Structure–function analysis using monoclonal antibodies (mAbs), chimeric proteins and deletion mutagenesis indicated that all the domains contribute to C3b and C4b binding and functional activities [42], [43], [44] and [45]. Further, our recent domain swapping study with human complement regulators revealed that of the four domains, domain 1 imparts decay of the protease subunit from the C3-convertase and domains HSP inhibitor 2 and 3 recruit factor I for imparting proteolytic inactivation of C3b and C4b [46]. The importance of VCP in VACV pathogenesis was addressed initially in a study using rabbit and guinea

pig intradermal models where it was shown that VACV devoid of VCP produced comparatively smaller

lesions than the wild type virus [36]. More recently, similar experiments were also performed using mice ear pinnae model in complement sufficient and C3−/− mice [38]. Though these studies clearly demonstrated the importance of VCP in pathogenesis, the significance of complement inhibition by VCP as a contributing factor in the pathogenesis based on the individual known functions is still not clear. In the present study, we have generated a panel of mAbs against find more VCP and used them to dissect the regions on VCP that play a role in the complement regulatory activity. We then asked whether VCP could serve as a virulence determinant by targeting complement and if so, which complement regulatory activity of VCP plays a predominant role in virulence in vivo. The vaccinia virus strain Western Reserve (VACV-WR) was a kind gift from Dr. Sekhar Chakrabarti (National Institute of Cholera & Enteric Diseases, Kolkata, India). The virus was cultured Thymidine kinase by infecting African green monkey kidney cells (CV-1) followed by purification on a sucrose gradient. Recombinant VCP (rVCP) and its truncation

mutants (CCP 1–3, CCP 2–4, CCP 1–2, CCP 2–3 and CCP 3–4) were expressed and purified as previously described [42]. The complement proteins C1, C2, C4, C4b and factor I were purchased from Calbiochem (La Jolla, CA), and complement proteins factor H, C3 [42] and C3b [41] were purified as described earlier. Human soluble CR1 (sCR1) was a generous gift from Dr. Henry Marsh (AVANT Immunotherapeutics, Inc., Needham, MA). Cobra venom factor (CVF) was purified from Naja naja kaouthia venom as described earlier with minor modifications [47]. In brief, the lyophilized venom was dissolved in 20 mM sodium phosphate buffer, pH 7.4, loaded onto Source Q column (12 cm × 9.5 cm, GE Healthcare Bio-Sciences) in the same buffer and eluted with a linear salt gradient to 1 M NaCl.