TREM2 and neurodegenerative disease: Since the discovery of TREM2 (triggering receptor expressed on myeloid cells 2) variants as genetic risk factors for Alzheimer's disease (AD) (Guerreiro et al., 2013; Jonsson et al., 2013), TREM2 biology has become a focal point in research efforts to better understand how the innate immune system impacts AD and other neurodegenerative diseases (Jay et al., 2017b; Ulrich et al., 2017; Yeh et al., 2017).