Interleukin 10 (IL-10), originally identified as a TH2 helper T-cell product able to inhibit cytokine production by THI cells, is highly homologous to BCRF1

CD40 is a 45- to 50-kilodalton transmembrane glycoprotein expressed on B lymphocytes, epithelial cells, and some carcinoma cell lines. Human resting B lymphocytes entered a state of sustained proliferation when incubated with both the mouse fibroblastic Ltk- cell line that had been transfected with the human Fc receptor (Fc gamma RII/CDw32) and monoclonal antibodies to CD40. In combination with interleukin-4, factor-dependent long-term normal human B cell lines were generated that were consistently negative for Epstein-Barr viral infection. Thus, cross-linking of CD40 is likely to represent an important phenomenon in the clonal expansion of B cells.

SummarySince CD40/CD40 ligand (CD40Lig) interactions are essential in vivo for the generation of germinal center B cells that express Fas (Apo-1/CD95), we explored whether CD40 engagement may modulate Fas expression and function on human B lymphocytes. Resting tonsil B cells, isolated by density gradient centrifugation, express either absent or low levels of Fas. They could be induced to promptly express Fas after ligation of their CD40, however, using either a recombinant human CD40Lig or a cross-linked anti-CD40 mAb. In contrast, engagement of the B cell antigen receptor by immobilized anti-K and -k antibodies did not turn on Fas expression. Addition of anti-Fas mAb CH11 inhibited the later phases of CD40-induced B cell growth as a result of apoptotic cell death. Furthermore, Fas ligation inhibited proliferation and Ig secretion of CD40-activated B cells in response to recombinant cytokines such as interleukin (IL)-2, IL-4, and IL-10, as well as a cytokine-rich supernatant ofphytohemagglutinin-activated T cells, indicating that none of those B cell tropic factors were able to prevent the Fas-induced death. Taken together, the present results show that engagement of CD40 antigen on B cells induces Fas expression and sensitizes them to Fas-mediated apoptosis. The delayed functional response to Fas ligation after CD40 activation may represent a way to limit the size of a specific B cell clone that is generated during T-B cell interactions.

SummaryHuman resting B lymphocytes enter a state of sustained proliferation when incubated with both mouse fibroblastic L cells stably expressing FcyRII/CDw32 and anti-CD40 antibodies . We have explored the effects of 11 recombinant human cytokines (CKs) on induced cell proliferation and immunoglobulin (Ig) production. Interleukin 4 (IL4) was the only CK able to enhance anti-C1340-induced B cell multiplication as measured by enumeration of viable cells, and interferon y (IFN-y) further stimulated this induced proliferation. IL4 enhanced the production of IgM and IgG by B cells and induced them to produce IgE. Combinations of 114 and 11,2 resulted in the production of large amounts of IgM and IgA. Interestingly, IFN-'y did not inhibit the production of IgE by cells stimulated with anti-CD40 and IL4. None of the tested CK combinations resulted in the production of large quantities of IgG. Therefore, this new culture system represents a unique model to study isotype regulation in highly purified human B lymphocytes, in addition to allowing the generation of long-term factor-dependent human B cell lines.T he maturation of resting B lymphocytes into Ig-secreting cells is a highly regulated phenomenon, thought to be coupled to considerable proliferation, which requires the participation of antigen (Ag),' T cells, and probably accessory cells (3) acting through cell-cell interactions via specific membrane Ags and through the release of cytokines (CKs). An approach to understanding the phenomena involved in B cell activation has been made possible through the availability of polyclonal activators including antibodies to surface Ags (4) and recombinant CKs (3). These studies have shown that antibodies against the B cell Ag receptors (sIg) as well as other non-Ig molecules can deliver activation signals to B cells. In particular, the CD40 molecule has recently emerged as a functionally important B cell surface Ag (5, 6) . The CD40 Ag is a 277-amino acid glycoprotein whose 172-amino acid extracellular domain has homology with the nerve growth factor receptor (7)

In this study, we describe human FDF03, a novel member of the Ig superfamily expressed as a monomeric 44-kDa transmembrane glycoprotein and containing a single extracellular V-set Ig-like domain. Two potential secreted isoforms were also identified. The gene encoding FDF03 mapped to chromosome 7q22. FDF03 was mostly detected in hemopoietic tissues and was expressed by monocytes, macrophages, and granulocytes, but not by lymphocytes (B, T, and NK cells), indicating an expression restricted to cells of the myelomonocytic lineage. FDF03 was also strongly expressed by monocyte-derived dendritic cells (DC) and preferentially by CD14+/CD1a− DC derived from CD34+ progenitors. Moreover, flow cytometric analysis showed FDF03 expression by CD11c+ blood and tonsil DC, but not by CD11c− DC precursors. The FDF03 cytoplasmic tail contained two immunoreceptor tyrosine-based inhibitory motif (ITIM)-like sequences. When overexpressed in pervanadate-treated U937 cells, FDF03 was tyrosine-phosphorylated and recruited Src homology-2 (SH2) domain-containing protein tyrosine phosphatase (SHP)-2 and to a lesser extent SHP-1. Like engagement of the ITIM-bearing receptor LAIR-1/p40, cross-linking of FDF03 inhibited calcium mobilization in response to CD32/FcγRII aggregation in transfected U937 cells, thus demonstrating that FDF03 can function as an inhibitory receptor. However, in contrast to LAIR-1/p40, cross-linking of FDF03 did not inhibit GM-CSF-induced monocyte differentiation into DC. Thus, FDF03 is a novel ITIM-bearing receptor selectively expressed by cells of myeloid origin, including DC, that may regulate functions other than that of the broadly distributed LAIR-1/p40 molecule.