Linker for activation of T cells (LAT) is an adaptor molecule indispensable for development of ab and cd T lymphocytes. Surprisingly, using a new model of LAT-deficient mice we found that despite arrested thymic development, a discrete population of cells with active Lat promoter, expressing Thy1 molecules, accumulated in peripheral lymphoid organs of homozygous (Lat Inv/Inv ) mutant mice. By measuring frequencies of TCR gene rearrangements in conjunction with a panel of cell surface Ag, we dissected two subsets of these Thy1 1 cells. Thy1 dull cells expressed markers of NK lymphocytes and contained low frequency of TCR-c gene rearrangements without detectable TCR-d rearrangements. Thy1 high cells resembled immature CD44 1 CD25 1 thymocytes and contained high frequency of non-productive TCR-c and TCR-d rearrangements, indicating that cells displaying molecular signatures of commitment toward cd T-cell lineage can develop and populate lymphoid tissues of LAT-deficient mice. Phenotypically similar Thy1 high cells were also found in lymph nodes of lymphocyte-deficient (Rag2 À/À ) mice but not in T lymphocyte proficient, heterozygous Lat 1/Inv mice suggesting that Thy1 high cells of LAT-deficient mice identified in this study accumulate in peripheral lymphoid organs as a result of congenital lymphopenia.Key words: Extrathymic lymphopoiesis . LAT . Thymopoiesis . Thy1 . g/d T cells Supporting Information available online IntroductionLinker for activation of T cells (LAT) belongs to a family of transmembrane adaptor proteins involved in transduction of immunoreceptor-mediated signals [1,2].It is expressed in a number of hematopoietic cell lineages including T cells [3], pre-B cells [4], NK lymphocytes [5], megakaryocytes [6] and mast cells [7]. However, physiological roles of LAT adaptor in these cell lineages seem variable. In LAT knockout mice, NK lymphocytes, megakaryocytes and mast cells show seemingly normal development [8] with only discrete deficiencies of particular signaling pathways [5][6][7]. During B-cell maturation LAT deficiency causes a partial developmental block at the pre-B-cell stage [9,10]. In contrast, development of ab and gd T cells is entirely dependent on LAT adaptor, which is essential for signaling at the pre-TCR and ''TCR-gd quality control'' checkpoints [8,11]. Neither ab nor gd T cells have previously been found to develop and/or colonize the periphery of LAT knockout mice.In the thymus the incoming earliest CD4 À CD8 À double negative (DN) T-cell precursors progress through discrete 2596developmental stages, which may be characterized by cell surface expression of c-kit (CD117), CD25 and CD44 molecules and rearrangement status of g,. Transitions from the most immature and heterogeneous DN1 stage (c-kit high CD25 À CD44 1 ) through DN2 stage (c-kit int CD25 1 CD44 1 ) toward DN3 stage (c-kit low CD25 1 CD44 À ) are TCR independent, since only at DN3 stage most cells committed to T-cell lineages express TCR/CD3 complexes and undergo gd or b selection resulting in transition to DN4 (c-kit...