A novel cholera vaccine developed in India provides more protection


An Indian cholera vaccine now available produces only 53 per cent protection after two doses.

Using a novel approach, scientists in India have developed a live oral cholera vaccine that is not only more efficacious and hence more protective than the currently available ones but also able to elicit better protection with just one dose. The results of the human clinical trial of the vaccine have been published in July this year in the journal PLoS ONE.

“We were able to achieve 65.9 per cent sero-conversion using only one dose of the vaccine,” said Amit Ghosh who is currently an Emeritus Scientist at the National Institute of Cholera and Enteric Diseases (NICED) in Kolkata. An Indian cholera vaccine now available produces only 53 per cent protection after two doses.

The difference between the existing three vaccines and the candidate vaccine — VA1.4 — being tested goes beyond the level of protection achieved. The most important one from the public health perspective is that the higher protection was achieved using only one dose of the vaccine.

‘Shanchol’, marketed by Hyderabad-based Shantha Biotechnics requires two doses to achieve 53 per cent protection, with the second dose given 14 days after the first. The other two vaccines too need to be given in two doses.

But the biggest public health challenge when a vaccine is given as two doses is to make sure that people come back for the second dose. In reality, there could be a significant number of people not turning up for the second dose; this greatly impacts the achievement of the primary objective of preventive vaccination, especially during cholera outbreaks.

“It is difficult to say” whether it was the use of a live cholera strain (unlike the killed ones used in the other three cholera vaccines) in the vaccine that produced better protection Dr. Ghosh noted. “It’s a speculation that if some antigen that may induceprotective immunity is made by the Vibrio bug only when it is in the intestine, then this protective antigen is absent in the killed bug [used in other vaccines],” he explained.

But the biggest differentiating factor is that unlike the other three vaccines, the strain used in the VA1.4 vaccine does not have the gene that produces the cholera toxin.

“It does happen in nature that due to various reasons one bug may not have the gene responsible for producing cholera toxin,” he said. “NICED [National Institute of Cholera & Enteric Diseases, Kolkatta] screened 1,000s of cholera strains. They identified one and sent it to me at IMTECH [Institute of Microbial Technology, Chandigarh] in mid 1990s to genetically engineer the bug.”

While the general trend at that time was to take a live virulent cholera strain and remove the cholera toxin gene thereby preventing the strain from causing cholera (when the vaccine containing the live bacteria is given), the vaccine still causedsome adverse effects.

“When they remove the toxin gene, other secondary virulent factors present in the cholera bacteria whose adverse effects are normally masked by the presence of the cholera toxin gene emerge,” Dr. Ghosh explained. “These [secondary virulent factors] cause diarrhoea.”

“So we wanted to take a Vibrio bug which is completely devoid of all virulent factors and then manipulate it so the bug has only the immunogenic subunit of the cholera toxin,” he said. The live oral vaccine VA 1.4 was developed by isolating a ‘Vibrio cholerae O1 El Tor’ strain.

The cholera toxin gene is a combination of two different subunits. Subunit A of the toxin gene is the one that causes cholera disease, while subunit B is the immunogenic subunit that is necessary for the virus to produce antigen. The human immune system produces antibodies in response to the antigen produced by a bacteria/virus; antibodies so produced are responsible for killing the bacteria. “We genetically engineered the strain to produce the subunit B,” he said.

“We were probably lucky the approach worked,” he said. “We got a U.S. patent in 10 months of filing it.”

A human clinical trial conducted last year had 44 subjects on whom the vaccine was tested; 43 got a placebo. Two doses were given — the second dose was given 14 days after the first one. The sero-conversion was about 66 per cent on day seven after the first dose was given, and it did not increase further after the second dose was administered. The trial was done in collaboration with the Society for Applied Sciences, Kolkatta.

“The trial was beyond Phase I,” Dr. Ghosh said, “because it looked at sero-conversion [efficacy] and not just safety.” The main objective of Phase I trials is to check for the safety of a candidate drug/vaccine. A larger trial involving more human subjects is being planned.

The vaccine was developed by a collaborative effort of three institutes in India — IMTECH, NICED the Indian Institute of Chemical Biology (IICB), Kolkata. DBT funded the project.

 

Cholera is Altering the Human Genome.


Cholera kills thousands of people a year, but a new study suggests that the human body is fighting back. Researchers have found evidence that the genomes of people in Bangladesh—where the disease is prevalent—have developed ways to combat the disease, a dramatic case of human evolution happening in modern times.

 

Cholera has hitchhiked around the globe, even entering Haiti with UN peacekeepers in 2010, but the disease’s heartland is the Ganges River Delta of India and Bangladesh. It has been killing people there for more than a thousand years. By the time they are 15 years old, half of the children in Bangladesh have been infected with the cholera-causing bacterium, which spreads in contaminated water and food. The microbe can cause torrential diarrhea, and, without treatment, “it can kill you in a matter of hours,” says Elinor Karlsson, a computationalgeneticist at Harvard and co-author of the new study.

The fact that cholera has been around so long, and that it kills children—thus altering the gene pool of a population—led the researchers to suspect that it was exerting evolutionary pressure on the people in the region, as malaria has been shown to do in Africa. Another hint that the microbe drives human evolution, notes Regina LaRocque, a study co-author and infectious disease specialist at Massachusetts General Hospital, Boston, is that many people suffer mild symptoms or don’t get sick at all, suggesting that they have adaptations to counter the bacterium.

To tease out the disease’s evolutionary impact, Karlsson, LaRocque, and their colleagues, including scientists from the International Centre for Diarrhoeal Disease Research in Bangladesh, used a new statistical technique that pinpoints sections of the genome that are under the influence of natural selection. The researchers analyzed DNA from 36 Bangladeshi families and compared it to the genomes of people from northwestern Europe, West Africa, and eastern Asia. Natural selection has left its mark on 305 regions in the genome of the subjects from Bangladesh, the team reveals online today in Science Translational Medicine.

The researchers bolstered the case that cholera was the driving force behind the genomic changes by contrasting DNA from Bangladeshi cholera patients with DNA from other residents of the country who remained healthy despite living in the same house as someone who fell ill with the disease. Individuals who were susceptible to cholera typically carried DNA variants that lie within the region that shows the strongest effect from natural selection.

One category of genes that is evolving in response to cholera, the researchers found, encodes potassium channels that release chloride ions into the intestines. Their involvement makes sense because the toxin spilled by the cholera bacterium spurs such channels to discharge large amounts of chloride, leading to the severe diarrhea that’s characteristic of the disease.

A second category of selected genes helps manage the protein NF- kB, the master controller of inflammation, which is one of the body’s responses to the cholera bacterium. A third category involves genes that adjust the activity of the inflammasome, a protein aggregation inside our cells that detects pathogens and fires up inflammation. However, the researchers don’t know what changes natural selection promotes in these genes to strengthen defenses against the cholera bacterium.

Researchers have identified other examples of infectious diseases driving human evolution, such as malaria in Africa favoring the sickle cell allele, a gene variant that provides resistance to the illness. But they are just starting to search the entire genome for signs of disease effects, and this study is the first to use such methods for cholera.

“I think it’s a great example of the impact infectious diseases have had on human evolution,” says infectious disease specialist William Petri of the University of Virginia School of Medicine in Charlottesville, who wasn’t involved with the study. “It’s ambitious, fairly extensive, and very well done,” adds medical microbiologist Jan Holmgren of the University of Gothenburg in Sweden. One strength of the work is that it flags genes, such as those involved with the inflammasome, that researchers have implicated in other intestinal illnesses such as inflammatory bowel disease, says genetic epidemiologist Priya Duggal of the Johns Hopkins Center for Global Health in Baltimore, Maryland. “Overall, they make a very nice case.”

The findings probably won’t lead to new cholera treatments, says LaRocque, because current measures—which rapidly replace the water and electrolytes patients lose—work very well. “The real issue with cholera,” she says, “is how do we prevent it,” a difficult problem in areas without clean water supplies. But understanding how humans have evolved in response to cholera might help researchers devise more potent vaccines that would provide better protection against this killer, she says.

Source: sciencemag.org