Precursor Frequencies of Two Types of Cytotoxic

In the first paper of this series (1), we described a limiting dilution assay designed for the determination of any precursor T cell whose effector function can be determined in vitro. For this assay, normal T ceils are used that are polyclonally activated in bulk cuhure with concanavalin A (Con A) 1 and thereafter expanded by limiting dilution in the presence of T cell growth factor (TCGF). The expansion step ensures a clone size of 500-1,000 so that T cell functions can be easily determined. The use of this assay for the determination of antigen-specific T helper cells provided for the first time information on the precursor frequencies for helper cells in nonimmune T cells. Furthermore, two types of helper cell precursors could be identified, the more frequent one being sensitive to and the less frequent one being insensitive to suppressor T cells. In the present paper, we apply the same method to the determination of precursor frequencies for cytotoxic T cells (CTL). Although, in contrast to helper T cell precursors, a host of information on C T L precursor frequencies is available (2-10), we decided to undertake this study to compare the frequencies determined in potyclonally activated T ceils with those determined in alloantigen-activated T cells. We found that C T L precursor frequencies determined in both assays were similar. However, in addition to the single type of C T L precursor found by us and others in alloantigenactivated T cells, polyclonally activated T cells show an additional C T L precursor of lesser frequency. As in the case of frequent and rare T helper cells, frequent C T L precursors are sensitive to suppression, whereas rare C T L precursors are not. The determination of C T L precursor frequencies in selected Lyt-1 and Lyt-23 T cells showed an absolute depletion of all C T L precursors in Lyt-1 and a slight depletion of frequent C T L precursors in Lyt-23 T ceils. This was the case for specifically and polyclonally activated cells, and it confirms previous data from our laboratory showing that the majority of C T L precursors are of Lyt-123 phenotype (11). Rare C T L precursors are enriched in the Lyt-23 population, a finding that is