Apoptotic U1–70 kd is antigenically distinct from the intact form of the U1–70‐kd molecule

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Abstract
Objective To determine whether immune responses to an apoptotically modified form of a human lupus autoantigen can be distinguished from immune responses to the intact form of the same antigen. Methods Immunoblot and enzyme-linked immunosorbent assay techniques were used to test human autoimmune sera for the presence of antibodies to apoptotic forms of the U1– 70-kd small nuclear RNP antigen, while antibody recognition of intact U1–70 kd was blocked. Results Apoptosis-specific U1–70-kd antibodies were identified by immunoblot in 15 of 29 sera with antibodies to intact U1–70 kd and in 2 of 25 sera without measurable antibodies to intact U1–70 kd. Bacterially produced, purified, caspase-cleaved U1–70 kd without additional posttranslational modifications was a target of apoptosis-specific antibodies in 3 of 9 U1–70-kd–positive sera tested. Conclusion The apoptotic form of U1–70 kd displays B cell epitopes that are not displayed on the intact form of U1–70 kd. Caspase cleavage in the absence of additional posttranslational modifications is sufficient to induce the display of some of these epitopes. Immunity to apoptotically modified proteins can develop against caspase-cleaved forms or against forms that undergo additional posttranslational modification.

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