Acne vulgaris is a great example to study the interaction among the environment, microbes, and host immune response. Acne vulgaris is a highly prevalent chronic inflammatory skin disease thought to be caused by increased in sebum secretion, bacterial colonization by the ubiquitous bacterium Cutibacterium acnes (C. acnes), and host inflammatory response. As a major factor implicated acne pathology, sebum and its composition, squalene, are unique to humans. Sebaceous lipids accumulate in the follicular duct are oxidized by C. acnes lipase leading to bacterial proliferation. The direct cause-and-effect relationship between the bacteria has been difficult to establish given that C. acnes is a ubiquitous bacterium and that there was no quantitative difference in the number of bacteria between subjects with and without acne. Recent genomic and phenotypic analyses on acne clinical samples revealed different phylotypes of C. acnes coexist in the pilosebaceous unit with other Cutibacterium spp. included C. granulosum and C. humerusii. Studies also showed strains phylotype IA1 are more abundant with the skin on acne patients (CA) and phylotype II are more associated with healthy skin (CH). However, the mechanisms underlying the lipid and bacterial interaction to cause the onset of inflammation and subsequent development of acne are still not understood.
This dissertation begins with a detailed analysis using single cell RNA-seq to deconstruct the genetic and molecular profiles of individual cells to study differences in lesional and non-lesional skin of acne patients. Particularly we were interested in changes within the same cell types to understand how normal skin becomes inflamed acne skin. We identified that a TREM2 macrophage subpopulation, expressing a lipid gene metabolism program, is associated with acne lesions. We further discovered that squalene induces TREM2+ macrophages in vitro to have enhanced phagocytosis but minimal antimicrobial activity against C. acnes. As a result, the squalene induced TREM2+ macrophage perpetuates inflammation linking the excess lipid production in acne to inflammation in acne.
Next, we investigated clinical strains of C. acnes, CA and CH, to understand how the microbiome interact in acne lesions. These strains differ in their ability to trigger inflammation with CA induces higher pro-inflammatory cytokine secretion compared to CH in cell culture and mouse infection model. However, the virulence factor that accounts for the differences in immune response between CA and CH are still unknown. In this study, we showed that C. acnes activates the innate immune system through RNA species that are usually reserved for viral detection. Interestingly, RNA species from CA and CH have different bioanalyzer profiles and can trigger distinct immune response as seen with live bacteria. We also showed that CA RNA stimulate the immune response through a TLR-8/IL-18/IL-12p40 pathway. Our in vitro data correlates well with our scRNA-seq data showing the abundance of IFN- and TLR-8 in acne lesions.
Taken together, the results enhanced our understanding of the impact of squalene, a sebum lipid, and resident microbes on the immunological functions of the skin. This work advanced our knowledge of the environment-microbe-host interplay occurring at the pilosebaceous follicle and introduced a novel way of thinking about the mechanisms underlying the initiation and pathogenesis of acne vulgaris and inflammatory skin conditions.
