Different Developmental Arrest Points in RAG-2 -/- and SCID Thymocytes on Two Genetic Backgrounds Developmental Choices and Cell Death Mechanisms before TCR Gene Rearrangement

To analyze the early development of T cell precursors in the absence of TCR gene rearrangement, recombinase-activating gene-deficient (RAG-2 -/-) thymocytes were compared with thymocytes from SCID mice on the C.B-17 (BALB) and B6 genetic backgrounds. RAG-2 -/- thymocytes accumulate as quiescent cells with a heat-stable Ag (HSA)-positive CD25+ CD44- c-kit(low) phenotype, resembling normal cells just before selection for functional TCR beta-chain expression. CD44 and c-kit progressively down-regulate in the HSA+ subset, providing a background-independent and TCR-independent developmental clock. On this basis, compared with RAG-2 -/- thymocytes, SCID thymocytes 1) arrest at more heterogeneous, and generally earlier, stages; 2) accumulate to lower overall cell numbers; and 3) maintain higher populations of cycling and activated G1 cells, showing both increased responsiveness and increased cell death. B6-SCID thymocytes appear to die particularly early. Low levels of Fas were observed on "advanced" HSA+ SCID thymocytes but not on any RAG-2 -/- thymocytes, suggesting a potential difference in activation state or mechanism of death. In both RAG-2 -/- and SCID thymocytes, there are also two discrete subsets of HSA(low) CD25- CD44+ c-kit+ cells: a Sca-1+ CD44++ CD122- NK1.1- putative progenitor subset and an NK-like Sca-1- CD44+(+) CD122+ NK1.1+ subset. The absolute cell numbers in these HSA(low) subsets and the extent of NK cell differentiation, measured by perforin expression, are nearly constant in all the mutant strains analyzed, in contrast to the HSA+ CD25+ population, which was expanded in the RAG-2 -/-. Thus, the SCID thymocytes appear to undergo a normal generation but a premature death as compared with the RAG-2 -/- thymocytes