We first investigated whether microglia induce A1 reactive astrocytes because LPS is a strong inducer of A1s1 and is an activator of TLR4 signaling, a receptor expressed specifically by microglial in the rodent CNS12–15. We took advantage of Csf1r−/− knock-out mice that lack microglia16 (Extended Data Fig. 1) to ask whether A1s can be produced without microglia. To assess astrocyte reactivity, we used a microfluidic qPCR screen to determine gene expression changes in astrocytes purified by immunopanning from saline- and LPS-treated wild type control or Csf1r−/− mice. As expected, wild-type littermate controls had a normal response to LPS injection5,17, with robust induction of an A1 response (Fig. 1a), however astrocytes from Csf1r−/− mice failed to activate A1s. These findings show reactive microglia are required to induce A1 reactive astrocytes in vivo.
