Are Potent Antigen-presenting Cells Research Articles

TLR5-deficiency controls dendritic cell subset development in an autoimmune diabetes-susceptible model.

The incidence of the autoimmune disease, type 1 diabetes (T1D), has been increasing worldwide and recent studies have shown that the gut microbiota are associated with modulating susceptibility to T1D. Toll-like receptor 5 (TLR5) recognizes bacterial flagellin and is widely expressed on many cells, including dendritic cells (DCs), which are potent antigen-presenting cells (APCs). TLR5 modulates susceptibility to obesity and alters metabolism through gut microbiota; however, little is known about the role TLR5 plays in autoimmunity, especially in T1D. To fill this knowledge gap, we generated a TLR5-deficient non-obese diabetic (NOD) mouse, an animal model of human T1D, for study. We found that TLR5-deficiency led to a reduction in CD11c+ DC development in utero, prior to microbial colonization, which was maintained into adulthood. This was associated with a bias in the DC populations expressing CD103, with or without CD8α co-expression, and hyper-secretion of different cytokines, both in vitro (after stimulation) and directly ex vivo. We also found that TLR5-deficient DCs were able to promote polyclonal and islet antigen-specific CD4+ T cell proliferation and proinflammatory cytokine secretion. Interestingly, only older TLR5-deficient NOD mice had a greater risk of developing spontaneous T1D compared to wild-type mice. In summary, our data show that TLR5 modulates DC development and enhances cytokine secretion and diabetogenic CD4+ T cell responses. Further investigation into the role of TLR5 in DC development and autoimmune diabetes may give additional insights into the pathogenesis of Type 1 diabetes.

Therapeutic induction of tolerogenic dendritic cells via aryl hydrocarbon receptor signaling.

Dendritic cells (DCs) are potent antigen-presenting cells (APCs), which sample the exogenous and endogenous cues to control adaptive immunity, balancing effector and regulatory components of the immune response. Multiple subsets of DCs, such as plasmacytoid and conventional DCs, have been defined based on specific phenotypic markers, functions and regulatory transcriptional programs. Tolerogenic DCs (tolDCs) have been functionally defined based on their ability to expand the regulatory T-cell compartment and suppress immune responses. However, it is still unclear whether tolDCs represent a homogeneous population, a specific DC subset and/or a heterogeneous collection of DC activation/maturation states. The ligand-activated transcription factor aryl hydrocarbon receptor (AHR) has been shown to control transcriptional programs associated to tolDCs. In this review, we discuss the role of AHR in the control of tolDCs, and also AHR-targeted approaches for the therapeutic induction of tolDCs in autoimmune diseases and allergy.

Cellular protein profiles altered by PRRSV infection of porcine monocytes-derived dendritic cells

Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that play an important role in inducing primary antigen-specific immune responses. Some viruses have evolved to specifically target DCs to circumvent the host immune responses for their persistence in the host. One example is porcine reproductive and respiratory syndrome virus (PRRSV) that causes a persistent infection in pigs through modulating DC-mediated antiviral response. To study the cellular protein responses in PRRSV-infected monocyte-derived dendritic cells (MoDCs), two-dimensional liquid chromatography-tandem mass spectrometry coupled with isobaric tags for relative and absolute quantification (iTRAQ) labeling was employed to quantitatively identify the differentially expressed proteins in PRRSV-infected MoDCs and the control cells. A total of 252 cellular proteins in MoDCs that were significantly altered at different time periods post-infection were identified. Differentially expressed proteins that are involved in the endocytosis pathway, actin cytoskeleton network, antigen processing and presentation, JAK-STAT signaling pathway and PRRSV receptors were identified and further analyzed. Among them, the expression changes of STAT1, Mx1, PICALM and SLA-DR were further verified by Western blotting. The protein profiles associated with PRRSV infection of MoDCs should offer novel insights to further investigation of PRRSV-mediated antiviral evasion mechanism and its pathobiology in swine.

Photochemical Treatment of Drosophila APCs Can Eliminate Associated Viruses and Maintain the APC Function for Generating Antigen-Specific CTLs Ex Vivo.

Drosophila cells transfected with MHC class I and a number of costimulation molecules including B7.1, ICAM, LFA-3, and CD70 are potent antigen-presenting cells (APCs) for the generation of antigen-specific cytotoxic T cells (CTLs) in vitro. Using Drosophila APCs, CTLs specific for melanoma antigens have been generated in vitro and adoptively transferred to melanoma patients. However, the recent discovery that Drosophila cells can carry insect viruses raises the potential risk of Drosophila APCs transmitting xenogenic viruses to patient CTLs. In this study, we have investigated photoreactive methods to inactivate insect viruses in APC. A clinical grade psoralen compound, 8-MOP (UVADEX) in combination with UVA treatment (5 joules/cm2) can be used to inactivate Drosophila cell viruses. UVADEX treatment is sufficient to inactivate insect viruses but does not affect the expression of MHC class I molecules and costimulation molecules on Drosophila APCs. In fact, UVADEX treatment prevents Drosophila APC growth while maintaining APC function. Furthermore, UVADEX-treated Drosophila APCs maintain or have enhanced APC function as determined by enhanced T cell activation, proliferation, and CTL generation. Thus, the use of UVADEX-treated Drosophila APCs may provide a valuable tool for immunotherapy to generate tumor antigen-specific CTLs.

Transdermal delivery of vaccines - Recent progress and critical issues.

In 2010, the number of deaths from infectious diseases globally was approximately 15 million. It has been reported that two-thirds of deaths from infections are caused by around 20 species, mainly bacteria and viruses. Transnational migration caused by war and the development of transportation facilities have led to the global spread of infectious diseases. Subcutaneous vaccination, though widespread, has a number of problems: the need for trained healthcare personnel, pain, needle-related injuries as well as storage difficulties. Two layers of the human skin- epidermis and dermis- are populated by dendritic cells (DCs), which are potent antigen-presenting cells (APCs). Transcutaneous immunization has therefore become an attractive and alternative route for vaccination. In this review, the various techniques for enhancing vaccine delivery are discussed. These techniques include iontophoresis, elastic liposomes as well as microneedles. Progress made so far with these techniques and the critical issues facing scientists will be highlighted.

Dendritic-Tumor Fusion Cell-Based Cancer Vaccines.

Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that play a critical role in the induction of antitumor immunity. Therefore, various strategies have been developed to deliver tumor-associated antigens (TAAs) to DCs as cancer vaccines. The fusion of DCs and whole tumor cells to generate DC-tumor fusion cells (DC-tumor FCs) is an alternative strategy to treat cancer patients. The cell fusion method allows DCs to be exposed to the broad array of TAAs originally expressed by whole tumor cells. DCs then process TAAs endogenously and present them through major histocompatibility complex (MHC) class I and II pathways in the context of costimulatory molecules, resulting in simultaneous activation of both CD4+ and CD8+ T cells. DC-tumor FCs require optimized enhanced immunogenicity of both DCs and whole tumor cells. In this context, an effective fusion strategy also needs to produce immunogenic DC-tumor FCs. We discuss the potential ability of DC-tumor FCs and the recent progress in improving clinical outcomes by DC-tumor FC-based cancer vaccines.

Identification of proteins derived from Listeria monocytogenes inducing human dendritic cell maturation.

Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that can promote antitumor immunity when pulsed with tumor antigens and then matured by stimulatory agents. Despite apparent progress in DC-based cancer immunotherapy, some discrepancies were reported in generating potent DCs. Listeria monocytogenes as an intracellular microorganism is able to effectively activate DCs through engaging pattern-recognition receptors (PRRs). This study aimed to find the most potent components derived from L. monocytogenes inducing DC maturation. The preliminary results demonstrated that the ability of protein components is higher than DNA components to promote DC maturation and activation. Protein lysate fractionation demonstrated that fraction 2 HIC (obtained by hydrophobic interaction chromatography) was able to efficiently mature DCs. F2HIC-matured DCs are able to induce allogeneic CD8(+) T cells proliferation better than LPS-matured DCs and induce IFN-γ producing CD8(+) T cells. Mass spectrometry results showed that F2HIC contains 109 proteins. Based on the bioinformatics analysis for these 109 proteins, elongation factor Tu (EF-Tu) could be considered as a PRR ligand for stimulating DC maturation.

The cancer vaccine therapy using DCs derived from iPS cells (iPSDCs) expressing TAA gene.

768 Background: Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that play a critical role in the initiation of anti-tumor immune responses. Many cancer patients have previously been treated by the cancer vaccine therapy using DCs worldwide. We have employed a study of a cancer vaccine therapy using genetically modified DCs expressing tumor-associated antigen (TAA) gene. Clinically DCs are generated from the peripheral blood monocytes of patients. Thus the number of monocytes and potential of them are limited, so they are serious obstacle. Recent studies have revealed that induced pluripotent stem (iPS) cells can be generated from murine fibroblasts. Furthermore, it has been reported that DCs can be successfully derived from murine iPS cells (iPSDCs). If the therapeutic efficacy of iPSDCs is equivalent to that of naive DCs, then the above-mentioned problems may be solved. Methods: We have induced iPSDCs from murine iPS cells by 4 steps and examined the efficacy as APCs of iPSDCs compared with naive DCs. We also examined whether a vaccine therapy using genetically modified iPSDCs can induce strong therapeutic antitumor immunity compared with naive DCs. Next, we examined the therapeutic antitumor immunity of iPSDCs expressing CEA gene compared with that of naive DCs in pre-clinical study with CEA transgenic mice. Results: We have clarified that genetically modified iPSDCs have an equal efficacy as APCs and TAA-specific therapeutic antitumor immunity, equivalent to naive DCs. And we also have clarified that genetically modified iPSDCs expressing CEA gene have a TAA-specific therapeutic antitumor immunity. Conclusions: This vaccine strategy using genetically modified iPSDCs has an equal capacity with naive DCs in terms of a therapeutic efficacy. Now, we are engaging another pre-clinical study with human, in an effort to apply in a clinical setting.

Genome-wide transcriptional profiling reveals that HIV-1 Vpr differentially regulates interferon-stimulated genes in human monocyte-derived dendritic cells

Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that directly link the innate and adaptive immune responses. HIV-1 infection of DCs leads to a diverse array of changes in gene expression and play a major role in dissemination of the virus into T-cells. Although HIV-1 Vpr is a pleiotropic protein involved in HIV-1 replication and pathogenesis, its exact role in APCs such as DCs remains elusive. In this study, utilizing a microarray-based systemic biology approach, we found that HIV-1 Vpr differentially regulates (fold change >2.0) more than 200 genes, primarily those involved in the immune response and innate immune response including type I interferon signaling pathway. The differential expression profiles of select genes involved in innate immune responses (interferon-stimulated genes [ISGs]), including MX1, MX2, ISG15, ISG20, IFIT1, IFIT2, IFIT3, IFI27, IFI44L, and TNFSF10, were validated by real-time quantitative PCR; the results were consistent with the microarray data. Taken together, our findings are the first to demonstrate that HIV-1 Vpr induces ISGs and activates the type I IFN signaling pathway in human DCs, and provide insights into the role of Vpr in HIV-1 pathogenesis.

Shark cartilage 14 kDa protein as a dendritic cells activator

Low molecular weight components of shark cartilage are reported to have anti-tumor as well as immuno-stimulating effects. Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that have a key role in establishment of anti-cancer immune response. In this study, the effect of 14 kDa protein from shark cartilage was investigated on stimulation and maturation of dendritic cells. The isolated 14 kDa protein from shark cartilage extract was added to DCs medium during overnight culture and their maturation and T cells stimulation potential was investigated. The majority of shark-cartilage-treated DCs expressed higher levels of maturation markers and were more effective in stimulation of allogenic T cells compared with non-treated DCs (p < 0.05). Our results showed that shark cartilage 14 kDa protein can potentially be used in DC-mediated T-cells stimulation and induction of desirable immune responses in clinical trials such as cancer immunotherapy. However, further studies are required to examine this proposal.

Development and Clinical Evaluation of Dendritic Cell Vaccines for HPV Related Cervical Cancer - a Feasibility Study

Human papillomavirus infection (HPV) and HPV related immune perturbation play important roles in the development of cervical cancer. Since mature dendritic cells (DCs) are potent antigen-presenting cells (APC), they could be primed by HPV antigens against cervical cancers. In this study we were able to generate, maintain and characterize, both phenotypically and functionally, patient specific dendritic cells in vitro. A randomized Phase I trial with three arms--saline control (arm I), unprimed mature DC (arm II) and autologous tumor lysate primed mature DC (arm III) and fourteen patients was conducted. According to WHO criteria, grade 0 or grade one toxicity was observed in three patients. One patient who received tumor lysate primed dendritic cells and later cis-platin chemotherapy showed a complete clinical response of her large metastatic disease and remained disease free for more than 72 months. Our findings indicate that DC vaccines hold promise as adjuvants for cervical cancer treatment and further studies to improve their efficacy need to be conducted.

MSCs inhibit bone marrow-derived DC maturation and function through the release of TSG-6

Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that are characterized by the ability to take up and process antigens and prime T cell responses. Mesenchymal stem cells (MSCs) are multipotent cells that have been shown to have immunomodulatory abilities, including inhibition of DC maturation and function in vivo and in vitro; however, the underlying mechanism is far from clear. In this study we found that MSCs can inhibit the maturation and function of bone marrow-derived DCs by releasing TSG-6. In the presence of MSCs, lower expression of mature DC surface phenotype (CD80, CD86, MHC-II, and CD11c) was observed. In addition, typical DC functions, such as the production of IL-12 and the ability to prime T cells, were decreased when co-cultured with MSCs. In contrast, knockdown of TSG-6 reduced the inhibitory effect of MSCs on DC. Moreover, we found that TSG-6 can suppress the activation of MAPKs, and NF-κB signaling pathways within DCs during Lipopolysaccharides (LPS) stimulation. In conclusion, we suggest that TSG-6 plays an important role in MSCs-mediated immunosuppressive effect on DC.

Abstract B17: Heat shock-induced danger signals in tumor cells are crucial for a rapid differentiation of monocyte to therapeutic dendritic cells

Abstract Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that initiate a cell-mediated antitumor immune response. Recently, we showed that monocyte-derived DCs loaded with a heat-shocked melanoma cell lysate (TRIMEL®) induce melanoma-specific immune responses, thus prolonging patient survival. Here, we describe the rapid effect of TRIMEL on human monocytes differentiation to tumor-antigen presenting cells (TAPCells®). TAPCells showed a mature DC-like phenotype and effectively activated melanoma associated antigen (MAA)-specific CD4+ and CD8+ T lymphocytes. Moreover, heat shock (HS) pre-conditioning of TRIMEL composing cells increased calreticulin (CRT) cell surface translocation and released high-mobility group box 1 protein (HMGB1), which was closely associated to the induction of APC maturation and efficient MAAs cross-presentation. Finally, 26 out of 43 patients vaccinated with TAPCells showed a TRIMEL-specific delayed type hypersensitivity reaction (DTH) associated to anti-melanoma T cell recognition, reduced rates of progression and prolonged patient survival. Our results suggest that heat-shocked tumor cell lysates are optimal source of MAAs, inducing activated APCs with improved MAAs cross-presentation capacity and clinically effective immunogenicity. Citation Information: Clin Cancer Res 2010;16(7 Suppl):B17

Statins Inhibit the Function of Human Antigen-Presenting Cells

Statins are lipid-lowering drugs that reduce cholesterol production by inhibition of HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway. It recently has been shown that statins, apart from their lipid-lowering action, have pleiotropic effects on the immune system. They possess immunomodulatory and anti-inflammatory activity. Our analysis of gene expression data comparing resting B cells with CD40-activated B cells, which are potent antigen-presenting cells (APC), showed that following CD40-activation expression of the enzymes in the mevalonate pathway was up-regulated. These results together with the known ability of some statins to directly interfere with the interaction of lymphocyte function-associated antigen 1 (LFA-1), an adhesion molecule involved in activation of APCs, with its ligands provided the rational for our investigation of the effects of HMG-CoA reductase inhibition on the function of human APCs. Addition of simvastatin led to inhibition of the proliferation and adhesion of B cells activated by via CD40 or TLR9. Likewise, the expression of MHC class II (HLA-DR) and of the costimulatory molecules CD80 and CD86 was reduced. The antigen presenting capacity of simvastatin-treated B cells was reduced as was shown by a reduction of T cell proliferation induced by allogeneic simvastatin-treated B cells. Simvastatin inhibited activation of resting B cells as well as already activated B cells. The inhibition was dose-dependent and doses as low as 10nM Simvastatin led to signification reduction of B cell activation. We observed similar effects on human dendritic cells. Interestingly, the concentration of simvastatin required for inhibition of T cell proliferation in response to stimulation by CD3/CD28-coated beads was 1–2 logs higher than for B cells. The inhibitory action of statins was only partially dependent on the mevalonate pathway since addition of excess mevalonate did not fully restore B cell activation. Our findings of the effects of statins on human APCs in conjunction with the observation made by others that preemptive treatment with a HMG-CoA reductase inhibitor protected mice from graft-versus-host-disease (GVHD) without affecting the graft-versus-leukemia effect provide the rational for further testing of statins in patients undergoing allogeneic stem cell transplantion. Since APCs are required for the development of acute GVHD the modulation of antigen presentation by DCs and B cells might be an important mechanism of action of statins. The exquisite sensitivity of APCs to statin-mediated inhibition could synergize with the T cell-directed immunosuppression of conventional immunosuppressants. In summary, statins represent a promising new immunomodulatory drug for the treatment immune-mediated diseases such as GVHD and autoimmune disease.

Tumor Protection Following Vaccination With Low Doses of Lentivirally Transduced DCs Expressing the Self-antigen erbB2

Gene therapy strategies may accelerate the development of prophylactic immunotherapy against cancer. We synthesized a lentiviral (LV) vector encoding a kinase-deficient form of erbB2 (erbB2tr) to transduce murine dendritic cells (DCs) efficiently. Murine erbB2 models a clinically relevant tumor-associated self-antigen; its human homolog (HER-2/neu) is overexpressed in breast cancer and in 80% of metastatic prostate cancers. Following one infection, approximately 47% of DCs overexpressed erbB2tr. To determine whether low doses of transduced DCs could protect mice from prostate cancer cells, we performed prime/boost vaccinations with 2 x 10(3) or 2 x 10(5) erbB2tr-transduced DCs. Six weeks after vaccination, mice were simultaneously bilaterally challenged with the aggressive RM-1 prostate cancer cell line and an erbB2tr-expressing variant (RM-1-erbB2tr). Whereas control mice developed both tumors, all recipients of 2 x 10(5) erbB2tr-transduced DCs developed only wild-type RM-1 tumors. One-third of mice vaccinated with just 2 x 10(3) erbB2tr-transduced DCs also demonstrated erbB2tr-specific tumor protection. Protection against RM-1-erbB2tr tumors was associated with sustained levels of anti-erbB2tr antibody production and also correlated with erbB2tr-specific Th1 cytokine secretion. Depletion of CD4(+), CD8(+), or natural killer (NK) cells prior to tumor challenge underscored their role in mediating tumor protection. We conclude that administration of DCs expressing a self-antigen through efficient LV-based gene transfer activates cellular and humoral immunity, protecting host animals against specific tumor challenge.

Professional Antigen-Presenting Cells of the Skin

The skin functions as an important pro-inflammatory and immune organ. Accordingly, the epidermis and dermis are highly populated by dendritic cells (DC), which are potent antigen-presenting cells (APC) with important immunostimulatory and migratory activities. Whereas the biological characteristics and immunological functions of epidermal DC known as Langernahs cells (LC) have been the focus of intense research in the past, less is known regarding their dermal counterparts named dermal dendritic cells (DDC). Although it has been widely accepted that LC are the more relevant skin-resident APC, recent experimental evidence challenges this concept and proposes a different role for these important cell populations. In this article we compile recent scientific advances regarding the function of different skin-resident DC and we try to reconcile the new observations with the previously established paradigm.

Differentiation of Peripheral Blood Monocytes into Dendritic Cells

Dendritic cells (DCs) are potent antigen-presenting cells (APC) that are important in the initiation and control of cellular immune responses. Commonly used in T cell-stimulation experiments, DCs are typically "matured" in vitro with microbial products or proinflammatory cytokines, and then loaded with antigens from any number of sources, including peptides, whole proteins, cell lysates, RNA, microbes, or killed tumor cells. This unit presents a simple and commonly used method for the generation of mature human dendritic cells--differentiating them from peripheral blood monocytes.

Lack of phenotypic and functional impairment in dendritic cells from chimpanzees chronically infected with hepatitis C virus.

Dendritic cells (DCs), which are potent antigen-presenting cells (APCs), are used as adjuvants for the treatment of cancer and infectious diseases in human and nonhuman primates, with documented clinical efficacy. The hepatitis C virus (HCV)-chimpanzee model is the best available model for testing the immunotherapeutic effects of DCs in the setting of a chronic infection, as chimpanzees develop a persistent infection resembling that seen in humans. However, several reports have suggested that DCs derived from chronically infected individuals or nonhuman primates are functionally compromised. As a prelude to clinical studies, we evaluated whether functionally mature DCs could be generated in chimpanzee plasma by good manufacturing practice using CD14(+) mononuclear precursors from chronically infected chimpanzees. DCs generated in a medium with HCV-negative plasma and treated with a defined cocktail of cytokines or a CD40 ligand trimer matured fully, as measured by the induction of CD83 expression and the upregulation of costimulatory molecules. Furthermore, the expression of CCR7 was induced, suggesting an acquisition of migration capacity. Mature DCs were capable of stimulating allogeneic T cells, antigen-specific memory CD4(+) T cells, and HCV-specific CD8(+)-T-cell clones. In all cases, there was no evidence of HCV infection in DCs. Furthermore, these DCs maintained their phenotype and APC function after cryopreservation. Finally, no discernible differences were noted between DCs derived from HCV-infected and uninfected chimpanzees. In summary, precursor cells from HCV-infected chimpanzees are fully capable of differentiating into functional, mature DCs, which can now be reproducibly prepared for investigations of their immunotherapeutic potential in the setting of chronic HCV infection.

Lentiviral transduction of human dendritic cells.

Dendritic cells (DCs) are potent antigen-presenting cells (APCs) that play a pivotal role in stimulating antigen-specific T cells in vivo. The cardinal properties of DCs are: the ability to take up, process, and present antigens; (2) the ability to migrate through different tissues into lymphoid organs; and (3) the ability to interact with and stimulate T cells (3). Because of their unique capability of generating primary CD4+ and CD8+ T-cell responses, DCs are of particular interest for immunotherapeutic approaches to infectious disease and cancer. There are many ways to load DCs with antigen and to subsequently induce specific immune responses in vivo and in vitro. One strategy relies on genetically modified peripheral blood mononuclear cell (PBMC)-derived DCs to establish expression of a gene that encodes a specific antigen. In contrast to pulsing DCs with antigenic peptides or proteins, genetic engineering of DCs has the advantages of providing multiple epitopes for major histocompatibility class (MHC) class I-and MHC class II-restricted immune responses as well as a continuous supply of antigen for presentation alone or together with immunomodulatory proteins (cytokines, for example).

Involvement of tumour necrosis factor-alpha-related apoptosis-inducing ligand in enhanced cytotoxicity of lipopolysaccharide-stimulated dendritic cells to activated T cells.

Dendritic cells (DC) are potent antigen-presenting cells (APC) specialized in T-cell mediated immune responses, and also play critical roles in the homeostasis of T cells for controlling immune responses. In the present study, we demonstrated that during mouse bone-marrow-derived DC activation of ovalbumin (OVA)-specific Ia-kb-restricted T hybridoma cells, MF2.2D9 and OVA257-264-specific H-2kb-restricted RF33.70 T cells, respectively, both hybridomas undergo cell death, partially mediated via apoptotic ligand-tumour necrosis factor-alpha (TNF-alpha)-related apoptosis-inducing ligand (TRAIL). Lipopolysaccharide enhanced the cytotoxic effect on the two activated T hybridoma cells, which was correlated with up-regulation of TRAIL-expression on DC to some extent. The activation of caspase-3 in activated T hybridoma cells cocultured with DC contributed to the programmed cell death pathway T cells underwent. Therefore, our results show that activation-induced cell death of T hybridoma cells can be influenced by DC, suggesting that DC may be involved in elimination of activated T cells at the end of primary immune responses.