Cortical neurons selectively inhibit MHC class II induction in astrocytes but not in microglial cells

Astrocytes have been shown to act as potent accessory cells for MHC class II-restricted T cell responses in vitro after treatment with interferon-gamma. In contrast, even under conditions of severe central nervous system (CNS) inflammation, they seem to express little, if any, class II molecules in vivo. Thus the role of astroglial cells as accessory cells in immune responses in the CNS remains to be determined. We have studied neuron--glia interactions with respect to induction of MHC class II molecules. Surprisingly, in a co-culture system, viable neurons inhibited the induction of class II restriction elements on astrocytes. This effect was only observed when neurons had contact to astrocytes; neuron derived soluble factors alone were insufficient. Most interestingly, the suppressive effect of neurons on class II inducibility operated specifically on astrocytes, while microglial cells were left unaffected.