Fetal thymic pre-T cells neither demonstrate nor develop natural killer cell activity

The postulate made by some investigators that functional NK cells are of pre-T lineage remains highly controversial. The present study tested this possibility by adoptive transfer of fetal thymocytes, rich in pre-T cells, into aged syngeneic mice which show thymic involution and low or no splenic NK activity. Splenic NK activity of aged CBA/J mice at 1 or 3 days after intravenous injection of fetal thymocytes was measured with a 4-hr chromium-release assay. Another group received saline only and a third group received either thymocytes as above or saline, and were subsequently injected with the interferon inducer, poly(I:C), 12 hr prior to removal of their spleens for the NK assay. The results revealed that reconstitution with fetal thymocytes did not restore NK activity in aged mice at 1 or 3 days after thymocyte inoculation. Poly(I:C) treatment caused an elevation of NK activity in both thymocyte-injected and control mice. The spleens of aged hosts given young adult splenocytes, on the other hand, had significantly elevated NK activity over controls. Collectively, the results demonstrate that while the adult spleen contains functional NK cells, the fetal thymus, rich in pre-T cells, does not contain functional NK cells or NK precursors as detected by maturation into functional NK cells within 3 days in the aged secondary hosts.