IgG depletion was confirmed by electrophoresis (data not shown). molecule expressed in the skin lesions caused by injected lupus serum. Our studies demonstrated that lupus serum IgG causes skin injury by involving the TNFR1 signaling pathway and monocyte differentiation to DCs. Accordingly, disruption of the TNFR1-mediated signaling pathway and blockade of DC generation may prove to be of therapeutic value in Mirk-IN-1 patients with cutaneous lupus erythematosus. Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by high levels of autoantibody and multiorgan tissue damage, including kidney and skin (1, 2). Skin is involved in three quarters of patients with SLE (3), whereas, in one fifth, skin lesions represent the first manifestation (4). Cutaneous lupus erythematosus (LE) has been reported as the third most common cause of industrial disability among dermatological diseases (5). The molecular and cellular mechanisms involved in the expression of cutaneous LE remain unclear (3, 6, Mirk-IN-1 7). There are three clinical subtypes of cutaneous LE: acute cutaneous LE (ACLE), subacute cutaneous LE, and chronic cutaneous LE (also known Mirk-IN-1 as discoid LE). Localized ACLE is often referred to as the malar rash or butterfly rash of SLE, whereas generalized ACLE is frequently referred to as the SLE rash (8). They share the following features of lichenoid tissue reaction: hyperkeratosis; epidermal atrophy; liquefactive degeneration of the epidermal basal cell layer; mononuclear cell infiltrates focused at the dermo-epidermal junction, perivascular areas, and perifollicular areas; thickening of the basal membrane; and melanin pigment incontinence. ACLE typically presents abruptly in the context of a systemic disease, and almost all patients develop SLE (9). Production of ATP1A1 autoantibodies directed against nuclear Ags and myriad other autoantigens characterize SLE. The cellular debris released as a result of apoptosis represents the source of nuclear and cytoplasmic Ags in SLE (2). Also, during apoptosis, Ags that are normally buried within the cells are exposed on the cell surface, and they may trigger an immune response (6). In the skin, the invariably observed keratinocyte apoptosis may result in the cell surface expression and release of self-Ags, including DNA, Ro/Sj?gren syndrome A, La/Sj?gren syndrome B, and histones (10). Although exposure to UV light and other environmental triggers may contribute to the initiation of cutaneous LE by triggering apoptosis of keratinocytes, it is unclear how the inflammatory process begins and is sustained (10). IFN- was reported to be important in the pathogenesis of cutaneous LE (11), and increased numbers of IFN-producing plasmacytoid dendritic cells (pDCs) have been found in skin lesions (12, 13). Tissue damage in SLE is associated with autoantibody production and immune complex formation and deposition (2). Anti-dsDNA Ab has been linked to kidney (2) and brain pathology (14). Anti-Ro Abs are associated with photosensitive skin disease (3) and have been found deposited in the skin of patients with SLE (15). It was proposed that they bind to keratinocytes undergoing apoptosis and contribute to the inflammatory process (2). These observations indicate that autoantibodies may play an important Mirk-IN-1 role in the expression of cutaneous lesions in SLE; however, the cellular and cytokine requirements for the initiation and propagation of skin injury have not been investigated. We report that skin inflammation develops in normal mice following intradermal injection of serum from patients with SLE and lupus prone-mice but not from healthy controls and normal mice. The development of skin lesions required the presence of functional TNFR1 and the differentiation of monocytes into dendritic cells (DCs). The identified cytokine and cellular components of the lupus serum-induced skin lesions strongly suggest the exploitation of agents that block TNFR1 signaling and monocyte differentiation in the treatment of skin lesions in patients with SLE. Materials and Methods Mice and reagents The mice (C57BL/6, BALB/c, Swiss, MRL/mice of different ages and normal C57BL/6 mice. Animal- and human-use protocols were approved by appropriate Beth Israel Deaconess Medical Center committees. Injection protocol and cell-depletion procedures Lupus serum or CpG oligonucleotide (CpG DNA) was injected intradermally in the back of the neck of different mice strains. Macrophage depletion was accomplished by s.c. injection.