Structure–function analyses of a stereotypic rheumatoid factor unravel the structural basis for germline-encoded antibody autoreactivity

Abstract

Rheumatoid factors (RFs) are autoantibodies against the fragment-crystallizable (Fc) region of IgG. In individuals with hematological diseases such as cryoglobulinemia and certain B cell lymphoma forms, the RFs derived from specific heavy- and light-chain germline pairs, so-called "stereotypic RFs," are frequently produced in copious amounts and form immune complexes with IgG in serum. Of note, many structural details of the antigen recognition mechanisms in RFs are unclear. Here we report the crystal structure of the RF YES8c derived from the / germline pair, the most common of the stereotypic RFs, in complex with human IgG1-Fc at 2.8 Å resolution. We observed that YES8c binds to the CH2-CH3 elbow in the canonical antigen-binding manner involving a large antigen-antibody interface. On the basis of this observation, combined with mutational analyses, we propose a recognition mechanism common to IGHV1-69/IGKV3-20 RFs: (1) the interaction of the Leu-His region of Fc enables the highly variable complementarity-determining region (CDR)-H3 to participate in the binding, (2) the hydrophobic tip in the CDR-H2 typical of IGHV1-69 antibodies recognizes the hydrophobic patch on Fc, and (3) the interaction of the highly conserved RF light chain with Fc is important for RF activity. These features may determine the putative epitope common to the IGHV1-69/IGKV3-20 RFs. We also showed that some mutations in the binding site of RF increase the affinity to Fc, which may aggravate hematological diseases. Our findings unravel the structural basis for germline-encoded antibody autoreactivity.

COVID‐19, varying genetic resistance to viral disease and immune tolerance checkpoints

“…Broadly neutralizing antibodies against hepatitis C virus primarily use the IGHV1‐69 gene, which encodes structural features that target a key site on the virus envelope E2 protein 113–115 . However, these same IGHV1‐69 ‐encoded structural features confer polyreactivity with many other molecules, including frequent binding to the Fc domain of human immunoglobulin G as a rheumatoid factor autoantibody 116,117 . Indeed a subset of people with chronic hepatitis C virus develop IGHV1‐69 rheumatoid factors that become extremely pathogenic causing mixed cryoglobulinemic vasculitis 118,119 .…”

Immunoglobulin G structure and rheumatoid factor epitopes

“…The epitopes of RFs have been characterised in several cases and reside in the Fc part of IgG, often in the CH2/CH3 groove or the CH3/CH3 groove (Fig 1a) [9–12]. Since RFs can be measured in sera from RA patients and as the concentration of IgG in serum is roughly 5–10 times higher than the concentration of IgM and IgA [1], an apparent paradox is how the RFs can circulate in blood of patients with rheumatoid diseases alongside their antigen (IgG) in large amounts without reacting with it and being cleared from the circulation.…”

Germinal centers and autoantibodies

“…SHM is required to generate the high-affinity RhF antibodies in RA and mixed cryoglobulinemia because reversion to germline decreases RhF activity. 54,55 It is possible that the SHM events that lead to increased RhF affinity can occur outside the GC, as has been reported in RhF-transgenic mice. 56 By contrast, murine B cells carrying a humanderived RhF transgene can form GCs when alloreactive T-cell help is present.…”

“…72 A stereotypic RhF (YES8c) encoded by the heavy chain IGHV1-69 has the identical complementarity region 2 residues Leu53 and Phe54 directly contacting IgG. 55 Mutation of either residue causes a significant decrease in RhF activity. The direct contribution of Leu53 and Phe54 to HCV E2 binding and RhF activity suggests cross-reactive molecular mimicry and could explain the high frequency of RhF autoantibodies detected in chronic HCV patients.…”