Immune protection of the body cavities depends on the swift activation of innate and adaptive immune responses in nonclassical secondary lymphoid organs known as fat-associated lymphoid clusters (FALCs). Compared with classical secondary lymphoid organs such as lymph nodes and Peyer's patches, FALCs develop along distinct differentiation trajectories and display a reduced structural complexity. Although it is well established that fibroblastic reticular cells (FRCs) are an integral component of the immune-stimulating infrastructure of classical secondary lymphoid organs, the role of FRCs in FALC-dependent peritoneal immunity remains unclear. Using FRC-specific gene targeting, we found that FRCs play an essential role in FALC-driven immune responses. Specifically, we report that initiation of peritoneal immunity was governed through FRC activation in a myeloid differentiation primary response 88 (MYD88)-dependent manner. FRC-specific ablation of MYD88 blocked recruitment of inflammatory monocytes into FALCs and subsequent CD4 T cell-dependent B-cell activation and IgG class switching. Moreover, containment of infection was compromised in mice lacking MYD88 expression in FRCs, indicating that FRCs in FALCs function as an initial checkpoint in the orchestration of protective immune responses in the peritoneal cavity.

There are multiple, distinct B-cell populations in human beings and other animals such as mice. In the latter species, there is a well-characterized subset of B-cells known as B1 cells, which are enriched in peripheral sites such as the peritoneal cavity but are rare in the blood. B1 cells can be further subdivided into B1a and B1b subsets. There may be additional B1 subsets, though it is unclear if these are distinct populations or stages in the developmental process to become mature B1a and B1b cells. A limitation in understanding B1 subsets is the relative paucity of specific surface markers. In contrast to mice, the existence of B1 cells in human beings is controversial and more studies are needed to investigate the nature of these enigmatic cells. Examples of B1b antigens include pneumococcal polysaccharide and the Vi antigen from Salmonella Typhi, both used routinely as vaccines in human beings and experimental antigens such as haptenated-Ficoll. In addition to inducing classical T-dependent responses some proteins are B1b antigens and can induce T-independent (TI) immunity, examples include factor H binding protein from Borrelia hermsii and porins from Salmonella. Therefore, B1b antigens can be proteinaceous or non-proteinaceous, induce TI responses, memory, and immunity, they exist in a diverse range of pathogenic bacteria, and a single species can contain multiple B1b antigens. An unexpected benefit to studying B1b cells is that they appear to have a propensity to recognize protective antigens in bacteria. This suggests that studying B1b cells may be rewarding for vaccine design as immunoprophylactic and immunotherapeutic interventions become more important due to the decreasing efficacy of small molecule antimicrobials.

The effective establishment of antiviral protection requires a coordinated interplay between the innate and adaptive immune system. Using osteopetrotic (op−/−) mice, this study investigated the influence of marginal zone macrophages in controlling and initiating a protective immune response against a cytopathic vs a non- or low-cytopathic virus. Despite the generation of potent adaptive immune responses, antiviral protection against cytopathic vesicular stomatitis virus critically depended on the presence of marginal zone macrophages. Infection with low doses (100 PFU) of non- or low-cytopathic lymphocytic choriomeningitis virus was rarely cleared and usually resulted in a carrier state in the majority of mice. This shows that the early innate immune system provides an important preparatory phase to the adaptive immune system and is particularly important for antiviral protection.

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Mice infected with vesicular stomatitis virus (VSV), a cytopathic virus closely related to rabies virus, mount a virus-neutralizing antibody response protecting against lethal disease. VSVneutralizing monoclonal IgGs isolated from primary immune responses were devoid of somatic mutations, whereas most secondary and all hyperimmune response IgGs tested were hypermutated. A comparative analysis of recombinant single-chain antibody fragments (scFv-C) revealed that even the germ-line precursor of one hypermutated antibody bound and neutralized VSV. Four somatic amino acid substitutions in VH increased by 300-fold the binding strength of monovalent scFv-C. The multivalent binding avidity of germ-line scFv-C was increased by more than 10-fold compared with the monovalent binding strength. In contrast, hypermutated scFv-C did not show such avidity effects. Thus the overall binding difference between the germ-line and the hypermutated VSV-neutralizing antibody was only 10-to 15-fold. This may explain why primary germ-line antibodies and secondary hypermutated antibodies directed against pathogens such as viruses and bacteria expressing repetitive antibody determinants show rather similar binding qualities, whereas monovalently binding hapten-specific antibodies can show ''affinity maturation'' effects of up to 1000-fold. N eutralizing antibodies against many human pathogenic viruses such as polio virus, influenza virus, and rabies virus are generated early-i.e., within the first week after infection (1-7). Similarly, after infection of mice with vesicular stomatitis virus (VSV), a cytopathic unsegmented negative-strand RNA virus closely related to rabies virus (8), an early antibody response is observed, which can prevent dissemination of the virus to neuronal tissues and protects the host from a lethal progressive paralytic disease (9, 10). Within the first hours after VSV infection, when neutralizing antibodies are not yet induced, the interferon system critically contributes to the survival of mice (11, 12). As early as 2 days after infection, neutralizing antibodies are detectable in the serum (13). By 4 days after infection, hybridomas have been isolated that secrete VSV-neutralizing germ-line IgGs (14). All VSV-neutralizing antibodies analyzed were directed against one major antigenic site of the protein moiety of the viral glycoprotein (VSV-G) (14, 15).The majority of primary response IgGs isolated 4 and 5 days after infection used V gene fragments of the heavy and the light chain (V H and V ) belonging to the V H Q52 and the V 19-28 gene families. These antibodies bound one defined subsite within the major antigenic site of VSV-G. In contrast, IgGs isolated from VSV-specific secondary and hyperimmune responses were predominantly V H 7183-positive and V H J558-positive (14). The comparison of VSV-specific monoclonal antibodies isolated from secondary and hyperimmune responses did not reveal evidence of further, drastic avidity increases or enhanced specific neutralizing capacities (16). The neutralizing capacity ...

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Abs play a significant role in protection against the intracellular bacterium Salmonella Typhi. In this article, we investigated how long-term protective IgM responses can be elicited by a S. Typhi outer-membrane protein C– and F–based subunit vaccine (porins). We found that repeated Ag exposure promoted a CD4+ T cell–dependent germinal center reaction that generated mutated IgM-producing B cells and was accompanied by a strong expansion of IFN-γ–secreting T follicular helper cells. Genetic ablation of individual cytokine receptors revealed that both IFN-γ and IL-17 are required for optimal germinal center reactions and production of porin-specific memory IgM+ B cells. However, more profound reduction of porin-specific IgM B cell responses in the absence of IFN-γR signaling indicated that this cytokine plays a dominant role. Importantly, mutated IgM mAbs against porins exhibited bactericidal capacity and efficiently augmented S. Typhi clearance. In conclusion, repeated vaccination with S. Typhi porins programs type I T follicular helper cell responses that contribute to the diversification of B cell memory and promote the generation of protective IgM Abs.

TNFR1−/− mice have been shown to lack networks of mature follicular dendritic cells (FDCs) and they do not form germinal centers. With nonreplicating Ags, IgG titers were inefficiently induced and not maintained. In this study, the neutralizing Ab response and the establishment of B cell memory in TNFR1−/− mice after infection with vesicular stomatitis virus (VSV) were analyzed histologically and functionally. Immunization with VSV-derived protein Ags without adjuvant induced only IgM but no IgG Abs in TNFR1−/− mice, whereas VSV glycoprotein emulsified in CFA or IFA induced IgM and IgG responses that were short-lived and of moderate titer. However, infection with live VSV induced excellent neutralizing IgM and IgG responses in TNFR1−/− mice, and adoptively transferable B cell memory was generated and persisted for more than 300 days. In contrast, IgG levels and Ab-forming cells in the bone marrow declined within 300 days by 90–95% compared with controls. These findings suggest that 1) increased Ag dose and time of Ag availability can substitute for FDC-stored Ab-complexed Ag in the induction of efficient IgG responses in TNFR1−/− mice devoid of classical germinal centers; 2) the induction and maintenance of adoptively transferable B cell memory can occur in the absence of Ag bound to mature FDCs; and 3) the long-term maintenance of elevated IgG titers is largely dependent on FDC-associated persisting Ag. However, about 5–10% of the Ab production remained in the absence of detectable persisting Ag in TNFR1−/− mice, probably either due to immature FDCs being partially functional and/or due to long-lived plasma cells.