Isolation of viable antigen-specific CD4 T cells by CD40L surface trapping

T-cell activation-induced marker assays in health and disease

Activation-induced marker (AIM) assays have proven to be an accessible and rapid means of antigen-specific T-cell detection. The method typically involves short-term incubation of whole blood or peripheral blood mononuclear cells with antigens of interest, where autologous antigen-presenting cells process and present peptides in complex with major histocompatibility complex (MHC) molecules. Recognition of peptide-MHC complexes by T-cell receptors then induces upregulation of activation markers on the T cells that can be detected by flow cytometry. In this review, we highlight the most widely used activation markers for assays in the literature while identifying nuances and potential downfalls associated with the technique. We provide a summary of how AIM assays have been used in both discovery science and clinical studies, including studies of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immunity. This review primarily focuses on AIM assays using human blood or peripheral blood mononuclear cell samples, with some considerations noted for tissue-derived T cells and nonhuman samples. AIM assays are a powerful tool that enables detailed analysis of antigen-specific T-cell frequency, phenotype and function without needing to know the precise antigenic peptides and their MHC restriction elements, enabling a wider analysis of immunity generated following infection and/or vaccination.

Pathogenic Autoimmunity in Atherosclerosis Evolves From Initially Protective Apolipoprotein B100-Reactive CD4+ T-Regulatory Cells

BACKGROUND

Throughout the inflammatory response that accompanies atherosclerosis, autoreactive CD4⁺ T-helper cells accumulate in the atherosclerotic plaque. Apolipoprotein B₁₀₀ (apoB), the core protein of low-density lipoprotein, is an autoantigen that drives the generation of pathogenic T-helper type 1 (T_H1) cells with proinflammatory cytokine secretion. Clinical data suggest the existence of apoB-specific CD4⁺ T cells with an atheroprotective, regulatory T cell (T_reg) phenotype in healthy individuals. Yet, the function of apoB-reactive T_regs and their relationship with pathogenic T_H1 cells remain unknown.

METHODS

To interrogate the function of autoreactive CD4⁺ T cells in atherosclerosis, we used a novel tetramer of major histocompatibility complex II to track T cells reactive to the mouse self-peptide apo B_978-993 (apoB⁺) at the single-cell level.

RESULTS

We found that apoB⁺ T cells build an oligoclonal population in lymph nodes of healthy mice that exhibit a T_reg-like transcriptome, although only 21% of all apoB⁺ T cells expressed the T_reg transcription factor FoxP3 (Forkhead Box P3) protein as detected by flow cytometry. In single-cell RNA sequencing, apoB⁺ T cells formed several clusters with mixed T_H signatures that suggested overlapping multilineage phenotypes with pro- and anti-inflammatory transcripts of T_H1, T helper cell type 2 (T_H2), and T helper cell type 17 (T_H17), and of follicular-helper T cells. ApoB⁺ T cells were increased in mice and humans with atherosclerosis and progressively converted into pathogenic T_H1/T_H17-like cells with proinflammatory properties and only a residual T_reg transcriptome. Plaque T cells that expanded during progression of atherosclerosis consistently showed a mixed T_H1/T_H17 phenotype in single-cell RNA sequencing. In addition, we observed a loss of FoxP3 in a fraction of apoB⁺ T_regs in lineage tracing of hyperlipidemic Apoe^-/- mice. In adoptive transfer experiments, converting apoB⁺ T_regs failed to protect from atherosclerosis.

CONCLUSIONS

Our results demonstrate an unexpected mixed phenotype of apoB-reactive autoimmune T cells in atherosclerosis and suggest an initially protective autoimmune response against apoB with a progressive derangement in clinical disease. These findings identify apoB autoreactive T_regs as a novel cellular target in atherosclerosis.

The domestic pig as human-relevant large animal model to study adaptive antifungal immune responses against airborne Aspergillus fumigatus

Pulmonary mucosal immune response is critical for preventing opportunistic Aspergillus fumigatus infections. Although fungus-specific CD4⁺ T cells in blood are described to reflect the actual host-pathogen interaction status, little is known about Aspergillus-specific pulmonary T-cell responses. Here, we exploit the domestic pig as human-relevant large animal model and introduce antigen-specific T-cell enrichment in pigs to address Aspergillus-specific T cells in the lung compared to peripheral blood. In healthy, environmentally Aspergillus-exposed pigs, the fungus-specific T cells are detectable in blood in similar frequencies as observed in healthy humans and exhibit a Th1 phenotype. Exposing pigs to 10⁶ cfu/m³ conidia induces a long-lasting accumulation of Aspergillus-specific Th1 cells locally in the lung and also systemically. Temporary immunosuppression during Aspergillus-exposure showed a drastic reduction in the lung-infiltrating antifungal T-cell responses more than 2 weeks after abrogation of the suppressive treatment. This was reflected in blood, but to a much lesser extent. In conclusion, by using the human-relevant large animal model the pig, this study highlights that the blood clearly reflects the mucosal fungal-specific T-cell reactivity in environmentally exposed as well as experimentally exposed healthy pigs. But, immunosuppression significantly impacts the mucosal site in contrast to the initial systemic immune response.

Impact of Early Antiretroviral Therapy Initiation on HIV-Specific CD4 and CD8 T Cell Function in Perinatally Infected Children

Early initiation of antiretroviral therapy (ART) in vertically HIV-infected children limits the size of the virus reservoir, but whether the time of treatment initiation (TI) can durably impact host immune responses associated with HIV infection is still unknown. This study was conducted in PBMC of 20 HIV-infected virally suppressed children on ART (mean age 9.4 y), classified as early treated (ET; age at ART initiation ≤0.5 y, n = 14) or late treated (LT; age at ART initiation 1-10 y, n = 6). Frequencies and functions of Ag-specific CD4 (CD40L⁺) and CD8 (CD69⁺) T cells were evaluated by intracellular IL-2, IFN-γ, and TNF-α production with IL-21 in CD4 or CD107a, granzyme B and perforin in CD8 T cells following stimulation with HIV gp140 protein (ENV) or GAG peptides by multiparameter flow cytometry. ET showed a higher proportion of cytokine-producing ENV- and GAG-specific CD4 and CD8 T cells compared with LT. In particular, ET were enriched in polyfunctional T cells. RNA sequencing analysis showed upregulation of immune activation pathways in LT compared with ET. Our results suggest that timing of TI in HIV-infected children has a long-term and measurable impact on the quality of the HIV-specific T cell immune responses and transcriptional profiles of PBMC, reinforcing the importance of early TI.

Dysfunctional peripheral T follicular helper cells dominate in people with impaired influenza vaccine responses: Results from the FLORAH study

Antigen-primed cluster of differentiation (CD) 4+ T follicular helper (Tfh) cells interact with B cells in the germinal centers (GCs) of lymph nodes to generate vaccine-induced antibody (Ab) responses. In the circulation, peripheral Tfh (pTfh) cells, a subset of memory CD4 T cells, serve as surrogates for GC Tfh because of several functional and phenotypic similarities between them. We investigated features of H1N1 influenza antigen-specific pTfh (Ag.pTfh) in virologically controlled HIV+ volunteers on antiretroviral therapy (ART) and healthy control (HC) participants selected from a seasonal influenza vaccine responsiveness study. Selection of the participants was made based on age, defined as young (18-40 y) and old (>60 y) and on their classification as a vaccine responder (VR) or vaccine nonresponder (VNR). VRs demonstrated expansion of CD40L+ and CD69+ Ag.pTfh, with induction of intracellular interleukin 21 (IL-21) and inducible costimulator (ICOS) post vaccination; these responses were strongest in young HC VRs and were less prominent in HIV+ individuals of all ages. Ag.pTfh in VNRs exhibited dramatically different characteristics from VRs, displaying an altered phenotype and a cytokine profile dominated by cytokines IL-2, tumor necrosis factor alpha (TNF-α), or IL-17 but lacking in IL-21. In coculture experiments, sorted pTfh did not support the B cell IgG production in VNRs and were predominantly an inflammatory T helper 1 (Th1)/T helper 17 (Th17) phenotype with lower ICOS and higher programmed cell death protein 1 (PD1) expression. Induction of IL-21 and ICOS on Ag.pTfh cells are negatively affected by both aging and HIV infection. Our findings demonstrate that dysfunctional Ag.pTfh cells with an altered IL-21/IL-2 axis contribute to inadequate vaccine responses. Approaches for targeting inflammation or expanding functional Tfh may improve vaccine responses in healthy aging and those aging with HIV infection.

Activation-induced Markers Detect Vaccine-Specific CD4⁺ T Cell Responses Not Measured by Assays Conventionally Used in Clinical Trials

Immunogenicity of T cell-inducing vaccines, such as viral vectors or DNA vaccines and Bacillus Calmette-Guérin (BCG), are frequently assessed by cytokine-based approaches. While these are sensitive methods that have shown correlates of protection in various vaccine studies, they only identify a small proportion of the vaccine-specific T cell response. Responses to vaccination are likely to be heterogeneous, particularly when comparing prime and boost or assessing vaccine performance across diverse populations. Activation-induced markers (AIM) can provide a broader view of the total antigen-specific T cell response to enable a more comprehensive evaluation of vaccine immunogenicity. We tested an AIM assay for the detection of vaccine-specific CD4⁺ and CD8⁺ T cell responses in healthy UK adults vaccinated with viral vectored Ebola vaccine candidates, ChAd3-EBO-Z and MVA-EBO-Z. We used the markers, CD25, CD134 (OX40), CD274 (PDL1), and CD107a, to sensitively identify vaccine-responsive T cells. We compared the use of OX40⁺CD25⁺ and OX40⁺PDL1⁺ in CD4⁺ T cells and OX40⁺CD25⁺ and CD25⁺CD107a⁺ in CD8⁺ T cells for their sensitivity, specificity, and associations with other measures of vaccine immunogenicity. We show that activation-induced markers can be used as an additional method of demonstrating vaccine immunogenicity, providing a broader picture of the global T cell response to vaccination.

Pathogen-Reactive T Helper Cell Analysis in the Pig

There is growing interest in studying host–pathogen interactions in human-relevant large animal models such as the pig. Despite the progress in developing immunological reagents for porcine T cell research, there is an urgent need to directly assess pathogen-specific T cells—an extremely rare population of cells, but of upmost importance in orchestrating the host immune response to a given pathogen. Here, we established that the activation marker CD154 (CD40L), known from human and mouse studies, identifies also porcine antigen-reactive CD4⁺ T lymphocytes. CD154 expression was upregulated early after antigen encounter and CD4⁺CD154⁺ antigen-reactive T cells coexpressed cytokines. Antigen-induced expansion and autologous restimulation enabled a time- and dose-resolved analysis of CD154 regulation and a significantly increased resolution in phenotypic profiling of antigen-responsive cells. CD154 expression identified T cells responding to staphylococcal Enterotoxin B superantigen stimulation as well as T cells responding to the fungus Candida albicans and T cells specific for a highly prevalent intestinal parasite, the nematode Ascaris suum during acute and trickle infection. Antigen-reactive T cells were further detected after immunization of pigs with a single recombinant bacterial antigen of Streptococcus suis only. Thus, our study offers new ways to study antigen-specific T lymphocytes in the pig and their contribution to host–pathogen interactions.

Vaccination Expands Antigen-Specific CD4+ Memory T Cells and Mobilizes Bystander Central Memory T Cells

CD4+ T helper memory (Thmem) cells influence both natural and vaccine-boosted immunity, but mechanisms for their maintenance remain unclear. Pro-survival signals from the common gamma-chain cytokines, in particular IL-7, appear important. Previously we showed in healthy volunteers that a booster vaccination with tetanus toxoid (TT) expanded peripheral blood TT-specific Thmem cells as expected, but was accompanied by parallel increase of Thmem cells specific for two unrelated and non cross-reactive common recall antigens. Here, in a new cohort of healthy human subjects, we compare blood vaccine-specific and bystander Thmem cells in terms of differentiation stage, function, activation and proliferative status. Both responses peaked 1 week post-vaccination. Vaccine-specific cytokine-producing Thmem cells were predominantly effector memory, whereas bystander cells were mainly of central memory phenotype. Importantly, TT-specific Thmem cells were activated (CD38High HLA-DR+), cycling or recently divided (Ki-67+), and apparently vulnerable to death (IL-7RαLow and Bcl-2 Low). In contrast, bystander Thmem cells were resting (CD38Low HLA-DR- Ki-67-) with high expression of IL-7Rα and Bcl-2. These findings allow a clear distinction between vaccine-specific and bystander Thmem cells, suggesting the latter do not derive from recent proliferation but from cells mobilized from as yet undefined reservoirs. Furthermore, they reveal the interdependent dynamics of specific and bystander T-cell responses which will inform assessments of responses to vaccines.

Preformed CD40L is stored in Th1, Th2, Th17, and T follicular helper cells as well as CD4+ 8- thymocytes and invariant NKT cells but not in Treg cells

CD40L is essential for the development of adaptive immune responses. It is generally thought that CD40L expression in CD4⁺ T cells is regulated transcriptionally and made from new mRNA following antigen recognition. However, imaging studies show that the majority of cognate interactions between effector CD4⁺ T cells and APCs in vivo are too short to allow de novo CD40L synthesis. We previously showed that Th1 effector and memory cells store preformed CD40L (pCD40L) in lysosomal compartments and mobilize it onto the plasma membrane immediately after antigenic stimulation, suggesting that primed CD4⁺ T cells may use pCD40L to activate APCs during brief encounters. Indeed, our recent study showed that pCD40L is sufficient to mediate selective activation of cognate B cells and trigger DC activation in vitro. In this study, we show that pCD40L is present in Th1 and follicular helper T cells developed during infection with lymphocytic choriomeningitis virus, Th2 cells in the airway of asthmatic mice, and Th17 cells from the CNS of animals with experimental autoimmune encephalitis (EAE). pCD40L is nearly absent in both natural and induced Treg cells, even in the presence of intense inflammation such as occurs in EAE. We also found pCD40L expression in CD4 single positive thymocytes and invariant NKT cells. Together, these results suggest that pCD40L may function in T cell development as well as an unexpectedly broad spectrum of innate and adaptive immune responses, while its expression in Treg cells is repressed to avoid compromising their suppressive activity.

Induction of strong HIV-1-specific CD4+ T-cell responses using an HIV-1 gp120/NefTat vaccine adjuvanted with AS02A in antiretroviral-treated HIV-1-infected individuals

BACKGROUND

Induction of HIV-1-specific CD4(+) T-cell responses by therapeutic vaccination represents an attractive intervention to potentially increase immune control of HIV-1.

METHODS

We performed a double-blinded, randomized, placebo-controlled clinical trial to determine the safety and immunogenicity of GlaxoSmithKline Biologicals' HIV-1 gp120/NefTat subunit protein vaccine formulated with the AS02(A) Adjuvant System in subjects with well-controlled chronic HIV-1 infection on highly active antiretroviral therapy. Ten individuals received the vaccine; whereas adjuvant alone or placebo was given to 5 subjects each. Immunogenicity was monitored by intracellular cytokine flow cytometry and carboxyfluorescein succinimidyl ester-based proliferation assays.

RESULTS

The vaccine was well tolerated with no related serious adverse events. Vaccine recipients had significantly stronger gp120-specific CD4(+) T-cell responses which persisted until week 48 and greater gp120-specific CD4(+) T-cell proliferation activity as compared with controls. In the vaccine group, the number of participants who demonstrated positive responses for both gp120-specific CD4(+) T-cell interleukin-2 production and gp120-specific CD8(+) T-cell proliferation were significantly higher at week 6.

CONCLUSIONS

The gp120/NefTat/AS02(A) vaccine induced strong gp120-specific CD4(+) T-cell responses and a higher number of vaccinees developed both HIV-1-specific CD4(+) T-cell responses and CD8(+) T-cell proliferation. The induction of these responses may be important in enhancing immune-mediated viral control.

Comparison between major histocompatibility complex class II tetramer staining and surface expression of activation markers for the detection of allergen-specific CD4⁺ T cells

BACKGROUND

Major histocompatibility complex (MHC) class II tetramers (tetramers) allow to detect allergen-specific CD4(+) T cells at a single-cell level. Limits to this technology include HLA restriction and the need to identify immunodominant T cell epitopes.

OBJECTIVE

Assessing the expression of various activation markers following allergen stimulation to replace tetramer staining.

METHODS

Peripheral blood mononuclear cells (PBMCs) from 25 birch pollen, grass pollen or house dust mite allergic individuals were stimulated with peptide mixes encompassing immunodominant epitopes from corresponding major allergens. After 2 weeks of in vitro amplification, cells were stained with both the appropriate tetramer and antibodies directed to CD25, CD30, CD39, CD69, CD137, CD154, GITR, HLA-DR and ICOS, before FACS analysis.

RESULTS

Following allergen stimulation, percentages of tetramer(+) cells among CD4(+) CD154(+) cells range from 5% to 87%, depending upon donors. As for CD154, a large inter-individual variability is observed in terms of surface expression for all activation markers tested in allergen-stimulated PBMCs. T cells reactive with either tetramers (0.4-10.4% CD4(+) T cells) or anti-marker antibodies (2.2-32.7% CD4(+) T cells), but not both, are observed, reflecting the presence of anergic as well as non-specifically activated cells. Tetramer(+) /marker(+) , tetramer(+) /marker(-) and tetramer(-) /marker(+) cells were compared for their capacity to express cytokines, demonstrating that only the former represent bona fide allergen-specific activated CD4(+) T cells, based upon a higher expression of cytokines or corresponding genes in presence of the allergen.

CONCLUSION AND CLINICAL RELEVANCE

No strict correlation exists between tetramer staining and the expression of multiple activation markers in stimulated CD4(+) T cells. Dual staining allows to discriminate functional tetramer(+) /marker(+) vs. anergic (tetramer(+) /marker(-) ) allergen-specific T cells or non-specifically activated (tetramer(-) /marker(+) ) T cells. Combining tetramer staining with the detection of activation markers helps understanding patient heterogeneity regarding specific CD4(+) T cell responses. This approach has immediate relevance for monitoring immune changes induced during specific immunotherapy.

CD4+T cells in CIKs (CD4+ CIKs) reversed resistance to fas-mediated apoptosis through CD40/CD40L ligation rather than IFN-gamma stimulation

BACKGROUND

Cytokine-induced killer cells (CIKs) are nonspecific antitumor effectors with superior advantages. CD4+ CIKs can induce Fas-dependent apoptosis in sensitive Raji cells. Here, a Fas-dependent apoptosis was detected in resistant breast cancer MDA-MB-231 cells, and underlying mechanisms were discriminated.

METHODS

Amplification of CIKs and purification of CD4+ CIKs were performed in 15 patients with malignant solid tumors. The expression of CD40L and soluble cytokines in CD4+ CIKs were analyzed. The apoptotic rates of tumor cells and the expression of Fas on membranes were detected using flow cytometry assay. The specific blocking antibodies against FasL, CD40L, and interferon-gamma (IFN-gamma) were added to abolish their effects. The changes of 4 apoptosis-related genes (Bcl-2, Bax, Fas-associating protein with death domain [FADD], and FLICE inhibitory protein [c-FLIP]) in MDA-MB-231 cells cocultured with CD4+ CIKs were measured by real-time quantitative reverse-transcriptase polymerase chain reaction after 6 hours and 24 hours with or without blocking antibodies.

RESULTS

Upregulated expression of membrane-attached CD40L and dramatically increased secretion of soluble CD40L and IFN-gamma were identified in CD4+ CIK. The susceptibility to Fas-mediated apoptosis of insensitive MDA-MB-231 cells was elevated after being pretreated with supernatants from CD4+ CIK. After coculture with CD4+ CIK, apoptosis in MDA-MB-231 cells paralleled with enhanced expression of Fas was blocked fully by either anti-FasL or anti-CD40L, but only partly by anti-IFN-gamma antibodies. The anti-CD40L monoclonal antibody (McAb) rather than anti-IFN-gamma McAb induced significant increase of c-FLIP, which negatively correlated with the apoptosis observed in MDA-MB-231 cells.

CONCLUSIONS

Apoptosis in MDA-MB-231 cells induced by CD4+ CIK is Fas-dependent. The reversion of Fas resistance is mediated through CD40/CD40L ligation rather than IFN-gamma stimulation by inhibiting synthesis of c-FLIP.

Identification and isolation of murine antigen-reactive T cells according to CD154 expression

T helper (Th) cells are central regulators of adaptive immune responses. However, the detection of the small number of Th cells specific for a particular antigen or pathogen is still a major challenge. CD154 was recently introduced as a marker for antigen-specific Th cells. To date, this technology was not applicable for mice - arguably the most important immunological model system. CD154 is difficult to detect due to its rapid removal from the cell surface upon binding to CD40 during antigen-specific activation by APC. We present an efficient strategy to block the degradation of murine CD154 by combined use of antibodies against CD40 and CD154. This strategy makes CD154 easily accessible for surface staining, which allows isolation and expansion of rare antigen specific T cells. Importantly, CD154 identified all specific T cells in strongly Th1- or Th2-polarized immune responses against pathogens like Salmonella typhimurium and Heligmosomoides polygyrus, independent of their potential to produce cytokines. We demonstrate that CD154 can in fact be used as a reliable marker for antigen-specific CD4 T cells in mice, offering a unique option to analyze, isolate and rapidly expand the entire pool of Th-cells generated during a physiological T cell response in vivo.

Preformed CD40 ligand exists in secretory lysosomes in effector and memory CD4+ T cells and is quickly expressed on the cell surface in an antigen-specific manner

CD40 ligand (CD40L) is an essential effector cytokine for macrophage activation, dendritic cell licensing, and T-cell-dependent antibody responses. Although CD40L is known to be made de novo following antigen recognition, several reports have described surface mobilization of preformed, intracellular CD40L in certain CD4(+) effector T cells. Here we show that rapid surface expression of preformed CD40L following antigen recognition is a general property of both effector and memory CD4(+) T cells, including in vitro and in vivo activated T-cell-receptor transgenic T cells, memory phenotype CD4(+) T cells from pathogen-free naive mice, and polyclonal virus-specific effector and memory T cells. Intracellular CD40L is stored in secretory lysosomes, and colocalizes more strongly with Fas ligand than with CTLA-4, two other molecules that are delivered to the cell surface following antigen recognition. Stimulated surface expression of preformed CD40L is found in memory CD4(+) T cells from CD40-deficient mice, indicating that it does not depend on CD40-induced internalization for delivery to the secretory compartment. We suggest that delivery of preformed CD40L to antigen-presenting cells (APCs) could enable antigen-specific activation of APCs in transient interactions that are too brief to permit de novo synthesis of CD40L.

Live-cell assay to detect antigen-specific CD4+ T-cell responses by CD154 expression

This protocol details a method to identify CD4+ T cells that respond to antigens. The method relies on detection of CD154, a costimulatory cell surface protein that is expressed by CD4+ T cells upon activation, and can be used to purify live CD4+ T cells of diverse function. To detect CD154, fluorescently labeled antibodies are cultured with cell samples, peptides (or whole antigens) and monensin during a 6- to 24-h stimulation period. (Note that the assay is not compatible with brefeldin A.) After stimulation, cells are stained with any other antibodies of interest and then are analyzed by flow cytometry or purified by cell sorting. Unlike other assays, this method allows simultaneous assessment of other cell phenotypes or functions, is compatible with downstream RNA-based assays and preserves cell viability. This protocol can be completed in 9 h.

Induction of a virus-specific effector-memory CD4+ T cell response by attenuated SIV infection

We investigated simian immunodeficiency virus (SIV)-specific CD4⁺ T cell responses in rhesus macaques chronically infected with attenuated or pathogenic SIV strains. Analysis of SIVΔnef-infected animals revealed a relatively high frequency of SIV-specific CD4⁺ T cells representing 4–10% of all CD4⁺ T lymphocytes directed against multiple SIV proteins. Gag-specific CD4⁺ T cells in wild-type SIV-infected animals were 5–10-fold lower in frequency and inversely correlated with the level of plasma viremia. SIV-specific CD4⁺ cells from SIVΔnef animals were predominantly CD27⁻CD28⁻CD45RA^lowCCR7⁻CCR5⁻, consistent with an effector–memory subset, and included a fully differentiated CD45RA⁺CCR7⁻ subpopulation. In contrast, SIV-specific CD4⁺ T cells from SIV-infected animals were mostly CD27⁺CD28⁺CD45RA⁻CCR7⁺CCR5⁺, consistent with an early central memory phenotype. The CD45RA⁺CCR7⁻CD4⁺ subset from SIVΔnef animals was highly enriched for effector CD4⁺ T cells, as indicated by the perforin expression and up-regulation of the lysosomal membrane protein CD107a after SIV Gag stimulation. SIV-specific CD4⁺ T cells in attenuated SIV-infected animals were increased in frequency in bronchioalveolar lavage and decreased in lymph nodes, consistent with an effector–memory T cell population. The ability of SIVΔnef to induce a high frequency virus-specific CD4⁺ T cell response with direct effector function may play a key role in protective immunity produced by vaccination with attenuated SIV strains.

CD4+ memory T cells: functional differentiation and homeostasis

CD4+ T cells are central regulators of both humoral and cellular immune responses. There are many subsets of CD4+ T cells, the most prominent being T-helper 1 (Th1), Th2, Th-17, and regulatory T cells, specialized in regulating different aspects of immunity. Without participation by these CD4+ T-cell subsets, B cells cannot undergo isotype switching to generate high-affinity antibodies, the microbicidal activity of macrophages is reduced, the efficiency of CD8+ T-cell responses and CD8+ T-cell memory are compromised, and downregulation of effector responses is impaired. It therefore stands to reason that memory CD4+ T cells are likely to fulfill an important facilitator role in the maintenance and control of protective immune responses. This review discusses some issues of importance for the generation of memory CD4+ T cells and focuses in particular on their heterogeneity and plasticity, with respect to both phenotypic characteristics and function. Finally, we discuss a number of factors that affect long-term maintenance of memory CD4+ T cells.