Decreased protective efficacy of reduced and alkylated human immune serum globulin in experimental infection with Haemophilus influenzae type b.

Conventionally prepared immune serum globulin frequently produces severe side effects when administered intravenously. A modified preparation in which 4 to 5 interchain disulfide bonds have been reduced and alkylated has been made for intravenous use. However, reduction and alkylation may affect Fc-mediated functions of immunoglobulin G, particularly its ability to fix complement by the classical pathway. To determine whether reduction and alkylation alters the protective activity of immune serum globulin in vivo we compared it with two less harshly prepared globulins (pH 4 treated or ultrafiltered) in an infant rat model of Haemophilus influenzae b infection. Antibody binding to the capsular and noncapsular components of H. influenzae b and in vitro bactericidal activity were similar in the globulin preparations. Infant rats were treated with various doses of globulins adjusted to provide identical concentrations of anticapsular antibodies as measured by the Farr radioactive antigen binding assay. At high doses of anticapsular antibody (greater than 1,500 ng per pup), all preparations protected well. At marginal doses (750 ng per pup), however, rats given reduced and alkylated globulin had a significantly greater incidence of bacteremia (P less than 0.05), meningitis (P less than 0.01), and death (P less than 0.05) and a higher magnitude of bacteremia (P less than 0.02) than rats who received pH4-treated or ultrafiltered globulins. These differences were not due to differences in anticapsular antibody concentrations achieved in the serum. The 50% protective serum concentrations of anticapsular antibody in this model were 200 to 300 ng/ml for reduced and alkylated globulin and 100 to 200 ng/ml for acid-treated globulin. Absorption of the globulins with purified H. influenzae b capsule reduced in vitro bactericidal activity and rat protective activity. However, the magnitude of bacteremia was lower in rats receiving absorbed pH 4-treated globulin than in those receiving absorbed reduced and alkylated globulin (P less than 0.05). We conclude that reduced and alkylated immunoglobulin G provides significantly less protective activity against H. influenzae b infection in this model than globulins not so modified, and we suggest that the altered Fc function of the immunoglobulin G, such as the decreased ability to fix complement by the classical pathway or decreased Fc-mediated opsonization, may be responsible for this impairment.