Deletion of Alzheimer's disease‐associated CD33 results in an inflammatory human microglia phenotype

Abstract

Genome‐wide association studies demonstrated that polymorphisms in the CD33/sialic acid‐binding immunoglobulin‐like lectin 3 gene are associated with late‐onset Alzheimer's disease (AD). CD33 is expressed on myeloid immune cells and mediates inhibitory signaling through protein tyrosine phosphatases, but the exact function of CD33 in microglia is still unknown. Here, we analyzed CD33 knockout human THP1 macrophages and human induced pluripotent stem cell‐derived microglia for immunoreceptor tyrosine‐based activation motif pathway activation, cytokine transcription, phagocytosis, and phagocytosis‐associated oxidative burst. Transcriptome analysis of the macrophage lines showed that knockout of CD33 as well as knockdown of the CD33 signaling‐associated protein tyrosine phosphatase, nonreceptor type 6 (PTPN6) led to constitutive activation of inflammation‐related pathways. Moreover, deletion of CD33 or expression of Exon 2‐deleted CD33 (CD33ΔE2/CD33m) led to increased phosphorylation of the kinases spleen tyrosine kinase (SYK) and extracellular signal‐regulated kinase 1 and 2 (ERK1 and 2). Transcript analysis by quantitative real‐time polymerase chain reaction confirmed increased levels of interleukin (IL) 1B, IL8, and IL10 after knockout of CD33 in macrophages and microglia. In addition, upregulation of the gene transcripts of the AD‐associated phosphatase INPP5D was observed after knockout of CD33. Functional analysis of macrophages and microglia showed that phagocytosis of aggregated amyloid‐β1‐42 and bacterial particles were increased after knockout of CD33 or CD33ΔE2 expression and knockdown of PTPN6. Furthermore, the phagocytic oxidative burst during uptake of amyloid‐β1‐42 or bacterial particles was increased after CD33 knockout but not in CD33ΔE2‐expressing microglia. In summary, deletion of CD33 or expression of CD33ΔE2 in human macrophages and microglia resulted in putative beneficial phagocytosis of amyloid β1‐42, but potentially detrimental oxidative burst and inflammation, which was absent in CD33ΔE2‐expressing microglia.

Regulation of microglia population dynamics throughout development, health, and disease

“…The human CD33 rs12459419C allele leads to the production of a long isoform of CD33 in microglia with increased AD risk compared to the rs12459419T allele that preferentially produces a short isoform of CD33 (CD33m) in microglia that reduces AD risk (Malik et al, 2013;Raj et al, 2014). Recently, there has been a push to understand the individual roles of these two ADassociated isoforms of CD33 in skewing microglia to a phenotype, particularly in relation to their ability to phagocytose Aβ (Ann Butler et al, 2021;Bhattacherjee et al, 2019;Bhattacherjee et al, 2021;Wissfeld et al, 2021). Although CD33 is proposed to be functionally linked to TREM2 in microglia (Chan et al, 2015), how these two isoforms of CD33 alternatively regulate TREM2 requires further examination.…”

Old and New Biomarkers for Infection, Inflammation, and Autoimmunity in Treatment-Resistant Affective and Schizophrenic Spectrum Disorders

“…CD33, a sialic-acid binding immunoglobulin-like lectin (Siglec-3), was elevated in monocytes of both patient groups. It interferes with monocyte and macrophage phagocytic function [ 27 ], regulates cell adhesion processes and endocytosis, and inhibits cytokine release [ 28 ]. In SZ, but not in AF patients, we found an inverse correlation of CD33 on monocytes with plasma anti-inflammatory IL-10, which further indicates inferior monocyte cell activation and function.…”

A tensor decomposition-based integrated analysis applicable to multiple gene expression profiles without sample matching

“…This dataset [27], which is denoted as dataset 2 (i.e., k = 2) in this study, was composed of four classes, including THP1 macrophages after knockout of CD33 and/or knockdown (silencing) of PTPN6 (6 samples per class). Thus, in total, it includes as few as 24 samples.…”