Regulation of In Vivo Anti Polysaccharide Responses to Intact Gram Positive and Gram Negative Extracellular Bacteria

Abstract

Proteins and polysaccharides (PS) are broadly categorized as T cell-dependent (TD) and T cell-independent (TI) antigens for the elicitation of immunoglobulin (Ig) responses. This dichotomy is derived mainly from studies using soluble protein and PS antigens. However, in any natural infection, the host samples these antigens in the context of the bacteria that cause invasive disease. Very little is known regarding the regulation of PS-specific Ig responses to the intact bacterium. Our lab has hypothesized that PS-specific Ig responses to intact bacteria may be regulated differently relative to that observed in response to isolated, soluble PS antigens. We believe that the basis for this is that intact bacteria coexpress their PS antigens in a particulate structure, along with various proteins and multiple adjuvanting moieties such as Toll-like receptor (TLR) ligands and scavenger receptor ligands. In particular, the association of PS with protein may convert the anti-PS response from TI to TD. Indeed, our previous studies using intact Streptococcus pneumoniae type 14 (Pn14), a Gram-positive extracellular bacterium demonstrated that the primary PS-specific IgG response was dependent on CD4+ T cells, newlineviii newlineCD40/CD40L interactions and costimulation dependent on B7-CD28 interactions. However, similar to that observed using soluble, isolated PS, the PS-specific IgG responses to Pn14 were ICOS-independent and failed to induce any secondary boosting. These latter observations were in distinct contrast to the classic TD IgG anti-PS responses to a soluble covalent conjugate of PS and protein (i.e. conjugate vaccine). These studies demonstrated that PS expressed by intact Pn14 behaved as a unique immunogen, exhibiting both TD and TI properties. However, these studies left unexplained whether the immunologic properties of intact Pn14 could be generalized to other Gram-positive, as well as Gram-negative bacteria. Thus, distinct Gram-positive and Gram-negative bacteria express unique attachments of capsular PS to the underlying