Cigarette smoke-induced reduction of C1q promotes emphysema

Alteration of innate immune cells in the lungs can promote loss of peripheral tolerance that leads to autoimmune responses in cigarette smokers. Development of autoimmunity in smokers with emphysema is also strongly linked to the expansion of autoreactive T helper (Th) cells expressing interferon gamma (Th1), and interleukin 17A (Th17). However, the mechanisms responsible for enhanced self-recognition and reduced immune tolerance in smoker with emphysema remain less clear. Here we show that C1q, a component of the complement protein 1 complex (C1), is downregulated in lung CD1a+ antigen presenting cells (APCs) isolated from emphysematous human, and mouse lung APCs after chronic cigarette smoke exposure. C1q potentiated the function of APCs to differentiate CD4+ T cells to Tregs, while it inhibited Th17 cell development and proliferation. Mice deficient in C1q that were exposed to chronic smoke exhibited exaggerated lung inflammation marked by increased Th17 cells, while reconstitution of C1q in the lungs enhanced Tregs abundance, dampened smoke-induced lung inflammation, and reversed established emphysema. Our findings demonstrate that cigarette smoke-mediated loss of C1q could play a key role in reduced peripheral tolerance, which could be explored to treat emphysema.