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A major human acute phase protein, the serum amyloid A protein, has been tested in vitro for its effect on lymphocyte proliferation, the formation of E-stable rosettes, as well as the growth of HeLa and MRC5 cell cultures. Serum amyloid A protein has been found to be markedly inhibitory at 30, 100, 200, and 300 micrograms/mL, and is a very potent inhibitor of in vitro biological functions.

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We have shown previously that serum from patients with rheumatoid arthritis (RA) contains a polyclonal B cell activator that is associated with α2‐macroglobulin (α2M). Some biologic effects of this activator appear to be due to a trypsin‐like protease attached to α2M. Therefore, in the present study, we used an anti‐α2M antibody solid‐phase assay, with Chromozym‐Try as a substrate, to determine the level of α2M‐protease complexes in plasma α2M. We found higher levels of these complexes in RA patients than in 2 control groups. Since α2M‐protease complexes have been shown to induce RA‐like inflammation in experimental animals and to be produced by lymphoid cells, we speculate that they may be involved in the pathogenesis of RA. However, the role of the other cells or enzyme systems in the formation of these complexes has not yet been ruled out. Results of these investigations could lead to another link between activation of the immune system and joint inflammation.

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