Objective To elucidate the mechanism of SSA/Ro formation after UV exposure,and its significance in the pathogenesis of photosensitive LE skin lesion.Methods Human keratinocytes(HKC)were cultured in Medium-154.After ultraviolet-B light(UVB)irradiation,the morphological change of apoptotic HKC was observed under phase contrast microscopy,the DNA fragment of apoptotic cell was de-tected by in situ nick end-labeling.And the expression of SSA/Ro was detected by indirect immunofluores-cence(IIF).As target cells,UVB irradiated keratinocytes were incubated with affinity-purified anti-SSA/Ro sera,with or without freshly prepared human sera.SSA/Ro antigen from supernatant of irradiated ker-atinocytes was also detected with an ELISA method.Results The treatment of keratinocytes with serial dos-es(52.8mJ/cm 2 ?105.6mJ/cm 2 ?158.4mJ/cm 2 ?200mJ/cm 2 ?211mJ/cm 2 )of UVB induced SSA/Ro expression on cell membrane and apoptosis,leading to apoptotic bleb formation on cell surface,which were proved con-taining SSA/Ro and ribonucleoprotein.SSA/Ro antigen expressed on UVB irradiated keratinocytes was proved to be bound to affinity purified anti-SSA/Ro sera and lead to complement-depend cytotoxicity.But no SSA/Ro in supernatant was detected.Conclusions UVB irradiation induces SSA/Ro expression on the HKC sur-face through apoptosis,and no SSA/Ro antigen was found in supernatant.The result suggests that SSA/Ro anti-gen expressed on UVB irradiated keratinocytes could be recognized and presented to immune cells by direct cell-cell contact.