Eur. J. Immunol. 1988.18: 289-294 Synthetic Fc peptides influence human B cell activation 289 zy Gabriella Sarmay', Denis R. Stanworth', Robert Szigeti", Eva Klein', Krisztina Reguly', Robert P61volgyio and Janos Gergely' Department of Immunology', Lorand Eotvos University, God, Department of Immunology', University of Birmingham, Birmingham, Department of Tumor Biology', Karolinska Institutet, Stockholm and Institute of Organic Chemistry, Lorand Eotvos University', Budapest The effect of synthetic peptides corresponding to Fc sequences in human IgG 1 on various steps in the B cell activation pathway The influence of synthetic peptides comprizing sequences in the exposed positions of the Fc region of human IgG 1 was tested on B lymphocyte activation. C H 2 domain peptides having an inhibitory effect on antibody-dependent cellular cytotoxicity, as well as the whole Fc fragment, induced the appearance of the early signs of activation on resting B lymphocytes such as increase in cell volume and HLA-DR antigen expression or leukocyte migration inhibitory factor production. The peptides did not induce proliferation of resting B cells even when B cell growth factor (BCGF)-con- taining supernatants were added. Exposure to Fc fragment, however, induced a weak proliferation which was significantly enhanced by BCGF. On the other hand, both the Fc fragment and the CH2 or CH3 domain peptides enhanced the IgM synthesis of human blood mononuclear cells when a suboptimal dose of pokeweed mitogen was present. This effect was lost when Fc fragment or the peptides were added on the third day of culture. These results suggest that the early steps of B cell activation can be induced by Fc fragment and by small molecular weight Fc peptides, which are potential ligands of Fc receptors. The Fc fragment activates B cells to the state where they respond to BCGF, but the peptides do not possess this activity. Furthermore, both Fc fragment and Fc peptides are able to enhance the IgM synthesis, when accessory cells and the appropriate differentiating factors are present. 1 Introduction B and T lymphocytes, K cells, monocytes and granulocytes express receptors (FcR) reactive with the Fc region of the IgG molecule [l-41. FcR are known to be involved in triggering several effector functions such as phagocytosis and antibody- dependent cellular cytotoxicity (ADCC), and also in the regu- lation of B cell proliferation and maturation [5-7]. In the latter system, both negative and positive signals have been shown to be mediated by the ligands of FcR [8-131. On the one hand, antigen-antibody complexes interacting with FcR or soluble, FcR-like immunoglobulin-binding factors (IBF) may inhibit the maturation of B cells [8-101. On the other hand, Fc frag- ment and a synthetic peptide corresponding to sequence 334-357 within the CH 3 domain were shown to induce prolif- eration and/or maturation of murine B lymphocytes [ll-131. We have shown earlier that synthetic peptides comprizing sequences within the CH2 (274-301) or C H 3 (407-416) domain of IgG1 inhibit ADCC, one of the FcR-mediated functions [14]. The amino acid sequences of these peptides partly overlap with two of the predicted antigenic epitopes of Fc [15]. These groups are located on the surface of the Ig molecule, where they can interact with the FcR of different cells. [I 63801 In the present study we have analyzed the influence of synthe- tic CH 2 and CH 3 domain peptides on the distinct steps in the human B cell activation pathway. This process can be divided into three distinct phases each with different requirements. The first step is the binding of the antigen which triggers the appearance of the early signs of activation such as expression of activation antigens, cell enlargement, increased expression of class I1 antigens and leukocyte migration inhibitory factor (LIF) production [16-181. To continue the process of activa- tion, B lymphocytes need factors originating from accessory cells, which induce cell proliferation (B cell growth factors, BCGF). B cell maturation into an Ig-producing plasma cells can be completed only in the presence of further factors, which induce this final step of differentiation (B cell differentiation factors, BCDF) [19]. In the absence of the factors necessary for triggering the next step in the activation pathway, the cells are captured in the lower activation state. Our results demonstrate that synthetic peptides corresponding sequences within the CH 2 and CH 3 domains of IgG 1, as well as the whole Fc fragment, directly induce the early phases of resting B lymphocyte activation without influencing prolifera- tion. Furthermore, they are also able to enhance the IgM pro- duction of zyxwv in zyxwvut vim cultures of human peripheral blood mono- nuclear cells (PBMC) stimulated with suboptimal amounts of pokeweed mitogen (PWM). Recipient of a research fellowship from the Swedish Cancer Society and also supported by the Swedish Society of Medicine and the Cancer Society (Nr. 86: zyxwvutsrqpo 56) of Stockholm. Correspondence: Gabriella Sarmay, Department of Immunology of L. Eotvos University, God, 2131, Javorka S. 14, Hungary Abbreviations: ADCC: Antibody-dependent cellular cytotoxicity LIF: Leukocyte migration inhibitory factor BCGF: B cell growth factor PWM: Pokeweed mitogen FcR: Receptors interacting with the Fc portion of IgG PHA: Phytohemagglutinin 2 Materials and methods 2.1 Reagents Monoclonal antibody specific for HLA-DR was kindly pro- vided by Dr. C. Navarrete (Dept. of Immunol. The London Hospital Medical College, London). Fluorescein isothiocyan- ate-labeled anti-mouse Ig (human serum absorbed) and anti- human IgM reagents were obtained from Dako Immunoglobu- zy 0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1988 0014-298018810202-0289$02.50/0