Antigen organization regulates cluster formation and induction of cytotoxic T lymphocytes by helper T cell subsets

Generation of effector cytotoxic T lymphocytes (CTLs) is a process tightly governed by regulatory helper T (Th) cells. The nature of cellular interactions as well as the precise role of distinct Th cell subsets involved in efficient CTL activation remains elusive. Employing in vitro cultures for primary induction of human, peptide-specific CTL, a strict requirement for Th cells and linkage of epitopes for helper and CTLs on the surface of antigen presenting cells was found, suggesting a three cell type cluster as minimal immune regulatory entity. Cognate and antigen-drivers interactions of T cells were neither essential nor sufficient to override the need for linked epitopes. Within the three cell type cluster complex, keyhole limpit hemocyanin or tetanus toxoid-reactive Th cells promoted generation of MAGE-3- or HIV-gag-specific CTL. Both type 1 and type 2 Th cells were recruited and induced by CTL. Interleukin 2 and interferon gamma were essential in early stages, and interleukin 4 was utilized in later stages, of CTL maturation. Synergistic effects of CD45RA(+) and CD45RO(+) Th cells were found. The data reported here suggest a critical link between the innate and adaptive immune system in the initiation process of cytolytic immune responses and offers the basis for efficient vaccine strategies.