The role of T cell subsets in the generation of secondary cytolytic responses in vitro against class I and class II major histocompatibility complex antigens.

Strain combinations generating cytotoxic T lymphocytes (CTL) specific for a single class I (K or D) or class II (A or E) MHC molecule were set up. The responder cells were separated into Ly subsets (Ly-1+2-, Ly-1-2+, and Ly-1+2+) on day 5 of culture by using lytic or non-lytic selection techniques and monoclonal Ly-specific antibodies. The separated subsets were restimulated on day 8 and tested for secondary CTL activity on day 12. Class II-specific secondary CTL could be generated from all three subsets, whereas class I-specific CTL developed only in the Ly-1+2+ and Ly-1-2+ subsets. The Ly-1+2+ cells underwent a phenotypic shift to Ly-1-2+ by day 12, whereas CTL generated from the Ly-1+2- and Ly-1-2+ subsets retained their phenotype up to the secondary effector stage. The cells separated according to their Ly phenotypes on day 5 were the progeny of unprimed progenitors expressing the same Ly phenotypes. Unprimed Ly-1+2+ cells gave rise to CTL in the absence of the other subsets, while unprimed Ly-1+2- and Ly-1-2+ cells required the help of Ly-1+2+ cells (or soluble factors) during priming to become non-lytic CTL precursors by day 5, and cytolytic cells after restimulation. The Ly-1+2- subset could generate class II-specific secondary CTL only in the absence of the other two subsets. Apparently, alloantigen-primed Ly-1+2+ and Ly-1-2+ cells suppressed the development of cytolytic activity in the Ly-1+2- subset. The combined data provide a comprehensive pathway of CTL differentiation from T cell subsets.