In schistosomiasis, the pathology resulting from chronic infection is predominantly induced by the host immune response to parasite eggs that are laid, in the case of Schistosoma mansoni, in the portal venous system and subsequently trapped in the liver and intestine. The egg induced granulomatous response eventually triggers significant liver fibrosis, which is the primary cause of chronic morbidity and mortality. Consequently, much of our current work is focused on elucidating the mechanisms of granulomatous inflammation and fibrosis. Progress was made in the following areas: 1) TSLP has been shown to be a critical mediator of allergen-induced Th2-dependent inflammation. However, whether TSLP/TSLPR signaling is required for the initiation and/or maintenance of helminth-induced Th2 responses remains unclear. In collaboration with Dr. Steve Ziegler (Benaroya Res Inst) and Mike Comeau (Amgen), we obtained TSLPR-/- mice and TSLP blocking antibodies and investigated the function of TSLP in schistosomiasis. In addition, S. mansoni eggs were injected into the lungs of nave and egg-sensitized mice to determine whether there were any tissue-specific differences in TSLP function. Additional studies were performed with a neutralizing TSLP-specific mAb to confirm the results with the genetically modified mice. The results from these studies demonstrated that while TSLP-signaling is involved in the development of polarized Th2 cytokine responses and for the optimal formation of primary granulomas in the lung, it is not necessary for secondary granulomatous inflammation or for the formation of Th2-dependent pathology in the liver and gut during chronic S. mansoni infection. Together, these findings suggest that while TSLPR-signaling serves a key role in allergen-driven Th2 responses, it exerts minimal regulatory activity during helminth-induced Th2 responses. 2) Retnla is a member of a family of cysteine-rich secreted proteins, referred to as resistin-like molecules (RELM) or found in inflammatory zone (FIZZ), which were originally identified in the lung. Retnla/Relmαis expressed in bronchial epithelial cells and in the wall of the large and small bowel and was originally hypothesized to regulate obesity and type-2 diabetes. Retnla increases significantly during allergic responses in the lung and expression is IL-4/IL-13 and Stat6-dependent. In addition to hypertrophic, hyperplastic bronchial epithelium, Retnla is also differentially expressed in alternatively- (AAM) and classically activated (CAM) macrophages, thus serving as a useful biomarker of AAMs. Despite its strong association with Th2 responses, however, the role of Retnla in Th2-driven inflammation is unknown. Recent studies suggested that Retnla is possibly involved in the induction of fibrosis in the lung by promoting the differentiation of myofibroblasts that mediate collagen deposition. Thus Retnla has been hypothesized to facilitate wound-repair and fibrosis during Th2-mediated inflammatory responses. In addition to being upregulated during Th2-associated pulmonary inflammation, Retnla is also found in abundance following infection with a variety of metazoan parasites. Nevertheless, whether Retnla is directly involved in the regulation of Th2-dependent pathology or susceptibility to GI nematode infection is unknown. Therefore, to elucidate the physiological function of Retnla, in collaboration with Regeneron Pharmaceuticals, mice with a targeted deletion of Retnla and insertion of LacZ were examined. To determine whether Retnla was involved in the regulation of Th2 driven inflammation and fibrosis, Retnla-/- knockout/reporter mice were compared with Retnla+/+ animals following both acute and chronic infection with S. mansoni. In addition, S. mansoni eggs were injected into the lungs to determine whether there were any anatomical differences in either the expression or activity of Retnla. Finally, studies were performed with the nematode N. brasiliensis to determine whether Retnla was required for the development of protective Th2 cytokine responses in the gut during GI nematode infection. We found that Retnla is not required for the development of helminth-induced Th2 responses. Instead, Th2 dependent pulmonary inflammation, S. mansoni-induced liver fibrosis, and GI nematode expulsion were all significantly enhanced in the absence of Retnla. Thus, these studies demonstrated that Retnla, although induced by IL-4/IL-13, functions as a feedback mechanism to suppress Th2 responses. In the case of S. mansoni, Retnla deficiency triggered severe inflammation in the lung and liver, leading to the accelerated development of hepatosplenic disease following infection, while in the case of N. brasiliensis, Retnla-/- mice expelled their parasites more rapidly from the gut. Thus, Retnla exhibited either protective or disease exacerbating activities by functioning as a negative regulator of Th2 responses. 3) In macrophages, TLRs activate protective immune responses, including recognition of pathogens, activation of anti-pathogen effector pathways and transition to protective adaptive responses. Classically activated macrophages (CAMs) are important for resistance to infections caused by intracellular pathogens. A key anti-pathogen effector of CAMs is nitric oxide (NO), which is required for host control of intracellular infections, including Mycobacteria species, T. gondii, Leishmania species and Trypanosoma cruzi, and has direct antimicrobial toxicity. TLR and interferon pathways synergistically trigger NO production by transcriptional and post-transcriptional mechanisms that enhance expression of inducible nitric oxide synthase (iNOS), the enzyme responsible for NO production from arginine in macrophages. As a countermeasure, some pathogens deploy NO scavengers or their own arginases or, in the case of M. tuberculosis, adapt to NO made by activated macrophages and exclude iNOS from phagosomes. However, mechanisms by which intracellular pathogens induce host responses that reduce or bypass NO remain largely unknown. Although a published report has shown that arginase activity is induced in the J774 macrophage-like cell line by infection with Mycobacterium bovis bacillus Calmette-Gurin (BCG), the isoform of arginase induced, the mechanism of induction and the biological consequences in primary macrophages and whole-animal models was unknown. In collaboration with Dr. Peter Murray (St. Jude Childrens Hospital) we found that Arg1 was induced to quite high levels in primary mouse macrophages following mycobacterial and T. gondii infection. Although macrophage Arg1 expression is more commonly linked to the hypothesized anti-worm functions of alternatively activated macrophages (AAMs), we found that Arg1 was induced in CAMs and functioned, in part, to suppress NO production. Consequently, we identified a loophole'in the TLR pathway that is advantageous to intracellular pathogens. These pathogens induced expression of Arg1 in mouse macrophages via a TLR-dependent mechanism. Surprisingly, in contrast to diseases dominated by TH2 responses, the TLR-mediated Arg1 induction was independent of the TH2-associated STAT6 pathway. The specific elimination of Arg1 in macrophages favored host survival in T. gondii infection and decreased lung bacterial loads during tuberculosis infection. Thus, arg1 was identified as important regulator of NO production and immunity during infection with intracellular pathogens.
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