Chronic Lymphocytic Leukemia Cells Recognize Conserved Epitopes Associated with Apoptosis and Catabolic Chemical Modifications

Chronic lymphocytic leukemia (CLL) is a monoclonal expansion of antigen-experienced human B cells. A key role for the B-cell antigen receptor (BCR) is suggested by the striking structural similarity among BCRs of unrelated patients. The goal of this study was to identify specific antigens or classes of antigens reacting with BCRs to better understand the role that such interactions play in the development, progression, and evolution of the disease. Since we and others recently described that CLL mAbs react with surfaces of apoptotic cells, we searched for reactivity of CLL mAbs with specific autoantigens associated with or created during apoptosis and other catabolic chemical processes. Reactivities of 28 recombinantly expressed CLL mAbs with defined autoantigens were studied using solid phase protein arrays. These experiments indicated significant reactivity with several intracellular autoantigens involved in systemic autoimmunity, some of which are displayed on the cell surface during apoptosis (e.g., Sm, Ku, snRNP A, BB¢, and C, and others). In addition, array analyses revealed reactivity with LDL modified by oxidation (oxidized-LDL) or derivatized with malondialdehyde (MDA-LDL). These data were provocative because, during apoptosis, cells can display oxidation-induced neoepitopes on lipoproteins, lipids, and proteins. In addition, mice immunized with syngeneic apoptotic cells can produce autoantibodies to oxidation-induced neoepitopes that result from conjugation to membrane molecules of metabolites of lipid peroxidation, such as MDA, 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC), and 4-hydroxynonenal (HNE). Therefore, we tested if CLL mAbs react with MDA-, POVPC-, and HNE-modified BSA. Next, we determined the extent of binding of the same mAbs with products of other metabolic processes linked to apoptosis, i.e., nitrosylation (nitrotyrosine-BSA) and glycosylation (advanced glycosylation end products, AGE-BSA). Seventeen of 28 mAbs reacted with nitrotyrosine and 9 with AGEs. All mAbs binding AGEs also interacted with MDA and all but one (16/17) nitrotyrosine-reactive mAbs bound MDA. Of note, the AGE-reactive mAbs exhibited greater reactivity with methylglyoxal- than carboxymethyllysine-modified BSA, two chemically defined AGEs. The subset of CLL mAbs reacting with apoptotic cells and neo-determinants generated by oxidation, nitrosylation, and glycosylation most frequently exhibits IGHV of minimal or no somatic mutations, However, even though the majority of CLL mAbs reactive with these epitopes were polyreactive and not somatically mutated, some unmutated mAbs failed to bind the targets and a few mutated mAbs did react. Thus, lack of somatic mutation does not necessarily confer autoantigen binding. In summary, our data suggest that CLL BCRs react with autoantigens associated with cellular apoptosis and chemical modifications of proteins and lipids that occur naturally in health and disease. The normal pre-immune repertoire is enriched in Abs reactive with the types of epitopes mentioned above. These “natural” antibodies serve housekeeping functions such as clearance of cellular debris and products of normal catabolism. They also represent the humoral arm of the innate-like B-cell immune responses, providing a first-line of defense against blood-borne pathogens. Therefore, our findings may have implications for understanding the normal cellular precursors of CLL cells.