The Role of Complement and IgG in Zymosan Opsonization

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Abstract
Zymosan particles require opsonization for optimal interaction with granulocytes and activation of their respiratory burst, In the present study we evaluated the serum factors necessary for zymosan opsonization. First, zymosan was treated with either normal serum or hypogammaglobulinemic serum (HGS), which is deficient in immunoglobulins (Ig) but has a normal concentration of complement components. Using granulocyte chemiluminescence to assay opsonization, the activity of particles treated with HGS was 66 ± 1.9% ( X ± SEM) of that with normal serum (p < 0.001), suggesting a role for Ig. HGS opsonic activity was restored to normal when the particles were also treated with IgG; however, neither heat-inactivated normal serum (56 °C, 30 min) nor pure human IgG alone had opsonic activity. The roles of the classical (CCP) and alternative (ACP) pathways of the complement system were also investigated. ACP activity seemed essential, since inactivation of the ACP (50 °C, 30 min) eliminated the activity of normal serum. However, when the ACP was intact, CCP action appeared to participate in opsonization, since selective CCP inactivation with EGTA and MgCl2 reduced the opsonic activity of normal serum by 26 ± 3.3% (p < 0.005). Thus, it is concluded that the ACP, CCP, and IgG all participate in zymosan opsonization.