Organs facing chronic injury or inflammation from a variety of causes may be susceptible to a prolonged repair process that ultimately creates a permanent scarring of the tissue, known as fibrosis, which can lead to organ dysfunction and even death. One estimate suggests that organ fibrosis may contribute to 45 percent of deaths in the US. Treatment options for fibrosis are scarce and have many limitations, and fewer still target fibrotic cells without affecting healthy ones.
A study published today (September 15) in Cell Stem Cell provides proof of principle of a potential new treatment based on vaccination against peptides that are only present in the cells that give rise to myofibroblasts—those responsible for the permanent scar tissue. Mice that received these vaccines showed reduced levels of fibrosis in their livers and lungs compared to those that received a control injection, the authors report.
In the past decade, various studies have found transient expression of certain genes—previously thought to be active only during embryogenesis—in cells associated with organ fibrosis. Using tracing tools, these “elegant papers” have shown that cells expressing these genes “basically give rise to all the fibroblasts that cause problems in fibrosis,” says Christian Stockmann, an immunologist at the University of Zurich’s Institute of Anatomy and coauthor of the new paper. Stockmann and his colleagues wondered whether these proteins could serve as tags for immune cells such as T cells to recognize profibrotic cells.
The researchers reasoned that fragments of these distinctive proteins would be degraded by major histocompatibility complex (MHC) class I molecules, which are located on the surfaces of most cells and display fragments of intracellular proteins to T cells. When a foreign peptide is among the presented fragments, an immune response is triggered and T cells kill the infected cells. So the scientists thought there might be a way to get the immune system to respond in a similar way to cells with the profibrotic proteins.
One of the identifying proteins the team focused on was ADAM12, which belongs to a family of proteins that counteract steps in blood clotting (although its function matters little for the intended approach). Stockmann and his colleagues conducted a series of tests in silico, in vitro, and in vivo to assess whether ADAM12 could be a potential candidate to activate T cells. When the analyses and experiments suggested it was, the team constructed a vaccine against it that contained a lentivirus shell housing a DNA sequence encoding Adam12 together with adjuvants to stimulate the immune system, which mice would receive as a prime immunization and, seven days later, as a booster.
The team tested the effects of the vaccine administered either before or after they induced liver fibrosis in mice. In both scenarios, mice showed reduced levels of fibrosis—based on molecular and cellular markers—after 6 weeks of treatment, compared with mice that received a control vaccine. Stockmann and his colleagues repeated the same process in mice with lung fibrosis using either the ADAM12 vaccine or a second vaccine targeting the transcription factor GLI1, which is also specific to profibrotic cells. Both injections lessened lung fibrosis in the treated mice at levels similar to those recorded in the vaccinated mice with liver fibrosis. The researchers also demonstrated that those antifibrotic effects were mediated by CD8+ T cells: When these cell populations were purposely depleted in the mice, the effect of the vaccines was completely abolished.
“I think they did a good job of using multiple different models of fibrosis—in the liver and in the lung—and two different targets,” says Benjamin Humphreys, a nephrologist at the Washington University School of Medicine in St. Louis who was not involved in the study and who has characterized the expression of Gli1 in fibrosis. Moreover, the treatment worked both before and after fibrosis began, he adds, and “that’s really critical because if you were to envision treating humans with this . . . vaccination strategy, it would always be after the disease has been initiated.”
Finally, Stockmann and his colleagues conducted a series of safety tests in mice to assess the potential risks of such a strategy, since it targets genes that are expressed at low levels in other parts of the body. Based on histological changes and blood markers, they found no evident damage to major organs in vaccinated mice four weeks after immunization. The animals showed no signs of problems in healing responses either.
These tests are “really important” since the vaccine targets the individual’s own proteins and could potentially affect other, normal processes in the body, says University of Notre Dame Australia immunologist Gerard Hoyne, who did not participate in the study. That “they didn’t see any evidence for that [is] a positive outcome,” he says. Nonetheless, Hoyne points out that if the immune system were to mount a lingering reaction to the vaccine that results in attacking healthy cells, that would not become evident for a long time, and the researchers looked only for short-term effects. “At least in the early stage, in the acute response to that vaccine, it doesn’t seem to be driving a general autoimmune response, which was good,” he says, but adds, “it doesn’t preclude that there maybe could be an effect later in the animal’s life which we don’t yet understand.”
Humphreys, who is a consultant for and has held research grants from Pfizer, Janssen, and Chinook Therapeutics, and owns equity in the latter, agrees that further preclinical investigation is needed. Researchers need “to better understand how does this therapeutic approach work across a diversity of fibrosis models, diversity of organs, and . . . make sure that we really understand effects in homeostasis, in healthy organs,” he says, concluding that it’s important to wait and learn more about the consequences of this approach before trying to translate it to humans.