Pulmonary metastasis of breast cancer requires recruitment and expansion of T regulatory cells (Tregs) that promote escape from host protective immune cells. However, it remains unclear precisely how tumors recruit Tregs to support metastatic growth. Here we report the mechanistic involvement of a unique and previously undescribed subset of regulatory B cells. These cells, designated tumor-evoked Bregs (tBregs), phenotypically resemble activated but poorly proliferative mature B2 cells (CD19+ CD25High CD69High) that express constitutively active Stat3 and B7-H1High CD81High CD86High CD62LLowIgMInt. Our studies with the mouse 4T1 model of breast cancer indicates that the primary role of tBregs in lung metastases is to induce TGFβ-dependent conversion of FoxP3+ Tregs from resting CD4+ T cells. In the absence of tBregs, 4T1 tumors can not metastasize into the lungs efficiently due to poor Treg conversion. Our findings have important clinical implications,since they suggest that tBregs must be controlled to interrupt the initiation of a key cancer-induced immunosuppressive event that is critical to support cancer metastasis.

PMID: 21444674 [PubMed - as supplied by publisher] Source: National Library of Medicine.