Finding markers for inflammation and auto-immunity

Professor Mark Hogarth has spent most of his career studying the role of Fc receptors in inflammatory and auto-immune disorders. Found on the surface of a variety of immune cells, these antibody-binding receptors are now prospective targets for new treatments of inflammation, therapeutic monoclonal antibodies in cancer and manipulation in vaccines for HIV-AIDS.

Professor Mark Hogarth was a postdoctoral fellow at the University of Melbourne when his team cloned their first Fc receptor three decades ago. He says that, at the time, his chosen research field was something of an obscure curiosity.

Today, a substantial part of a $50 billion industry revolves around the activity of Fc receptors. As of May this year, 352 monoclonal antibodies (mAbs) were either in clinical use or in clinical testing as new therapeutics.

The life-saving or life-changing activity of many of the effective mAbs for cancer therapy, along with several anti-inflammatory and anti-infectives, ultimately depends on them binding a complementary Fc receptor on the surface of an immune-system cell: a macrophage, neutrophil, monocyte, dendritic cell, B cell or mast cell. Most leucocytes have multiple types of Fc receptor.

“Therapeutically, Fc receptors are one of the most valuable classes of receptors,” Professor Hogarth said. “Monoclonal antibodies bind to their target in vivo and often their therapeutic benefit almost certainly involves harnessing or controlling powerful inflammatory processes induced by these Fc receptors - phagocytosis by macrophages, ADCC [antibody-dependent cellular cytotoxicity] by natural killer (NK) cells and inflammatory functions of mast cells.”

The rapidly expanding arsenal of precision-targeted mAb therapeutics includes those engineered to shrink or destroy solid tumours, leukaemias and lymphomas, as well as anti-inflammatory mAbs to quench the unfriendly fire of debilitating auto-immune disorders like rheumatoid arthritis and lupus. Other mAbs have been manufactured to quell severe allergic disorders like hay fever, allergic asthma and eczema, and yet more to eliminate chronic viral infections like HIV-AIDS and hepatitis.

From obscurity to central player

Professor Hogarth heads the Inflammation, Cancer and Infection Laboratory at Melbourne’s Burnet Institute. He has devoted most of his career to exploring how the Fc domains of antibody molecules - the flexible ‘stem’ of the antibody’s Y-shape - interact with complementary Fc receptors on the surface of leukocytes to induce particular responses from the immune system.

Over the years, Professor Hogarth’s teams have been pioneers in many areas of Fc receptor research.

“We were lucky initially to have a field that just about everyone else was totally uninterested in,” he said. “It’s been quite good to us - I guess it’s why we got the Gottschalk Medal in 1992.” The Australian Academy of Science’s prestigious Gottschalk Medal recognises outstanding medical research by young scientists within the first 15 years of completing their PhD.

Over the 30 years since, the Fc receptors have gone from obscurity to being central players in immunity and help underpin the success of a valuable industry.

Receptor-targeted therapies

Scientists and pharmaceutical companies searching to develop new therapies for cancer, chronic infections, and auto-immune and allergic disorders are exploring novel ways to manipulate Fc receptor function.

In April 2012, Professor Hogarth and long-time collaborator Dr Geoffrey Pietersz, of the Burnet Institute and Monash University’s Department of Immunology, reviewed the state of Fc receptor research and discussed opportunities for new therapies in a paper published in Nature Reviews Drug Discovery.

“There’s a big move today to exploit Fc receptors for therapeutic purposes,” Professor Hogarth said. “In the broadest context, Fc receptors are cell-surface receptors for antibodies, and they drive a very wide range of activities in human immunity.”

In their Nature Reviews paper, Hogarth and Pietersz describe how antibodies evolved as soluble mediators of immunological resistance to invading pathogens.

Over hundreds of millions of years, evolution has fine-tuned antibody activity, making them specific and efficient mediators of host protection. Sophisticated effector systems have co-evolved with antibodies and underpin their normally protective biological effects - Fc receptors present on immune cells activate these effector systems.

Fc receptors are known for each of the major antibody classes, IgG, IgE and IgA, which bind to receptors FcγR, FcεR and FcαR, respectively. Professor Hogarth says while these Fc receptors perform distinct biological functions, they are all fundamentally ‘sensors’ of immune complexes: complexes of antibody and antigen such as an antibody coated auto-antigen or allergen or cancer cell.

As docking sites for antibody-antigen complexes, Fc receptors provide the humoral immune system with a cellular effector arm that links the adaptive and innate immune systems. By acting as receptors for antigen-antibody immune complexes, Fc receptors induce powerful responses that activate, regulate and modulate immunity. Fc receptors fall into two basic categories: activating and inhibitory, with the latter working by repressing the function of activating receptors.

“There is enormous scope for manipulating Fc receptors with monoclonal antibodies or custom-designed synthetic molecules, to prime or direct the immune system assassins - like NK cells - to destroy established blood cancers and solid tumours or alternatively to block Fc receptor function in autoimmunity and allergy,” said Professor Hogarth.

“There are two strategies to indirectly manipulate receptors by manipulating their ligands. These involve modifying the Fc domain of the antibody molecule to cause preferential binding to an activating receptor, or do the reverse and selectively bind to the inhibitory receptor.

“In a global context, by manipulating antibodies in this way, you can manipulate the effector responses involved in pro- or anti-inflammatory activity,” he explained. “Depending on the context, inflammation can be ‘good’ or ‘bad’. On one hand, Fc receptors can induce ‘good’, well-regulated inflammation - that’s how the immune system resolves infections and how vaccines work to induce immunity to infection or parasites.

“On the other hand, ‘bad’ or destructive inflammation drives allergic reactions and auto-immune disorders like arthritis and lupus (systemic lupus erythematosus).”

In autoimmune diseases, antibodies complexed with auto-antigens act as powerful inducers of inflammation, activating cells of the innate immune system such as macrophages, mast cells and NK cells that attack tissues.

Failure of Fc receptor control also distorts the immune response of the adaptive immune system. In hay fever, antibodies complexed with normally innocuous antigens cause mast cells to release histamine.

Professor Hogarth says blockading activating receptors can prevent immune complex inflammation in autoimmunity, and induce profound inhibition of tissue destruction.

“One approach is to directly block receptors with anti-receptor antibodies, which induces them to turnover,” he explained. “Another is to use classical small drug molecules with molecular weights around 250 daltons.”

Preventing tissue destruction

After discovering and cloning several Fc receptors at Melbourne University with his student Margaret Hibbs in the 1980s, Hogarth worked with Tom Garrett and Peter Colman at the former Biomolecular Research Institute to solve the receptors’ structure in the 1990s.

“Geoff Pietersz was able to use the structures, and some intuition, to design drug molecules to bind to the receptors. By this stage we had realised that Fc receptors are very important in the induction of destructive inflammation caused by IgG immune complexes - Dr Maree Powell, who worked with me for 22 years, until last year, was very influential in this work.

“She showed that if you can design molecules to interfere with Fc receptor function, you can actually prevent immune complexes from triggering tissue destruction. That was a paradigm shift, because the prevailing wisdom was that tissue destruction was pretty much due to the action of complement.”

Complement proteins circulate in the blood and are usually activated by an antibody that has locked onto an antigen, after which they ‘complement’ the action of the antibody.

“We tested our drug molecules in transgenic mice expressing a human Fc receptor - the FcγIIa receptor - and found they were very specific, and quite potent in the mouse, but not yet potent enough to take into human trials.”

Hogarth says one particular receptor, the FcγIIb receptor, is primarily responsible for the immunomodulatory effects of Fc receptors. But recently, several overseas research groups have shown that in certain circumstances, some activating Fc receptors have a paradoxical ability to deliver inhibitory signals that inhibit immune responses.

“That’s why we’re also interested in how antibodies work. By exploiting the dualism of Fc receptor function, we should be able to design small molecules or other antagonists that inhibit Fc receptor signals. With our increasing understanding of how these receptors function, it may also be possible under different circumstances to produce anti- or pro-inflammatory responses.”

Hogarth says there is enormous scope for manipulating Fc receptors with monoclonal antibodies or custom-designed synthetic molecules. In this way, NK cells, for example, could be directed to destroy established blood cancers and solid tumours; or Fc activation could be inhibited to overcome pathological inflammation.

Therapeutic monoclonal antibodies are a spectacular example of this potential.

“Several anticancer antibodies - for example, rituximab - harness many of the Fc receptor-dependent destructive inflammatory processes that are seen in the ‘bad’ inflammation in autoimmune destruction,” said Hogarth.

Manipulating activating FcgR with engineered mAbs that will selectively increase binding to the activating receptor, or conversely, avoid inhibitory Fc receptor binding, could yield an even more destructive form of antibody therapy.

Professor Hogarth says the flip side of antibody-FcγR immunotherapy is immunomodulation to repress ‘bad’ inflammatory processes. Since the inhibitory FcγR inhibits all activating receptors, antibodies engineered to selectively engage FcγR offer a new avenue for treating inflammatory diseases as diverse as IgE-dependent allergy and lupus.

“Indeed we are very hopeful that we will soon have strategies to manipulate Fc receptors to quell the activity of the IgE receptor and fast-track allergic desensitisation therapy in life-threatening allergies. In lupus we could overcome autoimmune complex-driven activating IgG receptors.”

Vaccine-induced antibodies

Another area of promise for antibody therapies that harness Fc receptor functions is the treatment of persistent viral infections.

“One of the problems with HIV infections is that the virus is outstandingly successful in evading the immune system,” Professor Hogarth said.

“Much research effort has gone into developing vaccines against HIV, and recent trials point to a pivotal role for the Fc receptor function of antibodies in HIV resistance. Even neutralising anti-HIV antibodies depend on Fc receptor function for optimal effect”, which encourages the HIV research community to believe vaccine-induced antibodies could be the way to success.

“We’ve been working in this area ourselves for a while, and we’re getting some interesting data around anti-HIV antibodies and Fc receptors,” Professor Hogarth said, adding that the National Institutes of Health (NIH) in the US had just announced it is seeking applications for grants to investigate the role of Fc receptors in HIV vaccine responses.

Hogarth’s team has been working with Professor Stephen Kent, Dr Rob Centre and Dr Damien Purcell of the Peter Doherty Institute at the University of Melbourne and US collaborators on a project investigating how HIV evades vaccine-induced immunity and how the Fc receptor-dependent function of anti-HIV antibodies might be optimised in vaccines. They plan to apply for one of the NIH grants.

“So, once again we come back to the manipulation of Fc receptors as potential therapeutic strategies.”

Image credit: The structure of human Fc receptors. The alpha carbon (ribbon) skeleton of the receptor is superimposed on a droplet of FcγRIIa protein crystals that were used to visualise the first Fc receptor structure (Maxwell K, et al Nature Struct. Biol, 1999). The solid surface on the skeleton shows the area of the receptor that interacts with immune complexes (Ramsland et al J Immunology, 2011). This area is conserved throughout the Fc receptor family including the IgE receptor associated with allergy. Photo credit: Crystal image from Maree Powell.