Mouse lung transcriptional response to flagellin stimulation

Vaccine adjuvants enhance adaptive immunity to co-administered antigens. Whereas the modes of action are multiple, the activation of antigen-presenting cells (APC) like dendritic cells by adjuvants is a prerequisite. Detection of microbial signals by innate sensors like Toll-like receptors (TLR) is a major mechanism of APC activation. Most candidate or licensed vaccines assume that adjuvant activity of TLR agonists depends on direct effect on APCs. This study addressed whether TLR stimulation of non-hematopoietic cells could contribute to the adjuvant effect. Nasal administration of flagellin enhanced Tcell- and antibody-mediated immunity to co-administered antigens in a TLR5-dependent but inflammasome-independent manner. We found that lung radioresistant cells were sufficient to promote immunity, thereby suggesting that direct TLR5-mediated APC stimulation is dispensable to adjuvant activity. Consistent with this, radioresistant compartment is essential to stimulate the swift TLR5-dependent transcription. The transcriptional response was restricted to the epithelial compartment and was associated to the production of a narrow set of mediators including the chemokine CCL20, known to promote APC recruitment in mucosal tissues. Besides, flagellin was rapidly degraded in lower airways and was not transported into lung parenchyma or peripheral tissues. This study therefore suggests an unexpected mechanism for how TLR agonists act as adjuvant and how epithelium is instrumental to sense and integrate microbial signals to promote adaptive immunity. In conclusion, the immune-enhancing effect of adjuvants on epithelial cells can be harnessed for improving vaccines.

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