Collagen arthritis in the rat is initiated by CD4+ T cells and can be amplified by iron.

The role of iron in arthritis was studied by administering ferric citrate (Fe-cit) to age-matched, female Sprague-Dawley rats immunized with chick type II collagen on Day 0. Rats received intravenously (iv) either Fe-cit (7.7 mg/kg body wt) or an identical concentration of sodium citrate on varying days after immunization. Transferrin saturation peaked (88-95%) 1 hr post-Fe-cit and returned to baseline values within 24 hr. Injection of Fe-cit on either Day 3 or Day 5, but not on Day 7 or Day 9, significantly (P less than 0.03) increased the incidence of arthritis. Synovium from the infrapatellar fat pad was harvested on Days 0-10 for analysis by immunocytochemistry. The inceptual morphologic change in the synovium following collagen immunization in rats not injected iv was an increase in the number of CD4+ and transferrin receptor+ mononuclear cells in perivascular regions; compared to Day 0 both cell types had increased two- to threefold by Day 3. On Day 7, an increase in CD8+ mononuclear cells occurred and the first polymorphonuclear leukocytes were noted. These alterations resulted in a peak in the CD4-CD8 ratio on Day 3, with a gradual decline thereafter. Although Fe-cit administration promoted the ingress of these mononuclear cells, it did not change the CD4-CD8 ratio significantly or recruit polymorphonuclear leukocytes into the joint tissue. Serum antibody titers to type II collagen, measured 20 days after immunization by an enzyme-linked immunosorbent assay, and delayed-type hypersensitivity to collagen, measured by a radiometric ear assay on Day 23, did not differ significantly between the groups. As well as showing that the initial intrasynovial event in collagen arthritis is perivascular infiltration by members of the CD4+ T cell subset displaying a phenotypic sign of activation, these findings demonstrate that iron administered at a critical time after immunization enhances the induction of collagen arthritis. The coincidence of this brief period of susceptibility with maximum CD4-CD8 ratios within the synovium and its occurrence prior to the stage of neutrophil infiltration are consistent with the possibility that the augmenting effect of iron is mediated by the inducer T cell subset.