GM-CSF Production by Non-Classical Monocytes Controls Antagonistic LPS-driven Functions In Allergic Inflammation

Lipopolysaccharide (LPS) can either promote or prevent T helper-2 (Th2) cell-allergic responses. However, the underlying mechanism remains unknown. We show here that LPS activity switches from pro-pathogenic to protective depending on the production of GM-CSF by non-classical monocytes. In the absence of GM-CSF, LPS can favor pathogenic Th2 cell responses by supporting the trafficking of lung migratory dendritic cells (mDC2s) into the lung-draining lymph node. However, when non-classical monocytes produce GM-CSF, LPS and GM-CSF synergize to differentiate monocyte-derived DCs (moDCs) that instruct mDC2s for Th2 cell suppression. Importantly, only allergens with cysteine protease activity trigger GM-CSF production by non-classical monocytes. Hence, the therapeutic effect of LPS is restricted to allergens with this enzymatic activity. Treatment with GM-CSF, however, restores the protective effects of LPS. Thus, GM-CSF produced by non-classical monocytes acts as a rheostat that fine-tunes the pathogenic and therapeutic functions of LPS. Monocytes were sorted from bone marrow of B6 mice, stimulated with media +/- 5ng GM-CSF for 3 hours, and RNA-seq was performed.