IL-4 Promotes CCL17 in Human Dendritic Cells; Implications for Allergic Asthma

John Altpeter, Washington University School of Medicine St. Louis, MO, United States; Carol Webster, Washington University School of Medicine St. Louis, MO, United States; Michelle A. Gill, Washington University School of Medicine St. Louis, MO, United States

Background: Human Dendritic Cell (DC) responses are linked to asthma pathogenesis. Allergic ("T2") mediators including IgE and eosinophils disrupt DC responses; biologic therapies targeting these mediators in allergic asthma improve DC function (PMID 28870461, 35413355). IL-4 represents another allergic mediator contributing to asthma pathogenesis but its impact on human blood plasmacytoid (pDC) and myeloid (mDC) DCs remains undefined.

Objectives: To determine the effect of IL-4 on human pDC and mDC production of CCL17, a chemokine that mediates recruitment of pathogenic allergic T lymphocytes to the lung in asthma.

Design/Methods: Human pDCs and mDCs were purified from PBMCs from anonymous blood bank donors using antibody-labeled magnetic beads and cultured +/- IL-4 (10 ng/ml) for 20 hr. CCL17 was measured by ELISA in culture sups; CCL17, IRF4, STAT6 and IL-4Ra mRNA was measured by qPCR in RNA extracted from cell pellets. In select experiments, similar analyses were conducted on mDCs exposed to TSLP (Thymic Stromal Lymphopoietin; considered the master initiator of T2 allergic responses) +/- IL-4.

Results: IL-4 induced CCL17 secretion from both pDCs (65 vs 2.1; p< 0.01) and mDCs (234 vs 0.52; p< 0.01; Figure 1). CCL17 mRNA was similarly induced by IL-4 but significant only in mDCs (2454-fold increase in IL-4 vs control; p< 0.01; Table I). IL-4 promoted transcription of IRF4, a component of CCL17 signaling but did not impact STAT6 mRNA. IL-4 induced higher mDC CCL17 secretion than TSLP, a T2-promoting mediator known to drive mDC CCL17. Exposure to both IL-4+TSLP induced the greatest mDC CCL17 (Figure 2). The IL-4+TSLP combination also increased IL-4 receptor (IL-4Ra) mRNA expression in mDCs (Table I), suggesting a potential mechanism through which these 2 allergic mediators amplify mDC responses to IL-4.

Conclusion: IL-4, a mediator of T2 inflammation, promotes CCL17 responses in human DCs. mDCs displayed the greatest IL-4 responsiveness, with enhanced CCL17 protein and CCL17 and IRF4 mRNA resulting from IL-4 exposure. The finding that IL-4 induced higher mDC CCL17 responses than the master T2-promoting factor TSLP and that combined IL-4+TSLP exposure promoted even greater CCL17 responses highlights the potential impact of this cytokine on pathogenic mDC responses in the context of allergic asthma. This study provides the foundation to investigate the impact of Anti-IL-4Ra biologic therapy, a current strategy for treatment of allergic asthma, on mDC CCL17 responses and how these relate to clinical asthma outcomes.