Polymorphism of heavy chain immunoglobulin isotypes in the Mus subgenus. I. Limited gamma 3 polymorphism revealed by antibodies raised in SPE wild-derived inbred strain

IgG subclass and heavy chain domains contribute to binding and protection by mAbs to the poly γ-D-glutamic acid capsular antigen of Bacillus anthracis

Bacterial capsules are common targets for antibody-mediated immunity. The capsule of Bacillus anthracis is unusual among capsules because it is composed of a polymer of poly-γ-d-glutamic acid (γdPGA). We previously generated murine IgG3 monoclonal antibodies (mAbs) to γdPGA that were protective in a murine model of pulmonary anthrax. IgG3 antibodies are characteristic of the murine response to polysaccharide antigens. The goal of the present study was to produce subclass switch variants of the γdPGA mAbs (IgG3 → IgG1 → IgG2b → IgG2a) and assess the contribution of subclass to antibody affinity and protection. Subclass switch antibodies had identical variable regions but differed in their heavy chains. The results showed that a switch from the protective IgG3 to IgG1, IgG2b or IgG2a was accompanied by i) a loss of protective activity ii) a change in mAb binding to the capsular matrix, and iii) a loss of affinity. These results identify a role for the heavy chain constant region in mAb binding. Hybrid mAbs were constructed in which the CH1, CH2 or CH3 heavy chain constant domains from a non-protective, low binding IgG2b mAb were swapped into the protective IgG3 mAb. The IgG3 mAb that contained the CH1 domain from IgG2b showed no loss of affinity or protection. In contrast, swapping the CH2 or CH3 domains from IgG2b into IgG3 produced a reduction in affinity and a loss of protection. These studies identify a role for the constant region of IgG heavy chains in affinity and protection against an encapsulated bacterial pathogen.

Human IgG2 can form covalent dimers

Unlike IgA and IgM, IgG has not yet been shown to form covalent polymers. However in the presence of specific Ag, murine IgG3 has been shown to polymerize through noncovalent interactions. In contrast to the noncovalent oligomers found with murine IgG3, we have detected covalent dimers in three different recombinant human IgG2 Abs produced in myeloma cells. Both IgG2,kappa and IgG2,lambda can form dimers. In addition, analysis of pooled human gamma globulin and several normal sera revealed the presence of IgG2 dimers. The IgG2 dimers are in contrast to the noncovalent IgG dimers found in pooled sera of multiple donors resulting from idiotype/anti-idiotype (Id/anti-Id) interactions. Cyanogen bromide cleavage analysis suggests that one or more Cys residues in the gamma 2 hinge are involved in dimer assembly. The potential role of IgG2 dimers in immunity against carbohydrate Ags is discussed.

Intermolecular cooperativity: a clue to why mice have IgG3?

Mouse IgG3 subclass antibodies predominate in humoral responses to bacterial polysaccharide antigens. The reasons for this isotype restriction are not fully understood. Here, Neil Greenspan and Laurence Cooper propose that intermolecular cooperativity, a novel mechanism of antibody binding, may help to explain the preferential expression of IgG3 antibodies in these responses.

"New" allotypes of the murine gamma 1 immunoglobulins

SPE wild-derived inbred mice belonging to Mus spretus species can produce antibodies against gamma 1 isotypic determinants, common to European mice belonging to the Mus subgenus, with the exception of the Mus spretus species. They are also able to produce antibodies which define a "new" Igh-4 allotypic specificity (designated Igh-4.4).