nodes. In conclusion, our studies reveal that IL-12 p70 production by PDC is under strict control of two signals, an adequate exogenous microbial stimulus such as CpG ODN, and CD40L provided endogenously by activated T cells. Thus, CpG ODN acts as an enhancer of T cell help, while T cell-controlled restriction to foreign antigens is maintained.

Production and characterization of monoclonal antibodies reactive with the chicken interleukin-15 receptor alpha chain

We recently cloned the genes encoding chicken IL-15 and IL-15 receptor (R) alpha proteins. In this study, 12 monoclonal antibodies (mAbs) against recombinant chicken IL-15Ralpha were produced and characterized. By enzyme-linked immunosorbent assay (ELISA), all mAbs showed binding specificity for IL-15Ralpha, but not IL-2 or interferon-gamma, and identified a 25.0kDa protein by immunoblot analysis. Flow cytometric analysis revealed negligible expression of IL-15Ralpha on non-activated lymphocytes from the spleen, thymus or bursa, low but detectable expression on macrophages and high expression on concanavalin A-activated spleen lymphoblasts. Established chicken T cell (RP13) and macrophage (HD11) cell lines expressed substantially higher levels of IL-15Ralpha compared with a B cell line (RP9). Two mAbs inhibited IL-15 dependent proliferation of T cells suggesting that the tertiary structure of the protein domain of native IL-15Ralpha that binds to IL-15 is preserved in the recombinant receptor molecule. These mAbs will be useful reagents for further in vitro and in vivo studies of the biological functions of chicken IL-15 and its receptor.

Interleukin 15 skews monocyte differentiation into dendritic cells with features of Langerhans cells

Langerhans cells (LCs) represent a subset of immature dendritic cells (DCs) specifically localized in the epidermis and other mucosal epithelia. As surrounding keratinocytes can produce interleukin (IL)-15, a cytokine that utilizes IL-2Rgamma chain, we analyzed whether IL-15 could skew monocyte differentiation into LCs. Monocytes cultured for 6 d with granulocyte/macrophage colony-stimulating factor (GM-CSF) and IL-15 differentiate into CD1a(+)HLA-DR(+)CD14(-)DCs (IL15-DCs). Agents such as lipopolysaccharide (LPS), tumor necrosis factor (TNF)alpha, and CD40L induce maturation of IL15-DCs to CD83(+), DC-LAMP(+) cells. IL15-DCs are potent antigen-presenting cells able to induce the primary (mixed lymphocyte reaction [MLR]) and secondary (recall responses to flu-matrix peptide) immune responses. As opposed to cultures made with GM-CSF/IL-4 (IL4-DCs), a proportion of IL15-DCs expresses LC markers: E-Cadherin, Langerin, and CC chemokine receptor (CCR)6. Accordingly, IL15-DCs, but not IL4-DCs, migrate in response to macrophage inflammatory protein (MIP)-3alpha/CCL20. However, IL15-DCs cannot be qualified as "genuine" Langerhans cells because, despite the presence of the 43-kD Langerin, they do not express bona fide Birbeck granules. Thus, our results demonstrate a novel pathway in monocyte differentiation into dendritic cells.

Molecular, cellular, and functional characterization of chicken cytokines homologous to mammalian IL-15 and IL-2

DNA sequence analysis of a chicken interleukin (IL)-15 cDNA identified a 187 amino acid open reading frame encoding a protein with a predicted molecular weight of 21,964Da, two potential N-linked glycosylation sites, four highly conserved Cys residues, two out-of-frame AUG initiation codons in the 5' untranslated region, and an unusually long (66 amino acid) signal peptide such that the expected size of the mature protein is 14,462Da. Chicken IL-15 and IL-2 were compared with regard to their molecular, cellular, and functional characteristics. The predicted amino acid sequences of both chicken cytokines showed greater homologies with mammalian IL-15s compared with mammalian IL-2s. Northern hybridization and RT-PCR demonstrated chicken IL-15 gene transcripts in a wide variety of tissues and cell types while the chicken IL-2 gene was expressed only in concanavalin A (con A)-activated spleen cells. Both recombinant cytokines stimulated the growth of spleen T-cells and enhanced the activity of natural killer (NK) cells in vitro. Subcutaneous injection with an expression plasmid encoding IL-15 increased the percentage of CD3+ spleen T-lymphocytes whereas injection of an IL-2 cDNA augmented CD3+, CD4+, CD8+, T-cell receptor (TCR)1+, and TCR2+ T-cells. Collectively, these results indicate that chicken IL-15 and IL-2 are T-cell growth factors potentially capable of enhancing cell-mediated immunity in vivo.

An antagonist IL-15/Fc protein prevents costimulation blockade-resistant rejection

IL-15 is a powerful T cell growth factor (TCGF) with particular importance for the maintenance of CD8(+) T cells. Because costimulation blockade does not result in universal tolerance, we hypothesized that "escape" from costimulation blockade might represent a CD8(+) and IL-15/IL-15R(+)-dependent process. For this analysis, we have used an IL-15 mutant/Fcgamma2a protein, a potentially cytolytic protein that is also a high-affinity receptor site specific antagonist for the IL-15Ralpha receptor protein, as a therapeutic agent. The IL-15-related fusion protein was used as monotherapy or in combination with CTLA4/Fc in murine islet allograft models. As monotherapies, CTLA4/Fc and an IL-15 mutant/Fcgamma2a were comparably effective in a semiallogeneic model system, and combined treatment with IL-15 mutant/Fcgamma2a plus CTLA4/Fc produced universal permanent engraftment. In a fully MHC-mismatched strain combination known to be refractory to costimulation blockade treatment, combined treatment with both fusion proteins proved to be highly effective; >70% of recipients were tolerized. The analysis revealed that the IL-15 mutant/Fc treatment confers partial protection from both CD4(+) and CD8(+) T cell graft infiltration. In rejections occurring despite CTLA4/Fc treatment, concomitant treatment with the IL-15 mutant/Fcgamma2a protein blocked a CD8(+) T cell-dominated rejection processes. This protection was linked to a blunted proliferative response of alloreactive T cells as well silencing of CTL-related gene expression events. Hence, we have demonstrated that targeting the IL-15/IL-15R pathway represents a new and potent strategy to prevent costimulation blockade-resistant CD8(+) T cell-driven rejection.

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.

Selective expansion and partial activation of human NK cells and NK receptor-positive T cells by IL-2 and IL-15

IL-2 and IL-15 are lymphocyte growth factors produced by different cell types with overlapping functions in immune responses. Both cytokines costimulate lymphocyte proliferation and activation, while IL-15 additionally promotes the development and survival of NK cells, NKT cells, and intraepithelial lymphocytes. We have investigated the effects of IL-2 and IL-15 on proliferation, cytotoxicity, and cytokine secretion by human PBMC subpopulations in vitro. Both cytokines selectively induced the proliferation of NK cells and CD56(+) T cells, but not CD56(-) lymphocytes. All NK and CD56(+) T cell subpopulations tested (CD4(+), CD8(+), CD4(-)CD8(-), alphabetaTCR(+), gammadeltaTCR(+), CD16(+), CD161(+), CD158a(+), CD158b(+), KIR3DL1(+), and CD94(+)) expanded in response to both cytokines, whereas all CD56(-) cell subpopulations did not. Therefore, previously reported IL-15-induced gammadelta and CD8(+) T cell expansions reflect proliferations of NK and CD56(+) T cells that most frequently express these phenotypes. IL-15 also expanded CD8alpha(+)beta(-) and Valpha24Vbeta11 TCR(+) T cells. Both cytokines stimulated cytotoxicity by NK and CD56(+) T cells against K562 targets, but not the production of IFN-gamma, TNF-alpha, IL-2, or IL-4. However, they augmented cytokine production in response to phorbol ester stimulation or CD3 cross-linking by inducing the proliferation of NK cells and CD56(+) T cells that produce these cytokines at greater frequencies than other T cells. These results indicate that IL-2 and IL-15 act at different stages of the immune response by expanding and partially activating NK receptor-positive lymphocytes, but, on their own, do not influence the Th1/Th2 balance of adaptive immune responses.

Dressed to kill? A review of why antiviral CD8 T lymphocytes fail to prevent progressive immunodeficiency in HIV-1 infection

CD8 T cells play an important role in protection and control of HIV-1 by direct cytolysis of infected cells and by suppression of viral replication by secreted factors. However, although HIV-1-infected individuals have a high frequency of HIV-1-specific CD8 T cells, viral reservoirs persist and progressive immunodeficiency generally ensues in the absence of continuous potent antiviral drugs. Freshly isolated HIV-specific CD8 T cells are often unable to lyse HIV-1-infected cells. Maturation into competent cytotoxic T lymphocytes may be blocked during the initial encounter with antigen because of defects in antigen presentation by interdigitating dendritic cells or HIV-infected macrophages. The molecular basis for impaired function is multifactorial, due to incomplete T-cell signaling and activation (in part related to CD3zeta and CD28 down-modulation), reduced perforin expression, and inefficient trafficking of HIV-specific CD8 T cells to lymphoid sites of infection. CD8 T-cell dysfunction can partially be corrected in vitro with short-term exposure to interleukin 2, suggesting that impaired HIV-specific CD4 T helper function may play a significant causal or exacerbating role. Functional defects are qualitatively different and more severe with advanced disease, when interferon gamma production also becomes compromised.

Concurrent naive and memory CD8(+) T cell responses to an influenza A virus

Memory Thy-1(+)CD8(+) T cells specific for the influenza A virus nucleoprotein (NP(366-374)) peptide were sorted after staining with the D(b)NP(366) tetramer, labeled with CFSE, and transferred into normal Thy-1.2(+) recipients. The donor D(b)NP(366)(+) T cells recovered 2 days later from the spleens of the Thy-1.2(+) hosts showed the CD62L(low)CD44(high)CD69(low) phenotype, characteristic of the population analyzed before transfer, and were present at frequencies equivalent to those detected previously in mice primed once by a single exposure to an influenza A virus. Analysis of CFSE-staining profiles established that resting tetramer(+) T cells divided slowly over the next 30 days, while the numbers in the spleen decreased about 3-fold. Intranasal infection shortly after cell transfer with a noncross-reactive influenza B virus induced some of the donor D(b)NP(366)(+) T cells to cycle, but there was no increase in the total number of transferred cells. By contrast, comparable challenge with an influenza A virus caused substantial clonal expansion, and loss of the CFSE label. Unexpectedly, the recruitment of naive Thy-1.2(+)CD8(+)D(b)NP(366)(+) host D(b)NP(366)(+) T cells following influenza A challenge was not obviously diminished by the presence of the memory Thy-1.1(+)CD8(+)D(b)NP(366)(+) donor D(b)NP(366)(+) set. Furthermore, the splenic response to an epitope (D(b)PA(224)) derived from the influenza acid polymerase (PA(224-233)) was significantly enhanced in the mice given the donor D(b)NP(366)(+) memory population. These experiments indicate that an apparent recall response may be comprised of both naive and memory CD8(+) T cells.

Longevity of antigen presentation and activation status of APC are decisive factors in the balance between CTL immunity versus tolerance

Encounter of Ag by naive T cells can lead to T cell priming as well as tolerance. The balance between immunity and tolerance is controlled by the conditions of Ag encounter and the activation status of the APC. We have investigated the rules that govern this balance in case an environment that normally induces tolerance is reverted into a milieu that promotes T cell priming, using a minimal CTL epitope derived from human adenovirus type 5 E1A. Vaccination of mice s.c. with E1A peptide in IFA readily induces CTL tolerance, resulting in the inability to control E1A-expressing tumors. The present study shows that efficient CTL priming is achieved when this peptide vaccine is combined with systemic administration of APC-activating compounds like agonistic anti-CD40 mAb or polyriboinosinate-polyribocytidylate. Surprisingly, this CTL response is not long-lasting and therefore fails to protect against tumor outgrowth. Disappearance of CTL reactivity was strongly associated with systemic persistence of the peptide for >200 days. In contrast, peptide administered in PBS does not persist and generates long term CTL immunity capable of rejecting Ad5E1A-positive tumors, when combined with CD40 triggering. Thus, presentation of CTL epitopes in an appropriate costimulatory setting by activated APC, although being essential and sufficient for CTL priming, eventually results in tolerance when the Ag persists systemically for prolonged times. These observations are important for the development of immune intervention schemes in autoimmunity and cancer.

T lymphocytes need IL-7 but not IL-4 or IL-6 to survive in vivo

The role of IL-4, -6 and -7 in the survival of T lymphocytes was studied in vivo. The decay of polyclonal populations of CD4(+) and CD8(+) T cells was monitored in thymectomized anti-cytokine receptor mAb-treated and/or cytokine-deficient mice. The lack of IL-4 or -6 did not have any detectable effect on T cell survival, but IL-7 played an important role in the survival of the naive T cell compartment, especially of naive CD4(+) T cells.

CD4(+) T cell effectors can become memory cells with high efficiency and without further division

Whether memory T lymphocytes are derived directly from effector T cells or via a separately controlled pathway has long been debated. Here we present evidence that, after adoptive transfer, a large fraction of in vitro--derived effector CD4(+) T cells have the potential to become memory T cells and that this transition can occur without further division. This data supports a linear pathway from effector to memory cells and suggests that most properties of memory cells are predetermined during effector generation. We suggest, therefore, that evaluation of vaccine efficacy in the induction of memory CD4(+) T cells should focus on the effector stage.

Immunostimulatory DNA-based vaccines elicit multifaceted immune responses against HIV at systemic and mucosal sites

Immunostimulatory DNA sequences (ISS, also known as CpG motifs) are pathogen-associated molecular patterns that are potent stimulators of innate immunity. We tested the ability of ISS to act as an immunostimulatory pathogen-associated molecular pattern in a model HIV vaccine using gp120 envelope protein as the Ag. Mice immunized with gp120 and ISS, or a gp120:ISS conjugate, developed gp120-specific immune responses which included: 1) Ab production; 2) a Th1-biased cytokine response; 3) the secretion of beta-chemokines, which are known to inhibit the use of the CCR5 coreceptor by HIV; 4) CTL activity; 5) mucosal immune responses; and 6) CD8 T cell responses that were independent of CD4 T cell help. Based on these results, ISS-based immunization holds promise for the development of an effective preventive and therapeutic HIV vaccine.

IL-15 is expressed by dendritic cells in response to type I IFN, double-stranded RNA, or lipopolysaccharide and promotes dendritic cell activation

Cytokines that are induced by infection may contribute to the initiation of immune responses through their ability to stimulate dendritic cells (DCs). In this paper, we have addressed the role of IL-15 in DC activation, investigating its expression by DCs in response to three different signals of infection and examining its ability to stimulate DCs. We report that the expression of both IL-15 and the IL-15 receptor alpha-chain are increased in splenic DCs from mice inoculated with dsRNA (poly(I:C)), LPS, or IFN-alphabeta, and in purified murine splenic DCs treated with IFN-alphabeta in vitro. Furthermore, IL-15 itself was able to activate DCs, as in vivo or in vitro exposure of splenic DCs to IL-15 resulted in an up-regulation of costimulatory molecules, markedly increased production of IFN-gamma by DC and an enhanced ability of DCs to stimulate Ag-specific CD8(+) T cell proliferation. The magnitude of all of the IL-15-induced changes in DCs was reduced in mice deficient for the IFN-alphabeta receptor, suggesting a role for IFN-alphabeta in the stimulation of DCs by IL-15. These results identify IL-15 as a stimulatory cytokine for DCs with the potential for autocrine activity and link its effects to expression of IFN-alphabeta.

How alpha beta T cells deal with induced TCR alpha ablation

On deletion of the gene encoding the constant region of the T cell antigen receptor (TCR)alpha chain in mature T cells by induced Cre-mediated recombination, the cells lose most of their TCR from the cell surface within 7--10 days, but minute amounts of surface-bound TCR beta chains are retained for long periods of time. In a situation in which cellular influx from the thymus is blocked, TCR-deficient naive T cells decay over time, the decay rates being faster for CD8(+) cells (t(1/2) approximately 16 days) than for CD4(+) cells (t(1/2) approximately 46 days). TCR(+) naïve cells are either maintained (CD8(+)) or decay more slowly (CD4(+); t(1/2) approximately 78 days.) Numbers of TCR-deficient memory T cells decline very slowly (CD8(+) cells; t(1/2) approximately 52 days) or not at all (CD4(+) cells), but at the population level, these cells fail to expand as their TCR(+) counterparts do. Together with earlier data on T cell maintenance in environments lacking appropriate major histocompatibility complex antigens, these data argue against the possibility that spontaneous ligand-independent signaling by the alpha beta TCR contributes significantly to T-cell homeostasis.

IL-7 is critical for homeostatic proliferation and survival of naive T cells

In T cell-deficient conditions, naive T cells undergo spontaneous "homeostatic" proliferation in response to contact with self-MHC/peptide ligands. With the aid of an in vitro system, we show here that homeostatic proliferation is also cytokine-dependent. The cytokines IL-4, IL-7, and IL-15 enhanced homeostatic proliferation of naive T cells in vitro. Of these cytokines, only IL-7 was found to be critical; thus, naive T cells underwent homeostatic proliferation in IL-4(-) and IL-15(-) hosts but proliferated minimally in IL-7(-) hosts. In addition to homeostatic proliferation, the prolonged survival of naive T cells requires IL-7. Thus, naïve T cells disappeared gradually over a 1-month period upon adoptive transfer into IL-7(-) hosts. These findings indicate that naive T cells depend on IL-7 for survival and homeostatic proliferation.

Swift development of protective effector functions in naive CD8(+) T cells against malaria liver stages

We generated T cell receptor transgenic mice specific for the liver stages of the rodent malaria parasite Plasmodium yoelii and studied the early events in the development of in vivo effector functions in antigen-specific CD8(+) T cells. Differently to activated/memory cells, naive CD8(+) T cells are not capable of exerting antiparasitic activity unless previously primed by parasite immunization. While naive cells need to differentiate before achieving effector status, the time required for this process is very short. Indeed, interferon (IFN)-gamma and perforin mRNA are detectable 24 h after immunization and IFN-gamma secretion and cytotoxic activity are detected ex vivo 24 and 48 h after immunization, respectively. In contrast, the proliferation of CD8(+) T cells begins after 24 h and an increase in the total number of antigen-specific cells is detected only after 48 h. Remarkably, a strong CD8(+) T cell-mediated inhibition of parasite development is observed in mice challenged with viable parasites only 24 h after immunization with attenuated parasites. These results indicate that differentiation of naive CD8(+) T cells does not begin only after extensive cell division, rather this process precedes or occurs simultaneously with proliferation.

Overexpression of IL-15 in vivo enhances protection against Mycobacterium bovis bacillus Calmette-Guérin infection via augmentation of NK and T cytotoxic 1 responses

To investigate the immunomodulating effects of IL-15 in vivo on mycobacterial infection, we used IL-15-transgenic (Tg) mice, which were recently constructed with cDNA-encoding secretable isoform of IL-15 precursor protein under the control of a MHC class I promoter. The IL-15-Tg mice exhibited resistance against infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG), as assessed by bacteria growth. IFN-gamma level in serum was significantly higher in IL-15-Tg mice than in non-Tg mice after BCG infection. NK cells were remarkably increased, and Ag-specific T cytotoxic 1 response mediated by CD8+ T cells producing IFN-gamma was significantly augmented in the IL-15-Tg mice following BCG infection. Neutralization of endogenous IFN-gamma by in vivo administration of anti-IFN-gamma mAb deteriorated the clearance of the bacteria. Depletion of of NK cells or CD8+ T cells by in vivo administration of anti-asialo-GM(1) Ab or anti-CD8 mAb hampered the exclusion of bacteria. Thus, overexpression of IL-15 in vivo enhanced protection against BCG infection via augmentation of NK and T cytotoxic 1 responses.

T cell death and memory

In typical immune responses, contact with antigen causes naive T cells to proliferate and differentiate into effector cells. After the pathogen is destroyed, most effector T cells are eliminated-thereby preserving the primary T cell repertoire-but some cells survive and form long-lived memory cells. During each stage of this process, the life or death fate of T cells is strictly regulated.

Expression of IL-15 and IL-4 in IFN-gamma-independent control of experimental human Cryptosporidium parvum infection

We have previously demonstrated interferon gamma (IFN-gamma) in intestinal mucosa after experimental human Cryptosporidium parvum infection, but expression was limited to sensitized volunteers. To characterize IFN-gamma-independent mechanisms in control of infection, jejunal biopsies from immunocompetent volunteers experimentally challenged with C. parvum were examined by in situ hybridization for interleukin (IL-)15 and IL-4 mRNA with confirmation by immunohistochemistry. Cytokine expression was correlated with prechallenge anti- C. parvum IgG, symptoms, oocyst shedding, and prior IFN-gamma expression data. IL-15 expression was noted only in those without prior sensitization, who did not express IFN-gamma. By contrast, expression of IL-4 was associated with prior sensitization. IL-15 was only detected in those with symptoms (6/14 symptomatic vs 0/3 asymptomatic, P<0.05). Among 14 volunteers who did not express IFN-gamma, oocyst shedding was lower in those expressing IL-15. Overall, 14/15 volunteers who did not shed oocysts expressed either IFN-gamma or IL-15. There was no correlation between expression of IL-4 and symptoms or oocyst shedding. In conclusion, IL-15 expression was associated with control of oocyst shedding in those not expressing IFN-gamma. These data suggest that IL-15 is involved in IFN-gamma independent mechanisms of control of human cryptosporidiosis, perhaps via activation of the innate immune response.

Attrition of bystander CD8 T cells during virus-induced T-cell and interferon responses

Experiments designed to distinguish virus-specific from non-virus-specific T cells showed that bystander T cells underwent apoptosis and substantial attrition in the wake of a strong T-cell response. Memory CD8 T cells (CD8(+) CD44(hi)) were most affected. During acute viral infection, transgenic T cells that were clearly defined as non-virus specific decreased in number and showed an increase in apoptosis. Also, use of lymphocytic choriomeningitis virus (LCMV) carrier mice, which lack LCMV-specific T cells, showed a significant decline in non-virus-specific memory CD8 T cells that correlated to an increase in apoptosis in response to the proliferation of adoptively transferred virus-specific T cells. Attrition of T cells early during infection correlated with the alpha/beta interferon (IFN-alpha/beta) peak, and the IFN inducer poly(I:C) caused apoptosis and attrition of CD8(+) CD44(hi) T cells in normal mice but not in IFN-alpha/beta receptor-deficient mice. Apoptotic attrition of bystander T cells may make room for the antigen-specific expansion of T cells during infection and may, in part, account for the loss of T-cell memory that occurs when the host undergoes subsequent infections.

The Sushi domain of soluble IL-15 receptor alpha is essential for binding IL-15 and inhibiting inflammatory and allogenic responses in vitro and in vivo

IL-15 is a pleiotropic cytokine that plays important roles in both innate and adaptive immunity. It is associated with a range of immunopathology, including rheumatoid arthritis and allograft rejection. IL-15 functions through the trimeric IL-15R complex, which consists of a high affinity binding alpha-chain and the common IL-2R beta- and gamma-chains. Characterization of IL-15/IL-15R interactions may facilitate the development of improved IL-15 antagonists for therapeutic interventions. We previously constructed soluble murine IL-15Ralpha (sIL-15Ralpha) by deleting the cytoplasmic and transmembrane domains. To localize the functional domain of IL-15Ralpha, we have now constructed various truncated versions of sIL-15Ralpha. The shortest region retaining IL-15 binding activity is a 65-aa sequence spanning the Sushi domain of IL-15Ralpha. Sushi domains, common motifs in protein-protein interactions, contain four cysteines forming two disulfide bonds in a 1-3 and 2-4 pattern. Amino acid substitution of the first or fourth cysteine in sIL-15Ralpha completely abolished its IL-15 binding activity. This also abrogated the ability of sIL-15Ralpha to neutralize IL-15-induced proinflammatory cytokine production and anti-apoptotic response in vitro. Furthermore, the mutant sIL-15Ralpha lost its ability to inhibit carrageenan-induced local inflammation and allogenic cell-induced T cell proliferation and cytokine production in vivo. Thus, the Sushi domain is critical for the functional activity of sIL-15Ralpha.

Identification of CpG oligonucleotide sequences with high induction of IFN-alpha/beta in plasmacytoid dendritic cells

The immature plasmacytoid dendritic cell (PDC) is identical with the principal type I IFN-producing cell upon viral infection. Oligodeoxynucleotides which contain unmethylated CpG motifs (CpG ODN) are recognized by the vertebrate immune system. Previously, we described CpG ODN that strongly activate human B cells and human blood dendritic cells. Here we describe distinct CpG-containing oligonucleotide sequences which, in contrast to previously described CpG ODN, induced high amounts of IFN-alpha and IFN-beta in peripheral blood mononuclear cells (PBMC). Intracellular staining for IFN-alpha revealed that within PBMC CpG ODN-induced IFN-alpha is produced exclusively by PDC. Unlike IFN-alpha, TNF-alpha is up-regulated in PDC by all CpG ODN tested. Purified PDC responded to CpG ODN, demonstrating direct activation of PDC by CpG ODN. The most active sequence induced the production of up to 5 pg IFN-alpha per single PDC, resulting in more than 400 ng/ml IFN-alpha in the supernatant of PBMC enriched for PDC. The potency of CpG ODN to stimulate IFN-alpha correlated with their ability to stimulate NK cell lytic activity, while purified NK cells did not respond to CpG ODN. IFNgamma production in PBMC was dependent on CpG ODN-induced IFN-alpha/beta as demonstrated by IFN-alpha/beta blocking antibodies. IFN-alpha-inducing CpG ODN strongly supported IFN-gamma production of TCR-triggered CD4 T cells but were less active than other CpG ODN in stimulating B cells. In conclusion our results demonstrate that particular CpG ODN sequences exist which, due to high IFN-alpha/beta induction in PDC, induce a set of immune responses typical for viral infection.

IL-12 augments CD8+ T cell development for contact hypersensitivity responses and circumvents anti-CD154 antibody-mediated inhibition

During sensitization with dinitrofluorobenzene for contact hypersensitivity (CHS) responses, hapten-specific CD8(+) T cells develop into IFN-gamma-producing cells, and CD4(+) T cells develop into IL-4/IL-5-producing cells. Administration of IL-12 during sensitization skews CD4(+) T cell development to IFN-gamma-producing cells, resulting in exaggerated CHS responses. In the current report we tested the role of IL-12 on CD8(+) T cell development during sensitization and elicitation of CHS to dinitrofluorobenzene. Administration of IL-12 during hapten sensitization induced the expression of IL-12Rbeta2 on both CD4(+) and CD8(+) T cells, augmented IFN-gamma production by these T cell populations, and increased the magnitude and duration of the CHS response to hapten challenge. CHS responses were virtually identical in wild-type and IL-12 p40(-/-) mice. Since engagement of CD40 on APC may stimulate IL-12 production, we also tested the role of CD40-CD154 interactions on the development of IFN-gamma-producing CD4(+) and CD8(+) T cells following hapten sensitization. Development of IFN-gamma-producing CD4(+) T cells during hapten sensitization was absent in wild-type mice treated with anti-CD154 mAb or in CD154(-/-) mice. In contrast, the absence of CD40-CD154 signaling had little or no impact on the development of IFN-gamma-producing CD8(+) T cells. These results demonstrate that the development of hapten-specific Th1 effector CD4(+) T cells in CHS requires both CD40-CD154 interactions and IL-12, whereas the development of IFN-gamma-producing effector CD8(+) T cells can occur independently of these pathways.

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.

Reevaluation of the origin of CD44(high) "memory phenotype" CD8 T cells: comparison between memory CD8 T cells and thymus-independent CD8 T cells

CD44(high)CD8 T cells in naive mice, which increase with age, and are often referred to as memory CD8 T cells. However, since thymus-independent CD8 T cells have also been shown to be CD44(high), the origin of the CD44(high)CD8 T cells in naive mice remains unclear. In this study, we compared the characteristics of memory CD8 T cells and thymus-independent CD8 T cells in TCR transgenic mice to clarify the origin of the CD44(high)CD8 T cells in naive normal mice. The memory and thymus-independent CD8 T cells showed differences in surface molecules, spontaneous cell death, cytokine production, and response to IL-2R binding of cytokines. Importantly, the "memory phenotype" CD8 T cells in naive normal mice showed similar characteristics to the thymus-independent CD8 T cells, but differed greatly from "true" memory CD8 T cells in the TCR transgenic mice. Therefore, we conclude that a significant part of the CD44(high) memory phenotype CD8 T cells in naive normal mice represents thymus-independent CD8 T cells, which may participate in age-related changes in immune responses.

Il-12: keeping cell-mediated immunity alive

Interleukin (IL)-12 has dominated the field of cell-mediated immunity since its discovery more than 10 years ago, and clearly plays an essential role in the development of Th1 cells under a variety of conditions. Recent studies now indicate that the importance of IL-12 is not limited to initiating an immune response, but may contribute to maintaining immunity. Thus, Th1 responses rapidly wane in the absence of IL-12, leading to a loss in protective immunity against intracellular pathogens, such as Leishmania and Toxoplasma. Determining how IL-12 maintains Th1 cells, and consequently cell-mediated immunity, will provide new insights useful in controlling the immune response, and may thus influence the design of new vaccines and immunotherapies.

Cutting edge: antigen-independent CD8 T cell proliferation

Recent analyses of CD8 T cell responses to Listeria monocytogenes infection demonstrate that the duration of in vivo T cell proliferation is not determined by the amount or duration of Ag presentation. However, the extent to which T lymphocytes are capable of proliferating in the absence of Ag is unknown. Herein we demonstrate that CD8 T lymphocytes undergo up to eight rounds of proliferation in the absence of Ag following transient, 2.5-h in vitro antigenic stimulation. Ag-independent expansion of CD8 T cells is driven by IL-2 and is further augmented by IL-7 or IL-15. These experiments clearly demonstrate that CD8 T cells undergo prolonged proliferation following transient Ag exposure and support the notion that in vivo CD8 T cell expansion following infection can be uncoupled from Ag presentation.

An IFN-gamma-dependent pathway controls stimulation of memory phenotype CD8+ T cell turnover in vivo by IL-12, IL-18, and IFN-gamma

Unlike naive T cells, memory phenotype (CD44(high)) T cells exhibit a high background rate of turnover in vivo. Previous studies showed that the turnover of memory phenotype CD8(+) (but not CD4(+)) cells in vivo can be considerably enhanced by products of infectious agents such as LPS. Such stimulation is TCR independent and hinges on the release of type I IFNs (IFN-I) which leads to the production of an effector cytokine, probably IL-15. In this study, we describe a second pathway of CD44(high) CD8(+) stimulation in vivo. This pathway is IFN-gamma rather than IFN-I dependent and is mediated by at least three cytokines, IL-12, IL-18, and IFN-gamma. As for IFN-I, these three cytokines are nonstimulatory for purified T cells and under in vivo conditions probably act via production of IL-15.

Virus-specific and bystander CD8+ T-cell proliferation in the acute and persistent phases of a gammaherpesvirus infection

The cycling characteristics of CD8+ T cells specific for two lytic-phase epitopes of murine gammaherpesvirus 68 (gammaHV68) have been analyzed for mice with high or low levels of virus persistence. The extent of cell division is generally reflective of the antigen load and suggests that gammaHV68 may be regularly reactivating from latency for some months after the resolution of the acute phase of the infectious process. Although gammaHV68 infection is also associated with massive proliferation of lymphocytes that are not obviously specific for the virus, the level of "bystander-induced" cycling in a population of influenza virus-specific CD8+ T cells was generally fourfold lower than the extent of cell division seen for the antigen-driven, gammaHV68-specific response. The overall conclusion is that turnover rates substantially in excess of 5 to 10% over 6 days for CD8+ "memory" T-cell populations are likely to be reflective of continued antigenic exposure.

Generation and maintenance of memory T cells

Typical immune responses lead to the prominent clonal expansion of antigen-specific T cells followed by their differentiation into effector cells. Most effector cells die at the end of the immune response but some of the responding cells survive and form long-lived memory cells. The factors controlling the formation and survival of memory T cells are discussed. Recent evidence suggests that T memory cells arise from a subset of effector cells. The longevity of T memory cells may require continuous contact with cytokines, notably IL-15 for CD8(+) cells.

The CD7(-) subset of CD4(+) memory T cells is prone to accelerated apoptosis that is prevented by interleukin-15 (IL-15)

The CD7(-) subset of CD4(+) T cells reflects a stable differentiation state of post-thymic helper T cells with CD45R0(+)CD45RA(-) 'memory' phenotype. Here we report that CD4(+)CD7(-) T cells are prone to increased spontaneous apoptosis in vitro compared to CD4(+)CD7(+) T cells. Spontaneous apoptosis is prevented by IL-15, but not by IL-2. Moreover, IL-15 increases Bcl-2 and decreases CD95/Fas expression of CD7(-), but not of CD7(+) T cells. Because IL-15 is physiologically not secreted but expressed in a membrane-bound form, we cocultured T cells with TNF-alpha stimulated fibroblasts that expose membrane IL-15. TNF-alpha stimulated fibroblasts rescue CD4(+)CD7(-) T cells from apoptosis whereas unstimulated fibroblasts do not. Rescue from apoptosis requires cell-cell contact and is abolished by addition of neutralizing antibodies to IL-15. We conclude that membrane IL-15 prevents accelerated apoptosis of CD4(+)CD7(-) T cells. This mechanism may contribute to accumulation of CD7(-) T cells in chronic inflammatory skin lesions.

Expansion of HER2/neu-specific T cells ex vivo following immunization with a HER2/neu peptide-based vaccine

The identification and characterization of tumor antigens has facilitated the development of immune-based cancer prophylaxis and therapy. Cancer vaccines, like viral vaccines, may be effective in cancer prevention. Adoptive T-cell therapy, in contrast, may be more efficacious for the eradication of existing malignancies. Our group is examining the feasibility of antigen-specific adoptive T-cell therapy for the treatment of established cancer in the HER2/neu model. Transgenic mice overexpressing rat neu in mammary tissue develop malignancy, histologically similar to human HER2/neu-overexpressing breast cancer. These mice can be effectively immunized against a challenge with neu-positive tumor cells. Adoptive transfer of neu-specific T cells into tumor-bearing mice eradicates malignancy. Effective T-cell therapy relies on optimization of the ex vivo expansion of antigen-specific T cells. Two important elements of ex vivo antigen-specific T-cell growth that have been identified are (1) the preexisting levels of antigen-specific T cells and (2) the cytokine milieu used during ex vivo expansion of the T cells. Phase I clinical trials of HER2/neu-based peptide vaccination in human cancer patients have demonstrated that increased levels of HER2/neu-specific T-cells can be elicited after active immunization. Initiating cultures with greater numbers of antigen-specific T cells facilitates expansion. In addition, cytokines, such as interleukin-12, when added during ex vivo culturing along with interleukin-2 can selectively expand antigen-specific T-cells. Interleukin-12 also enhances antigen-specific functional measurements such as interferon-gamma and tumor necrosis factor-alpha release. Refinements in ex vivo expansion techniques may greatly improve the feasibility of tumor-antigen T-cell-based therapy for the treatment of advanced-stage HER2/neu-overexpressing breast malignancy.

Expansion of HBV-specific memory CTL primed by dual HIV/HBV genetic immunization during SHIV primary infection in rhesus macaques

We have previously shown the induction of humoral and cytotoxic responses specific for human immunodeficiency virus (HIV) and hepatitis B virus (HBV) antigens, following genetic immunization of rhesus macaques with a plasmid encoding both the third variable domain of the HIV-1 external envelope glycoprotein and the pseudo-viral particle of hepatitis B surface antigen (HBsAg) as presenting molecules. The DNA-immunized primates and two control animals were then challenged with a chimeric simian/human immunodeficiency virus (SHIV). They were all infected. Significant frequencies of SHIV specific cytotoxic T lymphocyte precursors (CTLp) were detected early in peripheral blood. But, in all DNA-immunized macaques, HBV envelope specific CTLp were detected during the primary infection, and they were correlated with the peak of SHIV viremia. Furthermore, HBV or SHIV specific cytotoxicity corresponded in part to CD8(+) T cells presenting a memory phenotype. Several mechanisms could account for this cellular response. But our results suggest that an expansion of memory cytotoxic CD8(+) cells, not restricted to SHIV specific effectors, could occur in peripheral blood during SHIV primary infection.

Intestinal and splenic T cell responses to enteric Listeria monocytogenes infection: distinct repertoires of responding CD8 T lymphocytes

Listeria monocytogenes is an intracellular bacterium that causes systemic infections after traversing the intestinal mucosa. Clearance of infection and long term protective immunity are mediated by L. monocytogenes-specific CD8 T lymphocytes. In this report, we characterize the murine CD8 T cell response in the lamina propria and intestinal epithelium after enteric L. monocytogenes infection. We find that the frequency of MHC class Ia-restricted, L. monocytogenes-specific T cells is approximately 4- to 5-fold greater in the lamina propria than in the spleen of mice after oral or i.v. infection. Although the kinetics of T cell expansion and contraction are similar in spleen, lamina propria, and intestinal epithelium, high frequencies of Ag-specific T cells are detected only in the lamina propria 1 mo after infection. In contrast to MHC class Ia-restricted T cells, the frequency of H2-M3-restricted, L. monocytogenes-specific T cells is decreased in the intestinal mucosa relative to that found in the spleen. In addition to this disparity, we find that MHC class Ia-restricted CD8 T cells specific for a dominant L. monocytogenes epitope have different TCR V beta repertoires in the spleen and intestinal mucosa of individual mice. These findings indicate that the intestinal mucosa is a depot where L. monocytogenes-specific effector CD8 T cells accumulate during and after infection irrespective of immunization route. Furthermore, our results demonstrate that CD8 T cell populations in these two sites, although overlapping in Ag specificity, are distinct in terms of their repertoire.

Memory phenotype CD8(+) T cells in IL-15 transgenic mice are involved in early protection against a primary infection with Listeria monocytogenes

We recently constructed IL-15 transgenic (Tg) mice using cDNA encoding a secretable isoform of the IL-15 precursor protein under the control of an MHC class I promoter. The IL-15 Tg mice exhibited resistance against a primary infection with Listeria monocytogenes. The numbers of memory CD8(+) T cells were markedly increased in the IL-15 Tg mice following Listeria infection accompanied by sustained IL-15 production. The increased CD44(+)CD8(+) T cells in the infected IL-15 Tg mice were not specialized to recognize Listeria-specific antigen but produced a large amount of IFN-gamma in response to bystander stimulation exogenous IL-15 in combination with IL-12. Furthermore, Listeria-specific Th1 response by CD4(+) T cells was significantly augmented in the IL-15 Tg mice compared with control mice following Listeria infection. In vivo depletion of the CD8(+) T cells by anti-CD8 monoclonal antibody and adoptive transfer of the T cells from naive IL-15 Tg mice indicated that the CD8(+) T cells functioned not only to eliminate bacteria at the early stage of infection but also to promote Th1 response to L. monocytogenes. Overexpression of IL-15 shed light on a novel role of memory CD8(+) T cells in early protection and promotion of Th1 response against a primary infection with L. monocytogenes.