Previous study demonstrated that Streptococcus gallolyticus subsp. pasteurianus (SP) designated as AL101002 strain, a highly virulent strain induced high mortality in duckings with splenic lesions. In this study, forty-two ducklings were subcutaneously inoculated with SP to study the dynamic change of splenic lesions. The spleens from these ducklings were significant enlarged by congestion, edema, and/or showing multiple marbled appearance in 14 days post inoculation (dpi). SP was re-isolated from the spleens and blood and confirmed by immunohistochemistry (IHC) using anti-SP antibody. Histopathologically the main lesion was macrophage necrosis in the spleens from 1 to 7 dpi. TUNEL assay, transmission electron microscope (TEM) and IHC of CD68 demonstrated that macrophage necrosis was necroptosis and further confirmed by qRT-PCR analysis. Two major factors of apoptosis caspase 3 and caspase 8 were not significant changes during the SP infection suggested that apotosis signals were not activated. While the key factor of necroptosis MLKL was increased significantly from day 1 to 14 dpi. Inflammatory cytokine IL-1β and IL-6 were significantly up-regulated expression in the spleens on day 1 dpi. TNF-α was down regulated from day 1 to 5 dpi, but it was increased from day 7 to 14 dpi. Our results demonstrated that SP mainly infects macrophages and resulted in macrophage necroptosis and suggested that macrophage necroptosis in spleens is involved in the pathogenesis of SP infection in ducklings.

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