Germinal center entry not selection of B cells is controlled by peptide-MHCII complex density

Abstract

B cells expressing high affinity antigen receptors are advantaged in germinal centers (GC), perhaps by increased acquisition of antigen for presentation to follicular helper T cells and improved T-cell help. In this model for affinity-dependent selection, the density of peptide/MHCII (pMHCII) complexes on GC B cells is the primary determinant of selection. Here we show in chimeric mice populated by B cells differing only in their capacity to express MHCII (MHCII+/+ and MHCII+/−) that GC selection is insensitive to halving pMHCII density. Alone, both B cell types generate identical humoral responses; in competition, MHCII+/+ B cells are preferentially recruited to early GCs but this advantage does not persist once GCs are established. During GC responses, competing MHCII+/+ and MHCII+/− GC B cells comparably accumulate mutations and have indistinguishable rates of affinity maturation. We conclude that B-cell selection by pMHCII density is stringent in the establishment of GCs, but relaxed during GC responses.

T cell interactions with B cells during germinal center formation, a three‐step model

“…144 Remarkably, the minimal BCR affinity required for GC seeding is very low, and in the absence of competition, B cells bearing BCRs with inefficient cognate antigen binding can still receive sufficient T-cell help and seed GCs. 147 Similarly, we demonstrated that ICAMs on B cells are required for T-cell interaction with B cells; however, B cells deficient for ICAMs can still seed GCs. 93,145 In a recent study that examined whether the amount of MHCII on the surface of the B cells plays a role in GC seeding, it was found that B cells bearing two-fold less MHCII are outcompeted by WT B cells prior to GC coalescence.…”

Germinal center responses to complex antigens

“…In consequence, plasmacytes, which do not divide, cannot be studied by Nojima cultures. GC B cells, which may be fragile (1) or predisposed to differentiate into plasmacytes, have a lower cloning efficiencies (~25%) than naïve or memory B cells (70–80%) (38, 43). Despite these limitations, we have found Nojima cultures to be a useful tool for BCR repertoire analysis and an effective pathway for the identification and study of “interesting” Abs (38, 40).…”

“…We quantitatively tested this hypothesis by generating chimeric mice in which B cells differing only by their capacity to express MHCII could compete for entry into and success within GCs. Control of MHCII expression was by haploinsufficiency, that is, B cells either were homozygous or were heterozygous for functional MHCII loci (MHCII +/+ or MHCII +/− , respectively)(43). In some instances, we fixed the initial BCR affinity in both populations by a shared VDJ knockin; in others, we allowed both populations to express the full BCR repertoires available to them (43).…”

“…Control of MHCII expression was by haploinsufficiency, that is, B cells either were homozygous or were heterozygous for functional MHCII loci (MHCII +/+ or MHCII +/− , respectively)(43). In some instances, we fixed the initial BCR affinity in both populations by a shared VDJ knockin; in others, we allowed both populations to express the full BCR repertoires available to them (43). Absent competition, both MHCII +/+ and MHCII +/− B cells produced identical humoral responses, demonstrating that MHCII +/− and MHCII +/+ B cells are equally competent to mount Ab and GC responses.…”

“…The equivalent fitness of MHCII +/+ and MHCII +/− B cells during affinity maturation in GCs demonstrates that whatever the driver of affinity maturation in GCs may be, it is insensitive to two-fold differences in pMHCII expression. Given that the GC reaction is the paradigm for affinity-driven selection, it is remarkable that entry into the GC response exhibits a greater dependence on pMHCII density than affinity maturation during GC the GC response (43). How affinity-driven selection acts in GCs, and it surely does, remains something of a mystery.…”