Streptococcus pyogenes infections and toxic shock syndrome : molecular epidemiology and immunotherapy

Abstract

Streptococcus pyogenes, also known as group A streptococcus (GAS), is an important human pathogen causing a wide variety of diseases. One of the most severe diseases is streptococcal toxic shock syndrome (STSS), which is associated with high mortality rates. Toxic shock syndrome (TSS) may also be caused by Staphylococcus aureus. Superantigens have been identified as key mediators of STSS and staphylococcal TSS. Intravenous polyspecific immunglobulin (IVIG) has been suggested as adjunctive therapy in TSS, since it contains neutralising antibodies against streptococcal and staphylococcal superantigens, as well as bacterial opsonising antibodies.

To assess the safety and efficacy of IVIG as adjunctive therapy in STSS, we conducted a multicenter placebo-controlled trial (paper I). The trial was prematurely terminated due to a low incidence of disease in the participating countries. Results were obtained from 21 enrolled patients; 10 of whom received IVIG and 11 placebo. The primary objective was mortality over 28 days, and a trend to decreased mortality was observed in the IVIG group, 10% versus 36% in the placebo group. A significant decrease in sepsis-related organ failure assessment (SOFA) score was noted in the IVIG-group, whereas no change was seen in the placebo group. The IVIG cases obtained a significantly increased plasma superantigen-neutralising activity against their own isolate following IVIG administration, whereas no change could be noted among patients in the placebo group.

In paper II we tested whether superantigen-containing culture supernatants from streptococcal and staphylococcal severe sepsis isolates were inhibited to an equal extent by IVIG. Three different IVIG preparations were tested and found to be highly efficient in neutralising the superantigens. Most supernatants were completely inhibited at concentrations between 0.5 - 2.5 mg IVIG/ml. Importantly, culture supernatants from S. pyogenes isolates were consistently inhibited to a higher extent as compared to those of S. aureus isolates.

In paper III and IV, results from active surveillance of invasive GAS infections in Denmark and Sweden during 2001-02 and 2002-04, respectively, are described. The yearly incidences were similar, 2.0-3.4 /100 0000 inhabitants, as was the prevalence of the severe manifestations STSS and necrotising fasciitis (NF), which were seen in approximately 10% of the cases. However, differences were observed in outcome with a mortality rate of 25% and 14.5% in Denmark and Sweden, respectively. Also the GAS type distribution varied between the two studies. emm1 was the most prevalent type (32%) in the Danish study in comparison to the new types emm89 (16%) and 81 (14%) that dominated in the Swedish study.

Non-invasive GAS isolates were collected and analysed in parallel with the invasive in both studies, and the type distribution differed significantly from the invasive isolates. Differences in presence of superantigen genes were seen between isolates of different emm-types and also between invasive and non-invasive isolates. A combination of PFGE-analysis and superantigen profiling revealed subclones within the emm-types with higher invasiveness than others (paper IV). Further, IVIG treatment in patients with STSS was significantly associated with improved outcome. 20 out of 72 patients with STSS were given IVIG, with a mortality of 15% as compared to 48% among patients not receiving IVIG (paper IV).

This thesis provides further support of IVIG therapy in STSS, and the in vitro analyses revealed that a higher dose of IVIG may be needed in staphylococcal TSS in order to achieve protective antibody levels. The thesis also provides new insights in the molecular epidemiology of invasive GAS disease.