FAP and neoplasm: In preclinical models of Lewis lung carcinoma (LL2) and pancreatic ductal adenocarcinoma (PDAC), a distinct subset of HO-1+ F4/80hi TAMs with an M2 phenotype co-expressing the fibroblast activation protein alpha (FAP+ HO-1+ TAMs) and representing 10% of the total TAMs population, has been described to be the major tumor source of HO-1 (51).