Kinetic measurement of the interaction of rheumatoid factor with IgG-coated latex particles and the influence of the first component of human complement.

Interaction between isolated rheumatoid factor (RF) of the IgM class or sera from patients with rheumatoid arthritis (RA) and IgG-coated latex particles has been studied kinetically by means of standard aggregometer equipment. The agglutination of particles mediated by isolated RF or RA-sera is inhibited by fresh normal human serum (NHS). The RF-inhibiting principle is heat-labile and recovered in the high molecular weight fractions of NHS separated on a G-200 column. Partially purified first component of complement, C1, also inhibits RF-mediated particle agglutination and disintegrates preformed RF-IgG-latex particle agglutinates. Addition of C1 to heated (56 degrees C, 30 min) NHS restores its RF-inhibiting activity. The most probable basis of this serum activity is competition between C1 with higher affinity for IgG bound to particles and RF. After about 5 min of incubation of NHS with IgG-latex particles the RF-inhibiting activity is gradually lost and interpreted to mean that C1 during the activation of the complement system is discharged from IgG bound to particles. The RF-inhibiting activity of NHS gradually decreases by incubation of serum with increasing doses of activators of the classical complement pathway probably due to the inability of activated C1 to hinder RF-interaction with IgG-particles. The presence in certain RA-sera of C1 in mainly activated form explains why such sera, even if they are fresh are able to agglutinate IgG-particles.