SOLUBLE-ANTIGEN CAN IMPEDE AFFINITY MATURATION AND THE GERMINAL CENTER REACTION BUT ENHANCE EXTRAFOLLICULAR IMMUNOGLOBULIN PRODUCTION

The primary immune response to T cell-dependent Ags develops in two pa thways. These comprise the extrafollicular pathway, in which foci of A b-secreting cells develop, and the intrafollicular pathway that gives rise to germinal centers and affinity maturation. We have previously s hown that de-aggregated (4-hydroxy-3-nitrophenyl)acetyl (NP) coupled t o the protein carrier human serum albumin (HSA), (NP-HSA), injected 6 days after challenge immunization with aggregated NP-HSA, resulted in impaired development of NP-specific, higher-affinity cells. Studies pr esented here describe the cellular basis underlying this impairment of affinity maturation. Using multiparameter flow cytometry, we show tha t mice injected with soluble NP-HSA (''tolerant'' mice) develop signif icantly fewer NP-binding IgG1(+) B220(+) cells of germinal center orig in than do the control (''immune'') mice. In addition, using immunohis tology, we noted that the spleens oi tolerant mice had a marked reduct ion in the number of germinal centers that contained lambda-bearing ce lls, these being characteristic of the NP response in C57BL/6 mice. Cu riously, germinal centers in the spleens of tolerant mice had more tha n twice the volumes of those in the immune spleens. In contrast to its effect on the germinal center pathway, soluble Ag enhanced the extraf ollicular pathway, reflected by the increased numbers of B cells secre ting IgM and IgG1 Abs specific for NP, HSA, and undefined Ags. Thus, s oluble NP-HSA given after challenge immunization can impede affinity m aturation of NP-specific cells, but enhance the extrafollicular pathwa y. These results are discussed in the context of the known capacity of some persisting Ags, e.g., malaria parasites, to frustrate affinity m aturation and memory cell generation.