Effects of BCG on Immune Response

Overview

In the present study, we want to investigate whether BCG-vaccination enhances the innate immune response in humans in vivo during (single) human endotoxemia. In a future experiment we will investigate whether BCG-vaccination can reverse the tolerant state observed upon a second LPS administration. Our goal is to ultimately translate our results into clinic applications to reverse for example sepsis-induced immunoparalysis.

Sepsis is a major medical challenge associated with a high mortality rate. Release of pro-inflammatory mediators can result in hemodynamic instability, coagulation abnormalities and end-organ dysfunction. Previous strategies have aimed to treat sepsis by inhibition of pro-inflammatory mediators, however, most of these approaches have failed. This might be due to the fact that the majority of septic patients do not succumb to the initial pro-inflammatory "hit", but die at a later time-point in a pronounced immunosuppressive state. This so-called 'immunoparalysis', which renders patients extremely vulnerable to secondary infections, results from the triggering of counter-regulatory anti-inflammatory pathways along with the pro-inflammatory response, already starting in the beginning of sepsis. Immunoparalysis is increasingly being recognized as the overriding immune dysfunction during sepsis. As a consequence, reconstitution of immunocompetence is now emerging as a new and promising therapeutic target to improve outcome in sepsis patients. Bacille Calmette-Guérin (BCG) is one of the most commonly administered vaccines worldwide. In addition to protection against tuberculosis, evidence suggests that BCG immunization has a number of additional beneficial non-specific immunological effects, hereby protecting against infections with pathogens other than tuberculosis. The underlying immunologic mechanisms are not fully elucidated. Recently it was demonstrated that monocytes can be functionally reprogrammed to an enhanced and lasting phenotype after vaccination with BCG. Production of pro-inflammatory cytokines by monocytes isolated from volunteers after BCG vaccination, was found to be enhanced upon ex vivo stimulation with non-related pathogens, even months after BCG vaccination. The observed effects are proposed to be due to modulation of the innate immune system in a process called 'trained immunity'. Upon stimulation with a pathogen, the innate immune system becomes primed and is able to react faster and more efficient to a secondary (and non-related) stimulus, even months later. Monocyte "training" was shown to rely on epigenetic reprogramming, namely increased methylation of histone 3 at lysine 4 (H3K4me3) at the level of cytokine and TLR4 gene promoter regions. Considering these potentiating effects of BCG on innate host defense, it could be a viable treatment option for sepsis-induced immunoparalysis. However, the effects of BCG vaccination on the innate immune response in humans have hitherto only been shown ex vivo. It has yet to be established whether these findings can be extrapolated to the human in vivo situation, because previous data from our group indicates that ex vivo measurements do not accurately reflect the in vivo situation. The human endotoxemia model, in which healthy volunteers receive lipopolysaccharide (LPS) derived from Escherichia coli, is widely used to study the effects of systemic inflammation in humans in vivo and is considered a safe and highly reproducible method to investigate the innate immune response. Furthermore, LPS administration results in a hyporesponsive state towards a second LPS administration called "endotoxin tolerance", which resembles sepsis-induced immunoparalysis, and can thus be used as a model to investigate therapeutic interventions to reverse this condition. The intended target group for this novel therapy, sepsis patients, are immunocompromised. Therefore, use of a live attenuated vaccine such as BCG could present a risk of disseminated mycobacterial infection. Therefore, we will use γ-irradiated (inactivated) BCG vaccine in this study. Recent, yet unpublished results of the group of Prof. Netea have shown that the effects of γ-irradiated BCG on monocyte training are comparable to those of the live vaccine. Furthermore, a study by the group of Prof. Netea (CMO 2013/319) with this type and dose of γ-irradiated BCG in healthy volunteers to study ex vivo responses has been previously approved by the ethics committee. Study design: A randomized double-blind placebo-controlled pilot study in healthy human volunteers during experimental endotoxemia. In this pilot study, we will enrol 20 subjects. On day 1, 10 subjects will receive γ-irradiated BCG-vaccination and 10 subjects will receive placebo. On day 6, all subjects will undergo experimental endotoxemia.

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Outcomes