David Beers thought he and his colleagues in Stanley Appel’s lab had a great idea to stop ALS in its tracks: Take out the T cells inflaming motor neurons. So they crossed ALS model mice with mice lacking an immune system (Sep 2008 news; Beers et al., 2008).
“We were totally wrong,” recalled Beers, of the Houston Methodist Neurologic Institute in Texas. “It made the animals worse.”
In more recent years, Appel’s team discovered that a certain subset of T cells, known as regulatory T cells (Tregs), protect motor neurons in ALS. And, the greater the number of these cells, the slower the progression of the disease (Henkel et al., 2013).
“Tregs are the master suppressors of inflammation,” explained Appel. But in ALS, over time, these cells tend to become few and far between, potentially fueling progression.
Boosting Treg levels may therefore help fight ALS, by reducing neuroinflammation in the disease. That’s what Brad Turner, Steve Vucic and colleagues at the Universities of Melbourne and Sydney in Australia recently confirmed – at least in ALS model mice (Sheean et al., 2018).
“I was thrilled,” said Appel; he, Beers, and another team member, Weihua Zhao, penned a commentary about the results for the journal JAMA Neurology (Beers et al., 2018).
Clinical researchers around the globe are now testing several potential ALS treatments they hope will increase the number of Tregs and their ability to reduce inflammation, and thereby slow down the disease. “You can sort of fine-tune the peripheral immune system to target that population of T cells, they’ll start to infiltrate to where the motor neurons are in crisis, and assist them,” explained Turner.
In the early 1990s, Appel’s team discovered that T cells infiltrate the spinal cord of people with ALS (Engelhardt et al., 1993). “That led us to the thought that maybe these T cells are toxic, [and] are being called in to kill motor neurons,” said Beers.
That’s why he and the late Jenny Henkel sought to eliminate them. But upon removal of T cells from ALS model mice, their disease progressed more rapidly. And, the mice died about two weeks earlier. When they added back the T cells, the animals’ survival improved (Beers et al., 2008).
The T cells, deployed into the CNS, appeared to be protecting the motor neurons against the disease, Beers explained.
Until that 2008 report, scientists had not paid much attention to the role of the peripheral immune system in ALS, said Turner. “That’s where all the excitement started.” (see Sep 2009 news)
Meanwhile, at Harvard Medical School in Boston, scientists also wondered whether T cells could be of benefit in ALS. They bred ALS model mice missing a gene for T cell receptors, thus blocking development of T cells known to infiltrate the brain and spinal cord. Average lifespan dropped by about 10 days, suggesting that these cells may play a protective role in the disease (Chiu et al., 2008).
But T cells come in many flavors. Which ones were helping? Around the same time, scientists at the University of Nebraska Medical Center in Omaha were also investigating the role of T cells in ALS. They noted that in ALS model mice, the spleens—where T cells can be produced—were shrunken. When the researchers infused these mice with wild-type Tregs, disease onset was delayed by a few weeks, and their lifespans rose by up to two months, suggesting that Tregs helped protect motor neurons in the disease (Banerjee et al., 2008).
Back in Texas, Beers and colleagues were also zeroing in on Tregs. They observed that Tregs were initially present at high levels in ALS model mice, but their numbers plummeted over time. And, when they transplanted Tregs into ALS model mice that lacked them, survival went up by about a month (Beers et al., 2011). “It was miraculous to see that,” said Appel.
While immune activity early in the disease seems to be a good thing, over time, neuroinflammation damages the spinal cord, driving progression of the disease.
“Decreasing inflammation is definitely something that’s interesting in ALS,” said Steven Perrin, chief executive officer of the ALS Therapy Development Institute (ALS-TDI) in Cambridge, Massachusetts. “The challenge I think we have had, historically, is that immunosuppression does not work.”
For example, in a recent study led by Emory University’s Jonathan Glass in Atlanta, researchers tested a four-drug immunosuppressive regime in 31 people with ALS, but found no change in their motor abilities (Fournier et al., 2018).
Appel thinks he knows why. There are “good guys” and “bad guys” in the immune system, he explained. Tregs are the good guys, but they’re counterbalanced by other cell types such as effector T cells (Teffs) that incite inflammation. Shutting down the immune response doesn’t change anything. Rather, he suggested, treatments must finely tune the immune system to restore balance: “The key thing that we hope to be doing is, in fact, to be increasing the good guys so that the ratio of good to bad is changing.”
Turner and Vucic, meanwhile, had been following Appel’s work, and were brainstorming treatment strategies to help the immune system fight ALS.
He came across an approach already being explored as a therapy for multiple sclerosis and other autoimmune diseases: IL-2 (Webster et al., 2009). It’s a cytokine that stimulates T cell proliferation.
Of course, activating Teffs could lead to increased inflammation. That’s why the dosing is crucial. Resting Teffs have low affinity IL-2 receptors that only bind IL-2 and become activated at high doses. Tregs, however, have higher affinity receptors.
“The idea is to give very low doses,” explained Jerome Ritz of the Dana-Farber Cancer Institute in Boston. “So only cells that are very sensitive to that low concentration can respond.”
The strategy is based on an emerging treatment for graft-versus-host-disease, a common complication that occurs after a bone marrow transplant (Matsuoka et a., 2013). “The patients actually got better,” said Ritz, who is developing this treatment with colleagues John Koreth and Rob Soiffer. Some have been on daily, low-dose IL-2 for up to three years now.
The idea of using IL-2 in ALS is similar. By reducing neuroinflammation, the strategy may help slow disease progression.
“If you have a lot of inflammation going on, then regulatory T cells can come in…and they may help quiet the inflammatory response,” said Ritz.
Turner and colleagues gave it a go. They used low-dose IL-2 bound to IL-2-antibody, plus rapamaycin. This is thought to provide a longer-lasting treatment that helps ensure that Tregs, not Teffs, multiply (see Battaglia et al., 2005; Strauss et al., 2007; Spangler et al., 2015).
The number of T cells expressing the T cell transcription factor FoxP3 quadrupled, reaching 18.9% of the CD4+ T cell population, in the treated mice. (Rapamycin, alone, had no effect.) The animals also lived an average of two weeks longer (Sheean et al., 2018). “We were pretty chuffed,” said Turner.
Turning their focus to people, Turner and colleagues found that the faster the disease was moving, the fewer inflammation-fighting Tregs, known as effector Tregs, in the bloodstream.
That lines up nicely with results from the team in Texas, who also found that people with rapidly progressing ALS had lower Treg numbers (Beers et al., 2011). They also tended to have lower levels of FoxP3, suggesting that their cells were less likely to suppress inflammation, potentially speeding up the progression of their disease (Henkel et al., 2013; Beers et al., 2017).
“You could actually use Tregs as a biomarker for the rate of someone’s progression,” suggested Turner.
More studies, however, are needed to validate this approach to prognose the disease, said Perrin. “It needs to be replicated, in multiple clinical cohorts, probably from multiple different geographical areas.”
Tregs on Trial
Nevertheless, several groups of ALS researchers are steaming ahead with early-stage trials of Treg-boosting therapeutics that they hope will delay disease progression.
One approach, being developed by Appel’s team, is to boost Treg populations ex vivo. When they isolated these cells from the blood of people with ALS, and expanded them in the lab, the Tregs regained their ability to suppress inflammation (Beers et al., 2017).
Jason Thonhoff, also in Appel’s laboratory, and colleagues have infused these cells back into three people so far (Thonhoff et al., 2018). The main goal of this open-label, phase 1 study is to assess safety and tolerability, Appel said. He’s not sure how long the new Tregs will remain in circulation, keeping inflammation levels down. So far, he said, “The results seem encouraging.” He expects to embark upon a larger, double-blind, placebo-controlled clinical trial soon.
Meanwhile, Gilbert Bensimon of the Assistance Publique – Hôpitaux de Paris (AP-HP) in France, in collaboration with Brighton and Sussex Medical School’s Nigel Leigh in England, is developing a potential low-dose IL-2 therapy for ALS, administering the drug intravenously. The approach is at the phase 2 stage. The strategy builds, in part, on previous work led by AP-HP’s David Klatzmann, who is developing the approach as a treatment for type 1 diabetes (Hartemann et al., 2013).
The phase 2 trial, known as MIROCALS, is currently ongoing in France and the UK. An estimated 216 people with ALS will receive, randomly, either placebo or low-dose IL-2 injections, monthly. No rapamycin is administered. The researchers aim to complete the 18-month study in 2020.
Ritz, who is not involved in the trial, said he’s confident IL-2 should amplify Treg populations. “Whether it’ll help the ALS, I don’t know,” he said. “I think it’s definitely worth a shot.”
Back in Australia, a phase 2 study of another MS drug, dimethyl fumarate (Tecfidera), is just taking off. The approach may help protect neurons from further damage in at least some forms of MS by increasing the percentage of Tregs and improving their ability to regulate inflammation in the immune system (Gross et al., 2016; Schloder et al., 2017).
The researchers—led by Vucic and Matthew Kiernan, also at the University of Sydney—will determine if Tecfidera is safe and tolerable, and if it shows signs of efficacy, potentially to support a larger trial in the future. 100 to 150 people with ALS are expected to participate in the randomized, placebo-controlled 42-week study.
And at ALS-TDI, scientists also have immunomodulation on their minds. One approach, Novartis’ MS medication fingolimod (Gilenya), aimed to reduce inflammation in ALS by keeping T cells inside the lymph nodes, so they couldn’t reach the CNS. In a phase 2a clinical trial, TDI found fingolimod to be safe. And it sequestered lymphocytes—including, unfortunately, Tregs (Berry et al., 2017). Lowering levels of fingolimod kept Tregs in circulation – at least in mice. However, lower doses aren’t safe for people, due to potential heart complications.
Though Perrin’s team has shelved Gilenya, they continue to develop strategies targeting inflammation in the disease. Another approach they’re pursuing is to block CD40 ligand (see Mar 2010 news; Lincecum et al., 2010). The antibodies, being developed by its subsidiary Anelixis Therapeutics, may also increase Treg levels. Perrin hopes to begin a phase 1 study this summer, with a phase 2 to start later in the year if all goes well.
None of these treatments would freeze the disease in its tracks, said Turner. “What we know about Tregs is that they affect the rate of progression of disease, but they don’t stop it from occurring.” But these therapeutics might just buy people with ALS more time.
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