Date on Master's Thesis/Doctoral Dissertation

5-2021

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Microbiology and Immunology

Degree Program

Microbiology and Immunology, PhD

Committee Chair

Kosiewicz, Michelle

Committee Co-Chair (if applicable)

Alard, Pascale

Committee Member

Alard, Pascale

Committee Member

Jala, Venkatakrishna

Committee Member

Yan, Jun

Committee Member

Zhang, Xiang

Author's Keywords

lupus; microbiota; autoimmune; macrophage phagocytosis; metabolite; sex-bias

Abstract

Systemic lupus erythematosus (SLE) is a complex and an insidious disease that is still not completely understood, and has very few treatment options. Those that are available are ineffective and/or have serious side effects. Both genetic and environmental factors contribute to the susceptibility and resistance to SLE, and understanding the environmental factors underlying this disease could lead to more effective prevention and/or treatment strategies. Like most autoimmune diseases, SLE is much more prevalent in females than males. While there are numerous factors that contribute to this lupus susceptibility, there is increasing evidence that the microbiota can strongly influence lupus progression, and that sex-based differences in microbiota composition and function may play a role in the sex bias (i.e., susceptibility in females and resistance in males) of disease. The NZBxNZW F1 (BWF1) mouse model of lupus has many of the same disease features seen in humans including the sex bias, making it an ideal model for studying sex-based differences in microbiota and how they affect lupus progression. Previous findings from our laboratory have shown that not only do female and male BWF1 microbiota profiles differ significantly, but male BWF1 microbiota can suppress lupus when cecal contents are transferred into female BWF1 recipients. The overall goal of this dissertation was to identify the players involved and begin to understand the potential mechanisms underlying the suppressive effect of the male microbiota on disease. Three aims were designed to address these issues. The goal of aim 1 was to identify the bacterial populations in the microbiota of female and male BWF1 mice that may either cause or suppress disease. The goal of aim 2 was to analyze the function of female and male microbiota by identifying metabolites that may be involved in the suppression of disease. In aims 1 and 2, we also investigated the impact of androgens on bacterial populations and metabolite profiles, respectively, by comparing intact and castrated male mice. In aim 3, the goal was to investigate the potential immune mechanisms that could underlie the suppression of disease mediated by male microbiota. The results of the first aim indicate that changes in the abundances of Bacteroides, Clostridium, and Alistipes strongly correlated with the ability of transferred male BWF1 microbiota to suppress disease in female BWF1 recipients. We took advantage of a change in animal facility that had a significant impact on our mouse colony and its microbiota populations, and gave us an opportunity to analyze and compare bacterial populations during periods of time when microbiota transfers had varying results. Specifically, we found that Bacteroides (primarily the Bacteroides acidifaciens species) abundance was high, Clostridium (primarily the Clostridium leptum species) abundance was low, and Alistipes was present during the periods when transfer of male microbiota was effective at suppressing disease in female BWF1 mice. Conversely, Bacteroides abundance was low, Clostridium abundance was high, and Alistipes was absent when male microbiota lost that capability. We concluded from our thorough microbiota analyses that a high Bacteroides/Clostridium ratio in the male microbiota may be a reliable predictor of disease-suppressing capability, since it correlated strongly with disease suppression in female recipients of male microbiota. The second aim of the project used analysis of metabolomic profiles to investigate functional differences in female and male BWF1 microbiota. Differential production of immunomodulatory metabolites is a major mechanism by which the gut microbiota influences the immune system. By measuring the fecal metabolite profiles, we identified phytol as a potential mediator of lupus suppression by male microbiota. Phytol is produced by the microbiota and converted into phytanic acid by host enzymes. Both phytol and phytanic acid were significantly more abundant in intact male than either female or castrated male BWF1 mice. Both phytol and phytanic acid have potent RXR and PPARγ agonist properties, all of which can directly influence many different immune functions. The third aim of this project investigated the differences in female and male macrophage efferocytosis (i.e., phagocytosis of apoptotic cells) efficacy and how phytol and/or phytanic acid could affect this immune function. Deficiencies in efferocytosis, particularly by macrophages, are a major risk factor for SLE because they result in the accumulation of debris that stimulates autoantibody production. We found male BWF1 splenic macrophages were more efficient at efferocytosis than female splenic macrophages, and treatment with phytanic acid in vitro or in vivo could enhance female splenic macrophage efferocytosis. Overall, we found that higher Bacteroides and lower Clostridium abundances correlated with lupus suppression in female BWF1 recipients of male microbiota, and speculate that this protection could be due, at least in part, to higher levels of phytol and phytanic acid production in males. Furthermore, phytol and phytanic acid produced by males may suppress disease, again at least in part, via enhancement of macrophage efferocytosis. Taken together, these data may provide the basis for a mechanistic understanding of the impact that the microbiota can have on autoimmune diseases such as lupus, and for the development of novel therapies.

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