Immunization induces a transient mononuclear inflammatory response in the footpad of <i>M. ulcerans</i> challenged mice.

<p>Mice were either non-immunized (A, D, and I) or immunized with BCG (B, E, G, and J) or with mycolactone-negative <i>M. ulcerans</i> 5114 (C, F, H, K) two months before challenge. All mice were infected in the footpad with 2 log₁₀ CFU of <i>M. ulcerans</i> 98–912. The footpads were harvested at on day 60 (A–C), day 70 (D–F and I), or day 150–200 (G–H and J–K) and processed for histology. Tissue sections were stained with HE (A–H) or ZN (I–K). (A, D, and I) Non-immunized mice showed a persistent mixed inflammatory infiltrate composed of mononuclear cells (arrowheads) and neutrophils (arrows). Necrotic areas harbored abundant extracellular bacilli (I) and cells with apoptotic morphology (triangles). (B, C, E, F, G, H, J, and K) The footpads of vaccinated mice presented a mononuclear inflammatory infiltrate (arrowheads) throughout the initial weeks of experimental infection, which was later replaced by a predominantly neutrophilic infiltrate (arrows) and/or apoptosis (triangles) and extracellular bacilli (J, K). Images are representative of 4 footpads per group. Original magnification: 10×. Original magnification of HE inserts: 100×. Original magnification of ZN: 100×.