TLR-mediated immune activation is a strong suppressor of the Treg-attracting chemokine CCL22

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Abstract The immunoregulatory chemokine CCL22 is mainly secreted by dendritic cells (DC) in lymph nodes and plays a crucial role in maintaining the balance between immune tolerance and efficient immune response by induction of intercellular contacts between DC and regulatory T cells (Treg) expressing the only so far known CCL22 receptor CCR4. In the absence of CCL22, the immune response to vaccination and cancer has been proven to be enhanced. In this work, we aimed to understand how activation of Toll-like receptors (TLR) modulates CCL22 levels mechanistically in vitro and in vivo. We observed that a bacterial infection with salmonella typhimurium and treatment with different TLR agonists such as Poly (I:C), LPS, R848 and CpG led to a potent and long-lasting suppression of systemic CCL22 in vivo. Furthermore, using in vitro co-cultures of DC, B cells and T cells, we identified TLR-mediated CCL22 suppression to be mediated by soluble factors derived from non-DC. Additionally, we pursued to investigate the underlying mechanism and could show in vitro that CCL22 suppression is particularly driven by redundant action of IL-10, IFN-α and IFN-γ. Moreover, inhibition of NF-κB reverted the CCL22-suppressing effect of CpG on splenocytes. In CpG-treated mice, Treg/DC contacts were diminished to the level known from Ccl22−/− mice, an observation which links TLR-mediated CCL22 suppression to an immunological relevance. Taken together, we demonstrate that TLR signaling results in CCL22 downregulation via different cytokines, a mechanism which is potentially important to enable the induction of a strong immune response and efficient pathogen clearance in the setting of microbial infection.

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