Induction of osteoclastogenesis and bone loss by human autoantibodies against citrullinated vimentin, Harre et al, 2012
First, some definitions to help you understand this article:
Vimentin is a part of the skeleton of many connective tissue cells, including bone cells.
Citrullination is a chemical change in a protein that changes its amino acid makeup (check out this page for more information about proteins and amino acids). Citrullination commonly occurs as a result of inflammation.
Citrullination is a chemical change in a protein that changes its amino acid makeup (check out this page for more information about proteins and amino acids). Citrullination commonly occurs as a result of inflammation.
There are several types of bone cells. This article focuses on osteoclasts, which are responsible for resorbing (destructing) bone. Osteoclasts don't always exist, although they generally can be found somewhere in your body. You always have the stem cells that become osteoclasts in your body (called precursors, below). These cells are activated by a member of the tumor necrosis (TNF) family--I will talk about TNF in a future article. Bone resorption is not always a bad thing. It is a necessary process of life. When structural bone cells (osteocytes) are damaged by injury or inflammation, osteoclasts take away the damaged tissue so that is can be replaced with new, healthy bone.
In rheumatoid arthritis, osteoclasts resorb bone but it is not able to be repaired at the pace that it is destroyed. This leads to joint damage and eventually complete joint destruction.
Anti-citrullinated protein antibody (ACPA) was discussed briefly in the previous article. These are antibodies found almost exclusively in people with RA and approximately 2/3 of RA patients have detectable ACPA in their blood. These are antibodies that attack bind to proteins that have undergone citrullination. An antibody titer is just a measure of the amount of antibody that is in a sample. For more about antibodies in general, see this article.
In rheumatoid arthritis, osteoclasts resorb bone but it is not able to be repaired at the pace that it is destroyed. This leads to joint damage and eventually complete joint destruction.
Anti-citrullinated protein antibody (ACPA) was discussed briefly in the previous article. These are antibodies found almost exclusively in people with RA and approximately 2/3 of RA patients have detectable ACPA in their blood. These are antibodies that attack bind to proteins that have undergone citrullination. An antibody titer is just a measure of the amount of antibody that is in a sample. For more about antibodies in general, see this article.
What did they study?
The researchers started by measuring levels of bone resorption biomarkers (levels of which indicate how much bone is being resorbed) in the blood of people with normal (non-RA), seronegative RA, ACPA-positive RA, and RF-positive RA. The RA patients were all newly diagnosed patients who had not been treated with drugs that affect the immune system.
They then collected blood from patients with high titers of ACPA and examined how these antibodies bind to certain proteins, to see if they have a binding preference for citrullinated vimentin and to see if they would bind to other citrullinated proteins.
To understand how ACPA promotes bone destruction, the researchers cultured precursor cells with ACPA-positive and negative serum (from RA patients) to see if the presence of ACPA encouraged the cells to differentiate into osteoclasts. They also tested to see if the osteoclasts that differentiated in the presence of ACPA were better at breaking down bone using slices of cow bones.
Finally, they isolated ACPA that specifically binds to citrullinated vimentin and administered these antibodies to mice with impaired immune systems (make fewer white blood cells). After four weeks, they measured the size and density of bones as well as bone resorptive, bone building markers, and TNF-alpha. They also measured the susceptibility of precursor cells to become osteoclasts.
They then collected blood from patients with high titers of ACPA and examined how these antibodies bind to certain proteins, to see if they have a binding preference for citrullinated vimentin and to see if they would bind to other citrullinated proteins.
To understand how ACPA promotes bone destruction, the researchers cultured precursor cells with ACPA-positive and negative serum (from RA patients) to see if the presence of ACPA encouraged the cells to differentiate into osteoclasts. They also tested to see if the osteoclasts that differentiated in the presence of ACPA were better at breaking down bone using slices of cow bones.
Finally, they isolated ACPA that specifically binds to citrullinated vimentin and administered these antibodies to mice with impaired immune systems (make fewer white blood cells). After four weeks, they measured the size and density of bones as well as bone resorptive, bone building markers, and TNF-alpha. They also measured the susceptibility of precursor cells to become osteoclasts.
What were the results?
In patients that were positive for ACPA, the researchers found significantly higher levels of bone resorption factors and the amount increased substantially depending on the titer of ACPA in the blood. Seronegative and RF-postive only patients did not have increased levels compared to non-RA patients. The antibodies from ACPA-positive patients bound to citrullinated vimentin, but not to vimentin in its normal state.
When precursor cells were cultured with ACPA-positive serum more cells differentiated into osteoclasts compared to cells treated with normal or negative RA serum. These results were confirmed in the mice. The animals that were administered ACPA had significantly lower bone density and high bone resorptive markers. Furthermore, there were more osteoclasts observed. The ACPA-dosed mice did not, however, have increased bone building factors compared to normal mice.
Precursor cells from these animals were also more susceptible to becoming osteoclasts as they appeared to be more sensitive to factors that induce this differentiation. ACPA-administered mice had higher levels of TNF-alpha, which the researchers confirmed in cell culture has a sensitizing effect on precursor cells. Inflammation and cellular injury are key factors in increasing the amount of citrullinated vimentin that is expressed in these cells. The antibodies binding to the citrullinated vimentin in cells appears to further encourage the cells to differentiate into osteoclasts.
What does this mean?
I mentioned in the previous article that ACPA-positive patients tend to have more bone and joint destruction than seronegative patients. In this article, we come understand why.
TNF-alpha is a pro-inflammatory cytokine (chemical signal in the immune system), and is present in high levels in RA patients. In fact, there are many drugs now available that interfere with TNF-alpha to reduce severity of disease. It appears from this study that the presence of TNF-alpha makes precursor cells more likely to become osteoclasts.
These results may indicate that physicians should consider the earlier use of TNF-alpha inhibiting drugs in treated ACPA-positive patients. The results also further underline the importance of controlling inflammation in patients, regardless of ACPA status.
