Using CRISPR to develop a cutaneous leishmaniasis vaccine

Scientists are planning a Phase I trial of a leishmaniasis vaccine developed using CRISPR gene-editing technology.

Leishmaniasis is a neglected tropical disease caused by the Leishmania parasite. The parasite infects humans through the bite of phlebotomine sand flies and can cause either cutaneous or visceral disease (skin sores or internal organ damage, respectively). There are an estimated 1.5 million new cases of cutaneous leishmaniasis, which this vaccine aims to protect against, worldwide each year. While the disease primarily infects those in tropical and sub-tropical regions, climate change means that the disease is now spreading further into developed nations such as the US.

To develop their vaccine the team applied modern CRISPR technology to the century-old Middle Eastern practice of leishmanization – deliberately introducing the live Leishmania parasite to the skin to create a small infection that, once healed, leads to life-long immunity against further disease. Abhay Satoskar, a co-lead investigator of the work and professor of pathology and microbiology at The Ohio State University, US, explained: “Live vaccines like that are the best vaccines, but there’s a potential risk of causing serious disease in some people. We refined the concept using modern technology, making a parasite that does not cause clinical disease but allows for induction of immunity.”

Using CRISPR the researchers edited the genome of the Leishmania major parasite, deleting cetrin – the gene for a protein which supports the parasite’s physical structure – and an antibiotic resistance marker gene that is needed to be introduced into the parasite for removal of the centrin gene.

To cause infection Leishmania parasites hijack immune cells and use those host cells to replicate indefinitely. In their study, the mutant parasite lacking centrin was able to enter host cells and replicate but only for a short period. Satoskar explained: The parasites are unable to proliferate, so they die. But they persist in the body for eight or nine months, which is long enough to generate acquired immunity.”

In murine models, including immune-deficient animals, the mutant parasites did not cause skin lesions, but natural parasites did. In additional experiments, vaccinated and unvaccinated mice were subjected to injections of live parasites and bites from infected sand flies seven weeks after their shots. Ten weeks later, most non-immunised mice developed large skin lesions, but only one vaccinated mouse developed a visible lesion.

“The multiple animal tests also made sure the genome didn’t revert back to normal,” Satoskar said. “And we found that if a sand fly were to bite at the site of the vaccine and take mutated parasites into the wild, the parasites cannot survive. So, it is environmentally safe.”

The standard treatment of severe cases of leishmaniasis can be expensive, costing up to $200 a dose. It also requires multiple daily injections of drugs that can cause unpleasant side effects. This results in poor patient compliance, enabling the parasites can develop drug resistance. Satoskar said their new vaccine is likely to cost less than $5 a dose.

According to the team, they are also using CRISPR to mutate the genome of the Leishmania donovani strain, which causes potentially fatal visceral leishmaniasis. They concluded that preliminary data suggests a safe vaccine to prevent visceral leishmaniasis could be on the horizon.

The research was published in Nature Communications.