Canonical Wnt signaling is not activated in vitro or in vivo by Lawsonia intracellularis infection

Intestinal infection by Lawsonia intracellularis in swine causes epithelial hyperplastic lesions in crypts that resemble those induced by aberrant activation of canonical Wnt signaling. The objective of this study was to evaluate evidence of canonical Wnt signaling activation induced by L. intracellularis infection in vitro and in vivo. Cell lines Caco-2, intestinal porcine enterocytes (IPEC-J2), rat intestinal epithelial cells (IEC-18), mouse McCoy cells, a Wnt reporter cell line, and swine ileal enteroids were infected with L. intracellularis. Cell culture media and McCoy cell lysate were collected after 48 h and tested for Wnt activation using a Wnt Reporter Assay. No increase in Wnt activation was detected among non-infected and infected cell lines (p > 0.05) or enteroids (p = 0.1704). Wnt signaling activation across samples was not different from reporter cells exposed to media alone. RNA from cell lysates was evaluated for differential gene expression (DGE) of Wnt target genes CyclinD1 and c-Myc by qPCR. There were no differences in gene expression between non-infected and infected McCoy (p = 0.2666, p = 0.0558), IEC-18 (p = 0.160, p = 0.2009), IPEC-J2 (p = 0.1274, p = 0.2659), or reporter (p = 0.2740, p = 0.3278) cells. Archival samples of pig intestines presenting hyperplastic lesions caused by experimental infection with L. intracellularis were evaluated for cellular localization of β-catenin and DGE. Immunostaining showed membrane but no nuclear β-catenin in lesions and surrounding normal tissue. DGE analysis of non-infected and infected tissue revealed no difference in CyclinD1 (p > 0.9999) or c-Myc (p > 0.9999) expression. In summary, we found no evidence of canonical Wnt signaling activation induced by L. intracellularis in vitro or in pig tissues.