Interactions between Streptococcus suis serotype 2 and cells of the myeloid lineage in the palatine tonsil of the pig

Streptococcus suis is a major pathogen in pigs and causes significant morbidity and mortality in herds world-wide. A major problem with S. suis is the presence of asymptomatic carrier animals which can spread the organism within and between herds. The palatine tonsil is one of the main sites where the organism can be recovered, both in infected and carrier animals. The use of multiple-colour immunohistology allowed identification of the cell types associated with bacteria in the tonsils of infected gnotobiotic piglets. Bacteria were never associated with T-cells or B-cells but were always associated with cells of the myeloid lineage. Expression of CD16 and CD163 on these leukocytes suggested an association with mature macrophages in tonsil, which may lead to clearance or control of the micro-organism.