Assignment #2 - Research Article Summary and Critique

Type IX collagen deficiency enhances the binding of cartilage-specific antibodies and arthritis severity

Stefan Carlsen, Kutty Selva Nandakumar and Rikard Holmdahl


Summary

Rheumatoid arthritis (RA) affects joint articular cartilage, synovium and bone by joint-specific autoimmune attack. This inflammatory disease is degenerative, leading to loss of function and joint deformity. In this article, researchers use animal arthritis models to determine whether the severity of correlated rheumatoid arthritis is dependent on the preexisting cartilage quality.

As mentioned in the previous blog, articular cartilage (in synovial joints) is of the hyaline cartilage type. Hyaline cartilage’s strong yet mildly flexible physical characteristics can be attributed to the type II collagen fibers that are most predominant in hyaline cartilage. Type IX and type XI collagen fibers are also found in hyaline cartilage, and it is thought that type IX collagen provides the necessary association between collagen fibrils and matrix macromolecules for integrity and stability of the cartilage. Type IX collagen associates with type II collagen by forming three polypeptide chains that periodically cover the surface of the collagen II/collagen XI heterofibril. (Carlsen et al.) Therefore, by disrupting or arresting the formation of one of the collagen IX polypeptide chains (α1) in a group of transgenic mice, the resulting absence of collagen IX can be used to test the contribution of collagen XI to the long-term stability of cartilage, as well as the vulnerability of the defective cartilage to inflammatory attack by monoclonal antibodies.

In order to simulate the susceptibility of collagen XI deficient mice to the rheumatoid arthritis inflammatory disease, collagen XI development was prevented in mice by backcrossing a transgenic disruption of the col9a1 gene. In addition, researchers used three animal models for arthritis; collagen-induced arthritis (CIA), collagen-antibody-induced arthritis (CAIA) and stress induced arthritis (SIA).

The CIA disease model has a similar disease course to that of RA, and therefore was used to test whether or not preexisting cartilage disorder changed the course of rheumatoid arthritis by allowing better accessibility to the immune response macrophages and neutrophils. CIA was induced in mice by injection of type II collagen from rat chondrosarcoma. The severity of the resulting arthritis was assessed by observation of the four paws, and assigning numbers to swelling or redness observed in one toe, more than one toe/ankle affected, or the whole paw affected. The two assay collagen IX deficient mice of different genetic backgrounds were found to develop more severe arthritis than of the control mice. In addition, they measured no change in antibody titers (a measurement of how much antibody was produced that recognizes a particular epitope) reinforcing that no new T-cells were produced to prime the inflammation because CIA disease is T-cell independent.

The CAIA model, inflammation is mediated primarily by neutrophils and macrophages, and therefore researchers could use this model to study the inflammation while ignoring the dependence of the immune response on initial complementary binding of Fcγ receptors. In addition, the regulatory role of T and B cells in cartilage stability can be investigated because it is known that with the absence of either T or B cells, the CAIA disease is enhanced. CAIA was induced by injecting the mice with purified IgG from B-cell CIIC1 antibodies (which binds to the C1 epitope [antibody recognition region of macromolecule]). Arthritis was observed to develop rapidly in both collagen IX deficient mouse strains, even affecting the knee. After further extending the study to test for the effect of different types of antibodies on arthritis, it was found that the different antibodies recognized different portions of the C1 epitope, and that antibody response overpowers the immune response (by macrophages and neutrophils) in the CIA model.

The last animal disease model, SIA appears to cause similar symptoms as CIA (edema and deformity of the hind paws) but is distinct because it is T-cell independent and causes proliferation of fibroblasts and periarticular enthesophyte formation in cartilage and bone, which can eventually lead to ankylosis (a joint inflammation). Since SIA is spontaneous and tend to occur in old males grouped together from different litters, SIA was induced in that exact way—grouping male mice together isolated from different litters. As in the above models, the developing arthritis was seen to be more severe in type IX collagen deficient mice than in the control mice.

These findings accentuate type IX collagen’s role in maintaining cartilage stability and integrity, and that genetic disorders which may disrupt cartilage integrity may be more vulnerable to inflammatory attack by pathogenic antibodies due to the increased accessibility of type II collagen fibrils.



Critique


This research paper was very clear to explain the methods and techniques used to draw their conclusions. Though the experiment was quite interesting, it was sometimes difficult to follow due to abundance of biochemical acronyms used.

The conclusions drawn from their experiment strongly supported their hypothesis, and they even chose at times to further extend their research (as in CAIA, by assessing the difference in severity of arthritis as a result of different antibodies) or reinforce their conclusions by retesting on a larger number of mice. In addition, they explored the
direct effect of CIIC1 antibodies on collagen II, by adding CIIC1 antibodies to chondrocytes cultured in vitro. The results observed (collagen II fibril disorganization and increased matrix synthesis) further strengthened their conclusion that the CIIC1 antibodies contribute directly to cartilage damage, and that type IX collagen deficient mice have type II collagen that is more accessible to the CIIC1 antibodies.

An obvious flaw to mention, concerning the processed data, is that the author did not refer to either the tables or figures in the discussion section. Furthermore, it was surprising that the author did not choose to include any photographs of the inflamed mouse paws, any figures depicting histological cartilage destruction, nor any cartoon images to illustrate the structures and interactions of type II and type IX collagen. Overall, this research paper was well organized, and the result tables that were provided were intuitively obvious to understand.