39 The institutional ethical committee BKM120 approved this study. The statistical significance of the results was determined using Student’s t-test. The results are presented as mean ± standard deviation (SD). The 16-kDa recombinant protein coded by Rv2626c was expressed in the E. coli BL21plys (DE3) strain and purified using metal affinity chromatography, giving a yield of 10 mg/l culture. The purified rRv2626c when analysed by SDS–PAGE (Fig. 1) or even after silver staining (data not shown) did not reveal any major contaminating protein band. The endotoxin content in the purified recombinant protein was checked using the amoebocyte lysate

assay and was found to be extremely low (0·05 pg/μg of protein). Previous studies have revealed that Rv2626c is a secretory protein, indicating that Rv2626c could influence the host immune response by interacting with macrophage surface receptors. In order to assess the ability of rRv2626c to bind to the surface of RAW 264·7 macrophages,

cells were incubated Bcl-2 inhibitor with 10 μg of rRv2626c for various times and the bound rRv2626c was investigated using anti-rRv2626c antibody in a FACS analysis. The binding of rRv2626c with macrophages could be seen as early as 5–10 min after the start of incubation, and remained noticeably high until 60 min (Fig. 2). It could be seen (Fig. 2, brown curve) that the binding of rRv2626c to macrophages was inhibited when the cells were incubated with anti-Rv2626c antibody preincubated with rRv2626c. This clearly indicates that rRv2626c binds with high affinity and specificity to the surface of RAW 264·7 macrophages. Similar observations were obtained for phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 macrophages (data not shown). Having demonstrated binding of Rv2626c to the surface of murine macrophage cells cultured in vitro, the ability of rRv2626c to induce NO production via de novo expression of iNOS in the macrophages was assessed. RAW 264·7 macrophages

were stimulated with different concentrations of rRv2626c aminophylline protein (Fig. 3a; bars 3, 4 and 5). Stimulants such as LPS and IFN-γ were used as positive controls (Fig. 3a; bar 2) for NO production and iNOS expression (Fig. 3b; lane 2) in RAW 264·7 macrophages. NO production increased in RAW 264·7 macrophages with the addition of rRv2626c in a dose-dependent manner (Fig. 3a; bars 3, 4 and 5). Similar observations were obtained in J774·1 macrophages (data not shown). NO production by the cells was not observed when cells were stimulated with proteinase K-treated rRv2626c protein (Fig. 3a; bar 6), indicating that the NO production was specifically attributable to the presence of rRv2626c and was not a result of endotoxin contamination in the protein preparation. This increased NO production correlated well with the increase in iNOS expression in cells stimulated with rRv2626c (Fig. 3b; lane 3) as compared with the unstimulated group (Fig. 3b; lane 1).