Molecular Basis of 9G4 B Cell Autoreactivity in Human Systemic Lupus Erythematosus

Abstract

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.

Survival of human lymphoma cells requires B-cell receptor engagement by self-antigens

“…3C). The germ-line-encoded V H 4-34 sequence confers intrinsic autoreactivity, which is directed against self-antigens on the surface of red blood cells in cold agglutinin disease (32), or B cells in systemic lupus erythematosis (SLE) (33). This autoreactivity requires two motifs within the FR1, Q 100% is scaled to the degree of rescue by re-expression of the endogenous IgH V region and 0% is scaled to the degree of rescue by an empty vector.…”

Redemption of autoantibodies on anergic B cells by variable-region glycosylation and mutation away from self-reactivity

“…hydrophobic patch in framework region 1 not present in other IGHV4 family elements, is independent of complementaritydetermining region (CDR)3 H or light-chain sequence, and is abolished if the AVY residues are individually mutated (11)(12)(13)(14) (Fig. 1A).…”

Absence of surrogate light chain results in spontaneous autoreactive germinal centres expanding VH81X-expressing B cells

“…In humans, usage of the autoreactive 9G4 idiotype in the naive B-cell pool of SLE patients is not different from that of healthy individuals and RA patients 55,56 . However, owing to its expansion in GCs, and exit into the memory B and long-lived plasma cell pools, 9G4 autoantibodies are abundant in SLE patients, whereas in healthy subjects and RA patients B cells expressing the 9G4 idiotype are not even allowed to enter GCs 55 .…”