ANALYSIS OF MUTATIONS IN IMMUNOGLOBULIN HEAVY-CHAIN VARIABLE REGION GENES OF MICRODISSECTED MARGINAL ZONE (MGZ) B-CELLS SUGGESTS THAT THE MGZ OF HUMAN SPLEEN IS A RESERVOIR OF MEMORY B-CELLS

The splenic marginal zone (MGZ), which surrounds the mantle zone (MTZ) in human splenic white pulp, contains a phenotypically and morphologi cally distinct population of B cells. The origin of MGZ B cells is unc ertain. Whereas some experiments in rodents have suggested that they a re a distinct cell lineage responsible for the immune response to T-in dependent type 2 antigens, others have suggested that they are memory B cells derived from a germinal center (GC) response. The progeny of a GC reaction is expected to have rearranged immunoglobulin (Ig) genes that are mutated. The distribution of mutations would be expected to r eflect the selection of Ig by its affinity for antigen. We have analyz ed rearranged Ig heavy chain variable region (V-H) 6 and V-H 4.21 gene s in MGZ and MTZ B cells microdissected from frozen sections of human spleen to determine whether these genes have the properties of an affi nity-selected memory B cell population. MTZ B cells contained germline Ig V-H genes, confirming previous reports and providing an internal c ontrol for mutational analysis. MGZ B cells contained Ig V-H genes tha t were mutated, showing that these cells had been subjected to a mutat ional mechanism characteristically active in the GC. The rearranged V- H 6 genes showed patterns of mutation indicative of an antigen selecti on process, whereas the distribution of mutations in V-H 4.21 genes wa s not characteristic of gene selection by conventional T-dependent ant igen. Our studies provide the first evidence that the human splenic MG Z is a reservoir of memory B cells.