Figure 3. Stat3 activation is maintained 2 months post AP mainly in inflammatory cells and ADM area. (A–D) IHC for phospho-Stat3 in N/K pancreata 2 months post caerulein treatment. (A) Low magnification picture of a representative area. Phospho-Stat3 is rarely expressed in developing PanIN lesions but is observed in the cells that display a normal ductal morphology (B, arrows); (it is mainly expressed in inflammatory cells (C), in cells undergoing ADM (C, D, arrows) as well as in morphologically normal acinar cells (D, arrowheads). Scale bar: 20 mm. doi:10.1371/journal.pone.0027725.g003 morphology was maintained and low to high-grade PanINs were also observed (Fig. 4E–F). Large areas of the pancreas displayed extensive ADM (not shown). As observed in human PDAC, the Acute Pancreatitis and Pancreatic Cancer tumors were highly proliferative as shown by Ki67 expression (Fig. 4G) and surrounded by conspicuously reactive stroma characterized by smooth muscle actin (SMA) expression (Fig. 4H). While we also observed high levels of Cox2 in PanINs, its expression was low in the cancer cells which is surprising as it is known to be upregulated in human PDAC (Fig. 4I). Nestin is expressed in human and mouse PanIN Following AP, the pancreas undergoes intense proliferation and regeneration, and increases in Nestin expression have been observed by different groups [26,27]. It is unclear if this increase occurs via an expansion of an already existing population of adult Nestin-expressing cells or is due to a transient expression of Nestin in dedifferentiating and proliferating acinar cells. We now show by immunohistochemistry that Nestin is consistently expressed in mouse PanIN from low to high-grade as well as in human PanINs and PDAC (Fig. 5). Discussion We showed previously that N/K and the P/K mouse models develop low grade PanINs with similar timing and efficiency between 2 and 4 months of age [6]. These data suggested that PDAC initiation occurred mainly in the Nestin cell lineage, which consists in the pancreas of acinar cells and rare adult pancreatic Nestin-expressing cells. Still, the limited survival of the N/K mice past 6 months prevented us from pursuing the comparison to the PDAC stage. In the present study, we show that two close episodes of caerulein-induced AP in 2 month-old NK mice not only cause rapid PanIN progression from low- to high-grade but also with low frequency leads to PDAC. The progression efficiency appears Figure 4. NK mice develop pancreatic ductal adenocarcinoma. High-grade lesions display microinvasion (A, arrowheads) and (B) as shown by CK19 expression (adjacent section) in cells in the adjacent stroma (C); (D) Low magnification shows the local invasion of fat and fibrous tissues (arrows). (E) Nuclei are enlarged, crowded and glandular morphology is maintained. (F) Low grade PanINs (stars) are observed in proximity with adenocarcinoma (arrows). (G) Adenocarcinoma cells are highly proliferative as shown by Ki67 expression. They are surrounded by reactive stroma characterized by SMA expression (H). Cox2 expression while high in the PanINs (arrows) is absent from the adenocarcinoma per se (arrowheads)(I). Scale bar: 20 mm. doi:10.1371/journal.pone.0027725.g004 PLoS ONE | www.plosone.org 4 November 2011 | Volume 6 | Issue 11 | e27725