A mouse with chronic rheumatoid arthritis
The close evolutionary relationship between humans and mice makes rodents good models to study the basic processes that underlie most human diseases. But the short lifespan and other features of mice often make it hard to study chronic conditions such as rheumatoid arthritis. This disease starts as an acute inflammation of one or more joints, followed by a long-term autoimmune response. The immune system attacks healthy cells in synovial joints – those we can flex – leading to the destruction of bone, a calcification of cartilage, and a lifetime of crippling pain and stiffness. The acute, inflammatory phase of the disease has been replicated in some strains of mice, but the condition usually recedes and fails to trigger a sustained immune reaction that spreads to other joints.
Top left: Healthy BALB/c mice exhibit a unique _periarticular lymph node_(paLN) near the knee joint (oval violet spot). Ten days into the development of chronic rheumatoid arthritis, the node expands to a huge size (bottom left). The right panel shows the node at a lower resolution (middle, violet spot) in early chronic arthritis, showing its position in a cross-section of the knee near the joint, as well as the _popliteal lymph node_(pLN) at the bottom, which is also present in C57BL/6 mice.
So there has been no good chronic model for human rheumatoid arthritis – until now. Senior scientist Gerd Müller, PhD student Uta Baddack and other members of Martin Lipp’s group at the MDC have now developed a new method to induce chronic arthritis in a widely used strain of laboratory mouse. They accomplished this by combining two methods currently used to artificially induce arthritis in the animals. Along the way, they discovered a unique feature of the lymph system in a strain of mouse that helps explain the long-term autoimmune problems that accompany the disease. The work appeared in a recent issue of Nature Communications.
“We thought it was important to start with commonly used mice,” Gerd says, “for which many knock out strains exist, and which allow for a detailed analysis of the mechanisms underlying the disease. There are a few specialized strains to study rheumatoid arthritis, but even they generally fail to replicate some features of the human disease.”
Human arthritis begins as an acute response to an injury or infection in or near synovial joints. Immune cells inflitrate the cushion of liquid and cartilage that separates bones. This causes the typical signs of inflammation: a buildup of cells, painful pressure, and a rise in temperature. In healthy patients, the symptoms subside after a few days. But in those with rheumatoid arthritis, the immune response continues. For some reason, molecules produced by cells in the joints are interpreted as “foreign”. The immune system creates antibodies against them and attacks cells in other joints that produce them. A blood test for one of these antibodies, which targets proteins called ACPA (anticitrullinated peptide/protein antibodies), is commonly used in diagnosing rheumatoid arthritis. But mice usually fail to produce the antibodies over the long term, probably explaining their failure to develop chronic arthritis.
Martin’s group started with strains of mice called BALB/c and C57BL/6, commonly used in research laboratories around the world. First they triggered a model form of arthritis by injecting the animals with a foreign molecule called mBSA, which would train their immune systems to recognize it and mount a response. Next they injected mBSA into a synovial joint. White blood cells called granulocytes rushed into the region, causing an inflammation and a build-up of liquid. In 2007 Martin’s lab managed to optimize this method to induce chronic inflammations in C57BL/6 mice which, however, did not result in joint-specific autoimmunity.
To overcome this, Gerd and his colleagues combined this method of antigen-induced arthritis (AIA) with another that causes the mouse immune system to respond to one of its own molecules called collagen II. “This protein is normally present in synovial joints,” Gerd says. “To trick the body into thinking it’s foreign, we injected a chicken version of the molecule accompanied by an ‘immune booster’ composed of inactive bacteria.” In this method, called CIA, the mice mount a transient immune response against collagen II in the joint that is injected, but here, too, the disease fails to spread.
Combining the two approaches – a procedure the lab calls ACIA – stimulated a strong inflammation in the knee joint of the two strains of mice. In BALB/c mice, the symptoms progressed and the animals developed chronic arthritis. They experienced a massive erosion of bone and a calcification of cartilage – two symptoms of the human disease. Their blood samples revealed long-lasting antibodies against synovial molecules. And after a few weeks, the problem had spread across the body to affect other joints.
The chronic condition was much weaker in the second strain, C57BL/6 mice – and the group wondered why. They began a careful investigation of the lymph system of the two strains. This network of nodes and ducts and vessels is spread across the body and plays a crucial role in the immune system. It filters bodily fluids in search of foreign molecules and produces white blood cells that recognize them – while training the cells not to respond to the body’s own molecules.
The scientists discovered that BALB/c mice have a structure close to their knee joints that resembles a small lymph node. C57BL/6 mice don’t have it. In BALB/c however, this lymph node, which Martin’s lab identified for the first time, becomes larger through an infiltration of immune cells when the joint suffers an inflammation. Martin’s lab used a cell-tracking system to demonstrate that the knee joint drains exclusively into this node; appropriately, the group christened it the “perarticular lymphe node.”
“Such peripheral lymph nodes can play a role in autoimmune diseases,” Martin says. “They train cells to cope with a region of the body where a particular protein rarely appears, which explains why they might mistake a healthy molecule for something foreign. They produce cells with antibodies against it, and if this immune reaction is strong enough, it can spread to other parts of the body.”
Overcoming the chronic symptoms of arthritis in human patients will probably require finding a way to shut down this long-term response. It’s a tricky problem given the fine line that separates autoimmune reactions from the healthy functions of the immune system.
“Peeling these two things apart is something we’ll have to investigate in a complex animal,” Martin says. “Our new combined method in this common strain of mouse is a good place to start. It is the first laboratory model that replicates the key features of human rheumatoid arthritis.”
– Russ Hodge
Highlight Reference:
Baddack U, Hartmann S, Bang H, Grobe J, Loddenkemper C, Lipp M, Müller G. A chronic model of arthritis supported by a strain-specific periarticular lymph node in BALB/c mice. Nat Commun. 2013;4:1644. Wengner AM, Höpken UE, Petrow PK, Hartmann S, Schurigt U, Bräuer R, Lipp M. CXCR5- and CCR7-dependent lymphoid neogenesis in a murine model of chronic antigen-induced arthritis. Arthritis Rheum. 2007 Oct;56(10):3271 – 83.
