Stöth M, et al. - Researchers determined if and how primary tumor and metastatic growth, as well as the formation of an inflammatory niche, was influenced by splenectomy. The number of infiltrating tumor-associated macrophages and neutrophils and tumor-infiltrating dendritic cells within primary tumors was attenuated by splenectomy, though no impact on tumor growth was seen, in a murine 4T1 and E0771 breast and Panc02 pancreatic cancer model. Splenectomy caused a reduction in the number of hematogenous breast cancer lung metastases, however, failed to influence breast or pancreatic lymph node metastases. In breast cancer bearing mice, a change in the immune composition of the pre-metastatic niche in lungs was evident, and this could result in attenuation of metastases. Overall, an impact of splenectomy on the immune microenvironment of primary tumors as well as of pre-metastatic and metastatic sites was highlighted.