Background/Purpose: Patients with SLE have an activated type I interferon (IFN) system due to an ongoing IFN-alpha synthesis by plasmacytoid dendritic cells (pDCs) stimulated by endogenous nucleic-acid containing immune complexes (ICs). The IFN-alpha production by pDC is strongly promoted by NK cells via MIP-1beta and LFA-1-mediated cell contact. In this study we aimed to further identify molecules of importance in the pDC-NK cell cross-talk.

Methods: Healthy donor PBMC were stimulated with medium or IC consisting of purified SLE-IgG and U1snRNP particles (SLE-IC) for 6 h. Surface expression of 45 different molecules was determined on pDCs, CD56^dim and CD56^brightNK cells by flow cytometry. Regulation of CD319 (CRACC) and CD229 (LY9) was analyzed in isolated or cocultured pDCs and NK cells. Intracellular IFN-alpha was correlated to the expression of CRACC and LY9 on pDCs and the expression of these molecules on different immune cells was compared between SLE patients and healthy controls. Expression of the adaptor molecules SAP and EAT-2 was determined by western blot. The functional role of CRACC and LY9 in pDC and NK cells was investigated.

Results: SLE-IC induced expression of CRACC and LY9 on pDCs, and CRACC on CD56^dim NK cells in PBMC (Mean fold increased MFI 3.7, 2.2 and 2.0, respectively). CRACC and LY9 were not regulated in purified pDC but addition of NK cells restored the up-regulation on pDCs. IL-3 or GM-CSF increased the expression of CRACC and LY9 on SLE-IC-stimulated pDCs as efficiently as NK cells. IFN-alpha producing pDCs had a higher expression of CRACC and LY-9 compared to IFN-alpha negative pDCs. Patients with SLE displayed an increased frequency of CRACC+ CD56^bright NK cells and B cells (p=0.03; 17±2% vs. 10±2% and p<0.001; 10±3% vs 3±0.3%, respectively, (mean±SEM)) and a decreased frequency of CRACC+ CD56^dim NK cells (p=0.01; 59±5% vs. 76±3%, (mean±SEM)). No expression of SAP or EAT-2 was found in pDCs, while NK cells expressed both molecules. Cross-linking CRACC or LY9 by plate-bound mAbs did not affect IFN-alpha production by SLE-IC-stimulated pDCs. Cross-linking CRACC in a redirected cytotoxicity assay resulted in a low frequency of degranulating NK cells, whereas co-ligation of CRACC together with DNAM-1 (CD226) significantly increased the degranulation (4% and 29% CD107a+ CD56^dimNK cells, respectively).

Conclusion: SLE-ICs up-regulates the expression of the costimulatory molecules CRACC and LY9 on pDCs and these molecules also have an increased expression on IFN-alpha producing pDCs. Because these SLAM molecules are self-ligands and involved in the activation of several types of immune cells, the expression on both pDCs and NK cells might facilitate the pDC-NK cell interaction and the function of these cells. In contrast to the positive signal delivered by CRACC and LY9 to NK cells, the lack of SAP and EAT-2 adaptor molecules in pDCs suggests an inhibitory function in these cells. The precise roles of CD319 and LY9 in the autoimmune disease process remains to be established, but are important to clarify given the altered expression of CRACC in SLE on both B cells and NK cells.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/systemic-lupus-erythematosus-immune-complexes-upregulate-the-expression-of-cd319-and-cd229-on-plasmacytoid-dendritic-cells/