Killing without collateral damage: new hope for sepsis therapy

Sepsis is a common condition, and more than 19 million cases of severe sepsis (that is, cases where organ dysfunction has occurred as a result of sepsis) occur each year.1 Despite many decades of research, the mortality rate remains ~30%, and survivors are often left with a significant morbidity burden, including neurocognitive impairment, neuromuscular weakness and persistent organ dysfunction.2 End-organ dysfunction and injury ensue in part as a result of innate immune cells (including neutrophils) failing to control pathogen invasion, which leads to microbial proliferation, release of inflammatory mediators, endothelial dysfunction and tissue injury. In this issue, Ruchaud-Sparagano et al.3 present novel data investigating the hypothesis that monophosphoryl lipid A (MPLA), a derivative of lipopolysaccharide (LPS), can ameliorate the release of cytotoxic mediators from neutrophils without impairing bacterial clearance. Maintenance of efficient neutrophil phagocytic and bactericidal function in the absence of significant release of cytotoxic mediators would be an attractive therapeutic strategy for septic patients. Using clinically relevant in vitro models, the authors show that exposing neutrophils to MPLA, in contrast to LPS, does not result in the release of reactive oxygen species and myeloperoxidase. In contrast to LPS, pre-exposure of MPLA-treated neutrophils to a blocking antibody for TLR4 augmented myeloperoxidase release, suggesting that LPS and MPLA modulate TLR4 differentially. Despite ameliorated production of superoxide and myeloperoxidase, MPLA-exposed neutrophils retained Pseudomonas aeruginosa-killing abilities, implying that bacterial killing may not be related to the amount of enzymes released by neutrophils, and, intriguingly, that superoxide-independent killing mechanisms may exist, at least in MPLA-exposed neutrophils. Ruchaud-Sparagano et al. also observed no difference in the release of tumour necrosis factor α, interleukin (IL)-1β and IL-10 from MPLA- and LPS-treated neutrophils. However, there was a significant reduction in the release of the potent neutrophil chemoattractant IL-8 from MPLA-treated cells, mediated via the phosphoinositide 3-kinase pathway. LPS exposure was also found to increase neutrophil cell surface TLR4 expression, a phenomenon that did not occur with MPLA. The contrasting effects of MPLA and LPS are summarized in Table 1. The observations that administration of MPLA after LPS exposure reduced superoxide and myeloperoxidase release, and that MPLA abrogated increased TLR4 cell surface expression in LPS-treated neutrophils while preserving efficient bacterial killing support the notion that MPLA may hold promise as adjunctive therapy for Gram-negative sepsis. Persistent activation of TLR pathways, as occurs in the early stages of sepsis, has been associated with excessive release of inflammatory cytokines and the development of tissue injury in sepsis.4,5 In contrast to the current paper, a study of 20 patients treated with a synthetic lipid A analogue prior to intravenous endotoxin, demonstrated decreased serum concentrations of the pro-inflammatory cytokines TNFα, IL-1β, IL-8 and IL-6, in the presence of increased circulating neutrophil numbers.6 It would seem reasonable to speculate that reduced IL-8 release, coupled with preserved chemotaxis in MPLA-treated cells, to other chemoattractants such as LTB4 may serve to optimize neutrophil recruitment to sites of pathogen invasion without the sequelae of exuberant, mediator-induced tissue injury. The current work also seeks to determine the mechanistic basis of the contrasting responses of neutrophils to LPS and MPLA, demonstrating the differential signaling responses of LPS and MPLA downstream of TLR4. TLR4 activation can lead to activation of the MyD88 and TRIF pathways. IL-8 production requires recruitment of MyD88 to TLR4; based on the ameliorated concentrations of IL-8 produced by MPLA-exposed neutrophils, it is plausible that the differential TLR4 effects of LPS and MPLA may be mediated via differential activation of pathways downstream of TLR4. LPS, but not MPLA, triggered IκB kinase phosphorylation, which results from MyD88 activation, whereas both LPS and MPLA induced interferon regulatory factor 3 (IRF3) activation, which is downstream of TRIF. Both LPS and MPLA were able to trigger phosphorylation of p38 mitogen-activated protein kinase (an event downstream from both MyD88 and TRIF), and LPS, but not MPLA, induced JNK phosphorylation, which occurs downstream from MyD88, and to a less extent TRIF. Thus, this differential activation of MyD88 and TRIF likely explains the different effects of LPS and MPLA on TLR4 upregulation. Sepsis is a heterogeneous and poorly defined clinical syndrome. The continued absence of sepsis therapies suggests that our knowledge of the underlying biology of sepsis remains limited, and a paucity of mechanistic and preclinical data to support the interventions trialled continues to hamper the rational development of therapeutic interventions for sepsis (for example, Kruger et al.,7 Bernard et al.8 and Ranieri et al.9). Attempts to target (and antagonize) single molecules implicated in sepsis have proved futile in clinical trials, reinforcing the notion that therapies that modulate the body’s innate immune response, such as MPLA, may be more likely to demonstrate clinical benefit as adjuncts to standard therapy (antibiotics). We should therefore welcome preclinical human data such as these that inform the field of neutrophil biology and may lead to the development of new therapies for this widespread and often devastating condition. The authors declare no conflict of interest. Dr Proudfoot is a Wellcome Trust Clinical Research Training Fellow (WT097248MA) and Dr Summers is a Wellcome Trust Postdoctoral Clinical Research Training Fellow (WT101692MA).