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Herpesviruses are enveloped DNA viruses which are dependent on two fusion steps for efficient replication in the host cell. First, they have to fuse their envelope with the cellular plasma membrane or with the vesicle membrane after endocytic uptake to enter the host cell and second, they have to export the newly generated nucleocapsids from the site of assembly to the cytoplasm by fusion of the primary virion envelope with the outer nuclear membrane (ONM). The main goal of this project was to provide a better understanding of how herpesvirus capsids exit the nucleus. On the one hand this thesis aimed at finding cellular proteins involved in nuclear egress (Paper I), while on the other the focus was on further characterization of the viral nuclear egress complex (NEC, Paper II) and its interaction with the capsid (Paper III).
It is the hallmark of viruses, including herpesviruses, to hijack host cell proteins for their efficient replication. Some of those interactions are well characterized, while others might not yet have been discovered. In the last step of the nuclear egress, where the primary virion membrane fuses with the ONM, most likely a cellular machinery is involved. The presented work focused on Torsin, the only known AAA+ ATPase localizing in the endoplasmic reticulum and the perinuclear space (PNS). For this, the effect of overexpression of WT and mutant proteins, as well as CRISPR/Cas9 generated knock-out cell lines, on PrV replication was analyzed. Neither single overexpression nor single knockouts of TorA or TorB had any significant effects on virus titers. However, infection of TorA/B double knockout cells revealed reduced viral titers and an accumulation of primary virions in the PNS at early infection times, indicating a delay in nuclear egress.
The process of nuclear egress has been intensively investigated without revealing all its details. To address some of the missing aspects we generated monoclonal antibodies (mAbs) against the NEC and its components (pUL31 and pUL34) for a better visualization of the process in transfected as well as infected cells. These mAbs provide a useful tool for future analyses.
The publication of the NEC crystal structure formed the basis for intensive research on the molecular details of the NEC formation and its interaction with the nucleocapsid. Recently, our lab showed that lysine (K) at position 242 in the membrane-distal part of pUL31 is crucial for incorporation of the nucleocapsid into budding vesicles. Replacing K by alanine (A) resulted in accumulations of vesicles in the PNS, while mature capsids were not incorporated. To test whether this is due to electrostatic interference or structural restrictions we substituted K242 by different aa to determine the requirements for nucleocapsid uptake into the nascent primary particles. To analyze whether the defect of pUL31-K242A can be compensated by second-site mutations, PrV-UL31-K242A was passaged and mutations in revertants were analyzed. Different mutations have been identified compensating for the K242A defect. A considerable number of mutations indicates that the NEC is much more flexible than previously thought. Further, we gained information that the K at position 242 is not directly involved in capsid interaction, while it is more likely involved in rearrangements within the NEC coat.
Rainbow trout (Oncorhynchus mykiss) represents the third most produced species of diadromous fish, with the total production of 0,732 million tonnes in 2009. More than one third of this production comes from Europe, where it is dominated by Norway, Italy and France. Germany is the fifth biggest producer in Europe, producing 21 thousand tonnes of rainbow trout in the value of 6,1 million Euro. However, the conditions in the intensive aquaculture often increase the disease susceptibility to many pathogens. One of the highest economic threats for a salmonids aquaculture is the causative agent of furunculosis, Aeromonas salmonicida subsp. salmonicida. Several strategies have been developed to protect the fish, but the traditional methods are either laborious or represent a potential risk for the environment. The selective breeding established more than 35 years ago in the brackish waters of Baltic Sea represent a attractive alternative, delivering a novel strain of rainbow trout better adapted to the brackish environment and exhibiting reduced mortality in the infection with A.salmonicida. Nevertheless, no information was available about the fundaments of this phenomenon. Thus, the aim of presented study was the identification of immune adaptations, which occurred during the 30 years of selection and favoured increased survival of “born” trout to the bacterial diseas es. In the presented work, the peritoneal cavity of rainbow trout has been used as a model for the investigation of disease resistance in fish. In the first chapter, the peritoneal cavity has been described as a unique niche of teleost immune system and the kinetic of peritoneal leukocytes induced by the stimulation has been analysed. Furthermore, a unique set of monoclonal antibodies has been used to evaluate the contribution of distinct cell populations on the inflammation and its resolution. In the second part of the study, the transcriptional changes of peritoneal leukocytes have been evaluated using the GRASP microarray. The following analysis provided unique insights into the local immune response in rainbow trout. The unprecedented combination of both data sets offers an unparalleled description of the local immune response in teleost fish and can be summarized into following facts. In general, the obtained results revealed, that the unstimulated peritoneal cavity is populated predominantly by lymphocytes with IgM+ Bcells being the major cells type. The rapid changes in the composition induced by the stimulation were underlined by the upregulation of major proinflammatory molecules such as IL1β, IL8 and TNFα within 12hpi. Although the initial phase of the reaction was dominated by myeloid cells, the cavity underwent within 72 hours two complete changes in the composition corresponding with the massive changes in the transcriptome. Eventually, the resolution of inflammation was marked by an increasing number of lymphocytes and correlated with the downregulation of pro-inflammatory genes to the initial level and upregulation of anti-inflammatory cytokines IL10 and TGFβ. Besides the general observations common to all treatments and both strains, our experiments revealed also remarkable differences between the antigenic stimulation and reaction towards pathogen. From these differences following conclusions can be drawn; the infection induces comparable reaction pattern as the stimulation, although the intensity of the reaction and number of cells is higher. These observations correlated with the higher expression of inflammatory molecules after the infection. Viable bacteria also prolong the myeloid phase of the reaction and delay the resolution of inflammation. Finally, model of peritoneal inflammation caused by A. salmonicida has been applied also to the second strain of rainbow trout, known for its higher resistance to infection. The comparison of obtained data suggested that resistant trout reacted to the antigenic stimulation and infection with a lower number of cells despite minor differences in the expression level of major pro-inflammatory molecules during early stages of the infection. Eventually, the resolution of inflammation and onset of adaptive immune response occurred in resistant trout almost 24 hours earlier and was correlating with an increased expression of anti-inflammatory cytokines IL10 and TGFβ. Notably, the increased survival of resistant strain correlates with the increased expression of antibacterial proteins such as NRAMP and hepcidin. Taken together, obtained data provided unprecedented insights into the local immune response in teleost fish and identified features conserved during the selection breeding in the brackish water of Baltic Sea. Additionally, combination of cellular and molecular data elucidates the peritoneal inflammation in fish and suggested high conservation of the immune response in the evolution.