We have revealed that 100–200 clusters, filled with closely packed lymphocytes, can be found throughout the length of the antimesenteric wall of the mouse small intestine. They are composed of a large B cell area, including a germinal center, and epithelia overlying the clusters contain M cells. A large fraction of B cells displays B220+CD19+CD23+IgMlowIgDhighCD5−Mac-1− phenotype, and the composition of IgA+ B cells is smaller but substantial. To our knowledge, these clusters are the first identification of isolated lymphoid follicles (ILF) in mouse small intestine. ILF can be first detected at 7 (BALB/c mice) and 25 (C57BL/6 mice) days after birth, and lymphoid clusters equivalent in terms of cellular mass to ILF are present in germfree, athymic nude, RAG-2−/−, TCR-β−/−, and Ig μ-chain mutant (μm−/−) mice, although c-kit+ cells outnumber B220+ cells in germfree and athymic nude mice, and most lymphoid residents are c-kit+B220− in RAG-2−/−, TCR-β−/−, and μm−/− mice. ILF develop normally in the progeny of transplacentally manipulated Peyer’s patch (PP)-deficient mice, and decreased numbers of conspicuously atrophied ILF are present in IL-7Rα−/− PPnull mice. Neither ILF nor PP are detectable in lymphotoxin α−/− and aly/aly mice that retain well-developed cryptopatches (CP) and thymus-independent subsets of intraepithelial T cells, whereas ILF, PP, CP, and thymus-independent subsets of intraepithelial T cells disappear from common cytokine receptor γ-chain mutant mice. These findings indicate that ILF, PP, and CP constitute three distinct organized gut-associated lymphoid tissues that reside in the lamina propria of the mouse small intestine.

After pulmonary virus infection, virus-binding B cells ectopically accumulate in the lung. However, their contribution to protective immunity against reinfecting viruses remains unknown. Here, we show the phenotypes and protective functions of virus-binding memory B cells that persist in the lung following pulmonary infection with influenza virus. A fraction of virus-binding B-cell population in the lung expressed surface markers for splenic mature memory B cells (CD73, CD80, and CD273) along with CD69 and CXCR3 that are up-regulated on lung effector/memory T cells. The lung B-cell population with memory phenotype persisted for more than 5 mo after infection, and on reinfection promptly differentiated into plasma cells that produced virus-neutralizing antibodies locally. This production of local IgG and IgA neutralizing antibody was correlated with reduced virus spread in adapted hosts. Our data demonstrates that infected lungs harbor a memory B-cell subset with distinctive phenotype and ability to provide protection against pulmonary virus reinfection.

Murine CD4+CD25+ regulatory cells have been reported to express latency-associated peptide (LAP) and TGF-β on the surface after activation, and exert regulatory function by the membrane-bound TGF-β in vitro. We have now found that a small population of CD4+ T cells, both CD25+ and CD25−, can be stained with a goat anti-LAP polyclonal Ab without being stimulated. Virtually all these LAP+ cells are also positive for thrombospondin, which has the ability to convert latent TGF-β to the active form. In the CD4+CD45RBhigh-induced colitis model of SCID mice, regulatory activity was exhibited not only by CD25+LAP+ and CD25+LAP− cells, but also by CD25−LAP+ cells. CD4+CD25−LAP+ T cells were part of the CD45RBlow cell fraction. CD4+CD25−LAP−CD45RBlow cells had minimal, if any, regulatory activity in the colitis model. The regulatory function of CD25−LAP+ cells was abrogated in vivo by anti-TGF-β mAb. These results identify a new TGF-β-dependent regulatory CD4+ T cell phenotype that is CD25− and LAP+.

Background:Lactobacillus casei is a nonpathogenic gram-positive bacterium widely used in dairy products and has been shown to enhance the cellular immunity of the host. Methods: To examine the inhibitory effect of L. casei on IgE production, splenocytes obtained from ovalbumin (OVA)-primed BALB/c mice were restimulated in vitro with the same antigen in the presence of heat-killed L. casei. The effect of this bacterium on T helper (Th) phenotype development was also examined with naive T cells from OVA-specific T cell receptor-transgenic mice. Results:L. casei induced IFN-γ, but inhibited IL-4 and IL-5 secretion, and markedly suppressed total and antigen-specific IgE secretion by OVA-stimulated splenocytes. The inhibitory effect of L. casei on IgE, IL-4, and IL-5 production was partially abrogated by addition of neutralizing antibody to IFN-γ. Augmented IL-12 production was also observed in the cell cultures containing L. casei, and anti-IL-12 monoclonal antibody completely restored the IgE, IL-4, and IL-5 production to the control levels. The IL-12 augmentation by L. casei was macrophage-dependent. The Th cell development assay showed the ability of L. casei to induce Th1 development preferentially. This effect was also completely blocked by anti-IL-12 antibody. Conclusions: This is the first demonstration that a nonpathogenic microorganism, L. casei, can inhibit antigen-induced IgE production through induction of IL-12 secretion by macrophages. The findings suggest a potential use of this organism in preventing IgE-mediated allergy.

Peyer’s patch (PP) dendritic cells (DCs) have been shown to exhibit a distinct capacity to induce cytokine secretion from CD4+ T cells compared with DCs in other lymphoid organs such as the spleen (SP). In this study, we investigated whether PP DCs are functionally different from DCs in the SP in their ability to induce Ab production from B cells. Compared with SP DCs, freshly isolated PP DCs induced higher levels of IgA secretion from naive B cells in DC-T cell-B cell coculture system in vitro. The IgA production induced by PP DCs was attenuated by neutralization of IL-6. In addition, the induction of IgA secretion by SP DCs, but not PP DCs, was further enhanced by the addition of exogenous IL-6. Finally, we demonstrated that only PP CD11b+ DC subset secreted higher levels of IL-6 compared with other DC subsets in the PP and all SP DC populations, and that PP CD11b+ DC induced naive B cells to produce higher levels of IgA compared with SP CD11b+ DC. These results suggest a unique role of PP CD11b+ DCs in enhancing IgA production from B cells via secretion of IL-6.