Long-lived protective immunity to Listeria is conferred by immunization with particulate or soluble listerial antigen preparations coadministered with IL-12

Antibody against biologically active p40 subunit of porcine interleukin-12 expressed in Escherichia coli

A truncated p40 subunit of porcine interleukin-12 (pIL-12) gene without the N-terminal signal peptide sequence was amplified by PCR and cloned into prokaryotic expression vector pGEX-6P-1. The resulting recombinant plasmid pGEX-IL12-40 was transformed into host cells BL21(DE3)pLysS, and the expression of the p40 subunit was induced using isopropyl β-D-thiogalactoside (IPTG). An anti-p40 polyclonal antibody was generated by immunizing a rabbit with the purified protein. Immunoreactivities of the p40 protein and the antibody were confirmed by immunoblotting. At the same time, a recombinant plasmid expressing the entire pIL-12 consisting of p35 and p40 genes was constructed by splicing by overlap extension (SOE)-PCR and transiently transfected into BHK-21 cells. Expression of p40 subunit on the surface of the transfected cells was identified using the anti-p40 antibody. The p40 protein and the specific antibody are biologically active and can be used as detecting reagents.

Effects of Clostridium perfringens alpha-toxin (PLC) and perfringolysin O (PFO) on cytotoxicity to macrophages, on escape from the phagosomes of macrophages, and on persistence of C. perfringens in host tissues

Clostridium perfringens is the most common cause of clostridial myonecrosis (gas gangrene). Polymorphonuclear cells (PMNs) appear to play only a minor role in preventing the onset of myonecrosis in a mouse animal model of the disease (unpublished results). However, the importance of macrophages in the host defense against C. perfringens infections is still unknown. Two membrane-active toxins produced by the anaerobic C. perfringens, alpha-toxin (PLC) and perfringolysin O (PFO), are thought to be important in the pathogenesis of gas gangrene and the lack of phagocytic cells at the site of infection. Therefore, C. perfringens mutants lacking PFO and PLC were examined for their relative cytotoxic effects on macrophages, their ability to escape the phagosome of macrophages, and their persistence in mouse tissues. C. perfringens survival in the presence of mouse peritoneal macrophages was dependent on both PFO and PLC. PFO was shown to be the primary mediator of C. perfringens-dependent cytotoxicity to macrophages. Escape of C. perfringens cells from phagosomes of macrophage-like J774-33 cells and mouse peritoneal macrophages was mediated by either PFO or PLC, although PFO seemed to play a more important role in escape from the phagosome in peritoneal macrophages. At lethal doses (10(9)) of bacteria only PLC was necessary for the onset of myonecrosis, while at sublethal doses (10(6)) both PFO and PLC were necessary for survival of C. perfringens in mouse muscle tissue. These results suggest PFO-mediated cytotoxicity toward macrophages and the ability to escape macrophage phagosomes may be important factors in the ability of C. perfringens to survive in host tissues when bacterial numbers are low relative to those of phagocytic cells, e.g., early in an 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.

CD8 T cell responses to infectious pathogens

CD8 T cells respond to viral infections but also participate in defense against bacterial and protozoal infections. In the last few years, as new methods to accurately quantify and characterize pathogen-specific CD8 T cells have become available, our understanding of in vivo T cell responses has increased dramatically. Pathogen-specific T cells, once thought to be quite rare following infection, are now known to be present at very high frequencies, particularly in peripheral, nonlymphoid tissues. With the ability to visualize in vivo CD8 T cell responses has come the recognition that T cell expansion is programmed and, to a great extent, independent of antigen concentrations. Comparison of CD8 T cell responses to different pathogens also highlights the intricate relationship between microbially induced innate inflammatory responses and the kinetics, magnitude, and character of long-term T cell responses. This review describes recent progress in some of the major murine models of CD8 T cell-mediated immunity to viral, bacterial, and protozoal infection.

IL-12-assisted immunization generates CD4+ T cell-mediated immunity to Listeria monocytogenes

Mice infected with virulent Listeria monocytogenes develop long-lived acquired immunity. We previously reported that acquired immunity to Listeria could also be elicited by immunizing mice with non-viable Listeria or listerial proteins/peptides in combination with IL-12. Here we show that this IL-12-assisted immunization strategy was effective in class I but not in class II MHC-deficient mice, suggesting that antigen-specific CD4(+) T cells are selectively generated using this adjuvant system. We have also evaluated the importance of endogenous production of IFN-gamma and IL-12 for the efficacy of IL-12-assisted immunization. IFN-gamma-deficient mice immunized with HKLM and IL-12 failed to produce effective Listeria-specific responses. In contrast, IL-12-deficient mice were able to generate protective antigen-specific T cell responses in response to immunization with HKLM and IL-12, indicating that exogenous IL-12 is sufficient to initiate a cytokine cascade that results in a potent T(H)1 response. IL-12-assisted immunization provides a model in which both the generation and effector mechanisms of anti-bacterial antigen-specific CD4(+) effector cells can be analyzed.

Optimized expression and specific activity of IL-12 by directed molecular evolution

DNA delivery of IL-12 has shown promise in reducing the toxic side effects associated with administration of recombinant human (h)IL-12 protein while maintaining the ability to inhibit tumor growth and abolish tumor metastases in animal models. We have developed a more potent version of IL-12 by using DNA shuffling and screening to improve its expression in human cells and specific activity on human T cells. The most improved evolved IL-12 (EvIL-12) derived from seven mammalian genes encoding both the p35 and p40 subunits of IL-12 showed a 128-fold improvement in human T cell proliferation compared with native hIL-12 during the initial screening of supernatants from transected cells. When purified hIL-12 and EvIL-12 proteins were compared in vitro in human T cell proliferation and Th1 differentiation assays, it was demonstrated that EvIL-12 exhibited a concomitant 10-fold increase in the specific activity of the protein compared with hIL-12. Furthermore, DNA shuffling improved the level of expression and homogeneity of the heterodimer synthesized by 293 human embryonic kidney cells transfected with EvIL-12 by at least 10-fold. Molecular analysis of the variant revealed strategic placement of amino acid substitutions that potentially may facilitate heterodimer formation and product expression. The enhanced expression and biological activity of EvIL-12 may improve the effectiveness of IL-12 gene-based vaccines and therapeutics without the toxic side effects sometimes associated with hIL-12 protein administration.

Adjuvanticity of an IL-12 fusion protein expressed by recombinant deltaG-vesicular stomatitis virus

The remarkable immunomodulatory and adjuvant properties of rIL-12 have been well described. Many early studies documenting the adjuvanticity of IL-12 were performed using the murine model of Listeria monocytogenes infection. In this report, we describe the construction of an attenuated recombinant vesicular stomatitis virus (VSV-deltaG) that encodes a single-chain IL-12 fusion protein (IL-12F), and the use of this virus as an expression vector to produce large quantities of IL-12F. VSV-expressed IL-12F (vIL-12F) was then co-administered to mice along with a poorly immunogenic listerial antigen preparation as a vaccine regimen and the resulting immune responses were monitored. The vIL-12F was found to have adjuvant properties similar to those observed for rIL-12. Co-administration of vIL-12F and listerial antigen elicited powerful cell-mediated immune responses that conferred long-lived protective listerial immunity. These studies demonstrated that VSVdeltaG-IL12F-infected cells secrete bioactive single-chain IL-12, and laid the foundation for studies using VSVdeltaG-IL12F as a vector for delivery of IL-12F in vivo.

Type 1/Type 2 immunity in infectious diseases

T helper type 1 (Th1) lymphocytes secrete secrete interleukin (IL)-2, interferon-gamma, and lymphotoxin-alpha and stimulate type 1 immunity, which is characterized by intense phagocytic activity. Conversely, Th2 cells secrete IL-4, IL-5, IL-9, IL-10, and IL-13 and stimulate type 2 immunity, which is characterized by high antibody titers. Type 1 and type 2 immunity are not strictly synonymous with cell-mediated and humoral immunity, because Th1 cells also stimulate moderate levels of antibody production, whereas Th2 cells actively suppress phagocytosis. For most infections, save those caused by large eukaryotic pathogens, type 1 immunity is protective, whereas type 2 responses assist with the resolution of cell-mediated inflammation. Severe systemic stress, immunosuppression, or overwhelming microbial inoculation causes the immune system to mount a type 2 response to an infection normally controlled by type 1 immunity. In such cases, administration of antimicrobial chemotherapy and exogenous cytokines restores systemic balance, which allows successful immune responses to clear the infection.

The anaerobic pathogen Clostridium perfringens can escape the phagosome of macrophages under aerobic conditions

Clostridium perfringens is the most common cause of gas gangrene (clostridial myonecrosis), a disease that begins when ischaemic tissues become contaminated with C. perfringens vegetative cells or spores. An aerotolerant anaerobe, C. perfringens quickly multiplies in ischaemic tissues and spreads to healthy areas, leading to a high level of morbidity and mortality. As a species, the bacterium can synthesize 13 different toxins, and these are thought to be the major virulence factors of the disease. However, we present evidence here that C. perfringens can also persist inside macrophages, under aerobic conditions, by escaping the phagosome into the cytoplasm. C. perfringens was not killed by the cells of a clone (J774-33) of the macrophage-like murine cell line J774A.1 under aerobic or anaerobic conditions, whereas the non-pathogenic bacterium Bacillus subtilis was killed by J774-33 cells under both conditions. Electron microscopy images showed that C. perfringens cells were intact and resided mostly in the cytoplasm of J774-33 cells, whereas B. subtilis was in the phagosome. Immunofluorescence microscopy showed that intracellular C. perfringens bacteria failed to co-localize with the late endosome-lysosomal marker glycoprotein LAMP-1, whereas B. subtilis did co-localize with LAMP-1. C. perfringens also appeared to escape the phagosome of both activated and unactivated mouse peritoneal macrophages, but not as efficiently as was seen with the J774-33 cell line. In addition, cytochalasin D was used to show that phagocytosis of C. perfringens was dependent on actin polymerization and that the bacteria attach to J774-33 cells at distinct areas of the cell membrane. We propose that the ability to escape the phagosome and persist inside macrophages is an important factor in the early stages of a gangrene infection, when bacterial numbers are low and phagocytic cells are present.

Effect of Alloiococcus otitidis and three pathogens of otitis media in production of interleukin-12 by human monocyte cell line

Alloiococcus otitidis is detected in middle ear effusion of otitis media with effusion (OME). Only a limited number of studies are available concerning the immunological profile of A. otitidis. We have studied the ability of A. otitidis and three other representative pathogens of otitis media to stimulate the production of interleukin-12 (IL-12) from a monocytic cell line THP-1. Viable A. otitidis induced the production of IL-12 in THP-1 cells but IL-12 production was reduced if glutaraldehyde-fixed bacteria were used as stimulants. When viable bacteria were physically separated from THP-1 cells during the stimulation period, remarkable reductions of IL-12 secretion were shown after challenge with gram-positive bacteria A. otitidis and S. pneumoniae. When stimulated with soluble extracts of A. otitidis, THP-1 secreted IL-12 in a dose-dependent manner. The subfraction with a molecular mass over 100 kDa showed a strong ability to induce IL-12 production. Our results show that A. otitidis has immunostimulatory capacity with regard to IL-12 production. We also show that soluble antigen(s) of A. otitidis can modulate the immune response in OME.

Interleukin-12 as an adjuvant promotes immunoglobulin G and type 1 cytokine recall responses to major surface protein 2 of the ehrlichial pathogen Anaplasma marginale

Anaplasma marginale is a tick-transmitted pathogen of cattle closely related to the human ehrlichiae, Ehrlichia chaffeensis and the agent of human granulocytic ehrlichiosis (HGE). These pathogens have in common a structurally conserved outer membrane protein (OMP) designated the major surface protein 2 (MSP-2) in A. marginale and HGE and OMP-1 in E. chaffeensis. Protective immunity against ehrlichial pathogens is believed to require induction of gamma interferon (IFN-gamma) and opsonizing immunoglobulin (Ig) subclasses directed against OMP epitopes that, in concert, activate macrophages for phagocytosis and killing. Because interleukin-12 (IL-12) acts as an adjuvant for protein immunization to induce IFN-gamma and protective immunity against intracellular pathogens, we hypothesized that as an adjuvant with MSP-2, IL-12 would augment type 1 recall responses to A. marginale. IL-12 was coadsorbed with MSP-2 to alum and shown to significantly enhance IFN-gamma production by lymph node cells (LNC) and LNC-derived CD4(+) T-cell lines from immunized calves following recall stimulation with A. marginale. LNC proliferation and IL-2 production were also enhanced in IL-12-treated calves. Elevated recall proliferative responses by peripheral blood mononuclear cells were still evident 9 months after immunization. Serum IgG levels were consistently increased in IL-12 immunized calves, predominantly due to higher IgG1 responses. The results support the use of IL-12 coadsorbed with OMP of ehrlichial pathogens in alum to amplify both antibody and type-1 cytokine responses important for protective immunity.