Differential Induction of Cytokine Genes and Mitogen-Activated Protein Kinase (MAPK) family by Soluble CD40 Ligand (sCD40L) and TNF in Human Follicular Dendritic Cell Line, HK

다국어 초록 (Multilingual Abstract)

CD40 is a member of the TNF receptor family whose function has been primarily characterized in B lymphocytes. In vitro stimulation of CD40, along with IL-4, on B cells leads to B cell activation, proliferation, Ig production, and isotype class switching; those which represent in vivo characteristic events occurring at the germinal centers(GC) of lymphoid follicles. The crucial role of CD40/CD40L interaction in vivo is evident from patients with X-linked hyper-IgM syndrome, leading to the lack of circulating IgG and IgA and the absence of GC. CD40 is found to be expressed on cells other than mature B cells including follicular dendritic cells(FDC). FDC are in close contact with B cells within the GC, and the interaction between these two cell types via CD40/CD40L recognition is believed to be necessary for the differentiation of B cells into memory B cells. In addition to FDC-B cell interaction, FDC appears to be able to bind to activated T cells, which also involves CD40/CD40L interaction. Therefore, CD40/CD40L recognition is regarded as a critical event in the three-way communication among FDC, T and B cells. However, in spite of the functional importance of FDC in GC reactions, studies on the biological effects of CD40 triggering on FDC has been hampered by difficulty to obtain sufficient amount of purified FDC for in vitro experiments. Several laboratories have succeeded in establishing FDC-like or FDC cell lines. Among them, HK cells were chosen in our study because their phenotypic and functional characteristics similar to physiological FDC has been relatively well studied. HK cells delayed apoptosis of GC B cells and stimulated the growth and differentiation of GC B cells in the presence of anti-CD40L antibody. In addition, activated T cells augmented the proliferation of HK cells, which was analogous to the in vivo observation that T cells are required for GC reaction and FDC development. An additional evidence that HK cells are related to FDC is that a monoclonal antibody, termed 3C8 prepared by immuni
In the present study, using HK cells we address the following questions. First, are CD40 molecules expressed on HK cells functional? Second, IS CD40 triggering on HK cells accompanied by the activation of mitogen-activated protein kinases(MAPK) family? Third, are there any differences between CD40-mediated and TNF receptor-mediated signal transduction pathways in HK cells? To answer these questions, we prepared a recombinant soluble form of CD40L(sCD40L) as a CD40 triggering agent, and examined first whether its treatment on HK cells causes cytokine gene induction. We showed that HK cells spontaneously produced M-CSF and G-CSF mRNAs, and their levels were markedly enhanced after treatment of sCD40L, but no IL-6 mRNA induction was observed, which differs from other cell types(e.g., fibroblasts, endothelial cells and monocytes) exhibiting a strong IL-6 gene induction upon CD40 stimulation. In addition, CD40 engagement on HK cells resulted in the activation of ERK1/2 and p38 without significant increase in JNK activity. Lack of JNK induction following CD40 ligation is different from the reports that most of B cells respond to CD40 stimulation by activating JNK activity strongly. These results suggest a uniqueness of CD40 signaling in FDC. When compared the effects of TNF with those of sCD40L on HK cells, besides the ability of TNF to induce JNK activity as well as IL-6 mRNA, the proximal signaling elements activated by the two molecules appeared to differ at least in the requirement of protein kinase C(PKC) activity, even though the two signals converge on the same consequences such as ERK and p38 activation and further downstream event of M-CSF and G-CSF mRNAs induction.