Marko A. Radic, PhD
University of Tennessee, Memphis, TN
2009 B Cells
2005 Environmental Triggers
Defects in the body’s removal of dead cells stimulate the production of damaging autoantibodies in lupus
Dr. Marko Radic and his team at University of Tennessee Health Science Center (UTHSC) in Memphis published findings in the September 15, 2011 issue of Journal of Immunology showing that despite a harmless appearance, certain B cells can in fact carry a potentially dangerous chemical cargo of immune complexes — large chemical networks connecting antibodies with antigens — which can get stuck in tissues, causing inflammation and organ damage. In lupus, immune complexes of autoantibodies and antigens are known to particularly damage the kidneys.
Current understanding of immune complexes suggests that they form after the antibodies have been released from B cells, the cells involved in making antibodies. However, Dr. Marko Radic and his colleagues have revealed an unexpected exception to this accepted pathway.
The researchers had previously observed that while some potentially dangerous, self-reactive B cells go through a process that should render them harmless, they can continue to produce antibodies that react against the body’s own cells. It was further determined that the antibody produced by these B cells accumulates inside the cells in the Golgi apparatus, a part of the cell considered the distribution and shipping center for its secreted products. Not only did the antibodies accumulate in the Golgi, but they also formed large, round immune complexes that were eventually ejected from the cells. Because of their shape, the team named these complexes “spherons.”
“We consider B cells that produce spherons as “Trojan Horses” because they may arise in a normally functioning immune system and appear as innocuous cells,” explained Dr. Radic. “However, once activated, they may release their pre-formed immune complexes that could go on and cause tissue damage.”
The researchers’ next steps will be to find out whether spherons are produced in mice with lupus and if they cause kidney damage. These discoveries are the outgrowth of investments LRI made in the work of Dr. Radic and his colleague Dr. Salar Khan at UTHSC as well as a research consortium with Dr. Martin Weigert at University of Chicago and Dr. Jason Zhang at the University of Nebraska.
“LRI support was critical for this study, even from the inception of the idea. They encourage you to explore novel ideas, which normally you shrink back from if applying to the NIH.” – Dr. Radic
The human immune system is programmed to ignore the proteins that make up our own cells. But this state of tolerance can break down, causing the immune system to launch a devastating attack on the body itself.
What causes tolerance to break down? One theory is that proteins are chemically modified in such a way that the immune system no longer recognizes them as ‘self’ and so attacks them as if they were invading viruses or bacteria.
Researchers are especially interested in how tolerance to DNA packaging proteins, known as histones, is broken as these proteins are a focus of immune attack in lupus.
Dr. Radic and his team at the University of Tennessee Health Sciences Center in Memphis suspected that a particular modification of proteins known as citrullination could break tolerance to histones as patients with the related autoimmune disease rheumatoid arthritis have antibodies directed at citrullinated proteins. They wanted to know what causes histones to become citrullinated.
Dr. Radic knew that white blood cells known as neutrophils were the most adept at citrullination. And he expected that citrullination would happen in dying cells. But he quickly discovered that this was not the case; neutrophils only citrullinated histones when they were activated to respond to infection (and were very much alive).
Moreover, Dr. Radic found that activated neutrophils expel citrullinated histones as part of a sticky rope-like structure, which they throw out in order to trap and kill bacteria. This explains how a modified protein normally found inside cells might be ‘seen’ by the immune system—and trigger an autoimmune attack.
This new theory to explain the triggering of autoimmunity is supported by observations in people. For example, patients with autoimmune diseases often have an infection (and therefore lots of neutrophil activity) just before their symptoms flare up.
Histone deimination as a response to inflammatory stimuli in neutrophils. Neeli I, Khan SN, Radic M. J Immunol. 2008 Feb 1;180(3):1895-902.
Regulation of Extracellular Chromatin Release from Neutrophils. Journal of Innate Immunity. I Neeli, N Dwivedi, S Khan, M Radic. J Innate Immunol. 2009 1(3):194-201.
Intra-Golgi Formation of IgM-Glycosaminoglycan Complexes Promotes Ig Deposition. Khan SN, Cox JV, Nishimoto SK, Chen C, Fritzler MJ, Hendershot LM, Weigert M, Radic M. J Immunol. 2011 Sep 15;187(6):3198-207. Epub 2011 Aug 12.
Dr. Radic won a $600,000 grant from the Dana Foundation, along with LRI researchers Dr. Zhang and Dr. Weigert, to continue working on an aspect of his lupus research.
Rev. March 2012
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