Binding of99mTc-labelled polyclonal human immunoglobulin to bacteria as a mechanism for scintigraphic detection of infection

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Abstract
The aim of the present study was to determine whether99mTc-labelled polyclonal human immunoglobulin (99mTc-HIG) binds to bacteria in vitro as well as in vivo. In vitro, the binding of99mTc-HIG to various gram-positive and gram-negative bacteria was determined. In vivo, mice were infected withStaphylococcus aureus Cowan I (protein A rich) orS. aureus EMS (protein A deficient) in a thigh muscle and then99mTc-HIG or99mTc-labelled human serum albumin (99mTc-HSA) was administered; scintigrams were made 1, 4, and 18 h later. In vitro binding of99mTc-HIG to bacteria was higher for gram-positive than for gram-negative forms. A positive correlation was found between the protein A content and the degree of binding toS. aureus. This was also found in vivo. The accumulation of99mTc-HIG at the site of infection was significantly (P < 0.01) higher than that of99mTc-HSA, for both strains ofS. aureus. It is concluded that vascular permeability cannot fully explain the accumulation of99mTc-HIG at the site of infection and that binding of99mTc-HIG to bacteria plays a role in this respect.