Cross-priming versus cross-tolerance: are two signals enough?

Depending on their state of maturation, dendritic cells (DCs) can cross-prime or cross-tolerize T cells. Mature DCs provide T-cell-receptor ligands (signal 1) and costimulatory molecules (signal 2), and activate T cells. Immature DCs provide signal 1, but not signal 2, and tolerize T cells. However, new data show that it is not the expression of costimulatory molecules alone that determines whether DCs induce immunity or tolerance, because mature DCs can induce tolerance also.