Clostridial & group A streptococcal myonecrosis
Necrotizing infections are defined as any mono- or polymicrobial bacterial infection that causes rapid destruction of skin, subcutaneous tissue, fascia and/or muscle. Of those attributable to a single etiologic agent, clostridial and group A streptococcal myonecrosis are two of the most fulminant infections of humans. Tissue destruction associated with Streptococcus pyogenes infection progresses rapidly to involve an entire extremity Citation[1]. Rapid destruction of viable, healthy tissue is also characteristic of gas gangrene due to Clostridium perfringens. Indeed, in victims of traumatic injury – whether on the battlefield or following accidents and natural disasters such as earthquakes – clostridial myonecrosis can become well established in as little as 6–8 h and the destruction of adjacent healthy muscle can progress several inches per hour despite appropriate antibiotic coverage. Amputation remains the single best life-saving treatment for established gas gangrene Citation[2]. Aggressive surgical debridement of necrotic tissue, often including amputation, is also recommended for patients with group A streptococcal myonecrosis Citation[2]. In both cases, survivors must undergo prolonged hospitalization and rehabilitation. Though the rapidly progressive nature of tissue destruction in these infections has alarmed clinicians and patients for centuries, the mechanisms have, until recently, remained enigmatic.
Both group A streptococcal necrotizing fasciitis/myonecrosis and clostridial gas gangrene are also characterized by excruciating pain at the infection site Citation[1]. In patients with streptococcal toxic shock syndrome with myonecrosis, the onset of this pain occurs well before shock, renal impairment and acute respiratory distress syndrome manifest Citation[1]. Similarly, the onset of severe pain in gas gangrene is “sometimes so sudden as to suggest a vascular catastrophe” Citation[3].
Certainly, vascular compromise may occur immediately as a result of the injury itself and in traumatic gas gangrene, an injury severe enough to interrupt the arterial blood supply is necessary to establish infection. However, in spontaneous gas gangrene and in cryptic group A streptococcal myonecrosis, a dramatic, penetrating soft-tissue injury is not required to initiate infection. However, propagation of both trauma-associated and spontaneous/cryptic necrotizing infections, with rapid destruction of previously viable tissue and expansion of the bacterial niche, is now known to result from bacterial toxin-mediated vascular dysfunction. Though the cellular mechanisms are subtly different between the two bacterial species, the end result is the same: intravascular thrombosis leading to ischemic necrosis of tissue. Bacterial toxin-induced vascular dysfunction also likely accounts for the characteristic pain associated with these infections. Indeed, such pain is a prominent feature in other clinical conditions that involve occlusion of the arterial blood supply, such as acute arterial embolism or myocardial infarction.
Last, on the cellular level, both infections are also characterized by the lack of a tissue inflammatory response. Instead leukocytes are seen amassed in vessels adjacent to the active site of infection. Without adequate inflammatory infiltration into infected tissues, bacterial replication proceeds unchecked. In C. perfringens gas gangrene and likely in group A streptococcal myonecrosis, the absence of a tissue inflammatory response is also mediated by toxin-induced disruption of the functional platelet/neutrophil/endothelial axis Citation[4].
Most of what we know about bacterial toxin-mediated vascular dysfunction in these necrotizing infections comes from studies involving phospholipase C (PLC), the principal lethal (alpha) toxin of C. perfringens (reviewed in Citation[5]). Our recent studies have shown that within minutes, PLC stimulates the formation of large intravascular aggregates of platelets and leukocytes that irreversibly block blood flow Citation[6,7] and impair leukocyte extravasation into infected tissues Citation[4,8]. The θ toxin of C. perfringens (known as perfringolysin O) also contributes to aggregate formation and aggregate-mediated vascular occlusion. Specifically, θ toxin functionally upregulates adhesins on both platelets and leukocytes Citation[4,9], which promotes their co-aggregation and facilitates their tethering to the activated vascular endothelium Citation[9]. We have subsequently shown that streptolysin O (SLO; a cholesterol-dependent cytolysin homologous to C. perfringens perfringolysin O) also induces the co-aggregation of platelets and neutrophils and that SLO-induced intravascular platelet/leukocyte aggregates also contribute to tissue destruction Citation[10]. Thus, our findings have established a new paradigm for the pathogenesis of aggressive, necrotizing infections in which toxin-induced vascular occlusion mediates the rapid ischemic destruction of tissue, the continued expansion of the bacterial niche and the thwarting of the host immune response.
Mechanisms of toxin-induced vascular dysfunction
Platelet activation is essential for normal hemostasis after traumatic vessel injury, however, uncontrolled stimulation of these cells contributes to many arterial thrombotic events such as myocardial infarction. In both the physiologic and pathologic settings, an initial injury to the endothelium is followed by localized platelet adhesion and activation of various platelet receptors. Key among these is the platelet fibrinogen receptor, gpIIbIIIa, which when activated supports adherence of additional platelets to the growing thrombus to staunch blood loss (physiologic) or blood flow (pathologic). Conversely, therapeutic strategies targeting activated gpIIbIIa (e.g., abciximab and tirofiban) have proven to be useful in preventing platelet aggregation in those at risk for an occlusive thrombotic event but many also dramatically increase the risk of bleeding.
Increased cytosolic calcium and protein kinase C (PKC) activation independently regulate the functional status of gpIIbIIIa. Most physiologic platelet agonists activate both pathways to elicit the platelet’s full physiological response. Our studies demonstrated that PLC-induced activation of gpIIbIIIa is highly calcium dependent, but surprisingly independent of the principal PKC isoforms found in platelets Citation[11]. Furthermore, unlike other physiologic platelet agonists, the PLC-induced rise in intracellular calcium begins with a depletion of internal calcium stores, followed by an influx of calcium through store-sensitive plasma membrane calcium channels – a process known as ‘store-operated calcium entry’ Citation[11]. Pharmacologic inhibition of store-sensitive calcium channels blocked PLC-induced activation of gpIIbIIIa in vitro, but had no effect on that induced by either ADP (a physiologic platelet agonist) or phorbol 12-myristate 13-acetate (a pharmacologic platelet agonist) Citation[11].
Implications for advances in treatment of necrotizing infections
Our findings suggest that C. perfringens PLC-induced platelet activation follows a signaling mechanism that is uniquely different from the physiologic mechanisms employed to maintain hemostasis following vessel injury. Specifically, blockade of either a novel PLC-specific isoform of platelet PKC or store-operated calcium channels may specifically prevent PLC-induced vascular thrombosis in vivo while maintaining platelet responsiveness to other physiologic agonists, thereby minimizing the risk of a bleeding diathesis in patients undergoing surgical debridement. Understanding the unique mechanism of PLC-induced platelet activation may open the door to development of a novel form of targeted therapy. Such pathway-focused therapeutic strategies are currently under investigation in our laboratory. These novel agents could prevent vascular occlusion, maintain tissue perfusion and thereby reduce the need for amputation in patients with devastating necrotizing infections.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
References
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