To the best of our knowledge, this is the most extensive surveillance of patients with iGAS infections in Japan. This study revealed that 18.4% of all patients developed STSS, which was significantly associated with NF, bacteraemia without primary focus, WBC count < 4,000 cells/µL, serum CK ≥ 300 U/L on admission, underlying kidney disease, and emm1.

Bacteraemia without a primary focus and NF were associated with STSS. In approximately 50% of the cases iGAS infections can lead to severe, life-threatening complications, including NF, a necrotizing soft-tissue infection subgroup, complicated by STSS. The results of this study are consistent with those of previous studies [3, 7, 20]. Therefore, iGAS infections should be suspected in patients with signs of systemic inflammation in cases with skin or soft-tissue infections, and a diagnosis of invasive GAS infection should be promptly established through a positive GAS culture from a normally sterile site. However, GAS bacteraemia can occur in the absence of a clear localizing source, as in this study, with bacteraemia without primary focus being the most common manifestation (16.4%, n = 83). Moreover, STSS is frequently associated with deep soft-tissue infections with nonspecific symptoms. Therefore, source control can be difficult in these patients, resulting in a poor prognosis.

Laboratory findings on admission, including WBC < 4,000 cells/µL and serum CK ≥ 300 U/L, were important biomarkers and correlated strongly with STSS development. Few studies have described biomarkers as mortality predictors in patients with STSS. In the first case series of STSS association with GAS, leukocytosis with a significant left shift, elevated serum Cr concentration, hypoalbuminemia, hypocalcaemia, and elevated serum CK concentration were observed, suggesting necrosis [3]. Another study on STSS reported that patients who died had significantly fewer leukocytes and platelets and elevated serum Cr concentrations [5]. Similarly, a WBC count < 4,000 cells/µL was an important biomarker with strong correlation with STSS in this study. International guidelines [21] on severe conditions in patients with systemic inflammatory response syndrome and sepsis emphasize leukopenia (WBC count < 4,000 cells/µL). Moreover, the serum CK concentration, associated with muscle cell or fascia disintegration, increases in patients with STSS. Thus, elevated serum CK concentrations suggest extensive and deep soft-tissue infections caused by GAS. Previously, a laboratory risk indicator for NF (LRINEC) score, based on the WBC count and haemoglobin, sodium, glucose, Cr, and CRP levels, was proposed [22]. This tool has variable sensitivity, but it should not be used to exclude necrotizing soft-tissue infections. Nevertheless, robust and rapid measurable biomarkers to improve STSS identification and management are urgently needed. Renal dysfunction occurs in nearly all patients with STSS within 48–72 h [3]. Elevated serum Cr concentrations precede hypotension development due to septic shock. Hypotension, myoglobinuria, and haemoglobinuria due to toxin-induced haemolysis may further contribute to acute renal failure, as demonstrated by the laboratory findings in this study, including Cr ≥ 2.0 mg/dL. STSS management focuses on supportive management by preventing dehydration in patients with life-threatening hypotension as well as sufficient kidney marker monitoring.

Regarding the emm type of iGAS isolates, emm1 was the most predominant emm type and accounted for approximately three-quarters of patients with STSS, indicating a significant association between emm1 and STSS. Of the > 275 M protein types of GAS, emm1 has been one of the most prevalent types in Japan [23, 24] and in other countries [8, 25, 26]. The M1UK lineage of GAS is a hypertoxigenic clone within the serotype M1 GAS strain that has been associated with increased iGAS infection incidence [10, 27,28,29]. The M protein, encoded by the emm gene, protects the organism against phagocytosis by polymorphonuclear leukocytes [30]. The binding of complement regulatory proteins also contributes to resistance to phagocytosis by limiting the deposition of opsonins, including C3b, on the bacterial surface. Streptococcal inhibitor of complement (SIC), a GAS virulence factor, is a 305-amino-acid extracellular protein that inactivates the complement membrane attack complex [31]. SIC has been reported only in serotypes M1 (emm1) and M57. The sic. gene is located in the mga regulon of M1 protein and is directly adjacent to the emm gene. By secreting SIC, GAS can evade destruction by the membrane attack complex (C5–C9) generated by either the alternative or classical pathway. SIC can also interfere with human innate immunity against GAS, including the antibacterial activity of chemokines [32]. Moreover, SIC from M1 GAS interacts with TLR2 and CD14 proteins on monocytes, leading to activation of the NF-κB and p38 MAPK pathways and release of several pro-inflammatory cytokines (e.g., TNFα and IFNγ) [33]. The pathology of massive inflammation, termed the cytokine storm in STSS, can be attributed to the pro-inflammatory properties of SIC. The detailed behaviour of inflammatory cytokines and molecular mechanisms during iGAS infections should be clarified by future studies.

Our study has some limitations. First, although we evaluated the patients according to their clinical features and laboratory findings using the STSS diagnostic criteria, the number of patients may have been underestimated. In particular, indicators of disseminated intravascular coagulation, including D-dimer and fibrin/fibrinogen degradation products, were not sufficiently assessed. Moreover, the other biomarkers and indicators observed commonly in patients with STSS including bands, metamyelocytes, myelocytes, low albumin, and the acidosis markers including blood pH, serum bicarbonate, and lactic acid levels were not evaluated in this study. These parameters should be considered in future studies. Moreover, we do not have any access to pathological reports of surgical procedures or autopsies as the aim of this study was to identify biomarkers and emm type associated with STSS development. However, pathological findings of surgical procedures and autopsies are clinically relevant. This should also be assessed in future studies. Second, the clinical course of the patients with STSS in this study varied. Some patients with STSS might not have been categorized correctly as the data were obtained at the time of admission. Symptoms, including shock, acute respiratory distress syndrome, or renal failure, may not be apparent at the time of admission; moreover, symptoms and abnormal laboratory findings associated with STSS may manifest within 24 h after admission. Based on our findings, we propose that clinical and laboratory evaluations should be performed at two time points to detect clinically relevant temporal trend changes, even though the time from onset to hospital admission is relatively uniform owing to the universal health insurance in Japan. Third, 1–2 g/kg intravenous immunoglobulin (IVIG) administered as a single dose has been suggested as a potential adjunctive therapy. IVIG has been proposed to inhibit T-cell activation by blocking or inactivating the superantigens, resulting in a decrease in cytokine release [34]. We do not have enough data on IVIG. Owing to the multicentre observational nature of the study, data collection was performed using a standardized questionnaire. However, as the questionnaire did not include questions on IVIG administration, relevant data were not available for analysis. However, adjunctive IVIG therapy with antibiotics was administered for a few patients for life-threatening infections.

Finally, the relationship between emm1 and STSS suggests the need for a rapid emm1 GAS detection method that does not require genome sequencing. We are already developing a rapid diagnostic method that simultaneously detects the sic. gene specific to emm1 as well as GAS (data not shown). SIC and its interaction with monocytes may be potential therapeutic targets.

In conclusion, our results may provide evidence for the early diagnosis of STSS based on iGAS on admission. The prevalence and burden of iGAS infection, affecting younger and older adults without underlying diseases, are likely to increase worldwide. Rapid prediction and diagnosis of STSS by molecular testing and timely initiation of targeted therapy may save lives. Strategies to reduce the mortality risk of iGAS infections require further investigation of the dynamic molecular epidemiology, rapid diagnostic STSS tools, and the development of effective standard prevention and treatment.