Dr. Guoyu Lan's latest paper was published in the Annals of Neurology. The objective of this study was triggering receptor expressed on myeloid cells-2 (TREM2) and progranulin (PGRN) are critical regulators of microglia activation and can be detected in cerebrospinal fluid (CSF). However, whether microglial reactivity is detrimental or neuroprotective for Alzheimer disease (AD) is still debatable. We identified 663 participants with baseline β-amyloid (Aβ) positron emission tomography (PET) and CSF biomarker data, including phosphorylated tau181 (p-Tau181), soluble TREM2 (sTREM2), PGRN, and growth-associated protein-43 (GAP-43). Among them, 254 participants had concurrent longitudinal CSF biomarkers. We used multivariate regression analysis to study the associations of CSF microglial biomarkers with Aβ PET, CSF p-Tau181, and CSF GAP-43 cross-sectionally and longitudinally. A Chinese aging cohort's independent CSF samples (n = 65) were analyzed as a validation. Results: Higher baseline levels of CSF microglial biomarkers were related to faster rates of CSF sTREM2 increase and CSF PGRN decrease. Elevated CSF p-Tau181 was associated with higher levels of CSF microglial biomarkers and faster rates of CSF sTREM2 increase and CSF PGRN decrease. In both cohorts, higher Aβ burden was associated with attenuated CSF p-Tau181 effects on CSF microglial biomarker increases. Independent of Aβ PET and CSF p-Tau181 pathologies, higher levels of CSF sTREM2 but not CSF PGRN were related to elevated CSF GAP-43 levels and faster rates of CSF GAP-43 increase. Interpretation: these findings suggest that higher Aβ burden may attenuate the p-Tau-associated microglial responses, and TREM2-related microglial reactivity may independently correlate with GAP-43-related presynaptic loss. This study highlights the two-edged role of microglial reactivity in AD and other neurodegenerative diseases.