Comparison of Lean, Obese, and Weight Loss Models Reveals TREM2 Deficiency Attenuates Breast Cancer Growth Uniquely in Lean Mice and Alters Clonal T Cell Populations

Obesity is an established risk factor for breast cancer development and poor prognosis. The adipose environment surrounding breast tumors, which is inflamed in obesity, has been implicated in tumor progression, and TREM2, a transmembrane receptor expressed on macrophages in adipose tissue and tumors, is an emerging therapeutic target for cancer. A better understanding of the mechanisms for the obesity-breast cancer association and the potential benefits of weight loss could help inform treatment strategies. Here, we utilized lean, obese, and weight loss mouse models to examine the impacts of TREM2 deficiency (Trem2+/+ and Trem2-/-) on postmenopausal breast cancer depending on weight history conditions. Trem2 deficiency constrained tumor growth in lean, but not obese or weight loss, mice. Single-cell RNA sequencing, in conjunction with VDJ sequencing of tumor and tumor-adjacent mammary adipose tissue (mATTum-adj) immune cells, revealed differences in the immune landscapes across the different models. Tumors of lean Trem2-/- mice exhibited a shift in clonal CD8+ T cells from an exhausted to an effector memory state, accompanied increased clonality of CD4+ Th1 cells, that was not observed in any other diet-genotype group. Notably, identical T cell clonotypes were identified in the tumor and mATTum-adj of the same mouse. Finally, anti-PD-1 therapy restricted tumor growth in lean and weight loss, but not obese, mice. These findings indicate that weight history could impact the efficacy of TREM2 inhibition in postmenopausal breast cancer. The reported immunological interactions between tumors and the surrounding adipose tissue highlight significant differences under obese and weight loss conditions.