Schematic presentation of the various gene transfer methods that are used in experimental models of rheumatoid arthritis. Central to gene therapy is the transfer of therapeutic genes to the site of inflammation. In the 'ex vivo' method autologous or allogeneic fibroblasts that are retrovirally transduced to express therapeutic genes are transplanted into inflamed joint. In 'adoptive cellular gene therapy', the transduced cells are either antigen-presenting cells (dendritic cells, macrophages, or B cells) or T cells (primary or hybridoma cells) that have the capacity to home to the site of inflammation. In the direct or in vivo method the therapeutic gene constructs (viral or nonviral) are directly transferred to the animal either locally (intra-articular, periarticular) or systemically (intramuscular, intravenous). All of these routes of gene delivery have been successful in gene therapy for experimental arthritis, and some of them also exhibited a 'contralateral' effect (i.e. protection of remote untreated joints). The numbers in brackets indicate reference numbers. AAV, adeno-associated virus; DC, dendritic cell; HSV, herpes simplex virus; Mφ, macrophage.

Schematic presentation of potential transcriptionally regulated transgene expression constructs for arthritis. Ideally, the expression of the therapeutic gene should follow the intermittent course of the disease in rheumatoid arthritis, and this can be achieved by using disease-regulating promoters (reg. promoter) for upregulation (promoters from interleukin [IL]-6, complement factor 3 [C3], serum amyloid A [SAA], tumour suppressor gene [TSG]6, heat shock protein [HSP]70) or downregulation (promoters from collagen type II, IL-1 receptor antagonist [IL-1Ra], osteocalcin, tumour necrosis factor receptor [TNFR]) of expression. Disease (cytokine balance) will regulate the expression of rtTA and/or tTS that, only in the presence of doxycycline, can regulate the expression of the therapeutic transgene using the drug-regulable expression system (tet-on/tet-off system). Tissue-specific expression elements either in front or downstream of the promoter must restrict the expression of the site of interest. Furthermore, insulator sequences must prevent cis-acting promoter activities and epigenetic interference on the disease-regulating promoter system. The performance of the transcriptionally regulated expression system will also depend on the vector (integrating/episomal), route of delivery (Fig. 2) and transgene (Fig. 2). Because safety is paramount, it must be possible to delete the transduced cells (e.g. by introducing a thymidine kinase gene in case of a worst scenario; not included in the illustration). I, insulator; Enh, enhancer elements; rtTA, reverse tetracycline-modulated transcription factor; tetO, tetracycline repressor-binding sequence; tTS, tetracycline-modulated transcriptional suppressor.

Schematic presentation of the various gene therapeutic approaches that are used in experimental models of rheumatoid arthritis. We discriminate cytokine targeting and cell targeting as the two main gene therapeutic approaches to arthritis. In cytokine targeting, the objective is to restore the (local) cytokine balance in arthritis in order to silence the inflammatory process and/or to stop the destruction of cartilage and bone. Cell targeting is the elimination of cells from the inflamed joint in order to silence the disease. Genetic synovectomy is the strategy of killing transformed synovial fibroblasts with a connective tissue aggressive phenotype. Selective cell targeting of antigen-specific lymphocytes has major consequences for the inflammatory process, and inhibition of angiogenesis results in reduced pannus formation and synovial hyperplasia. The numbers in brackets indicate reference numbers. CTLA4, cytotoxic lymphocyte antigen 4; FasL, Fas ligand; HSV, herpes simplex virus; IKKβ, IκB kinase-β; IκBαDN, IκBα dominant negative; IL-1R, interleukin-1 receptor; IL-1Ra, IL-1 receptor antagonist; IL-1RAcP, IL-1 receptor accessory protein; IL-18BPc, IL-18 binding protein c; NFκB, nuclear factor-κB; ODN, oligonucleotides; SOCS, suppressor of cytokine signalling; TACI, transmembrane activator and calcium modulator and cyclophilin ligand interactor; TK, thymidine kinase; TNF, tumour necrosis factor; TRAIL, TNF-related apoptosis inducing ligand; VEGFR, vascular endothelial growth factor receptor; vIL-10, viral IL-10; TGF, transforming growth factor. The strategy to protect cartilage and chondrocytes in arthritis given in this illustration are reviewed in detail by van der Kraan and coworkers [111] and by Evans and coworkers [112].