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- Institut für Immunologie u. Transfusionsmedizin - Abteilung Immunologie (2) (remove)
SUMMARY To date, Staphylococcus aureus is the most common cause of nosocomial infections and the species is becoming increasingly resistant to antibiotics. Beyond this, S. aureus colonises the nasal mucosa of circa 35% of the healthy population, so-called carriers. Importantly, S. aureus nasal carriage is a major risk factor for the development of S. aureus infections, which are commonly caused by the colonising strain. This underlines the importance of host factors for the outcome of S. aureus-host interactions. Despite the clinical importance of nasal carriage, little is known about humoral immune responses triggered by colonisation. Therefore, this thesis was focussed on the anti-staphylococcal antibody responses of S. aureus carriers and noncarriers. Staphylococcal superantigens (SAgs) served as indicator antigens for our studies. SAgs are virulence factors with extraordinary variability in the species S aureus and act as extremely potent T cell mitogens. To date, 19 different SAg gene loci are known in the species S. aureus, but molecular-epidemiological studies on the distribution of these genes are limited. Therefore, we established five multiplex PCRs for the detection of all known SAgs. With this robust and high-throughput technique we analysed the SAg gene patterns of more than 300 isolates, including 107 nasal isolates of S. aureus carriers and 88 blood culture isolates of hospital patients from Western Pomerania. The SAg gene patterns were highly heterogeneous, which can be explained by their localisation on mobile genetic elements (MGE), such as genomic islands, pathogenicity islands, phages and plasmids. Most isolates (~80%) harboured SAg genes, on average five to six, and SAgs of the enterotoxin gene cluster (egc) were by far the most prevalent. Additionally, we observed a strict correlation between the presence of SAg genes and the T cell mitogenic potency of clinical isolates. SAg-encoding MGEs can be distributed by two distinct mechanisms: horizontal transfer by bacteriophages and vertical transmission to daughter cells. To investigate the distribution of SAg genes within the S. aureus population, we determined the clonal relationship of our isolates by spa genotyping. Interestingly, SAg-gene encoding MGEs were not randomly distributed, but rather closely linked to clonal lineages. Each clonal lineage was characterised by defined combinations of SAg genes. These data suggest that the simultaneous assessment of virulence gene profiles and the genetic background strongly enhances the discriminatory power of genetic investigations into the mechanisms of S. aureus virulence. Indeed, the comparison of virulence genes within each clonal complex indicated a role in invasiveness for some MGEs, e.g. the exfoliative toxin D-encoding pathogenicity island, while rendering it unlikely for SAgs. It is known that neutralising serum antibodies against the SAgs SEA, SEB, SEC, SED and TSST-1 are frequently present in healthy individuals. However, the neutralising antibody profiles against more recently described SAgs or complex SAg cocktails as secreted by clinical isolates had not been determined so far. Therefore, we screened more than 100 sera for their SAg neutralising capacity with a neutralisation assay. We observed a marked heterogeneity and surprisingly large “gaps” in the neutralising capacity. Interestingly, the egc SAgs were inhibited only rarely (5-10%), whereas between 32 and 86% of the tested sera neutralised “classical” SAgs. This “egc gap” in the SAg-neutralising antibody profiles of healthy individuals was unexpected, since egc SAgs are by far the most prevalent SAgs. We could demonstrate that the “egc gap” is probably not due to different T cell activating properties of egc SAgs compared to classical SAgs, but rather to a differential regulation of SAg gene expression. S. aureus carriers have an increased risk of developing an S. aureus bacteraemia, which is in most cases caused by the colonising strain. Intriguingly, a large prospective clinical trial revealed a considerably higher mortality in noncarriers with invasive S. aureus strains compared to carriers with invasive disease. To explain these paradoxical findings, we hypothesised that in carriers partial immunity against the colonising strain may contribute to their improved outcome. We used SAgs as strain-specific indicator antigens. Importantly, sera from persistent carriers neutralised SAgs of their colonising strain with significantly higher efficiency than sera from noncarriers. This antibody response was strain-specific, since the antibody response of carriers against other SAgs did not differ from that of noncarriers. Thus, colonisation with S. aureus confers a strong and strain-specific antibody response against staphylococcal SAgs. We suggest that in carriers neutralising antibodies directed against SAgs and other staphylococcal virulence factors confer partial protection during systemic infections. This could explain the better prognosis of carriers with S. aureus bacteraemia compared to noncarriers. Moreover, our data imply that the key to understanding the pathogenesis of S. aureus disease may lie in the identification of host factors rather than bacterial factors. Such host factors could be the immune status and gene polymorphisms that contribute to colonisation, susceptibility to infection and outcome of infection. Finally, while the treatment of S. aureus bacteraemia with pooled immunoglobulins was performed in the past without significant success, our findings on strain-specific antibody profiles suggest that therapies with customised cocktails of monoclonal antibodies could have a higher efficacy.
Expression of the T cell regulatory molecule ICOS (CD278) and LICOS (CD275) on human blood cells
(2006)
Expression of the T cell regulatory molecule ICOS (CD278) and LICOS (CD275) on human blood cells Summary General bacterial infections, which can lead to the clinical picture of sepsis, are a major concern in intensive care units (ICU) and mortality remains high. Recent data have shown that, besides an overreaction of the immune system, also immunosuppression also plays a role in the pathogenesis of sepsis. Immunosuppression has been documented in patients with polytrauma, stroke and burn wounds, which all confer a high risk of severe bacterial infection. Moreover, it has been shown that T cells have an important role in sepsis. A shift of a Th1 dominated T cell response towards a Th2 response has been described as a potential mechanism of immune suppression in patients with sepsis. One of the molecules on the surface of T cells that is involved in the Th2-mediated immune response is the Inducible Costimulator of T cells (ICOS). Its ligand, LICOS, is expressed on the surface of B cells and monocytes. ICOS ligation induces the production of anti-inflammatory cytokines, especially of IL-10. However, nothing is known about the expression of ICOS on T cells and that of LICOS on APCs in patients with severe trauma and stroke. Therefore, in the present study, in a first step, a recombinant human LICOS-Ig fusion protein was generated, which was then used as an antigen for the generation of anti-LICOS monoclonal antibodies. In three fusion experiments, 5,000 primay clones were screened and a single hybridoma was obtained, which produced monoclonal antibodies that specifically reacted with recombinant LICOS, both in form of the LICOS-Ig fusion protein and on the surface of a cell line transfected with a full-length LICOS transgene. Since, it turned out that the antibodies did not bind with high affinity to wild type LICOS, as it is expressed on primary human blood cells, phenotypic analyses were carried out with another anti-LICOS monoclonal antibody, which had become commercially available. Next, the expression of HLA-DR, CD86, LICOS, and ICOS, on the surface of monocytes (CD14+), B cells (CD19+) and T cells (CD3+, CD4+) in whole blood was measured by flow cytometry. Six patients with severe trauma and nine stroke patients were compared with 32 healthy donors. On CD14+ monocytes from healthy donors, the expression levels of HLA-DR and CD86 were over 90%, while the expression of LICOS was much lower (7,5%). In critically ill patients, HLA-DR, CD86 and LICOS expression were strongly reduced. CD86 and HLA-DR were co-regulated, while HLA-DR and LICOS were not. In healthy donors, virtually all B cells expressed HLA-DR and the majority of them co-expressed LICOS (72%), while only a small fraction were CD86+ (14%). After trauma and stroke, HLA-DR, as well as LICOS expression on these cells remained normal; CD86 had a tendency towards being downregulated in most of the trauma patients, while most of the stroke patients exhibited normal CD86+ levels. The levels of HLA-DR and LICOS on T cells in trauma and stroke patients were low and very similar to those of healthy donors. The fraction of CD3+ T lymphocytes or their CD4+ subpopulation, which expressed measurable levels of ICOS (64% and 48%, respectively), did not change after stoke or trauma. However, within the ICOS+ T cell population two subpopulations could be distinguished: ICOSbright and ICOSdim T cells. Interestingly, the ICOSbright subpopulation, but not the ICOSdim and ICOSnegative subpopulations, was markedly increased in all trauma patients and in most of the stroke patients. Given that CD86 was co-regulated with HLA-DR on monocytes it appears that, similar to HLA-DR, CD86 expression could discriminate between patients with a low and high risk of sepsis. In contrast, because of its low basal expression on monocytes and its low signal-noise ratio, LICOS expression levels are not informative. Since ICOS expression on T cells is tightly connected to IL-10 secretion, the high proportion of ICOS bright cells in critically ill patients might contribute to the high IL-10 serum concentrations, which have been reported to be linked to immunosuppression in these patients.