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.