Epitope Masking during a Recall Response: Implications for Covid and Hiv Vaccines

Some of the first immunological experiments of the 20th century-on antibody feedback inhibition-demonstrated that past exposure does not merely expedite the humoral response to secondary challenges but, rather, alters the nature of that response. Despite this history, the mechanisms underpinning these classic observations and their relationship to modern vaccine design remain obscure. Circulating antibodies produced during a primary challenge can have enhancing or, conversely, inhibitory effects on later humoral responses. Using preclinical vaccine models, we dissected this apparent contradiction to find that the interaction between serum antibodies and the entry of cognate naive B cell lineages to germinal centers was determined by the interplay of breadth, affinity, and titer. Epitope-focused vaccine designs for HIV-1 elicit circulating antibodies capable of entirely obstructing their cognate naive B cells, with important implications for the ongoing design of boostphase immunogens. Conversely, SARS-CoV receptor binding domain (RBD) immunization elicits a polyclonal serum response that enhances the proportion of high-affinity cognate B cells in germinal centers, providing a partial explanation for the effectiveness of current boost protocols but also presaging diminishing returns unless new epitopes are introduced. The resolution of these surface-level contradictions points toward a unified interactive model and emphasizes that, to move vaccinology forward, the basic biology of the humoral immune system cannot remain a black box.

Advances in understanding the formation and fate of B-cell memory in response to immunization or infection.

Immunological memory has the potential to provide lifelong protection against recurrent infections. As such, it has been crucial to the success of vaccines. Yet, the recent pandemic has illuminated key gaps in our knowledge related to the factors influencing effective memory formation and the inability to predict the longevity of immune protection. In recent decades, researchers have acquired a number of novel and powerful tools with which to study the factors underpinning humoral memory. These tools have been used to study the B-cell fate decisions that occur within the germinal centre (GC), a site where responding B cells undergo affinity maturation and are one of the major routes for memory B cell and high-affinity long-lived plasma cell formation. The advent of single-cell sequencing technology has provided an enhanced resolution for studying fate decisions within the GC and cutting-edge techniques have enabled researchers to model this reaction with more accuracy both in vitro and in silico. Moreover, modern approaches to studying memory B cells have allowed us to gain a better appreciation for the heterogeneity and adaptability of this vital class of B cells. Together, these studies have facilitated important breakthroughs in our understanding of how these systems operate to ensure a successful immune response. In this review, we describe recent advances in the field of GC and memory B-cell biology in order to provide insight into how humoral memory is formed, as well as the potential for generating lasting immunity to novel pathogens such as severe acute respiratory syndrome coronavirus 2.

Tfh cells and the germinal center are required for memory B cell formation & humoral immunity after ChAdOx1 nCoV-19 vaccination.

Emergence from the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has been facilitated by the rollout of effective vaccines. Successful vaccines generate high-affinity plasma blasts and long-lived protective memory B cells. Here, we show a requirement for T follicular helper (Tfh) cells and the germinal center reaction for optimal serum antibody and memory B cell formation after ChAdOx1 nCoV-19 vaccination. We found that Tfh cells play an important role in expanding antigen-specific B cells while identifying Tfh-cell-dependent and -independent memory B cell subsets. Upon secondary vaccination, germinal center B cells generated during primary immunizations can be recalled as germinal center B cells again. Likewise, primary immunization GC-Tfh cells can be recalled as either Tfh or Th1 cells, highlighting the pluripotent nature of Tfh cell memory. This study demonstrates that ChAdOx1 nCoV-19-induced germinal centers are a critical source of humoral immunity.

Antibody Dynamics and Durability in Coronavirus Disease-19.

Severe acute respiratory syndrome coronavirus 2 (COVID)-19 has emerged as the greatest global health threat in generations. An unprecedented mobilization of researchers has generated a wealth of data on humoral responses to SARS-CoV-2 within a year of the pandemic's beginning. The rapidly developed understanding of acute-phase antibody induction and medium-term antibody durability in COVID-19 is important at an individual level to inform patient care and a population level to help predict transmission dynamics. In this brief review, we will describe the development and maintenance of antibody responses to immunization and infections generally and the specific antibody dynamics observed for COVID-19. These crucial features of the humoral response have implications for the use of antibody therapeutics against the virus and can inform the likelihood of reinfection of individuals by the virus.

Innate immune mechanisms of mRNA vaccines.

The lipid nanoparticle (LNP)-encapsulated, nucleoside-modified mRNA platform has been used to generate safe and effective vaccines in record time against COVID-19. Here, we review the current understanding of the manner whereby mRNA vaccines induce innate immune activation and how this contributes to protective immunity. We discuss innate immune sensing of mRNA vaccines at the cellular and intracellular levels and consider the contribution of both the mRNA and the LNP components to their immunogenicity. A key message that is emerging from recent observations is that the LNP carrier acts as a powerful adjuvant for this novel vaccine platform. In this context, we highlight important gaps in understanding and discuss how new insight into the mechanisms underlying the effectiveness of mRNA-LNP vaccines may enable tailoring mRNA and carrier molecules to develop vaccines with greater effectiveness and milder adverse events in the future.

B Cells Fit for Germinal Center Activity Predict Response to a Third Dose of SARS-CoV-2 Vaccine in Solid Organ Transplant Recipients

Purpose: While SARS-CoV-2 vaccination has dramatically reduced COVID-19 severity in the general population, fully vaccinated solid organ transplant recipients (SOTRs) demonstrate reduced seroconversion and increased breakthrough infection rates. Furthermore, a third vaccine dose only increases antibody and T cell responses in a proportion of SOTRs. We sought to investigate the underlying mechanisms resulting in varied humoral responses in SOTRs. Method(s): Within a longitudinal prospective cohort of SOTRs, anti-spike IgG, total and spike-specific B cells were evaluated in 44 SOTR participants before and after a third vaccine dose using high dimensional flow cytometry to assess immunologic and metabolic phenotypes. B cell phenotypes were compared to those of 10 healthy controls who received a standard two-dose mRNA series. Result(s): Notably, even in the absence anti-spike antibody after two doses, spikespecific B cells were detectable in most SOTRs (76%). While 15% of participants were seropositive before the third dose, 72% were seropositive afterward. B cells, however, were differentially skewed towards non-class switched B cells in SOTRs as compared to healthy control B cells. Expansion of spike-specific class-switched B cells in SOTRs following a third vaccine dose correlated with increased classswitched (IgG) antibody titers. Antibody response to a third vaccine dose was associated with expanded populations of germinal center-like (CD10+CD27+) B cells, as well as CD11c+ alternative lineage B cells with specific upregulation of CPT1a, the rate limiting enzyme of fatty acid oxidation and a preferred energy source of germinal center B cells. Conclusion(s): This analysis defines a distinct B cell phenotype in SOTRs who respond to a third SARS-CoV-2 vaccine dose, specifically identifying fatty acid oxidation as pathway that could be targeted to improve vaccine response such as through targeted immunosuppressive modulation. (Figure Presented).

Characterization of the humoral response to Orf virusvectored immunization

Recombinant Orf virus (rORFV) based vectors are under clinical development for COVID-19 vaccination. Little is known, however, about the cellular correlates of antibody responses to this poxviral vector platform. To monitor antigen-specific B cell responses to vaccination, we adoptively transferred to mice indicator populations of monoclonal B cells recognizing the glycoproteins (GPs) of either vesicular stomatitis virus or lymphocytic choriomeningitis virus and epitope variants thereof. Immunizations of mice with rORFV expressing the respective GPs stimulated the transferred B cells to engage in a protracted germinal center (GC) response, which was maintained longer-term when the delivered antigen was of lower affinity. GPspecific CD8 and CD4 T cells responses were also induced, and the latter included T follicular helper cells (Tfh). These T cell responses contracted over time but re-expanded upon homologous rORFV booster vaccination, alongside with an augmentation in antigen-specific memory B cells. Pre-existing rORFV-specific anti-vector immunity suppressed CD8 T cell responses to ORFV-vectored cargo whereas CD4 T cell and B cell responses were unaffected. Importantly, rORFV-based vaccination conferred long-term antibodymediated protection against VSV challenge. This study demonstrates the versatility of rORFV-vectored vaccination including its capacity to induce substantial GC B cell as well as Tfh responses. Limited interference by anti-vector immunity should facilitate the challenging task of maintaining protective antibody immunity by prime - boost vaccination.

B CELL CHARACTERISTICS at BASELINE PREDICT HUMORAL RESPONSE UPON SARS-COV-2 VACCINATION among PATIENTS TREATED with RITUXIMAB

Background: Vaccination is considered efficient in controlling infections incl. SARS-CoV-2. Prior studies showed that patients receiving rituximab (RTX) with low B cell counts are at increased infectious risk (1) and risk of inadequate vaccination responses (2, 3). Thus, the ability to further defne and predict vaccination responses in these patients may guide their optimal protection. Objectives: To assess predictive biomarkers of vaccination responses upon SARS-CoV-2 vaccination in RTX treated patients. Methods: B cell characteristics before vaccination were evaluated to predict responses in 15 patients with autoimmune infammatory rheumatic diseases receiving RTX. 11 patients with rheumatoid arthritis on other therapies (RA), 11 kidney transplant recipients (KTR) and 15 healthy volunteers (HC) served as controls. A multidimensional analysis of B cell subsets and a correlation matrix were performed to identify predictive biomarkers. Results: Signifcant differences regarding absolute B cell counts and specifc subset distribution pattern between the groups were validated at baseline. Here, the majority of B cells from vaccination responders of the RTX group (RTX IgG+) comprised naïve and transitional B cells, whereas vaccination non-responders (RTX IgG-) carried preferentially plasmablasts and double negative (CD27-IgD-) B cells (Figure 1). Moreover, there was a positive correlation between neutralizing antibodies and absolute B cell numbers with B cells expressing HLA-DR and CXCR5 (involved in antigen presentation and germinal center formation) as well as an inverse correlation with CD95 expression and CD21low expression (marker for activation and exhaustion) on B cells. Conclusion: Substantial repopulation of naïve B cells upon RTX therapy appears to be essential for an adequate vaccination response requiring germinal center formation. In contrast, expression of exhaustion markers (CD21low, CXCR5-, CD95+) indicate negative predictors of vaccination responses. These results may guide optimized vaccination strategies in RTX treated patients clearly requiring antigen-inexperienced B cells for appropriate protection.

Persistent but atypical germinal center reaction among 3rd SARS-CoV-2 vaccination after rituximab exposure.

Background: Durable vaccine-mediated immunity relies on the generation of long-lived plasma cells and memory B cells (MBCs), differentiating upon germinal center (GC) reactions. SARS-CoV-2 mRNA vaccination induces a strong GC response in healthy volunteers (HC), but limited data is available about response longevity upon rituximab treatment. Methods: We evaluated humoral and cellular responses upon 3rd vaccination in seven patients with rheumatoid arthritis (RA) who initially mounted anti-spike SARS-CoV-2 IgG antibodies after primary 2x vaccination and got re-exposed to rituximab (RTX) 1-2 months after the second vaccination. Ten patients with RA on other therapies and ten HC represented the control groups. As control for known long-lived induced immunity, we analyzed humoral and cellular tetanus toxoid (TT) immune responses in steady-state. Results: After 3rd vaccination, 5/7 seroconverted RTX patients revealed lower anti-SARS-CoV-2 IgG levels but similar neutralizing capacity compared with HC. Antibody levels after 3rd vaccination correlated with values after 2nd vaccination. Despite significant reduction of circulating total and antigen-specific B cells in RTX re-exposed patients, we observed the induction of IgG+ MBCs upon 3rd vaccination. Notably, only RTX treated patients revealed a high amount of IgA+ MBCs before and IgA+ plasmablasts after 3rd vaccination. IgA+ B cells were not part of the steady state TT+ B cell pool. TNF-secretion and generation of effector memory CD4 spike-specific T cells were significantly boosted upon 3rd vaccination. Summary: On the basis of pre-existing affinity matured MBCs within primary immunisation, RTX re-exposed patients revealed a persistent but atypical GC immune response accompanied by boosted spike-specific memory CD4 T cells upon SARS-CoV-2 recall vaccination.

Antibodies from primary humoral responses modulate the recruitment of naive B cells during secondary responses.

Vaccines generate high-affinity antibodies by recruiting antigen-specific B cells to germinal centers (GCs), but the mechanisms governing the recruitment to GCs on secondary challenges remain unclear. Here, using preclinical SARS-CoV and HIV mouse models, we demonstrated that the antibodies elicited during primary humoral responses shaped the naive B cell recruitment to GCs during secondary exposures. The antibodies from primary responses could either enhance or, conversely, restrict the GC participation of naive B cells: broad-binding, low-affinity, and low-titer antibodies enhanced recruitment, whereas, by contrast, the high titers of high-affinity, mono-epitope-specific antibodies attenuated cognate naive B cell recruitment. Thus, the directionality and intensity of that effect was determined by antibody concentration, affinity, and epitope specificity. Circulating antibodies can, therefore, be important determinants of antigen immunogenicity. Future vaccines may need to overcome-or could, alternatively, leverage-the effects of circulating primary antibodies on subsequent naive B cell recruitment.

Histomorphological patterns of regional lymph nodes in COVID-19 lungs.

BACKGROUND: A dysregulated immune response is considered one of the major factors leading to severe COVID-19. Previously described mechanisms include the development of a cytokine storm, missing immunoglobulin class switch, antibody-mediated enhancement, and aberrant antigen presentation. OBJECTIVES: To understand the heterogeneity of immune response in COVID-19, a thorough investigation of histomorphological patterns in regional lymph nodes was performed. MATERIALS AND METHODS: Lymph nodes from the cervical, mediastinal, and hilar regions were extracted from autopsies of patients with lethal COVID-19 (n = 20). Histomorphological characteristics, SARS-CoV­2 qRT-PCR, and gene expression profiling on common genes involved in immunologic response were analyzed. RESULTS: Lymph nodes displayed moderate to severe capillary stasis and edema, an increased presence of extrafollicular plasmablasts, mild to moderate plasmacytosis, a dominant population of CD8+ T­cells, and CD11c/CD68+ histiocytosis with hemophagocytic activity. Out of 20 cases, 18 presented with hypoplastic or missing germinal centers with a decrease of follicular dendritic cells and follicular T­helper cells. A positive viral load was detected by qRT-PCR in 14 of 20 cases, yet immunohistochemistry for SARS-CoV-2 N-antigen revealed positivity in sinus histiocytes of only one case. Gene expression analysis revealed an increased expression of STAT1, CD163, granzyme B, CD8A, MZB1, and PAK1, as well as CXCL9. CONCLUSIONS: Taken together, our findings imply a dysregulated immune response in lethal COVID-19. The absence/hypoplasia of germinal centers and increased presence of plasmablasts implies a transient B­cell response, implying an impaired development of long-term immunity against SARS-CoV­2 in such occasions.

COVID vaccination associated immunological cutaneous reactions

While our knowledge about the short-term side-effects of COVID-19 vaccination in adults has rapidly evolved, data about the long-term systemic side-effects and potential new onset autoimmune disorders has been limited. Here we present a case series of patients with new onset autoimmune skin conditions between 10 days and 4 weeks post mRNA COVID-19 vaccination and discuss the underlying pathophysiological changes contributing to these side-effects. Exclusions included any patients who have previously tested positive for COVID-19 or had COVID-19 symptoms. Our cases include new onset discoid lupus, localized cutaneous lupus, dermatomyositis, linear IgA bullous disease, pemphigus vulgaris, bullous pemphigoid, lichen planus pemphigoides, erosive lichen planus, psoriasis and vitiligo. In addition, we are reporting significant flare-up of pre-existing autoimmune skin conditions after a long period of remission. These include three cases of psoriasis, two cases of systemic lupus, one pemphigus vulgaris koebnerizing within a previous shingles site, and a case of pyoderma gangrenosum flare. The BNT162b2 vaccine is a potent activator of the T- and B-cell pathways. The production of interleukin (IL)-17 and IL- 21 seems to play an important role in vaccine-induced immunological protection, which is also linked to germinal centre activation linked to autoimmune disorders. This report improves our knowledge regarding some rarer potential sideeffects associated with these new vaccines and highlights the importance of further studies.

PUSHING the ENVELOPE: MRNA VACCINES for COVID-19, HIV, and OTHER PATHOGENS

Vaccines prevent 4-5 million deaths a year making them the principal tool of medical intervention worldwide. Nucleoside-modified mRNA was developed over 15 years ago and has become the darling of the COVID-19 pandemic with the first 2 FDA approved vaccines based on it. These vaccines show greater than 90% efficacy and outstanding safety in clinical use. The mechanism for the outstanding immune response induction are the prolonged production of antigen leading to continuous loading of germinal centers and the adjuvant effect of the LNPs, which selectively stimulate T follicular helper cells that drive germinal center responses. Vaccine against many pathogens, including HIV, HCV, HSV2, CMV, universal influenza, coronavirus variants, pancoronavirus, nipah, norovirus, malaria, TB, and many others are currently in development. Nucleoside-modified mRNA is also being developed for therapeutic protein delivery. Finally, nucleoside-modified mRNA-LNPs are being developed and used for gene therapy. Cas9 knockout to treat transthyretin amyloidosis has shown success in phase 1 trials. We have developed the ability to target specific cells and organs, including lung, brain, heart, CD4+ cells, all T cells, and bone marrow stem cells, with LNPs allowing specific delivery of gene editing and insertion systems to treat diseases such as sickle cell anemia. Nucleoside-modified mRNA will have an enormous potential in the development of new medical therapies.

A Novel Bacterial Protease Inhibitor Adjuvant in RBD-Based COVID-19 Vaccine Formulations Containing Alum Increases Neutralizing Antibodies, Specific Germinal Center B Cells and Confers Protection Against SARS-CoV-2 Infection in Mice.

In this work, we evaluated recombinant receptor binding domain (RBD)-based vaccine formulation prototypes with potential for further clinical development. We assessed different formulations containing RBD plus alum, AddaS03, AddaVax, or the combination of alum and U-Omp19: a novel Brucella spp. protease inhibitor vaccine adjuvant. Results show that the vaccine formulation composed of U-Omp19 and alum as adjuvants has a better performance: it significantly increased mucosal and systemic neutralizing antibodies in comparison to antigen plus alum, AddaVax, or AddaS03. Antibodies induced with the formulation containing U-Omp19 and alum not only increased their neutralization capacity against the ancestral virus but also cross-neutralized alpha, lambda, and gamma variants with similar potency. Furthermore, the addition of U-Omp19 to alum vaccine formulation increased the frequency of RBD-specific geminal center B cells and plasmablasts. Additionally, U-Omp19+alum formulation induced RBD-specific Th1 and CD8+ T-cell responses in spleens and lungs. Finally, this vaccine formulation conferred protection against an intranasal severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) challenge of K18-hACE2 mice.

Analysis of mRNA vaccination-elicited RBD-specific memory B cells reveals strong but incomplete immune escape of the SARS-CoV-2 Omicron variant.

The SARS-CoV-2 Omicron variant can escape neutralization by vaccine-elicited and convalescent antibodies. Memory B cells (MBCs) represent another layer of protection against SARS-CoV-2, as they persist after infection and vaccination and improve their affinity. Whether MBCs elicited by mRNA vaccines can recognize the Omicron variant remains unclear. We assessed the affinity and neutralization potency against the Omicron variant of several hundred naturally expressed MBC-derived monoclonal IgG antibodies from vaccinated COVID-19-recovered and -naive individuals. Compared with other variants of concern, Omicron evaded recognition by a larger proportion of MBC-derived antibodies, with only 30% retaining high affinity against the Omicron RBD, and the reduction in neutralization potency was even more pronounced. Nonetheless, neutralizing MBC clones could be found in all the analyzed individuals. Therefore, despite the strong immune escape potential of the Omicron variant, these results suggest that the MBC repertoire generated by mRNA vaccines still provides some protection against the Omicron variant in vaccinated individuals.

Intra tumoral electroporation of IL-12 and SARS-Cov-2 spike plasmids drives a coordinated vaccine response and elicits robust anti-tumor immunity

Despite extensive clinical evidence on the efficacy and safety of SARS-CoV-2 vaccines, there remains a paucity of data on their effectiveness in cancer patients who are actively receiving antineoplastic therapeutics. A recent study demonstrated only ∼30% of cancer patients had positive serologic test following 2 doses of FDA-authorized SARS-CoV-2 vaccines, in contrast to ∼80% positivity rate in healthy individuals, regardless of the age. Therefore, furtherinvestigation into novel approaches to boost immune response to SARS-CoV-2 vaccines in cancer patients isrequired. Our previous preclinical and clinical studies have established intratumoral IL-12 plasmid (TAVO)electroporation (EP) induces localized expression of IL-12p70, converting immune-excluded tumors into inflamedimmunogenic lesions, thereby generating objective responses in both treated and untreated, distant tumors. Basedon the enhancement of immunotherapy efficacy by IL-12, we leveraged the flexibility of our DNA plasmid-EPplatform to express SARS-CoV-2 spike protein in addition to IL-12 (CORVax12) as an intratumoral vaccine candidate which we hypothesized could not only drive anti-SARS-CoV-2 immune responses but also generate aproductive anti-tumor response. Naïve mice were vaccinated via intradermal injection of SARS-CoV-2 spike plasmidfollowed immediately by EP with or without plasmid-encoded mIL-12 on days 1 and 21. Longitudinal serum samples were collected to interrogate virus-specific cellular responses as well anti-spike IgG antibody. A surrogate viralneutralization test (sVNT) assessed serum blockade of soluble human ACE2 binding to immobilized SARS-CoV-2spike. Our data demonstrated that intradermally electroporated CORVax12 elicits significantly higher anti-SARS-CoV-2 spike IgG antibodies and neutralization when compared with EP of SARS-CoV-2 spike alone. Next, we askedif improved SARS-CoV-2 immune response may be observed when CORVax12 is incorporated into intratumoral EPin single-tumor bearing mice. CORVax12 robustly inhibited tumor growth, induced high percentages of germinal-center B cells and class switched B cells in tumor draining lymph nodes, and generated high of anti-spike IgG and neutralization antibodies. To further investigate systemic effects of this combination, we continued with contralateraltumor mice models. In both CT26 and B16-F10 tumor models, CORVax12 intratumoral EP induced strong systemicanti-tumor responses similar to IL-12 EP alone while also producing high serum levels of anti-SARS-CoV-2 spikeIgG and neutralization antibodies. Critically, this anti-viral immunity did not limit this IL-12-based intratumoral anti-tumor therapy. In summary, our preclinical data indicates that intratumoral EP of CORVax12 can induce IgGresponses to SARS-CoV-2 spike as well as apparent viral neutralizing activity all while maintaining local and systemic anti-tumor effects expected from TAVO Treatment. This combined intratumoral therapy represents a novelstrategy to address both tumor burden and anti-SARS-CoV-2 immunity in patients with cancer.

Follicular T cells optimize the germinal center response to SARS-CoV-2 protein vaccination in mice.

Follicular helper T (Tfh) cells promote, whereas follicular regulatory T (Tfr) cells restrain, germinal center (GC) reactions. However, the precise roles of these cells in the complex GC reaction remain poorly understood. Here, we perturb Tfh or Tfr cells after SARS-CoV-2 spike protein vaccination in mice. We find that Tfh cells promote the frequency and somatic hypermutation (SHM) of Spike-specific GC B cells and regulate clonal diversity. Tfr cells similarly control SHM and clonal diversity in the GC but do so by limiting clonal competition. In addition, deletion of Tfh or Tfr cells during primary vaccination results in changes in SHM after vaccine boosting. Aged mice, which have altered Tfh and Tfr cells, have lower GC responses, presenting a bimodal distribution of SHM. Together, these data demonstrate that GC responses to SARS-CoV-2 spike protein vaccines require a fine balance of positive and negative follicular T cell help to optimize humoral immunity.

Germinal center responses to SARS-CoV-2 mRNA vaccines in healthy and immunocompromised individuals.

Vaccine-mediated immunity often relies on the generation of protective antibodies and memory B cells, which commonly stem from germinal center (GC) reactions. An in-depth comparison of the GC responses elicited by SARS-CoV-2 mRNA vaccines in healthy and immunocompromised individuals has not yet been performed due to the challenge of directly probing human lymph nodes. Herein, through a fine-needle aspiration-based approach, we profiled the immune responses to SARS-CoV-2 mRNA vaccines in lymph nodes of healthy individuals and kidney transplant recipients (KTXs). We found that, unlike healthy subjects, KTXs presented deeply blunted SARS-CoV-2-specific GC B cell responses coupled with severely hindered T follicular helper cell, SARS-CoV-2 receptor binding domain-specific memory B cell, and neutralizing antibody responses. KTXs also displayed reduced SARS-CoV-2-specific CD4 and CD8 T cell frequencies. Broadly, these data indicate impaired GC-derived immunity in immunocompromised individuals and suggest a GC origin for certain humoral and memory B cell responses following mRNA vaccination.