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ABSTRACTEnterotoxigenic Escherichia coli(ETEC) is a major cause of travelers’ diarrhea and of diarrhea among young children in developing countries. Experimental challenge studies in adult volunteers have played a pivotal role in establishing ETEC as an enteric pathogen, elucidating its pathogenesis by identifying specific virulence attributes, characterizing the human immune response to clinical and sub-clinical ETEC infection and assessing preliminarily the clinical acceptability, immunogenicity and efficacy of prototype ETEC vaccines. This review provides a historical perspective of experimental challenge studies with ETEC. It summarizes pioneering early studies carried out by investigators at the University of Maryland School of Medicine to show how those studies provided key information that influenced the directions taken by many research groups to develop vaccines to prevent ETEC. In addition, key experimental challenge studies undertaken at other institutions will also be cited.

The Zika virus epidemic and associated congenital infections have prompted rapid vaccine development. We assessed two new DNA vaccines expressing premembrane and envelope Zika virus structural proteins.

Influenza causes substantial morbidity and mortality despite available treatments. Anecdotal reports suggest that plasma with high antibody titres to influenza might be of benefit in the treatment of severe influenza.

ABSTRACTStaphylococcus aureusproduces several enterotoxins and superantigens, exposure to which can elicit profound toxic shock. A recombinant staphylococcal enterotoxin B (rSEB) containing 3 distinct mutations in the major histocompatibility complex class II binding site was combined with an alum adjuvant (Alhydrogel) and used as a potential parenteral vaccine named STEBVax. Consenting healthy adult volunteers (age range, 23 to 38 years) participated in a first-in-human open-label dose escalation study of parenteral doses of STEBVax ranging from 0.01 µg up to 20 µg. Safety was assessed by determination of the frequency of adverse events and reactogenicity. Immune responses to the vaccination were determined by measurement of anti-staphylococcal enterotoxin B (anti-SEB) IgG by enzyme-linked immunosorbent assay and a toxin neutralization assay (TNA). Twenty-eight participants were enrolled in 7 dosing cohorts. All doses were well tolerated. The participants exhibited heterogeneous baseline antibody titers. More seroconversions and a faster onset of serum anti-SEB IgG toxin-neutralizing antibodies were observed by TNA with increasing doses of STEBVax. There was a trend for a plateau in antibody responses with doses of STEBVax of between 2.5 and 20 µg. Among the participants vaccinated with 2.5 µg to 20 µg of STEBVax, ~93% seroconverted for SEB toxin-neutralizing antibody. A strong correlation between individual SEB-specific serum IgG antibody titers and the neutralization of gamma interferon production was found in vitro. STEBvax appeared to be safe and immunogenic, inducing functional toxin-neutralizing antibodies. These data support its continued clinical development. (This study has been registered at ClinicalTrials.gov under registration no. NCT00974935.)

ABSTRACTThe adjuvant AS03 is stockpiled for future formulations with new and existing vaccines for the control of pandemic influenza virus. We previously reported the immunogenicity of an A/H5N1 vaccine extemporaneously mixed with the AS03 adjuvant for 42 days following vaccination. This report extends those findings to 1 year after vaccination.

ABSTRACTNoroviruses are the leading cause of acute gastroenteritis worldwide, and norovirus vaccine prevention strategies are under evaluation. The immunogenicity of two doses of bivalent genogroup 1 genotype 1 (GI.1)/GII.4 (50 µg of virus-like particles [VLPs] of each strain adjuvanted with aluminum hydroxide and 3-O-desacyl-4'monophosphoryl lipid A [MPL]) norovirus vaccine administered to healthy adults in a phase 1/2 double-blind placebo-controlled trial was determined using virus-specific serum total antibody enzyme-linked immunosorbent assay (ELISA), IgG, IgA, and histoblood group antigen (HBGA)-blocking assays. Trial participants subsequently received an oral live virus challenge with a GII.4 strain, and the vaccine efficacy results were reported previously (D. I. Bernstein et al., J Infect Dis 211:870–878, 2014, doi:10.1093/infdis/jiu497). This report assesses the impact of prechallenge serum antibody levels on infection and illness outcomes. Serum antibody responses were observed in vaccine recipients by all antibody assays, with first-dose seroresponse frequencies ranging from 88 to 100% for the GI.1 antigen and from 69 to 84% for the GII.4 antigen. There was little increase in antibody levels after the second vaccine dose. Among the subjects receiving the placebo, higher prechallenge serum anti-GII.4 HBGA-blocking and IgA antibody levels, but not IgG or total antibody levels, were associated with a lower frequency of virus infection and associated illness. Notably, some placebo subjects without measurable serum antibody levels prechallenge did not become infected after norovirus challenge. In vaccinees, anti-GII.4 HBGA-blocking antibody levels of >1:500 were associated with a lower frequency of moderate-to-severe vomiting or diarrheal illness. In this study, prechallenge serum HBGA antibody titers correlated with protection in subjects receiving the placebo; however, other factors may impact the likelihood of infection and illness after virus exposure. (This study is registered at ClinicalTrials.gov under registration number NCT1609257.)

ABSTRACTEnterotoxigenic Escherichia coli(ETEC) organisms are a leading cause of infectious diarrhea in developing countries. A live, attenuated cholera strain that expresses high levels of the nontoxic B subunit of cholera toxin, which might also serve as an ETEC protective antigen, was evaluated for safety, excretion, and immunogenicity in healthy volunteers. We enrolled four inpatient dose-escalation cohorts of 15 to 16 eligible subjects to randomly (3:1) receive a single oral dose of vaccine or placebo (buffer alone), evaluating 1 ×107, 1 ×108, 1 ×109, and 1 ×1010CFU of the vaccine. The vaccine was well tolerated, although some subjects experienced moderate diarrhea. The serum Inaba vibriocidal antibody response appeared to display a dose-response relationship with increasing dosages of vaccine, plateauing at the 109-CFU dosage. The serum antitoxin (cholera toxin and heat-labile enterotoxin) antibody seroconversion rate (4-fold increase over baseline) also appeared to display a dose-response relationship. The vaccine strain was excreted in stool cultures, displaying a dose-response relationship. A single oral dose of Peru-15 pCTB at dosages up to 1 ×1010CFU was safe and immunogenic in this first-in-human trial. These encouraging data support the ongoing clinical development of this candidate combined cholera and ETEC vaccine. (This study has been registered at ClinicalTrials.gov under registration no. NCT00654108.)

ABSTRACTCurrently, no cholera vaccine is available for persons traveling from the United States to areas of high cholera transmission and who for reasons of occupation or host factors are at increased risk for development of the disease. A single-dose oral cholera vaccine with a rapid onset of protection would be particularly useful for such travelers and might also be an adjunct control measure for cholera outbreaks. The attenuated Vibrio choleraeO1 vaccine strain CVD 103-HgR harbors a 94% deletion of the cholera toxin A subunit gene (ctxA) and has a mercury resistance gene inserted in the gene encoding hemolysin A. We undertook a phase I randomized placebo-controlled two-site trial to assess the safety and immunogenicity of a preliminary formulation of CVD 103-HgR prepared from new master and working cell banks. Healthy young adults were randomized (5:1 vaccinees to placebo recipients) to receive a single oral dose of ~4.4 × 108CFU of vaccine or a placebo. Blood serum vibriocidal and cholera toxin-specific IgG antibodies were measured before and 10, 14, and 28 days following vaccination or placebo. Excretion of the vaccine strain in the stool was assessed during the first week postvaccination. A total of 66 subjects were enrolled, comprising 55 vaccinees and 11 placebo recipients. The vaccine was well tolerated. The overall vibriocidal and anti-cholera toxin seroconversion rates were 89% and 57%, respectively. CVD 103-HgR is undergoing renewed manufacture for licensure in the United States under the auspices of PaxVax. Our data mimic those from previous commercial formulations that elicited vibriocidal antibody seroconversion (a correlate of protection) in ~90% of vaccinees. (This study has been registered at ClinicalTrials.gov under registration no. NCT01585181.)

ABSTRACTEnterotoxigenic Escherichia coli(ETEC) is a primary cause of traveler's diarrhea for which there is no licensed vaccine. This phase 1 trial determined the safety and immunogenicity of a recombinantly produced double mutant heat-labile enterotoxin (dmLT) of ETEC. It was administered as a single oral dose of dmLT in escalating doses of 5 µg, 25 µg, 50 µg, and 100 µg, followed by a 72-h inpatient observation, outpatient visits at 8, 14, and 28 days, and telephone calls at 2 and 6 months postvaccination. Safety was assessed by frequency of adverse events, and immune responses determined after immunization included dmLT-specific serum IgA and IgG, fecal IgA, antibody-secreting cells (ASC), and antibodies in lymphocyte supernatant (ALS) responses. All doses were well tolerated by the 36 healthy adults enrolled. Immune responses were limited in the 5- and 25-µg dose recipients. The 50-µg dose recipients trended toward stronger responses than the 100-µg dose recipients by serum IgA (67% versus 33%, P= 0.22), serum IgG (58% versus 33%, P= 0.41), and fecal IgA (58% versus 33%, P= 0.41). By day 14 postvaccination, there were significantly more positive responders (=4-fold increase from baseline) among the 50- versus 100-µg dose recipients for serum IgA (P= 0.036) but not serum IgG (P= 0.21). In conclusion, a single oral dose of dmLT was well tolerated and immunogenic, with immune responses plateauing at the 50-µg dose. (This clinical trial is registered at www.clinicaltrials.gov, registration number NCT01147445.)

When given passively or elicited actively, antibodies induced by a detoxified Escherichia coli Rc chemotype (J5) mutant lipopolysaccharide (J5dLPS)—group B meningococcal outer membrane protein (OMP) complex vaccine protected animals from lethal sepsis. The protection from sepsis is believed to be dependent on high levels of antibodies against the core glycolipid (CGL), a region of LPS that is rather conserved among Enterobacteriaceae. The addition of unmethylated deoxycytidyl-deoxyguanosine dinucleotide (CpG)-containing oligodeoxynucleotides (ODN) was used as an immuno-adjuvant to improve antibody responses. In preparation for a Phase I human trial, we elucidated potential contributions by which the sepsis vaccine (J5dLPS—OMP) and CpG ODN might enhance the antibody response and provide evidence that the generation of immune responses is Toll-like receptor (TLR) dependent. Toll-like receptor 2, TLR4, and TLR9 were each essential for generating robust cytokine and antibody responses. The signature cytokine of dendritic cells, interleukin-12, was one of the cytokines that demonstrated synergy with the optimal TLR ligand/ engagement combination. We conclude that the involvement of multiple TLRs upon immunization was critical for the generation of optimal antibody responses. These observations provide further evidence for the inclusion of innate immune-based adjuvants during the development of next-generation vaccines.

Vaccination is the most effective means for the prevention of influenza, including pandemic strains. An ideal pandemic influenza vaccine should provide effective protection with the fewest number of doses in the shortest amount of time, and among the greatest proportion of the population. The current manufacturing processes required for embryonated chicken-egg-based influenza vaccines are limited in their ability to respond to pandemic situations - these limitations include problems with surge capacity, the need for egg-adapted strains, the possibility of contamination, and the presence of trace egg protein. Several vaccine strategies to circumvent the deficiencies intrinsic to an egg-based influenza vaccine are in various phases of development. These include the use of cell-culture-based growth systems, concomitant use of adjuvants, whole virus vaccines, recombinant protein vaccines, plasmid DNA vaccines, virus-like particle vaccines, and universal flu vaccines.

Herpes zoster, also known as shingles, is a disease that results from the reactivation of a latent infection of the varicella zoster virus, which is usually encountered during early childhood. Aging is associated with an increased risk for herpes zoster and its complications. Boosting immunological memory is the key strategy for keeping the latent varicella zoster virus infection under control. A live attenuated virus vaccine is safe, effective and approved for use among healthy elderly adults aged 60 years or older. However, significant problems remain in the prevention of herpes zoster with the current vaccine. Future studies for improved vaccines and studies into the epidemiology of herpes zoster are required in order to address this significant public health burden.

When given passively or elicited actively, antibodies induced by a detoxified Escherichia coliJ5 mutant lipopolysaccharide (J5dLPS)-group B meningococcal outer membrane protein (-OMP) vaccine previously protected animals from lethal sepsis. To assess the use of this vaccine for the treatment of Gram-negative bacillary pneumonia, we vaccinated mice, with or without the adjuvant CpG, by intranasal (i.n.) or intraperitoneal (i.p.) routes of administration. Local and systemic IgG levels were 2—3 logs higher following i.p. immunization compared to i.n. However, i.n. immunization elicited both local and systemic IgA, unlike i.p. administration. The addition of CpG to the vaccine, by either route of administration, elicited greater levels of antibody. Intranasal immunization protected mice against lethal heterologous Gram-negative bacillary pneumonia and post-immunization serum and broncho-alveolar lavage fluid mediated enhanced bacterial killing with peritoneal and alveolar macrophages in vitro. We conclude that further studies on the use of J5dLPS-OMP for the prevention of nosocomial pneumonia are warranted.

Immunization is a highly effective public health measure that reduces the incidence of infectious diseases, yet there has been relatively little effort toward the development of vaccines for nosocomial infections. Many nosocomial infections originate on mucosal surfaces (e.g., respiratory or gastrointestinal mucosa). As patients who are hospitalized once are more likely to be hospitalized again, we propose a prime‐boost immunization strategy, whereby a priming dose of vaccine for a nosocomial infection is administered mucosally. Upon readmission, a parenteral boost would elicit a rapid immune response locally and systemically. Such a strategy could reduce or ameliorate nosocomial infections and perhaps limit dissemination of nosocomial pathogens. Thus, a more aggressive effort to develop vaccines for nosocomial infections is warranted.

The innate immune response of macrophages (M) to spores, the environmentally acquired form of Bacillus anthracis, is poorly characterized. We therefore examined the early M cytokine response to B. anthracis spores, before germination. M were exposed to bacilli and spores of Sterne strain 34F2 and its congenic nongerminating mutant (gerH), and cytokine expression was measured by real-time PCR and an enzyme-linked immunosorbent assay. The exosporium spore layer was retained (exo+) or removed by sonication (exo–). Spores consistently induced a strong cytokine response, with the exo–spores eliciting a two- to threefold-higher response than exo+spores. The threshold for interleukin-1{szligbeta} (IL-1{szligbeta}) production by wild-type M was significantly lower than that required for tumor necrosis factor alpha expression. Cytokine production was largely dependent on MyD88, suggesting Toll-like receptor involvement; however, the expression of beta interferon in MyD88–/–M suggests involvement of a MyD88-independent pathway. We conclude that (i) the B. anthracis spore is not immunologically inert, (ii) the exosporium masks epitopes recognized by the M, (iii) the M cytokine response to B. anthracis involves multiple pattern recognition receptors and signaling pathways, and (iv) compared to other cytokines, IL-1{szligbeta} is expressed at a lower spore concentration.

Because the induction of interleukin-1β (IL-1β) is critical to antibacterial host defenses and its excessive generation is a prominent component of sepsis, regulation of this proinflammatory cytokine is a critical factor in the immune response to lipopolysaccharide (LPS). We previously showed that LPS-induced IL-1β expression was regulated by a Stat1-dependent, nitric oxide (NO)-mediated mechanism. Subsequent in vivo studies showed that whereas Stat1 had a role in the downregulation of IL-1β expression, it had a more significant effect on its initial induction. Although both interferon-β (IFN-β) and IFN-γ activate Stat1, the early appearance of IFN-β in the circulation after LPS administration suggested its pivotal role in Stat1-mediated IL-1β expression in vivo. Further in vitro analysis of peritoneal macrophages from IFN-β–/–, Stat1–/–, and caspase-1–/– mice and their wild-type controls following LPS stimulation demonstrated that IL-1β mRNA was expressed in these mice but not in macrophages from MyD88–/– mice. Despite the presence of IL-1β mRNA, IL-1β protein was markedly reduced in the absence of Stat1 activation in macrophages derived from IFN-β–/– and Stat1–/– mice or in the absence of caspase-1 activity, which itself was dependent on Stat1 activation. These studies support the hypothesis that the expression of IL-1β requires both the MyD88-dependent induction of IL-1β mRNA and pro-IL-1β as well as the MyD88-independent, Stat1-mediated processing of that gene product into active cytokine.

Beyond the deterministic limit where the initial value sensitive predictability can hardly be found, the boundary condition dependent potential predictability is examined. The tropical anomalies of the opposite phases of El Niño/Southern Oscillation (ENSO) can significantly change the extratropical natural variability on a wide range of spatial and temporal scales. We address the impact on the low‐frequency variability, such as the persistent blocking flows, and the high‐frequency variability, represented primarily by the storm tracks. The NCEP/NCAR reanalyses are used for this investigation. Several diagnostics tools help to reveal the dynamical processes leading to the large change of the natural variability and the potential predictability in the extratropical latitudes between these two phases of the ENSO cycle. During El Niño winters, the principal storm tracks are steered more into the southern and Baja California region, by the much eastward extended subtropical jets. On the other hand, the storms are diverted more into the higher latitudes (Aleutians and Gulf of Alaska) during La Niña winters, when jetstreams are much weaker east of 160°W. Although being passively steered to widely different regions, the high‐frequency transients do feed back actively to strengthen and maintain the subtropical jet across the central North Pacific and also act to slow down the equatorward flank of the jet. The feedback by the transients is stronger during the El Niño than the La Niña winters, helping in maintaining stronger signals from the tropics for the El Niño winters. There is also a large change of low‐frequency variability: much larger magnitude of kinetic energy and height variance during La Niña than El Niño winters. The local barotropic energy diagnosis reveals that, on average, the low‐frequency components extract more energy from time‐mean flows during La Niña than El Niño winters, helping in explaining the presence of much larger low‐frequency variability during the La Niña winters. With stronger ENSO signals and weaker natural variability during El Niño winters, the potential predictability in the north Pacific sector is significantly higher, on these two counts, than during the La Niña winters.

AbstractA low-resolution version of the National Meteorological Center's global spectral model was used to generate a 10-year set of simulated daily meteorological data. Wintertime low-frequency large-amplitude anomalies were examined and compared with those observed in the real atmosphere. The geographical distributions of the mean and variance of model and real atmosphere show some resemblance. However, careful comparisons reveal distinct regions where short-term climate anomalies prefer to develop. The model's low-frequency anomalies (LFAS) over the North Pacific (North Atlantic) tend to occur about 1500 miles east (southeast) of those observed, locating themselves much closer to the western continents. Because of the Displacement of the model's LFA centers, their associated circulation patterns deviate substantially from those observed.The frequency distributions of the LFAs for both the model and reality display large skewness. The positive and negative large LFAs were, therefore, examined separately, and four-way intercomparisons were conducted between the model, the observed, the positive, and the negative LFAS. The separate analyses resulted in distinguishable circulation patterns between the positive and negative large LFAS, which cannot possibly be identified if a linear analysis tool, such as an empirical orthogonal function analysis, were used to extract the most dominant mode of the circulations. Despite pronounced misplacement of large LFAs of both polarities and a general underestimation of their magnitudes, the model dm have the capability of persisting its short-term climate anomaly at certain geographical locations. Over the North Pacific, the model's positive LFAs persist as long or longer than those found in reality, while its negative LFAs persist only one-fourth as long (10 versus 40 days).The principal storm tracks and mean zonal wind at 250 mb (U250) were also examined to supplement the low-frequency anomaly investigation. Contrasting with observations, the model's U250s display considerable eastward extension and its storm tracks near the jet exit show substantial equatorward displacement over both the North Pacific and the North Atlantic oceans. These model characteristics are consistent with the behavior that the model's large LFAs also prefer to develop over the regions far east and southeast of those observed in the real atmosphere.

The characteristics of extratropical low-frequency variability are examined using a comprehensive atmospheric general circulation model. A large experiment consisting of 13 45-yr-long integrations forced by prescribed sea surface temperature (SST) variations is analyzed. The predictability of timescales of seasonal to decadal averages is evaluated. The variability of a climate mean contains not only climate signal arising from external boundary forcing but also climate noise due to the internal dynamics of the climate system, resulting in various levels of predictability that are dependent on the forcing boundary conditions and averaging timescales. The focus of this study deviates from the classic predictability study of Lorenz, which is essentially initial condition sensitive. This study can be considered to be a model counterpart of Madden’s “potential” predictability study.

We assessed whether age modified the association between methicillin-resistant Staphylococcus aureus(MRSA) anterior nares colonization and subsequent infection. Among 7,405 patients (9,511 admissions), MRSA colonization was significantly associated with infection (adjusted odds ratio, 13.7 [95% confidence interval, 7.325.7]) but did not differ significantly by age group.

ABSTRACTReactive immunization with a single-dose cholera vaccine that could rapidly (within days) protect immunologically naive individuals during virgin soil epidemics, when cholera reaches immunologically naive populations that have not experienced cholera for decades, would facilitate cholera control. One dose of attenuated Vibrio choleraeO1 classical Inaba vaccine CVD 103-HgR (Vaxchora) containing =2 × 108CFU induces vibriocidal antibody seroconversion (a correlate of protection) in >90% of U.S. adults. A previous CVD 103-HgR commercial formulation required =2 × 109CFU to elicit high levels of seroconversion in populations in developing countries. We compared the vibriocidal responses of Malians (individuals 18 to 45 years old) randomized to ingest a single =2 × 108-CFU standard dose (n= 50) or a =2 × 109-CFU high dose (n= 50) of PaxVax CVD 103-HgR with buffer or two doses (n= 50) of Shanchol inactivated cholera vaccine (the immunologic comparator). To maintain blinding, participants were dosed twice 2 weeks apart; CVD 103-HgR recipients ingested placebo 2 weeks before or after ingesting vaccine. Seroconversion (a =4-fold vibriocidal titer rise) between the baseline and 14 days after CVD 103-HgR ingestion and following the first and second doses of Shanchol were the main outcomes measured. By day 14 postvaccination, the rates of seroconversion after ingestion of a single standard dose and a high dose of CVD 103-HgR were 71.7% (33/46 participants) and 83.3% (40/48 participants), respectively. The rate of seroconversion following the first dose of Shanchol, 56.0% (28/50 participants), was significantly lower than that following the high dose of CVD 103-HgR (P= 0.003). The vibriocidal geometric mean titer (GMT) of the high dose of CVD 103-HgR exceeded the GMT of the standard dose at day 14 (214 versus 95, P= 0.045) and was ~2-fold higher than the GMT on day 7 and day 14 following the first Shanchol dose (P> 0.05). High-dose CVD 103-HgR is recommended for accelerated evaluation in developing countries to assess its efficacy and practicality in field situations. (This study has been registered at ClinicalTrials.gov under registration no. NCT02145377.)

ABSTRACTNeuraminidases (NAs) are critical virulence factors for several microbial pathogens. With a highly conserved catalytic domain, a microbial NA “superfamily” has been proposed. We previously reported that murine polymorphonuclear leukocyte (PMN) sialidase activity was important in leukocyte trafficking to inflamed sites and that antibodies to Clostridium perfringensNA recognized a cell surface molecule(s), presumed to be a sialidase of eukaryotic origin on interleukin-8-stimulated human and murine PMNs. These antibodies also inhibited cell sialidase activity both in vitroand, in the latter instance, in vivo. We therefore hypothesized that mammalian sialidases share structural homology and epitopes with microbial NAs. We now report that antibodies to one of the isoforms of C. perfringensNA, as well as anti-influenza virus NA serum, recognize human NEU3 but not NEU1 and that antibodies to C. perfringensNA inhibit NEU3 enzymatic activity. We conclude that the previously described microbial NA superfamily extends to human sialidases. Strategies designed to therapeutically inhibit microbial NA may need to consider potential compromising effects on human sialidases, particularly those expressed in cells of the immune system.IMPORTANCEWe previously reported that sialidase activity of human neutrophils plays a critical role in the host inflammatory response. Since the catalytic domains of microbial neuraminidases are highly conserved, we hypothesized that antibodies against Clostridium perfringensneuraminidase might inhibit mammalian sialidase activity. Before the recognition of four mammalian sialidase (Neu) isoforms, we demonstrated that anti-C. perfringensneuraminidase antibodies inhibited human and murine sialidase activity in vivoand in vitro. We now show that the antibodies to microbial neuraminidase (C. perfringensand influenza virus) recognize human NEU3, which is important for neural development and cell signaling. Since many microbes that infect mucosal surfaces express neuraminidase, it is possible that the use of sialidase inhibitors (e.g., zanamivir), might also compromise human sialidase activity critical to the human immune response. Alternatively, sialidase inhibitors may prove useful in the treatment of hyperinflammatory conditions.

ABSTRACTOne potential advantage of live attenuated bacterial vaccines is the ability to stimulate responses to antigens which are only expressed during the course of infection. To determine whether the live attenuated cholera vaccine CVD 103-HgR (Vaxchora) results in antibody responses to the in vivo-induced toxin-coregulated pilus antigen TcpA, we measured IgA and IgG responses to Vibrio choleraeO1 El Tor TcpA in a subset of participants in a recently reported experimental challenge study. Participants were challenged with V. choleraeO1 El Tor Inaba N16961 either 10 days or 90 days after receiving the vaccine or a placebo. Neither vaccination nor experimental infection with V. choleraealone resulted in a robust TcpA IgG or IgA response, but each did elicit a strong response to cholera toxin. However, compared to placebo recipients, vaccinees had a marked increase in IgG TcpA antibodies following the 90-day challenge, suggesting that vaccination with CVD 103-HgR resulted in priming for a subsequent response to TcpA. No such difference between vaccine and placebo recipients was observed for volunteers challenged 10 days after vaccination, indicating that this was insufficient time for vaccine-induced priming of the TcpA response. The priming of the response to TcpA and potentially other antigens expressed in vivoby attenuated V. choleraemay have relevance to the maintenance of immunity in areas where cholera is endemic.