S1 E). a separate window Introduction Alzheimers disease (AD) is the most common form of senile dementia. It affects one in eight Americans over the age of 65 yr and is the sixth leading cause of death in the United Mmp11 States (https://www.alz.org/alzheimers-dementia/facts-figures). AD is characterized by memory and executive function deficits, followed by progressive, global cognitive decline (Long and Holtzman, 2019; Sarlus and Heneka, 2017). Brain AD pathology consists of extracellular aggregates of amyloid (A) oligomers and large insoluble plaques, intraneuronal tau hyperphosphorylation, synaptic dysfunction, and neuronal cell death (Long and Holtzman, 2019; Sarlus and Heneka, 2017). AD lesions trigger a secondary ITK inhibitor 2 expansion of reactive microglia, which cluster around A plaques, limiting their spreading (Long and Holtzman, 2019; Sarlus and Heneka, 2017). Profiling of microglia transcriptome in mouse models of A accumulation has revealed that this increase in microglia numbers is associated with a robust transcriptional activation signature on a per-microglia basis, which has been referred to as disease-associated microglia (DAM), which is quite distinct from that of homeostatic microglia (Keren-Shaul et al., 2017). Recently, the analysis of the human microglial transcriptome in AD by single-nucleus RNA sequencing (RNA-seq) has revealed a microglial transcriptional response that in part recapitulates the mouse DAM signature (Mathys et al., 2019; Zhou et al., 2020). Studies of genetic risk for sporadic AD have suggested that microglia not only respond to disease but also modulate disease course (Karch and Goate, 2015; Lambert et al., 2013). Most notably, a hypomorphic missense mutation in the microglia receptor TREM2, R47H, increases the risk of AD severalfold, as do other TREM2 variants, such as R62H, although with reduced penetrance (Jonsson et al., 2013; Guerreiro et al., 2013). TREM2 is a lipid receptor expressed in microglia and other tissue macrophages, which promotes their survival and proliferation by transmitting intracellular activating signals through the adaptor ITK inhibitor 2 DAP12. Impaired TREM2 function in the 5XFAD mouse model of A pathology restricts the ability of microglia to proliferate and accumulate around A plaques to limit their pathogenic potential (Wang et al., 2015). TREM2-deficient microglia can acquire an incomplete DAM profile, or stage 1 DAM, but fail to develop a completely activated profile, or stage 2 DAM (Keren-Shaul et al., 2017). This defective microglial response leads to greater neuritic dystrophy adjacent to A plaques (Yuan et al., 2016; Wang et al., 2016). The beneficial role of TREM2-dependent microglial activation has been further supported by an in ITK inhibitor 2 vivo study showing that 5XFAD mice develop less A pathology when crossed to transgenic (Tg) mice overexpressing human TREM2 (hTREM2; Lee et al., 2018). Moreover, a recent study showed that myeloid cells with potentially beneficial effects on neurodegeneration can be generated in vitro with an agonist TREM2 antibody (Cheng et al., 2018). Taken together, these findings suggest that TREM2-dependent microglial activation can delay AD onset and/or progression. In this study, we examined the potential therapeutic impact ITK inhibitor 2 of a mouse anti-hTREM2 agonistic mAb named AL002c, which is a variant of a mAb, called AL002, that has recently been studied in a phase I clinical trial (https://clinicaltrials.gov/ct2/show/”type”:”clinical-trial”,”attrs”:”text”:”NCT03635047″,”term_id”:”NCT03635047″NCT03635047). The antibody was tested in Tg mice that express either the common variant (CV) or the R47H variant of hTREM2, but lack the endogenous gene (referred to as CV knockout [CV-KO] and R47H-KO, respectively; Song et al., 2018). We had previously shown that 5XFAD mice crossed to CV-KO (CV-KO-5XFAD) show more microglia activation and plaque coverage than 5XFAD mice crossed to R47H-KO mice (R47H-KO-5XFAD). We found that a single injection of AL002c expanded unique subpopulations of metabolically active and proliferating microglia in both CV-KO-5XFAD and R47H-KO-5XFAD mice, as assessed by single-cell RNA-seq (scRNA-seq). Moreover, prolonged treatment of both mouse models with AL002c.