Targeting T cells responsive to the priming epitope prevent the relapsing phase of experimental autoimmune encephalomyelitis.

Upon recovery from the initial episode of experimental autoimmune encephalomyelitis (EAE), virtually all SJL mice develop relapsing/remitting episodes of disease. These relapses may occur due to the reactivation of memory T cells initially stimulated as part of the disease-inducing protocol or naïve T-cell populations stimulated by distinct encephalitogens derived from the inflammatory disease process (epitope spread). We have used encephalitogen-specific non-linear peptide octamers to modify the course of relapsing EAE (rEAE) in SJL mice immunized with an oliogodendrocyte-specific protein peptide (OSP 55-71). Our studies show that the peptide-octamers, which target the T cells stimulated by the priming encephalitogen, but not other candidate encephalitogens, prevent rEAE.

Amplification of an alternate transporter gene suppresses the avirulent phenotype of glucose transporter null mutants in Leishmania mexicana.

A glucose transporter null mutant of the parasitic protozoan Leishmania mexicana, in which three linked glucose transporter genes have been deleted by targeted gene replacement, is unable to replicate as amastigote forms within phagolysomes of mammalian host macrophages and is avirulent. Spontaneous suppressors of the null mutant have been isolated that partially restore replication of parasites within macrophages. These suppressor mutants have amplified the gene for an alternative hexose transporter, the LmGT4 permease (previously called the D2 permease), on a circular extrachromosomal element, and they overexpress LmGT4 mRNA and protein. The suppressors have also regained the ability to transport hexoses, and they have reverted other phenotypes of the null mutant exhibiting enhanced resistance to oxidative killing, heat shock and starvation for nutrients, as well as augmented levels of the storage carbohydrate beta-mannan, increased cell size and increased growth as insect stage promastigotes compared with the unsuppressed mutant. Complementation of the null mutant with the LmGT4 gene on a multicopy episomal expression vector also reverted these phenotypes, confirming that suppression results from amplification of the LmGT4 gene. These results underscore the importance of hexose transporters for the infectious stage of the parasite life cycle.

A novel prfA mutation that promotes Listeria monocytogenes cytosol entry but reduces bacterial spread and cytotoxicity.

Listeria monocytogenes is an environmental bacterium that becomes a pathogen following ingestion by a mammalian host. The transition from environmental organism to pathogen requires significant changes in gene expression, including the increased expression of gene products that contribute to bacterial growth within host cells. PrfA is an L. monocytogenes transcriptional regulator that becomes activated upon bacterial entry into mammalian cells and induces the expression of gene products required for virulence. How PrfA activation occurs is not known, however several mutations have been identified that increase PrfA activity in strains grown in vitro (prfA mutations). Here we describe a novel prfA mutation that enhances extracellular PrfA-dependent gene expression but in contrast to prfA mutants inhibits the cytosol-mediated induction of virulence genes. prfA Y154C strains entered cells and escaped from phagosomes with an efficiency similar to wild type bacteria, however the mutation prevented efficient L. monocytogenes actin polymerization and reduced spread of bacteria to adjacent cells. The prfA Y154C mutation severely attenuated bacterial virulence in mice but the mutant strains did generate target antigen specific CD8(+) effector cells. Interestingly, the prfA Y154C mutant was significantly less cytotoxic for host cells than wild type L. monocytogenes. The prfA Y154C mutant strain may therefore represent a novel attenuated strain of L. monocytogenes for antigen delivery with reduced host cell toxicity.

Stimulation of enhanced CD8 T cell responses following immunization with a hyper-antigen secreting intracytosolic bacterial pathogen.

The intracytosolic niche for replication of Listeria monocytogenes (Lm) facilitates delivery of bacteria-derived Ags into the MHC class I pathway for subsequent stimulation of CD8 effector T cells. Using Lm strains that are equivalent for in vivo virulence yet express marked differences in the level of secretion of a protective target Ag, we have evaluated how these specific differences in secretion levels influences the magnitude and effector function of Ag-specific CD8 T cell responses following Lm injection. Immunization with low doses of a hyperantigen-secreting Lm strain stimulated enhanced target-Ag specific CD8 T cell responses compared with the magnitude stimulated following immunization with the same dose of wild-type Lm. The enhanced determinant-specific response was also evident by in vivo CTL activity, increased numbers of memory cells 4 wk following immunization, and enhanced antilisterial protection following a challenge infection. Initiation of antibiotic treatment 24 h following infection with wild-type Lm markedly reduced the magnitude of the effector CD8 T cell response. In contrast, antibiotic treatment initiated 24 h following immunization with the hyperantigen secreting strain of Lm did not impact the frequency of the target-Ag specific CD8 T cells. Thus, immunization with a low dose of a hyperantigen secreting Lm strain, followed by antibiotic treatment to limit the extent of the infection, may represent a safe strategy for the stimulation of enhanced effector CD8 T cell responses to specific Ag by a rLm vaccine.

Phenotypic characterization of a glucose transporter null mutant in Leishmania mexicana.

Glucose is a major source of energy and carbon in promastigotes of Leishmania mexicana, and its uptake is mediated by three glucose transporters whose genes are encoded within a single cluster. A null mutant in which the glucose transporter gene cluster was deleted by homologous gene replacement was generated previously and shown to grow more slowly than wild type promastigotes but not to be viable as amastigotes in primary tissue culture macrophages or in axenic culture. Further phenotypic characterization demonstrates that the null mutant is unable to import glucose, mannose, fructose, or galactose and that each of the three glucose transporter isoforms, LmGT1, LmGT2, and LmGT3, is capable of transporting each of these hexoses. Complementation of the null mutant with each isoform is able to restore growth in each of the four hexoses to wild type levels. Null mutant promastigotes are reduced in size to about 2/3 the volume of wild type parasites. In addition, the null mutants are significantly more sensitive to oxidative stress than their wild type counterparts. These results underscore the importance of glucose transporters in the parasite life cycle and suggest reasons for their non-viability in the disease-causing amastigote stage.

Directed antigen delivery as a vaccine strategy for an intracellular bacterial pathogen.

We have developed a vaccine strategy for generating an attenuated strain of an intracellular bacterial pathogen that, after uptake by professional antigen-presenting cells, does not replicate intracellularly and is readily killed. However, after degradation of the vaccine strain within the phagolysosome, target antigens are released into the cytosol for endogenous processing and presentation for stimulation of CD8(+) effector T cells. Applying this strategy to the model intracellular pathogen Listeria monocytogenes, we show that an intracellular replication-deficient vaccine strain is cleared rapidly in normal and immunocompromised animals, yet antigen-specific CD8(+) effector T cells are stimulated after immunization. Furthermore, animals immunized with the intracellular replication-deficient vaccine strain are resistant to lethal challenge with a virulent WT strain of L. monocytogenes. These studies suggest a general strategy for developing safe and effective, attenuated intracellular replication-deficient vaccine strains for stimulation of protective immune responses against intracellular bacterial pathogens.

Killed but metabolically active microbes: a new vaccine paradigm for eliciting effector T-cell responses and protective immunity.

We developed a new class of vaccines, based on killed but metabolically active (KBMA) bacteria, that simultaneously takes advantage of the potency of live vaccines and the safety of killed vaccines. We removed genes required for nucleotide excision repair (uvrAB), rendering microbial-based vaccines exquisitely sensitive to photochemical inactivation with psoralen and long-wavelength ultraviolet light. Colony formation of the nucleotide excision repair mutants was blocked by infrequent, randomly distributed psoralen crosslinks, but the bacterial population was able to express its genes, synthesize and secrete proteins. Using the intracellular pathogen Listeria monocytogenes as a model platform, recombinant psoralen-inactivated Lm DeltauvrAB vaccines induced potent CD4(+) and CD8(+) T-cell responses and protected mice against virus challenge in an infectious disease model and provided therapeutic benefit in a mouse cancer model. Microbial KBMA vaccines used either as a recombinant vaccine platform or as a modified form of the pathogen itself may have broad use for the treatment of infectious disease and cancer.

The role of protein kinase A anchoring via the RII alpha regulatory subunit in the murine immune system.

Intracellular cAMP may inhibit T cell activation and proliferation via activation of the cAMP-dependent protein kinase, PKA. PKA signaling is maintained through interactions of the regulatory subunit with A-kinase anchoring proteins (AKAPs). We demonstrated that T cells contain AKAPs and now ask whether PKA anchoring to AKAPs via the RIIalpha regulatory subunit is necessary for cAMP-mediated inhibition of T cell activation. We studied the immune systems of mice lacking the RIIalpha regulatory subunit of PKA (-/-) and the ability of cells isolated from these mice to respond to cAMP. Dissection of spleen and thymus from wild-type (WT) and -/- mice, single cell suspensions generated from these organs, and flow cytometry analysis illustrate that the gross morphology, cell numbers, and cell populations in the spleen and thymus of the -/- mice are similar to WT controls. In vitro, splenocytes from -/- mice respond to anti-CD3/anti-CD28 and PMA/ionomycin stimulation and produce IL-2 similar to WT. Cytokine analysis revealed no significant difference in Th1 or Th2 differentiation. Finally, equivalent frequencies of CD8(+) IFN-gamma producing effector cells were stimulated upon infection of WT or -/- mice with Listeria monocytogenes. These data represent the first study of the role of RIIalpha in the immune system in vivo and provide evidence that T cell development, homeostasis, and the generation of a cell-mediated immune response are not altered in the RIIalpha -/- mice, suggesting either that RIIalpha is not required for normal immune function or that other proteins are able to compensate for RIIalpha function.

The development of functional CD8 T cell memory after Listeria monocytogenes infection 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.

Listeria monocytogenes regulates flagellar motility gene expression through MogR, a transcriptional repressor required for virulence.

Previous studies have shown that Listeria monocytogenes flagellar motility genes, including flaA, encoding flagellin, are transcriptionally down-regulated at 37 degrees C. For some L. monocytogenes strains, temperature-dependent motility gene expression is less stringent. By using flaA-lacZ transcriptional fusions, we identified regions upstream of the -35/-10 promoter elements that are necessary for temperature-dependent expression of flaA in L. monocytogenes strain EGDe. Whereas the sequence of the flaA promoter region was identical in L. monocytogenes strain 10403S, transcriptional activity was only partially down-regulated at 37 degrees C in 10403S. This finding suggested that a transacting regulatory protein with differential expression or activity in EGDe might be involved in temperature-dependent transcription of flaA. Indeed, a protein factor capable of specifically binding to the flaA promoter region was identified in cytoplasmic extracts of EGDe by using affinity purification and MS. Deletion of the factor-encoding gene (lmo0674) resulted in loss of temperature-dependent flaA expression and an increase in flaA promoter activity. Expression of other motility genes was also deregulated in the lmo0674 deletion. We have designated lmo0674 as mogR, indicating its role as a motility gene repressor. In tissue culture models, MogR repression of flaA during intracellular infection was independent of temperature and a deletion of mogR reduced the capacity for cell-to-cell spread. During in vivo infection, a deletion of mogR resulted in a 250-fold decrease in virulence. These studies indicate that regulation of flagellar motility gene expression and/or other genes controlled by MogR is required for full virulence of L. monocytogenes.

Listeria monocytogenes as a vaccine vector: virulence attenuation or existing antivector immunity does not diminish therapeutic efficacy.

The bacterium L. monocytogenes is a proposed vaccine carrier based upon the observation that this pathogen replicates within the intracytoplasmic environment facilitating delivery of Ag to the endogenous Ag processing and presentation pathway with subsequent stimulation of peptide specific MHC class I-restricted CD8(+) effector cells. In this report, we evaluate virulence-attenuated strains of Listeria monocytogenes as vaccine vectors and examine whether existing antivector (antilisterial) immunity limits or alters its efficacy as a therapeutic cancer vaccine. Following immunization with virulence-attenuated mutants, we found that the effectiveness of L. monocytogenes as a recombinant cancer vaccine remains intact. In addition, we found that antibiotic treatment initiated 24 or 36 h following therapeutic immunization with recombinant L. monocytogenes allows full development of the antitumor response. We also demonstrate that the vaccine vector potential of L. monocytogenes is not limited in animals with existing antilisterial immunity. For these latter studies, mice previously immunized with wild-type L. monocytogenes were infused with melanoma cells and then 5 days later challenged with recombinant tumor Ag expressing L. monocytogenes. Collectively, these results add additional support for the use of L. monocytogenes as a vaccine vector and underscore its potential to be used repeatedly for stimulation of recall responses concomitant with primary cell-mediated responses to newly delivered heterologous tumor-associated epitopes.

Evidence implicating the 5' untranslated region of Listeria monocytogenes actA in the regulation of bacterial actin-based motility.

The ActA protein of Listeria monocytogenes is a major virulence factor, essential for the recruitment and polymerization of host actin filaments that lead to intracellular motility and cell-to-cell spread of bacteria within the infected host. The expression of actA is tightly regulated and is strongly induced only when L. monocytogenes is within the host cytosol. Intracellular induction of actA expression is mediated through a single promoter element that directs the expression of a messenger RNA with a long (150 bp) 5' untranslated region (UTR). Deletion of the actA+3 to +130 upstream region was found to result in bacterial mutants that were no longer capable of intracellular actin recruitment or cell-to-cell spread, thus indicating that this region is important for actA expression. L. monocytogenes strains that contained smaller deletions (21-23 bp) within the actA upstream region demonstrated a range of actA expression levels that coincided with the amount of bacterial cell-to-cell spread observed within infected monolayers. A correlation appeared to exist between levels of actA expression and the ability of L. monocytogenes to transition from uniform actin accumulation surrounding individual bacteria (actin clouds) to directional assembly and the formation of actin tails. Bacterial mutants containing deletions that most significantly altered the predicted secondary structure of the actA mRNA 5' UTR had the largest reductions in actA expression. These results suggest that the actA 5' UTR is required for maximal ActA synthesis and that a threshold level of ActA synthesis must be achieved to promote the transition from bacteria-associated actin clouds to directional actin assembly and movement.

Isolation of Listeria monocytogenes mutants with high-level in vitro expression of host cytosol-induced gene products.

The facultative intracellular bacterial pathogen Listeria monocytogenes dramatically increases the expression of several key virulence factors upon entry into the host cell cytosol. actA, the protein product of which is required for cell-to-cell spread of the bacterium, is expressed at low to undetectable levels in vitro and increases in expression more than 200-fold after L. monocytogenes escape from the phagosome. To identify bacterial factors that participate in the intracellular induction of actA expression, L. monocytogenes mutants expressing high levels of actA during in vitro growth were selected after chemical mutagenesis. The resulting mutant isolates displayed a wide range of actA expression levels, and many were less sensitive to environmental signals that normally mediate repression of virulence gene expression. Several isolates contained mutations affecting actA gene expression that mapped at least 40 kb outside the PrfA regulon, supporting the existence of additional regulatory factors that contribute to virulence gene expression. Two actA in vitro expression mutants contained novel mutations within PrfA, a key regulator of L. monocytogenes virulence gene expression. PrfA E77K and PrfA G155S mutations resulted in high-level expression of PrfA-dependent genes, increased bacterial invasion of epithelial cells and increased virulence in mice. Both prfA mutant strains were significantly less motile than wild-type L. monocytogenes. These results suggest that, although constitutive activation of PrfA and PrfA-dependent gene expression may enhance L. monocytogenes virulence, it may conversely hamper the bacterium's ability to compete in environments outside host cells.

Intracellular induction of Listeria monocytogenes actA expression.

Following entry into the host cytosol, the bacterial pathogen Listeria monocytogenes dramatically increases the expression of several key virulence factors. The expression of actA, whose protein product is required for L. monocytogenes actin-based intracellular motility, is increased by more than 200-fold in cytosolic bacteria in comparison to broth-grown cultures. Two distinct promoter elements have been reported to regulate actA expression. One promoter is located immediately upstream of actA coding sequences, while the second promoter is contributed by the upstream mpl gene via the generation of an mpl-actA-plcB transcript. A series of L. monocytogenes mutants were constructed to define the contributions of individual promoter elements to actA expression. The intracellular induction of actA expression was found to be dependent upon the actA proximal promoter; the mpl promoter appeared to contribute to the extracellular induction of actA but did not affect intracellular levels of expression. The actA promoter is dependent upon a regulatory factor known as PrfA for transcriptional activation; however, no increase in actA expression was detected following the introduction of a high-affinity PrfA binding site within the actA promoter. The presence of a mutationally activated form of PrfA, known as PrfA*, increased overall actA expression in broth-grown cultures of both wild-type and actA promoter mutant strains, but the levels of induction observed were still approximately 50-fold lower than those observed for intracellularly grown L. monocytogenes. Collectively, these results indicate that the dramatic induction of actA expression that occurs in the host cell cytosol is mediated through a single promoter element. Furthermore, intracellular induction of actA appears to require additional steps or factors beyond those necessary for the activation and binding of PrfA to the actA promoter.

Frequencies of HCV-specific effector CD4+ T cells by flow cytometry: correlation with clinical disease stages.

Hepatitis C virus (HCV) is the leading cause of chronic hepatitis, affecting approximately 2% of the world's population. The immune mechanisms responsible for the highly variable natural history in a given individual are unknown. We used a multiparameter flow cytometric technique to functionally and phenotypically characterize HCV-specific effector T cells in the peripheral blood of 32 individuals with different stages of hepatitis C disease (resolved, mild chronic, advanced chronic) and normal controls. We found the highest frequencies of virus-specific effector cells with an activated memory phenotype (CD45RO+CD69+) in subjects who had resolved HCV infection, either spontaneously or with antiviral therapy. Effector cells from patients with resolved infection produced Th1 type cytokines following stimulation with nonstructural antigens (NS3 and NS4), whereas effector cells from chronically infected patients produced Th1 type cytokines predominantly following stimulation with the HCV core antigen. Stimulation with superantigen staphylococcal enterotoxin (SEB) induced the same levels of cytokine production in the different patient groups. Among the HCV-seropositive patients, viral load inversely correlated with the Th1 effector cell response to NS3. Interleukin (IL)-4 was produced only in response to the control antigens, but not in response to the HCV recombinant proteins. Taken together, these findings suggest that a vigorous HCV-specific CD4+ Th1 response, particularly against the nonstructural proteins of the virus, may be associated with viral clearance and protection from disease progression. Prospective studies using this new flow cytometric assay will be required to determine whether antiviral therapy modifies the frequency, specificity, and function of these virus-specific effector cells.