The results of research by scientists at Scripps Research suggest that experimental antibody therapies for Parkinson’s disease (PD) and Alzheimer’s disease (AD) may have the unintended adverse effect of triggering brain inflammation. The team carried out a series of in vitro studies and experiments in mice carrying brain grafts of human induced pluripotent stem cell (hiPSC)-derived microglia (hiMG). The findings indicated that administration of antibodies targeting the misfolded proteins that accumulate in PD and AD diseases triggers the NLRP3 inflammasome and leads to cell death.
“Our findings provide a possible explanation for why antibody treatments have not yet succeeded against neurodegenerative diseases,” suggested study co-senior author Stuart Lipton, MD, PhD, Step Family Foundation endowed chair in the department of molecular medicine and founding co-director of the Neurodegeneration New Medicines Center at Scripps Research.
Lipton, also a clinical neurologist, noted that the study marks the first time that researchers have examined antibody-induced brain inflammation in a human context. Prior research was conducted in mouse brains, whereas the current study used human brain cells. “These findings may have important implications for antibody therapies aimed at depleting misfolded/aggregated proteins from the human brain, as they may paradoxically trigger inflammation in human microglia,” the authors noted. “… these results have important potential implications for immunotherapies aimed at neurodegenerative disorders because misfolded protein–antibody complexes may trigger a profound microglial inflammatory response in the human context but not in the mouse, where most preclinical experiments have been performed.”
Their results are reported in Proceedings of the National Academy of Sciences (PNAS), in a paper titled, “Soluble α-synuclein-antibody complexes activate the NLRP3 inflammasome in hiPSC-derived microglia.”
Neurodegenerative diseases such as PD and AD affect more than six million people in the United States. Such diseases are commonly characterized by the development of abnormal protein clusters in the brain, with different mixes of proteins predominating in different disorders. PD is characterized by accumulation of α-synuclein (αSyn) into aggregates, while abnormal clusters, or aggregates, of amyloid-β peptide (Aβ) accumulate in AD.
The aggregates have included not only the large clusters that pathologists observe in patients’ brains at autopsy, but also the much smaller and harder-to-detect clusters called oligomers that are now widely considered the most harmful to the brain.
Exactly how these protein clusters damage brain cells is an area of active investigation, but inflammation is a likely contributing factor. In Alzheimer’s, for example, amyloid-β (Aβ) oligomers are known to shift brain immune cells called microglia to an inflammatory state in which they can damage or kill healthy neurons nearby. “Microglial cells contribute to neuroinflammation, specifically that mediated by the inflammasome,” the authors explained. “In particular, the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family pyrin domain-containing 3 (NLRP3) inflammasome has been associated with several neurodegenerative disorders …” although other types of inflammation may also be involved, they pointed out.
One potential treatment strategy that has been in development since the 1990s for neurodegenerative disorders such as PD and AD, is the use of antibodies that specifically target and clear abnormal protein aggregates. Experimental antibodies are in development as potential treatments for both PD and for AD the scientists pointed out that “… antibodies targeting misfolded proteins are being tested in human clinical trials for several neurodegenerative diseases, including AD and PD.” However, despite promising results in mice, such potential treatments so far have not seen much success in clinical trials. And as the authors further pointed out, in the case of PD, for example, it’s not clear whether antibodies to αSyn might affect the inflammatory response.
Lipton and colleagues were studying the ability of α-synuclein oligomers to trigger an inflammatory state in microglial cells when they surprisingly found that, while the oligomers on their own triggered inflammation in microglia derived from human stem cells, adding therapeutic antibodies made this inflammation worse, not better. The team traced this effect not to the antibodies per se but to the complexes formed with antibodies and their α-synuclein targets.
Virtually all prior studies of the effects of experimental antibody treatments have been carried out using mouse microglia, whereas the key experiments in the team’s newly reported study were carried out using the human-derived microglia—either in cell cultures or transplanted into the brains of mice whose immune system had been engineered to accommodate the human microglia. And interestingly, as Lipton noted, “We see this inflammation in human microglia, but not in mouse microglia, and thus this massive inflammatory effect may have been overlooked in the past.”
Amyloid-β aggregates often co-exist with the α-synuclein aggregates seen in Parkinson’s brains, just as α-synuclein often co-exists with amyloid-β in Alzheimer’s brains. In their reported study, the researchers added amyloid-β oligomers to their mix, mimicking what would happen in a clinical case, and they found that it worsened inflammation.
“Oligomerized amyloid-β peptide, as found in Alzheimer’s disease brains, exacerbates this neuroinflammation,” they wrote.
Adding anti-amyloid-β antibodies then worsened it even further. The results indicated that both α-synuclein antibodies and amyloid-β antibodies made inflammation worse when they successfully hit their oligomer targets. “Importantly, we found that misfolded proteins such as αSyn and Aβ, when bound to antibody and then presented to hiMG, result in enhancement of the proinflammatory response of the NLRP3 inflammasome,” the investigators wrote.
“Collectively, these results provide evidence that αSyn-stimulated hiMG can induce NLRP3 inflammasome activation and neuronal cell death in vivo, and these adverse effects are markedly increased by the presence of Aβ–αSyn complexes … the present study shows that the αSyn- and particularly αSyn/Aβ driven pathogenesis may manifest a component of neuroinflammation mediated by human microglia that, in contrast to mouse, is enhanced by anti-αSyn or anti-Aβ antibodies.”
Microglial inflammation of the kind observed in the study, Lipton suggested, could conceivably reverse any benefit of antibody treatment in a patient without being clinically obvious. Lipton and colleagues are developing an experimental drug that may be able to counter this inflammation and thereby restore any benefit of antibody treatment in the human brain.