Molecular Analysis of a Short-term Model of β-Glucans-Trained Immunity Highlights the Accessory Contribution of GM-CSF in Priming Mouse Macrophages Response

Abstract

β-Glucans (BGs) are glucose polymers present in the fungal cell wall (CW) and, as such, are recognized by innate immune cells as microbial-associated pattern through Dectin-1 receptor. Recent studies have highlighted the ability of the pathogenic yeast Candida albicans or its CW-derived β(1,3) (1,6)-glucans to increase human monocytes cytokine secretion upon secondary stimulation, a phenomenon now referred as immune training. This ability of monocytes programming confers BGs an undeniable immunotherapeutic potential. Our objective was to determine whether BGs from Saccharomyces cerevisiae, a non-pathogenic yeast, are endowed with such a property. For this purpose, we have developed a short-term training model based on lipopolysaccharide re-stimulation of mouse bone marrow-derived macrophages primed with S. cerevisiae BGs. Through a transcriptome analysis, we demonstrated that BGs induced a specific gene expression signature involving the PI3K/AKT signaling pathway as in human monocytes. Moreover, we showed that over-expression of Csf2 (that encodes for GM-CSF) was a Dectin-1-dependent feature of BG-induced priming of macrophages. Further experiments confirmed that GM-CSF up-regulated Dectin-1 cell surface expression and amplified macrophages response along BG-mediated training. However, the blockade of GM-CSFR demonstrated that GM-CSF was not primarily required for BG-induced training of macrophages although it can substantially improve it. In addition, we found that mouse macrophages trained with BGs upregulated their expression of the four and a half LIM-only protein 2 (Fhl2) in a Dectin-1-dependent manner. Consistently, we observed that intracellular levels of FHL2 increased after stimulation of macrophages with BGs. In conclusion, our experiments provide new insights on GM-CSF contribution to the training of cells from the monocytic lineage and highlights FHL2 as a possible regulator of BG-associated signaling.

Evidence of Trained Immunity in a Fish: Conserved Features in Carp Macrophages

“…To this end, a well-established in vitro culture of macrophages derived from the head kidney (21), the fish equivalent to mammalian bone marrow, was adapted to study the mechanisms of trained immunity in the common carp. Recent studies have shown trained immunity in monocytes and in bone marrow-derived macrophages (3,22) and even comparable trained immunity profiles between these two cell populations (23). These findings support the use of a heterogeneous myeloid primary cell culture, such as the head kidney-derived macrophage culture in carp.…”

Targeting SHIP-1 in Myeloid Cells Enhances Trained Immunity and Boosts Response to Infection

“…Dectin-1 sensing of β-glucan induces trained immunity in myeloid cells, including PBMCs ( Ifrim et al., 2013 ) or bone marrow-derived macrophages (BMDMs) ( Walachowski et al., 2017 ). We initially stimulated BMDMs with purified particulate β-glucan from S. cerevisiae , a well-known ligand for Dectin-1 ( Rosas et al., 2008 ).…”

Innate immune memory: An evolutionary perspective

“…In addition, recent studies have suggested that microbiota can induce long‐term functional reprogramming of bone marrow progenitors, and subsequently dendritic cells, to induce protection against Entamoeba histolytica . Which are the signals mediating the transfer of innate immune memory from the periphery to the bone marrow is not yet known in detail, although IL‐1β and GM‐CSF have been proposed as candidates. More studies are needed to understand in detail these processes.…”