Development and function of cytotoxic T lymphocytes (CTL). I. In vivo maturation of CTL precursors in the absence of detectable proliferation results as a normal consequence of alloimmunization

In the experiments reported here, we examine the need for cell division as a critical component in the clonal expansion of alloreactive CTL precursors. Unlike previous attempts to inhibit DNA synthesis and cell division non-specifically, we have chosen to follow the normal unimpeded development of CTL in two of the most commonly used in vivo and in vitro allograft systems. The development and relative contribution of CTL lymphoblast-associated cytotoxicity has been followed by density gradient separation and functional analysis of the various fractions of lymphocytes obtained throughout the entire course of sensitization. In addition to the physical parameters (size and density), even more convincing data have been obtained from in vivo administration of 3H-TdR during the entire allograft reaction. The results presented here clearly confirm CTL precursor proliferation in vitro but provide strong evidence that in vivo CTL normally arise via a mechanism independent of blast formation and cellular proliferation. Interpretations of these findings in relationship to the concept of "clonal expansion" for the generation of mature CTL are discussed.