To address these issues, we developed an in vitro system using microglia derived from human or mouse brains to evaluate whether microglia can take up the seed-capable form of tau that has been implicated in neuron to neuron propagation of tau in Alzheimer disease, and, if so, whether they degrade and/or secrete it back to the extracellular space. Here, MAPT is linked to early-onset autosomal dominant Alzheimer disease.