Vaccines are the most effective intervention currently available, offering protective immunity against targeted pathogens. The emergence of the coronavirus disease 2019 pandemic has prompted rapid development and deployment of lipid nanoparticle encapsulated, mRNA-based vaccines. While these vaccines have demonstrated remarkable immunogenicity, concerns persist regarding their ability to confer durable protective immunity to continuously evolving severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. This review focuses on human B cell responses induced by SARS-CoV-2 mRNA vaccination, with particular emphasis on the crucial role of germinal center reactions in shaping enduring protective immunity. Additionally, we explored observations of immunological imprinting and dynamics of recalled pre-existing immunity following variants of concern-based booster vaccination. Insights from this review contribute to comprehensive understanding B cell responses to mRNA vaccination in humans, thereby refining vaccination strategies for optimal and sustained protection against evolving coronavirus variants.

Following transplantation, human CD4+T cells can respond to alloantigen using three distinct pathways. Direct and semi-direct responses are considered potent, but brief, so contribute mostly to acute rejection. Indirect responses are persistent and prolonged, involve B cells as critical antigen presenting cells, and are an absolute requirement for development of donor specific antibody, so more often mediate chronic rejection. Novel in vitro techniques have furthered our understanding by mimicking in vivo germinal centre processes, including B cell antigen presentation to CD4+ T cells and effector cytokine responses following challenge with donor specific peptides. In this review we outline recent data detailing the contribution of CD4+ T follicular helper cells and antigen presenting B cells to donor specific antibody formation and antibody mediated rejection. Furthermore, multi-parametric flow cytometry analyses have revealed specific endogenous regulatory T and B subsets each capable of suppressing distinct aspects of the indirect response, including CD4+ T cell cytokine production, B cell maturation into plasmablasts and antibody production, and germinal centre maturation. These data underpin novel opportunities to control these aberrant processes either by targeting molecules critical to indirect alloresponses or potentiating suppression via exogenous regulatory cell therapy.

Follicular helper CD4hi T cells (TFH) are a major cellular pool for the maintenance of the HIV reservoir. Therefore, the delineation of the follicular (F)/germinal center (GC) immune landscape will significantly advance our understanding of HIV pathogenesis. We have applied multiplex confocal imaging, in combination with the relevant computational tools, to investigate F/GC in situ immune dynamics in viremic (vir-HIV), antiretroviral-treated (cART HIV) People Living With HIV (PLWH) and compare them to reactive, non-infected controls. Lymph nodes (LNs) from viremic and cART PLWH could be further grouped based on their TFH cell densities in high-TFH and low-TFH subgroups. These subgroups were also characterized by different in situ distributions of PD1hi TFH cells. Furthermore, a significant accumulation of follicular FOXP3hiCD4hi T cells, which were characterized by a low scattering in situ distribution profile and strongly correlated with the cell density of CD8hi T cells, was found in the cART-HIV low-TFH group. An inverse correlation between plasma viral load and LN GrzBhiCD8hi T and CD16hiCD15lo cells was found. Our data reveal the complex GC immune landscaping in HIV infection and suggest that follicular FOXP3hiCD4hi T cells could be negative regulators of TFH cell prevalence in cART-HIV.

The successful development of germinal centers (GC) relies heavily on innate mechanisms to amplify the initial inflammatory cascade. In addition to their role in antigen presentation, innate cells are essential for the redirection of circulating lymphocytes toward the draining lymph node (dLN) to maximize antigen surveillance. Sphingosine-1-Phosphate (S1P) and its receptors (S1PR1-5) affect various aspects of immunity; however, the role of S1PR4 in regulating an immune response is not well understood. Here we use a footpad model of localized TH1 inflammation to carefully monitor changes in leukocyte populations within the blood, the immunized tissue, and the dLN. Within hours of immunization, neutrophils failed to adequately mobilize and infiltrate into the footpad tissue of S1PR4-/- mice, thereby diminishing the local vascular changes thought to be necessary for redirecting circulating cells toward the inflamed region. Neutrophil depletion with anti-Ly6G antibodies significantly reduced early tissue edema as well as the redirection and initial accumulation of naïve lymphocytes in dLN of WT mice, while the effects were less prominent or absent in S1PR4-/- dLN. Adoptive transfer experiments further demonstrated that the lymphocyte homing deficiencies in vivo were not intrinsic to the donor S1PR4-/- lymphocytes, but were instead attributed to differences within the S1PR4-deficient host. Reduced cell recruitment in S1PR4-/- mice would seed the dLN with fewer antigen-respondent lymphocytes and indeed, dLN hypertrophy at the peak of the immune response was severely diminished, with attenuated GC and activation pathways in these mice. Histological examination of the S1PR4-/- dLN also revealed an underdeveloped vascular network with reduced expression of the leukocyte tethering ligand, PNAd, within high endothelial venule regions, suggesting inadequate growth of the dLN meant to support a robust GC response. Thus, our study reveals that S1PR4 may link early immune modulation by neutrophils to the initial recruitment of circulating lymphocytes and downstream expansion and maturation of the dLN, thereby contributing to optimal GC development during an adaptive response.

The myocyte enhancer factor 2B (MEF2B) transcription factor is frequently mutated in germinal center (GC)-derived B-cell lymphomas. Its ammino (N)-terminal mutations drive lymphomagenesis by escaping interaction with transcriptional repressors, while the function of carboxy (C)-terminal mutations remains to be elucidated. Here, we show that MEF2B C-tail is physiologically phosphorylated at specific residues and phosphorylation at serine (S)324 is impaired by lymphoma-associated mutations. Lack of phosphorylation at S324 enhances the interaction of MEF2B with the SWI/SNF chromatin remodeling complex, leading to higher transcriptional activity. In addition, these mutants show an increased protein stability due to impaired interaction with the CUL3/KLHL12 ubiquitin complex. Mice expressing a phosphorylation-deficient lymphoma-associated MEF2B mutant display GC enlargement and develop GC-derived lymphomas, when crossed with Bcl2 transgenic mice. These results unveil converging mechanisms of action for a diverse spectrum of MEF2B mutations, all leading to its dysregulation and GC B-cell lymphomagenesis.

When mature B cells are activated by antigens, the selection of these activated B cells takes place particularly during T cell-dependent immune responses in which an improved antibody repertoire is generated through somatic hypermutation in germinal centers (GCs). In this process the importance of antigen presentation by GC B cells, and subsequent T follicular helper (Tfh) cell help in positive selection of GC B cells, has been well appreciated. By contrast, the role of B cell receptor (BCR) signaling per se remains unclear. Strong experimental support for the involvement of BCR signaling in GC B cell selection has now been provided. Interestingly, these studies suggest that several checkpoints operating through the BCR ensure affinity maturation.

Memory B cells (MBCs) formed over the individual\'s lifetime constitute nearly half of the circulating B cell repertoire in humans. These pre-existing MBCs dominate recall responses to their cognate antigens, but how they respond to recognition of novel antigens is not well understood. Here, we tracked the origin and followed the differentiation paths of MBCs in the early anti-spike (S) response to mRNA vaccination in SARS-CoV-2-naive individuals on single-cell and monoclonal antibody levels. Pre-existing, highly mutated MBCs showed no signs of germinal center re-entry and rapidly developed into mature antibody-secreting cells (ASCs). By contrast, and despite similar levels of S reactivity, naive B cells showed strong signs of antibody affinity maturation before differentiating into MBCs and ASCs. Thus, pre-existing human MBCs differentiate into ASCs in response to novel antigens, but the quality of the humoral and cellular anti-S response improved through the clonal selection and affinity maturation of naive precursors.

The differentiation and specificity of human CD4+ T follicular helper cells (TFH cells) after influenza vaccination have been poorly defined. Here we profiled blood and draining lymph node (LN) samples from human volunteers for over 2 years after two influenza vaccines were administered 1 year apart to define the evolution of the CD4+ TFH cell response. The first vaccination induced an increase in the frequency of circulating TFH (cTFH) and LN TFH cells at week 1 postvaccination. This increase was transient for cTFH cells, whereas the LN TFH cells further expanded during week 2 and remained elevated in frequency for at least 3 months. We observed several distinct subsets of TFH cells in the LN, including pre-TFH cells, memory TFH cells, germinal center (GC) TFH cells and interleukin-10+ TFH cell subsets beginning at baseline and at all time points postvaccination. The shift toward a GC TFH cell phenotype occurred with faster kinetics after the second vaccine compared to the first vaccine. We identified several influenza-specific TFH cell clonal lineages, including multiple responses targeting internal influenza virus proteins, and found that each TFH cell state was attainable within a clonal lineage. Thus, human TFH cells form a durable and dynamic multitissue network.

The mammalian Brg1/Brm-associated factor (BAF) complexes are major regulators of nucleosomal remodeling that are commonly mutated in several cancers, including germinal center (GC)-derived B cell lymphomas. However, the specific roles of different BAF complexes in GC B cell biology are not well understood. Here we show that the AT-rich interaction domain 1a (Arid1a) containing canonical BAF (cBAF) complex is required for maintenance of GCs and high-affinity antibody responses. While Arid1a-deficient B cells undergo initial activation, they fail to sustain the GC program. Arid1a establishes permissive chromatin landscapes for B cell activation and is concomitantly required to suppress inflammatory gene programs. The inflammatory signatures instigated by Arid1a deficiency promoted the recruitment of neutrophils and inflammatory monocytes. Dampening of inflammatory cues through interleukin-1β blockade or glucocorticoid receptor agonist partially rescued Arid1a-deficient GCs, highlighting a critical role for inflammation in impeding GCs. Our work reveals essential functions of Arid1a-dependent cBAF in promoting efficient GC responses.

Ligation of the B cell antigen receptor (BCR) initiates humoral immunity. However, BCR signaling without appropriate co-stimulation commits B cells to death rather than to differentiation into immune effector cells. How BCR activation depletes potentially autoreactive B cells while simultaneously primes for receiving rescue and differentiation signals from cognate T lymphocytes remains unknown. Here, we use a mass spectrometry-based proteomic approach to identify cytosolic/nuclear shuttling elements and uncover transcription factor EB (TFEB) as a central BCR-controlled rheostat that drives activation-induced apoptosis, and concurrently promotes the reception of co-stimulatory rescue signals by supporting B cell migration and antigen presentation. CD40 co-stimulation prevents TFEB-driven cell death, while enhancing and prolonging TFEB\'s nuclear residency, which hallmarks antigenic experience also of memory B cells. In mice, TFEB shapes the transcriptional landscape of germinal center B cells. Within the germinal center, TFEB facilitates the dark zone entry of light-zone-residing centrocytes through regulation of chemokine receptors and, by balancing the expression of Bcl-2/BH3-only family members, integrates antigen-induced apoptosis with T cell-provided CD40 survival signals. Thus, TFEB reprograms antigen-primed germinal center B cells for cell fate decisions.