Biomarkers of Necrotising Soft Tissue Infections Aspects of the Innate Immune Response.

Necrotising soft tissue infection (NSTI) is a life-threatening and rapidly progressing bacterial infection involving one or more layers of the soft tissue compartments causing necrosis. The amputation and mortality rates remain high despite increased focus on the patients. Timely treatment, including surgical intervention, reduces the risk of severe disability and death. However, the lack of pathognomonic signs impedes early diagnosis and treatment. Moreover, the rarity of the disease makes it difficult to conduct large prospective studies, thus prospective research is almost non-existent in this group of patients. Instead data regarding biomarkers are extrapolated from the wide and heterogenic group of patients with sepsis, even though the immunological responses are likely to differ because of the large amount of necrotic tissue seen in patients with NSTI.   We performed the largest prospective, observational studies to date of patients with NSTI in Scandinavia sampled over more than two years with up to a 2.7-year follow-up. Blood samples were taken on admission (baseline) and the following three days and subsequently analysed for relevant plasma biomarkers. We elaborated on three aspects of the innate immune response, which included the investigation of acute-phase proteins, pattern recognition molecules of the lectin complement pathway, and inflammatory cytokines. The objective was to investigate aspects of the innate immune response in patients with NSTI, focusing on biomarkers as prognostic markers of disease severity and mortality. The overall hypothesis was that plasma biomarkers, representing the early innate immune response, can be used as prognostic markers of disease severity and mortality assessed by ICU scoring systems (SAPS II and SOFA score), the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score, presence of septic shock, microbial aetiology, renal replacement therapy, and amputation.   In Study 1, we assessed the following acute-phase proteins in 135 patients with NSTI: pentraxin-3 (PTX3), procalcitonin, and C-reactive protein. We found that a high baseline PTX3 level above the median was significantly associated with the presence of septic shock, amputation, and 180-day mortality, albeit PTX3 was not an independent predictor of mortality. PTX3 and procalcitonin performed equally well, whereas C-reactive protein correlated poorly with clinically relevant outcomes.      In Study 2, we assessed the following plasma pattern recognition molecules in the same cohort as in Study 1: mannose-binding lectin, Ficolin-1, Ficolin-2, and Ficolin-3. We found that baseline Ficolin-2 level below the median was associated with short- and long-term mortality and correlated with the SAPS II, whereas low levels of mannose-binding lectin and Ficolin-3 were associated only with short-term mortality.   In Study 3, we assessed the following inflammatory cytokines in 159 patients with NSTI: interleukin-1β, interleukin-6, interleukin-10, and tumor necrosis factor-α. We found no significant association between the LRINEC score and baseline cytokine levels. In addition, interleukin-6 had the strongest correlation with the disease severity scores (SAPS II and SOFA score), whereas interleukin-1β and interleukin-10 had the strongest association with 30-day mortality. Moreover, patients with β-haemolytic streptococcal infection had higher levels of interleukin-6 and tumor necrosis factor-α compared with each subgroup stratified by microbial aetiology.    This thesis provides new knowledge on the aspects of the innate immune response in patients with NSTI. The results prove that NSTI is characterised by a pronounced inflammatory response and that the innate immune response differs according to disease severity, microbial aetiology, and mortality. Through the three studies we have identified relevant biomarkers that are useful in the risk stratification of patients with NSTI, thus perhaps enhancing prognostication and decision making in these critically ill patients.