Germinal centres (GC) are lymphoid structures where B cells acquire affinity-enhancing somatic hypermutations (SHM), with surviving clones differentiating into memory B cells (MBCs) and long-lived bone marrow plasma cells (BMPCs) [1][2][3][4][5] . SARS-CoV-2 mRNA vaccination induces a persistent GC response that lasts for at least six months in humans [6][7][8] . The fate of responding GC B cells as well as the functional consequences of such persistence have not been elucidated. We detected SARS-CoV-2 spike (S)-specific MBCs in 42 individuals who had received two doses of BNT162b2, a SARS-CoV-2 mRNA vaccine six months earlier. S-specific IgG-secreting BMPCs were detected in 9 out of 11 participants. Using a combined approach of sequencing the B cell receptors of responding blood plasmablasts and MBCs, lymph node GC and plasma cells and BMPCs from eight individuals and expression of the corresponding monoclonal antibodies (mAbs), we tracked the evolution of 1540 S-specific B cell clones. We show that early blood S-specific plasmablasts -on averageexhibited the lowest SHM frequencies. In comparison, SHM frequencies of S-specific GC B cells increased by 3.5-fold within six months after vaccination. S-specific MBCs and BMPCs accumulated high levels of SHM, which corresponded with enhanced anti-S antibody avidity in blood and affinity as well as neutralization capacity of BMPC-derived mAbs. This study documents how the striking persistence of SARS-CoV-2 vaccination-induced GC reaction in humans culminates in affinity-matured long-term antibody responses that potently neutralize the virus. B cell response to mRNA vaccinationWe have previously shown that vaccination of humans with The Pfizer-BioNTech SARS-CoV-2 mRNA vaccine, BNT162b2 induces a robust but transient circulating plasmablast (PB) response and a persistent germinal centre (GC) reaction in the draining lymph nodes 6 . Whether these persistent GC responses lead to the generation of affinity-matured memory B cells (MBCs) and long-lived bone marrow-resident plasma cells (BMPCs) remains unclear. To address this question, we analyzed long-term B cell responses in the participants enrolled in our previously described observational study of 43 healthy participants (13 with a history of SARS-CoV-2 infection) who received two doses of BNT162b2 (Extended Data Tables 1) 6,7 . Long-term blood samples (n=42) and fine needle aspirates (FNAs) of the draining axillary lymph nodes (n=15) were collected 29 weeks post-vaccination (Fig. 1a). Bone marrow aspirates were collected 29 (n=11) and 40 weeks (n=2) post-vaccination, with the latter time point used only for B cell receptor (BCR) repertoire profiling (Fig. 1a). None of the participants who contributed FNA or bone marrow specimens had SARS-CoV-2 infection history. GC B cells were detected in FNAs from all 15 participants (Fig. 1b, c, left panels, Extended Data Fig. 1a, Extended Data Table 2). All 14 participants with FNAs collected prior to week 29 generated S-binding GC B cell responses of varying magnitudes (Fig 1b, c, r...

Plasmodium parasite-specific antibodies are critical for protection against malaria, yet the development of long-lived and effective humoral immunity against Plasmodium takes many years and multiple rounds of infection and cure. Here we report that the rapid development of short-lived plasmablasts during experimental malaria unexpectedly hindered parasite control by impeding germinal center (GC) responses. Metabolic hyperactivity of plasmablasts resulted in nutrient deprivation of the GC reaction limiting the generation of memory B cell and long-lived plasma cell responses. Therapeutic administration of a single amino acid to experimentally infected mice was sufficient to overcome the metabolic constraints imposed by plasmablasts and enhanced parasite clearance and the formation of protective humoral immune memory responses. Thus, our studies not only challenge the current paradigm describing the role and function of blood-stage Plasmodium -induced plasmablasts, but also reveal new targets and strategies to improve anti- Plasmodium humoral immunity.

The primary two-dose SARS-CoV-2 mRNA vaccine series are strongly immunogenic in humans, but the emergence of highly infectious variants necessitated additional doses of these vaccines and the development of new variant-derived ones. SARS-CoV-2 booster immunizations in humans primarily recruit pre-existing memory B cells (MBCs). It remains unclear, however, whether the additional doses induce germinal centre (GC) reactions where reengaged B cells can further mature and whether variant-derived vaccines can elicit responses to novel epitopes specific to such variants. Here, we show that boosting with the original SARS-CoV-2 spike vaccine (mRNA-1273) or a B.1.351/B.1.617.2 (Beta/Delta) bivalent vaccine (mRNA-1273.213) induces robust spike-specific GC B cell responses in humans. The GC response persisted for at least eight weeks, leading to significantly more mutated antigen-specific MBC and bone marrow plasma cell compartments. Interrogation of MBC-derived spike-binding monoclonal antibodies (mAbs) isolated from individuals boosted with either mRNA-1273, mRNA-1273.213, or a monovalent Omicron BA.1-based vaccine (mRNA-1273.529) revealed a striking imprinting effect by the primary vaccination series, with all mAbs (n=769) recognizing the original SARS-CoV-2 spike protein. Nonetheless, using a more targeted approach, we isolated mAbs that recognized the spike protein of the SARS-CoV-2 Omicron (BA.1) but not the original SARS-CoV-2 spike from the mRNA-1273.529 boosted individuals. The latter mAbs were less mutated and recognized novel epitopes within the spike protein, suggesting a naïve B cell origin. Thus, SARS-CoV-2 boosting in humans induce robust GC B cell responses, and immunization with an antigenically distant spike can overcome the antigenic imprinting by the primary vaccination series.

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Germinal centres (GC) are lymphoid structures where vaccine-responding B cells acquire affinity-enhancing somatic hypermutations (SHM), with surviving clones differentiating into memory B cells (MBCs) and long-lived bone marrow plasma cells (BMPCs)1–4. Induction of the latter is a hallmark of durable immunity after vaccination5. SARS-CoV-2 mRNA vaccination induces a robust GC response in humans6–8, but the maturation dynamics of GC B cells and propagation of their progeny throughout the B cell diaspora have not been elucidated. Here we show that anti-SARS-CoV-2 spike (S)-binding GC B cells were detectable in draining lymph nodes for at least six months in 10 out of 15 individuals who had received two doses of BNT162b2, a SARS-CoV-2 mRNA vaccine. Six months after vaccination, circulating S-binding MBCs were detected in all participants (n=42) and S-specific IgG-secreting BMPCs were detected in 9 out of 11 participants. Using a combined approach of single-cell RNA sequencing of responding blood and lymph node B cells from eight participants and expression of the corresponding monoclonal antibodies, we tracked the evolution of 1540 S-specific B cell clones. SHM accumulated along the B cell differentiation trajectory, with early blood plasmablasts showing the lowest frequencies, followed by MBCs and lymph node plasma cells whose SHM largely overlapped with GC B cells. By three months after vaccination, the frequency of SHM within GC B cells had doubled. Strikingly, S+ BMPCs detected six months after vaccination accumulated the highest level of SHM, corresponding with significantly enhanced anti-S polyclonal antibody avidity in blood at that time point. This study documents the induction of affinity-matured BMPCs after two doses of SARS-CoV-2 mRNA vaccination in humans, providing a foundation for the sustained high efficacy observed with these vaccines.

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.

Immunity against malaria depends on germinal center (GC)-derived antibody responses that are orchestrated by T follicular helper (TFH) cells. Emerging data show that the regulatory cytokine IL-10 plays an essential role in promoting GC B cell responses during both experimental malaria and virus infections. Here we investigated the cellular source and temporal role of IL-10, and whether IL-10 additionally signals to CD4 T-cells to support anti-Plasmodium humoral immunity. Distinct from reports of virus infection, we found that IL-10 expressed by conventional, Foxp3-negative effector CD4 T cells and functioned in a B cell-intrinsic manner only during the first 96 hours of Plasmodium infection to support humoral immunity. The critical functions of IL-10 manifested only before the orchestration of GC responses and were primarily localized outside of B cell follicles. Mechanistically, our studies showed that the rapid and transient provision of IL-10 promoted B cell expression of anti-apoptotic factors, MHC class II, CD83, and cell-cell adhesion proteins that are essential for B cell survival and interaction with CD4 T cells. Together, our data reveal temporal features and mechanisms by which IL-10 critically supports humoral immunity during blood-stage Plasmodium infection, information that may be useful for developing new strategies designed to lessen the burden of malaria.

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