HtrA aggravates C. jejuni induced colonic apoptosis and inflammation. Immediately after weaning, infant mice were perorally infected with C. jejuni NCTC11168 (WT, closed circles) or the mutant strain NCTC11168∆htrA (∆htrA, open circles). Uninfected animals (Naïve; open diamonds) served as negative controls. The average numbers of apoptotic cells (positive for caspase-3, panel A), proliferating cells (positive for Ki67, panel B), T lymphocytes (positive for CD3, panel C), and regulatory T cells (Treg, positive for Foxp3; panel D) from at least six high power fields (HPF, 400× magnification) per animal were determined microscopically in immunohistochemically stained colonic sections at day 7 p.i. In addition, representative photomicrographs of colonic Casp3-positive apoptotic (indicated by black arrows; A, right panel) and Ki67-positive proliferating (B, right panel) cells are shown (Naïve left, WT middle, ∆htrA right photo; 400× magnification, scale bar 20 μm). Numbers of analyzed animals are given in parentheses. Means (black bars) and levels of significance (P-values) determined by the Mann–Whitney-U test are indicated. Data shown were pooled from three independent experiments.