# Retinoic acid differently modulates NOD1/NOD2-mediated inflammatory responses in human macrophage subsets

## TL;DR

Retinoic acid affects how different types of human macrophages respond to inflammation, with distinct effects depending on the macrophage type.

## Contribution

This study reveals for the first time that retinoic acid modulates NOD1/NOD2-induced cytokine secretion differently in macrophage subsets.

## Key findings

- GM-MФ produce higher pro-inflammatory cytokines like IL-6, IL-8, TNF-α, and IL-1β compared to M-MФ.
- ATRA pre-treatment alters cytokine secretion patterns differently in GM-MФ and M-MФ.
- The effect of retinoic acid on cytokine secretion is subpopulation-specific and context-dependent.

## Abstract

Macrophages are indispensable in homeostasis and innate immune responses in multiple tissues, while their polarization and functional characteristics are determined by the activating stimuli and their tissue microenvironment. The vitamin A derivative retinoic acid shows inhomogeneous distribution among the tissues and has an important modulatory role in inflammatory responses. However, its effects on the cytokine secretion induced by the cytosolic pattern-recognition receptors NOD1 and NOD2 are unclear. In our study, we used human monocyte-derived macrophages differentiated in the presence of GM-CSF or M-CSF to generate inflammation inducing (GM-MФ) or inflammation resolving (M-MФ) cells, respectively. We activated the cells with either a NOD1- or NOD2 specific agonist and, using ELISA, we determined the pattern and dynamics of cytokines secreted by the macrophage subpopulations. Furthermore, we studied the effect of all-trans retinoic acid (ATRA) pre-treatment on the NOD1- and NOD2-induced cytokine release. Our comparative analysis shows subpopulation-characteristic pattern of cytokine secretion, as GM-MФ produce significantly higher pro-inflammatory IL-6, IL-8, TNF-α and IL-1β, while M-MФ secret higher anti-inflammatory IL-10. However, IL-18 and IFNβ secretion was comparable between the MФ subpopulations. We also show for the first time that ATRA has marked impact on cytokine secretion triggered by NOD1 and NOD2. Importantly however, the ATRA-induced changes of cytokine secretion follow opposite tendency in two MФ subpopulations. In conclusion, these results show that NOD1/NOD2-induced cytokine secretion by macrophage subsets is highly context-dependent and our results highlight the importance of the retinoic acid content of the local tissue environment in shaping macrophage function in health and disease.