T cell repopulation from functionally restricted splenic progenitors: 10,000-fold expansion documented by using limiting dilution analyses

Mice depleted of T cells by thymectomy, lethal irradiation, and reconstitution with Thy-1-depleted syngeneic bone marrow were given graded doses of splenic T cells to see whether post-thymic cells had the ability to regenerate immune function in these hosts. Using limiting dilution methods to estimate the number of antigen- and mitogen-responsive cells in recipients 12 to 20 wk after reconstitution, we found that new helper and cytotoxic precursor cells were produced, but attained levels only 10 to 20% of normal. Because these repopulated mice were able to produce nearly normal levels of helper and cytotoxic activity in conventional, high density cultures, despite their relative paucity of precursors, we infer that their normal function in conventional assays may reflect a balanced deficiency of effector and regulatory cell types. Surface phenotyping of the progenitor cells responsible for repopulation showed that Lyt-2- cells were required for helper cell regeneration and that Lyt-2+ cells acted as progenitors only for the cytotoxic lineage, contrary to earlier speculation that the splenic Lyt-1+ 2+ (Ly-123) pool included cells antecedent to both effector lineages. Comparison of the number of injected progenitors needed to produce repopulation with the number of new precursor cells eventually produced suggests that the relevant progenitors are able to undergo 10,000-fold expansion in 12 to 20 wk. Numerical expansion in the periphery from thymic-processed cells could well be a major source of new lymphocytes in adult mice.