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-alpha-dependent apoptosis and blocking of dermatitis. However, a role for cellular FLICE-inhibitory protein in TNF-alpha-independent cell death remains unclear. Objective: We investigated contribution of TNF-alpha-dependent and TNF-alpha-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(1/2) and Cflar(E-KO); Tnfrsf1a(-/-) mice. Mice were treated with neutralizing antibodies against Fas ligand and TNF-related apoptosis-inducing ligand to block TNF-alpha-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-alpha antibody alone is not sufficient.