A system immunology approach to understanding human langerhans cell tolerogenic function

Abstract

Langerhans cells (LC) maintain skin homeostasis through orchestrating immunogenic and tolerogenic immune responses in steady-state epidermis and at local lymph nodes after migration.While the mechanisms promoting activation of immune responses has been elucidated, little is known about the molecular mechanisms underpinning LC induced tolerance. We hypothesised that heterogeneous human LC populations existed in situ, which specialised in regulation of immunogenic vs tolerogenic regulation in healthy skin. Within the transcriptome of tolerance regulating LC, we sought to identify key molecular mediators of tolerogenic programming.

Previously published transcriptomic data containing a wide selection of DC subpopulations, including LC was analysed. Here, core mechanisms of tolerogenic programming across DCs was investigated. Overall, LC transcriptomic programming was largely distinct, although common pathways of suppression of stimuli responsiveness were identified in steady-state LCs.

The Drop-seq scRNA-seq protocol was optimised and implemented on steady-state and migrated LCs, to explore heterogeneity amongst populations and identify key molecular regulators of tolerogenic programming. This identified tolerogenic programmes (including IDO1) to be upregulated in migrated LCs, alongside enhanced immunocompetent programming compared to steady-state LCs. The latter populations were split into two subpopulations defined by immaturity and immunocompetency. Transcription factors (TFs) that correlated with enhanced migrated LC tolerogenic programmes included IRF4 and RELB.

Using in vitro experimentation, the importance of LC immunocompetency for mediation of Treg induction was revealed in steady-state LC populations (Immunocompetent/CD86High and Immature/CD86Low). Furthermore, immunocompetent migrated LCs displayed enhanced induction of functionally suppressive Tregs. Inhibition of IDO1 reduced migrated LC tolerogenic potential, revealing the criticalness of tolerogenic programming for LC tolerogenicity.

Mathematical modelling using TFs identified from analyses thought to control
immunogenic (IRF1±IRF4) vs tolerogenic (IRF4, MAP3K14, RELB) programming reflected observations from previous studies in which unstimulated migrated LC display both immunogenic and tolerogenic potential whilst, inflammatory stimuli (TNFa) increases favouring of immunogenic responses.

Overall, our analysis identified critical mechanisms which equip LCs with tolerogenic function and revealed potential mechanisms by which immunogenic vs tolerogenic responses are initiated.

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Identifiers

ORCID for Benjamin Macarthur: orcid.org/0000-0002-5396-9750

ORCID for Donna Davies: orcid.org/0000-0002-5117-2991

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