Interleukin 6 plays a key role in the development of antigen-induced arthritis

Abstract

To investigate the direct role of interleukin (IL) 6 in the development of rheumatoid arthritis, IL-6-deficient (IL-6 -/-) mice were backcrossed for eight generations into C57BL/6 mice, a strain of mice with a genetic background of susceptibility for antigen-induced arthritis (AIA). Both histological and immunological comparisons were made between IL-6-deficient (IL-6 -/-) mice and wild-type (IL-6 +/+) littermates after the induction of AIA. Although all IL-6 +/+ mice developed severe arthritis, only mild arthritis was observed in IL-6 -/- mice. Safranin O staining demonstrated that articular cartilage was well preserved in IL-6 -/- mice, whereas it was destroyed completely in IL-6 +/+ mice. In addition, comparable mRNA expression for both IL-1beta and tumor necrosis factor alpha, but not for IL-6, was detected in the inflamed joints of IL-6 -/- mice, suggesting that IL-6 may play a more crucial role in cartilage destruction than either IL-1beta or tumor necrosis factor alpha. In immunological comparisons, both antigen-specific in vitro proliferative response in lymph node cells and in vivo antibody production were elicited in IL-6 -/- mice, but they were reduced to less than half of that found in IL-6 +/+ mice. Lymph node cells of IL-6 -/- mice produced many more Th2 cytokines than did IL-6 +/+ mice with either antigen-specific or nonspecific stimulation in in vitro culture. Taken together, these results indicate that IL-6 may play a key role in the development of AIA at the inductive as well as the effector phase, and the blockade of IL-6 is possibly beneficial in the treatment of rheumatoid arthritis.

Figure 1

Representative histopathologies of the knee joints stained with hematoxylin and eosin (×100). Thirty-five days after the first immunization, mice were sacrificed and subjected to histopathological examination. (A) Wild-type (IL-6 +/+) mice induced AIA (grade 4); massive cellular infiltration, synovial hyperplasia, neovasculization, and erosion of cartilage and bone were observed in the knee joint. (B) IL-6-deficient (IL-6 −/−) mice (grade 2); only limited synovial lining cell hyperplasia was detected. (C) Wild-type (IL-6 +/+) mice injected with saline as a control (grade 0); no features of synovitis were detected.

Figure 2

Femoropatellar sections of the knee joints stained with Safranin O/fast green (×100). (A) Wild-type (IL-6 +/+) mice induced AIA; severe synovitis with pannus formation in patella (upper side) and femur (lower side) was observed. Moreover, Safranin O was not stained, indicating complete cartilage destruction. (B) IL-6-deficient (IL-6 −/−) mice induced AIA; only limited synovial hyperplasia was detected. Safranin O was stained strongly in patella and femur (red color), indicating cartilage was completely preserved. (C) Wild-type (IL-6 +/+) mice injected saline as a control; no features of synovitis were detected, and Safranin O was strongly stained.

Figure 3

Semiquantitative RT-PCR analysis for the expression of IL-6, IL-1β, and TNFα in the synovia of knee joints from two each of both IL-6-deficient and wild-type mice at 25 days after immunization. As a control, synovia of saline-injected knee joints from two wild-type mice at day 25 were used. Lanes 1 and 2: wild-type (IL-6 +/+) mice injected saline as a control. Lanes 3 and 4: IL-6-deficient (IL-6 −/−) mice induced AIA. Lanes 5 and 6: wild-type (IL-6 +/+) mice induced AIA.