Interleukin-6 is essential for zwitterionic polysaccharide-mediated abscess formation.

Abscess formation associated with secondary peritonitis causes severe morbidity and can be fatal. Formation of abscesses requires the presence of CD4+ T-cells. Zwitterionic polysaccharides (ZPSs) represent a novel class of immunomodulatory bacterial antigens that stimulate CD4+ T-cells in a major histocompatibility complex (MHC) class II-dependent manner. The capsular polysaccharide Sp1 of Streptococcus pneumoniae serotype 1 possesses a zwitterionic charge with free amino groups and promotes T-cell-dependent abscess formation in an experimental mouse model. So far, nothing is known about the function of Interleukin (IL)-6 in intraperitoneal abscess formation. Here, we demonstrate that macrophages and dendritic cells (DCs), the most prevalent professional antigen-presenting cells involved in the formation of abscesses, secrete Interleukin (IL)-6 and are incorporated in the abscess capsule. Sp1 inhibits apoptosis of CD4+ T-cells and causes IL-17 expression by CD4+ T-cells in an IL-6-dependent manner. Abrogation of the Sp1-induced pleiotropic effects of IL-6 in IL-6-deficient mice and mice treated with an IL-6-specific neutralizing antibody results in significant inhibition of abscess formation. The data delineate the essential role of IL-6 in the linkage of innate and adaptive immunity in polysaccharide-mediated abscess formation.