Expanding T_Reg cell numbers in vivo

There is great interest in harnessing the suppressive powers of regulatory T (T_Reg) cells in the clinic. Although it is possible to numerically expand T_Reg cells ex vivo, concerns over their purity and stability remain. An alternative approach is to expand pre-existing populations of T_Reg cells in vivo. This study showed that a monoclonal antibody specific for CD45RB (which has a role in T cell receptor signalling) increased the frequency and absolute number of T_Reg cells in mice. This increase required the presence of cognate antigen and was due to homeostatic proliferation of thymus-derived T_Reg cells rather than increased thymic output or conversion from effector T cells. CD45RB-specific antibody was shown to inhibit T_Reg cell motility, thereby increasing the contact time of the cells with dendritic cells. This increased contact time resulted in increased activation of nuclear factor of activated T cells (NFAT), which is required for T_Reg cell proliferation. These data describe a novel method by which T_Reg cell numbers may be increased in vivo.