Defined culture conditions improve functional properties of mature iPSC-derived macrophages for therapeutic screening

Background: Generation and use of human macrophages in vitro are essential for a variety of (therapeutic) applications. Recently, there has been growing interest in macrophages derived from induced pluripotent stem cells (iPSC-Mac), which has led to the development of numerous differentiation protocols. These protocols typically involve a stepwise differentiation process using well-defined culture media, though the final differentiation stage often relies on undefined animal serum components, such as fetal bovine serum (FBS). This study aimed to elucidate protocol-dependent effects on iPSC-Mac usability. Methods: We assessed the impact of a serum-supplemented medium (RPMI + FBS) compared to a fully defined medium (X-VIVO 15™) on the function of terminally differentiated human iPSC-Mac. Upon harvest, the cells were differentiated for three to four days in the respective media. Subsequently, phenotypic analysis was performed by microscopy and flow cytometry, as well as transcriptome and proteome analysis. Functional responsiveness to stimuli such as polarizing cytokines and lipopolysaccharide (LPS) was evaluated using flow cytometry and ELISA assay. The pharmaceutical screening potential was examined using secondary stimuli with dexamethasone. Results: While both media compositions effectively produced fully functional human macrophages, we observed significant differences in their stage of activation and their functional responsiveness. Macrophages differentiated under the defined, serum-free conditions showed a more neutral activation state, an enhanced cholesterol metabolism, and were more sensitive to post pro- or anti-inflammatory stimulation compared to serum-grown cells. Most important, iPSC-Mac from these cultures also exhibit a more robust and reproducible response to co-stimulatory signals with IFNγ or LPS in combination with the immune-suppressive agent dexamethasone. In contrast, serum-grown iPSC-Mac demonstrated a more pre-activated state, with noteworthy background levels of interleukin 6 (IL-6). Upon stimulation, these cells showed reduced sensitivity and responsiveness towards secondary signals, underscoring potential challenges when using FBS-based media for drug screening or immunomodulatory assessments. Conclusions: The enhanced responsiveness to co-stimulatory signals makes iPSC-Mac from defined cultures more suitable for testing immunomodulatory drugs, novel bio-assays and in vivo applications.