Activation of Toll-like receptors (TLRs) induces inflammatory responses involved in immunity to pathogens and autoimmune pathogenesis, such as in patients with systemic lupus erythematosus (SLE). Although TLRs are differentially expressed across the immune system, a comprehensive analysis of how multiple immune cell subsets respond in a system-wide manner has not been described.

We sought to characterize TLR activation across multiple immune cell subsets and subjects, with the goal of establishing a reference framework against which to compare pathologic processes.

Peripheral whole-blood samples were stimulated with TLR ligands and analyzed by means of mass cytometry simultaneously for surface marker expression, activation states of intracellular signaling proteins, and cytokine production. We developed a novel data visualization tool to provide an integrated view of TLR signaling networks with single-cell resolution. We studied 17 healthy volunteer donors and 8 patients with newly diagnosed and untreated SLE.

Our data revealed the diversity of TLR-induced responses within cell types, with TLR ligand specificity. Subsets of natural killer cells and T cells selectively induced nuclear factor κ light chain enhancer of activated B cells in response to TLR2 ligands. CD14^hi monocytes exhibited the most polyfunctional cytokine expression patterns, with more than 80 distinct cytokine combinations. Monocytic TLR-induced cytokine patterns were shared among a group of healthy donors, with minimal intraindividual and interindividual variability. Furthermore, autoimmune disease altered baseline cytokine production; newly diagnosed untreated SLE patients shared a distinct monocytic chemokine signature, despite clinical heterogeneity.

Mass cytometry defined a systems-level reference framework for human TLR activation, which can be applied to study perturbations in patients with inflammatory diseases, such as SLE.