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ABSTRACTVibrio choleraeand enterotoxigenic Escherichia coli(ETEC) are noninvasive mucosal pathogens that cause acute watery diarrhea in people in developing countries. Direct assessment of the mucosal immune responses to these pathogens is problematic. Surrogate markers of local mucosal responses in blood are increasingly being studied to determine the mucosal immune responses after infection. However, the volume of blood available in children and infants has limited this approach. We assessed whether an approach that first isolates ß7-positive cells from a small volume of blood would allow measurement of the antigen-specific immune responses in patients with cholera and ETEC infection. ß7 is a cell surface marker associated with mucosal homing. We isolated ß7-expressing cells from blood on days 2, 7, and 30 and used an enzyme-linked immunosorbent spot (ELISPOT) assay to assess the gut-homing antibody-secreting cells (ASCs) specific to pathogen antigens. Patients with ETEC diarrhea showed a significant increase in toxin-specific gut-homing ASCs at day 7 compared to the levels at days 2 and 30 after onset of illness and to the levels in healthy controls. Similar elevations of responses to the ETEC colonization factors (CFs) CS6 and CFA/I were observed in patients infected with CS6- and CFA/I-positive ETEC strains. Antigen-specific gut-homing ASCs to the B subunit of cholera toxin and cholera-specific lipopolysaccharides (LPS) were also observed on day 7 after the onset of cholera using this approach. This study demonstrates that a simple ELISPOT assay can be used to study the mucosal immunity to specific antigens using a cell-sorting protocol to isolate mucosal homing cells, facilitating measurement of mucosal responses in children following infection or vaccination.

ABSTRACTInfection with Vibrio choleraeand oral cholera vaccines (OCVs) induce transient circulating plasmablast responses that peak within approximately 7 days after infection or vaccination. We previously demonstrated that plasmablast responses strongly correlate with subsequent levels of V. cholerae-specific duodenal antibodies up to 6 months after V. choleraeinfection. Hence, plasmablast responses provide an early window into the immunologic memory at the mucosal surface. In this study, we characterized plasmablast responses following V. choleraeinfection using a flow cytometrically defined population and compared V. cholerae-specific responses in adult patients with V. choleraeO1 infection and vaccinees who received the OCV Dukoral (Crucell Vaccines Canada). Among flow cytometrically sorted populations of gut-homing plasmablasts, almost 50% of the cells recognized either cholera toxin B subunit (CtxB) or V. choleraeO1 lipopolysaccharide (LPS). Using a traditional enzyme-linked immunosorbent spot assay (ELISPOT), we found that infection with V. choleraeO1 and OCVs induce similar responses to the protein antigen CtxB, but responses to LPS were diminished after OCV compared to those after natural V. choleraeinfection. A second dose of OCV on day 14 failed to boost circulating V. cholerae-specific plasmablast responses in Bangladeshi adults. Our results differ from those in studies from areas where cholera is not endemic, in which a second vaccination on day 14 significantly boosts plasmablast responses. Given these results, it is likely that the optimal boosting strategies for OCVs differ significantly between areas where V. choleraeinfection is endemic and those where it is not.

ABSTRACTThe avidity of antibodies to specific antigens and the relationship of avidity to memory B cell responses to these antigens have not been studied in patients with cholera or those receiving oral cholera vaccines. We measured the avidity of antibodies to cholera toxin B subunit (CTB) and Vibrio choleraeO1 lipopolysaccharide (LPS) in Bangladeshi adult cholera patients (n= 30), as well as vaccinees (n= 30) after administration of two doses of a killed oral cholera vaccine. We assessed antibody and memory B cell responses at the acute stage in patients or prior to vaccination in vaccinees and then in follow-up over a year. Both patients and vaccinees mounted CTB-specific IgG and IgA antibodies of high avidity. Patients showed longer persistence of these antibodies than vaccinees, with persistence lasting in patients up to day 270 to 360. The avidity of LPS-specific IgG and IgA antibodies in patients remained elevated up to 180 days of follow-up. Vaccinees mounted highly avid LPS-specific antibodies at day 17 (3 days after the second dose of vaccine), but the avidity waned rapidly to baseline by 30 days. We examined the correlation between antigen-specific memory B cell responses and avidity indices for both antigens. We found that numbers of CTB- and LPS-specific memory B cells significantly correlated with the avidity indices of the corresponding antibodies (P< 0.05; Spearman's ?= 0.28 to 0.45). These findings suggest that antibody avidity after infection and immunization is a good correlate of the development and maintenance of memory B cell responses to Vibrio choleraeO1 antigens.

Vibrio cholerae O1 is a noninvasive enteric pathogen and serves as a model for studies of mucosal immunity. Although symptomatic V. cholerae infection induces durable protection against subsequent disease, vaccination with oral killed whole-cell V. cholerae stimulates less long-lasting protection against cholera. In this study, we demonstrated that cholera induces an early proinflammatory cellular immune response that results in priming of Th1- and Th17-type cytokine responses to ex vivo antigenic stimulation and an increase in the ratio of Th1 to Th2 CD4+T-cell responses. Comparable priming of Th1 and Th17 responses, with an increased ratio of Th1 to Th2 CD4+T-cell responses, was not observed in subjects who received two doses of the oral cholera vaccine Dukoral (a whole-cell cholera toxin B subunit containing [WC-CTB] vaccine). These findings suggest that natural V. cholerae infection induces an early, proinflammatory cellular immune response, despite the apparent lack of clinical signs of inflammation. The failure of the WC-CTB vaccine to activate equivalent, CD4+T-cell responses is a potential explanation for the shorter duration of protection following immunization with this vaccine. Additional studies are needed to determine whether these early T-cell-mediated events predict the subsequent duration of immunologic memory.

ABSTRACTVibrio choleraeO1 is a noninvasive enteric pathogen and serves as a model for studies of mucosal immunity. Although symptomatic V. choleraeinfection induces durable protection against subsequent disease, vaccination with oral killed whole-cell V. choleraestimulates less long-lasting protection against cholera. In this study, we demonstrated that cholera induces an early proinflammatory cellular immune response that results in priming of Th1- and Th17-type cytokine responses to ex vivoantigenic stimulation and an increase in the ratio of Th1 to Th2 CD4+T-cell responses. Comparable priming of Th1 and Th17 responses, with an increased ratio of Th1 to Th2 CD4+T-cell responses, was not observed in subjects who received two doses of the oral cholera vaccine Dukoral (a whole-cell cholera toxin B subunit containing [WC-CTB] vaccine). These findings suggest that natural V. choleraeinfection induces an early, proinflammatory cellular immune response, despite the apparent lack of clinical signs of inflammation. The failure of the WC-CTB vaccine to activate equivalent, CD4+T-cell responses is a potential explanation for the shorter duration of protection following immunization with this vaccine. Additional studies are needed to determine whether these early T-cell-mediated events predict the subsequent duration of immunologic memory.

ABSTRACTTo better understand the innate immune response to Vibrio choleraeinfection, we tracked gene expression in the duodenal mucosa of 11 Bangladeshi adults with cholera, using biopsy specimens obtained immediately after rehydration and 30 and 180 days later. We identified differentially expressed genes and performed an analysis to predict differentially regulated pathways and upstream regulators. During acute cholera, there was a broad increase in the expression of genes associated with innate immunity, including activation of the NF-κB, mitogen-activated protein kinase (MAPK), and Toll-like receptor (TLR)-mediated signaling pathways, which, unexpectedly, persisted even 30 days after infection. Focusing on early differences in gene expression, we identified 37 genes that were differentially expressed on days 2 and 30 across the 11 participants. These genes included the endosomal Toll-like receptor gene TLR8, which was expressed in lamina propria cells. Underscoring a potential role for endosomal TLR-mediated signaling in vivo, our pathway analysis found that interferon regulatory factor 7 and beta 1 and alpha 2 interferons were among the top upstream regulators activated during cholera. Among the innate immune effectors, we found that the gene for DUOX2, an NADPH oxidase involved in the maintenance of intestinal homeostasis, was upregulated in intestinal epithelial cells during cholera. Notably, the observed increases in DUOX2and TLR8expression were also modeled in vitrowhen Caco-2 or THP-1 cells, respectively, were stimulated with live V. choleraebut not with heat-killed organisms or cholera toxin alone. These previously unidentified features of the innate immune response to V. choleraeextend our understanding of the mucosal immune signaling pathways and effectors activated in vivofollowing cholera.

ABSTRACTWe characterized the acute B cell response in adults with cholera by analyzing the repertoire, specificity, and functional characteristics of 138 monoclonal antibodies (MAbs) generated from single-cell-sorted plasmablasts. We found that the cholera-induced responses were characterized by high levels of somatic hypermutation and large clonal expansions. A majority of the expansions targeted cholera toxin (CT) or lipopolysaccharide (LPS). Using a novel proteomics approach, we were able to identify sialidase as another major antigen targeted by the antibody response to Vibrio choleraeinfection. Antitoxin MAbs targeted both the A and B subunits, and most were also potent neutralizers of enterotoxigenic Escherichia coliheat-labile toxin. LPS-specific MAbs uniformly targeted the O-specific polysaccharide, with no detectable responses to either the core or the lipid moiety of LPS. Interestingly, the LPS-specific antibodies varied widely in serotype specificity and functional characteristics. One participant infected with the Ogawa serotype produced highly mutated LPS-specific antibodies that preferentially bound the previously circulating Inaba serotype. This demonstrates durable memory against a polysaccharide antigen presented at the mucosal surface and provides a mechanism for the long-term, partial heterotypic immunity seen following cholera.IMPORTANCECholera is a diarrheal disease that results in significant mortality. While oral cholera vaccines are beneficial, they do not achieve equivalent protection compared to infection with Vibrio cholerae. Although antibodies likely mediate protection, the mechanisms of immunity following cholera are poorly understood, and a detailed understanding of antibody responses to cholera is of significance for human health. In this study, we characterized the human response to cholera at the single-plasmablast, monoclonal antibody level. Although this approach has not been widely applied to the study of human bacterial infection, we were able to uncover the basis of cross-reactivity between different V. choleraeserotypes and the likely impact of prior enterotoxigenic Escherichia coliexposure on the response to cholera, as well as identify novel antigenic targets. In addition to improving our understanding of the repertoire and function of the antibody response to cholera in humans, this study has implications for future cholera vaccination efforts.