The aim of this dissertation is to increase the understanding of the complex cellular communication network underlying skin immunity, with a focus on the role of fibroblasts. Dermal fibroblasts are appreciated for supporting tissue architecture and scar formation but have historically been ignored in terms of immunology. I begin this dissertation with a literature review on the topic of fibroblast immune function, which led to my innovative hypothesis that subsets of fibroblasts are critical for skin inflammation and host defense. To test my hypothesis, I performed an unbiased network analysis of mouse skin during Staphylococcus aureus infection and discovered CXCL12+ immune acting fibroblast (IAF) subsets with a dominant role in neutrophil communication. Subsequent experiments to characterize this novel cell population demonstrated that CXCL12+ IAFs predominantly reside in the reticular dermis, express adipocyte lineage markers, detect IL-17 and TNF⍺, and promote robust neutrophil recruitment through NFKBIZ-dependent release of CXCR2 ligands and CXCL12. Targeted deletion of Il17ra in mouse fibroblasts resulted in greatly reduced neutrophil recruitment and increased infection by S. aureus, demonstrating the significance of CXCL12+ IAFs. I next asked if CXCL12+ IAFs are involved in psoriasis, a human inflammatory skin disease responsive to drugs targeting IL-17 and TNF⍺. Using single-cell transcriptomics, analogous CXCL12+ IAFs were identified in human psoriatic skin. Importantly, therapeutic targeting of IL-17 decreased fibroblast expression of neutrophil chemokines, demonstrating that IL-17 inhibitors work, in part, by inhibiting CXCL12+ IAF communication with neutrophils. Finally, to characterize the cell-cell interactions underlying human neutrophilic dermatoses, I performed bulk, single-nuclei, and spatial transcriptomics on skin biopsies from patients with Sweet’s syndrome (SS), pustular psoriasis, and pyoderma gangrenosum. This analysis led to the identification interferon (IFN)-activated IAFs enriched in SS. To test the significance of the IFN signature, I performed experiments with cultured fibroblasts and found that type 1 IFN induced fibroblast expression of neutrophil chemokines, implicating IFN-activated IAFs in SS pathogenesis. In summary, this dissertation unveils the pivotal role of dermal IAFs in orchestrating skin immunity. Using a combination of bioinformatics, in vitro systems, murine models, and clinical samples, this work provides unprecedented insight into the intricate cellular communication network underlying skin inflammation and host defense. These findings contribute significantly to our understanding of immunology and lay the groundwork for the development of novel treatments for infectious and inflammatory diseases.

The skin serves a vital role as our body’s first line of defense against the external environment and the potentially pathogenic microbes we encounter. However, abundant, diverse communities of microbial species colonize the skin shortly after birth and evolve with us over time. Thus, the skin exists in a unique immunological state, where it must tolerate and avoid reacting to these resident microbes under normal conditions while remaining poised to respond rapidly and effectively at the first sign of danger. To accomplish this feat, tight regulation of the inflammatory and immune processes present in the skin is crucial.

This dissertation begins with a description of the processes relevant to the maintenance of immunological homeostasis in the skin. In particular, the complex network of immune functions present in the skin is discussed, with a focus on the innate immune processes active in the epidermis and the role of keratinocytes in host defense. Additionally, we introduce the concept of epigenetic regulation of gene expression and discuss the roles of histone modifications, chromatin structure, and regulatory nuclear factors. The skin microbiome is described in detail, as are the impacts of these skin-resident microbes on cutaneous immunological processes and the consequences that can arise when their composition is altered.

Next, the hypothesis that the microbiome can influence epigenetic regulation of inflammatory gene expression in keratinocytes is explored. This work describes how a particular member of the skin microbiome can generate molecules that inhibit histone deacetylase (HDAC) activity, which has distinct effects in different cell types of the cutaneous innate immune system—specifically, an enhancement of TLR-mediated proinflammatory gene expression in keratinocytes. Furthermore, HDAC8 and HDAC9 are identified as key regulators of inflammatory gene expression in keratinocytes, as depletion of either of these enzymes greatly enhances the response to TLR activation.

Taken together, these results enhance our understanding of the impact of resident microbes on the immunological functions of the skin, and identify a mechanism by which tolerance to these microbes can be broken. This work advances our knowledge of the host-microbe interactions occurring at epithelial surfaces, and introduces a novel way of thinking about the mechanisms underlying the initiation and pathogenesis of inflammatory skin conditions.