T cell lineages in the thymus of lpr/lpr mice. Evidence for parallel pathways of normal and abnormal T cell development.

Autoimmune mice homozygous for the lpr/lpr (lpr) gene develop a profound lymphadenopathy resulting from the accumulation of immature Thy-1+ Lyt-2- L3T4- cells in peripheral lymphoid tissues. The source of these cells is not known although the presence of a thymus is necessary to manifest both the lymph node enlargement and the autoimmunity. For this reason and the fact that the abnormal lpr cell phenotypically resembles immature thymocytes, we studied the thymus in lpr mice. Adult lpr thymuses were found to contain an immature population phenotypically identical to the peripherally accumulating cells, including the expression of B220 and Pgp-1 antigens as well as the presence of surface T cell receptor molecules as defined by the antibody KJ16-133. Evidence is presented that some of these lpr precursor T cells are capable of intrathymic differentiation, whereas the vast majority are exported unchanged to the lymph nodes where a portion differentiate further into mature T cells. This lpr-specific lineage could be distinguished from a normal component of the lpr thymus by surface phenotype and immunohistology. The results suggest that the massive accumulation of cells in lpr lymph nodes is not so much the result of abnormal proliferation of T cells as abnormal intrathymic differentiation. In addition, a minor subpopulation of normal Lyt-2- L3T4- thymocytes was identified that resembles the phenotype of the lpr cell and similarly expresses surface T cell receptor molecules. The presence of two parallel lineages in the lpr thymus thus also provides insight into normal T cell development.