Inhibition of T cell activation and adhesion functions by soluble CD2 protein.

The CD2 (T11) molecule belongs to a family of cell-surface glycoproteins that function as adhesion molecules in the immune system. Human CD2 is found exclusively on cells of the T lineage: peripheral T lymphocytes, NK cells, and thymocytes. CD2 binds specifically to the surface glycoprotein LFA-3. CD2/LFA-3 adhesion is the basis for the formation of rosettes between T cells and sheep erythrocytes (SRBC) which bear the sheep homologue of LFA-3. More importantly, CD2/LFA-3 adhesion functions in the immune system to augment T cell activation; it initiates conjugate formation between participating T cells and antigen-presenting cells (APC). We investigated the effects of soluble forms of CD2 (sCD2), produced in either baculovirus or CHO expression systems, on the rosetting of T cells with SRBC and on the activation of T cells by antigen plus major histocompatibility complex (MHC) molecules. Rosette formation between T cells and SRBC was completely inhibited by as little as 1 microM sCD2. Furthermore, sCD2 effectively inhibited (at micromolar concentrations) the T cell proliferative response to recall antigens including rubella, tetanus toxoid, and herpes simplex virus (HSV-1), as well as alloantigens in a mixed lymphocyte culture. These findings are consistent with the notion that the CD2/LFA-3 interaction augments antigen-specific T cell functions. The use of a CD2 "decoy" molecule rather than anti-CD2 or anti-LFA-3 antibodies to block the CD2/LFA-3 interaction rules out secondary antibody effects, via the Fc portion, as the basis for inhibition of T cell activation and directly stresses the importance of this adhesion interaction in T cell responses.

Interferon-gamma inhibits the action of B cell stimulatory factor (BSF)-1 on resting B cells.

B cell stimulatory factor-1 (BSF-1) stimulates resting B cells to increase in volume and prepares these cells to enter the S phase in response to anti-IgM and other B cell mitogens. Interferon-gamma (IFN-gamma) blocks both the volume enlargement and preparation for DNA synthesis caused by BSF-1, although it has little effect on B cells already stimulated by BSF-1. The capacity of IFN-gamma to inhibit the action of BSF-1 on resting B cells suggests a mutual regulatory interaction between these two T cell-derived products.

B cell stimulatory factor 1 (BSF-1) prepares resting B cells to enter S phase in response to anti-IgM and lipopolysaccharide.

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.

B-cell stimulatory factor 1 activates resting B cells.

B-cell stimulatory factor 1 (BSF-1) is a T-cell-derived lymphokine that acts together with low concentrations of anti-IgM antibodies to stimulate resting B cells to enter the G1 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 approximately equal to 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.

Structure of glycosylated and unglycosylated gag polyproteins of Rauscher murine leukemia virus: carbohydrate attachment sites.

The structural relationships among the gag polyproteins Pr65gag, Pr75gag, and gPr80gag 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 one-dimensional mapping. Endoglycosidase H reduced the size of gPr80gag to that of Pr75gag. By comparing fragments of gPr80gag and the apoprotein Pr75gag, the former was shown to contain two mannose-rich oligosaccharide units. By comparing fragments of Pr65gag and Pr75gag, the latter was shown to differ from Pr65gag 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 gPr80gag has been specified. One occurs in the carboxyl-terminal half of the polyprotein at asparagine177 of the p30 sequence and the other is found in a 23,000-dalton fragment located in the amino-terminal region of gPr80gag and containing the additional amino acid sequences not found in Pr65gag plus a substantial portion of p15.