Early expansion of CD38+ICOS+ GC Tfh in draining lymph nodes during influenza vaccination immune response

Help me help you: emerging concepts in T follicular helper cell differentiation, identity, and function

Effective high-affinity, long-term humoral immunity requires T cell help provided by a subset of differentiated CD4+ T cells known as T follicular helper (Tfh) cells. Classically, Tfh cells provide contact-dependent help for the generation of germinal centers (GCs) in secondary lymphoid organs (SLOs). Recent studies have expanded the conventional definition of Tfh cells, revealing new functions, new descriptions of Tfh subsets, new factors regulating Tfh differentiation, and new roles outside of SLO GCs. Together, these data suggest that one Tfh is not equivalent to another, helping redefine our understanding of Tfh cells and their biology.

Ultrasound-guided lymph node fine-needle aspiration for evaluating post-vaccination germinal center responses in humans

The lymph node (LN) is a critical biological site for immune maturation after vaccination as it includes several cell populations critical for priming the antibody response. Here, we present a protocol for sampling the LN and isolating cell populations to evaluate immunogens targeting germline cells. We describe steps for media and tube preparation and sample collection using an ultrasound-guided LN fine-needle aspiration procedure. This protocol is safe, quick, low-cost, and less invasive than excisional biopsy. For complete details on the use and execution of this protocol, please refer to Leggat et al. (2022).1.

Dissecting the Protective Effect of CD8+ T Cells in Response to SARS-CoV-2 mRNA Vaccination and the Potential Link with Lymph Node CD8+ T Cells

SARS-CoV-2 vaccines have played a crucial role in effectively reducing COVID-19 disease severity, with a new generation of vaccines that use messenger RNA (mRNA) technology being administered globally. Neutralizing antibodies have featured as the heroes of vaccine-induced immunity. However, vaccine-elicited CD8⁺ T cells may have a significant impact on the early protective effects of the mRNA vaccine, which are evident 12 days after initial vaccination. Vaccine-induced CD8⁺ T cells have been shown to respond to multiple epitopes of SARS-CoV-2 and exhibit polyfunctionality in the periphery at the early stage, even when neutralizing antibodies are scarce. Furthermore, SARS-CoV-2 mRNA vaccines induce diverse subsets of memory CD8⁺ T cells that persist for more than six months following vaccination. However, the protective role of CD8⁺ T cells in response to the SARS-CoV-2 mRNA vaccines remains a topic of debate. In addition, our understanding of CD8⁺ T cells in response to vaccination in the lymph nodes, where they first encounter antigen, is still limited. This review delves into the current knowledge regarding the protective role of polyfunctional CD8⁺ T cells in controlling the virus, the response to SARS-CoV-2 mRNA vaccines, and the contribution to supporting B cell activity and promoting immune protection in the lymph nodes.

Dysregulated IFN-γ signals promote autoimmunity in STAT1 gain-of-function syndrome

Heterozygous signal transducer and activator of transcription 1 (STAT1) gain-of-function (GOF) mutations promote a clinical syndrome of immune dysregulation characterized by recurrent infections and predisposition to humoral autoimmunity. To gain insights into immune characteristics of STAT1-driven inflammation, we performed deep immunophenotyping of pediatric patients with STAT1 GOF syndrome and age-matched controls. Affected individuals exhibited dysregulated CD4⁺ T cell and B cell activation, including expansion of T_H1-skewed CXCR3⁺ populations that correlated with serum autoantibody titers. To dissect underlying immune mechanisms, we generated Stat1 GOF transgenic mice (Stat1^GOF mice) and confirmed the development of spontaneous humoral autoimmunity that recapitulated the human phenotype. Despite clinical resemblance to human regulatory T cell (T_reg) deficiency, Stat1^GOF mice and humans with STAT1 GOF syndrome exhibited normal T_reg development and function. In contrast, STAT1 GOF autoimmunity was characterized by adaptive immune activation driven by dysregulated STAT1-dependent signals downstream of the type 1 and type 2 interferon (IFN) receptors. However, in contrast to the prevailing type 1 IFN-centric model for STAT1 GOF autoimmunity, Stat1^GOF mice lacking the type 1 IFN receptor were only partially protected from STAT1-driven systemic inflammation, whereas loss of type 2 IFN (IFN-γ) signals abrogated autoimmunity. Last, germline STAT1 GOF alleles are thought to enhance transcriptional activity by increasing total STAT1 protein, but the underlying biochemical mechanisms have not been defined. We showed that IFN-γ receptor deletion normalized total STAT1 expression across immune lineages, highlighting IFN-γ as the critical driver of feedforward STAT1 elevation in STAT1 GOF syndrome.

T follicular helper cell responses to SARS-CoV-2 vaccination among healthy and immunocompromised adults

The worldwide rollout of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccinations in the last 2 years has produced a multitude of studies investigating T-cell responses in the peripheral blood and a limited number in secondary lymphoid tissues. As a key component to an effective immune response, vaccine-specific T follicular helper (Tfh) cells are localized in the draining lymph node (LN) and assist in the selection of highly specific B-cell clones for the production of neutralizing antibodies. While these cells have been noted in the blood as circulating Tfh (cTfh) cells, they are not often taken into consideration when examining effective CD4+ T-cell responses, particularly in immunocompromised groups. Furthermore, site-specific analyses in locations such as the LN have recently become an attractive area of investigation. This is mainly a result of improved sampling methods via ultrasound-guided fine-needle biopsy (FNB)/fine-needle aspiration (FNA), which are less invasive than LN excision and able to be performed longitudinally. While these studies have been undertaken in healthy individuals, data from immunocompromised groups are lacking. This review will focus on both Tfh and cTfh responses after SARS-CoV-2 vaccination in healthy and immunocompromised individuals. This area of investigation could identify key characteristics of a successful LN response required for the prevention of infection and viral clearance. This furthermore may highlight responses that could be fine-tuned to improve vaccine efficacy within immunocompromised groups that are at a risk of more severe disease.

High titre neutralizing antibodies in response to SARS-CoV-2 infection require RBD-specific CD4 T cells that include proliferative memory cells

Background

Long-term immunity to SARS-CoV-2 infection, including neutralizing antibodies and T cell-mediated immunity, is required in a very large majority of the population in order to reduce ongoing disease burden.

Methods

We have investigated the association between memory CD4 and CD8 T cells and levels of neutralizing antibodies in convalescent COVID-19 subjects.

Findings

Higher titres of convalescent neutralizing antibodies were associated with significantly higher levels of RBD-specific CD4 T cells, including specific memory cells that proliferated vigorously in vitro. Conversely, up to half of convalescent individuals had low neutralizing antibody titres together with a lack of receptor binding domain (RBD)-specific memory CD4 T cells. These low antibody subjects had other, non-RBD, spike-specific CD4 T cells, but with more of an inhibitory Foxp3+ and CTLA-4+ cell phenotype, in contrast to the effector T-bet+, cytotoxic granzymes+ and perforin+ cells seen in RBD-specific memory CD4 T cells from high antibody subjects. Single cell transcriptomics of antigen-specific CD4+ T cells from high antibody subjects similarly revealed heterogenous RBD-specific CD4+ T cells that comprised central memory, transitional memory and Tregs, as well as cytotoxic clusters containing diverse TCR repertoires, in individuals with high antibody levels. However, vaccination of low antibody convalescent individuals led to a slight but significant improvement in RBD-specific memory CD4 T cells and increased neutralizing antibody titres.

Interpretation

Our results suggest that targeting CD4 T cell epitopes proximal to and within the RBD-region should be prioritized in booster vaccines.

Human lymphoid tissue sampling for vaccinology

Long-lived plasma cells (LLPCs) - largely resident in the bone marrow - secrete antibody over months and years, thus maintaining serum antibody concentrations relevant for vaccine-mediated immunity. Little is known regarding factors that can modulate the induction of human LLPC responses in draining lymph node germinal centres, or those that maintain LLPCs in bone marrow niches following vaccination. Here, we review human and non-human primate vaccination studies which incorporate draining lymph node and/or bone marrow aspirate sampling. We emphasise the key contributions these samples can make to improve our understanding of LLPC immunology and guide rational vaccine development. Specifically, we highlight findings related to the impact of vaccine dosing regimens, adjuvant/vaccine platform selection, duration of germinal centre reactions in draining lymph nodes and relevance for timing of tissue sampling, and heterogeneity in bone marrow plasma cell populations. Much of this work has come from recent studies with SARS-CoV-2 vaccine candidates or, with respect to the non-human primate work, HIV vaccine development.