T lymphocytes express B7 family molecules following interaction with dendritic cells and acquire bystander costimulatory properties.

Dendritic cells (DC) play a pivotal role in the initiation, maintenance and regulation of the immune response. Here we obtained the first evidence that DC, in the absence of any foreign antigens, induce the expression of B7 family costimulatory molecules, such as CD80, CD86, B7-H1, PD-L2, B7-H3, and B7RP-1, on autologous T lymphocytes. Cell-to-cell contact between DC and T cells was needed in order to obtain this expression on T cells. De novo expressed B7 molecules on T cells were functional since B7+ T cells were able to costimulate the proliferation of highly purified T cells. While both autologous and allogeneic DC were able to induce similar levels of costimulatory molecule expression, the chemokine receptor repertoire on B7+ T cells after interaction with DC varied depending on the presence of allo-antigens during the interaction (CCR7-, CCR5+) or the absence of antigens (CCR7+, CCR5-). In accordance with this different pattern of chemokine receptors in the two conditions, we propose that, after the encounter with DC in lymphoid organs, this peculiar T cell population should reside in the T cell areas of the lymph nodes or migrate to peripheral sites of inflammation, providing a second signal for activating or switching off, respectively, naive or peripheral effector T cells.