Early and Enhanced Germinal Center Formation and Antibody Responses in Mice after Primary Stimulation with Antigen-Isologous Antibody Complexes as Compared with Antigen Alone

Abstract

Primary antibody responses elicited in mice by injection into the hind leg footpads of tetanus toxoid complexed with isologous antibody at equivalence were enhanced as compared with responses to antigen alone. Serum antitoxin titers in animals given immune complexes started to rise approximately 4 days earlier and reached peak values more than 15 times higher than in mice given antigen alone. In both groups of animals, however, the doubling time of serum antitoxin concentrations was similar and approximated 30 hr as compared with 8 hr in anamnestic responses to tetanus toxoid. These differences were reflected in de novo appearance and growth of germinal centers in popliteal lymph nodes. The median number of germinal centers/lymph node cross-section of complex-injected animals began to rise significantly 4 days after stimulation and throughout days 4 to 16 exceeded corresponding values for mice given tetanus toxoid. The median numbers of germinal centers in both groups of stimulated animals was maximal around day 12 and fell to near-control values at day 20, indicating that many of these structures underwent involution within 3 wk following de novo formation. The potentiating effect of antigen complexes as compared with antigen was seen in the significantly larger size of newly formed germinal centers from day 8 through day 24 following stimulation. The best estimate for volume doubling time of developing germinal centers after injection of immune complexes was 10 hr or less, which was near corresponding values proposed previously for germinal center growth in regional lymph nodes during anamnestic responses. Labeling patterns 1 hr after i.v. injection of 3H-thymidine and mitotic activity of lymphoid germinal center cells revealed no significant differences between the two groups of stimulated mice. These results were discussed in view of available evidence that identified lymphoid germinal center cells as precursors of antibody-forming elements possibly related to production of a certain class or subclass of immunoglobulins. The question was raised whether in vivo formation of immune complexes is also instrumental in triggering the germinal center system in the course of a true primary antibody response to antigen alone.