Serosuppression in experimental filariasis.

Both antigen specific and non-specific immunoregulation by cells have been described in jirds infected with Brugia pahangi, but the contribution of serum factors to immunoregulatory phenomena in this infection has not been examined. The present study determined the effect of serum from normal or B. pahangi infected jirds on the mitogen responsiveness of spleen cells from uninfected animals and on the antigen responsiveness of lymph node cells (LNC) from infected jirds. Addition of heat-inactivated jird serum to cultures of cells supplemented with 1% fetal bovine serum demonstrated that serum from chronically infected (greater than or equal to 20 weeks post-infection), but not normal jirds consistently suppressed responsiveness of LNC from infected jirds to B. pahangi extracts in a dose-dependent manner (0.25%-1% concentration). A comparison of sera from jirds at different times post-infection demonstrated that sera (1%) from chronically infected (30 weeks; 100% suppression), but not acutely infected (4 weeks; 0% suppression) or recently microfilaremic (10 weeks; 11% suppression) animals were capable of suppressing antigen reactivity of LNC. In contrast, the inhibitory effect of serum on lymphocyte reactivity to the mitogens, PHA and PWM, was observed intermittently throughout the course of the infection and was less than the effect of chronic serum on antigen responsiveness. The B. pahangi antigen response of spleen cells from infected jirds depleted of suppressor cells by fractionation over nylon wool was also inhibited by chronic sera. Following fractionation of chronic sera by Sephadex G-200 chromatography, suppressor activity was observed in the void volume and IgG peaks. Suppressor activity was not associated with protein A, anti-jird Ig, or B. pahangi antigen bound fractions, nor with polyethylene glycol precipitable material.