A thymus-independent IgG response against dextran B512 can be induced in C57BL but not in CBA mice, even though both strains possess a VHdex gene.

CBA and C57BL mice both possess a VHdex gene coding for antibodies against the alpha 1-6 epitope of dextran B512. After immunization with dextran, CBA mice produce IgM plaque-forming cells (PFC) only, which show regular cyclical fluctuations. The second PFC peak disappeared after injection of dextranase, indicating that it is antigen-dependent. The anti-dextran response in C57BL mice is characterized by only one IgM peak, followed 1 day later by an IgG peak that may exceed the IgM response by a factor of 10. The IgG anti-dextran response in C57BL mice was thymus-independent. CBA mice gave an IgG response to the hapten fluorescein isothiocyanate (FITC) after immunization with FITC-dextran, indicating that dextran can function as a carrier for an IgG response in this strain. Attempts to induce IgG PFC against dextran by immunizing CBA mice with thymus-dependent dextran-protein conjugates consistently failed, althouth the conjugates induced IgG fc in C57BL mice. Spleen cells from CBA mice failed to produce IgG antibodies against dextran after transfer into lethally irradiated C57BL mice, whereas the C57BL spleen cells produced IgG PFC after transfer into CBA mice. The lack of IgG synthesis against the alpha 1-6 epitope of dextran in CBA mice appears to be regulated exclusively at the B cell level and is restricted specifically to the VHdex gene product.