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The origin of T cells in permanently reconstituted old athymic nude rats. Analysis using chromosome or allotype markers.

Athymic PVG-rnu/rnu rats receiving a single intravenous injection of syngeneic euthymic thoracic duct lymphocytes (TDL) develop normal levels of CD4+ T lymphocytes and survive for more than 2 years in a conventional animal house. We investigated the origin of the T cells (and B cells) in reconstituted nude recipients by transferring TDL carrying either the 3T chromosome marker or the RT6b + Igk-1b allotype or the RT7b (leucocyte-common) allotype markers. Karyotype analysis of spleen and lymph node (LN) cells from 1- to 2-year-old PVG-3T/3T-reconstituted nude recipients, stimulated in vitro with phytohaemagglutinin (PHA), unexpectedly revealed that a majority (79-97%) of dividing cells were of nude origin. However, extensive nude cell division was also recorded in PHA-stimulated cultures using mixtures of euthymic (PVG-3T/3T) and unreconstituted nude spleen cells; the assumption that only T cells divide in PHA-stimulated cultures thus appears to be erroneous. In contrast to the karyotype analysis, sIg- RT6b+ LN cells obtained from nude recipients reconstituted 2 years earlier with PVG-RT6b allotype-marked TDL, were all of donor origin with no indication of a nude-derived sIg- RT6a+ population. Igk-1b+ donor B cells were not found in these same recipients. Dual fluorescence analysis of TDL from 18- to 20-month RT7b-reconstituted nudes showed that 91-100% of CD4+ cells were donor-derived. When tested functionally, sIg- RT7b+ (donor) cells, but not sIg- RT7b- (nude-derived) cells, were able to reject skin allografts and induce local graft-versus-host (GVH) responses. Donor T cells, in contrast to CD4+ cells of nude origin, divided extensively in nude recipients; FACS-purified RT7b+ (donor) TDL retransferred from 17-month primary reconstituted nude rats, expanded further (60-100-fold) in secondary nude recipients. In conclusion, only the donor-derived CD4+ cells in reconstituted nude rats displayed T-cell function; evidence to the contrary from karyotype analysis was flawed. At no stage in their life did uninjected or T-cell reconstituted nude rats develop endogenous cells that in any way resembled CD4+ products of the thymus.