The presence of MOMA-2+ macrophages in the outer B cell zone and protection of the splenic micro-architecture from LPS-induced destruction depend on secreted IgM

Tissue compartmentalization enables Salmonella persistence during chemotherapy

Antimicrobial chemotherapy can fail to eradicate the pathogen, even in the absence of antimicrobial resistance. Persisting pathogens can subsequently cause relapsing diseases. In vitro studies suggest various mechanisms of antibiotic persistence, but their in vivo relevance remains unclear because of the difficulty of studying scarce pathogen survivors in complex host tissues. Here, we localized and characterized rare surviving Salmonella in mouse spleen using high-resolution whole-organ tomography. Chemotherapy cleared >99.5% of the Salmonella but was inefficient against a small Salmonella subset in the white pulp. Previous models could not explain these findings: drug exposure was adequate, Salmonella continued to replicate, and host stresses induced only limited Salmonella drug tolerance. Instead, antimicrobial clearance required support of Salmonella-killing neutrophils and monocytes, and the density of such cells was lower in the white pulp than in other spleen compartments containing higher Salmonella loads. Neutrophil densities declined further during treatment in response to receding Salmonella loads, resulting in insufficient support for Salmonella clearance from the white pulp and eradication failure. However, adjunctive therapies sustaining inflammatory support enabled effective clearance. These results identify uneven Salmonella tissue colonization and spatiotemporal inflammation dynamics as main causes of Salmonella persistence and establish a powerful approach to investigate scarce but impactful pathogen subsets in complex host environments.

Distinct Cellular Pathways for Induction of CD4+ T Cell-Dependent Antibody Responses to Antigen Expressed by Intact Bacteria Versus Isolated Soluble Antigen

Uptake of intact bacteria and soluble Ags by APCs is mediated by phagocytosis and endocytosis or pinocytosis, respectively. Thus, we predicted that injection of clodronate-containing liposomes (CLs), which selectively deplete cells efficient in phagocytosis, would inhibit murine CD4(+) T cell-dependent IgG responses to Ags expressed by intact bacteria but not isolated soluble Ags. Surprisingly, injection of CLs markedly inhibited protein-specific IgG responses to intact, heat-killed Streptococcus pneumoniae, as well as a soluble OVA-polysaccharide conjugate or OVA alone. IgG anti-polysaccharide responses to bacteria and conjugate were also reduced, but more modestly. In both instances, CL-mediated inhibition was associated with a significant reduction in induced germinal centers and CD4(+) germinal center T follicular helper cells. However, CL injection, which largely abrogated the proliferative response of adoptively transferred OVA peptide-specific-transgenic CD4(+) T cells in response to immunization with S. pneumoniae expressing OVA peptide, did not inhibit T cell proliferation in response to OVA-polysaccharide conjugate or OVA. In this regard, monocyte-derived cells, depleted by CLs, internalized S. pneumoniae in vivo, whereas CD11c(low) dendritic cells, unaffected by CL injection, internalized soluble OVA. Ex vivo isolation and coculture of these respective APCs from S. pneumoniae- or OVA-immunized mice with OVA-specific T cells, in the absence of exogenous Ag, demonstrated their selective ability to induce T cell activation. These data suggest that, although distinct APCs initiate CD4(+) T cell activation in response to Ag expressed by intact bacteria versus Ag in soluble form, CL-sensitive cells appear to be necessary for the subsequent IgG responses to both forms of Ag.

Pneumococcal Surface Protein A Plays a Major Role in Streptococcus pneumoniae-Induced Immunosuppression

Intact, inactivated Streptococcus pneumoniae [including the unencapsulated S. pneumoniae, serotype 2 strain (R36A)] markedly inhibits the humoral immune response to coimmunized heterologous proteins, a property not observed with several other intact Gram-positive or Gram-negative bacteria. In this study, we determined the nature of this immunosuppressive property. Because phosphorylcholine (PC), a major haptenic component of teichoic acid in the S. pneumoniae cell wall, and lipoteichoic acid in the S. pneumoniae membrane were previously reported to be immunosuppressive when derived from filarial parasites, we determined whether R36A lacking PC (R36A(pc-)) was inhibitory. Indeed, although R36A(pc-) exhibited a markedly reduced level of inhibition of the IgG response to coimmunized chicken OVA (cOVA), no inhibition was observed when using several other distinct PC-expressing bacteria or a soluble, protein-PC conjugate. Further, treatment of R36A with periodate, which selectively destroys PC residues, had no effect on R36A-mediated inhibition. Because R36A(pc-) also lacks choline-binding proteins (CBPs) that require PC for cell wall attachment, and because treatment of R36A with trypsin eliminated its inhibitory activity, we incubated R36A in choline chloride, which selectively strips CBPs from its surface. R36A lacking CBPs lost most of its inhibitory property, whereas the supernatant of choline chloride-treated R36A, containing CBPs, was markedly inhibitory. Coimmunization studies using cOVA and various S. pneumoniae mutants, each genetically deficient in one of the CBPs, demonstrated that only S. pneumoniae lacking the CBP pneumococcal surface protein A lost its ability to inhibit the IgG anti-cOVA response. These results strongly suggest that PspA plays a major role in mediating the immunosuppressive property of S. pneumoniae.

B cell acquisition of antigen in vivo

The fate of B lymphocytes is dictated in large part by cognate antigen and the environment in which it is encountered. Yet we are only now beginning to understand where and how B cells acquire antigen. Recent studies identify multiple pathways by which lymph-borne antigens enter the B cell follicles of LNs. Size is a major factor as particulate antigens and large IC are bound by subcapsular sinus macrophages. By contrast, small antigens (under 70kDa) are rapidly channeled into follicles via conduits secreted by fibroblastic reticular cells (FRC). Interestingly, the conduits not only deliver antigen to follicular dendritic cells (FDC) but also provide a rich source of B cell chemokine, that is, CXCL-13. Thus, the follicular conduits provide an 'antigen highway' for B cells trafficking within the LN. These new findings provide an important discovery in understanding how B cells acquire cognate antigen.