In response to T cell-dependent antigens, B cells proliferate extensively to form germinal centres (GC), and then differentiate into memory B (B mem ) cells or long-lived plasma cells (LLPCs) by largely unknown mechanisms. Here we show a new culture system in which mouse na ï ve B cells undergo massive expansion and isotype switching, and generate GC-phenotype B (iGB) cells. The iGB cells expressing IgG1 or IgM / D, but not IgE, differentiate into B mem cells in vivo after adoptive transfer and can elicit rapid immune responses with the help of cognate T cells. Secondary culture with IL-21 maintains the proliferation of the iGB cells, while shifting their in vivo developmental fate from B mem cells to LLPCs, an outcome that can be reversed by withdrawal of IL-21 in tertiary cultures. Thus, this system enables in vitro manipulation of B-cell fate, into either B mem cells or LLPCs, and will facilitate dissection of GC-B cell differentiation programs.

Immunotherapies such as adoptive transfer of T cells or natural killer cells, or monoclonal antibody (MoAb) treatment have recently been recognized as effective means to treat cancer patients. However, adoptive transfer of B cells or plasma cells producing tumor-specific antibodies has not been applied as a therapy because long-term culture and selective expansion of antigen-specific B cells has been technically very difficult. Here, we describe a novel cancer immunotherapy that uses B-cell adoptive transfer. We demonstrate that germinal-center-like B cells (iGB cells) induced in vitro from mouse naïve B cells become plasma cells and produce IgG antibodies for more than a month in the bone marrow of non-irradiated recipient mice. When transferred into mice, iGB cells producing antibody against a surrogate tumor antigen suppressed lung metastasis and growth of mouse melanoma cells expressing the same antigen and prolonged survival of the recipients. In addition, we have developed a novel culture system called FAIS to selectively expand antigen-specific iGB cells utilizing the fact that iGB cells are sensitive to Fas-induced cell death unless their antigen receptors are ligated by membrane-bound antigens. The selected iGB cells efficiently suppressed lung metastasis of melanoma cells in the adoptive immunotherapy model. As human blood B cells can be propagated as iGB cells using culture conditions similar to the mouse iGB cell cultures, our data suggest that it will be possible to treat cancer-bearing patients by the adoptive transfer of cancer-antigen-specific iGB cells selected in vitro. This new adoptive immunotherapy should be an alternative to the laborious development of MoAb drugs against cancers for which no effective treatments currently exist.

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