B-cell stimulatory factor 1 (BSF-1) is a T-cellderived lymphokine that acts together with low concentrations of anti-IgM antibodies to stimulate resting B cells to enter the G, phase of the cell cycle and to synthesize DNA. We show here that supernatants from EL-4 cells, rich in BSF-1 activity, and BSF-1 purified by high-pressure liquid chromatography (HPLC-BSF-1) act on resting B cells, in the absence of anti-IgM antibodies, to prepare them to respond to anti-IgM and BSF-1. A 24-hour preculture with BSF-1 speeds the entry into S phase of B cells subsequently cultured with anti-IgM and BSF-1 by =12 hours and causes substantial increase in cell volume of all resting B cells. Both of these effects, stimulated either by EL-4 supernatants or by HPLC-BSF-1, are inhibited by a monoclonal anti-BSF-1 antibody. These results lead us to propose that BSF-1 should be regarded as a B-cell activation factor.The T-cell-derived lymphokine B-cell stimulatory factor 1 (BSF-1) was initially described as a required costimulant for DNA synthesis responses of resting B cells to low concentrations of anti-IgM antibodies (1). This factor had initially been designated B-cell growth factor (1, 2) and early studies were interpreted to indicate that it functioned in B-cell responses by stimulating activated B cells to enter S phase much in the manner that interleukin 2 (IL-2) acted upon activated T cells to stimulate their proliferation (3).This view was first challenged by time course studies that demonstrated that a delay of even 4 hr in the addition of BSF-1-rich supernatants (SNs) to resting B cells cultured with anti-IgM caused a diminution in the magnitude of entry into S phase at 30-42 hr after initiation of culture; delays of 12 hr or greater in the addition of BSF-1 completely abrogated the uptake of [3H]thymidine between 30 and 42 hr of culture (4). Such studies indicated that BSF-1 acted on very early G1 phase or, perhaps, on Go B cells. More recently, it has been shown that B-cell populations cultured for 24 hr with BSF-1, or with T-cell SNs rich in BSF-1, but in the absence of anti-IgM display substantial increases in cell volume (5) and in expression of class II major histocompatibility complex (MHC) molecules (6, 7). These experiments strongly suggest that BSF-1 can act upon resting B cells.In the current work, we wished to determine whether the volume changes and enhancement in class II MHC molecule expression stimulated by BSF-1 were indicative of an actual preparation of resting B cells to enter and/or to move through the cell cycle more rapidly upon subsequent stimulation. Our results indicate that culture of highly enriched, small dense B cells with preparations of BSF-1 purified by high-pressure liquid chromatography (HPLC) prepares those cells to enter S phase more rapidly when they are stimulated with anti-IgM and a BSF-1-rich T-cell SN. A 24-hr initial culture speeds the subsequent entry into S phase by =12 hr. This effect is inhibited by incorporation of a monoclonal antibody to BSF-1 into the initial cultu...

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BSF-1 prepares resting BALB/c, DBA/2, and BDF1 B cells to enter S phase more promptly in response to subsequent culture with anti-IgM-based stimulants. It prepares DBA/2 and BDF1 B cells to respond to LPS, but its preparative effect for LPS responses of BALB/c B cells is both inconstant and meager. Preparation mediated by BSF-1 requires extended contact of B cells with the stimulant for full effect. Half-maximal preparation requires approximately 12 h of contact, as judged by delayed addition of BSF-1 or by inhibition of BSF-1 action with anti-BSF-1 mAbs. BSF-1 preparative action on resting DBA/2 B cells is mimicked by anti-Lyb-2.1 antibody. B cell blasts prepared by culture with BSF-1 and anti-IgM show modest responses to high concentrations of BSF-1; large B cells directly isolated from the spleen are not stimulated to enter S phase by BSF-1. These results lead us to conclude that BSF-1 functions principally as an activation factor for resting B cells.

The structural relationships among the gag polyproteins Pr655"9, Pr75 a8, and gPr80959 of Rauscher murine leukemia virus were studied by endoglycosidase H digestion and formic acid cleavage. Fragments were identified by precipitation with specific antisera to constituent virion structural proteins followed by onedimensional mapping. Endoglycosidase H reduced the size of gPr80959 to that of Pr759'a. By comparing fragments of gPr80g'w and the apoprotein Pr759'9, the former was shown to contain two mannose-rich oligosaccharide units. By comparing fragments of Pr65""9 and Pr755 ¶9, the latter was shown to differ from Pr659"9 at the amino terminus by the presence of a leader peptide approximately 7,000 daltons in size. The internal and carboxyl-terminal peptides of the two unglycosylated polyproteins were not detectably different. The location of the two N-linked carbohydrate chains in gPr80"g" has been specified. One occurs in the carboxyl-terminal half of the polyprotein at asparagine,77 of the p30 sequence and the other is found in a 23,000-dalton fragment located in the amino-terminal region of gPr809a9 and containing the additional amino acid sequences not found in Pr65gag plus a substantial portion of p15. MATERIALS AND METHODS Cells and cell culture. JLS-V9 cells (36) chronically infected with R-MuLV were obtained from the Viral Resources Laboratory (Frederick Cancer Research Center, Frederick, Md.). Fetal calf serum was purchased from GIBCO Laboratories (Grand Island, N.Y.). Eagle minimal essential medium (EMEM) and methionine-free EMEM were obtained from Flow Laboratories (Rockville, Md.). Cells were grown on Corning plastic ware in EMEM supplemented with 10% heat-inactivated fetal calf serum, penicillin (100 IU/ml), and streptomycin (100 ,ug/ 581

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