Studies of the human skin microbiome suggest that Propionibacterium acnes strains may contribute differently to skin health and disease. However, the immune phenotype and functions of T helper type 17 (Th17) cells induced by healthy (PH) versus acne (PA) skin-associated P. acnes strains are currently unknown. We stimulated peripheral blood mononuclear cells from healthy donors and observed that PA strains induce higher IL-17 levels than PH strains. We next generated PH and PA strain-specific Th17 clones and show that P. acnes strains induce Th17 cells of varied phenotype and function that are stable in the presence of IL-2 and IL-23. Although PH- and PA-specific clones expressed similar levels of LL-37 and DEFB4, only PH-specific clones secreted molecules sufficient to kill P. acnes. Furthermore, electron microscopic studies showed that supernatants derived from activated PH and not PA-specific clones exhibited robust bactericidal activity against P. acnes, and complete breaches in the bacterial cell envelope were observed. This antimicrobial activity was independent of IL-26, because both natural IL-26 released by Th17 clones and rhIL-26 lacked antimicrobial potency against P. acnes. Overall, our data suggest that P. acnes strains may differentially modulate the CD4+ T-cell responses, leading to the generation of Th17 cells that may contribute to either homeostasis or acne pathogenesis.

Regulatory T cells (Tregs) play an important role in maintaining immune tolerance and homeostasis by modulating how the immune system is activated. Several studies have documented the critical role of Tregs in suppressing the functions of effector T cells and antigen-presenting cells. Under certain conditions, Tregs can lose their suppressive capability, leading to a compromised immune system. For example, mutations in the Treg transcription factor, Forkhead box P3 (FOXP3), can drive the development of autoimmune diseases in multiple organs within the body. Furthermore, mutations leading to a reduction in the numbers of Tregs or a change in their function facilitate autoimmunity, whereas an overabundance can inhibit anti-tumor and anti-pathogen immunity. This review discusses the characteristics of Tregs and their mechanism of action in select autoimmune skin diseases, transplantation, and skin cancer. We also examine the potential of Tregs-based cellular therapies in autoimmunity.

Merkel cell carcinoma (MCC) is a rare and frequently lethal skin cancer with neuroendocrine characteristics. MCC can originate from either the presence of MCC polyomavirus (MCPyV) DNA or chronic ultraviolet (UV) exposure that can cause DNA mutations. MCC is predominant in sun-exposed regions of the body and can metastasize to regional lymph nodes, liver, lungs, bone, and brain. Older, light-skinned individuals with a history of significant sun exposure are at the highest risk. Previous studies have shown that tumors containing a high number of tumor-infiltrating T-cells have favorable survival, even in the absence of MCPyV DNA, suggesting that MCPyV infection enhances T-cell infiltration. However, other factors may also play a role in the host antitumor response. Herein, we review the impact of tumor infiltrating lymphocytes (TILs), mainly the CD4⁺, CD8⁺, and regulatory T-cell (Tregs) responses on the course of MCC, including their role in initiating MCPyV-specific immune responses. Furthermore, potential research avenues related to T-cell biology in MCC, as well as relevant immunotherapies are discussed.

Aryl polyenes (APEs) are specialized polyunsaturated outer membrane lipids that protect their producers from oxidative stress and contribute to biofilm formation. APEs are produced by an abundant biosynthetic gene cluster (BGC) family conserved across Gram-negative bacterial clades. The APE biosynthesis pathway involves 11 different enzymes and cumulates in the attachment of APEs to an anchor molecule in the Gram-negative outer membrane. Unlike most other small molecule BGCs, the APE BGC does not contain a dedicated regulatory gene that controls production of its metabolically costly compounds. Building from our prior observations of APEs role in acute oxidative stress protection, we here use a uropathogenic Escherichia coli (UPEC) strain to show that APE expression conveys a potential competitive advantage characterized by increased early-stage growth, sensitization of the bacterial oxidative stress response, and dampening of the redox stress of innate immune cells after in vitro infection. Our data indicate that APEs could act as a UPEC fitness factor, and in future work we aim to study their contribution to overall bacterial pathogenicity and survival, as well as how APEs could facilitate the transition from an oxygen poor environment such as the gut to the oxygen rich environment of the urinary tract. Importance: Bacterial pathogens use various mechanisms to achieve a competitive advantage under harsh conditions such as upon interaction with their host. We studied the function of aryl polyenes (APEs), specialized polyunsaturated fatty acids in the outer membrane, in the context of a uropathogenic E. coli strain. APE expression is induced by an oxidative environment and contributes to early-stage growth and sensitization of the oxidative stress response. Furthermore, APE-expressing E. coli dampens the intracellular oxidative milieu of target host phagocytes. These findings suggest a role for APEs as a fitness factor, and create opportunities to study their in vivo function, and explore them as a potential drug target.