Apoptotic splenocytes drive the autoimmune response to poly(ADP-ribose) polymerase 1 in a murine model of lupus

Although defects in apoptosis have been linked to both human and murine lupus, the exact mechanisms remain unknown. Moreover, it is not clear whether such defects are primary or secondary events in disease pathogenesis. To address these issues, we used an induced model of murine lupus, the parent-into-F(1) model of chronic graft-versus-host disease (cGVHD) in which a lupus-like phenotype highly similar to human systemic lupus erythematosus is reliably induced in normal F(1) mice. We addressed the role of nuclear Ags modified by caspases during apoptosis as potential targets of the autoantibody response and our results identify poly(ADP-ribose) polymerase 1 (PARP-1) as a frequently targeted autoantigen. Additional proteins cleaved during apoptosis were also targeted by the immune response. Importantly, female mice exhibited significantly greater numbers of apoptotic cells in germinal centers and higher serum anti-PARP-1 Ab levels compared with male cGVHD mice. Serum anti-PARP-1 levels in male cGVHD mice could be elevated to levels comparable to those of female cGVHD mice by the injection of apoptotic syngeneic F(1) splenocytes early in the disease course. These results provide a mechanism by which lupus autoantibodies target apoptotic molecules. Specifically, T cell-driven polyclonal B cell activation characteristic of systemic lupus erythematosus is sufficient to saturate otherwise normal apoptotic clearance mechanisms, permitting apoptotic material to accumulate, serve as autoantigens, and drive autoantibody production.