Alzheimer’s disease (AD) is the leading cause of dementia affecting the elderly in the United States, with as many as 5 million Americans currently diagnosed. The cost to patients and caregivers measures in the billions of dollars, and will increase in the coming decades. As the population ages, developing effective treatments for AD will become even more critical.
One of the ways researchers are trying to treat the disease, is by targeting the lesions which form in the patients’ brains. In AD, there are two major types of lesion, plaques which form outside the cell composed of amyloid-beta peptide and intracellular neurofibrillary tangles composed of highly phosphorylated tau protein.
Much of the focus has been on amyloid plaques; however, research suggests that once symptoms occur, removing the plaques may not stop the progress of the disease. At this stage, it may be better to target tau as its pathology correlates better with the severity of dementia than plaque deposition. Eventually, when the disease can be diagnosed in its earliest stages, both of these hallmark pathologies should be targeted together to attempt to prevent memory impairments.
Active and passive immunotherapy
One promising approach for targeting tau lesions is immunotherapy, in which antibodies are used to bind to the pathological tau protein and promote its clearance. There are two types of immunotherapy, active and passive, which have their own advantages and drawbacks.
As most of pathological tau protein resides within neurons, the best therapeutic antibodies may be those that can work inside and outside the cell.
In active immunotherapy, the organism is given the target protein along with an adjuvant to encourage an immune response. Using this type, the body produces its own antibodies and the effects are long lasting.
However, because in the case of AD this would mean eliciting an immune response against a protein that naturally occurs in the brain, there is a greater risk of side effects than with the passive approach.
In passive immunotherapy, the patient is given antibodies that are generated in a laboratory. The effects will last only as long as the antibodies remain in the patient’s system, meaning that they will not be permanent.
However, there is a reduced risk of side effects using this method, and the antibodies which are then given to the patient about once per month can be highly specific for the protein of interest.
It was once thought that using therapeutic antibodies against an intracellular protein like tau would be impossible, but two key findings suggest that these assumptions were wrong. First, several groups have shown that neurons, the brain cells that are mainly affected by tau pathology, can take up antibodies from the extracellular fluid.
Second, the tau protein has been shown to be released from cells and to be taken up by neighboring cells, which may lead to spread of the pathology throughout the brain. These findings indicate two ways in which antibodies could be used against tau pathology: 1) for intracellular clearance and 2) to block the spread by binding to tau outside the cell.
Studying antibody efficacy
In our most recent study, we tested the efficacy of two different tau antibodies, recognizing a similar site of the tau protein, in transgenic mice and cell culture models. In the animals, one of these antibodies reduced soluble pathological tau and improved memory. We then assessed whether these antibodies could prevent tau toxicity and its spread in cultured neurons using tau purified from an Alzheimer’s patient.
These results show that both extra and intracellular mechanisms are in play for developing effective immunotherapies.
Our results showed that the mechanism of action depended on where the pathological tau was located when the antibody was added. When we added the two to cultured cells at the same time, and the majority of the tau was extracellular, the antibody bound to the tau outside the cell and prevented it from being taken up and having toxic effects on the cells. However, when the antibody was added 24 hours after tau, the antibody entered the neurons and neutralized the tau protein intracellularly, preventing its toxicity.
These results show that both extra- and intracellular mechanisms are in play for developing effective immunotherapies. While our group and others have observed antibody uptake into neurons that was associated with beneficial effects, other laboratories have seen favorable outcome with antibodies that may work solely outside the cell. As most of pathological tau protein resides within neurons, the best therapeutic antibodies may be those that can work inside and outside the cell.
A surprising result
What is also interesting about our findings is that the effective antibody binds less tightly to most forms of the tau protein than the ineffective antibody. Further investigation revealed that its efficacy could be explained by its strong binding to a soluble form of the tau protein that was isolated from the human brain.
These results indicate that strong binding to various forms of the target protein does not predict efficacy and this should be taken into account for development of this approach for clinical trials.