Molecular basis of 9G4 B cell autoreactivity in human systemic lupus erythematosus

9G4⁺IgG Abs expand in systemic lupus erythematosus (SLE) in a disease-specific fashion and react with different lupus Ags including B cell Ags and apoptotic cells. Their shared use of VH4-34 represents a unique system to understand the molecular basis of lupus autoreactivity. In this study, a large panel of recombinant 9G4⁺mAbs from single naive and memory cells was generated and tested against B cells, apoptotic cells, and other Ags. Mutagenesis eliminated the framework-1 hydrophobic patch (HP) responsible for the 9G4 idiotype. The expression of the HP in unselected VH4-34 cells was assessed by deep sequencing. We found that 9G4 Abs recognize several Ags following two distinct structural patterns. B cell binding is dependent on the HP, whereas anti-nuclear Abs, apoptotic cells, and dsDNA binding are HP independent and correlate with positively charged H chain third CDR. The majority of mutated VH4-34 memory cells retain the HP, thereby suggesting selection by Ags that require this germline structure. Our findings show that the germline-encoded HP is compulsory for the anti-B cell reactivity largely associated with 9G4 Abs in SLE but is not required for reactivity against apoptotic cells, dsDNA, chromatin, anti-nuclear Abs, or cardiolipin. Given that the lupus memory compartment contains a majority of HP⁺VH4-34 cells but decreased B cell reactivity, additional HP-dependent Ags must participate in the selection of this compartment. This study represents the first analysis, to our knowledge, of VH-restricted autoreactive B cells specifically expanded in SLE and provides the foundation to understand the antigenic forces at play in this disease.