**Abstract**

The bone marrow is a dynamic organ where osteogenesis and bone remodeling take place side by side with hematopoiesis and the maintenance of immunological memory. It provides a unique microenvironment favoring the colonization and outgrowth of breast cancer cells. The outcome of breast-cancer-derived bone metastases depends on the formation of a pre-metastatic niche, which is initiated through "education" of non-tumoral cells present in the primary cancerous niche. Among other participants, immune cells and their secreted factors can boost the successful seeding of the distant disease. In this chapter, we discuss the reciprocal interplay between bone and T and B cells, particularly in pathological contexts. In the first part, we are exploring the knowledge brought by the osteoimmunology field, especially from the best studied disease in this area, rheumatoid arthritis. In the second part, we summarize the latest findings on underlying cellular and molecular mechanisms for breast-cancer-derived bone pre-metastatic niche formation. In addition, we explore the concept that breast-tumor-primed T and B cells function as messengers from the periphery to the bone marrow, alter bone turnover homeostasis in favor of osteoclasts, before tumor colonization, leading to a pre-metastatic niche formation to further the development of bone metastases.

**Keywords:** bone metastases, T cells, B cells, dendritic cells, osteoclasts, osteoblasts, breast tumor and pre-metastatic niche
