An investigation of the molecular basis of interactions between human monoclonal antibodies and antigens that are clinically relevant in systemic lupus erythematosus and the Antiphospholipid Syndrome.

Abstract

Autontibodies to a wide variety of antigens are associated with systemic lupus erythematosus (SLE) and the Antiphospholipid Syndrome (APS). Previous studies have demonstrated the importance of somatic mutations and arginine residues in the complementarity determining regions (CDRs) of pathogenic anti-dsDNA and antiphospholipid antibodies. This thesis describes the study of two human monoclonal IgG antibodies, B3 (anti-DNA) and IS4 (antiphospholipid) that were derived from a patient with active SLE and primary APS respectively. I have demonstrated in-vitro expression and mutagenesis of B3 and IS4 and used this expression system to investigate the importance of the arginine residues in B3VH and IS4VH. The mutant heavy chains, as well as the wild-type VH were expressed with different light chains and the resulting antibodies assessed for binding to nucleosomes, alpha-actinin, cardiolipin (CL), phosphatidylserine (PS), beta-2-glycoprotein I (foGPI), and the N-terminal domain of p2GPI (Domain I) using direct binding assays. The results obtained have shown that the presence of arginine at position 53 in B3VH was essential but not sufficient for binding to dsDNA and nucleosomes. Conversely, the presence of this arginine reduced binding to alpha-actinin, pzGPI and Domain I of P2GPI. The fact that the arginine to serine substitution at position 53 in B3VH significantly alters binding of B3 to different clinically relevant antigens, but in opposite directions implies that this arginine residue plays a critical role in the affinity maturation of the antibody B3. Furthermore, of four arginine residues in IS4VH CDR3 substituted to serine, two at positions 100 and 100g reduced binding to all antigens, while two at positions 96 and 97 reduced binding to fcGPI but increased or decreased binding to CL and PS. Only one H/L chain combination bound neutral phospholipid and none bound dsDNA hence, these effects are particularly relevant to antigens important in APS. Therefore, my findings suggest that these four arginine residues have developed as a result of somatic mutations driven by an antigen containing both phospholipid and frGPI. These results extend our knowledge of the structure-function relationship of human anti-DNA and anti phospholipid antibodies and aid in our understanding of how these antibodies lead to pathogenicity and what we need to target in the future for possible therapies