Transgenic mice showed a pronounced increase in CD4+ cells leading to an accumulation in the spleen and lymph nodes after 12-15 months. The number of T cells expressing the shared memory/activated CD44high CD62Llow CD45RBlow phenotype was elevated as compared to the wild-type animals. The expression of CD25, CD54, CD69, CD11b, and VLA-4 activation markers was not significantly increased in the CD4+ cells of the p65PI3K Tg mice between week 6 and month 10, suggesting that the CD4+ T cells were enriched in memory cells. In some 12-15 month old p65PI3K Tg mice, the expanding CD4+ cell population acquired a partially activated phenotype with increased expression of CD69 and CD54, but not CD25, CD11b, or VLA-4. The expansion of the CD4+ lymphocyte pool in the p65PI3K Tg mice was accompanied by an increased survival of CD4+ memory cells. Adult p65PI3K Tg mice developed a lymphoproliferative disorder with large lymphoid infiltrates in non-lymphoid organs. Flow cytometry of lung cell suspensions revealed a dominance (60%) of CD4+T cells. The majority of animals with advanced lymphoproliferative disease developed severe autoimmune glomerulonephritis that was accompanied by a polyclonal hypergammaglobulinemia with an increase in IgG1 and IgG2. Levels of anti-ds DNA autoantibodies were also elevated in the sera of p65PI3K Tg mice. No tissue destruction by T cells was observed. To examine whether PTEN counteracts the action of PI3K in cellular transformation, the ability of PTEN to inhibit p65PI3K-induced 3T3 cell transformation was examined. A focus formation assay, was used to assess the transformation of 3T3 cells. Ectopic expression of PTEN (two- to four-fold higher expression as compared to the endogenous enzyme) inhibited focus formation by p65PI3K alone or in combination with v-Raf, but did not affect foci induced by v-Raf alone or by v-Src.