Optimization of a neuron-microglia co-culture model to explore cell-to-cell interaction dynamics.

INTRODUCTION: The dialogue between the immune and nervous systems in the central nervous system (CNS) is a fundamental challenge in neuroscience and neuroimmunology. Microglia, the brain's resident immune cells, continuously communicate with neurons to maintain brain homeostasis, support development, and orchestrate responses to injury. METHODS: We adapted the CEBPA/SPI1 transcription factor overexpression approach to generate induced microglia-like cells (iMG) from human embryonic stem cells (hESCs), establishing a reproducible and standardized baseline framework for neuron-microglia co-culture. iMG characterization included assessment of key microglial markers (GPR34, MMP9, TMEM119, IBA1, P2Y12), phagocytic capability benchmarked against the human microglial cell line HMC3, and high-resolution scanning electron microscopy (SEM). iMGs were then co-cultured with hESC-derived NGN2-induced neurons (iNeu), with systematic optimization of seeding ratios and temporal parameters. RESULTS: iMGs expressed key microglial markers and demonstrated phagocytic capabilities comparable to HMC3 cells. In co-culture, iMGs exhibited remarkable phenotypic plasticity, with upregulation of both homeostatic and activation-associated microglial genes. Reciprocally, neurons showed increased expression of maturation and synaptic markers, indicating a bidirectional crosstalk. SEM revealed spatially variable iMG morphology - transitioning from ramified to amoeboid forms depending on proximity to neuronal structures. DISCUSSION: This contact-based in vitro system provides a versatile platform for investigating neuroimmune interactions, designed to be further integrated with additional CNS cell types, and applicable to the study of microglial activation, neuron-microglia signalling, and the impact of infections on CNS homeostasis.