Purification and characterization of novel toxin produced by Vibrio cholerae O1

Construction and evaluation of V. cholerae O139 mutant, VCUSM21P, as a safe live attenuated cholera vaccine

Cholera is a major infectious disease, affecting millions of lives annually. In endemic areas, implementation of vaccination strategy against cholera is vital. As the use of safer live vaccine that can induce protective immunity against Vibrio cholerae O139 infection is a promising approach for immunization, we have designed VCUSM21P, an oral cholera vaccine candidate, which has ctxA that encodes A subunit of ctx and mutated rtxA/C, ace and zot mutations. VCUSM21P was found not to disassemble the actin of HEp2 cells. It colonized the mice intestine approximately 1 log lower than that of the Wild Type (WT) strain obtained from Hospital Universiti Sains Malaysia. In the ileal loop assay, unlike WT challenge, 1×10⁶ and 1×10⁸ colony forming unit (CFU) of VCUSM21P was not reactogenic in non-immunized rabbits. Whereas, the reactogenicity caused by the WT in rabbits immunized with 1×10¹⁰ CFU of VCUSM21P was found to be reduced as evidenced by absence of fluid in loops administered with 1×10²–1×10⁷ CFU of WT. Oral immunization using 1×10¹⁰ CFU of VCUSM21P induced both IgA and IgG against Cholera Toxin (CT) and O139 lipopolysaccharides (LPS). The serum vibriocidal antibody titer had a peak rise of 2560 fold on week 4. Following Removable Intestinal Tie Adult Rabbit Diarrhoea (RITARD) experiment, the non-immunized rabbits were found not to be protected against lethal challenge with 1×10⁹ CFU WT, but 100% of immunized rabbits survived the challenge. In the past eleven years, V. cholerae O139 induced cholera has not been observed. However, attenuated VCUSM21P vaccine could be used for vaccination program against potentially fatal endemic or emerging cholera caused by V. cholerae O139.

Cholera toxin: a paradigm of a multifunctional protein

Cholera toxin (CT) was discovered exactly half a century ago by S.N. De. We have come a long way since this epoch-making discovery. Retrospectively, science had to wait a long time since Koch's prediction of the existence of a toxin, and its actual discovery by De. CT is not just another enterotoxin that causes the signs and symptoms of the dreaded disease, cholera. It is unique in many respects, starting from its structure to its functions. CT is a multifunctional protein that is capable of influencing the immune system in many ways. It not only has remarkable adjuvant properties, but also acts as an anti-inflammatory agent, by modulating specific signal transduction pathways. Its immunomodulatory properties can be harnessed for treatment of various autoimmune disorders, and have shown great promise in the area of immunotherapeutics. CT can truly be considered as a paradigm of a multifunctional protein.

A potential epidemic factor from the bacteria, Vibrio cholerae WO7

Certain species of Vibrio cholerae have evolved mechanisms to become pathogenic to humans, with the potential to cause a severe life-threatening diarrheal disease, cholera. Cholera can emerge as explosive outbreaks in the human population. V. cholerae illness is produced primarily through the expression of a potent toxin (cholera toxin) within the human intestine. The present study has been carried out on a novel toxin purified from V. cholerae W07, an epidemic cholera strain devoid of cholera toxin gene (ctx). A modified method of purification improved purification fold as well as yield of this toxin. Heating was found to be the essential and sufficient condition for dissociation of the two subunits (58 kDa and 40 kDa) of this toxin (pI 5.2). The 40-kDa subunit of the purified toxin was identified as the carbohydrate binding subunit. This toxin was found to induce apoptosis in HEp-2 cells. Thus, the WO7 toxin seems to have potential importance in the pathogenesis of disease associated with Vibrio cholerae WO7.

Vibrio cholerae infection of Drosophila melanogaster mimics the human disease cholera

Cholera, the pandemic diarrheal disease caused by the gram-negative bacterium Vibrio cholerae, continues to be a major public health challenge in the developing world. Cholera toxin, which is responsible for the voluminous stools of cholera, causes constitutive activation of adenylyl cyclase, resulting in the export of ions into the intestinal lumen. Environmental studies have demonstrated a close association between V. cholerae and many species of arthropods including insects. Here we report the susceptibility of the fruit fly, Drosophila melanogaster, to oral V. cholerae infection through a process that exhibits many of the hallmarks of human disease: (i) death of the fly is dependent on the presence of cholera toxin and is preceded by rapid weight loss; (ii) flies harboring mutant alleles of either adenylyl cyclase, Gsα, or the Gardos K⁺ channel homolog SK are resistant to V. cholerae infection; and (iii) ingestion of a K⁺ channel blocker along with V. cholerae protects wild-type flies against death. In mammals, ingestion of as little as 25 μg of cholera toxin results in massive diarrhea. In contrast, we found that ingestion of cholera toxin was not lethal to the fly. However, when cholera toxin was co-administered with a pathogenic strain of V. cholerae carrying a chromosomal deletion of the genes encoding cholera toxin, death of the fly ensued. These findings suggest that additional virulence factors are required for intoxication of the fly that may not be essential for intoxication of mammals. Furthermore, we demonstrate for the first time the mechanism of action of cholera toxin in a whole organism and the utility of D. melanogaster as an accurate, inexpensive model for elucidation of host susceptibility to cholera.

Cholera, the pandemic diarrheal disease caused by the gram-negative bacterium Vibrio cholerae, continues to be a major public health challenge in the developing world. Environmental studies have demonstrated a close association between V. cholerae and many species of arthropods, and insects have previously been implicated as vectors of this disease. Here researchers report the susceptibility of the fruit fly, Drosophila melanogaster, to oral V. cholerae infection through a process that exhibits many of the hallmarks of human disease. Furthermore, although ingestion of cholera toxin results in massive diarrhea in mammals, these researchers have found that ingestion of purified cholera toxin is not lethal to the fly. However, when co-ingested with a pathogenic strain of V. cholerae carrying a deletion of the cholera toxin genes, cholera toxin is lethal. These findings not only demonstrate the utility of D. melanogaster as an accurate, inexpensive model for elucidation of the host-pathogen interaction and identification of inhibitors of the action of cholera toxin; they also suggest that V. cholerae carries additional virulence factors that enable intoxication of an arthropod host. Based on these findings, the researchers suggest that the fly or a related arthropod may be a true host of V. cholerae in nature.

Effect of W07-toxin on gut physiological response in mice

A number of unknown secretogenic factor(s) from Vibrio cholerae have been implicated to play a role in inducing cholera-like symptoms observed in patients. The present study has been carried out on the novel W07-toxin (pI 5.2) from V. cholerae W07, an epidemic cholera strain devoid of the ctx gene. The toxin showed maximum binding to GM(1) and interacted with a 20 kDa glycoprotein present on the cell membrane of mice enterocytes in a GM(1) specific manner. The analysis of biochemical parameters in enterocytes triggered with this toxin revealed a significant increase in intracellular calcium concentration and a massive secretion of Cl(-). However, no absorption of Na(+) was observed under the same condition. This toxin also elevated the level of cyclic adenosine 3',5'-monophosphate (cAMP) as well as protein kinase A (PKA). Thus, the novel toxin, although distinct from cholera-toxin, showed some functional homology to it and may be one of the key players inducing electrolyte imbalance within intestinal cells in the cholera-like symptoms associated with V. cholerae W07.

Spectrum of gut immunologic reactions: selective induction of distinct responses to Vibrio cholerae WO7 and its toxin

Past studies with Vibrio cholerae have shown that cholera toxin (CT) is mainly responsible for inducing T helper type 2 (Th2) responses with systemic IgG1, IgE and mucosal secretory IgA (sIgA) antibodies. In this study, V. cholerae WO7, which produces novel toxin unrelated to CT, was given orally to mice in order to determine whether the strain V. cholerae WO7 differs from V. cholerae 569B, which produces CT, in the nature of responses generated at the gut and splenic level. The analysis of immune responses evoked by V. cholerae WO7 in the gut of mice revealed striking differences as compared to those elicited by V. cholerae 569B infection. To assess the T helper cell type responses, lymphocytes from Peyer's patches and the spleen were stimulated in vitro for studying the cytokine patterns. PP and SP lymphoid cells from V. cholerae WO7 infected animals elaborated significant amounts of IL-2, IFN-gamma and IL-12 by 7 days p.i., suggesting a Th1 type of response. However by 15 days p.i., the PP and SP lymphoid cells secreted only IL-6 and IL-10 with traces of IFN-gamma. On the other hand, infection with V. cholerae 569B yielded mainly Th2 type responses at Peyer's patches as well as the splenic level. Infection with both V. cholerae WO7 and 569B induced toxin-specific IgA secreting cells at the gut and splenic level along with IgG1 secreting cells, indicating that both V. cholerae WO7 and 569B evoke an antigen-specific Th2 type of response in the gut as well as spleen. The persistence of IgA along with Th1-type cytokines indicates an alternate induction mechanism since mucosal IgA responses are usually associated with Th2-type responses. These observations are suggestive of a common mechanism employed by the host to clear different strains of V. cholerae infection (569B and WO7 in this case), while the nature of toxins elaborated failed to modulate the net outcome of the infection caused by V. cholerae.

Purification and characterization of a cytotonic protein expressed In vitro by the live cholera vaccine candidate CVD 103-HgR

Cholera vaccines developed by the deletion of CTX genes from Vibrio cholerae induce a residual reactogenicity in up to 10% of vaccinees. A novel cytotonic agent named secreted CHO cell elongating protein (S-CEP) was purified from culture supernatants of CVD 103-HgR (Levine et al., Lancet ii:467-470, 1988). Five fractionation steps yielded electrophoretically pure S-CEP with an M(r) of 79,000. A partially purified preparation caused fluid accumulation in the sealed infant mouse model. The amino terminus bore a unique sequence with strong homology to a cytotonic toxin of El Tor V. cholerae.