Precursor cells to CD3-intermediate (CD3int) liver mononuclear cells in the adult liver: further evidence for the extrathymic development of CD3int liver mononuclear cells.

Liver mononuclear cells (LMNC) can be divided into CD3-high positive (CD3hi), CD3-intermediate positive (CD3int) and CD3- populations. CD3int LMNC, but not CD3hi LMNC, are considered to be extrathymically derived because they are found in the liver of nude mice and adult thymectomized lethally irradiated bone marrow chimeras. CD3int LMNC express NK1.1 and show a skewed T cell receptor (TCR) V region repertoire with relatively high levels of V beta 8 and V alpha 14 expression, further suggesting that they belong to a different lineage from conventional T cells, which lack NK1.1 expression and V region skewing. Since the liver has been proposed to be a site of extrathymic T cell differentiation in adult mice, we analyzed LMNC for the presence of T cell precursors. CD3- LMNC contained CD4- CD8- cells and CD4lo CD8- cells. Reverse transcription-polymerase chain reaction analysis showed both CD3- populations express the recombination-activating gene -1, which is indispensable in gene rearrangement of TCR and expressed by thymic T cell precursors. Furthermore, when electronically sorted CD3- LMNC were cultured in medium without any feeder cells or exogenously added cytokines for 24 h, CD3int cells, but not CD3hi cells, appeared. These results suggest that the adult liver contains T cell precursors that lack the expression of the CD3/TCR complex, but are strongly committed to differentiate into extrathymic liver CD3int T cells.