The Role of IgG Subclass in Antibody-Mediated Protection against Carbapenem-Resistant Klebsiella pneumoniae

Monoclonal antibodies (MAbs) have the potential to assist in the battle against multidrug-resistant bacteria such as carbapenem-resistant Klebsiella pneumoniae (CR-Kp). However, the characteristics by which these antibodies (Abs) function, such as the role of antibody subclass, must be determined before such modalities can be carried from the bench to the bedside. We performed a subclass switch on anticapsular monoclonal murine IgG(3) (mIgG(3)) hybridomas and identified and purified a murine IgG(1) (mIgG(1)) hybridoma line through sib selection. We then compared the ability of the mIgG(1) and mIgG(3) antibodies to control CR-Kp sequence type 258 (ST258) infection both in vitro and in vivo. We found by enzyme-limited immunosorbent assay (ELISA) and flow cytometry that mIgG(3) has superior binding to the CR-Kp capsular polysaccharide (CPS) and superior agglutinating ability compared to mIgG(1). The mIgG(3) also, predictably, had better complement-mediated serum bactericidal activity than the mIgG(1) and also promoted neutrophil-mediated killing at concentrations lower than that of the mIgG(1). In contrast, the mIgG(1) had marginally better activity in improving macrophage-mediated phagocytosis. Comparing their activities in a pulmonary infection model with wild-type as well as neutropenic mice, both antibodies reduced organ burden in a nonlethal challenge, regardless of neutrophil status, with mIgG(1) having the highest overall burden reduction in both scenarios. However, at a lethal inoculum, both antibodies showed reduced efficacy in neutropenic mice, with mIgG(3) retaining the most activity. These findings suggest the viability of monoclonal Ab adjunctive therapy in neutropenic patients that cannot mount their own immune response, while also providing some insight into the relative contributions of immune mediators in CR-Kp protection.