Lanthanide nanoparticles for high sensitivity multiparameter single cell analysis

Abstract

Mass cytometry (MC) is a high throughput multiparameter analytical technique for determining biomarker expression in cells. In MC, antibodies (Abs) are tagged with heavy metal isotopes via conjugation to metal chelating polymers (MCPs). To improve the sensitivity of MC towards low abundance biomarkers, we are developing nanoparticle (NP)-based reagents as mass tags for Abs. We examine the use of silica-coated NaHoF₄ NPs (d ∼ 12 nm) decorated with PEG5k conjugated to thiol-modified primary or secondary Abs for MC assays. We compare the sensitivity of NP–Ab conjugates to MCP–Ab conjugates towards seven biomarkers with varying expression levels across six cell lines. We also perform a multi-parameter assay using a cocktail of both NP- and MCP-based reagents to detect seven cellular markers in peripheral blood mononuclear cells (PBMCs). In the case of highly abundant markers, signal enhancements from NP–Ab conjugates offer minimal advantages over MCP–Ab conjugates, which already give strong signals. In the case of biomarkers with lower abundance, the level of signal enhancements depended on the nature of the biomarker being detected, or on the type of detection method used. When comparing the indirect detection of CD14 on THP-1 cells using NPs or MCPs conjugated to secondary Abs, the NP reagents offered little signal enhancements compared to the MCP reagents. However, in the case of direct CD14 detection on THP-1 or U937 cells using NPs or MCPs conjugated to primary Abs, a 30- or 450-fold signal enhancement was seen from the NP-based reagent. In the experiments where both NP–Ab and MCP–Ab conjugates were used together to stain PBMCs, we found that the presence of the NP–Ab conjugates did not affect the function of MCP–Ab conjugates, and the NP–Ab conjugates showed minimal non-specific interaction with cells without the target biomarker (CD14). Furthermore, these NP–Ab conjugates could be used to identify rare CD14⁺ monocytes from the PBMC mixture with a 20-fold signal increase when compared to the use of only MCP–Ab conjugates. Collectively, the strong signal amplification obtained from NP reagents demonstrate the potential of these reagents to be used in conjunction with MCP-reagents to detect rare cellular markers or cell types that may otherwise be overlooked when using MCP-reagents alone.