The antibody response of normal adult, of young and of nu/nu BALB/c mice to bacterial levan was characterized in terms of its relative specificity for levan and for inulin and of the expression of a family of cross-reactive idiotypes (IdX) found on BALB/c myeloma proteins which bind inulin and levan. Normal adult BALB/c mice immunized with levan develop prompt antibody responses which can be detected by serum hemagglutinin assays or by enumeration of plaque-forming cells. These antibodies are specific for levan and many also bind inulin. In addition, the IdX is expressed on a substantial fraction of the anti-levan antibodies, particularly those that bind both inulin and levan. The response to levan is thymus independent in that nu/nu BALB/c mice and lethally irradiated BALB/c mice reconstituted only with bone marrow cells produce anti-levan antibodies. However, the response of nu/nu BALB/c mice to levan is unusual in that the titer of anti-inulin antibodies and of serum IdX is very low. When young mice (2 to 4 weeks of age) are immunized with levan they respond quite well, but their anti-levan antibodies also fail to bind inulin or to express the IdX. These results suggest that the unusual specificity of the response of nu/nu BALB/c mice may reflect the need for a functional thymus in the normal maturation of the thymus-independent lymphocytes which respond to levan with the secretion of antibodies that bind inulin and express the IdX.
Finally, cells secreting antibodies specific for the IdX are regularly observed in the spleens of mice immunized with bacterial levan. This is consistent with regulation of the anti-levan response by autoanti-idiotype antibodies.