Structural characterization of human monoclonal cold agglutinins: Evidence for a distinct primary sequence‐defined VH4 idiotype

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Abstract
Cold agglutinins that bind the developmentally regulated I red cell determinant occur naturally among human monoclonal IgM proteins. These autoantibodies are known to use light chains that derive mainly from the minor kappa III (xIII) variable region subgroup. The xIII subgroup is also highly expressed in monoclonal rheumatoid factors. However, while most monoclonal rheumatoid factors use structurally homologous heavy chains that derive from the VH1 family, information regarding the structure of the cold agglutinin heavy chains remains fragmentary. We demonstrate here that the xIII cold agglutinin autoantibodies exclusively use heavy chains that derive from the VH4 family. Furthermore, these autoantibody heavy chains all express the same primary sequence‐defined idiotype, corresponding to the second hypervariable region. These data indicate that cold agglutinins use a remarkably homogeneous subset of heavy chain variable regions. Moreover, unique patterns of preferential VH and VL pairing clearly distinguish the anti‐I cold agglutinins from all other known monoreactive autoantibodies.