Cancer mortality is the second leading cause of death in the United States. It is estimated that 90% of these are attributed to stage IV metastatic disease, in which tumor cells have disseminated from the primary tumor to form clinically relevant macroscopic metastatic colonies in distant organs. While advances in early screening protocols, surgical intervention and treatment with chemotherapy, radiation therapy, immunotherapies, and targeted therapies have improved the management of primary tumors in earlystage cancer patients;​ cancers that have progressed to metastatic disease are often resistant to these strategies and generally represent a terminal illness. Furthermore, even aggressive treatment of some cancers, such as triple-negative breast cancer, can often leave behind significant residual cancer burden that can spontaneously relapse and culminate in metastatic disease, even years after initial treatments. Until advances throughout the last century, metastasis-associated cancer mortality was thought to be largely attributed to local complications due to widespread tumor burden, such as the physical crushing of vital organs and damage to blood circulation, neuron circuitry, and the gastrointestinal tract. And while these are often causes metastasis-associated mortality, studies have revealed that cancer metastasis is a systemic illness that dysregulates the host’s metabolism and immune system.

It is now well known that tumor cells themselves or through signaling with non-tumor cells – can release a milieu of soluble factors, exosomes, and metabolites that can systemically alter host metabolism, physiology, and immune regulation to promote metastasis and compromise natural homeostasis. These systemic alterations often culminate in a lethal condition known as cachexia, which is defined as:​ “a multifactorial syndrome characterized by an ongoing loss of skeletal muscle mass (with or without loss of fat mass) that cannot be fully reserved by conventional nutritional support and leads to progressive functional impairment”. Cancerassociated cachexia develops in about 80% of metastatic cancer patients, depending on cancer type, has no approved efficacious treatments, and is associated with 20% of all cancer-related deaths due to severe loss muscle mass and function.

Here, we show that the aberrant muscle-cell upregulation of the metal ion importer, ZIP14, expression and concomitant increase in intramuscular zinc is associated with the development of cachexia in Pan02 and FC1242 experimental metastasis models of pancreatic adenocarcinoma and a Bard1-deficient model of BRCA-like triple-negative breast cancer. Furthermore, we show that ZIP14 expression is highly upregulated human advanced pancreatic adenocarcinoma patients. These findings are consistent with our previous studies that show the ZIP14-zinc axis mediates the development of cachexia in metastatic models of breast, lung, and colon cancer and suggest that inhibition of ZIP14 function or zinc chelation strategies may provide a potential therapeutic option for the prevention or treatment of metastasis-induced cachexia.

In addition to systemic metabolic dysregulation, metastatic cancers also locally or systemically dysregulate immune responses and immune cell populations to promote tumor growth and help facilitate metastasis. As the tumor progresses and metastasizes, tumor cell- or tumor stroma-derived factors polarize many myeloid and lymphocyte cell populations into tumorpromoting subsets that can suppress anti-tumor immune response through the production of immunosuppressive cytokines or expression of co-inhibitory molecules, remodel the extracellular matrix, induce angiogenesis, and promote the survival, proliferation, and motility of tumor cells. These tumor-associated factors can also dysregulate the hematopoiesis of immune cells, resulting in a systemic alteration of immune cell populations, such as peripheral expansion of immunosuppressive, immature myeloid-derived suppressor cells or T-regulatory cells. While the functional roles and subsets of tumor-infiltrating myeloid and T lymphocytes have been extensively studied, the role and function of tumor-infiltrating B lymphocytes are less defined, especially in the metastatic context. Here, we show that a population of B220⁺ B cells is recruited to the invasive margin of lung metastases in triple-negative breast cancer patients and 4T1 and LM3 metastatic mouse models of triple-negative breast cancer. Furthermore, B220⁺ B cells isolated from the metastatic lungs of the 4T1 and LM3 triple-negative breast cancer models were found to enhance the invasion of lung metastasis-derived organoids in 3-dimensional co-culture and promote the migration of triple-negative breast cancer cell lines. We confirmed that 4T1 B cell-deficient mouse models exhibit impaired formation of lung metastases and a concomitant reduction in the proportion of cytokeratin-14⁺ invasive leader tumor cells and p-mTOR⁺ tumor cells per lung metastasis, suggesting that the B cell-associated invasion-promoting mechanism is mediated in part by p-mTOR. We further show that the lung metastases of triple-negative breast cancer patients with high tumor-infiltrating B cell density exhibit increased p-mTOR expression compared to patients with low tumor-infiltrating B cell density. Taken together, our findings provide evidence of a tumor invasion-promoting function for metastasis-infiltrating B cells in metastatic triple-negative breast cancer and suggest that further elucidation of the metastasisinfiltrating B cell phenotype and the role of p-mTOR in the tumor cell-B cell invasion-promoting mechanism may reveal promising targets for the treatment of triple-negative breast cancer patients.