Control of CD4 + T‐cell memory by cytokines and costimulators

Pretransplant, Th17 dominant alloreactivity in highly sensitized kidney transplant candidates

Introduction

Sensitization to donor human leukocyte antigen (HLA) molecules prior to transplantation is a significant risk factor for delayed access to transplantation and to long-term outcomes. Memory T cells and their cytokines play a pivotal role in shaping immune responses, thereby increasing the risk of allograft rejection among highly sensitized patients. This study aims to elucidate the precise contribution of different CD4+ memory T cell subsets to alloreactivity in highly sensitized (HS) kidney transplant recipients.

Methods and results

Stimulation of peripheral blood mononuclear cells (PBMC) with various polyclonal stimulating agents to assess non-specific immune responses revealed that HS patients exhibit elevated immune reactivity even before kidney transplantation, compared to non-sensitized (NS) patients. HS patients' PBMC displayed higher frequencies of CD4+ T cells expressing IFNγ, IL4, IL6, IL17A, and TNFα and secreted relatively higher levels of IL17A and IL21 upon stimulation with PMA/ionomycin. Additionally, PBMC from HS patients stimulated with T cell stimulating agent phytohemagglutinin (PHA) exhibited elevated expression levels of IFNγ, IL4 and, IL21. On the other hand, stimulation with a combination of resiquimod (R848) and IL2 for the activation of memory B cells demonstrated higher expression of IL17A, TNFα and IL21, as determined by quantitative real-time PCR. A mixed leukocyte reaction (MLR) assay, employing third-party donor antigen presenting cells (APCs), was implemented to evaluate the direct alloreactive response. HS patients demonstrated notably higher frequencies of CD4+ T cells expressing IL4, IL6 and IL17A. Interestingly, APCs expressing recall HLA antigens triggered a stronger Th17 response compared to APCs lacking recall HLA antigens in sensitized patients. Furthermore, donor APCs induced higher activation of effector memory T cells in HS patients as compared to NS patients.

Conclusion

These results provide an assessment of pretransplant alloreactive T cell subsets in highly sensitized patients and emphasize the significance of Th17 cells in alloimmune responses. These findings hold promise for the development of treatment strategies tailored to sensitized kidney transplant recipients, with potential clinical implications.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

DNA Methylation and Immune Memory Response

The generation of memory is a cardinal feature of the adaptive immune response, involving different factors in a complex process of cellular differentiation. This process is essential for protecting the second encounter with pathogens and is the mechanism by which vaccines work. Epigenetic changes play important roles in the regulation of cell differentiation events. There are three types of epigenetic regulation: DNA methylation, histone modification, and microRNA expression. One of these epigenetic changes, DNA methylation, occurs in cytosine residues, mainly in CpG dinucleotides. This brief review aimed to analyse the literature to verify the involvement of DNA methylation during memory T and B cell development. Several studies have highlighted the importance of the DNA methyltransferases, enzymes that catalyse the methylation of DNA, during memory differentiation, maintenance, and function. The methylation profile within different subsets of naïve activated and memory cells could be an interesting tool to help monitor immune memory response.

Mixing and oxygen transfer characteristics of a microplate bioreactor with surface-attached microposts

Bioprocess optimization for cell-based therapies is a resource heavy activity. To reduce the associated cost and time, process development may be carried out in small volume systems, with the caveat that such systems be predictive for process scale-up. The transport of oxygen from the gas phase into the culture medium, characterized using the volumetric mass transfer coefficient, k_L a, has been identified as a critical parameter for predictive process scale-up. Here, we describe the development of a 96-well microplate with integrated Redbud Posts to provide mixing and enhanced k_L a. Mixing in the microplate is characterized by observation of dyes and analyzed using the relative mixing index (RMI). The k_L a is measured via dynamic gassing out method. Actuating Redbud Posts are shown to increase rate of planar homogeneity (2 min) verse diffusion alone (120 min) and increase oxygenation, with increasing stirrer speed (3500-9000 rpm) and decreasing fill volume (150-350 μL) leading to an increase in k_L a (4-88 h^-1 ). Significant increase in Chinese Hamster Ovary growth in Redbud Labs vessel (580,000 cells mL^-1 ) versus the control (420,000 cells mL^-1 ); t(12.814) = 8.3678, p ≤ .001), and CD4⁺ Naïve cell growth in the microbioreactor indicates the potential for this technology in early stage bioprocess development and optimization.

Investigation of the Impact from IL-2, IL-7, and IL-15 on the Growth and Signaling of Activated CD4+ T Cells

While CAR-T therapy is a growing and promising area of cancer research, it is limited by high cost and the difficulty of consistently culturing T-cells to therapeutically relevant concentrations ex-vivo. Cytokines IL-2, IL-7 and IL-15 have been found to stimulate the growth of T cells, however, the optimized combination of these three cytokines for T cell proliferation is unknown. In this study, we designed an integrated experimental and modeling approach to optimize cytokine supplementation for rapid expansion in clinical applications. We assessed the growth data for statistical improvements over no cytokine supplementation and used a systems biology approach to identify genes with the highest magnitude of expression change from control at several time points. Further, we developed a predictive mathematical model to project the growth rate for various cytokine combinations, and investigate genes and reactions regulated by cytokines in activated CD4+ T cells. The most favorable conditions from the T cell growth study and from the predictive model align to include the full range of IL-2 and IL-7 studied, and at lower levels of IL-15 (6 ng/mL or 36 ng/mL). The highest growth rates were observed where either IL-2 or IL-7 was at the highest concentration tested (15 ng/mL for IL-2 and 80 ng/mL for IL-7) while the other was at the lowest (1 ng/mL for IL-2 and 6 ng/mL for IL-7), or where both IL-2 and IL-7 concentrations are moderate-corresponding to condition keys 200, 020, and 110 respectively. This suggests a synergistic interaction of IL-2 and IL-7 with regards to promoting optimal proliferation and survival of the activated CD4+ T cells. Transcriptomic data analysis identified the genes and transcriptional regulators up/down-regulated by each of the cytokines IL-2, IL-7, and IL-15. It was found that the genes with persistent expressing changes were associated with major pathways involved in cell growth and proliferation. In addition to influencing T cell metabolism, the three cytokines were found to regulate specific genes involved in TCR, JAK/STAT, MAPK, AKT and PI3K-AKT signaling. The developed Fuzzy model that can predict the growth rate of activated CD4+ T cells for various combinations of cytokines, along with identified optimal cytokine cocktails and important genes found in transcriptomic data, can pave the way for optimizing activated CD4 T cells by regulating cytokines in the clinical setting.

Simulation of Stimulation: Cytokine Dosage and Cell Cycle Crosstalk Driving Timing-Dependent T Cell Differentiation

Triggering an appropriate protective response against invading agents is crucial to the effectiveness of human innate and adaptive immunity. Pathogen recognition and elimination requires integration of a myriad of signals from many different immune cells. For example, T cell functioning is not qualitatively, but quantitatively determined by cellular and humoral signals. Tipping the balance of signals, such that one of these is favored or gains advantage on another one, may impact the plasticity of T cells. This may lead to switching their phenotypes and, ultimately, modulating the balance between proliferating and memory T cells to sustain an appropriate immune response. We hypothesize that, similar to other intracellular processes such as the cell cycle, the process of T cell differentiation is the result of: (i) pleiotropy (pattern) and (ii) magnitude (dosage/concentration) of input signals, as well as (iii) their timing and duration. That is, a flexible, yet robust immune response upon recognition of the pathogen may result from the integration of signals at the right dosage and timing. To investigate and understand how system's properties such as T cell plasticity and T cell-mediated robust response arise from the interplay between these signals, the use of experimental toolboxes that modulate immune proteins may be explored. Currently available methodologies to engineer T cells and a recently devised strategy to measure protein dosage may be employed to precisely determine, for example, the expression of transcription factors responsible for T cell differentiation into various subtypes. Thus, the immune response may be systematically investigated quantitatively. Here, we provide a perspective of how pattern, dosage and timing of specific signals, called interleukins, may influence T cell activation and differentiation during the course of the immune response. We further propose that interleukins alone cannot explain the phenotype variability observed in T cells. Specifically, we provide evidence that the dosage of intercellular components of both the immune system and the cell cycle regulating cell proliferation may contribute to T cell activation, differentiation, as well as T cell memory formation and maintenance. Altogether, we envision that a qualitative (pattern) and quantitative (dosage) crosstalk between the extracellular milieu and intracellular proteins leads to T cell plasticity and robustness. The understanding of this complex interplay is crucial to predict and prevent scenarios where tipping the balance of signals may be compromised, such as in autoimmunity.

Early programming and late-acting checkpoints governing the development of CD4 T-cell memory

CD4 T cells contribute to protection against pathogens through numerous mechanisms. Incorporating the goal of memory CD4 T-cell generation into vaccine strategies therefore offers a powerful approach to improve their efficacy, especially in situations where humoral responses alone cannot confer long-term immunity. These threats include viruses such as influenza that mutate coat proteins to avoid neutralizing antibodies, but that are targeted by T cells that recognize more conserved protein epitopes shared by different strains. A major barrier in the design of such vaccines is that the mechanisms controlling the efficiency with which memory cells form remain incompletely understood. Here, we discuss recent insights into fate decisions controlling memory generation. We focus on the importance of three general cues: interleukin-2, antigen and co-stimulatory interactions. It is increasingly clear that these signals have a powerful influence on the capacity of CD4 T cells to form memory during two distinct phases of the immune response. First, through 'programming' that occurs during initial priming, and second, through 'checkpoints' that operate later during the effector stage. These findings indicate that novel vaccine strategies must seek to optimize cognate interactions, during which interleukin-2-, antigen- and co-stimulation-dependent signals are tightly linked, well beyond initial antigen encounter to induce robust memory CD4 T cells.

Trying to See the Forest through the Trees: Deciphering the Nature of Memory Immunity to Mycobacterium tuberculosis

The purpose of vaccination against tuberculosis and other diseases is to establish a heightened state of acquired specific resistance in which the memory immune response is capable of mediating an accelerated and magnified expression of protection to the pathogen when this is encountered at a later time. In the earliest studies in mice infected with Mycobacterium tuberculosis, memory immunity and the cells that express this were definable both in terms of kinetics of emergence, and soon thereafter by the levels of expression of markers including CD44, CD62L, and the chemokine receptor CCR7, allowing the identification of effector memory and central memory T cell subsets. Despite these initial advances in knowledge, more recent information has not revealed more clarity, but instead, has created a morass of complications—complications that, if not resolved, could harm correct vaccine design. Here, we discuss two central issues. The first is that we have always assumed that memory is induced in the same way, and consists of the same T cells, regardless of whether that immunity is generated by BCG vaccination, or by exposure to M. tuberculosis followed by effective chemotherapy. This assumption is almost certainly incorrect. Second, a myriad of additional memory subsets have now been described, such as resident, stem cell-like, tissue specific, among others, but as yet we know nothing about the relative importance of each, or whether if a new vaccine needs to induce all of these, or just some, to be fully effective.

Calcium-dependent protein kinases are potential targets for Toxoplasma gondii vaccine

Toxoplasma gondii belongs to the Apicomplexa phylum that caused a widespread zoonotic infection in wide range of intermediate hosts. Over one-third of the world's population are latently infected with T. gondii and carry it. The complex life cycle of T. gondii indicates the presence of a plurality of antigenic epitopes. During the recent years, continuous efforts of scientists have made precious advances to elucidate the different aspects of the cell and molecular biology of T. gondii. Despite of great progresses, the development of vaccine candidates for preventing of T. gondii infection in men and animals is still remains a challenge. The calcium-dependent protein kinases (CDPKs) belongs to the superfamily of kinases, which restricted to the apicomplexans, ciliates, and plants. It has been documented that they contribute several functions in the life cycle of T. gondii such as gliding motility, cell invasion, and egress as well as some other critical developmental processes. In current paper, we reviewed the recent progress concerning the development of CDPK-based vaccines against acute and chronic T. gondii.

Role of B and T Lymphocyte Attenuator in Renal Transplant Recipients with Biopsy-Proven Acute Rejection

Background

Acute rejection is a common predisposing cause of allograft dysfunction in kidney transplantation. Recently, the B and T lymphocyte attenuator (BTLA)/herpes virus entry mediator (HVEM)/lymphotoxin (LIGHT)/CD160 pathway was found to be potentially involved in the regulation of T cell activation. This could mean that this pathway is involved in graft rejection in kidney transplantation; the present study aimed to explore this possibility.

Material/Methods

The expression of BTLA, HVEM, LIGHT and CD160 on peripheral CD4+, CD8+ and CD19+ lymphocytes were analyzed by flow cytometry in recipients with biopsy-proven acute rejection (BPAR) or stable allograft function, as well as in healthy volunteers. Moreover, we performed HE staining and immunohistochemical staining to assess the expression of BTLA and HVEM in kidney samples from recipients with BPAR and patients who underwent the surgery of radical nephrectomy.

Results

We observed the significantly lower expression of BTLA on CD4+ T cells in recipients from the BPAR group than in recipients from the stable group. The expression of BTLA on CD8+ T cells among recipients both from the BPAR and stable group was statistically increased than that in the healthy volunteers. A significant difference in the expression of CD160 in the stable group was found when compared with the BPAR group or control group. Moreover, there was no significance in the expression of HVEM, LIGHT or CD160 on other subtypes of T cells between the 3 groups or in the expression of BTLA on CD4+ T cells between the BPAR and control group.

Conclusions

The findings indicate that the BTLA/HVEM pathway does be involved in pathogenesis of acute rejection following kidney transplantation, as well as the induction of transplant tolerance. This pathway may therefore be a useful target for therapy against acute rejection after kidney transplantation.

Genetic Influence on the Peripheral Blood CD4+ T-cell Differentiation Status in CMV Infection

A latent infection with cytomegalovirus (CMV), a ubiquitous beta herpesvirus, is associated with an accumulation of late-differentiated memory T-cells, often accompanied by a reciprocal reduced frequency of early-differentiated cells (commonly also referred to as "naïve"). However, this impact of CMV on T-cell phenotypes is variable between individuals. Our previous findings in a subgroup of participants in the Leiden familial Longevity Study indicated an important role of genetics. For further testing, we have analyzed middle-aged monozygotic (MZ, n = 42) and dizygotic (DZ, n = 39) twin pairs from the Danish Twin Registry for their T-cell differentiation status, assessed by surface expression of CD27, CD28, CD57, and KLRG-1. We observed a significant intraclass correlation between cotwins of MZ, but not DZ pairs for the differentiation status of CD4⁺ and CD8⁺ subsets. Classical heritability analysis confirmed a substantial contribution of genetics to the differentiation status of T-cells in CMV infection. The humoral (IgG) response to different CMV antigens also seems to be genetically influenced, suggesting that a similar degree of immune control of the virus in MZ twins might be responsible for their similar T-cell differentiation status. Thus, the way T-cells differentiate in the face of a latent CMV infection, and the parallel humoral responses, both controlling the virus, are genetically influenced.

Co-administration of interleukins 7 and 15 with DNA vaccine improves protective immunity against Toxoplasma gondii

Toxoplasma gondii is an obligatory intracellular parasite, which can infect all warm-blooded animals including humans. Cytokines, including IL-15 and IL-7, play a critical role in the regulation of the homeostasis of naive and memory T cells. Co-administration the DNA vaccine with cytokines may improve its efficacy. IL-7 and IL-15 from splenic tissues of Kunming mice were cloned, and eukaryotic plasmid pVAX-IL-7-IL-15 was constructed. Kunming mice were administrated with DNA vaccine expressing T. gondii calcium-dependent protein kinase 1 (TgCDPK1), pVAX-CDPK1, in the presence or absence of IL-7 and IL-15 plasmids (pVAX-IL-7-IL-15), immune responses were analyzed including lymphoproliferative assay, cytokine and serum antibody measurements, flow cytometric surface markers on lymphocytes, and thus protective immunity against acute and chronic T. gondii infection was estimated. Mice injected with pVAX-CDPK1 supplemented with pVAX-IL-7-IL-15 showed higher Toxoplasma-specific IgG2a titers, Th1 responses associated with the production of IFN-γ, IL-2 as well as cell-mediated cytotoxic activity where stronger frequencies of IFN-γ secreting CD8+ and CD4+ T cells (CD8+/CD4+ IFN-γ+ T cells) compared to controls. Co-administration of pVAX-IL-7-IL-15 and pVAX-CDPK1 significantly (P < 0.05) increased survival time (18.07 ± 5.43 days) compared with pVAX-CDPK1 (14.13 ± 3.85 days) or pVAX-IL-7-IL-15 (11.73 ± 1.83 days) alone, and pVAX-IL-7-IL-15 + pVAX-CDPK1 significantly reduced the number of brain cysts (73.5%) in contrast to pVAX-CDPK1 (46.0%) or pVAX-IL-7-IL-15 alone (45.0%). Our results indicate that supplementation of DNA vaccine with IL-7 and IL-15 would facilitate specific humoral and cellular immune responses elicited by DNA vaccine against acute and chronic T. gondii infection in mice.

Spaceflight impairs antigen-specific tolerance induction in vivo and increases inflammatory cytokines

The health risks of a dysregulated immune response during spaceflight are important to understand as plans emerge for humans to embark on long-term space travel to Mars. In this first-of-its-kind study, we used adoptive transfer of T-cell receptor transgenic OT-II CD4 T cells to track an in vivo antigen-specific immune response that was induced during the course of spaceflight. Experimental mice destined for spaceflight and mice that remained on the ground received transferred OT-II cells and cognate peptide stimulation with ovalbumin (OVA) 323-339 plus the inflammatory adjuvant, monophosphoryl lipid A. Control mice in both flight and ground cohorts received monophosphoryl lipid A alone without additional OVA stimulation. Numbers of OT-II cells in flight mice treated with OVA were significantly increased by 2-fold compared with ground mice treated with OVA, suggesting that tolerance induction was impaired by spaceflight. Production of proinflammatory cytokines were significantly increased in flight compared with ground mice, including a 5-fold increase in IFN-γ and a 10-fold increase in IL-17. This study is the first to show that immune tolerance may be impaired in spaceflight, leading to excessive inflammatory responses.

Thymic stromal lymphopoietin signaling in CD4(+) T cells is required for TH2 memory

BACKGROUND

Thymic stromal lymphopoietin (TSLP) is a key factor in the development of allergic asthma. Numbers of TH2 memory cells gradually increase in allergic patients with the progression of disease and persist in the lungs during remission, although the mechanism is not clear.

OBJECTIVE

We sought to define the role of TSLP in TH2 memory cell generation and maintenance in vivo.

METHODS

Adoptive transfer of wild-type and thymic stromal lymphopoietin receptor (TSLPR)-deficient ovalbumin-specific CD4(+) T cells before TH2 sensitization was used to define T cell-specific TSLP effects. Atopic dermatitis and increased serum TSLP concentrations were induced by topical application of the vitamin D3 analog MC903. Memory cells in peripheral blood were monitored weekly with flow cytometry. Memory recall was tested after intranasal ovalbumin challenge.

RESULTS

TSLP signaling in CD4(+) T cells is required for the generation/maintenance of memory cells after in vivo priming. TSLPR-deficient CD4(+) T cells have no defects in proliferation but do not survive 1 week after sensitization, and increased TSLP expression during sensitization significantly increased the frequency of memory cells. Although in vitro-differentiated TSLPR-deficient TH2 cells develop into memory cells with equal efficiency to wild-type cells, the recall response to airway antigen challenge is impaired. Moreover, after antigen challenge of mice with established TH2 memory, TSLP signaling in CD4(+) T cells significantly affects memory cell generation/maintenance from secondary effector cells.

CONCLUSION

TSLP signaling in CD4(+) T cells is required for not only TH2 memory cell formation in vivo but also the recall response of the memory cells to local antigen challenge.

Improving vaccines by incorporating immunological coadjuvants

While vaccination continues to be the most successful interventionist health policy to date, infectious disease remains a significant cause of death worldwide. A primary reason that vaccination is not able to generate effective immunity is a lack of appropriate adjuvants capable of initiating the desired immune response. Adjuvant combinations can potentially overcome this problem; however, the possible permutations to consider, which include the route and kinetics of vaccination, as well as combinations of adjuvants, are practically limitless. This review aims to summarize the current understanding of adjuvants and related immunological processes and how this knowledge can and has been applied to the strategic selection of adjuvant combinations as components of vaccines against human infectious disease.

A transendocytosis perspective on the CD28/CTLA-4 pathway

T cell activation is a key event in the adaptive immune response and vital to the generation of both cellular and humoral immunity. Activation is required not only for effective CD4 T cell responses but also to provide help for B cells and the generation of cytotoxic T cell responses. Unsurprisingly, impaired T cell activation results in infectious pathology, whereas dysregulated activation can result in autoimmunity. The decision to activate is therefore tightly regulated and the CD28/CTLA-4 pathway represents this apical decision point at the molecular level. In particular, CTLA-4 (CD152) is an essential checkpoint control for autoimmunity; however, the molecular mechanism(s) by which CTLA-4 achieves its regulatory function are not well understood, especially how it functionally intersects with the CD28 pathway. In this chapter, we review the established molecular and cellular concepts relating to CD28 and CTLA-4 biology, and attempt to integrate these by discussing the transendocytosis of ligands as a new model of CTLA-4 function.

Distinctive patterns of naïve/memory subset distribution and cytokine expression in CD4 T lymphocytes in ZAP-70 B-chronic lymphocytic patients

BACKGROUND

ZAP-70 upregulation in B chronic lymphocytic leukemia (B-CLL) cells is a recognized marker of poor prognosis in these patients; the biological basis of this differential clinical outcome nonetheless remains unknown. ZAP-70 overexpression is considered a surrogate marker of a B-CLL cell subset. To test whether the differential biological characteristics of these patients also include the T helper population, we studied naïve, non-terminated memory (NTEM), terminated memory (TEM) and central memory (CM) cells, and cytokine expression by CD4 T lymphocytes from ZAP-70(+) and ZAP-70(-) B-CLL patients.

METHODS

Expression of CD3, CD8, CD45RA, CD27, and CD28 antigens and intracytoplasmic cytokine production (IFNγ, IL-2, IL-4, IL-10, and TNFα) were assessed simultaneously by nine-color flow-cytometry in peripheral blood lymphocytes from B-CLL patients. B cell ZAP-70 expression in B-CLL cells was also analyzed by flow cytometry.

RESULTS

Compared to ZAP-70(-) B-CLL patients, ZAP-70(+) B-CLL patients showed 1) significant reduction in the naïve T helper subset and expansion of NTEM and TEM subsets, 2) a decrease in the percentage of activated CD4 T lymphocytes expressing IFNγ, TNFα, and IL-2, and 3) an increase in the percentage of CD4 T lymphocytes expressing IL-4 or IL-10.

CONCLUSIONS

In conclusion, in early stage B-CLL patients, ZAP-70 upregulation is associated with distinct patterns of activation/differentiation stage subset distribution and of cytokine expression in CD4 T lymphocytes.

IL-27 receptor signaling regulates memory CD4+ T cell populations and suppresses rapid inflammatory responses during secondary malaria infection

Interleukin-27 (IL-27) is known to control primary CD4(+) T cell responses during a variety of different infections, but its role in regulating memory CD4(+) T responses has not been investigated in any model. In this study, we have examined the functional importance of IL-27 receptor (IL-27R) signaling in regulating the formation and maintenance of memory CD4(+) T cells following malaria infection and in controlling their subsequent reactivation during secondary parasite challenge. We demonstrate that although the primary effector/memory CD4(+) T cell response was greater in IL-27R-deficient (WSX-1(-/-)) mice following Plasmodium berghei NK65 infection than in wild-type (WT) mice, there were no significant differences in the size of the maintained memory CD4(+) T population(s) at 20 weeks postinfection in the spleen, liver, or bone marrow of WSX-1(-/-) mice compared with WT mice. However, the composition of the memory CD4(+) T cell pool was slightly altered in WSX-1(-/-) mice following clearance of primary malaria infection, with elevated numbers of late effector memory CD4(+) T cells in the spleen and liver and increased production of IL-2 in the spleen. Crucially, WSX-1(-/-) mice displayed significantly enhanced parasite control compared with WT mice following rechallenge with homologous malaria parasites. Improved parasite control in WSX-1(-/-) mice during secondary infection was associated with elevated systemic production of multiple inflammatory innate and adaptive cytokines and extremely rapid proliferation of antigen-experienced T cells in the liver. These data are the first to demonstrate that IL-27R signaling plays a role in regulating the magnitude and quality of secondary immune responses during rechallenge infections.

T-Cell Signaling in HIV-1 Infection

HIV exploits the T-cell signaling network to gain access to downstream cellular components, which serves as effective tools to break the cellular barriers. Multiple host factors and their interaction with viral proteins contribute to the complexity of HIV-1 pathogenesis and disease progression. HIV-1 proteins gp120, Nef, Tat and Vpr alter the T-cell signaling pathways by activating multiple transcription factors including NF-ĸB, Sp1 and AP-1. HIV-1 evades the immune system by developing a multi-pronged strategy. Additionally, HIV-1 encoded proteins influence the apoptosis in the host cell favoring or blocking T-cell apoptosis. Thus, T-cell signaling hijacked by viral proteins accounts for both viral persistence and immune suppression during HIV-1 infection. Here, we summarize past and present studies on HIV-1 T-cell signaling with special focus on the possible role of T cells in facilitating viral infection and pathogenesis

CD28-inducible transcription factor DEC1 is required for efficient autoreactive CD4+ T cell response

The transcription factor DEC1 is induced by CD28 ligation and is required for optimal CD4⁺ T cell responses and the development of EAE.

During the initial hours after activation, CD4⁺ T cells experience profound changes in gene expression. Co-stimulation via the CD28 receptor is required for efficient activation of naive T cells. However, the transcriptional consequences of CD28 co-stimulation are not completely understood. We performed expression microarray analysis to elucidate the effects of CD28 signals on the transcriptome of activated T cells. We show that the transcription factor DEC1 is highly induced in a CD28-dependent manner upon T cell activation, is involved in essential CD4⁺ effector T cell functions, and participates in the transcriptional regulation of several T cell activation pathways, including a large group of CD28-regulated genes. Antigen-specific, DEC1-deficient CD4⁺ T cells have cell-intrinsic defects in survival and proliferation. Furthermore, we found that DEC1 is required for the development of experimental autoimmune encephalomyelitis because of its critical role in the production of the proinflammatory cytokines GM-CSF, IFN-γ, and IL-2. Thus, we identify DEC1 as a critical transcriptional mediator in the activation of naive CD4⁺ T cells that is required for the development of a T cell–mediated autoimmune disease.

Interleukin-7: Fuel for the autoimmune attack

Interleukin-7 (IL-7) is a critical survival factor for lymphocytes and recent studies suggest targeting the IL-7/IL-7Rα pathway holds promise for the treatment of autoimmune diseases. Several lines of evidence, genetic as well as functional, indicate an important role for this cytokine in autoimmune inflammation: polymorphisms in the IL-7Rα have been associated with increased risk for autoimmune disease and blocking IL-7/IL-7Rα with antibodies showed therapeutic efficacy in several autoimmune mouse models. Insights are starting to emerge about the mechanisms underlying IL-7's role in autoimmunity and tolerance, revealing surprising novel functions beyond its traditional activity as a T cell survival factor. In the first part of this review, the functions of IL-7 in the immune system are concisely described, providing a basis for understanding their potential role in promoting autoimmune responses. In the second part, current knowledge about the role of IL-7 in various autoimmune conditions is reviewed.

An emerging role for Serine Protease Inhibitors in T lymphocyte immunity and beyond

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmo-regulation, tissue re-modeling and immunity to infection. T lymphocytes are required for adaptive cell mediated immunity and serine proteases are not only important for effector function but also homeostatic regulation of cell numbers. Serine Protease Inhibitors (Serpins) are the physiological regulators of serine proteases activity. In this review, I will discuss the role of serpins in controlling the recognition of antigen, effector function and homeostatic control of T lymphocytes through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This will be discussed in the context of the T lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. The potent anti-apoptotic properties of serpins can also work against adaptive cell immunity by protecting viruses and tumors from eradication by cytotoxic T cells (CTL). Recent insights from knock-out mouse models demonstrate that these serpins also are required for hematological progenitor cells and so are critical for the development of lineages other than T lymphocytes. Given the emerging role of serpins in multiple aspects of lymphocyte immunity and blood development I will review the progress to date in developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Impairment of immune systems in diabetes

Type 1 diabetes mellitus (T1DM) is an autoimmune disease that involves the progressive destruction of the insulin-producing beta cells in the islets of langerhans. It is a complex process that results from the loss of tolerance to insulin and other beta-cell-specific antigens. Various genetic and environmental factors have been studied so far, but precise causation has yet to be established. Numerous studies in rodents and human subjects have been performed in order to elucidate the role of B and T cells, which determine the risk of development and progression of diabetes. These studies have demonstrated that while T1DM is fundamentally a T-cell-mediated autoimmune response, the development of this disease results from complex interactions between the adaptive and innate immune systems, with numerous cell types thought to contribute to pathogenesis. Like any complex disease, the variation in severity and incidence of T1DM can be attributed to a combination of genetic and environmental factors.

Defining a functionally distinct subset of human memory CD4+ T cells that are CD25POS and FOXP3NEG

Surface expression of the IL-2 receptor α-chain (CD25) has been used to discriminate between CD4(+) CD25(HI) FOXP3(+) regulatory T (Treg) cells and CD4(+) CD25(NEG) FOXP3(-) non-Treg cells. However, this study reports that the majority of resting human memory CD4(+) FOXP3(-) T cells expresses intermediate levels of CD25 and that CD25 expression can be used to delineate a functionally distinct memory subpopulation. The CD25(NEG) memory T-cell population contains the vast majority of late differentiated cells that respond to antigens associated with chronic immune responses and are increased in patients with systemic lupus erythematosus (SLE). In contrast, the CD25(INT) memory T cells respond to antigens associated with recall responses, produce a greater array of cytokines, and are less dependent on costimulation for effector responses due to their expression of CD25. Lastly, compared to the CD25(NEG) and Treg-cell populations, the CD25(INT) memory population is lost to a greater degree from the blood of cancer patients treated with IL-2. Collectively, these results show that in humans, a large proportion of CD4(+) memory T cells express intermediate levels of CD25, and this CD25(INT) FOXP3(-) subset is a functionally distinct memory population that is uniquely affected by IL-2.

In vitro derivation of interferon-γ producing, IL-4 and IL-7 responsive memory-like CD4(+) T cells

CD4(+) memory is critical for successful protection against pathogenic challenge. As such, understanding the heterogeneity of cells that arise and survive after initial stimulation of naïve CD4(+) T cells will aid in the design of more successful vaccines. In previous studies, in vivo experimental systems have been extensively used to generate functional memory responses by lymphocytes. Here, we have attempted to develop an in vitro experimental system to generate memory CD4(+) T lymphocytes. CD4(+) T cells stimulated through the antigen receptor complex were examined for their memory-like characteristics after 3 weeks of cell culture. A subset of surviving cells expressed high levels of CD44 and low levels of CD45RB (CD44(hi)CD45(lo)), a phenotype that is similar to bonafide memory CD4(+) T cells. In vitro generated memory-like CD4(+) T cells secreted higher levels of IFN-γ, with rapid kinetics, upon re-stimulation than their naïve counterparts. In addition, these memory-like CD4(+) T cells did not produce either IL-2 or IL-4 but readily proliferated when cultured in the presence of IL-7 and IL-4. These observations suggest that CD4(+) cells surviving the expansion phase of immune response produce a Th1-signature cytokine and retain responsiveness to IL-4, a Th-2 cytokine, as well as to a well described survival factor, interleukin-7.

Long-lasting immune responses 4 years after GAD-alum treatment in children with type 1 diabetes

A phase II clinical trial with glutamic acid decarboxylase (GAD) 65 formulated with aluminium hydroxide (GAD-alum) has shown efficacy in preserving residual insulin secretion in children and adolescents with recent-onset type 1 diabetes (T1D). We have performed a 4-year follow-up study of 59 of the original 70 patients to investigate long-term cellular and humoral immune responses after GAD-alum-treatment. Peripheral blood mononuclear cells (PBMC) were stimulated in vitro with GAD₆₅. Frequencies of naïve, central and effector memory CD4+ and CD8+ T cells were measured, together with cytokine secretion, proliferation, gene expression and serum GAD₆₅ autoantibody (GADA) levels. We here show that GAD-alum-treated patients display increased memory T-cell frequencies and prompt T-cell activation upon in vitro stimulation with GAD₆₅, but not with control antigens, compared with placebo subjects. GAD₆₅-induced T-cell activation was accompanied by secretion of T helper (Th) 1, Th2 and T regulatory cytokines and by induction of T-cell inhibitory pathways. Moreover, post-treatment serum GADA titres remained persistently increased in the GAD-alum arm, but did not inhibit GAD₆₅ enzymatic activity. In conclusion, memory T- and B-cell responses persist 4 years after GAD-alum-treatment. In parallel to a GAD₆₅-induced T-cell activation, our results show induction of T-cell inhibitory pathways important for regulating the GAD₆₅ immunity.

Anti-apoptotic genes in the survival of monocytic cells during infection

Macrophages are cells of the immune system that protect organisms against invading pathogens by fulfilling critical roles in innate and adaptive immunity and inflammation. They originate from circulating monocytes and show a high degree of heterogeneity, which reflects the specialization of function given by different anatomical locations. Differentiation of monocytes towards a macrophage phenotype is also accompanied by an increase of resistance against various apoptotic stimuli, a required characteristic that allows macrophages to accomplish their function in a stressful environment.

Apoptosis, a form of programmed cell death, is a tightly regulated process, needed to maintain homeostasis by balancing proliferation with cellular demise. Caspases, a family of cysteine proteases that are highly conserved in multicellular organisms, function as central regulators of apoptosis. FLIP (FLICE-inhibitory protein), anti-apoptotic members of the Bcl2 family and inhibitors of apoptosis (IAP) are the main three groups of anti-apoptotic genes that counteract caspase activation through both the extrinsic and intrinsic apoptotic pathways.

Modulation of the apoptotic machinery during viral and bacterial infections, as well as in various malignancies, is a wellestablished mechanism that promotes the survival of affected cells. The involvement of anti-apoptotic genes in the survival of monocytes/macrophages, either physiological or pathological, will be described in this review. How viral and bacterial infections that target cells of the monocytic lineage affect the expression of anti-apoptotic genes is important in understanding the pathological mechanisms that lead to manifested disease. The latest therapeutic approaches that target anti-apoptotic genes will also be discussed.

Protective effector memory CD4 T cells depend on ICOS for survival

Memory CD4 T cells play a vital role in protection against re-infection by pathogens as diverse as helminthes or influenza viruses. Inducible costimulator (ICOS) is highly expressed on memory CD4 T cells and has been shown to augment proliferation and survival of activated CD4 T cells. However, the role of ICOS costimulation on the development and maintenance of memory CD4 T cells remains controversial. Herein, we describe a significant defect in the number of effector memory (EM) phenotype cells in ICOS^−/− and ICOSL^−/− mice that becomes progressively more dramatic as the mice age. This decrease was not due to a defect in the homeostatic proliferation of EM phenotype CD4 T cells in ICOS^−/− or ICOSL^−/− mice. To determine whether ICOS regulated the development or survival of EM CD4 T cells, we utilized an adoptive transfer model. We found no defect in development of EM CD4 T cells, but long-term survival of ICOS^−/− EM CD4 T cells was significantly compromised compared to wild-type cells. The defect in survival was specific to EM cells as the central memory (CM) ICOS^−/− CD4 T cells persisted as well as wild type cells. To determine the physiological consequences of a specific defect in EM CD4 T cells, wild-type and ICOS^−/− mice were infected with influenza virus. ICOS^−/− mice developed significantly fewer influenza-specific EM CD4 T cells and were more susceptible to re-infection than wild-type mice. Collectively, our findings demonstrate a role for ICOS costimulation in the maintenance of EM but not CM CD4 T cells.

Co-administration of IL-1+IL-6+TNF-α with Mycobacterium tuberculosis infected macrophages vaccine induces better protective T cell memory than BCG

BCG has been administered globally for more than 75 years, yet tuberculosis (TB) continues to kill more than 2 million people annually. Further, BCG protects childhood TB but is quite inefficient in adults. This indicates that BCG fails to induce long-term protection. Hence there is a need to explore alternative vaccination strategies that can stimulate enduring T cell memory response. Dendritic cell based vaccination has attained extensive popularity following their success in various malignancies. In our previous study, we have established a novel and unique vaccination strategy against Mycobacterium tuberculosis (M. tb) and Salmonella typhimurium by utilizing infected macrophages (IM). In short-term experiments (30 days), substantial degree of protection was observed. However, remarkable difference was not observed in long-term studies (240 days) due to failure of the vaccine to generate long-lasting memory T cells. Hence, in the present study we employed T cell memory augmenting cytokines IL-1+IL-6+TNF-α and IL-7+IL-15 for the induction of the enhancement of long-term protection by the vaccine. We co-administered the M. tb infected macrophages vaccine with IL-1+IL-6+TNF-α (IM-1.6.α) and IL-7+IL-15 (IM-7.15). The mice were then rested for a reasonably large period (240 days) to study the bona fide T cell memory response before exposing them to aerosolized M. tb. IM-1.6.α but not IM-7.15 significantly improved memory T cell response against M. tb, as evidenced by recall responses of memory T cells, expansion of both central as well as effector memory CD4 and CD8 T cell pools, elicitation of mainly Th1 memory response, reduction in the mycobacterial load and alleviated lung pathology. Importantly, the protection induced by IM-1.6.α was significantly better than BCG. Thus, this study demonstrates that not only antigen-pulsed DCs can be successfully employed as vaccines against cancer and infectious diseases but also macrophages infected with M. tb can be utilized with great efficacy especially in protection against TB.

CD28 and IL-4: two heavyweights controlling the balance between immunity and inflammation

The costimulatory receptor CD28 and IL-4Rα-containing cytokine receptors play key roles in controlling the size and quality of pathogen-specific immune responses. Thus, CD28-mediated costimulation is needed for effective primary T-cell expansion and for the generation and activation of regulatory T-cells (Treg cells), which protect from immunopathology. Similarly, IL-4Rα signals are required for alternative activation of macrophages, which counteract inflammation by type 1 responses. Furthermore, immune modulation by CD28 and IL-4 is interconnected through the promotion of IL-4 producing T-helper 2 cells by CD28 signals. Using conditionally IL-4Rα and CD28 deleting mice, as well as monoclonal antibodies, which block or stimulate CD28, or mAb that deplete Treg cells, we have studied the roles of CD28 and IL-4Rα in experimental mouse models of virus (influenza), intracellular bacteria (L. monocytogenes, M. tuberculosis), and parasite infections (T. congolense, L. major). We observed that in some, but not all settings, Treg cells and type 2 immune deviation, including activation of alternative macrophages can be manipulated to protect the host either from infection or from immunopathology with an overall beneficial outcome. Furthermore, we provide direct evidence that secondary CD8 T-cell responses to i.c. bacteria are dependent on CD28-mediated costimulation.

Regulation of CD4+ T-cell contraction during pathogen challenge

Signals orchestrating productive CD4+ T-cell responses are well documented; however, the regulation of contraction of CD4+ T-cell effector populations following the resolution of primary immune responses is not well understood. While distinct mechanisms of T-cell death have been defined, the relative importance of discrete death pathways during the termination of immune responses in vivo remains unclear. Here, we review the current understanding of cell-intrinsic and -extrinsic variables that regulate contraction of CD4+ T-cell effector populations through multiple pathways that operate both initially during T-cell priming and later during the effector phase. We discuss the relative importance of antigen-dependent and -independent mechanisms of CD4+ T-cell contraction during in vivo responses, with a special emphasis on influenza virus infection. In this model, we highlight the roles of greater differentiation and presence in the lung of CD4+ effector T cells, as well as their polarization to particular T-helper subsets, in maximizing contraction. We also discuss the role of autocrine interleukin-2 in limiting the extent of contraction, and we point out that these same factors regulate contraction during secondary CD4+ T-cell responses.

The potential of CD4 T-cell memory

While many aspects of memory T-cell immunobiology have been characterized, we suggest that we know only a fraction of the effector functions that CD4 T cells can bring to bear during secondary challenges. Exploring the full impact of memory CD4 T-cell responses is key to the development of improved vaccines against many prominent pathogens, including influenza viruses, and also to a better understanding of the mechanisms of autoimmunity. Here we discuss factors regulating the generation of memory CD4 T cells during the activation of naïve cells and how the nature of the transition from highly activated effector to resting memory upon the resolution of primary responses might impact memory CD4 T-cell heterogeneity in vivo. We stress that memory CD4 T cells have unique functional attributes beyond the secretion of T helper (Th) subset-associated cytokines that can shape highly effective secondary responses through novel mechanisms. These include the recruitment of innate inflammatory responses at early phases of secondary responses as well as the action of enhanced direct effector functions at later phases, in addition to well-established helper roles for CD8 T-cell and B-cell responses.

IL-7 is superior to IL-2 for ex vivo expansion of tumour-specific CD4(+) T cells

It is well established that tumours hinder both natural and vaccine-induced tumour-specific CD4(+) T-cell responses. Adoptive T-cell therapy has the potential to circumvent functional tolerance and enhance anti-tumour protective responses. While protocols suitable for the expansion of cytotoxic CD8(+) T cells are currently available, data on tumour-specific CD4(+) T cells remain scarce. We report here that CD4(+) T cells sensitized to tumour-associated Ag in vivo, proliferate in vitro in response to IL-7 without the need for exogenous Ag stimulation and accumulate several folds while preserving a memory-like phenotype. Both cell proliferation and survival accounts for the outgrowth of tumour-sensitized T cells among other memory and naive lymphocytes following exposure to IL-7. Also IL-2, previously used to expand anti-tumour CTL, promotes tumour-specific CD4(+) T-cell accumulation. However, IL-7 is superior to IL-2 at preserving lymphocyte viability, in vitro and in vivo, maintaining those properties, that are required by helper CD4(+) T cells to confer therapeutic efficacy upon transplantation in tumour-bearing hosts. Together our data support a unique role for IL-7 in retrieving memory-like CD4(+) T cells suitable for adoptive T-cell therapy.

Cell cycle progression following naive T cell activation is independent of Jak3/common gamma-chain cytokine signals

T cell proliferation following activation is an essential aspect of the adaptive immune response. Multiple factors, such as TCR signaling, costimulation, and signals from cytokines, each contribute to determine the magnitude of T cell expansion. In this report, we examine in detail the role of Jak3/common gamma-chain-dependent cytokines in promoting cell cycle progression and proliferation of naive T cells. Using naive CD4+ T cells from Jak3-deficient mice and wild-type CD4+ T cells treated with a small molecule inhibitor of Jak3, we find that these cytokine signals are not required for proliferation; instead, they are important for the survival of activated T cells. In addition, we show that the percentage of cells entering the cell cycle and the percentage of cells in each round of cell division are comparable between Jak3-deficent and wild-type T cells. Furthermore, cell cycle progression and the regulated expression of key cell cycle proteins are independent of Jak3/common gamma-chain cytokine signals. These findings hold true over a wide range of TCR signal strengths. However, when CD28 costimulatory signals, but not TCR signals, are limiting, Jak3-dependent cytokine signals become necessary for the proliferation of naive T cells. Because CD28 signaling has been found to be dispensable for autoreactive T cell responses, these data suggest the potential for interfering with autoimmune T cell responses by inhibition of Jak3 signaling.

Improved outcome with immunosuppressive monotherapy after renal transplantation in Schimke-immuno-osseous dysplasia

SIOD is a multisystem disorder caused by a mutant chromatin remodelling protein. The main clinical findings are spondyloepiphyseal dysplasia with disproportionate growth restriction, defective cellular immunity, and steroid-resistant nephrotic syndrome secondary to biopsy proven FSGS leading to ESRF. Concerning ESRF, kidney transplantation is the therapy of choice since FSGS does not recur in the graft. However, with respect to the underlying immune disorder and the increased susceptibility to life threatening infections, the question of the optimal immunosuppressive therapy after renal transplantation remains unresolved. Under conventional immunosuppressive regimens some SIOD patients have developed severe disseminated cutaneous papilloma virus infections or EBV associated lymphoproliferative disease. We present several cases of children with SIOD (four of five had SMARCAL1 mutations) and monotherapy maintenance immunosuppression after renal transplantation and compare them with 13 patients from the SIOD registry. We have found that post-renal transplantation immunosuppressive monotherapy results in a good outcome with a reduced number of severe infections. Due to the underlying immunodeficiency in SIOD, limited immunosuppression may be possible without increasing the risk of acute or chronic rejection.

NK cell enhancement of antigen presentation by B lymphocytes

Ag presentation to CD4 T cells can be mediated by a number of cell types depending on the anatomical site in which Ag is first encountered. For blood borne Ags, cells localized in situ in the spleen should be major players. There is now much evidence that B cell Ag presentation may be particularly important in the priming of memory T cells. The majority of NK cells are also localized the spleen. Inasmuch as we have previously shown that NK cells can modulate various aspects of B cell differentiation, we entertained the possibility that NK cells can also influence Ag presentation by B cells. By specific depletion of NK cells before immunization, we show herein that NK cells play an important role in modulating the ability of B cells to process and present Ag to T cells. These effects are particularly important in the generation of memory T cells. The findings are further substantiated by in vitro experiments showing that the enhancement does not require IFN-gamma but is mediated by direct cell-cell interaction. These results show, for the first time, that the rapid activation of a component of the innate response can even exert effects on the Ag-specific memory response.

Colitis immunoregulation by CD8+ T cell requires T cell cytotoxicity and B cell peptide antigen presentation

Deficient immunoregulation by CD4+ T cells is an important susceptibility trait for inflammatory bowel disease, but the role of other regulatory cell types is less understood. This study addresses the role and mechanistic interaction of B cells and CD8+ T cells in controlling immune-mediated colitis. The genetic requirements for B cells and CD8+ T cells to confer protective immunoregulation were assessed by cotransfer with colitogenic Galphai2-/- T cells into immune-deficient mice. Disease activity in Galphai2-/- T cell recipients was evaluated by CD4+ T intestinal lymphocyte abundance, cytokine production levels, and large intestine histology. B cells deficient in B7.1/B7.2, CD40, major histocompatibility complex (MHC) II (Abb), or native B cell antigen receptor (MD4) were competent for colitis protection. However, transporter-1-deficient B cells failed to protect, indicating a requirement for peptide MHC I presentation to CD8+ T cells. CD8+ T cells deficient in native T cell receptor repertoire (OT-1) or cytolysis (perforin-/-) also were nonprotective. These finding reveal an integrated role for antigen-specific perforin-dependent CD8+ T cell cytotoxicity in colitis immunoregulatory and its efficient induction by a subset of mesenteric B lymphocytes.

Semaphorin 6D regulates the late phase of CD4+ T cell primary immune responses

The semaphorin and plexin family of ligand and receptor proteins provides important axon guidance cues required for development. Recent studies have expanded the role of semaphorins and plexins in the regulation of cardiac, circulatory and immune system function. Within the immune system, semaphorins and plexins regulate cell-cell interactions through a complex network of receptor and ligand pairs. Immune cells at different stages of development often express multiple semaphorins and plexins, leading to multivariate interactions, involving more than one ligand and receptor within each functional group. Because of this complexity, the significance of semaphorin and plexin regulation on individual immune cell types has yet to be fully appreciated. In this work, we examined the regulation of T cells by semaphorin 6D. Both in vitro and in vivo T cell stimulation enhanced semaphorin 6D expression. However, semaphorin 6D was only expressed by a majority of T cells during the late phases of activation. Consequently, the targeted disruption of semaphorin 6D receptor-ligand interactions inhibited T cell proliferation at late but not early phases of activation. This proliferation defect was associated with reduced linker of activated T cells protein phosphorylation, which may reflect semaphorin 6D regulation of c-Abl kinase activity. Semaphorin 6D disruption also inhibited expression of CD127, which is required during the multiphase antigen-presenting cell and T cell interactions leading to selection of long-lived lymphocytes. This work reveals a role for semaphorin 6D as a regulator of the late phase of primary immune responses.

In vivo studies fail to reveal a role for IL-4 or STAT6 signaling in Th2 lymphocyte differentiation

The expression of interleukin-4 (IL-4) is viewed as the hallmark of a Th2 lymphocyte, whereas the subsequent action of IL-4 and IL-13, mediated through the STAT6 signaling pathway, is seen as a prerequisite for the full development of Th2 immune responses to parasites and allergens. G4 mice, whose IL-4 gene locus contains the fluorescent reporter eGFP, were used to quantify the number of Th2 cells that develop during Nippostrongylus brasiliensis- or allergen-induced immune responses under conditions where IL-4 or STAT6 was absent. Here, we show that deletion of IL-4 or STAT6 had little impact on the number or timing of appearance of IL-4-producing Th2 cells. These data indicate that in vivo differentiation of naïve CD4 T cells to Th2 status often occurs independently of IL-4 and STAT6 and that recently described pathways of Th2 cell differentiation may explain how allergens and parasites selectively induce Th2-mediated immunity.

Human CD25highFoxp3pos regulatory T cells differentiate into IL-17-producing cells

The effector T-cell lineage shows great plasticity. Th17 cells are acknowledged to be instrumental in the response against microbial infection, but are also associated with autoimmune inflammatory processes. Here, we report that human regulatory T cells (CD4(pos)CD25(high)Foxp3(pos)CD127(neg)CD27(pos)) can differentiate into IL-17-producing cells, when stimulated by allogeneic antigen-presenting cells, especially monocytes, in the presence of rhIL-2/rhIL-15. These regulatory T cell (Treg)-derived IL-17-producing cells showed high expression of the Th17-related transcription factor RORgammat and were positively identified by CCR6 expression. This differentiation process was enhanced by exogenous IL-1beta, IL-23, and IL-21, whereas IL-6 or TGFbeta did not affect the emergence of IL-17-producing cells. The addition of IL-1 receptor antagonist (IL-1Ra), but not anti-IL-23 antibody, reduced IL-17-producing cell numbers. When an histone deacetylase (HDAC) inhibitor trichostatin A (TSA) was evaluated, we found a profound negative effect on the emergence of IL-17-producing cells from Tregs, implying that Treg differentiation into IL-17-producing cells depends on histone/protein deacetylase activity. Thus, the data suggest that epigenetic modification underlies the phenomenon of Treg plasticity here described.

Immunosuppression in islet transplantation

Islet transplantation can temporarily cure type 1 diabetes mellitus (T1DM) but requires simultaneous immunosuppression to avoid allograft rejection. In this issue of the JCI, Monti et al. report that immune conditioning via use of the Edmonton protocol - a treatment approach in which T1DM patients infused with pancreatic islets from multiple cadaveric donors simultaneously receive immunosuppressive drugs - results in lymphopenia that is associated with elevated serum levels of the homeostatic cytokines IL-7 and IL-15, which causes in vivo expansion of the autoreactive CD8(+) T cell population (see the related article beginning on page 1806). Reemergence of autoreactivity is likely the main culprit underlying long-term islet graft failure, and new strategies will need to be tested to circumvent this homeostatic expansion and recurrent autoreactivity.

The effector to memory transition of CD4 T cells

The small number of antigen-specific memory CD4 T cells surviving long-term after antigen or pathogen challenge are often characterized by a surprising degree of phenotypic and functional heterogeneity. We here propose that the immune system has evolved to express this diversity in memory T-cell populations, in order to provide flexibility in recall responses, via a rapid transition from heterogeneous effector cells into correspondingly heterogeneous memory cells. Little attention has been paid to another important transition-from resting memory cell to re-activated effector. We would suggest that superior functional attributes of secondary effectors arising from memory CD4 T cells, as compared to primary effectors arising from naïve precursors, play an important and underappreciated role in protective secondary immune responses.