Rheumatoid arthritis (RA) is a chronic debilitating disease. GM-CSF is a 14kDa Th-2 cytokine, implicated in the development and maintenance of the arthritis phenotype (Campbell, et al., 1997; 1998). The aim of this study was to further investigate the therapeutic potential of GM-CSF neutralization in a mouse model of collagen-induced arthritis (CIA) and to compare the response to that of a steroid.

On experimental day 0, male DBA/1 mice (16-22g) were anaesthetised with isoflurane (induction 2-3%, maintenance 0.1-0.5%) and dosed with 100 µg bovine type II collagen in Freund’s complete adjuvant at the base of the tail (two intradermal sites, 50 µL/site). Paws were inspected daily for clinical signs of inflammation and visually scored from 0-4 (0=normal, 1=erythema or only one digit affected, 2=slight swelling, 3=pronounced swelling and 4=ankylosis). Onset (day 1) of arthritis was the first day that an animal scored 1. On the day of onset of arthritis, mice were allocated into treatment groups (n=9-10, all 0.1mL): vehicle (i.p.), prednisolone (3mg.kg^-1, p.o.), anti-GM-CSF antibody (1, 10 & 100 µg, i.p. MAB415, R&D Systems) and null rat IgG2a isotype control (100 µg, i.p. MAB006 R&D Systems) and dosed daily for 14 days. Arthritis was assessed daily in all animals for either 14 or 21 days. Animals were killed on either day 14 or 21 with an overdose of sodium pentobarbitone (12 mg.kg^-1, 0.3mL, i.p.) and knee and ankle joints taken for subsequent histological analysis. Data are presented in Table 1 as mean (± SEM) clinical score.

The first signs of arthritis appeared between day 15 and 50 after collagen injection (arthritis day 1). Disease severity was significantly reduced by daily treatment of prednisolone or 100 µg MAB415 compared to MAB006 (Table 1). Histological evaluation of joints corroborated the observed clinical score. By day 21, one week after cessation of treatment, prednisolone treated animals developed arthritis, which was comparable to that seen in the vehicle control group (clinical score at day 21, vehicle 7.2±1.1, prednisolone 8.0±2.1, n=4-5 per group). In contrast, the therapeutic effect of MAB415 (100 µg) was sustained (clinical score at day 21, MAB415 2.8±1.1, MAB006 8.3±0.8, n=4 per group).

Table 1. Effect of 14 days of treatment on arthritis severity. **p<0.01, and *p<0.05 using one way ANOVA with Dunnett’s multiple comparisons test.

This data shows that neutralization of GM-CSF with a specific monoclonal antibody can dose-dependently ameliorate clinical signs of CIA in mice. Whilst the magnitude of the anti-arthritic response of MAB415 was slightly less than prednisolone up to day 14, the therapeutic effect of MAB415 was sustained up to day 21, in contrast to prednisolone. This study suggests that inhibition of GM-CSF may provide therapeutic benefit to patients with RA.