[On-Target] AAV Targeting the Skin — Serotypes, Promoters & Dosages: All You Need in One Guide!

Skin is the body's largest organ, covering the entire external surface. It serves as a barrier against pathogens, ultraviolet radiation, chemical agents, and mechanical injury, while also sensing external stimuli, regulating temperature, and maintaining hydration. The skin is organized into three principal layers: the epidermis, dermis, and hypodermis.

The epidermis is the outermost layer and the body's first line of defense against the external environment. From the surface inward, it is further subdivided into the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The epidermis itself is avascular, deriving nutrients from the underlying dermis.

The dermis is the connective tissue layer immediately beneath the epidermis, comprising a superficial papillary layer and a deeper reticular layer that merge without a distinct boundary. It contains hair follicles, glands, blood vessels, and nerve endings.

The hypodermis (subcutaneous tissue) is the deepest skin layer, anchored to the dermis by collagen and elastin fibers. It contains adipose lobules, blood vessels, and nerves, and serves as the body's primary fat storage depot, as well as providing insulation and mechanical cushioning.

Figure 1. Schematic of skin structure (PMID: 29262154)

Skin diseases are among the most common and frequently encountered conditions in daily life, and many follow a chronic course. Psoriasis is a chronic inflammatory skin disease characterized by abnormal, rapid, and excessive proliferation of the epidermal keratinocytes, manifesting as well-demarcated or widespread scaly erythematous plaques. Acne vulgaris is another prevalent chronic inflammatory condition arising from follicular occlusion and inflammation, primarily presenting as papules, pustules, and scarring on the face and back; contributing factors include androgen excess, hypersecretion of sebaceous glands, and abnormal follicular keratinization. Vitiligo results from the destruction of epidermal melanocytes, leading to focal melanin deficiency and depigmented macules; its etiology involves autoimmunity, genetic predisposition, and neurochemical factors.

Figure 2. Pathogenesis of psoriasis (PMID: 30909615)

Research into these skin conditions frequently requires in vivo experimentation. Recombinant adeno-associated viral vectors (rAAV) have become an essential tool for in vivo gene delivery, valued for their serotype diversity, low immunogenicity, and capacity for long-term stable transgene expression — making them an ideal platform for gene therapy in skin biology.

AAV serotypes differ in their capsid proteins and, consequently, in their tropism for specific tissues and cell types. Based on published literature, AAV2, AAV5, AAV7, AAV8, AAV9, and AAVDJ have all been reported to transduce skin tissue. Among these:

• AAV5 and AAV8 transduce the epidermis with high efficiency and show moderate transduction of the dermis.

• AAV9 is widely used for general skin transduction in vivo.

• AAVDJ mediates efficient gene delivery across skin layers.

Figure 3. AAV5 and AAV8 transduction of the epidermis and dermis (PMID: 22713157)

Figure 4. Efficient skin transduction by AAVDJ (PMID: 30185909)

The skin contains a diverse array of cell types. The epidermis harbors keratinocytes, Langerhans cells, Merkel cells, and melanocytes. Dermal fibroblasts synthesize collagen and elastic fibers. The hypodermis is rich in adipocytes that store energy and provide cushioning. Cell-type-specific promoters enable precise targeting of each population.

Keratinocytes are the predominant cell type of the epidermis, accounting for approximately 95% of epidermal cells. They originate in the stratum basale, progressively migrate toward the surface as they mature and differentiate, gradually flattening and accumulating keratin, and ultimately desquamate as the stratum corneum to form the skin's protective barrier.

• K14 (encoding cytokeratin 14) and BK5 (encoding cytokeratin 5) are expressed specifically in basal layer keratinocytes. Accordingly, the K14 and BK5 promoters selectively target basal keratinocytes.

• K10 (encoding cytokeratin 10) is expressed in suprabasal, post-mitotic keratinocytes undergoing terminal differentiation. The K10 promoter therefore drives expression specifically in differentiating keratinocytes above the basal layer.

Melanocytes reside in the stratum basale of the epidermis, where they synthesize melanin — the primary pigment responsible for skin color — and protect the skin from ultraviolet radiation. The mTyr promoter (mouse tyrosinase promoter) confers melanocyte-specific transgene expression.

Figure 5. Cre recombinase expression driven by the melanocyte-specific mTyr promoter following cutaneous delivery induces melanoma formation (PMID: 31649959)

Table 1. Recommended AAV serotypes and promoters for skin-targeted gene delivery

The two most commonly used delivery methods for AAV-mediated skin gene transfer are subcutaneous (SC) injection and intradermal (ID) injection. Dosing parameters for each route in adult mice are summarized below.

Table 2. Injection routes and dosing guidelines for AAV-mediated skin gene delivery in adult mice

CellProliferation, 2022  |  PMID: 35851701

BritishJournal of Dermatology, 2024  |  PMID: 39009432

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[7] Huang Y, et al. Atypical chemokine receptor 1-positive endothelial cellsmediate leucocyte infiltration and synergize with SFRP2/ASPN⁺fibroblasts to promote skin fibrosis in systemic sclerosis. Br J Dermatol.2024;191(6):964–978. PMID:39009432