PIWIL4 and pneumonia: In pneumonia, PIWIL2 and PIWIL4 are upregulated in response to endoplasmic reticulum (ER) stress, and MIWI2 modulates immune responses in multiciliated airway epithelial cells in mice (Figure 8A) (Hogan et al., 2014; Wasserman et al., 2017; Gebert et al., 2018).