Differential expression pattern of the antiapoptotic proteins, Bcl-2 and FLIP, in experimental arthritis

Objective. To examine the relationship between apoptosis and the expression of antiapoptotic proteins in the pathogenesis of experimental inflammatory arthritis. Methods. Clinical and histologic assessment of adjuvant-induced arthritis ( AIA) was performed over a 42-day period. The induction of apoptosis was mea sured by TUNEL analysis, and the antiapoptotic proteins, Bcl-2 and FLIP, we re examined by immunohistochemistry with the use of monospecific antibodies . The percentage of Bcl-2- and FLIP-positive cells was correlated with hist ologic markers of AIA. Results. Arthritis developed by day 14 following adjuvant injection. Few TU NEL-positive cells were observed between days 0 and 21, indicating that apo ptosis did not occur at these time points. An increase in the number of TUN EL-positive cells was observed at day 28, particularly outside sites of car tilage or bone erosion, which dramatically declined by day 35. Immunohistoc hemical analyses of Bcl-2 and FLIP revealed that the synovium was positive for Bcl-2 and FLIP on day 0. On day 14, Bcl-2 was present at the sites of e arly erosions and correlated with the erosion and inflammation scores. FLIP was also highly expressed at sites of erosion and was localized to the pan nus starting on day 21. Although TUNEL positivity peaked at day 28, a time point in which Bcl-2 and FLIP were present, the areas that displayed intens e positivity for expression of Bcl-2 and FLIP were TUNEL negative. In addit ion, the number of neutrophils in the synovial lining and pannus significan tly decreased from day 28 to day 35, suggesting that the cells undergoing a poptosis were neutrophils. Furthermore, at day 42 when TUNEL-positive cells were absent, Bcl-2 expression was diminished, while FLIP remained highly e xpressed in the pannus. Conclusion. The overall percentage of TUNEL-positive cells in the ankle was <1% except on days 28 and 35 post-adjuvant injection, suggesting that in A IA, similar to rheumatoid arthritis, a lack of apoptosis may contribute to disease progression. Furthermore, Bcl-2 and FLIP are temporally and differe ntially expressed during the pathogenesis of AIA. Inhibition of these molec ules may augment synovial apoptosis and ameliorate the disease.