Vaccines prevent millions of deaths a year and have pushed some diseases to the edge of existence. Yet it’s proven highly challenging, if not impossible, to develop effective vaccines against a number of viruses.
Despite decades of research, there’s no approved vaccine that offers long-term protection against influenza, Epstein-Barr virus (EBV), or respiratory syncytial virus (RSV), a widespread pathogen that causes respiratory tract infections. For RSV, for instance, high-risk infants receive monthly injections of synthetically produced antibodies to protect them in the short term.
In recent years, several groups of researchers have explored a different approach: Taking the immune system’s own antibody-generating B cells and using CRISPR to engineer them to express antibodies against these hard-to-treat viruses—in bulk and on demand in the event of an infection.
The latest group to do so has successfully modified human and mouse B cells to express antibodies targeting HIV, EBV, influenza, and RSV. The researchers also demonstrate that the engineered B cells protect against RSV in a mouse model of infection. They report their findings today (May 17) in Science Immunology.
“It’s a really well-done study,” says Branden Moriarity, a genome engineer at the University of Minnesota, who wasn’t involved in the new research. “It’s a concept people have thought about for a long time.”
To modify the cells, Justin Taylor, an immunologist at the Fred Hutchinson Cancer Research Center, and his team turned to antibodies that had been proven effective against each of the four viruses in previous research. They used a routine CRISPR strategy to insert the antibody-coding DNA into a small cut made in the antibody genes of primary human B cells.
The researchers designed the insertion such that the expression of the new antibody protein would be regulated by the cells’ own promoter, allowing the cells to produce it as they normally would—when triggered by viral antigens, for instance. They then differentiated as many of the cells as possible into antibody-secreting plasma B cells, which express the antibodies on their cell surfaces as well as secrete them.
Once the researchers confirmed that a reasonable portion of the engineered B cells actually expressed the new antibodies, they investigated whether the cells could protect against viral infection in mice. They repeated the process for murine B cells, coaxing them into expressing RSV-targeting antibodies, infused a population of successfully modified cells into normal rodents, and waited.
Immune-deficient mice were capable of preventing RSV infection as late as 82 days after receiving the engineered B cells.
Six days later, RSV-specific antibodies appeared in their serum, while none appeared in control mice. Some of the animals then received an intranasal dose of RSV, which the modified B cells managed to ward off. After five days, the scientists could barely detect any concentration of RSV in the lungs of mice that had the engineered B cells in their blood, while they discovered high concentrations of the virus in control rodents that hadn’t received the cells.
Taylor says he thinks the modified B cells would be useful for patients who have undergone hematopoietic stem cell transplants. They’re typically on immunosuppressants and are very vulnerable to viral infection. RSV infection in these patients can be prevented with synthetically produced antibodies such as palivizumab, but these have to be injected on a regular basis. Engineered B cells, on the other hand, would theoretically only be infused once.
To investigate whether engineered B cells might offer protection in such a situation, the team turned to mutant mice that lack T and B cells. After they infused the cells into the rodents, they found that, surprisingly, the RSV-specific antibodies appeared to linger in the blood much longer in these mice compared to in wildtype animals. In fact, immune-deficient mice were capable of preventing RSV infection as late as 82 days after receiving the engineered B cells. Why that is, is “something we need to explore a little bit more,” explains Taylor, who received funding through a sponsored research agreement from California-based Vir Biotechnology for the research as well as from the Hartwell Foundation, a nonprofit funded by an anonymous donor.
On the whole, Taylor’s team’s approach is similar to that taken by other recent studies, such as one in eLife and another in the Journal of Experimental Medicine, both of which employed CRISPR to create B cells that express HIV-neutralizing antibodies. But it differs in the way the researchers have addressed some of the key difficulties in engineering antibody genes.
It’s tricky thing to do: for one, antibodies are synthesized from two genes—one for the heavy chain and one for the light chain. Once translated separately into proteins, they’re fused together to form the final antibody. A challenge with editing these genes is that sometimes, mispairing can occur, resulting in hybrid antibodies consisting of engineered heavy chains paired with the B cells’ own light chains. In some cases, these could cause autoimmune reactions. “The risk of this is low . . . but with a few million unique light chains from a few million B cells, this isn’t a risk worth taking,” Taylor explains.
Taylor’s team got around this by physically linking the DNA encoding the heavy chains and light chains together, and inserting this into a single CRISPR cut. (Other groups have devised different strategies to get around the issue, for instance, by knocking out the B cells’ own light chain genes.)
“I think that the engineering strategy they’re using to fuse the light chain to the heavy chain is a neat idea, that’s pretty cool,” remarks immunologist Richard James of the Seattle Children’s Research Institute who wasn’t involved in the study.
However, “in practice, this is going to be an expensive therapy,” he adds, noting that personalized CAR T-cell therapies are already thought to cost close to $500,000. And for now, B-cell therapies would have to be personalized: Because the cells express individual-specific cell-surface antigens, cells from one donor would be rejected by a different recipient. “Unless we can find a way to make an allogeneic B cell, and then it would become much more cost effective,” he adds.
Moriarity agrees. Taking engineered B cells prophylactically may only be a viable therapeutic option “when you have a disease that you’re going to die from and there’s really no other treatment,” he says. Nevertheless, he says the study offers other possibilities for future research, being one of the first to engineer antibodies in murine B cells. “I think that’ll be a big contribution to the basic biology field” for scientists studying B-cell differentiation and cellular and immune-related processes.
Michael Goldberg, an immune engineer formerly at Harvard Medical School who is now the CEO of the immunotherapy startup STIMIT, finds the study “a nice extension of previous work,” he writes to The Scientist in an email. For him, B-cell antibody engineering offers advantages beyond preventing viral infection: For instance, having antibody levels that are responsive to antigen concentrations in the blood could be useful in treating diseases such as rheumatoid arthritis, which is often treated with antibodies that neutralize the cytokine TNFalpha.
However, “the duration of antibody expression needs to be increased. Enhancing our understanding of engraftment processes and B cell culturing conditions will be important in this regard. Still, given that these were among the first experiments involving adoptive transfer of engineered B cells, the initial results are encouraging.”
Taylor agrees there’s still work to be done. “One thing we completely ignored in this study is really assessing the safety of what we’re doing, making sure that we don’t have any off-target effects, I think that’s always a concern for CRISPR-Cas9,” he says. “Before this goes into people, we have to be absolutely certain that it’s safe.”
H.F. Moffett et al., “B cells engineered to express pathogen-specific antibodies protect against infection,” Science Immunology, eaax0644, 2019.