Modulation of fine specificity of anti-DNA antibody by CDR3L residues

The light chain of 2C10, an anti-double stranded DNA (dsDNA) autoantibody, is not favorable for DNA binding and it was suggested that the light chain might modulate the specificity of the antibody in DNA binding. We studied several mutant scFvs expressing mutated VL and normal VH of 2C10 to explore the role of the light chain in determining the fine specificity of the antibody, which we define as the preferential binding to a specific sequence of bases or a helical conformation compared to dsDNA from calf thymus. The wild-type Fab and scFv of 2C10 bind to poly(dA-dC).(dG-dT) better than to dsDNA. However, in the absence of the light chain domain, the VH domain bound dsDNA better than poly(dA-dC).(dG-dT), indicating the possible involvement of the light chain in determining the fine specificity in DNA binding. The mutations we studied were located in either CDR1L or CDR3L of the antibody. The CDR1 mutants, D28A, D30A, D31A, and D32A have been previously shown to cause an increase in the affinity of 2C10 scFv to DNA. The fine specificity of 2C10 was not affected by the CDR1 mutants which bound to poly(dA-dC).(dG-dT) better than dsDNA. However, CDR3L mutants, D92A and N93A, which had been shown to be involved in direct interaction with DNA, preferred dsDNA to poly(dA-dC).(dG-dT) in their binding. Our results indicate that the fine specificity of 2C10 in DNA binding is modulated primarily by Asp at 92 and Asn at 93 in CDR3L. The effects of CDR1L mutations indicate that this region affects only the affinity but not the fine specificity of 2C10.