Simplistic representation of the formation of acne comedones. alt=Three images illustrating hair follicle anatomy alt=Image illustrating a flowchart of the pathophysiology of acne Acne vulgaris is a chronic skin disease of the pilosebaceous unit and develops due to blockages in the skin's hair follicles. These blockages occur as a result of the following four abnormal processes: increased oily sebum production (influenced by androgens), excessive deposition of the protein keratin leading to comedo formation, colonization of the follicle by Cutibacterium acnes (C. acnes) bacteria, and the local release of pro-inflammatory chemicals in the skin. The earliest pathologic change is the formation of a plug (a microcomedone), which is driven primarily by excessive growth, reproduction, and accumulation of skin cells in the hair follicle. In healthy skin, the skin cells that have died come up to the surface and exit the pore of the hair follicle. In people with acne, the increased production of oily sebum causes the dead skin cells to stick together. The accumulation of dead skin cell debris and oily sebum blocks the pore of the hair follicle, thus forming the microcomedone. The C. acnes biofilm within the hair follicle worsens this process. If the microcomedone is superficial within the hair follicle, the skin pigment melanin is exposed to air, resulting in its oxidation and dark appearance (known as a blackhead or open comedo). In contrast, if the microcomedone occurs deep within the hair follicle, this causes the formation of a whitehead (known as a closed comedo). The main hormonal driver of oily sebum production in the skin is dihydrotestosterone. Another androgenic hormone responsible for increased sebaceous gland activity is DHEA-S. The adrenal glands secrete higher amounts of DHEA-S during adrenarche (a stage of puberty), and this leads to an increase in sebum production. In a sebum-rich skin environment, the naturally occurring and largely commensal skin bacterium C. acnes readily grows and can cause inflammation within and around the follicle due to activation of the innate immune system. C. acnes triggers skin inflammation in acne by increasing the production of several pro-inflammatory chemical signals (such as IL-1α, IL-8, TNF-α, and LTB4); IL-1α is essential to comedo formation. C. acnes ability to bind and activate a class of immune system receptors known as toll-like receptors (TLRs), especially TLR2 and TLR4, is a core mechanism of acne-related skin inflammation. Activation of TLR2 and TLR4 by C. acnes leads to increased secretion of IL-1α, IL-8, and TNF-α. The release of these inflammatory signals attracts various immune cells to the hair follicle, including neutrophils, macrophages, and Th1 cells. IL-1α stimulates increased skin cell activity and reproduction, which, in turn, fuels comedo development. Furthermore, sebaceous gland cells produce more antimicrobial peptides, such as HBD1 and HBD2, in response to the binding of TLR2 and TLR4. C. acnes also provokes skin inflammation by altering the fatty composition of oily sebum. Oxidation of the lipid squalene by C. acnes is of particular importance. Squalene oxidation activates NF-κB (a protein complex) and consequently increases IL-1α levels. Additionally, squalene oxidation increases 5-lipoxygenase enzyme activity, which catalyzes the conversion of arachidonic acid to leukotriene B4 (LTB4). LTB4 promotes skin inflammation by acting on the peroxisome proliferator-activated receptor alpha (PPARα) protein. PPARα increases the activity of activator protein 1 (AP-1) and NF-κB, thereby leading to the recruitment of inflammatory T cells. C. acnes ability to convert sebum triglycerides to pro-inflammatory free fatty acids via secretion of the enzyme lipase further explains its inflammatory properties. These free fatty acids spur increased production of cathelicidin, HBD1, and HBD2, thus leading to further inflammation. This inflammatory cascade typically leads to the formation of inflammatory acne lesions, including papules, infected pustules, or nodules. If the inflammatory reaction is severe, the follicle can break into the deeper layers of the dermis and subcutaneous tissue and cause the formation of deep nodules. The involvement of AP-1 in the aforementioned inflammatory cascade activates matrix metalloproteinases, which contribute to local tissue destruction and scar formation. Along with the bacteria C. acnes, the bacterial species Staphylococcus epidermidis (S. epidermidis) also takes a part in the physiopathology of acne vulgaris. The proliferation of S. epidermidis with C. acnes causes the formation of biofilms, which blocks the hair follicles and pores, creating an anaerobic environment under the skin. This enables for increased growth of both C. acnes and S. epidermidis under the skin. The proliferation of C. acnes causes the formation of biofilms and a biofilm matrix, making it even harder to treat the acne.
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