Leukocyte sensitization against synovial components in rheumatoid arthritis: blocking by pregnancy serum

The leukocyte adherence inhibition (LAI) test in arthritis can serve as an in vitro model for investigating cellular cooperation between mononuclear cells sensitized by cytophilic IgG rheumatoid factor(s) present in rheumatoid arthritis (RA) serum, normal synovial components, and polymorphonuclear cells. Since inflammatory and immunologic responses are altered during pregnancy, we studied the effect of pregnancy serum on the LAI. The adherence of normal leukocytes once sensitized in RA serum is inhibited by a synovial extract, but when these leukocytes are exposed to pregnancy serum prior to sensitization with RA serum, the cells are unresponsive to subsequent stimulation by the synovial extract (p less than 0.01). This blocking effect on sensitization is only present in serum obtained after the first trimester of pregnancy and can be detected even at serum dilutions of 1:100. Leukocytes obtained directly from pregnant subjects, washed, and then incubated with RA serum failed to react. Since pregnancy serum has been shown to mask certain cell membrane receptors, it is possible that our results are due to the blocking of IgG rheumatoid factor binding to mononuclear cells. This finding could contribute to the subsidence during pregnancy of some disorders such as rheumatoid arthritis.