Antibodies to the hemagglutinin (HA) and neuraminidase (NA) glycoproteins are the major mediators of protection against influenza virus infection. Here, we report that current influenza vaccines poorly display key NA epitopes and rarely induce NA-reactive B cells. Conversely, influenza virus infection induces NA-reactive B cells at a frequency that approaches (H1N1) or exceeds (H3N2) that of HA-reactive B cells. NA-reactive antibodies display broad binding activity spanning the entire history of influenza A virus circulation in humans, including the original pandemic strains of both H1N1 and H3N2 subtypes. The antibodies robustly inhibit the enzymatic activity of NA, including oseltamivir-resistant variants, and provide robust prophylactic protection, including against avian H5N1 viruses, in vivo. When used therapeutically, NA-reactive antibodies protected mice from lethal influenza virus challenge even 48 hr post infection. These findings strongly suggest that influenza vaccines should be optimized to improve targeting of NA for durable and broad protection against divergent influenza strains.

In this study, we report that antigen-specific CD19+CD27+CD21lo (CD21lo) B cells are transiently induced 14-28 days after immunization, at the time germinal centers (GCs) peak. Although clonally related to memory B cells and plasmablasts, CD21lo cells form distinct clades within phylogenetic trees based on accumulated variable gene mutations, supporting exit from active GCs. CD21lo cells express a transcriptional program suggesting they are primed for plasma cell differentiation and are refractory to GC differentiation, although they do not spontaneously secrete antibody. Additionally, CD21lo cells differentially express multiple cell surface markers and have elevated intracellular levels of Blimp-1 and T-bet protein compared to memory B cells. Together, these data support a model in which CD21lo cells are recent GC graduates that represent a distinct population from CD27+ classical memory cells, are refractory to GC reentry and are predisposed to differentiate into long-lived plasma cells.

Antimalarial antibody responses are essential for mediating the clearance of Plasmodium parasite–infected RBCs from infected hosts. However, the rapid appearance of large numbers of plasmablasts in Plasmodium-infected hosts can suppress the development and function of durable humoral immunity. Here, we identify that the formation of plasmablast populations in Plasmodium-infected mice is mechanistically linked to both hemolysis-induced exposure of phosphatidylserine on damaged RBCs and inflammatory cues. We also show that virus and Trypanosoma infections known to trigger hemolytic anemia and high-grade inflammation also induce exuberant plasmablast responses. The induction of hemolysis or administration of RBC membrane ghosts increases plasmablast differentiation. The phosphatidylserine receptor Axl is critical for optimal plasmablast formation, and blocking phosphatidylserine limits plasmablast expansions and reduces Plasmodium parasite burden in vivo. Our findings support that strategies aimed at modulating polyclonal B cell activation and phosphatidylserine exposure may improve immune responses against Plasmodium parasites and potentially other infectious diseases that are associated with anemia.