Partial characterization of two lymphocyte-specific natural autoantibodies isolated from newborn mice.

The early antibody-repertoire is mainly composed of multi- and polyreactive IgM autoantibodies. Some of them (about 10%) also react with cell surface antigens. It is supposed that these latter antibodies promote--particularly under the maturation of the immune system--the expansion of different lymphocyte populations and subpopulations, namely the filling of the secondary lymphoid organs by immunocompetent cells. In order to verify this hypothesis we established hybridoma clones from newborn Balb/c mice. Almost all IgM antibodies produced by these hybridomas (94%) were found to be polyreactive, but only two--the 4/5 and the 8/8--react with cell surface structures. The 4/5 IgM recognizes a glicerin-phosphathydil-inositol (GPI)-anchored membrane component expressed on the early thymocytes. The 8/8 antibody reacts with resting splenic B lymphocytes as well as B and T cell blasts via an about 40 kD protein. They do not influence the spontaneous or the mitogen induced proliferation of the lymphocytes. Therefore we cannot confirm the hypothesis that increase of the expansion of peripheral lymphocytes would be the main function of these neonatal natural autoantibodies.