The role of hematogenous and intrinsic precursor cells in lymphocyte production in murine bone marrow and thymus

It is well recognized that the bone marrow contains cells that can repopulate a depleted thymus as well as cells that can be induced to express phenotypic markers characteristic of T cells. It is not known, however, to what extent thymocytopoiesis in the normal thymus relies on immigrant, bone marrow-derived cells, nor whether some T cell precursors have entered the bone marrow from the circulation. We used the parabiotic system to test whether thymocytopoiesis relies on progenitors intrinsic to the thymus or on cells that enter the organ from the circulation. In the same system, we have also investigated whether Thy-1- bone marrow lymphocytes that respond to phytohemagglutinin (PHA) by proliferation and Thy-1 expression are produced by myelogenous or hematogenous progenitors. Syngeneic CBA/HT6 and CBA/CaJ mice were joined in parabiotic union at 4-6 weeks of age. Cross circulation between the two partners was verified by the equilibration of Evans' blue dye injected into one partner and by the equilibration of PHA-responsive T cells in the spleen of the parabionts. Chromosome spreads were prepared from the PHA-stimulated T cell-depleted bone marrow and from spontaneously proliferating thymocytes as well as from thymocytes stimulated by PHA or Concanavalin A (Con A). The exchange of spleen colony-forming units (CFU-S) in the femoral marrow was assessed by karyotyping individual spleen colonies. Regardless of the length of parabiotic union, ranging from 4 to 20 weeks, Thy-l-, PHA-responsive bone marrow lymphocytes remained predominantly of the host type with only 3% being derived from the opposite partner. The same held true for CFU-S in the femur; only around 5% of this cell population were of the nonhost type. Thus, although some Thy-1-, PHA-responsive lymphocytes in the bone marrow may be derived from hematogenous stem cells, the majority of them are generated by precursors resident in the bone marrow. Likewise, regardless of the length of parabiotic union, at least 95% of spontaneously proliferating cells in the thymus of each partner possessed the karyotype of the host, and this held true also for PHA- or Con A-stimulated thymocytes, indicating that the small population of spontaneously proliferating immigrant cells cannot account for the production of the large number of postmitotic (mitogen-responsive) thymocytes. Our findings, therefore, demonstrate a high degree of self-maintenance for Thy-1-, PHA-responsive lymphocytes in the bone marrow and also for intrathymic T cell precursors. In the unperturbed, postnatal thymus, thymoctye production does not rely on cell input from the circulation; the vast majority of thymoctyes are generated by an intrathymic precursor pool that is independent of immigrant myelogenous T cell precursors.