Identification of a dysfunctional microglial population in human Alzheimer's disease cortex using novel single-cell histology image analysis

Abstract

In Alzheimer's disease (AD), microglia are affected by disease processes, but may also drive pathogenesis. AD pathology-associated microglial populations have been identified with single-cell RNA-Seq, but have not been validated in human brain tissue with anatomical context. Here, we quantified myeloid cell markers to identify changes in AD pathology-associated microglial populations. We performed fluorescent immunohistochemistry on normal (n = 8) and AD (n = 8) middle temporal gyri, co-labelling the pan-myeloid cell marker, Iba1, with one of 11 markers of interest (MOIs): CD45, HLA-DR, CD14, CD74, CD33, CD206, CD32, CD163, P2RY12, TMEM119, L-Ferritin. Novel image analyses quantified the single-cell abundance of Iba1 and each MOI. Each cell was gated into one Iba1-MOI population (Iba1^low MOI^high, Iba1^high MOI^high, or Iba1^high MOI^low) and the abundance of each population was compared between AD and control. Triple-labelling of L-Ferritin and Iba1 with a subset of MOIs was performed to investigate L-Ferritin-MOI co-expression on Iba1^low cells. Iba1^low MOI^high myeloid cell populations delineated by MOIs CD45, HLA-DR, CD14, CD74, CD33, CD32, and L-Ferritin were increased in AD. Further investigation of the Iba1^low MOI^high populations revealed that their abundances correlated with tau, but not amyloid beta, load in AD. The Iba1^low microglial population highly expressed L-Ferritin, reflecting microglial dysfunction. The L-Ferritin^high CD74^high HLA-DR^high phenotype of the Iba1^low population mirrors that of a human AD pathology-associated microglial subpopulation previously identified using single-cell RNA-Seq. Our high-throughput immunohistochemical data with anatomical context support the microglial dysfunction hypothesis of AD.

Keywords: Alzheimer’s disease; Amyloid beta; Dysfunction; Immunohistochemistry; Microglia; Single-cell analysis; Tau.