The crypt lymphoepithelium of the palatine tonsils of pigs : a study of its structure, function and role in the pathogenesis of Streptococcus suis infection

Abstract

The crypt epithelium of the palatine tonsils, which is the first barrier between external environment and lymphoid tissue, was investigated in 6-month-old healthy market weight pigs and in 3 week-old piglets experimentally infected with Streptococcus suis serotype 2. This pathogen enters via and persists in the palatine tonsils, and the pathogenesis of this infection is still unclear.

First, the subpopulations of intra-epithelial leukocytes were characterized and quantified using immunohistochemistry and monoclonal antibodies to porcine lymphocyte markers. Different lymphocyte subpopulations, namely CD4, CD8, and γδ T lymphocytes, and B lymphocytes, were identified. Next, the alterations in the intra­epithelial leukocyte subpopulations in response to experimental infection with S. suis were described. Myeloid cells, CD4 and CD8 T lymphocytes, and B lymphocytes were increased in the first 72h post-infection.

The transport of S. suis serotype 2 across the crypt epithelium was investigated by transmission electron microscopy. S. suis was observed within neutrophils, macrophages, and also in occasional epithelial cells as early as 18h post-infection, indicating that participation of the innate immune response and epithelial cellular invasion are early steps in this infection. Macrophages containing bacteria were noticed straddling the basal lamina, suggesting that they can transport S. suis from the epithelium to the subepithelial lymphoid tissue and subsequently may disseminate the bacteria systemically.

Last, the ability of epithelial cells to engulf S. suis and their ultrastructural morphology were correlated to determine whether cells compatible with M (microfold/membranous) cells were involved in bacterial uptake. Although many of the features described for M cells were observed in epithelial cells of the crypt epithelium, those containing S. suis had no specific common feature to permit classification as M cells.

Taken together, these data suggest that the tonsillar crypt epithelium actively participates in the early phase of S. suis infection and likely plays an important role in the initiation of immune response.