CA4 and infection: Our analyses have previously shown increased inflammation in the lungs of aged mice compared to young mice (Fig. 2C, Supplemental Fig. 2E); therefore, it was unsurprising that aged Car4+ ECs significantly upregulated Gdf15, a TGF-β family member and marker of inflammatory stress in lung damage and other pathological states64, post-infection compared to both young Car4+ ECs and conventional ECs (Supplemental Fig. 5A, C).