Systemic NKG2D down-regulation impairs NK and CD8 T cell responses in vivo

The immunoreceptor NKG2D stimulates activation of cytotoxic lymphocytes upon engagement with MHC class I-related NKG2D ligands of which at least some are expressed inducibly upon exposure to carcinogens, cell stress, or viruses. In this study, we investigated consequences of a persistent NKG2D ligand expression in vivo by using transgenic mice expressing MHC class I chain-related protein A (MICA) under control of the H2-K(b) promoter. Although MICA functions as a potent activating ligand of mouse NKG2D, H2-K(b)-MICA mice appear healthy without aberrations in lymphocyte subsets. However, NKG2D-mediated cytotoxicity of H2-K(b)-MICA NK cells is severely impaired in vitro and in vivo. This deficiency concurs with a pronounced down-regulation of surface NKG2D that is also seen on activated CD8 T cells. As a consequence, H2-K(b)-MICA mice fail to reject MICA-expressing tumors and to mount normal CD8 T cell responses upon Listeria infection emphasizing the importance of NKG2D in immunity against tumors and intracellular infectious agents.

Generation of antifungal effector CD8+ T cells in the absence of CD4+ T cells during Cryptococcus neoformans infection

Immunity to the opportunistic fungus Cryptococcus neoformans is dependent on cell-mediated immunity. Individuals with defects in cellular immunity, CD4(+) T cells in particular, are susceptible to infection with this pathogen. In host defense against a number of pathogens, CD8(+) T cell responses are dependent upon CD4(+) T cell help. The goal of these studies was to determine whether CD4(+) T cells are required for the generation of antifungal CD8(+) T cell effectors during pulmonary C. neoformans infection. Using a murine intratracheal infection model, our results demonstrated that CD4(+) T cells were not required for the expansion and trafficking of CD8(+) T cells to the site of infection. CD4(+) T cells were also not required for the generation of IFN-gamma-producing CD8(+) T cell effectors in the lungs. In CD4(-) mice, depletion of CD8(+) T cells resulted in increased intracellular infection of pulmonary macrophages by C. neoformans, increasing the pulmonary burden of the infection. Neutralization of IFN-gamma in CD4(-)CD8(+) mice similarly increased macrophage infection by C. neoformans, thereby blocking the protection provided by CD8(+) T cells. Altogether, these data support the hypothesis that effector CD8(+) T cell function is independent of CD4(+) T cells and that IFN-gamma production from CD8(+) T cells plays a role in controlling C. neoformans by limiting survival of C. neoformans within macrophages.

Dendritic cells cross-present exogenous fungal antigens to stimulate a protective CD8 T cell response in infection by Histoplasma capsulatum

The contribution of CD8 T cells in host defense against histoplasmosis is minor in the CD4 T cell-intact mouse, as it has been shown that depleting CD8 T cells only marginally affects fungal clearance. However, it remains to be determined whether the CD8 T cells are protective in a host lacking functional CD4 T cells. In this study, MHC class II-deficient mice infected with Histoplasma capsulatum (Histoplasma) kept the fungus in check for up to 16 wk, indicating that CD8 T cells are able to limit fungal replication. Ex vivo studies showed that CD8 T cells from Histoplasma-infected mice expressed both intracytoplasmic IFN-gamma and granzyme B. Furthermore, CD8 T cells exhibited cytotoxic activity against macrophage targets containing Histoplasma. We demonstrated that the macrophage, being the primary host cell as well as the effector cell, can also serve as Ag donor to dendritic cells. Histoplasma-specific CD8 T cells are stimulated by dendritic cells that present exogenous Histoplasma Ags, either through direct ingestion of yeasts or through uptake of apoptotic macrophage-associated fungal Ags, a process known as "cross-presentation." Based on these results, we present a model detailing the possible sequence of events leading to a cell-mediated immune response and fungal clearance in Histoplasma-infected hosts.

Effector functions of heparin-binding hemagglutinin-specific CD8+ T lymphocytes in latent human tuberculosis

BACKGROUND

Most individuals infected with Mycobacterium tuberculosis do not develop tuberculosis (TB) and can be regarded as being protected by an appropriate immune response to the infection. The characterization of the immune responses of individuals with latent TB may thus be helpful in the definition of correlates of protection and the development of new vaccine strategies. The highly protective antigen heparin-binding hemagglutinin (HBHA) induces strong interferon (IFN)- gamma responses during latent, but not active, TB. Because of the recently recognized importance of CD8(+) T lymphocytes in anti-TB immunity, we characterized the CD8(+) T lymphocyte responses to HBHA in subjects with latent TB.

RESULTS

HBHA-specific CD8(+) T lymphocytes expressed memory cell markers and synthesized HBHA-specific IFN- gamma . They also restricted mycobacterial growth and expressed cytotoxicity by a granule-dependent mechanism. This activity was associated with the intracellular expression of HBHA-induced perforin. Surprisingly, the perforin-producing CD8(+) T lymphocytes were distinct from the IFN- gamma -producing CD8(+) T lymphocytes.

CONCLUSION

During latent TB, the HBHA-specific CD8(+) T lymphocyte population expresses all 3 effector functions associated with CD8(+) T lymphocyte-mediated protective immune mechanisms, which supports the notion that HBHA may be protective in humans and suggests that markers of HBHA-specific CD8(+) T lymphocyte responses may be useful in the monitoring of protection.

In Vivo Modulation of T Cell Responses and Protective Immunity by TCR Antagonism during Infection

Infectious agents are known to express altered peptide ligands that antagonize T cells in vitro; however, direct evidence of TCR antagonism during infection is still lacking, and its importance in the context of infection remains to be established. In this study, we used a murine model of infection with recombinant Listeria monocytogenes and addressed three issues that are critical for assessing the role of TCR antagonism in the modulation of the immune response. First, we demonstrated that the antagonist peptide efficiently inhibited the ability of the agonist to prime naive TCR-transgenic T cells in vivo. Second, we showed clonal memory T cells were antagonized during recall responses, resulting in loss of protective immunity. Lastly, we observed that even in the context of a polyclonal response, TCR antagonism greatly inhibits the agonist-specific response, leading to altered hierarchy of immunodominance and reduced T cell memory and protective immunity. These results provide direct evidence of clonal TCR antagonism of naive and memory CD8 T cells during infection and demonstrate the effect of TCR antagonism on protective immunity. Thus, agonist/antagonist interactions may play an important role in determining the immunodominance and repertoire of T cell targets, and evaluation of immune responses and vaccine strategies may require examination of not only agonists but also antagonists and their interactions during an infection.

Dynamic regulation of IFN-gamma signaling in antigen-specific CD8+ T cells responding to infection

IFN-gamma plays a critical role in the CD8(+) T cell response to infection, but when and if this cytokine directly signals CD8(+) T cells during an immune response is unknown. We show that naive Ag-specific CD8(+) T cells receive IFN-gamma signals within 12 h after in vivo infection with Listeria monocytogenes and then become unresponsive to IFN-gamma throughout the ensuing Ag-driven expansion phase. Ag-specific CD8(+) T cells regain partial IFN-gamma responsiveness throughout the contraction phase, whereas the memory pool exhibits uniform, but reduced, responsiveness that is also modulated during the secondary response. The responsiveness of Ag-specific CD8(+) T cells to IFN-gamma correlated with modulation in the expression of IFN-gammaR2, but not with IFN-gammaR1 or suppressor of cytokine signaling-1. This dynamic regulation suggests that early IFN-gamma signals participate in regulation of the primary CD8(+) T cell response program, but that evading or minimizing IFN-gamma signals during expansion and the memory phase may contribute to appropriate regulation of the CD8(+) T cell response.

Dietary (n-3) Polyunsaturated Fatty Acids Do Not Affect the In Vivo Development and Function of Listeria-Specific CD4+ and CD8+ Effector and Memory/Effector T Cells in Mice

We previously reported that in a mouse model, a diet high in (n-3) PUFA diminishes host survival following an infection from Listeria monocytogenes, a gram-positive bacterial pathogen. In this study we investigated the impact of (n-3) PUFA on the adaptive immune response to L. monocytogenes. BALB/c mice were fed experimental diets either devoid of or rich in (n-3) PUFA from fish oil for 4 wk and then infected with 10(6) actA-deficient L. monocytogenes. At 7 and 35 d postchallenge, effector and memory/effector T cells in the spleen were enumerated by flow cytometry. Surprisingly, the number of Listeria-specific CD4(+) and CD8(+) effector and memory/effector T cells in the spleen was not affected by (n-3) PUFA. Also, the effector cells derived from mice fed either diet were equally capable of conferring protective immunity upon adoptive transfer to naive recipients. Despite our previous data, which demonstrated that (n-3) PUFA profoundly impaired host resistance to L. monocytogenes, pathogen-specific T cell responses were not substantially affected by dietary (n-3) PUFA.

A secreted low-molecular-weight protein from Helicobacter pylori induces cell-cycle arrest of T cells

BACKGROUND & AIMS

Although Helicobacter pylori is recognized by the human immune system, the bacteria are not eliminated and lead to a chronic inflammation of the gastric mucosa.

METHODS

We investigated the interaction of H. pylori with human lymphocytes. T and B lymphocytes were isolated from H. pylori-infected patients and stimulated with anti-CD3/CD28 or interleukin-6.

RESULTS

Proliferation of lymphocytes was abolished on co-incubation with different H. pylori strains (1-5 bacteria/cell) or with protein extracts of culture supernatants. Inhibition of proliferation was independent of known virulence factors. The factor is a protein or protein complex with an apparent molecular weight between 30 and 60 kilodaltons, clearly distinct from VacA. Although antigen-specific activation of T cells (as shown by nuclear factor of activated T cells [NFAT]-activation, interferon-gamma production, and CD25 or CD69 up-regulation) remained intact, cell-cycle analysis showed that S-phase entry of T cells was inhibited completely by H. pylori. Consequently, stimulated T cells arrested in the G1 phase of the cell cycle. Western blot analysis showed markedly reduced phosphorylation of the retinoblastoma protein (pRb), suggesting inhibition of G1 cyclin-dependent kinase activity. In line with this, activities of cyclin D3 and cyclin E were down-regulated, and levels of the cyclin-dependent kinase inhibitor p27Kip1 were increased. Mouse embryonic fibroblasts deficient in p27 showed a decrease in H. pylori-induced inhibition of cell proliferation, suggesting a central role for p27 in mediating H. pylori-induced G1 arrest.

CONCLUSIONS

Induction of cell-cycle arrest in lymphocytes may be of major significance for the chronic persistence of bacteria in the human stomach.

Fatal Mycobacterium tuberculosis infection despite adaptive immune response in the absence of MyD88

Toll-like receptors (TLRs) such as TLR2 and TLR4 have been implicated in host response to mycobacterial infection. Here, mice deficient in the TLR adaptor molecule myeloid differentiation factor 88 (MyD88) were infected with Mycobacterium tuberculosis (MTB). While primary MyD88(-/-) macrophages and DCs are defective in TNF, IL-12, and NO production in response to mycobacterial stimulation, the upregulation of costimulatory molecules CD40 and CD86 is unaffected. Aerogenic infection of MyD88(-/-) mice with MTB is lethal within 4 weeks with 2 log(10) higher CFU in the lung; high pulmonary levels of cytokines and chemokines; and acute, necrotic pneumonia, despite a normal T cell response with IFN-gamma production to mycobacterial antigens upon ex vivo restimulation. Vaccination with Mycobacterium bovis bacillus Calmette-Guerin conferred a substantial protection in MyD88(-/-) mice from acute MTB infection. These data demonstrate that MyD88 signaling is dispensable to raise an acquired immune response to MTB. Nonetheless, this acquired immune response is not sufficient to compensate for the profound innate immune defect and the inability of MyD88(-/-) mice to control MTB infection.

Reciprocal stimulation of gammadelta T cells and dendritic cells during the anti-mycobacterial immune response

Gammadelta T cells and dendritic cells (DC) are two distinct cell types of innate immunity that participate in early phases of immune response against Mycobacterium tuberculosis infection. Here we show that a close functional relationship exists between these cell populations. Using an in vitro coculture system, Vgamma1 T cells from Tcrb(-/- )mice were found to be activated by DC infected in vitro with BCG, as indicated by the elevated CD69 expression, IFN-gamma secretion and cytotoxic activity. This activation process was due to a non-cognate mechanism since it required neither cell to cell contact nor interaction between the TCR and a specific antigen, but was mediated by DC-derived IL-12. Reciprocally, Vgamma1 T cells provided a key cytokine, IFN-gamma, which increased IL-12 production by BCG-infected DC. Moreover, exposure of BCG-infected DC to Vgamma1 T cells conditioned the former to prime a significantly stronger anti-mycobacterial CD8 T cell response. Consequently, stimulation of gammadelta T cells and their non-cognate interaction with DC could be applied as an immune adjuvant strategy to optimize vaccine-induced CD8 T cell immunity.

Hematopoietic cells are required to initiate a Chlamydia trachomatis-specific CD8+ T cell response

Chlamydia trachomatis is a global human pathogen causing diseases ranging from blinding trachoma to pelvic inflammatory disease. To explore how innate and adaptive immune responses cooperate to protect against systemic infection with C. trachomatis L2, we investigated the role of macrophages (Mphi) and dendritic cells (DCs) in the stimulation of C. trachomatis-specific CD8(+) T cells. We found that C. trachomatis infection of Mphi and DCs is far less productive than infection of nonprofessional APCs, the typical targets of infection. However, despite the limited replication of C. trachomatis within Mphi and DCs, infected Mphi and DCs process and present C. trachomatis CD8(+) T cell Ag in a proteasome-dependent manner. These findings suggest that although C. trachomatis is a vacuolar pathogen, some Ags expressed in infected Mphi and DCs are processed in the host cell cytosol for presentation to CD8(+) T cells. We also show that even though C. trachomatis replicates efficiently within nonprofessional APCs both in vitro and in vivo, Ag presentation by hematopoietic cells is essential for initial stimulation of C. trachomatis-specific CD8(+) T cells. However, when DCs infected with C. trachomatis ex vivo were adoptively transferred into naive mice, they failed to prime C. trachomatis-specific CD8(+) T cells. We propose a model for priming C. trachomatis-specific CD8(+) T cells whereby DCs acquire C. trachomatis Ag by engulfing productively infected nonprofessional APCs and then present the Ag to T cells via a mechanism of cross-presentation.

Human CD8+T Cells Recognize the 60-kDa Cysteine-Rich Outer Membrane Protein fromChlamydia trachomatis

The intracellular bacterial pathogen Chlamydia is sequestered from the host cell cytoplasm by remaining within an inclusion body during its replication cycle. Nevertheless, CD8(+) T cells recognizing Chlamydia Ags in the context of MHC class I molecules are primed during infection. We have recently described derivation of Chlamydia-specific human CD8(+) T cells by using infected dendritic cells as a surrogate system to reflect Chlamydia-specific CD8(+) T cell responses in vivo. These CD8(+) T cell clones recognize chlamydial Ags processed via the conventional class Ia processing pathway, as assessed by treatment of infected APC with lactacystin and brefeldin A, suggesting that the Ags are translocated from the chlamydial inclusion into the host cell cytosol. In this study, outer membrane protein 2 (OmcB) was identified as the Ag recognized by one of these Chlamydia-specific human CD8(+) T cells, and we defined the HLA*A0101-restricted epitope from this Ag. CD8(+) T cell responses to this epitope were present at high frequencies in the peripheral blood of both of two HLA*A0101 donors tested. In vitro chlamydial growth was completely inhibited by the OmcB-specific CD8(+) T cell clone independently of lytic mechanisms. OmcB is a 60-kDa protein that has been postulated to be associated with the Chlamydia outer membrane complex. The subcellular localization of OmcB to the cytosol of infected cells, as determined by conventional MHC class I Ag processing and presentation, suggests the possibility of an additional, cytosolic-associated function for this protein.

Activation of CD8 T cells by mycobacterial vaccination protects against pulmonary tuberculosis in the absence of CD4 T cells

We have investigated whether both primary CD8 T cell activation and CD8 T cell-mediated protection from Mycobacterium tuberculosis challenge could occur in mycobacterial-vaccinated CD4 T cell-deficient (CD4KO) mice. Different from wild-type C57BL/6 mice, s.c. vaccination with bacillus Calmette-Guérin (BCG) in CD4KO mice failed to provide protection from secondary M. tuberculosis challenge at 3 wk postvaccination. However, similar to C57BL/6 mice, CD4KO mice were well protected from M. tuberculosis at weeks 6 and 12 postvaccination. This protection was mediated by CD8 T cells. The maintenance of protective effector/memory CD8 T cells in CD4KO mice did not require the continuous presence of live BCG vaccine. As in C57BL/6 mice, similar levels of primary activation of CD8 T cells in CD4KO mice occurred in the draining lymph nodes at 3 wk after BCG vaccination, but different from C57BL/6 mice, the distribution of these cells to the spleen and lungs of CD4KO mice was delayed, which coincided with delayed acquisition of protection in CD4KO mice. Our results suggest that both the primary and secondary activation of CD8 T cells is CD4 T cell independent and that the maintenance of these CD8 T cells is also independent of CD4 T cells and no longer requires the presence of live mycobacteria. However, the lack of CD4 T cells may result in delayed distribution of activated CD8 T cells from draining lymph nodes to distant organs and consequently a delayed acquisition of immune protection. Our findings hold implications in rational design of tuberculosis vaccination strategies for humans with impaired CD4 T cell function.

4-1BB and OX40 Act Independently to Facilitate Robust CD8 and CD4 Recall Responses

Mice deficient in OX40 or 4-1BB costimulatory pathways show defects in T cell recall responses, with predominant effects on CD4 vs CD8 T cells, respectively. However, OX40L can also stimulate CD8 T cells and 4-1BBL can influence CD4 T cells, raising the possibility of redundancy between the two TNFR family costimulators. To test this possibility, we generated mice deficient in both 4-1BBL and OX40L. In an adoptive transfer model, CD4 T cells expressed 4-1BB and OX40 sequentially in response to immunization, with little or no overlap in the timing of their expression. Under the same conditions, CD8 T cells expressed 4-1BB, but no detectable OX40. Thus, in vivo expression of 4-1BB and OX40 can be temporally and spatially segregated. In the absence of OX40L, there were decreased CD4 T cells late in the primary response and no detectable secondary expansion of adoptively transferred CD4 T cells under conditions in which primary expansion was unaffected. The 4-1BBL had a minor effect on the primary response of CD4 T cells in this model, but showed larger effects on the secondary response, although 4-1BBL(-/-) mice show less impairment in CD4 secondary responses than OX40L(-/-) mice. The 4-1BBL(-/-) and double knockout mice were similarly impaired in the CD8 T cell response, whereas OX40L(-/-) and double knockout mice were similarly impaired in the CD4 T cell response to both protein Ag and influenza virus. Thus, 4-1BB and OX40 act independently and nonredundantly to facilitate robust CD4 and CD8 recall responses.

Identification of a human HLA-E-restricted CD8+ T cell subset in volunteers immunized with Salmonella enterica serovar Typhi strain Ty21a typhoid vaccine

Our previous studies in volunteers immunized with Salmonella enterica serovar Typhi (S. Typhi) have suggested an important role for CD8+ T cells in host defense. In this study we describe a novel subset of nonclassical human HLA-E-restricted S. Typhi-specific CD8+ T cells derived from PBMC of Ty21a typhoid vaccinees. CD3+CD8+CD4-CD56- T cells effectively killed S. Typhi-infected targets regardless of whether they share classical HLA class I molecules with them, by a FAS-independent, granule-dependent mechanism, as evidenced by induction of granzyme B release and the blocking effects of concanamycin and strontium ions. The expression of HLA-E Ags, but not CD1-a, -b, or -c, on the membrane of S. Typhi-infected targets rendered them susceptible to lysis. Moreover, anti-HLA-E Abs partially blocked these responses. We also demonstrated that presentation of S. Typhi Ags via HLA-E could stimulate IFN-gamma production. Increases in the net frequency of IFN-gamma spot-forming cells were observed in the presence of targets coated with peptides that contain S. Typhi GroEL HLA-E binding motifs. These results demonstrate that HLA-E binds nonamer peptides derived from bacterial proteins and trigger CD8+-mediated lysis and IFN-gamma production when exposed to infected targets, raising the possibility that this novel effector mechanism might contribute to host defense against intracellular bacterial infections.

Analysis of human peripheral blood samples from fatal and nonfatal cases of Ebola (Sudan) hemorrhagic fever: cellular responses, virus load, and nitric oxide levels

Peripheral blood samples obtained from patients during an outbreak of Ebola virus (Sudan species) disease in Uganda in 2000 were used to phenotype peripheral blood mononuclear cells (PBMC), quantitate gene expression, measure antigenemia, and determine nitric oxide levels. It was determined that as the severity of disease increased in infected patients, there was a corresponding increase in antigenemia and leukopenia. Blood smears revealed thrombocytopenia, a left shift in neutrophils (in some cases degenerating), and atypical lymphocytes. Infected patients who died had reduced numbers of T cells, CD8(+) T cells, and activated (HLA-DR(+)) CD8(+) T cells, while the opposite was noted for patients who survived the disease. Expression levels of cytokines, Fas antigen, and Fas ligand (TaqMan quantitation) in PBMC from infected patients were not significantly different from those in uninfected patients (treated in the same isolation wards), nor was there a significant increase in expression compared to healthy volunteers (United States). This unresponsive state of PBMC from infected patients despite high levels of circulating antigen and virus replication suggests that some form of immunosuppression had developed. Ebola virus RNA levels (virus load) in PBMC specimens were found to be much higher in infected patients who died than patients who survived the disease. Similarly, blood levels of nitric oxide were much higher in fatal cases (increasing with disease severity), and extremely elevated levels (>/=150 microM) would have negatively affected vascular tone and contributed to virus-induced shock.

Duration of Infection and Antigen Display Have Minimal Influence on the Kinetics of the CD4+T Cell Response toListeria monocytogenesInfection

The T cell response to infection consists of clonal expansion of effector cells, followed by contraction to memory levels. It was previously thought that the duration of infection determines the magnitude and kinetics of the T cell response. However, recent analysis revealed that transition between the expansion and contraction phases of the Ag-specific CD8+ T cell response is not affected by experimental manipulation in the duration of infection or Ag display. We studied whether the duration of infection and Ag display influenced the kinetics of the Ag-specific CD4+ T cell response to Listeria monocytogenes (LM) infection. We found that truncating infection and Ag display with antibiotic treatment as early as 24 h postinfection had minimal impact on the expansion or contraction of CD4+ T cells; however, the magnitudes of the Ag-specific CD4+ and CD8+ T cell responses were differentially affected by the timing of antibiotic treatment. Treatment of LM-infected mice with antibiotics at 24 h postinfection did not prevent generation of detectable CD4+ and CD8+ memory T cells at 28 days after infection, vigorous secondary expansion of these memory T cells, or protection against a subsequent LM challenge. These results demonstrate that events within the first few days of infection stimulate CD4+ and CD8+ T cell responses that are capable of carrying out the full program of expansion and contraction to functional memory, independently of prolonged infection or Ag display.

Cross-Presentation ofListeria-Derived CD8 T Cell Epitopes Requires Unstable Bacterial Translation Products

Presentation of bacteria-derived CD8 T cell epitopes by dendritic cells (DC) requires either their direct infection or that DC acquire and cross-present Ags from other infected cells. We found that cross-presentation of Listeria monocytogenes-derived CD8 T cell epitopes was much stronger than direct Ag presentation by infected murine DC. Cross-presentation of Listeria-derived CD8 T cell epitopes showed unique physiological requirements. It was dependent upon the delivery of unstable bacterial translation products by infected, but still viable, Ag donor cells. Cross-presentation was enhanced both when unstable translation products in infected Ag donor cells were protected from proteasomal degradation and when the production of misfolded bacterial proteins was increased. The requirement of unstable translation products for cross-presentation may represent a novel pathway that functions to focus the CD8 T cell response toward epitopes derived from newly synthesized proteins.

The Development of Functional CD8 T Cell Memory afterListeria monocytogenesInfection Is Not Dependent on CD40

The immunologic requirements for generating long-lived protective CD8 T cell memory remain unclear. Memory CD8 populations generated in the absence of CD4 Th cells reportedly have functional defects, and at least a subset of CD8 T cells transiently express CD40 after activation, suggesting that direct CD4-CD8 T cell interactions through CD40 may influence the magnitude and functional quality of memory CD8 populations. To ascertain the role of CD40 in such direct T cell interactions, we investigated CD8 T cell responses in CD40-/- mice after infection with Listeria monocytogenes, an intracellular bacterium that induces APC activation and thus priming of CD8 T cells independently of CD4 Th cell help through CD40. In this study we show that memory CD8 T cells generated in CD40-deficient mice show in vivo cytotoxicity and cytokine production equivalent to CD8 memory T cells from wild-type mice. Upon secondary Listeria infection, CD40-/- memory CD8 T cells expand to greater numbers than seen in wild-type mice. These results indicate that CD40 ligation on CD8 T cells, although reportedly a part of CD8 T cell memory development in an H-Y-directed response, is not needed for the development of functional memory CD8 T cell populations after Listeria infection.

NK cells regulate CD8+ T cell effector function in response to an intracellular pathogen

We studied the role of NK cells in regulating human CD8+ T cell effector function against mononuclear phagocytes infected with the intracellular pathogen Mycobacterium tuberculosis. Depletion of NK cells from PBMC of healthy tuberculin reactors reduced the frequency of M. tuberculosis-responsive CD8+IFN-gamma+ cells and decreased their capacity to lyse M. tuberculosis-infected monocytes. The frequency of CD8+ IFN-gamma+ cells was restored by soluble factors produced by activated NK cells and was dependent on IFN-gamma, IL-15, and IL-18. M. tuberculosis-activated NK cells produced IFN-gamma, activated NK cells stimulated infected monocytes to produce IL-15 and IL-18, and production of IL-15 and IL-18 were inhibited by anti-IFN-gamma. These findings suggest that NK cells maintain the frequency of M. tuberculosis-responsive CD8+IFN-gamma+ T cells by producing IFN-gamma, which elicits secretion of IL-15 and IL-18 by monocytes. These monokines in turn favor expansion of Tc1 CD8+ T cells. The capacity of NK cells to prime CD8+ T cells to lyse M. tuberculosis-infected target cells required cell-cell contact between NK cells and infected monocytes and depended on interactions between the CD40 ligand on NK cells and CD40 on infected monocytes. NK cells link the innate and the adaptive immune responses by optimizing the capacity of CD8+ T cells to produce IFN-gamma and to lyse infected cells, functions that are critical for protective immunity against M. tuberculosis and other intracellular pathogens.

Extensive MHC class I-restricted CD8 T lymphocyte responses against various yeast genera in humans

The human cellular immune response against 14 distantly related yeast species was analyzed by intracellular cytokine staining of lymphocytes after ex vivo stimulation of whole blood. While the CD4 T cell response was marginal, extensive MHC class I-restricted CD8 T cell responses were detected against a number of species including spoiling, environmental and human pathogenic yeasts. The yeast-specific CD8 T cells expressed interferon-gamma but lacked expression of CD27 and CCR7, indicating that they were end-differentiated effector memory cells. Mainly intact yeast cells rather than spheroplasts were able to induce cytokine expression in T cells demonstrating that the dominant immunogens were located in the yeast cell wall. Together these data underline the importance of the cellular immune response in protecting humans against yeast and fungal infections. And, from another perspective, recombinant yeast suggests itself as a potential vaccine candidate to efficiently induce antigen-specific CD8 T cell responses.

Promiscuity of MHC class Ib-restricted T cell responses

Murine infection with the Gram-positive intracellular bacterium Listeria monocytogenes activates CD8(+) T cells that recognize bacterially derived N-formyl methionine peptides in the context of H2-M3 MHC class Ib molecules. Three peptides, fMIGWII, fMIVIL, and fMIVTLF, are targets of L. monocytogenes-specific CD8(+) T cells. To investigate epitope cross-recognition by H2-M3-restricted CD8(+) T cells, we deleted the sequence encoding fMIGWII from a virulent strain of L. monocytogenes. Infection with fMIGWII-deficient L. monocytogenes unexpectedly primed CD8(+) T cells that stain with fMIGWII/H2-M3 tetramers and lyse fMIGWII-coated target cells in vivo. Because the fMIGWII sequence is nonredundant, we speculated that other bacterially derived Ags are priming these responses. HPLC peptide fractionation of bacterial culture supernatants revealed several distinct L. monocytogenes-derived peptides that are recognized by fMIGWII-specific T cells. Our results demonstrate that the dominant H2-M3-restricted CD8(+) T cell population, although reactive with fMIGWII, is primed by other, non-fMIGWII peptides derived from L. monocytogenes. Although this degree of Ag receptor promiscuity is unusual for the adaptive immune system, it may be a more common feature of T cell responses restricted by nonpolymorphic MHC class Ib molecules.

Activation of antigen-specific CD8 T cells results in minimal killing of bystander bacteria

Memory CD8 T cells play a critical role in protective immunity against intracellular pathogens. In addition to their ability to specifically recognize and lyse infected targets, activated CD8 T cells secrete cytokines that induce phagocytic cells to engulf and kill bacterial pathogens. In this study, we asked whether activation of Ag-specific CD8 T cells results in nonspecific killing of bystander bacteria during a mixed infection. Mice with epitope-specific memory CD8 T cells were coinfected with two isogenic strains of recombinant Listeria monocytogenes that differ in the cognate epitope. Recall responses by epitope-specific CD8 T cells rapidly inhibited the growth of epitope-bearing bacteria, impeding the course of infection within 6 h after challenge. This rapid inhibition was highly specific and did not affect the growth of coinfecting bacteria without the epitope. CTL recall did not enhance activation of innate immune cells, as evidenced by the absence of inducible NO synthase production in infectious foci. Our observations demonstrate the remarkable specificity of the bactericidal mechanisms of CTL and reveal the possibility for escape mutants to prevail in the hostile environment of a specific immune response. This implication has a bearing on subunit vaccine design strategies and understanding failure of immunization against bacterial infection.

Depletion of CD4+T Cells during Immunization with NonviableListeria monocytogenesCauses Enhanced CD8+T Cell-Mediated Protection against Listeriosis

Immunization of mice with nonviable Listeria monocytogenes generates an insufficient CD8(+) T cell response and consequently only limited protection against subsequent L. monocytogenes infection. We have recently demonstrated that depletion of regulatory CD4(+) T cells during immunization significantly enhances CD8(+) T cell responses. In the present study, we determined the impact of CD4(+) T cell depletion on the CD8(+) T cell response against heat-killed Listeria. Treatment of mice with anti-CD4 mAb during boost immunization with heat-killed Listeria significantly increased numbers of Listeria-specific CD8(+) T cells and improved protection against subsequent infection with L. monocytogenes. During challenge infection, numbers of Listeria-specific CD8(+) T cells were enhanced, and these cells expressed effector functions in terms of IFN-gamma production. In summary, we demonstrate that combining nonviable L. monocytogenes vaccination and CD4(+) T cell depletion improves generation of long-lasting and functional Listeria-specific CD8(+) memory T cells.

Escherichia coliExpressing Recombinant Antigen and Listeriolysin O Stimulate Class I-Restricted CD8+T Cells following Uptake by Human APC

Vaccination against cancer or intracellular pathogens requires stimulation of class I-restricted CD8(+) T cells. It is therefore important to develop Ag delivery vectors that will promote cross-presentation by APCs and stimulate appropriate inflammatory responses. Toward this goal, we tested the potential of Escherichia coli as an Ag delivery vector in in vitro human culture. Bacteria expressing enhanced green fluorescent protein were internalized efficiently by dendritic cells, as shown by flow cytometry and fluorescence microscopy. Phenotypic changes in DC were observed, including up-regulation of costimulatory molecules and IL-12p40 production. We tested whether bacteria expressing recombinant Ags could stimulate human T cells using the influenza matrix protein as a model Ag. Specific responses against an immunodominant epitope were seen using IFN-gamma ELISPOT assays when the matrix protein was coexpressed with listeriolysin O, but not when expressed alone. THP-1 macrophages were also capable of stimulating T cells after uptake of bacteria, but showed slower kinetics and lower overall levels of T cell stimulation than dendritic cells. Increased phagocytosis of bacteria induced by differentiation of THP-1 increased their ability to stimulate T cells, as did opsonization. Presentation was blocked by proteasome inhibitors, but not by lysosomal protease inhibitors leupeptin and E64. These results demonstrate that recombinant E. coli can be engineered to direct Ags to the cytosol of human phagocytic APCs, and suggest possible vaccine strategies for generating CD8(+) T cell responses against pathogens or tumors.