Characterization of the specificity of a naturally-occurring monoclonal anti-thymocyte autoantibody derived from an unimmunized, neonatal Balb/c mouse.

The immune repertoire of healthy unimmunized Balb/c mice contains a significant proportion of B lymphocytes which produce natural autoantibodies. The majority of these predominantly CD5+ B lymphocytes, secrete autoantibodies which react with conserved intracellular autoantigens such as actin, myosin and DNA. Significantly fewer autoreactive B lymphocytes produce natural autoantibodies reactive with cell surface autoantigens. In the present study, the specificity of monoclonal IgM kappa anti-thymocyte autoantibodies from hybridoma NMT-1 (NMT-1 maAbs), derived from the spleen of an unimmunized 8-day-old inbred Balb/c mouse has been examined. Anti-thymocyte NMT-1 maAbs reacted with cell surface molecules on 86-87% thymocytes from mice 1-28 days of age. Thymus-restricted expression of the identified autoantigen was demonstrated by the lack of detectable reactivity of NMT-1 maAbs to cell surface molecules of Balb/c mouse splenocytes, PBLs, lymph node, peritoneal and bone marrow cells and tissues including brain, liver and kidney. Furthermore, multiparameter flow cytometry demonstrated an association between the expression of the cell surface autoantigen identified by NMT-1 maAbs and thymocyte maturation as 94-97% of the CD4+ CD8+ thymocytes expressed the identified autoantigen which was largely absent from CD3+ thymocytes and not expressed in the peripheral immune system. Tissue distribution, flow cytometry and competition analysis indicated differences between identified T lymphocyte markers, including Thy-1, and the autoantigen identified by NMT-1 maAbs in this study. Immunoprecipitation analysis, however, revealed that NMT-1 maAbs reacted with 14.5 and 18.3 kDa Thy-1-related autoantigens within Balb/c mouse thymocyte membrane extracts, possibly unique glycosylated forms of the Thy-1 molecule.