FIG. 4.

Cofactor activities of relapsing fever spirochete surface-bound C4BP (A) and FH (B). Bacteria (1 × 10⁹) were incubated in HI-NHS or with purified C4BP (10 μg/ml) and washed, and factor I was added in combination with ¹²⁵I-labeled C4b (A) or ¹²⁵I-labeled C3b (B). Samples from supernatants were run in reducing SDS-PAGE gels, after which the gels were dried and visualized by autoradiography. As positive controls, ¹²⁵I-labeled C4b (A) and ¹²⁵I-labeled C3b (B) were incubated in the presence of factor I and purified C4BP and FH, respectively. The cleavage of ¹²⁵I-labeled C4b is observed by the appearance of the C4d fragment, and that of ¹²⁵I-labeled C3b is observed by the appearance of the α′ 68-kDa, 46-kDa, and 43-kDa fragments.