In-vitro experimental model of trained innate immunity in human primary monocytes.
Innate immune memory or ‘trained immunity’ has recently been described as 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: β-glucan, the Bacille Calmette-Guérin (BCG) vaccine, and oxidized LDL (oxLDL).
We investigated and optimized a protocol of monocyte trained immunity induced by an initial training period with β-glucan, BCG or oxLDL, followed by washing and resting of the cells, and thereafter restimualtion with secondary bacterial stimuli. The training and resting time intervals varied to identify the optimal setting for long-term induction of trained immunity. Trained immunity was assessed in terms of secondary cytokine response, production of reactive oxygen species, cell morphology and induction of glycolysis.
Monocytes primed with β-glucan, BCG and oxLDL showed increased pro- and anti-inflammatory cytokine responses upon restimulation with non-related stimuli. Also, all three stimuli induced a switch to glycolysis (Warburg effect). These effects were most pronounced when training interval was 24h training and resting time interval was 6 days. BCG and oxLDL training also led to increased production of reactive oxygen species, whereas β-glucan training showed a decrease in reactive oxygen species production.
We describe the optimal conditions for an in vitro experimental model in human primary monocytes to study the induction of trained innate immunity by microbial and metabolic stimuli.