An antibody to the blood-clotting protein fibrin leaves coagulation alone but blocks the ensuing inflammatory cascade. Scientists led by Katerina Akassoglou, Gladstone Institutes, San Francisco, created the antibody to block a small domain on fibrin that normally binds a complement receptor and kicks off an inflammatory reaction. Called 5B8, the monoclonal leaves fibrin’s platelet-interacting site unperturbed. Writing in the October 15 Nature Immunology, Akassoglou reports how 5B8 blocked innate immune responses and neurodegeneration in animal models of both multiple sclerosis (MS) and Alzheimer’s disease.

  • Fibrin that leaks into the brain clots and causes inflammation.
  • An antibody that blocks the inflammation still lets fibrin clot.
  • In mouse models of neurodegeneration, the antibody rescued neurons without slowing clotting.

“This provides a way to target the inflammatory properties of fibrin without affecting its coagulation,” said Sidney Strickland, The Rockefeller University, New York, who was not involved in the work. “That’s potentially very important.”

“We knew that vascular pathology was a major contributor to disease,” Akassoglou told Alzforum. “However, until now we were unable to suppress the damaging effects of blood leaks in the brain. It wasn’t clear which protein to target in the blood, or what strategy to use.”

“It’s a very nice paper,” agreed Costantino Iadecola, Cornell University, New York. “It has significant therapeutic implications that will hopefully come to fruition one day.”

Fibrin’s γ Chain.

Inflammation involves fibrin’s CD11b-binding site (red), which lies adjacent to the region that binds platelets in blood clots (gray). [Image Courtesy of Ryu et al., 2018. Nature Immunology.]

Leaks in the blood-brain barrier (BBB) allow blood to seep into the brain, carrying a myriad of proteins, including fibrinogen. Akassoglou and colleagues previously reported that when the BBB is breached, fibrinogen is cleaved into fibrin, exposing amino acids 400 to 411, a region that binds platelets and forms a mesh-like matrix to plug the hole (for a review, see Petersen et al., 2018). At the same time, they found that amino acids 377 to 395 bind to the CD11b-CD18 integrin receptor—a.k.a. complement receptor 3—to activate microglia and macrophages (Davalos et al., 2012). This stimulates the release of free radicals that can damage axons, but it was not known how. It was also unclear if the inflammatory characteristics of fibrin could be disrupted without limiting its ability to clot.

To address the latter, first author Jae Ryu and colleagues designed 5B8 to bind amino acids 377 to 395, but not the platelet-binding region. The antibody also ignored fibrinogen, since amino acids 377-395 are tucked away in the full-length protein. Tested in bone marrow-derived macrophages, the antibody reduced the expression of genes involved in cell adhesion and suppressed inflammatory immune responses brought on by fibrin exposure. However, 5B8 allowed fibrin to polymerize normally in a test tube, and clotting time of mouse plasma remained unchanged.

How did 5B8 reduce inflammation? Examining the gene-expression patterns in treated macrophages lent a clue. The monoclonal suppressed a component of NADPH oxidase. This complex is a key producer of reactive oxygen species (ROS). In both mouse and human macrophages, 5B8 prevented NADPH activity and the release of ROS. The results suggest that fibrin spurs NADPH oxidase into action.

The antibody seemed to protect neurons. In mouse models of multiple sclerosis, 5B8 delayed disease onset, lessened the severity of symptoms and relapses, and reduced paralysis relative to untreated controls. The antibody also reduced the number of activated microglia and infiltrating monocytes in the brains of these mice, while preventing ROS, damage to axons, and demyelination.

In the 5XFAD mouse model of AD, where activation of the innate immune system is also part of the disease cascade, fibrin rose to detectable levels by three months of age and surrounded Aβ plaques and dystrophic neurites by five months. Treatment with 5B8 starting at 3.5 months lowered microglial activation around plaques and rescued cholinergic neurons. The number of plaques and macrophages encircling them remained unchanged, however. The antibody also reduced the expression of genes involved in innate immunity and oxidative stress.

“The efficacy of the antibody in two disease models of different etiologies suggests fibrin could be a shared mechanism that drives inflammatory responses in the brain towards neurotoxicity,” said Akassoglou. By cutting back the inflammation induced by fibrin, researchers may selectively reduce one source of neurotoxicity, but overall leave innate immune activity intact, she said. “It will be interesting to examine whether 5B8 can be developed further.” More tests will be needed to ensure that this antibody allows clotting to continue normally.

“I think it’s a very interesting and novel target, and it needs to be pursued,” said Greg Brewer, University of California, Irvine. He recommended lowering but not eliminating the fibrin-CD11b interaction, as partial function is likely needed for normal immune function.

Strickland said this fits with his work reporting that Aβ interacts with fibrin and makes blood clots hard to degrade (Cortes-Canteli et al., 2010). If more fibrin sticks around, it would become chronically pro-inflammatory, according to this paper, but the 5B8 antibody could attenuate that, he said.

Fibrinogen is one of many proteins that escape through damage in the blood-brain barrier, so there may be other rabble-rousing culprits. However, among them, fibrin is likely an important target, Iadecola said. He wondered whether reducing fibrin-induced inflammation improved learning and memory in the 5XFAD mice. “It would be nice to see if that’s sufficient to improve behavior in spite of all the plaques.”—Gwyneth Dickey Zakaib

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References

Research Models Citations

  1. 5xFAD

Paper Citations

  1. . Fibrinogen in neurological diseases: mechanisms, imaging and therapeutics. Nat Rev Neurosci. 2018 May;19(5):283-301. Epub 2018 Apr 5 PubMed.
  2. . Fibrinogen-induced perivascular microglial clustering is required for the development of axonal damage in neuroinflammation. Nat Commun. 2012;3:1227. PubMed.
  3. . Fibrinogen and beta-amyloid association alters thrombosis and fibrinolysis: a possible contributing factor to Alzheimer's disease. Neuron. 2010 Jun 10;66(5):695-709. PubMed.

Further Reading

Papers

  1. . Fibrin(ogen) and neurodegeneration in the progressive multiple sclerosis cortex. Ann Neurol. 2017 Aug;82(2):259-270. Epub 2017 Aug 19 PubMed.
  2. . Blood will out: vascular contributions to Alzheimer's disease. J Clin Invest. 2018 Feb 1;128(2):556-563. PubMed.

Primary Papers

  1. . Fibrin-targeting immunotherapy protects against neuroinflammation and neurodegeneration. Nat Immunol. 2018 Nov;19(11):1212-1223. Epub 2018 Oct 15 PubMed.