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Synovial tissues are frequent sites of inflammatory disorders in which dendritic cells (DCs) may play an important role. This study examines potential antigen-presenting cells obtained from synovium-rich tissues (SRTs) by vascular perfusion of rat hind limbs with collagenase and further enzymatic digestion of the disarticulated hind paws in vitro. The three sub-populations of interest were: CD45+MHC IIhi, mainly CD11c+ and CD163-; CD45+MHC IIlo, mainly CD11c- and CD163+ and CD45+MHC II-, mainly CD11c- and CD163+. Expression of CD11c and CD163 correlated with ruffled cell-surface (CD11c+CD163-) and highly vacuolated cytoplasm (CD11c-CD163+), respectively. Culture of the CD45+CD163- sub-population in granulocyte macrophage colony-stimulating factor (GM-CSF) yielded CD45+MHC IIhi CD11c+CD163- cells with veiled morphology, while the large vacuolated cells that expressed CD163 resembled type A synoviocytes in both surface antigen phenotype and morphology. These results demonstrate that SRTs contain indeterminate cells that can differentiate into mature DCs in vitro in response to GM-CSF, plus mature synovial lining macrophages.

Adoptive transfer of adjuvant-induced arthritis was used in this study to examine local macrophages and dendritic cells (DCs) during T cell-mediated synovial inflammation. We studied the influx of CD11b + CD11c + putative myeloid DCs and other nonlymphoid CD45 + cells into synovium-rich tissues (SRTs) of the affected hind paws in response to a pulse of autoreactive thoracic duct cells. Cells were prepared from the SRTs using a collagenase perfusion-digestion technique, thus allowing enumeration and phenotypic analysis by flow cytometry. Numbers of CD45 + cells increased during the first 6 days, with increases in CD45 + MHC (major histocompatibility complex) II + monocyte-like cells from as early as day 3 after transfer. In contrast, typical MHC II -monocytes, mainly of the CD4 -subset, did not increase until 12 to 14 days after cell transfer, coinciding with the main influx of polymorphonuclear cells. By day 14, CD45 + MHC II hi cells constituted approximately half of all CD45 + cells in SRT. Most of the MHC II hi cells expressed CD11c and CD11b and represented putative myeloid DCs, whereas only approximately 20% were CD163 + macrophages. Less than 5% of the MHC II hi cells in inflamed SRT were CD11b -, setting a maximum for any influx of plasmacytoid DCs. Of the putative myeloid DCs, a third expressed CD4 and both the CD4 + and the CD4 -subsets expressed the co-stimulatory molecule CD172a. Early accumulation of MHC II hi CD11c + monocyte-like cells during the early phase of T cell-mediated inflammation, relative to typical MHC II -blood monocytes, suggests that recruited monocytes differentiate rapidly toward the DC lineage at this stage in the disease process. However, it is possible also that the MHC II hi CD11c + cells originate from a specific subset of DC-like circulating mononuclear cells.