Identifying Immune Signalling Pathways in Response to Archaeal Stimuli : A Study on the Recognition of M. stadtmanae and M. smithii

Presence of the archaeon M. stadtmanae (MSS), among others, in the household microbiome has been associated with reduced asthma risk. In contrast to bacteria, which usually activate immune cells by diverse cell wall components and their nucleic acids through multiple pattern recognition receptors, only one such interaction has been identified for archaea, namely recognition of archaeal RNA via TLR8. There is limited understanding about the downstream signalling beyond TLR8 activation by these microbes.
This work identifies and differentiates immune signalling pathways activated upon recognition of archaeal cells of the species MSS and M. smithii (MBS).
A co-culture of primary CD4 T-cells with stimulated autologous monocyte-derived dendritic cells (moDC) was used to investigate the potential for T-cell polarisation by the tested archaea via moDC. Peripheral blood mononuclear cells (PBMC) from healthy donors were used to test the cytokine induction in fully differentiated effector cells by both archaeal species. To further elucidate the downstream mechanisms, site-specific knockouts of genes coding for signalling molecules downstream of TLR8 were generated by using CRISPR/Cas9 on the monocyte-like cell line BLaER1.
Stimulation of moDC with MSS drove TH1 polarisation in naïve T-cells and elicited a strong TH1 and IL-10 effector response from PBMC that strongly interacted with TLR4 and TLR2 signalling. Data of KO cell lines show IRF5 is essential for the response to MBS while it is involved in but not essential for the response to MSS. Negative feedback regulation by IKKα is not required for responses to any tested stimulants, while positive signal transduction by IRAK4 and IKKβ is essential. Unbiased analysis via RNA-Seq confirmed the TH1-inducing potential of MSS, uncovered possible immunoregulatory mechanisms engaged by MBS, and revealed a putative novel pathway of immunoregulation by archaea via complement.

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