Ultraviolet B radiation-induced cell death: Critical role of ultraviolet dose in inflammation and lupus autoantigen redistribution

The nuclear self-Ags targeted in systemic lupus erythematosus translocate to the cell membrane of UV-irradiated apoptotic keratinocytes and may represent an important source of self-immunization. It is hard to understand how the noninflammatory milieu accompanying most apoptosis might provoke an immunogenic response leading to autoantibodies. We have found that the precise amount of keratinocyte UV exposure is crucial in determining the rate of apoptosis, the amount of inflammatory cytokine production, and the degree of autoantigen translocation. Low doses of UVB (less than or equal to 15 mJ/cm(2)) promptly induced a normal, caspase-dependent apoptosis, while intermediate doses of UV-B (35 mJ/cm(2)) caused apoptosis with altered morphology, slower DNA fragmentation, and poly(ADP-ribose) polymerase degradation accompanied by increased Bcl-2. High doses of UVB (80 mJ/cm(2)) induced instead necrosis. We observed IL-1 production upon intermediate and high UVB doses. Nuclear Ag redistribution was also markedly UV dose dependent: at low doses, Sm, Ku, and DNA translocated to the surfaces of early apoptotic cells. At intermediate doses, these Ags concentrated on the cell membrane when the nucleus was still visible. At high doses, these autoantigens diffused into the cytoplasm and were released into the, supernatant. Taken together,. the results show that low-dose UVB induces prompt noninflammatory apoptosis. In contrast, intermediate and high doses of UVB induce proinflammatory apoptosis and necrosis, where the production of inflammatory cytokines is accompanied by exposure and release of autoantigens. The key importance of the UV dose on the fate of apoptotic keratinocytes and on their potential immunogenicity should help clarify the role of UVB in inducing systemic lupus erythematosus autoimmunity.