Change in the humoral response of athymic nude mice with ageing.

The evolution of the serum Ig levels of Balb/c-nu/nu mice was investigated between 1 month and 12 months of age. An increase as a function of age was observed in all classes and subclasses, which was, expressed in percentage of a nu/+ serum, from 130% to 230% for IgM, from 3% to 24% for IgG1, from 12% to 164% for IgG2a, from 28% to 62% for IgG2b, and from 10% to 50% for IgA. This increase correlates with an increase of plasma cells of each class in the bone marrow, whereas the number of plasma cells in the plasma cells in the spleen, the lymph node, and the intestinal mucosa did not change markedly with age. The humoral response after an injection of heterologous erythrocytes was compared in young and aged nu/nu mice; aged mice had a higher haemagglutination titre mainly due to direct (IgM) antibodies. The response of the spleen, as judged by plaque-forming cells (PFC), was similar in young and aged mice, but the bone marrow response, not detectable in young mice, was about as high in aged nude mice as in nu/+ mice. Although the content of Thy 1 cells in the spleen and lymph nodes was markedly higher in aged than in young nude mice, no T-cell function could be detected at any age, either in the response to phytohaemagglutinin or concanavalin A or in a graft-versus-host assay. Increase in the Ig production with age is interpreted as the result of progressive priming and hyperimmunization by environmental antigens, leading to a T-independent immune response (even against antigens considered to be T-dependent) predominantly located in the bone marrow.