The skin, as the first physical barrier for invading pathogens, also serves as an immunologically active organ. Breaching the skin barrier is thus essential for pathogens to enter the body. The skin contains various immune responsive cells that initiate both the innate and adaptive immune response upon invasion. Activated immune cells help to regulate cytokine response and their differentiation to promote or suppress the crucial immune response against invading pathogens. Human herpes simplex viruses (HHSVs) are the oldest pathogen that can escape immune surveillance of the human host by a well-developed escape mechanism within ganglia, as their evolutionary strategy. In primary infection, a non-specific defense of the host initiates the response against the invading virion. The initial direct antiviral action of the host is regulated by activated macrophages, via the release of cytokines like tumor necrosis factor (TNF), and type-1 interferon (IFN-1). The host-derived cytokines including IFN-12, TNF, and IFN-1 in turn induce natural killer (NK) cells to release IFN-γ. Their positive feedback with synergistic interactions collectively releases nitric oxide (NO) and reactive oxygen species (ROS) against the invading virus. Simultaneously, the combination of cytokines, macrophages, and other cells activates the immune system to eliminate the pathogen. However, the virus has also evolved various mechanisms to counter the host defense strategies. This review will highlight virus-mediated skin infections, especially by HSV, and portray a detailed role of virus-induced cytokines in host-immunity to challenge the invading virion during mucocutaneous HSV infection. Further, this review will discuss the viral-interference on host defense to provide a simplistic overview of the complications of cutaneous HSV infection.

Psoriasis is a skin disease characterized by scaly erythema, parakeratosis, and epidermal hyperplasia. Application of imiquimod (IMQ), a ligand for Toll-like receptor 7, produces a mouse model for psoriasis. IMQ application induces scaling, erythema, and thickness in skin lesions, and the symptoms are milder in interleukin-23 p19 (Il23p19)-deficient and Il17a-deficient mice than in wild-type mice; this suggests that the interleukin-23 (IL-23)/T helper 17 (Th17) axis and Th17 cell-secreting cytokines play essential roles in the IMQ-induced psoriasis model. It is notable that a genome-wide association study identified the human tyrosine kinase 2 (TYK2) gene within the psoriasis susceptibility locus. After IMQ application, mice lacking Tyk2, a mouse homologue of the human TYK2 gene, exhibited significantly lower symptom scores of psoriasis and diminished inflammatory cell infiltration in the skin lesions. Tyk2-deficient mice also failed to increase CD4⁺IL-17⁺ or CD4⁺ interferon-γ⁺ (IFN-γ⁺) T cells in the draining lymph nodes or to produce Th17 cell-related cytokines after IMQ application. Furthermore, Tyk2 deficiency led to diminished skin inflammation induced by IL-23 and IL-22 injections. These results indicate that Tyk2-mediated signals in mice contribute to multiple steps of immune and inflammatory responses during the development of psoriasis; therefore, TYK2 targeting may be a promising strategy to treat patients with psoriasis. Recent clinical trials have shown that TYK2 inhibitors have a high overall response rate with good tolerability in the management of psoriasis. This review describes the fundamental mechanisms of Tyk2 inhibition in immune/inflammatory diseases.