Anti-phospholipid antibodies (aPL) and apoptosis: prothrombin-dependent aPL as a paradigm for phospholipid-dependent interactions with apoptotic cells

Abstract

The natural targets of anti-phospholipid antibodies (aPL) and the stimuli that induce them remain unknown. Apoptotic cells have been proposed as both potential targets and immunogens for antiphospholipid antibodies. Demonstration of selective recognition by anti-phospholipid antibodies provides support for apoptotic cells as antigenic targets. Here, we summarize data showing that prothrombin (PT) binds to apoptotic, but not viable, cells, and that apoptotic-cell bound prothrombin provides a target for human polyclonal and murine monoclonal lupus anticoagulant (LA) antibodies. We discuss findings for two monoclonal lupus anticoagulant antibodies that have high (antibody 29J3-62) or low (antibody 29I4-24) affinity, respectively, for soluble prothrombin. Despite their very different affinities for soluble prothrombin, both monoclonal antibodies reacted similarly with prothrombin bound to phospholipid or apoptotic cells. Furthermore, both antibodies enhanced the binding of prothrombin to apoptotic cells. We propose that the recognition of apoptotic cells by these prothrombin-dependent monoclonal antibodies provides a paradigm for other anti-phospholipid autoantibodies. 29I4-24 is prototypical of phospholipid-dependent antiphospholipid antibodies, while 29J3-62 represents a prototype for phospholipid-independent antiphospholipid antibodies. Proteins such as prothrombin and β2-glycoprotein I (β2GPI) bind to apoptotic cells, thereby enhancing the recognition of apoptotic cells by anti-phospholipid antibodies. Furthermore, anti-phospholipid antibodies potentiate the interaction of these proteins with apoptotic cells. While it is unclear whether apoptotic cells are the inducing stimuli in patients with anti-phospholipid antibodies or even whether anti-phospholipid antibodies interact with apoptotic cells in vivo, it is nonetheless clear that anti-phospholipid antibodies have the potential to affect both the procoagulant activity and the uptake and clearance of apoptotic cells.

Do antiphospholipid antibodies develop for a purpose?

“…This is consistent with the data that these antibodies bind best to oxidized surfaces, since anionic PL are normally absent from the surface of cell membranes but redistribute to the outer membrane during apoptosis [23]. Two autoantibodies have been identified which actually enhance the binding of prothrombin to apoptotic cells [25]. These authors hypothesize that when antiphospholipid targeted proteins such as prothrombin and B2 bind to apoptotic cells, they serve to enhance the recognition of apoptotic cells by antiphospholipid antibodies and enhance their interactions [25].…”

“…Some studies suggest that prothrombin and B2 preferentially bind to apoptotic cells as opposed to resting, healthy cells [23,25], and that autoantibodies only target prothrombin when it is bound to apoptotic cells [25]. This is consistent with the data that these antibodies bind best to oxidized surfaces, since anionic PL are normally absent from the surface of cell membranes but redistribute to the outer membrane during apoptosis [23].…”

“…Apoptotic cells are known to be targets for antiphospholipid antibodies [25], which suggest that natural antiphospholipid antibodies play a significant role beneficial to the host during immune clearance. However, in patients who have defects in the immune clearance process it is possible that a resultant overproduction and/or loss of restrictions on antiphospholipid antibodies might lead to the evolution of a more pathologic autoantibody with pathologic consequences (ie, the antiphospholipid syndrome).…”

“…Two autoantibodies have been identified which actually enhance the binding of prothrombin to apoptotic cells [25]. These authors hypothesize that when antiphospholipid targeted proteins such as prothrombin and B2 bind to apoptotic cells, they serve to enhance the recognition of apoptotic cells by antiphospholipid antibodies and enhance their interactions [25]. This provides further evidence that antiphospholipid antibodies are part of the beneficial natural autoantibody repertoire with the potential to beneficially affect the uptake and clearance of apoptotic cells [25].…”

“…These authors hypothesize that when antiphospholipid targeted proteins such as prothrombin and B2 bind to apoptotic cells, they serve to enhance the recognition of apoptotic cells by antiphospholipid antibodies and enhance their interactions [25]. This provides further evidence that antiphospholipid antibodies are part of the beneficial natural autoantibody repertoire with the potential to beneficially affect the uptake and clearance of apoptotic cells [25]. If there are defects in that clearance process, again, this might serve as a trigger for breaking the restrictions on somatic hypermutation of these natural autoantibodies, leading to more diversity in the binding capacities of these antibodies, and to the generation of pathologic structures.…”