Apoptosis of the Teratocarcinoma Cell Line Tera-1 Leads to the Cleavage of HERV-K10 super(gag) Proteins by Caspases and/or Granzyme B

Redistribution, post-translational modifications and coclustering with viral antigens contribute to the immunogenicity of apoptotic cell-derived autoantigens. Almost all known targets of the humoral autoimmune response in systemic lupus erythematosus (SLE) are cleaved by caspases or granzyme B during apoptosis. Antibodies against retroviral proteins can frequently be detected in the sera of SLE patients without overt retroviral infections. These antibodies may represent cross-reactive antibodies or may have been induced by proteins encoded by endogenous retroviral sequences. We used Tera-1 cells that abundantly express a group-specific antigen of human endogenous retroviruses, HERV-K10 super(gag) polyprotein, to investigate its processing during apoptosis. Tera-1 cells induced to undergo apoptosis showed an altered HERV-K10 super(gag) processing compared with viable cells. In addition, granzyme B was able to cleave HERV-K10 super(gag) isolated from viable Tera-1 cells. Similar to nuclear autoantigens, endogenous retroviral proteins are cleaved during the execution phase of apoptosis. These post-translational modifications may result in the generation of T-cell neoepitopes or a changed epitope hierarchy of retroviral proteins. Therefore, immunogenicity of retroviral antigens in SLE patients may result from a similar mechanism as described for nuclear autoantigens.