Mucosal antibody responses play a major role in mediating homeostasis with the intestinal flora. It has been suggested that imbalance in the IgA + and IgG + intestinal B cell repertoire may be associated with the development of diseases such as inflammatory bowel disease. Despite this, little is known about the antibody specificity of human intestinal plasmablasts. Here, we have determined the reactivity profile of single isolated IgA + and IgG + plasmablasts from human terminal ileum using antibody cloning and in vitro expression. We found that approximately 25% of intestinal IgA and IgG plasmablast antibodies were polyreactive; the majority were antigen-specific. Antigen specificity was not only directed against enteropathogenic microbes but also against commensal microbes and self antigens. Regardless of their reactivity, all intestinal antibodies were somatically mutated and showed signs of antigen-mediated selection, suggesting that they developed from antigen-specific B cell responses. Together, our data indicate that antigen-specific immune responses to intestinal microbes are largely responsible for the maintenance of intestinal homeostasis and thus provide a basis for understanding the deregulated immune responses observed in patients with inflammatory bowel disease.

Antibodies against the NANP repeat of circumsporozoite protein (CSP), the major surface antigen of Plasmodium falciparum (Pf) sporozoites, can protect from malaria in animal models but protective humoral immunity is difficult to induce in humans. Here we cloned and characterized rare affinity-matured human NANP-reactive memory B cell antibodies elicited by natural Pf exposure that potently inhibited parasite transmission and development in vivo. We unveiled the molecular details of antibody binding to two distinct protective epitopes within the NANP repeat. NANP repeat recognition was largely mediated by germline encoded and immunoglobulin (Ig) heavy-chain complementarity determining region 3 (HCDR3) residues, whereas affinity maturation contributed predominantly to stabilizing the antigen-binding site conformation. Combined, our findings illustrate the power of exploring human anti-CSP antibody responses to develop tools for malaria control in the mammalian and the mosquito vector and provide a molecular basis for the structure-based design of next-generation CSP malaria vaccines.

Affinity maturation, the clonal selection and expansion of antigen-activated B cells expressing somatically mutated antibody variants that develop during T cell-dependent germinal center reactions, is considered pivotal for efficient development of protective B cell memory responses to infection and vaccination. Repeated antigen exposure promotes affinity maturation but each time also recruits antigen-reactive naïve B cells into the response. Here, we determined the relative impact of affinity maturation versus antigen-mediated clonal selection of naïve B cells to mount potent B cell memory responses in humans after repeated exposure to a complex pathogen, the malaria parasite (Pf). Using single-cell immunoglobulin (Ig) gene sequencing and production of recombinant monoclonal antibodies, we analyzed the origin, development, and quality of memory B cell responses to Pf circumsporozoite protein (PfCSP), the major sporozoite surface protein. We show that after repeated immunization of Pf-naïve volunteers with infectious Pf sporozoites (PfSPZ Challenge) under chloroquine prophylaxis (PfSPZ-CVac), the clonal selection of potent germline and memory B cell precursors against the central PfCSP NANP repeat outpaces affinity maturation because the majority of Ig gene mutations are affinity-neutral. Mathematical modeling explains how the efficiency of affinity maturation decreases strongly with antigen complexity. Thus, in the absence of long-term exposure, the frequency of antigen-reactive precursors and likelihood of their activation rather than affinity maturation will determine the quality of anti-PfCSP memory B cell responses. These findings have wide implications for the design of vaccination strategies to induce potent B cell memory responses against PfCSP and presumably other structurally complex antigens.

The leukemogenic effects of Myc drive recurrent trisomy in a mouse model of acute myeloid leukemia.

The present study was undertaken to unequivocally demonstrate the morphology, immunophenotype, and localization of Epstein Barr virus (EBV)-infected cells as well as the type of infection (latent versus productive) in tonsils of acute infectious mononucleosis. Paraffin sections from nine cases with clinical, serologic, and morphologic evidence of EBV infection were analyzed for the detection of small transcripts, designated EBER1 & 2, and BHLF1 by in situ hybridization (ISH) using nonisotopically labeled probes. ISH was combined with immunohistology, employing a broad panel of antibodies against B-, T-, epithelial-, macrophage-, and follicular dendritic cell (FDC)-antigens. All EBER-positive cells could be identified as lymphocytes, as they did not exhibit any morphologic or immunologic characteristics of epithelial cells, macrophages, or FDCs. A preferential accumulation of EBER-positive cells was noted around crypts, within surface squamous epithelium, and in the surroundings of necrosis. The majority of these lymphocytes could be shown to be B cells, which morphologically included Reed-Sternberg (RS)-like cells, immunoblasts, medium-sized lymphoid cells, as well as cells with plasmacytoid differentiation. In all cases, a varying number of EBER-positive T cells could be identified. ISH for BHLF1-RNA detection showed that almost all cases contained single positive small lymphoid cells, indicating a transition from latent to productive infection cycle. Such cells could also be detected within the crypt epithelium reaching up to its surface. Additional screening of 123 oropharyngeal mucosa samples from patients without evidence of acute EBV-infection, using the polymerase chain reaction for EBV-DNA detection combined with EBER- and BHLF1-ISH showed single latently infected lymphocytes in only one case. Our data imply that infected lymphocytes and not epithelial cells are, in fact, the reservoir for EBV infection, and that these are the cells that participate in the interindividual virus transfer.

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