Similarities/discrepancies between the in situ and in vitro phenotype of RA-SFB

Similarities
The cells obtained by negative isolation generally showed features similar to those reported in in situ analyses. The expression of Thy-1, CD13 (aminopeptidaseN), and vimentin exemplify this. The expression of these molecules, particularly their local upregulation by cytokines [54,55] and/or heterogeneous expression at different anatomical sites [9,49], may be pathogenetically relevant, either in terms of defining functionally heterogeneous SFB subpopulations [49] (reviewed in [50,51,52]), by providing pro-inflammatory enzymes [17,47,56,57,58,59], or as targets of autoimmune reactions [60].
MHC-II. The constitutive MHC-II expression in isolated RA-SFB and OA-SFB, confirming previous in situ observations in the RA and OA SM [13,21,22,28] and in systemic scleroderma [61], is probably related to the inflammatory microenvironment, since normal skin-FB (Table 4) and normal SFB [31] hardly express MHC-II. This is further supported by the a strong decrease of the percentage of MHC-II+ cells upon repeated passage (both conventional and following negative isolation), possibly due to lack of or a progressive decrease of external stimulation with pro-inflammatory mediators (Fig. 7 and Table 5). Notably, there was a significant, negative correlation between the percentage of MHC-II-positive SFB and treatment of the RA patients with Methotrexate (ρ=-0.866, P = 0.01, n = 7), indicating that effective antirheumatic therapy may be reflected by decreased MHC-II expression on RA-SFB [28].
Procollagen III. The significant increase of the MFI for intracellular procollagen III in RA-SFB and OA-SFB (Table 4) suggests that the pattern of SFB activation may be functional (among other things) to the increase of collagen metabolism; this is supported by the increased expression of collagen α2I and α1III mRNA in the RASM in situ [2,23,24], in comparison with normal or non-RA synovial tissue. Also, in as much as procollagen III is the fetal form of collagen used in wound healing and tissue repair (reviewed in [62,63]), ongoing fibrosis may be a considerable component of the disease process in RA (and OA), in analogy to systemic scleroderma [61] or interstitial kidney fibrosis [11,26]. A striking increase of procollagen I and III expression in RA-SFB from primary culture to fourth passage (increase of positive cells by ≥ 38%; Fig. 7 and Table 5), on the other hand, indicates that primary-culture cells do not exploit their full potential of matrix production.

Discrepancies
The expression of VCAM-1 [1,15,64,65] and Jun and Fos proto-oncogenes [23,25,66,67] showed some in situ / in vitro discrepancies.
VCAM-1. Surprisingly, only a moderate and variable percentage of RA-SFB (whether conventionally passaged or negatively isolated from primary culture) expressed the adhesion molecule VCAM-1 (Tables 4 and 5). There were also no significant differences between RA-SFB and OA-SFB or normal skin-FB (Table 4). This is in apparent contrast to the enhanced expression of VCAM-1 reported in the lining layer of RA and OA synovial tissue [64,65]; however, it is well compatible with the large variability in VCAM-1 expression observed in vitro (Table 4) [1,15,68] and in situ [1,15]. The significant, positive correlation between VCAM-1 expression in isolated primary-culture RA-SFB and the erythrocyte sedimentation rate in RA patients (ρ=1.00, P = 0.000, n = 4) indicates that the variability may depend on disease activity, as also suggested in recent reports [68,69,70].
Proto-oncogene expression. Negatively isolated RA-SFB expressed c-Fos, c-Jun, and Jun-D (Table 4), in analogy to the features of SFB in RASM [23,25,66]. The expression of c-Jun and Jun-D in the present study could be unequivocally demonstrated by FACS analysis (Table 4), while previous immunohistochemical studies had failed to detect these molecules in situ [23]. Notably, however, the degree of proto-oncogene expression (both percentage of positive cells and MFI) did not significantly differ from that of normal skin-FB (Fig. 6 and Table 4) or, as previously reported, of SFB from traumatic joint injury [67]. This finding supports in situ observations that, at a single-cell level, the degree of proto-oncogene expression in RA, OA, and joint trauma is similar [23], thereby questioning whether proto-oncogene expression in RA reflects per se cell transformation and/or severe metabolic abnormalities.
Notably, the expression of the proto-oncogenes c-Fos and Jun-D was strikingly increased in conventional fourth-passage RA-SFB as compared with isolated primary-culture RA-SFB (increase of positive cells ≥ 38%; Fig. 7 and Table 5). This increase was limited to RA, since the percentage of cells positive for these molecules was significantly decreased in OA-SFB and numerically decreased in normal skin-FB upon passaging. As a consequence of these reciprocal changes, the percentage of positive cells and/or MFI for c-Fos and Jun-D in conventional fourth-passage RA-SFB was significantly higher than in OA-SFB, under these circumstances confirming previously published data [25].
Finally, a significant, positive correlation between the percentage of c-fos+ and procollagen I and III+ isolated primary-culture SFB in both RA and OA patients (RA: c-fos/procollagen I, ρ=1.000, P = 0.00, n = 7; c-fos/procollagen III, ρ=0.964, P = 0.00, n = 7) suggests a link between c-fos expression and augmented matrix production by SFB in rheumatic disorders, in line with the known regulation of collagen expression by the activator protein-1 transcription factor (reviewed in [13]).