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A subline of the C57BL/6 mouse EL-4 thymoma produces upon stimulation with phorbol myristate acetate (PMA) a B cell growth factor (BCGF) that co-stimulates anti-IgM-stimulated splenic B cells. The BCGF present in the EL-4 thymoma culture supernatants is shown to be distinct from IL 2 by a variety of chromatographic criteria. Untreated BCGF exhibits a heterogeneity of isoelectric points (pH 6.4 to 6.6, 7.4 to 7.6, 8.5 to 8.7) all of which are more alkaline than that of IL 2 (pl 3.5 to 4.8). Neuraminidase-treated BCGF exhibits a single major pl of 9.3 that is also distinct from similarly treated IL 2. Moreover, IL 2 and BCGF exhibit distinct monomeric m.w. by SDS gel electrophoresis under nonreducing conditions (IL 2 is 21,000; BCGF is 11,000 and 15,000). The biologic activity of IL 2 is sensitive to trypsin inactivation as is BCGF activity. Thus, BCGF and IL 2 appear to be biologically and biochemically distinct glycoproteins.
Farrar et al. (Sat,) studied this question.