Presentation of Listeria monocytogenes antigens by major histocompatibility complex class I molecules to CD8 cytotoxic T lymphocytes independent of listeriolysin secretion and virulence

Development of a recombinant vaccine against foot and mouth disease utilizing mutant attenuated Listeria ivanovii strain as a live vector

The drawbacks of conventional inactivated Foot and Mouth Disease (FMD) vaccine, such as escaping of the virus during manufacture processes prompted researchers to explore novel types of vaccine to overcome these disadvantages. Listeria ivanovii (LI) is an intracellular microorganism that possesses immune-stimulatory properties, making it appropriate for use as a live bacterial vaccine vector. The Foot and mouth disease virus (FMDV) VP1 protein is the most immunogenic part of FMDV capsid, it has most of the antigenic sites for viral neutralization. The expression of antigen gene cassette in vitro was confirmed by Western blot analysis. Mice were able to eliminate LI△actAplcB-vp1 from the liver and spleen within few days revealed a safety of the candidate vaccine. Two doses of LI△actAplcB-vp1 with 14 days of interval were injected into mice. High levels of specific IgG antibodies and CD8⁺ and CD4⁺ T cells secreted cytokines including IFN-γ, TNF-α and IL-2 against FMDV-VP1 were achieved. Based on the obtained results, LI△actAplcB-vp1 candidate vaccine utilizing Listeria ivanovii as a live vector-based vaccine could enhance a specific cellular and humoral immune responses against the inserted FMDV-vp1 heterologous genes. LI△actAplcB-vp1 candidate vaccine could be a modern tool to overcome the disadvantages of the traditional inactivated FMD vaccine.

Expression of the VP1 protein of FMDV integrated chromosomally with mutant Listeria monocytogenes strain induced both humoral and cellular immune responses

Live vector-based vaccine is a modern approach to overcome the drawbacks of inactivated foot-and-mouth disease (FMD) vaccines such as improper inactivation during manufacture. Listeria monocytogenes (LM), an intracellular microorganism with immune-stimulatory properties, is appropriate to be utilized as a live bacterial vaccine vector. FMDV-VP1 protein has the capability to induce both cellular and humoral immune responses since it is considered the most immunogenic part of FMDV capsid and has the most of antigenic sites for viral neutralization. The codon-optimized vp1 gene was ligated to the integrative pCW702 plasmid to construct the target cassette. The antigen cassette was integrated successfully into the chromosome of mutant LM strain via homologous recombination for more stability to generate a candidate vaccine strain LM△actAplcB-vp1. Safety evaluation of recombinant LM△actAplcB-vp1 revealed it could be eliminated from the internal organs within 3 days as a safe candidate vaccine. Mice groups were immunized I.V. twice with the recombinant LM△actAplcB-vp1 at an interval of 2 weeks. Antigen-specific IgG antibodies and the level of CD4+- and CD8+-specific secreted cytokines were estimated to evaluate the immunogenicity of the candidate vaccine. The rapid onset immune response was detected, strong IgG humoral immune response within 14 days post immunization and augmented again after the booster dose. Cellular immunity data after 9 days post the prime dose indicated elevation in CD4+ and CD8+ secreted cytokine level with another elevation after the booster dose. This is the first report to explain the ability of attenuated mutant LM to be a promising live vector for FMDV vaccine.

Multiple effector mechanisms induced by recombinant Listeria monocytogenes anticancer immunotherapeutics

Listeria monocytogenes is a facultative intracellular gram-positive bacterium that naturally infects professional antigen presenting cells (APC) to target antigens to both class I and class II antigen processing pathways. This infection process results in the stimulation of strong innate and adaptive immune responses, which make it an ideal candidate for a vaccine vector to deliver heterologous antigens. This ability of L. monocytogenes has been exploited by several researchers over the past decade to specifically deliver tumor-associated antigens that are poorly immunogenic such as self-antigens. This review describes the preclinical studies that have elucidated the multiple immune responses elicited by this bacterium that direct its ability to influence tumor growth.

Antigen co-encapsulated with adjuvants efficiently drive protective T cell immunity

Compared to "live" vaccines, the immunogenicity of "subunit" vaccines based on recombinant antigen (Ag) is poor, presumably because exogenous Ag fails to effectively access the endosomal Ag-processing pathways of Ag-presenting cells (APC). To overcome this limitation, we exploited biodegradable poly(lactic-co-glycolic) microspheres (MP) co-entrapping Ag and Toll-like receptor (TLR) 9 or 7 ligands as an endosomal delivery device. In vitro, microspheres were rapidly phagocytosed by APC and translocated into phago-endosomal compartments, followed by degradation of the Ag and concurrent activation of endosomal TLR. As a consequence, full maturation of and cytokine secretion by APC as well as Ag-cross-presentation ensued. In vivo, "loaded" microspheres triggered clonal expansion of primary and secondary Ag-specific CD4 and CD8 T cells. The efficacy of CD8 T cell cross-priming was comparable to that of live vectors. The potency of T cell vaccination was demonstrated by protective and therapeutic interventions using infection- and tumor-model systems. These preclinical "subunit" vaccination data thus recommend MP as a generally applicable and powerful endosomal delivery device of exogenous Ag plus TLR-based adjuvants to vaccinate for protective and therapeutic CD4 and CD8 T cell immunity.

A role for the endoplasmic reticulum protein retrotranslocation machinery during crosspresentation by dendritic cells

Crosspresentation of exogenous antigens (Ags) to CD8(+) T cells by dendritic cells generally requires their entry into the cytosol. Here we show that both soluble and phagocytosed extracellular Ags accessed the cytosol via molecular components required for endoplasmic reticulum (ER)-associated degradation (ERAD). Exogenous Pseudomonas aeruginosa Exotoxin A, which inhibits protein translocation from the ER to the cytosol, abrogated crosspresentation. Exotoxin A also prevented the transporter associated with antigen processing (TAP) inhibitor, ICP47, from entering the cytosol and blocking TAP-mediated peptide transport. In an in vitro model of retrotranslocation, the AAA ATPase p97, an enzyme critical for ERAD, was the only cytosolic cofactor required for protein export from isolated phagosomes. Functional p97 was also required for crosspresentation but not conventional presentation. Thus, crosspresentation appears to result from an adaptation of the retrotranslocation mechanisms involved in the degradation of misfolded ER proteins.

Cellular mechanisms governing cross-presentation of exogenous antigens

The recent discovery of fusion of endoplasmic reticulum membrane with nascent phagosomes suggests that this peripheral compartment in macrophages and dendritic cells may serve as an organelle optimized for major histocompatibility complex (MHC) class I-restricted cross-presentation of exogenous antigens. The process allows intersection of the endosomal system with the endoplasmic reticulum, the classical site of MHC class I peptide loading, and may reconcile the seemingly conflicting evidence indicating both of these sites are crucial in cross-presentation. Here we discuss the potential mechanisms involved in loading exogenous antigens onto MHC class I molecules and the implications of this new evidence for the in vivo function of dendritic cells.

Bacterial and host factors involved in the major histocompatibility complex class Ib-restricted presentation of Salmonella Hsp 60: novel pathway

Previously, a peptide epitope derived from the Hsp 60 molecule of Salmonella that is presented by the major histocompatibility complex (MHC) class Ib molecule Qa-1 to CD8(+) cytotoxic T cells (CTLs) was described. In the present study we investigated the Salmonella-induced processing and presentation pathway for generating this Qa-1-restricted epitope. Live bacteria and, to a lesser extent, opsonized heat-killed bacteria are able to sensitize target cells for lysis by Salmonella-specific CTL. In contrast, heat-killed bacteria cannot sensitize target cells. Presentation of the Hsp 60 epitope appears independent of bacterial internalization, because cytochalasin D does not affect presentation. Moreover, Salmonella strains defective in the InvA or InvE operon, two critical components of the type III secretion pathway, are as efficient as wild-type Salmonella enterica serovar Typhimurium in sensitizing infected targets to lysis. Collectively, these results suggest the existence of a novel antigen-processing pathway in which exogenous antigens gain access to the cytosolic MHC class I processing machinery. Considering the abundant nature of bacterial Hsp 60 and the upregulation of this protein after Salmonella infection of eukaryotic cells, this mode of antigen presentation may be particularly relevant to understanding the host defense mechanisms against gram-negative bacteria.

T cell immunity to brucellosis

Brucellosis is an ancient disease of animals and man that still threatens the health and prosperity of many, primarily in the third world, who depend on animal agriculture for their livelihood. Further, its pathogenicity and the facts that it is zoonotic is effectively eradicated from many Western nations make it a dangerous bioterrorism threat. Targeted human vaccination may reduce the various threats brucellosis poses. Significant effort has been expended toward this goal and many candidate vaccines exist. However, the ideal vaccine would be a subunit vaccine that specifically targets only the critical aspects of the immune response necessary to induce immunity. Much about the immune response, in particular the T cell response, remains to be discovered in order to accomplish that goal. In this review we focus on T cell responses to brucellosis with particular attention to the specific roles of T cell subtypes. We also point out areas of research on T cell responses that may allow exploitation of cutting edge vaccine technologies for the next generation vaccine for brucellosis.

Bovine T lymphocyte responses to Brucella abortus

The long-held paradigm of T lymphocyte-mediated activation of mononuclear phagocytes (Mø) as the major mechanism of protection against facultative intracellular pathogens such as Brucella has been modified to include killing of infected Mø by various subsets of T lymphocytes. Remnants of killed infected cells are phagocytosed by immunologically-activated Mø, which are much more efficient at killing such pathogens. Most of the detailed information regarding immunity in general and that of brucellosis specifically has been obtained using murine infection models rather than in cattle. However, there has been considerable definition of cellular phenotypes, cytokines and functional characteristics of T lymphocytes in cattle over the last decade. This was mainly due to development of monoclonal antibodies against cell surface markers and application of molecular cloning and polymerase chain reaction (PCR) for isolation, characterization and detection of genes encoding bovine cytokines. This review discusses cellular and molecular immunity in bovine brucellosis as pertains to T lymphocyte interactions with the Mø. Although current knowledge directly obtained from brucellosis immunity studies in the bovine host is limited and incomplete, the many parallels between the bovine and murine immune systems allow for some extrapolation in the description of bovine host defense mechanisms. Direct information from studies with immunized cattle supports the concepts of coordinate activation of uninfected Mø and killing of Brucella-infected Mø by antigen-specific T lymphocytes as major mechanisms of host defense in bovine brucellosis. There also appears to be a bias in the T lymphocyte compartment towards recognition of particular bacterial stress proteins following immunization with live Brucella vaccines.

The involvement of class Ib molecules in the host response to infection with Salmonella and its relevance to autoimmunity

Class I molecules with limited polymorphism have been implicated in the host response to infectious agents. Following infection with Salmonella typhimurium, mice develop a CD8+ CTL response that specifically recognizes bacteria infected cells. An immunodominant component of the CTL response recognizes a peptide epitope derived from the Salmonella GroEL molecule that is presented by the non-polymorphic MHC class Ib molecule Qa-1. T cells recognizing the bacterial peptide also cross-recognize a homologous peptide from the mammalian hsp60 molecule. Since Qa-1 has a functional equivalent in humans, this observation may be relevant not only to the host response involved in clearing infection but also in understanding the link between infection with Gram-negative pathogens and autoimmune disease.

The effect of an anti-HLA-B27 immune response on CTL recognition of Chlamydia

The interplay between triggering bacteria and HLA-B27 in the pathogenesis of the spondyloarthropathies remains one of the most active areas of investigation in the rheumatic diseases. This has proved difficult to study systematically in the clinical setting, and in this study we utilized a rat model to address the influence that B27-related immunity may have on the process of generating anti-Chlamydia immunity. When splenocytes from HLA-B27 DNA-immunized Lewis (LEW) animals received restimulation in vitro with Chlamydia-treated cells from B27-transgenic LEW rats, we observed that in addition to the expected CTL recognition of HLA-B27, there was also anti-Chlamydia CTL killing of Chlamydia-sensitized syngeneic fibroblast targets. This was not seen when responding cells in vitro were naive LEW splenocytes. To confirm the existence of CTLs recognizing both HLA-B27 and Chlamydia, LEW rats were immunized with B27-transgenic LEW cells, instead of the B27 DNA construct. Splenocytes from the immune rats were restimulated in vitro with Chlamydia-treated B27-transgenic LEW cells. In this instance, the CTLs retained the allele-specific recognition of HLA-B27, as well as recognition of Chlamydia-sensitized syngeneic fibroblasts. Thus, if there is prior expansion of an immune response against HLA-B27, then the resulting splenocytes demonstrate a reduced threshold for generating a primary anti-Chlamydia CTL response. These studies implicate a dynamic interrelationship between recognition of HLA-B27 and Chlamydia trachomatis. The results may have implications for deciphering the cellular basis of Chlamydia-induced reactive arthritis.

Protective CTL response is induced in the absence of CD4+ T cells and IFN-gamma by gene gun DNA vaccination with a minigene encoding a CTL epitope of Listeria monocytogenes

Our work was undertaken to learn the mechanism of induction of protective cytotoxic T lymphocytes (CTL) by gene gun DNA vaccination with p91m encoding an H-2Kd-restricted T cell epitope of listeriolysin O (LLO). Vaccination with p91m induced vigorous antigen-specific CD8+ CTL that produce IFN-gamma and was able to confer partial protection against listerial challenge. However, the p91m-induced protective immunity was revealed to be independent of the IFN-gamma and CD4+ T cell help. The CTL induction is also suggested to require neither adjuvant activity of the plasmid used nor IFN-gamma. The data may be feasible for the design of CTL inducing vaccines in various immunodeficiencies.

Antigen Secreted from Noncytosolic Listeria monocytogenes Is Processed by the Classical MHC Class I Processing Pathway

Intracellular bacteria can reside in a vacuolar compartment, or they can escape the vacuole and become free living in the cytoplasm. The presentation of Ag by class I MHC molecules has been defined primarily for Ag present in the cytoplasm. It was therefore thought that Ags from bacteria that remain in a vacuole would not be presented by MHC class I molecules. Although some studies have provided data to support this idea, it is not necessarily true for all intracellular bacteria. For example, we have previously demonstrated that an epitope from the p60 protein secreted by LLO− Listeria monocytogenes, which does not reside in the cytoplasm, can be presented by MHC class I molecules to a T cell clone specific for the epitope, p60217–225. We have further examined the route by which Ag secreted by LLO− L. monocytogenes is presented by MHC class I molecules. Using pharmacological inhibitors, we demonstrate that MHC class I presentation of the p60 epitope derived from by LLO− L. monocytogenes requires phagolysosome fusion and processing by the proteasome. Lysosomal cathepsins, however, are not required for processing of the p60 epitope. Similarly, processing of the AttM epitope, secreted by LLO− L. monocytogenes and presented by H2-M3, also requires phagolysosome fusion and cleavage by the proteasome. Thus, p60 and AttM secreted by LLO−  L. monocytogenes are processed via the classical class I pathway for presentation by MHC class I molecules.

T Cell Responses to Gram-Negative Intracellular Bacterial Pathogens: A Role for CD8+ T Cells in Immunity to Salmonella Infection and the Involvement of MHC Class Ib Molecules

Despite being a major group of intracellular pathogens, the role of class I-restricted T cells in the clearance of Gram-negative bacteria is not resolved. Using a murine typhoid model, a role for class I-restricted T cells in the immune response to the Gram-negative pathogen Salmonella typhimurium is revealed. Class I-deficient β2-microglobulin−/− mice show increased susceptibility to infection with S. typhimurium. Following infection, CD8+ CTLs specific for Salmonella-infected targets can be readily detected. The Salmonella-specific CTLs recognize infected H-2-mismatched targets, suggesting the involvement of shared class Ib molecules. Studies using transfectants expressing defined class Ia and class Ib molecules indicate the involvement of the class Ib molecule, Qa-1. Ab-blocking studies and the measurement of bacteria-specific CTL frequencies identified Qa-1 as a dominant restricting element. The Qa-1-restricted CTL recognition depends on TAP and proteasome functions. Surprisingly, Qa-1-restricted CTLs recognized cells infected with other closely related Gram-negative bacteria. Taken together, these observations indicate that Salmonella-specific CTLs recognize a cross-reactive epitope presented by Qa-1 molecules and, as such, may be novel targets for vaccine development.

Enhancement of the Listeria monocytogenes p60-Specific CD4 and CD8 T Cell Memory by Nonpathogenic Listeria innocua

The contact of T cells to cross-reactive antigenic determinants expressed by nonpathogenic environmental micro-organisms may contribute to the induction or maintenance of T cell memory. This hypothesis was evaluated in the model of murine Listeria monocytogenes infection. The influence of nonpathogenic L. innocua on the L. monocytogenes p60-specific T cell response was analyzed. We show that some CD4 T cell clones raised against purified p60 from L. monocytogenes cross-react with p60 purified from L. innocua. The L. monocytogenes p60-specific CD4 T cell clone 1A recognized the corresponding L. innocua p60 peptide QAAKPAPAPSTN, which differs only in the first amino acid residue. In vitro experiments revealed that after L. monocytogenes infection of APCs, MHC class I-restricted presentation of p60 occurs, while MHC class II-restricted p60 presentation is inhibited. L. innocua-infected cells presented p60 more weakly but equally well in the context of both MHC class I and MHC class II. In contrast to these in vitro experiments the infection of mice with L. monocytogenes induced a strong p60-specific CD4 and CD8 T cell response, while L. innocua infection failed to induce p60-specific T cells. L. innocua booster infection, however, expanded p60-specific memory T cells induced by previous L. monocytogenes infection. In conclusion, these findings suggest that infection with a frequently occurring environmental bacterium such as L. innocua, which is nonpathogenic and not adapted to intracellular replication, can contribute to the maintenance of memory T cells specific for a related intracellular pathogen.

Kinetics of interferon-gamma production and its comparison with anti-listeriolysin O detection in experimental bovine listeriosis

The kinetics of the production of interferon gamma (IFN-gamma) in whole blood culture and its comparison with anti-listeriolysin O (ALLO) detection by ELISA were studied during oral infection of calves with Listeria monocytogenes. Culture filtrate antigen (CFA), listeriolysin O (LLO), and sonicated antigen (SA) were used to prime the peripheral blood mononuclear cells (PBMCs) and the plasma from orally infected calves. IFN-gamma and ALLO appeared as early as day 7 of an oral infection. IFN-gamma was detected earlier with LLO than with SA. The Max50 interleukin (IL-2) activity and IFN-gamma estimated in the culture supernatant from PBMCs primed in vitro with different antigens of L. monocytogenes revealed high induction of IL-2 and IFN-gamma by CFA, LLO and live antigen. IFN-gamma assay and ALLO detection were used for testing cases of repeat breeding in dairy cattle. It appeared that detection of IFN-gamma employing LLO can be used to diagnose listerial infections.

Suppression of major histocompatibility complex class I and class II gene expression in Listeria monocytogenes-infected murine macrophages

Macrophage cells play a central role during infection with Listeria monocytogenes by both providing a major habitat for bacterial multiplication and presenting bacterial antigens to the immune system. In this study, we investigated the influence of L. monocytogenes infection on the expression of MHC class I and class II genes in two murine macrophage cell lines. Steady-state levels of I-Abeta chain mRNA were decreased in both resting J774A.1 and P388D1 macrophages infected with L. monocytogenes whereas reduction of H-2K mRNA was only observed in P388D1 cells. In addition, L. monocytogenes suppressed induction of MHC class I and class II mRNAs in response to gamma-interferon as well as the maintenance of the induced state in activated P388D1 macrophages. Exposure to the non-pathogenic species L. innocua or a deletion mutant of L. monocytogenes, which lacks the lecithinase operon, did not cause a reduction in H-2K and I-Abeta mRNA levels nor suppress expression of Ia antigens. Inhibition of MHC gene expression may represent an important part of the cross-talk between L. monocytogenes and the macrophage that probably influences the efficiency of a T cell-mediated immune response and thus the outcome of a listerial infection.

Activation of CD8 T cells with specificity for mycobacterial heat shock protein 60 in Mycobacterium bovis bacillus Calmette-Guérin-vaccinated mice

Heat shock protein 60 (hsp60)-specific CD8 T cells lysed Mycobacterium bovis BCG-infected macrophages in vitro and adoptively transferred protection against mycobacterial infection. Moreover, CD8 T cells with this hsp60 specificity were activated in vivo by BCG vaccination. Our studies suggest there is participation of hsp60-specific CD8 T cells in BCG-induced immunity.

Interleukin-4 and listeriosis

Experimental infection of mice with Listeria monocytogenes (L. monocytogenes) has served as an appropriate model for analyzing Th1-cell-driven immune responses. Generally, Th2 responses are absent and IL-4 is not detectable. Here, we describe experimental settings under which IL-4 is detectable in listeriosis. Our data suggest that IL-4 is rapidly produced after infection. This prompt IL-4 burst seems to stimulate chemokine responses and, therefore, may participate in the regulation of the early antilisterial host response. Soon thereafter, IL-4 production wanes. At least partially this seems to be caused by downregulation of IL-4-producing CD4+ NK1+ TCR alpha beta int lymphocytes by IL-12. In the absence of IFN-gamma responsiveness, IL-4 production is demonstrable during acquired immunity against L monocytogenes, and this elevated IL-4 production apparently contributes to disease exacerbation. In conclusion, the data are consistent with a detrimental role of IL-4 in listeriosis and active control of IL-4 synthesis by the antilisterial immune response. The rapid, but transient, IL-4 burst in listeriosis probably contributes to host defense without impairing development of the acquired T-cell response because of its shortness.

Alternative antigen processing pathways in anti-infective immunity

Proteinaceous and nonproteinaceous antigens from exogenous microorganisms can be processed by the host for MHC class I restricted presentation to T cells. Macrophages, B cells, mast cells and dendritic cells are antigen-presenting cells that process such exogenous antigens through multiple pathways before MHC-restricted epitope presentation. New conceptual frameworks are emerging regarding the processing and presentation to T cells of peptide or nonpeptide epitopes from bacteria in the context of conventional MHC class I molecules, nonconventional MHC class I molecules, or CD1 molecules. Animal experiments have demonstrated that these pathways are of central importance for generating protective antibacterial T cell responses. These findings form the basis for new vaccine designs that specifically target MHC class I restricted T cell reactivity.

CD8+ T cells in intracellular bacterial infections of mice

In the normal course of an immune response, both CD4+ and CD8+ T cells respond to each of the bacterial pathogens we have discussed and both responses may be required for the most potent immunity to infection. In this discussion, we have focused on the ability of these organisms to prime CD8+ T-cell responses in vivo and the ability of CD8+ T cells as sole mediators of acquired immunity, to protect against infection. It is clear that the vacuolar location of bacterial pathogens such as Salmonella or Mycobacteria does not prevent in vivo priming of CD8+ T-cell responses to these pathogens. However, vacuolar localization may affect the potency of CD8+ T-cell responses under experimental conditions that assess the capacity of CD8+ T cells as the sole mediators of acquired immunity. In the case of cytoplasmic L. monocytogenes, clear evidence exists that antigen-specific CD8+ T cells, in the absence of immune CD4+ T cells, can provide substantial acquired immunity to naive mice. Similar clear experimental results with Salmonella and Mycobacteria are lacking. Such results would provide stronger support for vaccines that elicit CD8+ T-cell responses to these vacuolar pathogens. Although our discussion has focused on only three specific organisms, we suggest that detection of an in vivo CD8+ T-cell response to a bacterial antigen does not ensure that the response will be protective against infection in a vaccine setting. In the case of Salmonella and Mycobacteria, this issue remains unresolved.

Peptide epitopes from noncytosolic Listeria monocytogenes can be presented by major histocompatibility complex class I molecules

Listeria monocytogenes is an intracellular pathogen which escapes the phagosome and resides in the cytosol of the host cell. Using Listeria innocua and a mutant strain of L. monocytogenes (listeriolysin O negative), which do not enter the cytosol of the host cell, we demonstrate class I presentation of an epitope of p60, a protein secreted by L. monocytogenes, to a class I-restricted CD8+ cytotoxic T lymphocyte clone.

A new foreign policy: MHC class I molecules monitor the outside world

Although most cells exclusively use their major histocompatibility complex (MHC) class I molecules to present peptides from endogenous proteins, phagocytes also use them to present exogenous antigens. Here, Kenneth Rock describes how this novel antigen-presenting pathway may play an important role in immune surveillance for intracellular bacteria or parasites, as well as for viral infections and tumors affecting somatic tissues.

Superior efficacy of secreted over somatic antigen display in recombinant Salmonella vaccine induced protection against listeriosis

Vaccination provides the most potent measure against infectious disease, and recombinant (r) viable vaccines expressing defined pathogen-derived antigens represent powerful candidates for future vaccination strategies. In a new approach we constructed r-aroA- Salmonella typhimurium displaying p60 or listeriolysin (Hly) antigen of Listeria monocytogenes in secreted or somatic form in the host cell. Vaccination of mice with r-aroA- S. typhimurium induced protection against the intracellular pathogen L. monocytogenes only with secreted and not with somatic antigen. Secreted Hly was slightly more potent in inducing protective immunity than secreted p60. Both r-aroA- S. typhimurium secreting p60 in the endosome and r-aroA- S. typhimurium secreting Hly in the cytosol induced protective CD4+ and CD8+ T-cells suggesting CD8+ T-cell stimulation independent from intracellular residence of r-aroA- S. typhimurium carriers. Hence, not only the type of antigen but also its display by the r-carrier within the host cell critically influences vaccine efficacy.

Human gamma/delta T-cell response to Listeria monocytogenes protein components in vitro

Listeria monocytogenes is a facultative intracellular pathogen that replicates inside mononuclear phagocytes and induces specific cellular immunity. Listeriosis encompasses many clinical syndromes and meningitis is the most frequent clinical manifestation. Human alpha/beta and gamma/delta T cells have been shown to respond to L. monocytogenes antigens and to play an important role in resistance against listerial infection. We investigated the nature of listerial ligands and the influence of the major virulence factor, listeriolysin (hly), on the stimulation of human gamma/delta T cells from healthy individuals. We found that a listerial somatic protein ligand, which is sensitive to proteinase treatment, stimulated gamma/delta T cells in vitro; the majority of Listeria-responsive gamma/delta T cells expressed V gamma 9V delta 2 T-cell receptor chains and human leucocyte antigen-DR molecules; gamma/delta T-cell responses to hly+ and hly- Listeria strains were comparable; L. monocytogenes strains of different virulence stimulated gamma/delta T cells equally. Thus, protein components of L. monocytogenes unrelated to virulence activate human gamma/delta T cells in vitro.

Stimulation of protective CD8+ T lymphocytes by vaccination with nonliving bacteria

Infectious diseases caused by intracellular microbes are responsible for major health problems, and satisfactory control will ultimately depend on efficient vaccination strategies. The general assumption is that activation of protective immune responses against intracellular microbes dominated by CD8+ T cells are achieved only by live vaccines. In contrast, we here demonstrate stimulation of protective immunity in mice against the intracellular pathogen Listeria monocytogenes by vaccination with heat-killed listeriae. Vaccine-induced immunity comprised cytolytic and interferon gamma-producing CD8+ T lymphocytes. CD8+ T cells from vaccinated donor mice transferred protection against listeriosis. Moreover, vaccination with heat-killed listeriae induced production in CD4+ T-cell-deficient, H2-A beta gene-disrupted mutant mice. We conclude that antigens from killed listeriae are introduced into the major histocompatibility complex class I pathway and thus are recognized by CD8+ T cells. The practicability of killed vaccines against human infectious diseases therefore should be reevaluated.

Class I MHC presentation of exogenous soluble antigen via macropinocytosis in bone marrow macrophages

Extracellular proteins are not generally presented on class I MHC molecules in vitro, yet many studies show that a pathway exists in vivo for the presentation of extracellular material on class I molecules to prime CD8+ T cell responses. Here, we provide morphological evidence that proteins taken up by macropinocytosis can gain access to the cytosol and therefore into the conventional class I MHC pathway. Class I presentation of soluble ovalbumin by mouse bone marrow macrophages was dramatically enhanced by MCSF or phorbol ester and blocked by amiloride, which stimulate and inhibit membrane ruffling and macropinocytosis, respectively. Brefeldin A, gelonin, and a peptide aldehyde inhibitor of proteasomal processing each blocked presentation of macropinocytosed antigen, demonstrating that unusual access to the conventional class I MHC pathway was occurring. This novel cell type-specific endocytic pathway may facilitate presentation of exogenous material on class I MHC molecules, allowing induction of CD8+ T cell responses to soluble proteins, tumor cell fragments, and some pathogens.

CD8+ type 1 CD44hi CD45 RBlo T lymphocytes control intracellular Brucella abortus infection as demonstrated in major histocompatibility complex class I- and class II-deficient mice

Genetically engineered mice with a targeted disruption in the beta 2-microglobulin (beta 2-m) gene or the H2-I-A beta chain (A beta) which lack functional CD8+ or CD4+ T cells, respectively, were used to assess the role of T cell subsets in Brucella abortus infection. Murine brucellosis was markedly exacerbated in beta 2-m-deficient mice (beta 2-m-/-) compared to A beta mutant (A beta-/-) or C57BL/6 mice, strongly indicating that optimal resistance to B. abortus requires CD8+ T cells. Splenocytes from Brucella-primed beta 2-m-/-, A beta-/- and C57BL/6 mice exhibited a type 1 cytokine profile marked by elevated IFN-gamma mRNA expression and protein production, and basal levels of IL-2 and IL-4 transcripts. B. abortus did not induce secretion of TGF-beta 1, but substantial IL-10 activity was detected in spleen cell supernatants from all mouse strains studied. CD8+ T cells from A beta-/- and C57BL/6 mice displayed a CD44hi CD45RBlo phenotype and a type 1 cytokine transcription profile featuring high levels of IFN-gamma mRNA. Additionally, we have shown the ability of C57BL/6 CD8+ CTL to kill Brucella-infected macrophages. This study illustrates the predominant role of MHC class I-restricted T cells in controlling B. abortus infection.

Immune response to Mycobacterium bovis bacille Calmette Guérin infection in major histocompatibility complex class I- and II-deficient knock-out mice: contribution of CD4 and CD8 T cells to acquired resistance

Knock-out mice with defined major histocompatibility complex (MHC) deficiencies were infected intravenously with Mycobacterium bovis bacille Calmette Guérin (M. bovis BCG) to assess the relative impact of MHC class I- and II-dependent immune responses. Heterozygous control mice were capable of controlling growth of M. bovis BCG, although infection progressed chronically, as assessed by determination of colony-forming units. Furthermore, infected controls developed granulomatous lesions at the site of mycobacterial growth and delayed-type hypersensitivity (DTH) reactions after challenge with purified protein derivative of tuberculin. In vitro, spleen cells from heterozygous control mice produced high concentrations of interferon-gamma (IFN-gamma) after restimulation with mycobacterial antigens. In contrast, the MHC class II-deficient A beta-/- mice, which are virtually devoid of functional CD4 T cells, succumbed to M. bovis BCG infection. Furthermore, A beta-/- mice lacked DTH reactions to tuberculin and only few minute picnotic lesions were formed in livers of infected mice. Finally, spleen cells from infected A beta-/- mice failed to produce measurable IFN-gamma concentrations after restimulation in vitro with various mycobacterial antigen preparations. The capacity of beta 2-microglobulin (beta 2m)-deficient mice, which are devoid of CD8 alpha/beta T cells, to inhibit growth of M. bovis BCG was only slightly affected at low inocula, although significantly higher colony-forming units were detected in spleens. These knock-out mice developed strong DTH responses to tuberculin and their spleen cells produced high levels of IFN-gamma once reactivated by mycobacterial antigens. Furthermore, in livers of infected beta 2m-deficient mice, extravascular infiltrates developed which were more diffuse than those in infected control littermates. Remarkably, the beta 2m-deficient mice were substantially more susceptible to higher inocula of M. bovis BCG than their control littermates. Our data formally prove the essential role of MHC class II-dependent immune mechanisms in all relevant aspects of immunity to M. bovis BCG. In addition, our findings emphasize an important contribution of MHC class I-dependent immunity to effective anti-mycobacterial protection. We assume that CD4 T cells are highly effective in containing M. bovis BCG within distinct granulomatous lesions, but fail to eradicate their intracellular pathogens. It appears most likely that CD8 T cells are also required to achieve this goal.

Intracellular processing and presentation of T cell epitopes, expressed by recombinant Escherichia coli and Salmonella typhimurium, to human T cells

Vaccines based on recombinant attenuated bacteria represent a potentially safe and effective immunization strategy. A carrier system was developed to analyze in vitro whether foreign T cell epitopes, inserted in the outer membrane protein PhoE of Escherichia coli and expressed by recombinant bacteria, are efficiently processed and presented via human leukocyte antigen (HLA) class I and II molecules by bacterial infected human macrophages. A well-defined HLA-B27-restricted cytotoxic T cell (CTL) epitope and an HLA-DR53 restricted T helper (Th) epitope of the fusion protein of measles virus were genetically inserted in a surface-exposed region of PhoE, and the chimeric proteins were expressed in E. coli and Salmonella typhimurium. Macrophages infected with both recombinant bacteria presented the Th epitope to the specific CD4+ T cell clone, but failed to present the CTL epitope to the specific CD8+ T cell clone. Presentation of the Th epitope by the infected macrophages was inhibited by cytochalasin D, indicating that phagocytic processing of intact bacteria within infected macrophages was essential for antigen presentation via HLA class II. Presentation of the Th epitope to the CD4+ T cell clone by infected macrophages was blocked by brefeldin A and cycloheximide, indicating the requirement of nascent HLA class II molecules for presentation. The efficiency of macrophages to process and present the inserted Th epitope was similar for both the recombinant E. coli and S. typhimurium strains.

A phagosome-to-cytosol pathway for exogenous antigens presented on MHC class I molecules

Peptides from endogenous proteins are presented by major histocompatibility complex class I molecules, but antigens (Ags) in the extracellular fluids are generally not. However, pathogens or particulate Ags that are internalized into phagosomes of macrophages (M phi s) stimulate CD8 T cells. The presentation of these Ags is resistant to chloroquine but is blocked by inhibitors of the proteasome, a mutation in the TAP1-TAP2 transporter, and brefeldin A. Moreover, phagocytosis of a ribosomal-inactivating protein inhibited M phi protein synthesis. These results demonstrate that M phi s transfer Ags from phagosomes into the cytosol and that endogenous and exogenous Ags use a final common pathway for class I presentation.