Prognostic and diagnostic biomarkers of underlying pathological processes may represent promising tools to supplement clinical and laboratory assessment and improve triage and clinical management of SM. In this study, HMGB1 levels at clinical presentation were increased in a cohort of Ugandan children with severe P. falciparum malaria, confirming and extending previous reports of elevated extracellular HMGB1 in fatal paediatric falciparum cases
. Moreover, HMGB1 levels were predictive of malarial disease severity and clinical outcome, suggesting that quantification of extracellular HMGB1 may be a useful prognostic marker of severe and fatal malaria. HMGB1 performed better as a prognostic indicator than the peripheral blood parasite count, a parameter commonly used as a prognostic indicator in falciparum malaria. The predictive accuracy of HMGB1 was comparable to other acute phase biomarkers associated with inflammatory conditions, such as procalcitonin (PCT) (AUC = 0.72) and soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) (AUC = 0.76), previously reported to improve clinical performance (sensitivity >90% and specificity >80%) when used in biomarker combinations to predict mortality in children with severe malaria
. These results indicate that HMGB1 may represent a novel host-derived biomarker that may contribute unique information and further improve predicative accuracy when integrated into combinatorial biomarker panels.
Extracellular HMGB1 is believed to act as a danger signal and initiates a host immune response resulting in increased pro-inflammatory production. Using a peripheral blood mononuclear cell (PBMC)-P. falciparum co-culture approach, P. falciparum-PEs were shown in this study to induce HMGB1 release from human PBMCs, which may account for elevated plasma/serum levels observed in malaria patients. It was hypothesized that malaria-induced release of HMGB1 from immune effector cells could be involved in the propagation of inflammation leading to malaria-associated immunopathology. If so, HMGB1-based strategies might represent a novel therapeutic approach for severe P. falciparum infection, as proposed for sepsis
. In the P. berghei ANKA murine model, the development of ECM is highly dependent on host genetics and immune response to infection. Mice lacking key inflammatory mediators, such as IFN-γ and members of the TNF superfamily (e g, LTα), are protected against the development of ECM
[12–16]. Specific strategies to modulate the host immune response in this model have been reported to decrease disease severity and improve survival
In this study, HMGB1 release was modulated by Plasmodium infection and increased in the peripheral blood of ECM-susceptible mice following infection, similar to observation in human populations, suggesting a potential role for HMGB1 in disease progression. However, administration of a monoclonal anti-HMGB1 antibody (2G7), given prophylactically at a dose previously shown to confer protection in experimental sepsis models
, did not improve survival or modulate peripheral levels of key inflammatory markers. This study suggests that, unlike sepsis models, HMGB1-based interventions directed at the specific epitope targeted by anti-HMGB1 2G7 are not likely to be efficacious in the prevention of experimental SM in PbA-infected C57BL/6 mice.
Further studies are required to explain the failure of anti-HMGB-1 antibody-based intervention in this model. In the current study, anti-HMGB1 treatment did not affect circulating levels of inflammatory cytokines induced by PbA infection, although the same antibody has been shown to attenuate levels of inflammatory cytokines induced by CLP in an experimental sepsis model
. This could suggest that excessive pro-inflammatory responses in this model are not mediated by HMGB1, as has been previously described for other inflammatory diseases. Some, but not all, studies suggest that HMGB1 does not have direct cytokine activity but instead functions as a complex with TLR ligands (e g, LPS) to enhance or promote their effects, a function that may not be relevant for severe malaria pathology caused by malaria toxins or by-products. TLR4 has been identified as a principal receptor that meditates HMGB1-induced cytokine production and immunopathology
. Although the exact role for TLRs in CM remains to be elucidated, a number of studies suggest that the pathogenesis of PbA-induced SM is independent of TLR4
, unlike the pathogenesis of sepsis where mice deficient in TLR4 are highly resistant to the development of LPS-induced septic shock
. Although the study does not support the use of this anti-HMGB1 mAb, at the dose employed, as treatment in this context, it does not rule out a role for extracellular HMGB1 in the pathogenesis of P. falciparum-induced CM in humans. Further studies to elucidate the role of HMGB1, using strategies not employed and/or outside the scope of the current study, are warranted. It is also important to note that this study was carried out with a single neutralizing monoclonal antibody. It is possible that additional strategies to block HMGB1, including antibodies raised against the B box subunit domain of HMBG1, may yield more favourable outcomes.
Accumulating evidence indicates that the host response to infection contributes to the pathogenesis of SM. Improved understanding of the pathophysiological mechanisms of SM may lead to novel prognostic tools and therapeutic strategies to improve clinical outcome. In this study, HMGB1 levels at presentation were correlated with falciparum malaria disease severity in a cohort of paediatric patients, and there was a significant difference in admission HMGB1 levels between children who subsequently died from their infection versus those who survived. This study supports further investigation into the potential use of HMGB1 as a biomarker to assess disease severity and prognosis in paediatric malaria. Additional prospective, multicentre studies of SM in areas of varying malaria transmission are required to validate the clinical utility of HMGB1. However, based on the results of this study in a mouse model of SM, HMGB1 neutralization using anti-HMGB1 2G7 mAb does not appear to be a viable therapeutic strategy to improve clinical outcome in this model of severe malaria. Further studies are warranted to elucidate the role of HMGB1 in the pathogenesis of human and experimental SM and CM.