Potential Mechanisms of Mucin-Enhanced Acinetobacter baumannii Virulence in the Mouse Model of Intraperitoneal Infection

Abstract

Porcine mucin has been commonly used to enhance the infectivity of bacterial pathogens, including Acinetobacter baumannii, in animal models, but the mechanisms for enhancement by mucin remain relatively unknown. In this study, using the mouse model of intraperitoneal (i.p.) mucin-enhanced A. baumannii infection, we characterized the kinetics of bacterial replication and dissemination and the host innate immune responses, as well as their potential contribution to mucin-enhanced bacterial virulence. We found that mucin, either admixed with or separately injected with the challenge bacterial inoculum, was able to enhance the tissue and blood burdens of A. baumannii strains of different virulence. Intraperitoneal injection of A. baumannii-mucin or mucin alone induced a significant but comparable reduction of peritoneal macrophages and lymphocytes, accompanied by a significant neutrophil recruitment and early interleukin-10 (IL-10) responses, suggesting that the resulting inflammatory cellular and cytokine responses were largely induced by the mucin. Depletion of peritoneal macrophages or neutralization of endogenous IL-10 activities showed no effect on the mucin-enhanced infectivity. However, pretreatment of mucin with iron chelator DIBI, but not deferoxamine, partially abolished its virulence enhancement ability, and replacement of mucin with iron significantly enhanced the bacterial burdens in the peritoneal cavity and lung. Taken together, our results favor the hypothesis that iron at least partially contributes to the mucin-enhanced infectivity of A. baumannii in this model.

Host Innate Immune Responses to Acinetobacter baumannii Infection

“…The importance of macrophages in host defense against A. baumannii infection has been confirmed in mouse and rat models of A. baumannii infection (Breslow et al, 2011b;Qiu et al, 2012;Tsuchiya et al, 2012;Bruhn et al, 2015;Harris et al, 2019a;Kang et al, 2020;Lee et al, 2020). Macrophages phagocytose and kill A. baumannii at the early stage of the infection before large numbers of neutrophils are recruited to the site of infection (Qiu et al, 2012;Bruhn et al, 2015).…”

“…On the other hand, neutralization of endogenous IFN-γ or TNF-α by monoclonal antibodies showed no effect on the tissue bacterial burdens or inflammatory responses in A. baumannii-infected mice (Van Faassen et al, 2007). This result was perhaps not entirely surprising since later studies have shown that IFN-γ was not consistently elevated after A. baumannii infection (Van Faassen et al, 2007;Qiu et al, 2009aQiu et al, ,b, 2012Qiu et al, , 2016Harris et al, 2013Harris et al, , 2019a. Similarly, neutralization of endogenous CXCL1 had no effect on intraperitoneal A. baumannii infection in mice (Breslow et al, 2011a), despite the fact that CXCL1 is a key chemotactic factor for neutrophil recruitment and was consistently detected locally or systemically at early stage of A. baumannii infection (Van Faassen et al, 2007;Qiu et al, 2009aQiu et al, ,b, 2012Qiu et al, , 2016Harris et al, 2013Harris et al, , 2019a.…”

“…Although alcohol consumption had no direct effect on neutrophil recruitment to the lungs in uninfected mice, their leucocytes showed significantly reduced ROS production, phagocytosis, and bactericidal capacity, which leads to increased bacterial burden and severity of infection. Those results suggest that it is not only the number but also the quality and function of neutrophils that are important in host immunity against A. baumannii (Harris et al, 2019a ).…”

“…Our current knowledge on the interaction between macrophages and A. baumannii were largely derived from in vitro studies in different macrophage-like cell lines, murine BMDMs, human peripheral mononuclear cells as well as human and murine primary macrophages. Those in vitro studies consistently showed that both human and murine macrophages produced large amounts of proinflammatory cytokines and chemokines, and moderate amounts of iNOS, in response to A. baumannii infection (Breslow et al, 2011b ; Qiu et al, 2012 ; Tsuchiya et al, 2012 ; Jun et al, 2013 ; Kim et al, 2013 ; Bist et al, 2014 ; Bruhn et al, 2015 ; Li et al, 2015 ; Kang et al, 2017 , 2018 , 2020 ; An and Su, 2019 ; Choby et al, 2019 ; Harris et al, 2019a , b ; Sato et al, 2019 ; Lee et al, 2020 ). In addition, the in vitro uptake of A. baumannii by macrophages was microfilament- and microtubule-dependent (Qiu et al, 2012 ).…”

“…The chemokine and neutrophil responses were impaired in NK1.1 cell-depleted mice and 50% of the mice died within 1 day of infection. However, multiple redundant neutrophil chemokines (such as MIP-2 and CCL5) were induced during A. baumannii infection (Van Faassen et al, 2007 ; Harris et al, 2013 , 2019a , b ) so it is somewhat surprising to see such a dramatic effect on neutrophil recruitment and mortality by NK1.1 cell depletion. Others have suggested that the increased number and cytotoxic activity of NK cells seen in the mouse model of intraperitoneal infection with MDR A. baumannii might contribute to the efficacy of the combination therapy with colistin and teicoplanin/daptomycin (Cirioni et al, 2016 ).…”