ANTIGEN DRIVES VERY-LOW AFFINITY B-CELLS TO BECOME PLASMACYTES AND ENTER GERMINAL-CENTERS

In the first week of the primary immune response to the (4-hydroxy-3-n itrophenyl)acetyl (NP) hapten, plasmacytic foci and germinal centers ( GCs) in C57BL/6 mice are comprised of polyclonal populations of B lymp hocytes bearing the hi L-chain (lambda 1(+)). The Ig H-chains of these early populations of B cells are encoded by a variety of V-H and D ex ons undiversified by hypermutation while later, oligoclonal population s are dominated by mutated rearrangements of the V(H)186.2 and DFL16.1 gene segments. To assess directly Ab affinities within these defined splenic microenvironments, representative VDJ rearrangements were reco vered from B cells participating in the early immune response to NP, i nserted into Ig II-chain expression cassettes, and transfected into J5 58L (H-; lambda 1(+)) myeloma cells. These transfectoma Abs expressed a remarkably wide range of measured affinities (K-a = 5 x 10(4)-1.3 x 10(6) M-1) for NP. VDJs recovered from both foci and early GCs generat ed comparable affinities, suggesting that initial differentiation into these compartments occurs stochastically. We conclude that Ag normall y activates B cells bearing an unexpectedly wide spectrum of Ab affini ties and that this initial, promiscuous clonal activation is followed by affinity-driven competition to determine survival and clonal expans ion within GCs and entry into the memory and bone marrow plasmacyte co mpartments.