Mice homozygous for lpr or gld develop autoimmunity and progressive lymphoproliferative disease characterized by the accumulation of two unusual populations of B220+ TCR-alpha beta+ T cells, a predominant CD4-CD8- double-negative (DN) subset and a minor CD4dull+ subset. B220+ DN T cells appear to be derived from negatively selected thymocytes, but their immediate precursors have not been identified conclusively, and their relationship to CD4+B220+ T cells is unclear. Our previous studies of lpr and gld mice treated chronically with anti-CD8 mAb provided evidence that the majority of B220+ DN T cells are unrelated to CD4+B220+ T cells and may be descended from peripheral thymus-derived CD8+ T cells. To investigate the contributions of MHC class I-selected thymus-derived T cells to the production of B220+ DN T cells and to the accumulation of CD4+ T cell subsets, we studied C3H-lpr and -gld mice rendered deficient in CD8+ T cells by the introduction of disrupted beta 2-microglobulin (beta 2-m) genes. These CD8+ T cell-deficient mice developed massively enlarged lymph nodes, in which CD4+B220+ T cells and CD4+ T cells replaced B220+ DN T cells as the dominant T cell subsets. As a population, the CD4+B220+ T cells were depleted of autoreactive populations specific for endogenous retroviral superantigens and were enriched for V beta 8.3+ T cells. The deficiency of CD8+ T cells in beta 2-m(-/-)-lpr mice had no effects on the accumulation of primed CD4+ T cells or autoreactive B cells. The selective reduction in B220+ DN T cells and corresponding accumulation of CD4+B220+ T cells in beta 2-m(-/-)-lpr mice provide strong evidence that 1) the majority of B220+ DN T cells are unrelated to CD4+ T cells and their development and/or accumulation is dependent on MHC class I expression; and 2) CD4+B220+ T cells are a remarkably similar, but separate, lineage of cells that develop independently of thymus-derived CD8+ T cells and class I MHC expression.