The epidermal growth factor receptor (EGFR) pathway is induced in the gut and is required for ISC proliferation triggered by infection. (a) Reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) analysis of gut extracts shows that genes encoding components of the Imd (Diptericin (Dpt)), Janus kinase/signal transducer and activator of transcription (JAK/STAT) (upd3, Socs36E) and EGFR (vein, Keren, rhomboid, argos) pathways are induced upon oral ingestion with Ecc15. Values were normalized to RpL32 and set relative to their own maximum induction levels. (b) Immunostaining of guts of esgGal4 UAS-GFP flies with antibodies directed against the phospho-form of the extracellular signal-regulated kinase (ERK) kinase (red) and GFP (green) reveals that the ERK kinase was activated in enterocytes 1 hour postinfection and in both progenitor cells (green) and enterocytes at 4 hours. At 16 hours, no phospho-ERK signal was detected. (c) Ingestion of Ecc15 induces a marked increase in the number of esgGal4TS, UAS-GFP-positive cells (indicative of epithelium renewal), which was not observed when double-stranded RNA (dsRNA or RNAi) or dominant-negative forms of members of the EGFR pathway (UAS-EGFRDN, UAS-Ras-IR or UAS-Raf-IR) were expressed in ISCs. Overexpression of an active form of EGFR (UAS-EGFRACT) in ISCs is sufficient to induce a high level of epithelium renewal in the absence of infection. (d) Quantification of PH3-positive cells per midgut shows an increase in the number of mitotic cells upon Ecc15 infection in WT flies, but not in flies with reduced EGFR activity in ISCs (UAS-EGFRDN, UAS-Ras-IR or UAS-Raf-IR; P < 0.05). Overexpression of a constitutively activated form of EGFR in ISCs increases the mitotic index (esgGal4TS, UAS-EGFRACT, UAS-RasV12 or UAS-RafACT; P < 0.05). Mean values of five experiments (N = 10 to 20 guts each) ± SE are shown. Analysis of variance (ANOVA) F = 58.64. df = 14. P < 0.0001.