HMOX1 and neoplasm: As IL-6 was a regulator of the FAP+ HO-1+ TAM phenotype in this model, we analysed pulmonary metastasis in 4T1 tumours grown in Il6−/− mice, and in agreement, found significantly fewer metastases in the absence of host-derived IL-6 (Fig. 6k), demonstrating that IL-6 played a key role in the process.