Interleukin‐2 and phytohaemagglutinin stimulate proliferation of tunicate cells

Profilerative responses of cells in tunicate pharyngeal explants to human interleukins and mitogenic lectins were tested. Increased tritiated‐thymidine ([³H]‐TdR) uptake was detected among pharyngeal cells incubated with recombinant human interleukin‐2 (IL‐2), and phytohaemagglutinin‐P (PHA‐P). Responses to IL‐2 were dose‐dependent and affected lymphocyte‐like cells. Enhanced proliferation was stimulated by IL‐2 in the absence of co‐stimulants and was not synergized by co‐incubation with human interleukin‐l (IL‐1) or PHA‐P. Anti‐IL‐2 polyclonal antibody inhibited the stimulatory activity of recombinant human interleukin‐2 (rhIL‐2). Of three lectins tested (concanavalin‐A [Con‐A]. pokeweed mitogen [PWM] and PHA‐P), only PHA‐P proved to be mitogenic. Con‐A and PWM did not significantly increase proliferative activity even though both lectins were capable of binding pharyngeal cells as revealed by flow cytometry and fluorescence microscopy. Similarly, human IL‐I had no effect on [³H]‐TdR uptake either alone or in combination with IL‐2 and PHA‐P. These data suggest that the functions of some interleukin‐like cytokines have been conserved during evolution.