Microglia and astrocytes, two glial cell populations of the central nervous system, present Ag and stimulate T cell proliferation, but it is unclear whether they preferentially activate Th1 or Th2 responses. We have investigated the efficiency of microglia and astrocytes in the presentation of OVA peptide 323-339 or native OVA to Th1 and Th2 cell lines from DO11.10 TCR transgenic mice. Upon stimulation with IFN-gamma, microglia express MHC class II molecules, CD40, and ICAM-1 and efficiently present OVA 323-339, leading to T cell proliferation and production of IL-2 and IFN-gamma by Th1 and of IL-4 by Th2 cells. IFN-gamma-treated astrocytes, which express MHC class II and ICAM-1, present OVA 323-339 less efficiently to Th1 cells but are as efficient as microglia in inducing IL-4 secretion by Th2 cells. However, astrocytes are much less potent than microglia in presenting naturally processed OVA peptide to either T cell subset, indicating inefficient Ag processing. The capacity of astrocytes and microglia to stimulate Th1 and Th2 cells depends on their MHC class II expression and does not involve ICAM-1, B7-1, or B7-2 molecules. However, CD40-CD40L interactions contribute to Th1 activation by microglia. These data suggest that microglia may play a role in the activation of Th1 and Th2 cells, whereas astrocytes would restimulate mainly Th2 responses in the presence of appropriate peptides. This differential capacity of brain APC to restimulate Th1 and Th2 responses may contribute to the reactivation and regulation of local inflammatory processes during infectious and autoimmune diseases.