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LPAIV H9N2 and HPAIV H5N8 clade 2.3.4.4 viruses have been frequently isolated from domestic and wild birds in Germany and they are endemic in poultry worldwide. H9N2 is known to donate gene segments to other AIV with high case fatality rate in humans (e.g. H5N1, H7N9). Similarly, H5N8 devastated poultry worldwide since 2014 and has been recently isolated from humans. Therefore, it is important to understand the genetic predisposition for adaptation of H9N2 and H5N8 AIV in poultry and mammals. In the first publication, we focused on the variable hemagglutinin cleavage site (HACS) of European and Non-European H9N2 viruses, since the HACS is a main virulence determinant of AIV in birds. We found a preferential substitution of non-basic amino acids (G, A, N, S, D, K) in the HACS at position 319 of European H9N2 viruses compared to non-European H9N2 viruses. Recombinant viruses carrying different non-basic amino acids in the HACS modulated replication in vitro. While these non-basic amino acids did not affect virulence or transmission in chickens, they modulated virulence and replication in turkeys. Moreover, H9N2 viruses with non-basic amino acids in the HACS were able to replicate in mammalian brain cells for multiple cycles even without trypsin. In the second publication, we addressed the question whether reassortment between two recent German H9N2 and H5N8 clade 2.3.4.4. B viruses is possible and analysed the impact on virus fitness in mammals and birds. We found that H9N2 PB1 and NP segments were not compatible to generate infectious H5N8 viruses and this incompatibility was due to mutations outside the packaging region. However, H9N2 NS alone or in combination with PB2 and PA significantly increased replication of H5N8 in human cells. Moreover, H9N2 PB2, PA and/or NS segments increased virulence of H5N8 in mice. Interestingly, in chickens, reassortment with H9N2 gene segments, particularly NS, partially or fully impaired chicken-to-chicken transmission. These results indicate that the evolution of H9N2/H5N8 reassortants showing high virulence for mammals is unlikely to occur in chickens. In the third publication, we focused on the NS1 protein of different HPAIV H5N8 clade 2.3.4.4 viruses from 2013 to 2019 and studied the impact of its C-terminus (CTE) variation on virus fitness in chickens and ducks. Our findings revealed a preferential selection for a certain NS1 CTE length in 2.3.4.4. H5N8 clade A (237 aa) and B (217 aa) viruses over the common length of 230 aa. Indeed, the NS1 CTE can affect virus virulence and pathogenesis in a species and virus clade dependent manner. In chickens, although there was no impact on virulence, NS1 CTE of H5N8-A and H5N8-B, regardless of the length, have evolved towards higher efficiency to block the IFN response. In ducks, NS1 CTE contributed to efficient transmission, replication and high virulence of H5N8-B. In the fourth publication, we assessed the impact of variable length of NS1 on H5N8 virus replication in human cells and virulence in mice. We showed that NS1 of H5N8-B virus unlike the vast majority of NS1 of AIV, shared preferences for short NS1 similar to human and zoonotic influenza viruses. This virus (i) was able to efficiently block IFN and apoptosis induction which might be the first steps for efficient adaptation to human cells and (ii) without prior adaptation replicated at higher levels and was more virulent in mice than H5N8-A. The virulence of the latter virus increased after shortening the NS1 similar to H5N8-B virus. Therefore, it is conceivable that truncation in NS1 is a determinant for adaptation of H5N8 in mammals irrespective of its impact on virus fitness in poultry. Findings in this dissertation indicated that HA mutations in the European H9N2 and NS1 variations in H5N8 viruses play a role in virus fitness in poultry and/or mammals. These results improve our current understanding for AIV adaptation and are useful to assess the potential of these viruses to infect mammals.
Staphylococcus aureus is present in around a third of the human population as a constant commensal in the anterior nares, in a third as an intermittent commensal, and a third are non-carriers. However, S. aureus is also a dangerous pathogen, responsible for many types of infections. Recently, the emerging of methicillin-resistant S. aureus strains has aggravated the health problem. Treating infections caused by the invasive strains has become ineffective with conventional antibiotics. Noticeably, transmission of S. aureus has occurred not only in healthcare settings but also in the community; furthermore, transmission between humans and domestic animals has been reported. Although studies about host-pathogen interactions of S. aureus have advanced our knowledge in the last decades, we still have not fully understood mechanisms of the immune system in responses to S. aureus. The aim of this study is to unravel interactions of the human adaptive immune system to selected S. aureus virulence factors. In particular, the study focuses on two aspects: the reaction of human antibodies to the bacterial extracellular proteins in S. aureus-induced furunculosis with an emphasis on Panton-Valentine Leukocidin and responses of the adaptive immune system to membrane-bound lipoproteins of S. aureus. Furunculosis is a variety of hair follicle infection in which S. aureus is one of the chief causal pathogens involved. The corresponding bacterial strains are generally capable of producing of a pore-forming toxin, known as Panton-Valentine Leukocidin (PVL). Recently, the emerging of pvl-positive methicillin-resistant S. aureus has become a problem for treating the bacterially caused furuncles. Colonization with the bacteria is a risk factor for development of chronic or recurrent boils. It is not yet known why furunculosis patients are largely infants or young adults. In this context, we untangled the responses of antibody IgG antibodies to S. aureus extra-cellular factors, notably the PVL toxin, in families in which the patients were children. Multiplex PCR demonstrated that S. aureus clones, isolated from the patients’ wounds but also from the nares of family members, harbored genes coding for PVL toxin. Spa-typing highlighted that bacterial genotypes were very similar in each family. This suggests that transmission of pvl-positive S. aureus took place between family members. The finding also raises the question why only the young patients but not family members who were colonized by the same S. aureus clones suffered from furunculosis. 2D immune proteomics procedures showed a tendency of higher IgG titers against bacterial virulence factors in family healthy members than in patients. PVL-specific antibodies were measured using ELISA, in which patients’ PVL-specific IgG titers were low. This supports the idea that antibodies, probably in conjunction with T cells, might contribute to clinical protection in furunculosis. This research will serve as a foundation for future studies, in which our results should be validated in a larger cohort. Among S. aureus’ virulence factors are lipoproteins, which are anchored in the bacterial cell membrane. Lipoproteins perform various functions in colonization, immune evasion, and immunomodulation. These proteins are potent activators of the complex of innate immune receptors termed Toll-like receptors (TLR) 2 and 6. This study addressed the specific B-cell and T-cell responses to lipoproteins in human S. aureus carriers and non-carriers. 2D immune proteomics and ELISA approaches revealed that titers of serum antibody (IgG) binding to the S. aureus lipoproteins were very low or even unmeasurable in healthy individuals except for the lipoprotein SaeP. Only patients with cystic fibrosis or epidermolysis bullosa who were heavily exposed to the bacteria, generated an antibody response also to lipoproteins. Proliferation assays and cytokine profiling data showed only subtle responses of T cells in healthy individuals; three out of eight tested lipoproteins did not elicit proliferation. Hence, the robust activation of the innate immune system by S. aureus lipoproteins does not translate into a strong adaptive immune response. Reasons for this may be inaccessibility of lipoproteins for B cells as well as ineffective processing and presentation of the antigens to T cells. The main findings implicate that family members can serve as S. aureus reservoirs causing recurrent furunculosis in young patients and that antibodies may provide partial protection from such infections by S. aureus. We have found that, different from proteins that are secreted by S. aureus, lipoproteins which anchored in the bacterial cell membrane, do not trigger strong responses from the human adaptive immune system. This suggests that these proteins remain mostly hidden in the bacterial cell-wall.