Sea Star Factor Blocks Development of T-Dependent Antibody Secreting Clones by Preventing Lymphokine Secretion

Sea star factor (SSF), a protein of 39 kDa purified from macrophage-like coelomocytes of the echinoderm Asterias forbesi, has potent immunosuppressive effects on T-dependent but not T-independent antibody responses in vivo. SSF at a concentration of 0.5 μg/ml markedly inhibits T-dependent antibody production in vitro by fluorescein (Flu)-specific B cells responding in clonal microculture to antigenic stimulation with Flu-conalbumin via the conalbumin-specific T cells D10.G4.1 (D10). At this concentration of SSF, Ig secretion induced by a T cell-independent stimulus, lipopolysaccharide (LPS), is not affected. Inhibition of antibody production in T-dependent microcultures by SSF can be completely overcome in a dose-dependent fashion by addition of lymphokine-rich supernatants from stimulated cultures of D10 cells. The possibility that SSF suppresses production of requisite cytokine growth factors from T cells was substantiated by the finding that SSF diminishes concentrations of stimulatory cytokines detectable in supernatants from antigen-stimulated cultures. Nevertheless, levels of intracytoplasmic mRNA for IL-4 and IL-5 are not detectably altered by concentrations of SSF that suppress antibody production. Furthermore, when cultures of D10 cells stimulated in the presence of SSF are subjected to freezing and thawing to release intracytoplasmic lymphokines, total levels of stimulatory cytokines are not lower than those in cultures without SSF. These results suggest that SSF inhibits antibody responses by limiting the availability of lymphokines produced by helper T cells. The mechanism for this inhibition may involve either direct effects of SSF on T cells or a block in effective T cell-B cell interaction.