Publications

Pulmonary surfactant contains homeostatic and antimicrobial hydrolases. When Mycobacterium tuberculosis is initially deposited in the terminal bronchioles and alveoli, as well as following release from lysed macrophages, bacilli are in intimate contact with these lung surfactant hydrolases. We identified and measured several hydrolases in human alveolar lining fluid and lung tissue that, at their physiological concentrations, dramatically modified the M. tuberculosis cell envelope. Independent of their action time (15 min to 12 h), the effects of the hydrolases on the M. tuberculosis cell envelope resulted in a significant decrease (60-80%) in M. tuberculosis association with, and intracellular growth of the bacteria within, human macrophages. The cell envelope-modifying effects of the hydrolases also led to altered M. tuberculosis intracellular trafficking and induced a protective proinflammatory response to infection. These findings add a new concept to our understanding of M. tuberculosis-macrophage interactions (i.e., the impact of lung surfactant hydrolases on M. tuberculosis infection).

Porcine reproductive and respiratory syndrome (PRRS) is an immunosuppressive chronic respiratory viral disease of pigs that is responsible for major economic losses to the swine industry worldwide. The efficacy of parenteral administration of widely used modified live virus PRRS vaccine (PRRS-MLV) against genetically divergent PRRSV strains remains questionable. Therefore, we evaluated an alternate and proven mucosal immunization approach by intranasal delivery of PRRS-MLV (strain VR2332) with a potent adjuvant to elicit cross-protective immunity against a heterologous PRRSV (strain MN184). Mycobacterium tuberculosis whole cell lysate (Mtb WCL) was chosen as a potent mucosal adjuvant due to its Th1 biased immune response to PRRS-MLV. Unvaccinated pigs challenged with MN184 had clinical PRRS with severe lung pathology; however, vaccinated (PRRS-MLV+ Mtb WCL) pigs challenged with MN184 were apparently healthy. There was a significant increase in the body weight gain in vaccinated compared to unvaccinated PRRSV challenged pigs. Vaccinated compared to unvaccinated, virus-challenged pigs had reduced lung pathology associated with enhanced PRRSV neutralizing antibody titers and reduced viremia. Immunologically, an increased frequency of Th cells, Th/memory cells, γδ T cells, dendritic cells, and activated Th cells and a reduced frequency of T-regulatory cells were detected at both mucosal and systemic sites. Further, reduced secretion of immunosuppressive cytokines (IL-10 and TGF-β) and upregulation of the Th1 cytokine IFN-γ in blood and lungs were detected in mucosally vaccinated, PRRSV-challenged pigs. In conclusion, intranasal immunization of pigs with PRRS-MLV administered with Mtb WCL generated effective cross-protective immunity against PRRSV.

Mannosylated molecules on the Mycobacterium tuberculosis surface are important determinants in the immunopathogenesis of tuberculosis. To date, much attention has been paid to mannose-capped lipoarabinomannan, which mediates phagocytosis and intracellular trafficking of M. tuberculosis by engaging the macrophage mannose receptor and subsequently binds to intracellular CD1b molecules for presentation to T cells. Another important mannosylated lipoglycan on the M. tuberculosis surface is lipomannan (LM). Comparative structural detail of the LMs from virulent and avirulent strains is limited as is knowledge regarding their differential capacity to be recognized by the adaptive immune response. Here, we purified LM from the avirulent M. smegmatis and the virulent M. tuberculosis H(37)R(v), performed a comparative structural biochemical analysis, and addressed their ability to stimulate CD1b-restricted T cell clones. We found that M. tuberculosis H(37)R(v) produces a large neutral LM (TB-LM); in contrast, M. smegmatis produces a smaller linear acidic LM (SmegLM) with a high succinate content. Correspondingly, TB-LM was not as efficiently presented to CD1b-restricted T cells as SmegLM. Thus, here we correlate the structure-function relationships for LMs with CD1b-restricted T cell responses and provide evidence that the structural features of TB-LM contribute to its diminished T cell responsiveness.

Tuberculosis is still a major health problem in the world. Initial interactions between Mycobacterium tuberculosis and the host mark the pathway of infection and the subsequent host inflammatory response. This inflammatory response is tightly regulated by both the host and the bacterium during different stages of infection. As infection progresses, the initial intense pro-inflammatory response observed is regulated by suppressive mediators balancing inflammation. In this environment, M. tuberculosis battles to survive interfering with the host inflammatory response. In this review we discuss the major effector molecules involved in inflammation in relation to the different stages of M. tuberculosis infection.

Reactive oxygen species (ROS) are produced predominantly by phagocytic cells in response to microbial infections. When produced at optimal levels ROS have potent antimicrobial properties. However, excessive production of ROS induces apoptosis/necrosis of infected as well as bystander cells, resulting in inflammatory pathology. Previously we showed that vaccination of pigs with a modified live porcine reproductive and respiratory syndrome virus vaccine (PRRS-MLV) administered intranasally with a potent mucosal adjuvant M. tuberculosis whole-cell lysate (Mtb WCL) induces protective immunity against PRRSV challenge. In this study, using bronchoalveolar lavage fluid cells and peripheral blood mononuclear cells harvested from that study were quantified for the levels of ROS produced. Our results indicated that in vaccinated pigs, levels of ROS were lower compared to unvaccinated PRRSV-challenged pigs. In unvaccinated but PRRSV-challenged pigs, the higher ROS production was associated with increased inflammatory lung pathology. In conclusion, our results suggest that intranasal immunization using PRRS-MLV along with a potent mucosal adjuvant protects pigs against both homologous and virulent heterologous PRRSV challenge, which was associated with reduced ROS production and reduced lung pathology compared to control virus-challenged pigs.

Contact of Mycobacterium tuberculosis (M.tb) with the immune system requires interactions between microbial surface molecules and host pattern recognition receptors. Major M.tb-exposed cell envelope molecules, such as lipomannan (LM), contain subtle structural variations that affect the nature of the immune response. Here we show that LM from virulent M.tb (TB-LM), but not from avirulent Myocobacterium smegmatis (SmegLM), is a potent inhibitor of TNF biosynthesis in human macrophages. This difference in response is not because of variation in Toll-like receptor 2-dependent activation of the signaling kinase MAPK p38. Rather, TB-LM stimulation leads to destabilization of TNF mRNA transcripts and subsequent failure to produce TNF protein. In contrast, SmegLM enhances MAPK-activated protein kinase 2 phosphorylation, which is critical for maintaining TNF mRNA stability in part by contributing microRNAs (miRNAs). In this context, human miRNA miR-125b binds to the 3' UTR region of TNF mRNA and destabilizes the transcript, whereas miR-155 enhances TNF production by increasing TNF mRNA half-life and limiting expression of SHIP1, a negative regulator of the PI3K/Akt pathway. We show that macrophages incubated with TB-LM and live M.tb induce high miR-125b expression and low miR-155 expression with correspondingly low TNF production. In contrast, SmegLM and live M. smegmatis induce high miR-155 expression and low miR-125b expression with high TNF production. Thus, we identify a unique cellular mechanism underlying the ability of a major M.tb cell wall component, TB-LM, to block TNF biosynthesis in human macrophages, thereby allowing M.tb to subvert host immunity and potentially increase its virulence.

Mycobacterium tuberculosis enhances its survival in macrophages by suppressing immune responses in part through its complex cell wall structures. Peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor superfamily member, is a transcriptional factor that regulates inflammation and has high expression in alternatively activated alveolar macrophages and macrophage-derived foam cells, both cell types relevant to tuberculosis pathogenesis. In this study, we show that virulent M. tuberculosis and its cell wall mannose-capped lipoarabinomannan induce PPARgamma expression through a macrophage mannose receptor-dependent pathway. When activated, PPARgamma promotes IL-8 and cyclooxygenase 2 expression, a process modulated by a PPARgamma agonist or antagonist. Upstream, MAPK-p38 mediates cytosolic phospholipase A(2) activation, which is required for PPARgamma ligand production. The induced IL-8 response mediated by mannose-capped lipoarabinomannan and the mannose receptor is independent of TLR2 and NF-kappaB activation. In contrast, the attenuated Mycobacterium bovis bacillus Calmette-Guérin induces less PPARgamma and preferentially uses the NF-kappaB-mediated pathway to induce IL-8 production. Finally, PPARgamma knockdown in human macrophages enhances TNF production and controls the intracellular growth of M. tuberculosis. These data identify a new molecular pathway that links engagement of the mannose receptor, an important pattern recognition receptor for M. tuberculosis, with PPARgamma activation, which regulates the macrophage inflammatory response, thereby playing a role in tuberculosis pathogenesis.

Mycobacterium tuberculosis (the causal agent of TB) has co-evolved with humans for centuries. It infects via the airborne route and is a prototypic highly adapted intracellular pathogen of macrophages. Extensive sequencing of the M. tuberculosis genome along with recent molecular phylogenetic studies is enabling us to gain insight into the biologic diversity that exists among bacterial strains that impact the pathogenesis of latent infection and disease. The majority of the M. tuberculosis cell envelope is comprised of carbohydrates and lipids, and there is increasing evidence that these microbial determinants that are readily exposed to the host immune system play critical roles in disease pathogenesis. Studies from our laboratory and others have raised the possibility that M. tuberculosis is adapting to the human host by cloaking its cell envelope molecules with terminal mannosylated (i.e. Man-alpha-(1–>2)-Man) oligosaccharides that resemble the glycoforms of mammalian mannoproteins. These mannosylated biomolecules engage the mannose receptor (MR) on macrophages during phagocytosis and dictate the intracellular fate of M. tuberculosis by regulating formation of the unique vesicular compartment in which the bacterium survives. The MR is highly expressed on alveolar macrophages (predominant C-type lectin on human cells) and functions as a scavenger receptor to maintain the healthiness of the lung by clearing foreign particles and at the same time regulating dangerous inflammatory responses. Thus M. tuberculosis exploits MR functions to gain entry into the macrophage and survive. Key biochemical pathways and mycobacterial determinants involved in the development and maintenance of the M. tuberculosis phagosome are being identified. The phylogenetic diversity observed in M. tuberculosis strains that impact its cell wall structure together with the genetic diversity observed in human populations, including those elements that affect macrophage function, may help to explain the extraordinary evolutionary adaptation of this pathogen to the human host. Major developments in these areas are the focus of this review.

BACKGROUND: Inorganic polyphosphate (poly P) plays an important role in stress tolerance and virulence in many bacteria. PPK1 is the principal enzyme involved in poly P synthesis, while PPK2 uses poly P to generate GTP, a signaling molecule that serves as an alternative energy source and a precursor for various physiological processes. Campylobacter jejuni, an important cause of foodborne gastroenteritis in humans, possesses homologs of both ppk1 and ppk2. ppk1 has been previously shown to impact the pathobiology of C. jejuni.

METHODOLOGY/PRINCIPAL FINDINGS: Here, we demonstrate for the first time that the deletion of ppk2 in C. jejuni resulted in a significant decrease in poly P-dependent GTP synthesis, while displaying an increased intracellular ATP:GTP ratio. The Deltappk2 mutant exhibited a significant survival defect under osmotic, nutrient, aerobic, and antimicrobial stresses and displayed an enhanced ability to form static biofilms. However, the Deltappk2 mutant was not defective in poly P and ppGpp synthesis suggesting that PPK2-mediated stress tolerance is not ppGpp-mediated. Importantly, the Deltappk2 mutant was significantly attenuated in invasion and intracellular survival within human intestinal epithelial cells as well as in chicken colonization.

CONCLUSIONS/SIGNIFICANCE: Taken together, we have highlighted the role of PPK2 as a novel pathogenicity determinant that is critical for C. jejuni survival, adaptation, and persistence in the host environments. PPK2 is absent in humans and animals; therefore, can serve as a novel target for therapeutic intervention of C. jejuni infections.

Surfactant protein D (SP-D), a lectin that recognizes carbohydrates via its C-type carbohydrate recognition domains (CRDs), regulates Mycobacterium tuberculosis (M.tb)-macrophage interactions via recognition of M.tb mannosylated cell wall components. SP-D binds to, agglutinates, and reduces phagocytosis and intracellular growth of M.tb. Species-specific variations in the CRD amino acid sequence contribute to carbohydrate recognition preferences and have been exploited to enhance the antimicrobial properties of SP-D in vitro. Here, we characterized the binding interaction between several wild-type and mutant SP-D neck + CRD trimeric subunits (NCRDs) and pathogenic and nonpathogenic mycobacterial species. Specific amino acid substitutions (i.e., the 343-amino-acid position) that flank the carbohydrate binding groove led to significant increases in binding of only virulent and attenuated M.tb strains and to a lesser extent M. marinum, whereas there was negligible binding to M. avium complex and M. smegmatis. Moreover, a nonconserved mutation at the critical 321-amino-acid position (involved in Ca(2+) coordination) abrogated binding to M.tb and M. marinum. We further characterized the binding of NCRDs to the predominant surface-exposed mannosylated lipoglycans of the M.tb cell envelope. Results showed a binding pattern that is dependent on the nature of the side chain of the 343-amino-acid position flanking the SP-D CRD binding groove and the nature of the terminal mannosyl sugar linkages of the mycobacterial lipoglycans. We conclude that the 343 position is critical in defining the binding pattern of SP-D proteins to M.tb and its mannosylated cell envelope components.