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Immune defense mechanisms against Legionella longbeachae
The pulmonary epithelial barrier is the first line of defense against pathogens invading the lungs. If those are able to overcome this first barrier, myeloid cells of the innate immune system are instrumental for the antimicrobial defense and can directly eliminate invading microorganisms. This work aimed to identify novel mechanisms by which pulmonary epithelial cells and myeloid cells eliminate invading bacteria from the lungs. For this, infections with Legionella longbeachae were used to investigate a severe and often fatal form of pneumonia in humans known as Legionnaires’ disease in a mouse model. <br /> Following infection, infiltration of immune cells was dominated by neutrophils and, to a lesser extent, by monocytes. In addition to this, a significantly higher fraction of neutrophils contained L. longbeachae bacteria compared with other myeloid immune cells. Within host cells, bacteria translocated effector proteins mostly into neutrophils, and were residing in a vacuole resembling the Legionella-containing vacuole, as known from infections with L. pneumophila. However, neutrophils played an important role in the in vivo clearance of L. longbeachae, as mice depleted of this cell type exhibited significantly higher bacterial burden in the lungs. Besides neutrophils, monocytes also contributed to the control of pulmonary L. longbeachae infections, while lymphoid immune cells had no effect on the clearance of the bacteria. <br /> Molecularly, it is well known that IL18 is important in anti-bacterial defense by inducing lymphocytes to release IFNγ. However, IL18 receptor (IL18R) expression on lymphoid cells did apparently not promote L. longbeachae clearance. Instead, expression by pulmonary stromal cells was required and sufficient for elimination of the bacteria. Stromal expression of the IL18 receptor was almost confined to the ciliated epithelial cell compartment in the bronchioles. IL18R signaling in those cells did not promote mucus production but it rather enhanced the anti-bactericidal activity of neutrophils. Therefore, these results indicate a non-canonical role of IL18 in the defense against pulmonary L. longbeachae infection, linking non-immune pulmonary epithelial cells with inflammatory neutrophils....
The role of cytohesins in the regulation of immune responses
Cytohesins are guanine nucleotide exchange factors for adenosine diphosphate ribosylation factor (Arf) proteins and promote the switch of Arfs to the active GTP-bound form. Cytohesins have been shown in different in vitro settings to affect cell motility, cell adhesion and chemotaxis of various leukocytes, which are fundamental processes necessary for efficient innate and adaptive immune responses. Furthermore, due to their engagement in phagocytic processes, cytohesins are also targeted by different pathogens during bacterial invasion to evade the immune responses and to exert their full pathogenicity. However, all the evidence for the regulation of immunity by cytohesins has derived from in vitro studies. The primary impact(s) of different cytohesins on the regulation and coordination of the immune responses in the control of infection in vivo has not been elucidated.<br /> The aim of this PhD thesis was to investigate the in vivo function of cytohesin-1, cytohesin-2 and cytohesin-3 in the complex immune responses and in pathogenesis by using acute infection with the respiratory pathogens Legionella pneumophila and influenza A virus in knockout mice. L. pneumophila is a Gram-negative bacteria and the causative agent for Legionnaires’ Disease, and influenza A virus causes ”flu”, which occurs in seasonal and pandemic outbreaks.<br /> These studies revealed that cytohesin-1 promotes T cell responses in both bacterial and viral respiratory infections. Moreover, in influenza A infection, cytohesin-1 deficiency hampered development of cognate T cells and their response to cognate antigens. Cytohesin-1 was demonstrated experimentally to be involved in the initial activation phase of naïve T cells and was required for optimal metabolic switching of T cells following activation. Lack of cytohesin-1 impaired the differentiation of distinct helper T cells, but also different memory and effector cell types.<br /> Myeloid-specific deletion of cytohesin-2 transiently impaired cDC recruitment in the course of bacterial infection highlighting a potential intrinsic role in cDC biology. However, this did not have major effects on the overall phenotype of L. pneumophila or influenza A infection.<br /> Interestingly, cytohesin-3 had an opposing role on T cells compared to cytohesin-1 and suppressed T cell immune responses in both L. pneumophila and influenza A infection. Increased infiltration of several different T cell subpopulations to the site of infection and increased acquisition of antigen-specific responses was observed in cytohesin-3 deficient mice. Furthermore, cytohesin-3 deficient T cells were more reactive to cognate stimulation leading to enhanced cellular immune responses. Additionally, recovery from L. pneumophila infection was delayed in cytohesin-3 deficient mice, suggesting that cytohesin-3 is important for preventing overactivation of T cells and any resulting inflammatory disease.<br /> In conclusion, this PhD thesis provided for the first time a broad in vivo examination of the role(s) of different cytohesins in the immune responses to pulmonary infections. Although minor roles were found for cytohesins in regulating innate immune responses, the primary role(s) of cytohesin-1 and cytohesin-3 appear to lie in the regulation of T cells. Cytohesin-1 promotes T cell responses potentially by providing the optimal (signalling) threshold and by supporting the bioenergetic adaptation following T cell activation, while cytohesin-3 may suppress T cell responses by acting as an immune checkpoint....