HAPI Cells are SIM-A9-related Mouse Microglial Cells Useful for <i>In Vitro</i> Modeling of Microglial Immunometabolism

Highly aggressively proliferating immortalized (HAPI) cells were initially described as a spontaneously immortalized rat cell line isolated from a mixed neonatal rat glial population. It was demonstrated that HAPI cells are phagocytic, stain for macrophage-/microglia-specific markers like CD11b and GLUT5, and exhibit lipopolysaccharide (LPS)-induced nitric oxide (NO) and tumor necrosis factor-alpha (TNF-α) release. These characteristics led to their widespread use as a rat microglial cell line. Here, we report that HAPI cells are mouse cells, not rat cells, but further establish that they have a microglia-like identity and properties useful for <i>in vitro</i> modeling. Cell line authentication by short tandem repeat (STR) profiling, a method that detects identifying DNA signatures, indicates that HAPI cells are a 100% match for SIM-A9 cells, a mouse microglial cell line reported to be spontaneously immortalized from primary cell culture. We find that both HAPI cells and SIM-A9 cells express the microglia-selective gene <i>Tmem119</i>, as well as the microglia/macrophage-selective marker <i>Cx3cr1</i>, supporting a microglial origin. Like primary rodent microglia or macrophages, HAPI cells respond to combined stimulation with LPS and the Type II interferon, interferon-gamma (IFN-γ), with a pro-inflammatory morphology, NO production, NO-dependent suppression of mitochondrial oxygen consumption, and increased extracellular acidification (an indicator of glycolysis). The Type I interferon, interferon-alpha (IFN-α), also reduces mitochondrial oxygen consumption when administered alone or in combination with LPS. Overall, results indicate that HAPI cells are SIM-A9-related mouse cells of microglial origin and support their continued use to study microglial behavior <i>in vitro</i>, including immunometabolism.