Distribution of TNF‐α, TNF‐R55 and TNF‐R75 in the Rheumatoid Synovial Membrane: TNF Receptors are Localized Preferentially in the Lining Layer; TNF‐α is Distributed Mainly in the Vicinity of TNF Receptors in the Deeper Layers

The expression of TNF-alpha and its receptors in the rheumatoid synovial membrane was investigated using immunohistochemistry and immunocytofluorescence. TNF-alpha+ cells (< 10% of all cells) were found in all regions, predominantly in sublining and diffuse infiltrates. The highest percentage of TNF-R+ cells was found in the lining layer (50-90%), with a slight predominance of TNF-R55. In the sublining, fewer cells expressed TNF-R (approximately 50%), mostly TNF-R75. TNF-R75+ cells were also detectable in diffuse infiltrates and lymphoid aggregates (10-50%). These contained only individual TNF-R55+ cells. In diffuse infiltrates, there were slightly more TNF-R55+ cells than in lymphoid aggregates (in both cases < 10%). In sequential sections, TNF-alpha+ cells localized mostly in the vicinity of TNF-R+ cells. Macrophage-like cells appeared to be the predominant TNF-R+ cell type. CD3+ T cells in lymphoid aggregates expressed exclusively TNF-R75. Subsequently, the expression of membrane-bound TNF-alpha, TNF-R55 and TNF-R75 was tested by FACS analysis in isolated RA synoviocytes (n = 7 patients). Only four specimens expressed mTNF-alpha, and that on a low percentage of cells (2 +/- 2.4%; mean +/- SD). In contrast, all specimens expressed higher percentages of TNF-R55 and TNF-R75 (21 +/- 1% and 14 +/- 7.1%, respectively). These results demonstrate that: (1) the percentage of cells expressing soluble/transmembrane TNF-alpha is greatly outnumbered by the percentage of cells expressing TNF receptors; and (2) TNF-alpha-expressing cells are localized in regions expressing substantial levels of TNF receptors. Therefore, the known pro-inflammatory and pro-arthritic effects of TNF-alpha are probably mediated by local interactions between the receptors and their soluble and transmembrane ligands.