Ultrasound-guided CNBs are a safe, quick, and valid tool for the workup of lymphadenopathy. Yet, a benign diagnosis from CNB must be completed by a secondary biopsy if clinical presentation suggests malignant disease.

Bacterial capsular polysaccharides are virulence factors and are considered T cell–independent antigens. However, the capsular polysaccharide Sp1 from Streptococcus pneumoniae serotype 1 has been shown to activate CD4+ T cells in a major histocompatibility complex (MHC) class II–dependent manner. The mechanism of carbohydrate presentation to CD4+ T cells is unknown. We show in live murine dendritic cells (DCs) that Sp1 translocates from lysosomal compartments to the plasma membrane in MHCII-positive tubules. Sp1 cell surface presentation results in reduction of self-peptide presentation without alteration of the MHCII self peptide repertoire. In DM-deficient mice, retrograde transport of Sp1/MHCII complexes resulting in T cell–dependent immune responses to the polysaccharide in vitro and in vivo is significantly reduced. The results demonstrate the capacity of a bacterial capsular polysaccharide antigen to use DC tubules as a vehicle for its transport as an MHCII/saccharide complex to the cell surface for the induction of T cell activation. Furthermore, retrograde transport requires the functional role of DM in self peptide–carbohydrate exchange. These observations open new opportunities for the design of vaccines against microbial encapsulated pathogens.

Zwitterionic polysaccharides of the normal flora bacteria represent a novel class of antigens in that they correct systemic CD4؉ T-cell deficiencies and direct lymphoid organogenesis during colonization of the host. Presentation of these polysaccharides to CD4؉ T cells depends on major histocompatibility complex class IIand DM-dependent retrograde transport from lysosomes to the cell surface. Yet the phenotype and clonality of the immune response to the polysaccharide in the mature host immune system have not been studied. Capsular polysaccharides of the human physiologic bacterial flora are immunogenic components that first encounter the human immune system during initial colonization and at the time that the immune system is developing and maturing. As opposed to common negatively charged polysaccharides, the biologic activities of certain commensal bacterial polysaccharides are unique in their ability to stimulate CD4 ϩ T cells in vivo and in vitro. They direct the development of the systemic cellular immune response by correcting CD4 ϩ T-cell deficiencies and TH1/TH2 imbalances toward a TH1 immune response. Responses to the polysaccharides are conferred by CD4 ϩ T cells, not B cells or other T cells (14,18,28,(31)(32)(33). Examples of such bacteria are the ubiquitous anaerobic member of the gut flora Bacteroides fragilis, Staphylococcus aureus as a temporary member of the skin and mucosal flora, and Streptococcus pneumoniae of the upper respiratory tract flora. CD4 ϩ T-cell activation induced by these polysaccharides depends on their unique electrical charge: each repeating unit has a minimum of one positive and one negative charge, leading to their common three-dimensional configuration characterized by a right-handed helix with repeating negatively charged grooves, with the positive charges being on the outer surface of the lateral boundaries (5,14,31,36). Presentation of the so-called zwitterionic polysaccharide (ZPS) from S. pneumoniae serotype 1 (Sp1) by major histocompatibility complex (MHC) class II molecules requires its retrograde transport from lysosomes to the cell surface within tubules as a ZPS-MHC class II complex and also requires the DM molecule (12,22). DM is known to catalyze and edit the exchange of the self-peptide CLIP with processed antigen in MHC class II compartments. The requirement of DM for Sp1 presentation via MHC class II suggests presentation within the antigen binding groove, which is supported by recent studies demonstrating that binding of the ZPS PS A1 from B. fragilis to MHC class II molecules can be competed by peptides known to be presented in the antigen binding cleft (7).For protein-derived T-cell antigens, it is well established that their presence in the MHC class II binding groove leads to the recognition of their antigenic epitopes by the T-cell receptor (TCR), within the CDR3 antigen binding domain of the ␤-chain variable (BV) region, and to subsequent T-cell activation and oligoclonal T-cell proliferation. However, for ZPS, besides the requirement of engaging the ...

Zwitterionic capsular polysaccharides (ZPS) of commensal bacteria are characterized by having both positive and negative charged substituents on each repeating unit of a highly repetitive structure that has an α-helix configuration. In this paper we look at the immune response of CD8+ T cells to ZPSs. Intraperitoneal application of the ZPS Sp1 from Streptococcus pneumoniae serotype 1 induces CD8+CD28− T cells in the spleen and peritoneal cavity of WT mice. However, chemically modified Sp1 (mSp1) without the positive charge and resembling common negatively charged polysaccharides fails to induce CD8+CD28− T lymphocytes. The Sp1-induced CD8+CD28− T lymphocytes are CD122lowCTLA-4+CD39+. They synthesize IL-10 and TGF-β. The Sp1-induced CD8+CD28− T cells exhibit immunosuppressive properties on CD4+ T cells in vivo and in vitro. Experimental approaches to elucidate the mechanism of CD8+ T cell activation by Sp1 demonstrate in a dimeric MHC class I-Ig model that Sp1 induces CD8+ T cell activation by enhancing crosslinking of TCR. The expansion of CD8+CD28− T cells is independent, of direct antigen-presenting cell/T cell contact and, to the specificity of the T cell receptor (TCR). In CD8+CD28− T cells, Sp1 enhances Zap-70 phosphorylation and increasingly involves NF-κB which ultimately results in protection versus apoptosis and cell death and promotes survival and accumulation of the CD8+CD28− population. This is the first description of a naturally occurring bacterial antigen that is able to induce suppressive CD8+CD28− T lymphocytes in vivo and in vitro. The underlying mechanism of CD8+ T cell activation appears to rely on enhanced TCR crosslinking. The data provides evidence that ZPS of commensal bacteria play an important role in peripheral tolerance mechanisms and the maintenance of the homeostasis of the immune system.