Summary Molecular mechanisms that regulate tumour-associated macrophage (TAM) phenotype and function are incompletely understood. Here, we show that the pseudokinase TRIB1 is highly expressed by TAMs in breast cancer and that its expression correlates with response to chemotherapy and patient survival. We used immune-competent murine models of breast cancer to characterise the consequences of altered (reduced or elevated) myeloid Trib1 expression on tumour growth and composition of stromal immune cells. We found that both overexpression and knockout of myeloid Trib1 promote tumour growth, albeit through distinct molecular mechanisms. Myeloid Trib1 deficiency resulted in an early accelearation of tumour growth, paired with a selective reduction in perivascular macrophage numbers in vivo and enhanced oncogenic cytokine expression in vitro . In contrast, elevated levels of Trib1 in myeloid cells led to an increase in mammary tumour volume at late stages, together with a reduction of NOS2 expressing macrophages and an overall reduction of these cells in hypoxic tumour regions. In addition, we show that myeloid Trib1 is a previously unknown, negative regulator of the anti-tumour cytokine IL-15 and that increased expression of myeloid Trib1 leads to reduced IL-15 levels in mammary tumours, with a consequent reduction in the number of T-cells, that are key to anti-tumour immune responses. Together, these results define the different roles of TRIB1 in human breast cancer and provide a mechanistic understanding for the importance of myeloid TRIB1 expression levels in the development of this disease. Significance TRIB1 expression is strongly associated with response to chemotherapy in breast cancer patients with aggressive tumours. This protein is also highly expressed by tumour-associated macrophages. Thus, we used myeloid-specific alterations of Trib1 expression in mice ( Trib1 mKO and Trib1 mTg ), and characterised consequent changes in the growth rate and tumour microenvironment of mammary tumours. Both Trib1 mKO and Trib1 mTg enhanced tumour growth, but at different stages of tumour growth and via distinct mechanisms. Trib1 mKO significantly increased the expression of oncogenic cytokines, such as IL6, IL10, CCL20, PD-L1, and VEGF. In contrast, Trib1 mTg accelerated at the later stage of tumour growth via inhibition of hypoxic TAMs in the TME, as well as by reduced IL-15 expression thus leading to impaired naïve and cytotoxic T cell infiltration. These data define TRIB1 as a potential novel marker of therapeutic responses in breast cancer, as well as a key mechanistic regulator of the anti-tumour cytokine, IL-15 in myeloid cells.

TRIB1 regulates tumour growth via controlling tumour-associated macrophage phenotypes and is associated with breast cancer survival and treatment response