B Lymphocytes, Potent Antigen Presenting Cells for Preferential Expansion of Allo-Reactive FoxP3+ CD4 Regulatory T Cells

The naturally arising FoxP3+ CD4 T (nTr) cells are absolutely required for the induction and maintenance of immunological tolerance to self antigens. In addition to their physiological role, nTr cells can also be utilized effectively for treatment of autoimmune diseases, allograft rejection and graft versus host disease in animal models. Due to the very low frequency in the peripheral blood, nTr cells need to be expanded in ex vivo to generate sufficient numbers for therapeutic applications. The nTr cells can be expanded either polyclonally using anti-CD3/CD28 antibodies, or in an alloantigen-specific manner using allogeneic antigen presenting cells. The allospecific nTr cells are more therapeutically effective with less risk to cause non-specific immune suppression as compared to polyclonal nTr cells. Despite the success in expanding murine nTr cells, little success has been achieved in expanding human allospecific nTr cells, posing a major barrier to the development of nTr cell-based immunotherapy in humans. We have found that mouse B cells preferentially activate and induce expansion of nTr cells in allogeneic mixed lymphocyte reactions. The preferential expansion of Foxp3+ T cells can be further enhanced by a partial blockade of class II MHC-TCR interaction, suggesting that nTr cells preferentially respond to weak TCR stimulation. Extending the findings with murine B cells, we have further found that human B cells can efficiently expand allogeneic human nTr cells ex vivo. The expanded nTr cells express very high levels of FoxP3, maintain an anergic phenotype, and are potent suppressor cells capable of inhibiting the alloreactivity of thirdparty responder T cells at very low nTr-to-T effector cell ratios in an alloantigen-specific manner. The allospecificity possessed by the B cell-expanded nTr cells is not determined by the HLA haplotypes of the nTr cells, but it is induced and determined by the HLA haplotype of the B cells used to expand nTr cells. Our findings represent a significant advance in the development of nTr cell-based immunotherapy in humans and raise the possibility of using “off-the-shelf” third-party nTr cells for therapeutic applications. This review also outlines some patents on immunotherapy.