Targeting the serum marker interleukin 9 improves the underlying characterization and immune homeostasis in rheumatoid arthritis

Introduction:
Rheumatoid factor (RF) and anti-citrullinated peptide antibodies (ACPA) are serological markers used for diagnosing rheumatoid arthritis (RA), an autoimmune disease characterized by inflammatory joint damage. However, there is a subset of RA patients who test negative for both RF and ACPA, known as seronegative rheumatoid arthritis (SNRA).

Material and methods:
The levels of serum markers were examined in both clinical samples and a rat model of type II collagen-induced RA (CIA). The effect of interleukin 9 (IL-9) on RA was investigated using recombinant rat IL-9 (rrIL-9), anti-rat IL-9 neutralizing monoclonal antibody (mAb), and control IgG antibody in the CIA rat. The severity of arthritis was assessed. Treg and Th17 cells, M1 and M2 macrophages, and inflammatory cytokine levels were analyzed.

Results:
We observed higher levels of IL-9 in clinical samples from SNRA patients compared to the normal group. Rat models of CIA exhibit increased arthritis scores, weight loss, paw swelling, and severe joint damage. IL-9 was the most sensitive serum marker for the diagnosis of RA in serum assays of CIA rats. IL-9 increased arthritis scores and cartilage damage, whereas treatment with IL-9 inhibitors produced the opposite effect. IL-9 inhibitors promoted Treg/Th17 homeostasis, decreased M1 macrophages, increased M2 macrophages, and decreased levels of inflammatory cytokines in joint tissues.

Conclusions:
These results suggest that IL-9 has potential as a diagnostic marker for SNRA. Inhibition of IL-9 could reduce the severity of arthritis in CIA rats by ameliorating inflammation and modulating the Treg/Th17 immune balance, M2 and M1 macrophage activation.