FUNCTIONAL AND PHENOTYPIC ANALYSIS OF THYMOCYTES IN SCID MICE - EVIDENCE FOR FUNCTIONAL-RESPONSE TRANSITIONS BEFORE AND AFTER THE SCID ARREST POINT

Thymocytes from severe combined immune deficient (SCID) mice undergo d evelopmental arrest at an early stage, before most TCR gene rearrangem ent. They therefore represent a natural test case to assess those aspe cts of T cell development that are TCR independent. Multiparameter flo w cytometry was used to analyze the array of immature phenotypes prese nt in the SCID thymus at steady state, as defined by the markers CD4, CD5, Sca-1, NK1.1, CD44, heat-stable antigen (HSA), and IL-2Ralpha. Th e results suggest a simple developmental block in SCID mice rather tha n a program of aberrant differentiation. SCID thymocytes displayed eff icient, developmentally regulated functional responses. Approximately 20-25% of the cells, mostly within the IL-2Ralpha+HSA+CD44low fraction , could be induced to express IL-2. This IL-2 inducibility was highly dependent on IL-1 costimulation, in agreement with the behavior of nor mal immature thymocytes. These results formally demonstrate that compe tence to express IL-2 is developed independently of TCR expression or gene rearrangement. Comparison of the response properties of various S CID thymocyte subsets indicated that IL-2 inducibility is first likely to be acquired at an early (Sca-1++CD44++HSA(low)) stage. A later fun ctional transition was revealed by comparing patterns of IL-2Ralpha re gulation in normal and SCID IL-2Ralpha+HSA+CD44low thymocytes. The SCI D thymocytes uniformly maintained IL-2Ralpha expression on in vitro st imulation, whereas only a minority of the normal cells in the correspo nding subset could do so unless IL-1 was also added. The SCID arrest p oint thus appears to separate the IL-2Ralpha+HSA+CD44low stage into di stinct early (TCR independent) and late phases. Normal cells that prog ress beyond the SCID arrest point appear to lose, rather than gain, co mpetence to make various responses, even before they leave the IL-2Ral pha+HSA+CD44low stage. A model is proposed in which discrete changes i n functional competence define novel transitions in early thymocyte de velopment, at least some of which may be linked to TCR-beta gene rearr angement before positive or negative selection.