New insights into the proliferation and differentiation of early mouse thymocytes

Early thymocyte development was compared in normal, recombinase-activating gene 2-inactivated (RAG-2 KO) and pre-T cell receptor alpha-inactivated (pre-Tα KO) mice, mutants representing either a complete (RAG-2 KO) or partial (pre-Tα KO) block in progenitor development. Using three colour analysis with antibodies to CD117, CD44 and CD25, cell numbers in each progenitor subset were quantified, demonstrating an accumulation of cells prior to the block. Progenitor number was influenced both by the nature of the genetic block and thymus size, as shown in the enlarged thymus of a transgenic mouse line. By four colour staining for CD3, CD117, CD44 and CD25 and deliberately not gating out CD3^- cells, a novel aspect of gamma delta T cell development in pre-Tα KO mice was identified. 5-bromodeoxyuridine labelling and subsequent four colour staining for BrdU, CD117, CD44 and CD25 showed firstly that DN1 cells were cycling, secondly that the developmental block in pre-Tα KO mice corresponded to a decrease in DN4 cell proliferation, and thirdly provided a novel 'snapshot' of T cell receptor beta-selected cells transiting the DN3 to DN4 compartment. Taken together, these results emphasise the need for a more detailed qualitative and quantitative analysis of the progenitor compartment in the thymus.