Ultraviolet-radiation-induced keratinocyte apoptosis in C1q-deficient mice

Exposure to ultraviolet B radiation is an important trigger of both systemic and cutaneous disease flares in individuals with systemic lupus erythematosus. More than 90% of individuals with homozygous C1q deficiency develop a systemic-lupus-erythematosus-like illness, which is typically associated with a severe photosensitive rash. Apoptotic, human keratinocytes have been shown in vitro to bind C1q, in the absence of antibody. These observations, together with the hypothesis that a major source of the autoantigens driving the immune response in systemic lupus erythematosus comes from apoptotic cells, led us to investigate the effects of murine C1q deficiency on ultraviolet-radiation-induced keratinocyte apoptosis in vivo. In this work, we demonstrated C1q binding to apoptotic murine keratinocytes in vitro and showed for the first time that C1q is also present on sunburn cells in vivo. In addition to C1q, we detected C3 deposition on sunburn cells in both wild-type and C1q-deficient mice, suggesting activation of the alternative pathway. Following acute ultraviolet exposure in vivo, no difference in the rate of clearance of sunburn cells was found in C1q-deficient mice from three different genetic backgrounds, compared with strain-matched wild-type controls. Furthermore, chronic ultraviolet exposure did not result in the production of autoantibodies or the development of glomerulonephritis. Our findings suggest that C1q does not play a critical role in the physiologic clearance of apoptotic murine keratinocytes in vivo.