Migration stimulation factor (MStF), from murine B cells, constitutively produced by a T-B hybridoma

Hybridomas were established between murine spleen B cells and the thymoma cell line BW5147, to purify the migration stimulation factor (MStF), a molecule likely involved in immunosuppression. The parental B cells were from Lo/PHA mice previously shown to produce high levels of MStF after immunization by appropriate (tolerogenic) doses of ovalbumin. Among the positive clones, B9 was selected, since it produced high levels of MStF constitutively, and no immunoglobulin. This clone was shown to contain the genome of the B-cell fusion partner, since one of its L chain genes had undergone a VK-JK rearrangement. Isolation of MStF by size-exclusion chromatography showed 2 major peaks of activity, one of which eluted in a 20-kDa, almost protein-free fraction. This elution is unlikely to correspond to the true molecular mass, since MStF was found not to be a protein. Indeed, MStF was TCA-soluble, thermoresistant, highly hydrophobic and protease-resistant, but activity was abolished by neuraminidase digestion. The possibility of its being a small molecule transported by a protein carrier was also ruled out. These results suggest that MStF is a complex molecule containing both sialic residues and a lipid moiety. Experiments are planned to further investigate the chemical structure of this unusual B-cell factor.