Our results demonstrate that P60 PLAD but not P80 PLAD protein can suppress skin injury in lupus MRL/lpr mice. The fact that skin lesions express TNFR1 rather than TNFR2 may explain the mitigating effect of the TNFR1 inhibitor P60 PLAD. Our findings are in agreement with previous reports that TNF inhibition does not affect systemic autoimmunity, or it may, as it has been observed in patients receiving anti-TNF treatment (20
), instigate systemic autoimmunity manifested by the appearance of anti-nuclear antibodies. Treatment of MRL/lpr mice with P60 or P80 PLAD proteins did not improve systemic autoimmunity, but instead, it appeared to provoke a tendency towards worsening. Treatment with P60 PLAD did not even affect the deposition of immunoglobulin to the skin of MRL/lpr mice despite the fact that the inflammatory process was suppressed. This antithetic effect signifies the independence of local organ injury form systemic autoimmunity and calls for attention to local events which may execute tissue damage and which may become targets of treatment.
Keratinocyte apoptosis is a histologically recognizable feature of skin lesions in patients with lupus. It has been well established that apoptotic material may flood the immune system with autoantigens which may promote the appearance of high levels of autoantibodies (21
). On the other hand, since Fas-induced apoptosis is defective in MRL/lpr mice, blocking of TNF-α-induced apoptosis(5
) may enhance severity of defective apoptosis in MRL/lpr mice. TNFR1 represents the main receptor in the TNF-α-induced apoptosis (5
) and although TNFR2 lacks a death domain it activates NF-κB (5
) and is involved in T cell apoptosis, especially CD8+ T cells (22
). Thus, blockade of TNFR1 or TNFR2 using P60 PLAD or P80 PLAD may enhance defective apoptosis in T cells and promote the production of autoantibodies in MRL/lpr mice. This data indicates that TNF may protect against nephritis by reducing the production of autoantibodies and by promoting T cell apoptosis in MRL/lpr mice.
Tissue damage in SLE is characterized by autoantibody production and immune complex formation and deposition (2
). Antibody-nucleosome complexes and IgG have been found deposited in the renal glomerular basement membrane in lupus nephritis in humans and mice (23
). These immune complexes activate complement and initiate the glomerulonephritis which has been demonstrated in animal models (26
). The anti-dsDNA and anti-nucleosome antibodies cross-react with proteins such as α-actinin in the kidney and may have a direct pathogenic effect on renal cells (28
). The absence of TNFR1 in the Fas deficient mouse resulted in greatly accelerated glomerulonephritis and production of IgG and anti-dsDNA antibodies (30
). Although the levels of anti-dsDNA antibody in mice treated with P60 or P80 PLAD did not rise significantly more than in mice treated with PBS, both levels of IgG and anti-dsDNA antibody displayed a trend towards higher values (). It is possible that other autoantibodies including anti-histone, anti-phospholipid antibodies might be elevated in MRL/lpr mice treated with P60 or P80 PLAD protein and make contribution to development of proteinuria and kidney damage.
DCs are important resident cells in skin and the first to react to pathogen in the skin inflammation (13
). Activated DCs release cytokines and move to the draining lymph nodes where mature DCs activate T cells (14
). Monocytes from blood vessels are thought to be an important resource of DCs (15
). Serum from lupus patients promotes monocyte differentiation into DCs (9
) and serum IgG from MRL/lpr mice was shown here to induce monocyte differentiation into DCs. Although IFN-α has been shown to exert important role in lupus patient serum-induced monocyte differentiation into DCs (9
), our results demonstrate that lupus MRL/lpr serum-induced monocyte differentiation into DCs also requires TNFR1 signaling. Blocking TNFR1 signaling by P60 PLAD protein inhibited lupus MRL/lpr serum –induced monocyte differentiation into DCs. Lupus MRL/lpr serum-induced monocyte differentiation into DCs was markedly reduced in monocytes from TNFR1 but not TNFR2 knockout mice. TNFR1 has been shown to be required for differentiation of follicular DCs (31
). TNFR1 knockout mice are resistant to TNF-α-induced skin inflammation (32
In conclusion, this study shows that TNFR1 is important for the expression of skin lesion in the lupus prone MRL/lpr mouse. Blockade of TNFR1 signaling mitigates skin lesions despite the fact that it does not affect skin immunoglobulin deposition. In agreement with previous information and clinical experience, TNF inhibition with PLAD proteins displayed a tendency towards aggravated systemic autoimmunity and kidney pathology. The value of our results lies with the establishment of the importance of local factors in the expression of local organ injury and the possibility of considering the application of local TNFR1 inhibitors in the treatment of lupus skin lesions.