Blockade of TNF receptor superfamily 1 (TNFR1)–dependent and TNFR1-independent cell death is crucial for normal epidermal differentiation

Background: A delicate balance between cell death and keratinocyte proliferation is crucial for normal skin development. Previous studies have reported that cellular FLICE (FADD-like ICE)-inhibitory protein plays a crucial role in prevention of keratinocytes from TNF-α–dependent apoptosis and blocking of dermatitis. However, a role for cellular FLICE-inhibitory protein in TNF-α–independent cell death remains unclear. Objective: We investigated contribution of TNF-α–dependent and TNF-α–independent signals to the development of dermatitis in epidermis-specific Cflar-deficient (Cflar^E-KO) mice. Methods: We examined the histology and expression of epidermal differentiation markers and inflammatory cytokines in the skin of Cflar^E-KO;Tnfrsf1a^+/− and Cflar^E-KO;Tnfrsf1a^−/− mice. Mice were treated with neutralizing antibodies against Fas ligand and TNF-related apoptosis-inducing ligand to block TNF-α–independent cell death of Cflar^E-KO;Tnfrsf1a^−/− mice. Results: Cflar^E-KO;Tnfrsf1a^−/− mice were born but experienced severe dermatitis and succumbed soon after birth. Cflar^E-KO;Tnfrsf1a^+/− mice exhibited embryonic lethality caused by massive keratinocyte apoptosis. Although keratinocytes from Cflar^E-KO;Tnfrsf1a^−/− mice still died of apoptosis, neutralizing antibodies against Fas ligand and TNF-related apoptosis-inducing ligand substantially prolonged survival of Cflar^E-KO;Tnfrsf1a^−/− mice. Expression of inflammatory cytokines, such as Il6 and Il17a was increased; conversely, expression of epidermal differentiation markers was severely downregulated in the skin of Cflar^E-KO;Tnfrsf1a^−/− mice. Treatment of primary keratinocytes with IL-6 and, to a lesser extent, IL-17A suppressed expression of epidermal differentiation markers. Conclusion: TNF receptor superfamily 1 (TNFR1)–dependent or TNFR1-independent apoptosis of keratinocytes promotes inflammatory cytokine production, which subsequently blocks epidermal differentiation. Thus blockade of both TNFR1-dependent and TNFR1-independent cell death might be an alternative strategy to treat skin diseases when treatment with anti–TNF-α antibody alone is not sufficient.