The immune system has developed powerful weapons against pathogens. Now, scientists at Berlin’s Max Delbrück Center uncovered a new mechanism that acts as a counterbalance to this arsenal of weapons without losing the efficiency of the immune response. Their study demonstrates how interferon gamma uses four amino acids to bind to the extracellular matrix of connective tissue, which forms a web between individual cells and thus mediates intercellular contact.
The findings are published in Nature Immunology in a paper titled, “IFNγ binding to extracellular matrix prevents fatal systemic toxicity.”
“Interferon-γ (IFNγ) is an important mediator of cellular immune responses, but high systemic levels of this cytokine are associated with immunopathology,” wrote the researchers. “IFNγ binds to its receptor (IFNγR) and to extracellular matrix (ECM) via four positively charged C-terminal amino acids (KRKR), the ECM-binding domain (EBD). Across evolution, IFNγ is not well conserved, but the EBD is highly conserved, suggesting a critical function. Here, we show that IFNγ lacking the EBD (IFNγΔKRKR) does not bind to ECM but still binds to the IFNγR and retains bioactivity.”
The researchers started out using a mouse model developed by Thomas Kammertöns, a member of the team who also works at the Institute for Immunology at Charité, which allowed them to regulate the concentration of interferon-gamma that was produced. “We were already able to determine from this model that IFNγ becomes toxic very quickly, and that animals with high concentrations of this signaling molecule in their blood fall ill within a few days,” explained Kammertöns.
The researchers then turned to Ralf Kühn, PhD, head of the Genome Editing & Disease Models Lab at the Max Delbrück Center, to help develop a model that would produce interferon molecules without a KRKR motif. To do this, Kühn and his team removed the four amino acids from the cytokine in mice using CRISPR-Cas9. “For a long time, scientists have believed that the signaling molecule is dependent on this binding site to function at all,” Kammertöns said. “So we first had to prove that this is not the case.”
Usually, the immune system would then fight the viral infection and eventually eliminate it. However, for the mice lacking the four amino acids in their IFNγ, that was not the case. “The animals’ immune systems were still able to regulate immune responses for viruses that elicit only very brief inflammatory reactions.” Kammertöns reported saying that in these cases, the amount of IFNγ in the blood did initially increase but then fell again very quickly. “Yet when the mice were infected with LCM viruses, which cause a flu-like disease called lymphocytic choriomeningitis and keep the immune system busy for a longer period of time, the gene-edited mice quickly became ill due to the high concentrations of interferon-gamma in their blood.”
“In my view, it is clear from our research that our immune system has developed highly potent mechanisms to keep its own defenses in check,” said first author Josephine Kemna of the Max Delbrück Center. If these mechanisms fail to work properly, she said, the immune system can end up damaging its own organism due to the toxic effect of certain molecules as they continue to spread. “The mechanism we have uncovered shows that evolution has ensured toxic molecules generally act only where they are needed—that is, where the T cell recognizes a virus-infected cell.”
“This study is of fundamental importance for immunology and our understanding of many inflammatory diseases in the human body,” said Kammertöns. “We would never have made these new findings without the outstanding collaboration with our French colleague Hugues Lortat-Jacob, who has been researching extracellular matrices for more than 30 years and is one of the world’s leading experts in this field,” Kammertöns added.
Kammertöns is now planning the next phase of the study with his group leader Thomas Blankenstein, PhD, and scientists at University Medical Center Freiburg. Together, they are going to test their latest findings on a new model. “We want to work with so-called wildlings— mice that have already undergone several infections and whose immune systems, therefore, elicit a response more similar to that of a human,” Kammertöns said.