Obesity Modulates Anti-Bacterial Functions of Alveolar Macrophages from Male Mice, but not Female Mice

Abstract

Obesity often leads to chronic inflammation, termed meta-inflammation. Meta-inflammation contributes to chronic overactivation of macrophages and production of pro-inflammatory cytokines. However, obesity has also been associated with increased morbidity and mortality during bacterial pneumonia. Sexual dimorphism is seen in obesity induced inflammation and bacterial pneumonia responses with males having greater morbidity. As such, the theme of this dissertation is to further explore how obesity impacts alveolar macrophage function during infection and why males are at a disadvantage compared to females. This dissertation is comprised of three interrelated projects. The first set out to characterize the functional defects of alveolar macrophages in male high fat diet (HFD) mice and understand the mechanism that impacts host defense functions of their macrophages. HFD mice had increased numbers of alveolar macrophages, however male HFD alveolar macrophages showed a decreased ability to both phagocytize and kill bacteria compared to normal diet (ND) macrophages. PGE2 was elevated in male HFD macrophages and treatment of male alveolar macrophages with PGE2 led to decreased phagocytosis. Inhibition COX-2 which catalyzes PGE2 production improved bacterial phagocytosis and clearance ex vivo and in vivo, highlighting the role of PGE2 in obesity and how it impairs male macrophage function. In the second project, the aim was to characterize how fatty acids derived from a high fat diet and increased in circulation during obesity, impact alveolar macrophage phagocytosis and killing. The findings demonstrate that palmitic acid and arachidonic acid have detrimental effects on bacterial phagocytosis. Lipidomic analysis of both the alveolar macrophages themselves and the bronchoalveolar lavage fluid (BALF) showed male and female ND and HFD mice had distinct lipid signatures in both compartments. PGE2 was upregulated in HFD mice in both the alveolar macrophages and the BALF, consistent with previous findings. HETE metabolites were increased in HFD mice and HEPE metabolites were decreased in HFD mice, with both classes of metabolites having some impact on immune system function. RNAseq analysis was also performed on alveolar macrophages from male and female ND and HFD mice. Alveolar macrophages from male and female HFD mice showed differences in transcription of genes related to inflammatory processes, lipid utilization, and leukocyte migration. Treatment of AMs with both palmitate and arachidonic acid impaired phagocytosis function primarily in males. Taken together these data provide evidence on how dietary fatty acids impact host defense functions of macrophages and how high fat diet influences the lipidome and transcriptome of alveolar macrophages to impact function. In the third project, the aim was to understand the role of sex hormones in alveolar macrophage anti-bacterial function. This aim used a combination of mouse models influencing sex hormones, including gonadectomy, dihydrotestosterone treatment, and androgen receptor knockout mice. Castrated male mice showed improved ability to phagocytize bacteria and re-introduction of testosterone impaired this capability. Ovariectomized female mice showed a slight reduction in ability to phagocytize bacteria. Female mice that were heterozygotes for androgen receptor also showed a reduction in ability to phagocytize and kill bacteria. While this phenotype is likely multifactorial, this study highlights a role for sex hormones in macrophage function. In summary, this work has explored the role of obesity in impairment of alveolar macrophage function during bacterial pneumonia. This work highlights the role of sex in both obesity and infection models and identifies therapeutic targets that may lessen morbidity and mortality.