Edited by Wilhelm JustChlamydia psittaci causes psittacosis/ornithosis in birds and is an economically important pathogen for poultry farming. It also infects nonavian domestic animals as well as rodents, and is a zoonotic human pathogen responsible for atypical pneumonia. The bacterium efficiently disseminates in host organisms causing pulmonary and systemic disease. Its rapid entry, fast replication cycle, and tight control of intracellular transport routes contribute to the host-to-host transmission and efficient growth observed with C. psittaci. Recent studies have revealed that the pathogen copes better than other chlamydial strains with proinflammatory effectors produced during the early immune reaction of infected hosts. These features likely contribute to successful infections and might explain the potent adaptation and evasion characteristics of the agent. Current findings on cell-autonomous, innate, and adaptive defenses against C. psittaci provide novel insights into the concerted immune mechanisms involved in the clearance of the pathogen. Further in-depth studies on C. psittaci and other related agents in cellular as well as animal models are needed to develop more efficient antichlamydial therapies and vaccination strategies.

Biosynthesis of acetone and n-butanol is naturally restricted to the group of solventogenic clostridia with Clostridium acetobutylicum being the model organism for acetone-butanol-ethanol (ABE) fermentation. According to limited genetic tools, only a few rational metabolic engineering approaches were conducted in the past to improve the production of butanol, an advanced biofuel. In this study, a phosphotransbutyrylase-(Ptb) negative mutant, C. acetobutylicum ptb::int(87), was generated using the ClosTron methodology for targeted gene knock-out and resulted in a distinct butyrate-negative phenotype. The major end products of fermentation experiments without pH control were acetate (3.2 g/l), lactate (4.0 g/l), and butanol (3.4 g/l). The product pattern of the ptb mutant was altered to high ethanol (12.1 g/l) and butanol (8.0 g/l) titers in pH ≥ 5.0-regulated fermentations. Glucose fed-batch cultivation elevated the ethanol concentration to 32.4 g/l, yielding a more than fourfold increased alcohol to acetone ratio as compared to the wildtype. Although butyrate was never detected in cultures of C. acetobutylicum ptb::int(87), the mutant was still capable to take up butyrate when externally added during the late exponential growth phase. These findings suggest that alternative pathways of butyrate re-assimilation exist in C. acetobutylicum, supposably mediated by acetoacetyl-CoA:acyl-CoA transferase and acetoacetate decarboxylase, as well as reverse reactions of butyrate kinase and Ptb with respect to previous studies.

Dendritic cells (DCs) and natural killer (NK) cells are critically involved in the early response against various bacterial microbes. Functional activation of infected DCs and NK cell-mediated gamma interferon (IFN-γ) secretion essentially contribute to the protective immunity against Chlamydia. How DCs and NK cells cooperate during the antichlamydial response is not fully understood. Therefore, in the present study, we investigated the functional interplay between Chlamydia-infected DCs and NK cells. Our biochemical and cell biological experiments show that Chlamydia psittaci-infected DCs display enhanced exosome release. We find that such extracellular vesicles (referred to as dexosomes) do not contain infectious bacterial material but strongly induce IFN-γ production by NK cells. This directly affects C. psittaci growth in infected target cells. Furthermore, NK cell-released IFN-γ in cooperation with tumor necrosis factor alpha (TNF-α) and/or dexosomes augments apoptosis of both noninfected and infected epithelial cells. Thus, the combined effect of dexosomes and proinflammatory cytokines restricts C. psittaci growth and attenuates bacterial subversion of apoptotic host cell death. In conclusion, this provides new insights into the functional cooperation between DCs, dexosomes, and NK cells in the early steps of antichlamydial defense.

Chlamydiae are bacterial pathogens that grow in vacuolar inclusions. Dendritic cells (DCs) disintegrate these compartments, thereby eliminating the microbes, through auto/xenophagy, which also promotes chlamydial antigen presentation via MHC I. Here, we show that TNF-α controls this pathway by driving cytosolic phospholipase (cPLA)2-mediated arachidonic acid (AA) production. AA then impairs mitochondrial function, which disturbs the development and integrity of these energy-dependent parasitic inclusions, while a simultaneous metabolic switch towards aerobic glycolysis promotes DC survival. Tubulin deacetylase/autophagy regulator HDAC6 associates with disintegrated inclusions, thereby further disrupting their subcellular localisation and stability. Bacterial remnants are decorated with defective mitochondria, mito-aggresomal structures, and components of the ubiquitin/autophagy machinery before they are degraded via mito-xenophagy. The mechanism depends on cytoprotective HSP25/27, the E3 ubiquitin ligase Parkin and HDAC6 and promotes chlamydial antigen generation for presentation on MHC I. We propose that this novel mito-xenophagic pathway linking innate and adaptive immunity is critical for effective DC-mediated anti-bacterial resistance.

Natural killer (NK) cells are innate immune cells critically involved in the early immune response against various pathogens including chlamydia. Here, we demonstrate that chlamydia-infected NK cells prevent the intracellular establishment and growth of the bacteria. Upon infection, they display functional maturation characterized by enhanced IFN-γ secretion, CD146 induction, PKCϴ activation, and granule secretion. Eventually, chlamydia are released in a non-infectious, highly immunogenic form driving a potent Th1 immune response. Further, anti-chlamydial antibodies generated during immunization neutralize the infection of epithelial cells. The release of chlamydia from NK cells requires pKCϴ function and active degranulation, while granule-associated granzyme B drives the loss of chlamydial infectivity. Cellular infection and bacterial release can be undergone repeatedly and do not affect NK cell function. Strikingly, NK cells passing through such an infection cycle significantly improve their cytotoxicity. Thus, NK cells not only protect themselves against productive chlamydial infections but also actively trigger potent anti-bacterial responses. NK cells play an important role in the immune response against various pathogens including chlamydia 1. Through their interactions with other immune cells, they are important mediators between innate and adaptive immunity 2. NK cells express a set of activating/inhibiting receptors 3 , which generate signals whose balance determines which cellular program is chosen 4. They are activated by various cytokines 5 resulting in the activation of phospholipase C (PLC). PLC generates two messengers, 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3), which activate protein kinases C (PKCs) and mobilize Ca 2+ from intracellular stores. DAG promotes PKCϴ translocation to membranes and phospho-activation, regulating NK-mediated effector functions 6. To detect and lyse target cells, NK cells use distinct mechanisms: Antibody-dependent cell-mediated cytotoxicity (ADCC) and natural cytotoxic activity 7. In ADCC, the Fc part of target cell-bound IgG is recognized by the FcγRΙΙΙ receptor (CD16) on NK cells, upon which cytotoxic proteins are released in addition to IFN-γ. This leads to the cytotoxic killing of target cells 8. No prior sensitization is needed for natural cytotoxicity, allowing for rapid detection/killing by this mechanism 8. After direct contact with the target cell, secretory granules (containing granzymes and perforin) are released into the immunological gap 8. Moreover, NK cells can kill via TNF family ligands 9 as well as via the secretion of cytokines and chemokines 10. DAG-mediated activation of PKCs is sufficient to induce degranulation of NK cells, leading to the release of granzyme B 11. Granzyme B is initially synthesized as an inactive precursor whose propeptide is removed by cathepsin C 12 , generating the enzymatically active protease. Perforin mediates the entry of activated granzyme B into the cytoplasm of target cells, where a large number o...