In Vitro Experimental Model of Trained Innate Immunity in Human Primary Monocytes

Abstract

Innate immune memory, or trained immunity, has recently been described to be an important property of cells of the innate immune system. Due to the increased interest in this important new field of immunological investigation, we sought to determine the optimal conditions for an in vitro experimental protocol of monocyte training using three of the most commonly used training stimuli from the literature: ␤-glucan, the bacillus Calmette-Guérin (BCG) vaccine, and oxidized low-density lipoprotein (ox-LDL). We investigated and optimized a protocol of monocyte trained immunity induced by an initial training period with ␤-glucan, BCG, or oxLDL, followed by washing and resting of the cells and, thereafter, restimulation with secondary bacterial stimuli. The training and resting time intervals were varied to identify the optimal setting for the long-term induction of trained immunity. Trained immunity was assessed in terms of the secondary cytokine response, the production of reactive oxygen species, cell morphology, and induction of glycolysis. Monocytes primed with ␤-glucan, BCG, and oxLDL showed increased pro-and antiinflammatory cytokine responses upon restimulation with nonrelated stimuli. Also, all three stimuli induced a switch to glycolysis (the Warburg effect). These effects were most pronounced when the training interval was 24 h and the resting time interval was 6 days. Training with BCG and oxLDL also led to the increased production of reactive oxygen species, whereas training with ␤-glucan led to the decreased production of reactive oxygen species. We describe the optimal conditions for an in vitro experimental model with human primary monocytes for study of the induction of trained innate immunity by microbial and metabolic stimuli.T he immune response is a complex system of cellular and humoral components which have the ability to detect non-selfstructures and confer protection against invading pathogens, playing a crucial role in the survival of multicellular organisms. The immune response has traditionally been divided into the innate immune system and the adaptive immune system. Adaptive immunity, with T and B cells as cellular effectors, develops over several weeks after birth, is highly specific, and builds a specific immunological memory, leading to protection against reinfection. The innate immune system, on the other hand, is classically thought to act rapidly in a nonspecific and identical manner every time that it encounters a pathogen, without having the ability to build an immunological memory.Recently, the concept that innate immunity is nonspecific and completely lacks immunological memory has been challenged. First, pattern recognition receptors (PRRs) confer some specificity to innate immunity, and second, a growing body of literature shows that innate immunity can adapt its function after a first insult (1, 2). Several studies have shown that plants and invertebrates, organisms which lack an adaptive immune system, show enhanced immune responses upon reinfection (3). This phenomenon has r...

The role of Toll‐like receptor 10 in modulation of trained immunity

“…24 We induced trained immunity in vitro by incubating monocytes with b-glucan for 24 hr and re-stimulation at day 6 with LPS, as a non-specific stimulus. 24 We induced trained immunity in vitro by incubating monocytes with b-glucan for 24 hr and re-stimulation at day 6 with LPS, as a non-specific stimulus.…”

Whole Blood Profiling of Bacillus Calmette–Guérin-Induced Trained Innate Immunity in Infants Identifies Epidermal Growth Factor, IL-6, Platelet-Derived Growth Factor-AB/BB, and Natural Killer Cell Activation

“…It is also possible to isolate adult blood monocytes and train them directly with BCG and β-glucan in vitro (32). Although the assay described in this paper was designed primarily to look at secreted cytokines and chemokines, it was possible to harvest PBLs and to examine monocytes and NK cells for evidence of BCG vaccination-associated trained innate activation.…”

Different Regulation of Interleukin-1 Production and Activity in Monocytes and Macrophages: Innate Memory as an Endogenous Mechanism of IL-1 Inhibition

“…The concept, well known in invertebrates and also in mice, applies also to human innate immune cells in vivo and in vitro (Quintin et al, 2012; Arts et al, 2015; Bekkering et al, 2016a; van der Valk et al, 2016). In our study, we have used an in vitro system, based on human primary monocytes and tissue-like/deactivated monocyte-derived macrophages from the same donors, to examine the role of innate memory in determining the production of active IL-1β.…”