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. IMPORTANCE Carbapenem-resistant Klebsiella pneumoniae is an urgent public health threat that causes life-threatening infections in immunocompromised hosts. Its resistance to nearly all antibiotics necessitates novel strategies to treat it, including the use of monoclonal antibodies. Monoclonal antibodies are emerging as important adjuncts to traditional pharmaceuticals, and studying how they protect against specific bacteria such as Klebsiella pneumoniae is crucial to their development as effective therapies. Antibody subclass is often overlooked but is a major factor in how an antibody interacts with other mediators of immunity. This paper is the first to examine how the subclass of anticapsular monoclonal antibodies can affect efficacy against CR- Kp Additionally, this work sheds light on the viability of monoclonal antibody therapy in neutropenic patients, who are most vulnerable to CR- Kp infection.