Effects of UVB on the Synthesis of Complement Proteins by Keratinocytes

UVB exposure of the skin results in increased production of several cytokines by keratinocytes and infiltration of inflammatory cells. We hypothesized that UVB may increase the expression of complement (C) components and C-regulatory proteins by keratinocytes. In vivo , UVB may upregulate these proteins by direct effects or via cytokines released by keratinocytes or infiltrating inflammatory cells. In vitro , UVB may upregulate these proteins only directly, because of dilution of released cytokines in the medium. To test this, we exposed cultured human keratinocytes to UVB (0–64 J per m 2 ) and monitored C3 and Factor B release in the medium by enzyme-linked immunosorbent assay, and surface expression of decay accelerating factor, membrane cofactor protein, and CD59 by flow cytometry. Keratinocytes produced small amounts of C3 and Factor B, which remained unaffected by UVB. UVB (32 J per m 2 ) caused a transient upregulation of all three C-regulatory proteins. Decay accelerating factor expression was maximal at 48 h (1.81 ± 0.06-fold increase in mean fluorescence intensity over nonexposed cells), membrane cofactor protein at 72 h (2.13 ± 0.09-fold increase in mean fluorescence intensity), and CD59 at 120 h (1.96 ± 0.09-fold increase in mean fluorescence intensity), returning to baseline values within 96, 192, and 192 h, respectively. Exposure to 64 J per m 2 resulted in significant cell death; cells surviving this dose up to 48 h expressed a higher level of all the three proteins than those surviving 32 J per m 2 . In conclusion, UVB upregulated membrane cofactor protein, decay accelerating factor, and CD59 on keratinocytes without affecting the constitutive release of C3 and Factor B. Thus, UVB can increase the resistance of keratinocytes against their own C known to be produced excessively in response to cytokines of inflammatory cells that infiltrate the skin following UVB exposure.