A Biochemical Signature for Rapid Recall of Memory CD4 T Cells

Bidirectional Communication Between the Innate and Adaptive Immune Systems

Effective bidirectional communication between the innate and adaptive immune systems is crucial for tissue homeostasis and protective immunity against infections. The innate immune system is responsible for the early sensing of and initial response to threats, including microbial ligands, toxins, and tissue damage. Pathogen-related information, detected primarily by the innate immune system via dendritic cells, is relayed to adaptive immune cells, leading to the priming and differentiation of naive T cells into effector and memory lineages. Memory T cells that persist long after pathogen clearance are integral for durable protective immunity. In addition to rapidly responding to reinfections, memory T cells also directly instruct the interacting myeloid cells to induce innate inflammation, which resembles microbial inflammation. As such, memory T cells act as newly emerging activators of the innate immune system and function independently of direct microbial recognition. While T cell-mediated activation of the innate immune system likely evolved as a protective mechanism to combat reinfections by virulent pathogens, the detrimental outcomes of this mechanism manifest in the forms of autoimmunity and other T cell-driven pathologies. Here, we review the complexities and layers of regulation at the interface between the innate and adaptive immune systems to highlight the implications of adaptive instruction of innate immunity in health and disease.

T helper (Th) cell profiles and cytokines/chemokines in characterization, treatment, and monitoring of autoimmune diseases

Autoimmune diseases (AD) consist of a spectrum of disease entities whose etiologies are very complex and still not well understood. Every individual has the potential for developing AD under appropriate conditions because the body contains lymphocytes that are potentially reactive with self-antigens. The aims of this study are to (1) explore the flow cytometry method to identify the frequency of various circulating CD4+ T helper (Th) cell-subsets, including Th1, Th2, Th9, Th17, Th17.1, and Th22; (2) In parallel, to examine multiplex ELISA method for pathogenic inflammatory cytokines/chemokines, and (3) To assess the correlation of expression of T cell-subsets with serum cytokines/chemokines and understand its clinical importance with available AD treatments. We analyzed Th17, Th17.1, Th22, Th2, Th1, and Th9 Th cell populations and compared the concentrations of 67 cytokines/chemokines in healthy as well as AD-diagnosed patients. We observed that patients with autoimmune markers had significantly elevated percentages of naïve (Th17, Th22, and Th9) as well as memory (Th17 and Th22) Th cell-subsets, along with increased concentrations of cytokines/chemokines (Eotaxin, TNFβ, and FABP4). The percentage of Th cell-subsets correlated positively or negatively with the production of cytokines/chemokines of patients diagnosed with AD. Our study demonstrates that the naïve and memory Th cell-subsets with positive correlations to cytokines/chemokines show new diagnostic markers to predict the patients' outcome, while the negative correlation of cytokines/chemokines shows the response to autoimmune therapies. Our findings of Th cell-subsets by flow cytometry and cytokines/chemokines by multiplex ELISA suggest that CCR6+ Th cell-subsets (Th17, Th17.1, Th22, and Th9) contribute to our understanding of the pathogenesis of AD and identify the new onset of AD from the autoimmune spectrum. Our findings highlight the importance of CCR6+ as a possible marker in the characterization, treatment, and monitoring of AD.

Innate control of adaptive immunity and adaptive instruction of innate immunity: bi-directional flow of information

The ability of the innate and adaptive immune systems to communicate with each other is central to protective immune responses and maintenance of host health. Myeloid cells of the innate immune system are able to sense microbial ligands, perturbations in cellular homeostasis, and virulence factors, thereby allowing them to relay distinct pathogen-specific information to naïve T cells in the form of pathogen-derived peptides and a unique cytokine milieu. Once primed, effector T helper cells produce lineage-defining cytokines to help combat the original pathogen, and a subset of these cells persist as memory or effector-memory populations. These memory T cells then play a dual role in host protection by not only responding rapidly to reinfection, but by also directly instructing myeloid cells to express licensing cytokines. This means there is a bi-directional flow of information first from the innate to the adaptive immune system, and then from the adaptive back to innate immune system. Here, we focus on how signals, first from pathogens and then from primed effector and memory T cells, are integrated by myeloid cells and its consequences for protective immunity or systemic inflammation.

Multifunctional cytokine production reveals functional superiority of memory CD4 T cells

T cell protective immunity is associated with multifunctional memory cells that produce several different cytokines. Currently, our understanding of when and how these cells are generated is limited. We have used an influenza virus mouse infection model to investigate whether the cytokine profile of memory T cells is reflective of primary responding cells or skewed toward a distinct profile. We found that, in comparison to primary cells, memory T cells tended to make multiple cytokines simultaneously. Analysis of the timings of release of cytokine by influenza virus‐specific T cells, demonstrated that primary responding CD4 T cells from lymphoid organs were unable to produce a sustained cytokine response. In contrast CD8 T cells, memory CD4 T cells, and primary responding CD4 T cells from the lung produced a sustained cytokine response throughout the restimulation period. Moreover, memory CD4 T cells were more resistant than primary responding CD4 T cells to inhibitors that suppress T cell receptor signaling. Together, these data suggest that memory CD4 T cells display superior cytokine responses compared to primary responding cells. These data are key to our ability to identify the cues that drive the generation of protective memory CD4 T cells following infection.

miR-34a: a new player in the regulation of T cell function by modulation of NF-κB signaling

NF-κB functions as modulator of T cell receptor-mediated signaling and transcriptional regulator of miR-34a. Our in silico analysis revealed that miR-34a impacts the NF-κB signalosome with miR-34a binding sites in 14 key members of the NF-κB signaling pathway. Functional analysis identified five target genes of miR-34a including PLCG1, CD3E, PIK3CB, TAB2, and NFΚBIA. Overexpression of miR-34a in CD4⁺ and CD8⁺ T cells led to a significant decrease of NFΚBIA as the most downstream cytoplasmic NF-κB member, a reduced cell surface abundance of TCRA and CD3E, and to a reduction of T cell killing capacity. Inhibition of miR-34a caused an increase of NFΚBIA, TCRA, and CD3E. Notably, activation of CD4⁺ and CD8⁺ T cells entrails a gradual increase of miR-34a. Our results lend further support to a model with miR-34a as a central NF-κB regulator in T cells.

Histone acetyltransferase CBP is critical for conventional effector and memory T-cell differentiation in mice

Compared with naïve T cells, memory CD8⁺ T cells have a transcriptional landscape and proteome that are optimized to generate a more rapid and robust response to secondary infection. Additionally, rewired kinase signal transduction pathways likely contribute to the superior recall response of memory CD8⁺ T cells, but this idea has not been experimentally confirmed. Herein, we utilized an MS approach to identify proteins that are phosphorylated on tyrosine residues in response to Listeria-induced T-cell receptor (TCR) stimulation in both naïve and memory CD8⁺ T cells from mice and separated by fluorescence- and flow cytometry-based cell sorting. This analysis identified substantial differences in tyrosine kinase signaling networks between naïve and memory CD8⁺ T cells. We also observed that an important axis in memory CD8⁺ T cells couples Janus kinase 2 (JAK2) hyperactivation to the phosphorylation of CREB-binding protein (CBP). Functionally, JAK2-catalyzed phosphorylation enabled CBP to bind with higher affinity to acetylated histone peptides, indicating a potential epigenetic mechanism that could contribute to rapid initiation of transcriptional programs in memory CD8⁺ T cells. Moreover, we found that CBP itself is essential for conventional effector and memory CD8⁺ T-cell formation. These results indicate how signaling pathways are altered to promote CD8⁺ memory cell formation and rapid responses to and protection from repeat infections.

Th1-like Plasmodium-Specific Memory CD4+ T Cells Support Humoral Immunity

Effector T cells exhibiting features of either T helper 1 (Th1) or T follicular helper (Tfh) populations are essential to control experimental Plasmodium infection and are believed to be critical for resistance to clinical malaria. To determine whether Plasmodium-specific Th1- and Tfh-like effector cells generate memory populations that contribute to protection, we developed transgenic parasites that enable high-resolution study of anti-malarial memory CD4 T cells in experimental models. We found that populations of both Th1- and Tfh-like Plasmodium-specific memory CD4 T cells persist. Unexpectedly, Th1-like memory cells exhibit phenotypic and functional features of Tfh cells during recall and provide potent B cell help and protection following transfer, characteristics that are enhanced following ligation of the T cell co-stimulatory receptor OX40. Our findings delineate critical functional attributes of Plasmodium-specific memory CD4 T cells and identify a host-specific factor that can be targeted to improve resolution of acute malaria and provide durable, long-term protection against Plasmodium parasite re-exposure.

Priming of transcriptional memory responses via the chromatin accessibility landscape in T cells

Memory T cells exhibit transcriptional memory and “remember” their previous pathogenic encounter to increase transcription on re-infection. However, how this transcriptional priming response is regulated is unknown. Here we performed global FAIRE-seq profiling of chromatin accessibility in a human T cell transcriptional memory model. Primary activation induced persistent accessibility changes, and secondary activation induced secondary-specific opening of previously less accessible regions associated with enhanced expression of memory-responsive genes. Increased accessibility occurred largely in distal regulatory regions and was associated with increased histone acetylation and relative H3.3 deposition. The enhanced re-stimulation response was linked to the strength of initial PKC-induced signalling, and PKC-sensitive increases in accessibility upon initial stimulation showed higher accessibility on re-stimulation. While accessibility maintenance was associated with ETS-1, accessibility at re-stimulation-specific regions was linked to NFAT, especially in combination with ETS-1, EGR, GATA, NFκB, and NR4A. Furthermore, NFATC1 was directly regulated by ETS-1 at an enhancer region. In contrast to the factors that increased accessibility, signalling from bHLH and ZEB family members enhanced decreased accessibility upon re-stimulation. Interplay between distal regulatory elements, accessibility, and the combined action of sequence-specific transcription factors allows transcriptional memory-responsive genes to “remember” their initial environmental encounter.

Gastrointestinal Microbiome Dysbiosis in Infant Mice Alters Peripheral CD8+ T Cell Receptor Signaling

We recently reported that maternal antibiotic treatment (MAT) of mice in the last days of pregnancy and during lactation dramatically alters the density and composition of the gastrointestinal microbiota of their infants. MAT infants also exhibited enhanced susceptibility to a systemic viral infection and altered adaptive immune cell activation phenotype and function. CD8⁺ effector T cells from MAT infants consistently demonstrate an inability to sustain interferon gamma (IFN-γ) production in vivo following vaccinia virus infection and in vitro upon T cell receptor (TCR) stimulation. We hypothesize that T cells developing in infant mice with gastrointestinal microbiota dysbiosis and insufficient toll-like receptor (TLR) exposure alters immune responsiveness associated with intrinsic T cell defects in the TCR signaling pathway and compromised T cell effector function. To evaluate this, splenic T cells from day of life 15 MAT infant mice were stimulated in vitro with anti-CD3 and anti-CD28 antibodies prior to examining the expression of ZAP-70, phosphorylated ZAP-70, phospho-Erk-1/2, c-Rel, total protein tyrosine phosphorylation, and IFN-γ production. We determine that MAT infant CD8⁺ T cells fail to sustain total protein tyrosine phosphorylation and Erk1/2 activation. Lipopolysaccharide treatment in vitro and in vivo, partially restored IFN-γ production in MAT effector CD8⁺ T cells and reduced mortality typically observed in MAT mice following systemic viral infection. Our results demonstrate a surprising dependence on the gastrointestinal microbiome and TLR ligand stimulation toward shaping optimal CD8⁺ T cell function during infancy.

T Cell Receptor and Cytokine Signaling Can Function at Different Stages to Establish and Maintain Transcriptional Memory and Enable T Helper Cell Differentiation

Experienced T cells exhibit immunological memory via a rapid recall response, responding to restimulation much faster than naïve T cells. The formation of immunological memory starts during an initial slow response, when naïve T cells become transformed to proliferating T blast cells, and inducible immune response genes are reprogrammed as active chromatin domains. We demonstrated that these active domains are supported by thousands of priming elements which cooperate with inducible transcriptional enhancers to enable efficient responses to stimuli. At the conclusion of this response, a small proportion of these cells return to the quiescent state as long-term memory T cells. We proposed that priming elements can be established in a hit-and-run process dependent on the inducible factor AP-1, but then maintained by the constitutive factors RUNX1 and ETS-1. This priming mechanism may also function to render genes receptive to additional differentiation-inducing factors such as GATA3 and TBX21 that are encountered under polarizing conditions. The proliferation of recently activated T cells and the maintenance of immunological memory in quiescent memory T cells are also dependent on various cytokine signaling pathways upstream of AP-1. We suggest that immunological memory is established by T cell receptor signaling, but maintained by cytokine signaling.

Chromatin priming of genes in development: Concepts, mechanisms and consequences

During ontogeny, cells progress through multiple alternate differentiation states by activating distinct gene regulatory networks. In this review, we highlight the important role of chromatin priming in facilitating gene activation during lineage specification and in maintaining an epigenetic memory of previous gene activation. We show that chromatin priming is part of a hugely diverse repertoire of regulatory mechanisms that genes use to ensure that they are expressed at the correct time, in the correct cell type, and at the correct level, but also that they react to signals. We also emphasize how increasing our knowledge of these principles could inform our understanding of developmental failure and disease.

Chromatin priming elements establish immunological memory in T cells without activating transcription: T cell memory is maintained by DNA elements which stably prime inducible genes without activating steady state transcription

We have identified a simple epigenetic mechanism underlying the establishment and maintenance of immunological memory in T cells. By studying the transcriptional regulation of inducible genes we found that a single cycle of activation of inducible factors is sufficient to initiate stable binding of pre-existing transcription factors to thousands of newly activated distal regulatory elements within inducible genes. These events lead to the creation of islands of active chromatin encompassing nearby enhancers, thereby supporting the accelerated activation of inducible genes, without changing steady state levels of transcription in memory T cells. These studies also highlighted the need for more sophisticated definitions of gene regulatory elements. The chromatin priming elements defined here are distinct from classical enhancers because they function by maintaining chromatin accessibility rather than directly activating transcription. We propose that these priming elements are members of a wider class of genomic elements that support correct developmentally regulated gene expression.

A multi-omic analysis of human naïve CD4+ T cells

Background

Cellular function and diversity are orchestrated by complex interactions of fundamental biomolecules including DNA, RNA and proteins. Technological advances in genomics, epigenomics, transcriptomics and proteomics have enabled massively parallel and unbiased measurements. Such high-throughput technologies have been extensively used to carry out broad, unbiased studies, particularly in the context of human diseases. Nevertheless, a unified analysis of the genome, epigenome, transcriptome and proteome of a single human cell type to obtain a coherent view of the complex interplay between various biomolecules has not yet been undertaken. Here, we report the first multi-omic analysis of human primary naïve CD4+ T cells isolated from a single individual.

Results

Integrating multi-omics datasets allowed us to investigate genome-wide methylation and its effect on mRNA/protein expression patterns, extent of RNA editing under normal physiological conditions and allele specific expression in naïve CD4+ T cells. In addition, we carried out a multi-omic comparative analysis of naïve with primary resting memory CD4+ T cells to identify molecular changes underlying T cell differentiation. This analysis provided mechanistic insights into how several molecules involved in T cell receptor signaling are regulated at the DNA, RNA and protein levels. Phosphoproteomics revealed downstream signaling events that regulate these two cellular states. Availability of multi-omics data from an identical genetic background also allowed us to employ novel proteogenomics approaches to identify individual-specific variants and putative novel protein coding regions in the human genome.

Conclusions

We utilized multiple high-throughput technologies to derive a comprehensive profile of two primary human cell types, naïve CD4+ T cells and memory CD4+ T cells, from a single donor. Through vertical as well as horizontal integration of whole genome sequencing, methylation arrays, RNA-Seq, miRNA-Seq, proteomics, and phosphoproteomics, we derived an integrated and comparative map of these two closely related immune cells and identified potential molecular effectors of immune cell differentiation following antigen encounter.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0225-4) contains supplementary material, which is available to authorized users.

Development and Function of Protective and Pathologic Memory CD4 T Cells

Immunological memory is one of the defining features of the adaptive immune system. As key orchestrators and mediators of immunity, CD4 T cells are central to the vast majority of adaptive immune responses. Generated following an immune response, memory CD4 T cells retain pertinent information about their activation environment enabling them to make rapid effector responses upon reactivation. These responses can either benefit the host by hastening the control of pathogens or cause damaging immunopathology. Here, we will discuss the diversity of the memory CD4 T cell pool, the signals that influence the transition of activated T cells into that pool, and highlight how activation requirements differ between naïve and memory CD4 T cells. A greater understanding of these factors has the potential to aid the design of more effective vaccines and to improve regulation of pathologic CD4 T cells, such as in the context of autoimmunity and allergy.

Multi-layered epigenetic mechanisms contribute to transcriptional memory in T lymphocytes

Background

Immunological memory is the ability of the immune system to respond more rapidly and effectively to previously encountered pathogens, a key feature of adaptive immunity. The capacity of memory T cells to “remember” previous cellular responses to specific antigens ultimately resides in their unique patterns of gene expression. Following re-exposure to an antigen, previously activated genes are transcribed more rapidly and robustly in memory T cells compared to their naïve counterparts. The ability for cells to remember past transcriptional responses is termed “adaptive transcriptional memory”.

Results

Recent global epigenome studies suggest that epigenetic mechanisms are central to establishing and maintaining transcriptional memory, with elegant studies in model organisms providing tantalizing insights into the epigenetic programs that contribute to adaptive immunity. These epigenetic mechanisms are diverse, and include not only classical acetylation and methylation events, but also exciting and less well-known mechanisms involving histone structure, upstream signalling pathways, and nuclear localisation of genomic regions.

Conclusions

Current global health challenges in areas such as tuberculosis and influenza demand not only more effective and safer vaccines, but also vaccines for a wider range of health priorities, including HIV, cancer, and emerging pathogens such as Ebola. Understanding the multi-layered epigenetic mechanisms that underpin the rapid recall responses of memory T cells following reactivation is a critical component of this development pathway.

Human memory, but not naive, CD4+ T cells expressing transcription factor T-bet might drive rapid cytokine production

We found that after stimulation for a few hours, memory but not naive CD4(+) T cells produced a large amount of IFN-γ; however, the mechanism of rapid response of memory CD4(+) T cells remains undefined. We compared the expression of transcription factors in resting or activated naive and memory CD4(+) T cells and found that T-bet, but not pSTAT-1 or pSTAT-4, was highly expressed in resting memory CD4(+) T cells and that phenotypic characteristics of T-bet(+)CD4(+) T cells were CD45RA(low)CD62L(low) CCR7(low). After short-term stimulation, purified memory CD4(+) T cells rapidly produced effector cytokines that were closely associated with the pre-existence of T-bet. By contrast, resting naive CD4(+) T cells did not express T-bet, and they produced cytokines only after sustained stimulation. Our further studies indicated that T-bet was expressed in the nuclei of resting memory CD4(+) T cells, which might have important implications for rapid IFN-γ production. Our results indicate that the pre-existence and nuclear mobilization of T-bet in resting memory CD4(+) T cells might be a possible transcriptional mechanism for rapid production of cytokines by human memory CD4(+) T cells.

Dynamic functional modulation of CD4+ T cell recall responses is dependent on the inflammatory environment of the secondary stimulus

The parameters that modulate the functional capacity of secondary Th1 effector cells are poorly understood. In this study, we employ a serial adoptive transfer model system to show that the functional differentiation and secondary memory potential of secondary CD4+ effector T cells are dependent on the inflammatory environment of the secondary challenge. Adoptive transfer of TCR transgenic lymphocytic choriomeningitis virus (LCMV) Glycoprotein-specific SMARTA memory cells into LCMV-immune hosts, followed by secondary challenge with Listeria monocytogenes recombinantly expressing a portion of the LCMV Glycoprotein (Lm-gp61), resulted in the rapid emergence of SMARTA secondary effector cells with heightened functional avidity (as measured by their ability to make IFNγ in response to ex vivo restimulation with decreasing concentrations of peptide), limited contraction after pathogen clearance and stable maintenance secondary memory T cell populations. In contrast, transfer of SMARTA memory cells into naïve hosts prior to secondary Lm-gp61 challenge, which resulted in a more extended infectious period, resulted in poor functional avidity, increased death during the contraction phase and poor maintenance of secondary memory T cell populations. The modulation of functional avidity during the secondary Th1 response was independent of differences in antigen load or persistence. Instead, the inflammatory environment strongly influenced the function of the secondary Th1 response, as inhibition of IL-12 or IFN-I activity respectively reduced or increased the functional avidity of secondary SMARTA effector cells following rechallenge in a naïve secondary hosts. Our findings demonstrate that secondary effector T cells exhibit inflammation-dependent differences in functional avidity and memory potential, and have direct bearing on the design of strategies aimed at boosting memory T cell responses.

Blockade of Syk ameliorates the development of murine sclerodermatous chronic graft-versus-host disease

BACKGROUND

Murine sclerodermatous chronic graft-versus-host disease (Scl-cGVHD) is a model for human Scl-cGVHD and systemic sclerosis (SSc). Syk is expressed in most of hematopoietic cells, fibroblasts, and endothelial cells. Syk is a protein tyrosine kinase that has an important role in transmitting signals from a variety of cell surface receptors.

OBJECTIVE

This study aims to investigate the effect of R788 (fostamatinib sodium), an oral prodrug that is rapidly converted to a potent inhibitor of Syk, R406, on Scl-cGVHD.

METHODS

R788 was orally administered twice a day to allogeneic recipients from day 14 to day 42 after bone marrow transplantation (BMT). In vitro, proliferation of GVHD-derived CD4(+) T cells and CD11b(+) cells was analyzed by R406.

RESULTS

Allogeneic BMT increased Syk phosphorylation in T, B, and CD11b(+) cells. The administration of R788 attenuated severity and fibrosis of Scl-cGVHD. The elevated expressions of CXCR4 on T cells, B cells, and CD11b(+) cells were significantly down-regulated by R788 treatment. R788 reduced memory CD4(+) T cells (CD44(hi)CD62L(-)CD4(+)). R406 inhibited proliferation of GVHD CD4(+) T cells and CD11b(+) cells in vitro. In addition, R788 treatment, inhibited proliferation of CD11b(+) cells in Scl-cGVHD mice. R788 treatment also reduced skin mRNA expressions of MCP-1, MIP-1α, IFN-γ, IL-13, IL-17A, and TGF-β1, but not influenced RANTES, CXCL12, and TFN-α.

CONCLUSION

Blockade of Syk suppressed migration factor of immune cells and antigen-specific memory CD4(+) T cells and proliferation and activation of GVHD CD4(+) T cells and CD11b(+) cells. The current studies suggested that Syk inhibitor is a potential candidate for use in treating patients with Scl-cGVHD and SSc.

Early Decision: Effector and Effector Memory T Cell Differentiation in Chronic Infection

As effector memory T cells (Tem) are the predominant population elicited by chronic parasitic infections, increasing our knowledge of their function, survival and derivation, as phenotypically and functionally distinct from central memory and effector T cells will be critical to vaccine development for these diseases. In some infections, memory T cells maintain increased effector functions, however; this may require the presence of continued antigen, which can also lead to T cell exhaustion. Alternatively, in the absence of antigen, only the increase in the number of memory cells remains, without enhanced functionality as central memory. In order to understand the requirement for antigen and the potential for longevity or protection, the derivation of each type of memory must be understood. A thorough review of the data establishes the existence of both memory (Tmem) precursors and effector T cells (Teff) from the first hours of an immune response. This suggests a new paradigm of Tmem differentiation distinct from the proposition that Tmem only appear after the contraction of Teff. Several signals have been shown to be important in the generation of memory T cells, such as the integrated strength of “signals 1-3” of antigen presentation (antigen receptor, co-stimulation, cytokines) as perceived by each T cell clone. Given that these signals integrated at antigen presentation cells have been shown to determine the outcome of Teff and Tmem phenotypes and numbers, this decision must be made at a very early stage. It would appear that the overwhelming expansion of effector T cells and the inability to phenotypically distinguish memory T cells at early time points has masked this important decision point. This does not rule out an effect of repeated stimulation or chronic inflammatory milieu on populations generated in these early stages. Recent studies suggest that Tmem are derived from early Teff, and we suggest that this includes Tem as well as Tcm. Therefore, we propose a testable model for the pathway of differentiation from naïve to memory that suggests that Tem are not fully differentiated effector cells, but derived from central memory T cells as originally suggested by Sallusto et al. in 1999, but much debated since.

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.

A new look at T cell receptor signaling to nuclear factor-κB

Antigen stimulation of T cell receptor (TCR) signaling to nuclear factor (NF)-κB is required for T cell proliferation and differentiation of effector cells. The TCR-to-NF-κB pathway is generally viewed as a linear sequence of events in which TCR engagement triggers a cytoplasmic cascade of protein-protein interactions and post-translational modifications, ultimately culminating in the nuclear translocation of NF-κB. However, recent findings suggest a more complex picture in which distinct signalosomes, previously unrecognized proteins, and newly identified regulatory mechanisms play key roles in signal transmission. In this review, we evaluate recent data and suggest areas of future emphasis in the study of this important pathway.

Human CD4(+) effector T lymphocytes generated upon TCR engagement with self-peptides respond defectively to IL-7 in their transition to memory cells

The peripheral repertoire of CD4(+) T lymphocytes contains autoreactive cells that remain tolerant through several mechanisms. However, nonspecific CD4(+) T cells can be activated in physiological conditions as in the course of an ongoing immune response, and their outcome is not yet fully understood. Here, we investigate the fate of human naive CD4(+) lymphocytes activated by dendritic cells (DCs) presenting endogenous self-peptides in comparison with lymphocytes involved in alloresponses. We generated memory cells (Tmem) from primary effectors activated with mature autologous DCs plus interleukin (IL)-2 (Tmauto), simulating the circumstances of an active immune response, or allogeneic DCs (Tmallo). Tmem were generated from effector cells that were rested in the absence of antigenic stimuli, with or without IL-7. Tmem were less activated than effectors (demonstrated by CD25 downregulation) particularly with IL-7, suggesting that this cytokine may favour the transition to quiescence. Tmauto and Tmallo showed an effector memory phenotype, and responded similarly to polyclonal and antigen-specific stimuli. Biochemically, IL-7-treated Tmallo were closely related to conventional memory lymphocytes based on Erk-1/2 activation, whereas Tmauto were more similar to effectors. Autologous effectors exhibited lower responses to IL-7 than allogeneic cells, which were reflected in their reduced proliferation and higher cell death. This was not related to IL-7 receptor expression but rather to signalling deficiencies, according to STAT5 activation These results suggest that ineffective responses to IL-7 could impair the transition to memory cells of naive CD4(+) T lymphocytes recognizing self-peptides in the setting of strong costimulation.

Targeting T-cell memory: where do we stand?

PURPOSE OF REVIEW

The need to control donor-reactive T cell memory for successful organ transplantation is widely acknowledged. Alloreactive memory T cells are present in many human transplant recipients prior to transplantation and are less susceptible to the effects of currently used immunosuppression than naïve T cells. This review brings together recent advances in various fields of immunology that are potentially applicable for targeting memory T cells in sensitized transplant patients.

RECENT FINDINGS

The topics of the discussion are evoked by the latest findings on immunobiology of memory T cells and include functional diversity of T-cell memory, characteristic features of memory T-cell homeostasis and signaling, costimulatory requirements of memory T cells and their susceptibility to regulation.

SUMMARY

Despite apparent resistance of memory T cells to currently used therapies, recent findings suggest that recall responses by memory T cells can be controlled at many different levels. Use of this information may facilitate development of future tools managing T-cell memory in transplant settings.

TGF-β inhibits the uptake of modified low density lipoprotein by human macrophages through a Smad-dependent pathway: a dominant role for Smad-2

The anti-atherogenic cytokine, TGF-β, plays a key role during macrophage foam cell formation by modulating the expression of key genes involved in the control of cholesterol homeostasis. Unfortunately, the molecular mechanisms underlying these actions of TGF-β remain poorly understood. In this study we examine the effect of TGF-β on macrophage cholesterol homeostasis and delineate the role of Smads-2 and ‐3 during this process. Western blot analysis showed that TGF-β induces a rapid phosphorylation-dependent activation of Smad-2 and ‐3 in THP-1 and primary human monocyte-derived macrophages. Small interfering RNA-mediated knockdown of Smad-2/3 expression showed that the TGF-β-mediated regulation of key genes implicated in the uptake of modified low density lipoproteins and the efflux of cholesterol from foam cells was Smad-dependent. Additionally, through the use of virally delivered Smad-2 and/or Smad-3 short hairpin RNA, we demonstrate that TGF-β inhibits the uptake of modified LDL by macrophages through a Smad-dependent mechanism and that the TGF-β-mediated regulation of CD36, lipoprotein lipase and scavenger receptor-A gene expression was dependent on Smad-2. These studies reveal a crucial role for Smad signaling, particularly Smad-2, in the inhibition of foam cell formation by TGF-β through the regulation of expression of key genes involved in the control of macrophage cholesterol homeostasis.

Advances in direct T-cell alloreactivity: function, avidity, biophysics and structure

Although T-cell-based adaptive immunity plays a crucial role in protection against infectious pathogens and uncontrolled outgrowth of malignant cells, a large portion of these T cells are also capable of responding to allogeneic HLA molecules, violating the paradigm of self-major histocompatibility complex (MHC) restriction. Recent studies have provided insights into the mechanisms by which these T cells recognize allogeneic targets. The role of antiviral T cells in direct alloreactivity through peptide-dependent molecular mimicry and alternate peptide-MHC docking modes has emerged as major models for the human alloresponse. Here, we review in depth recent advances in this field and discuss how molecular interactions between T cells and HLA molecules drive the activation of these effector cells and its potential implications for alloreactivity in human transplantation.

A repertoire-independent and cell-intrinsic defect in murine GVHD induction by effector memory T cells

Effector memory T cells (T(EM)) do not cause graft-versus-host disease (GVHD), though why this is has not been elucidated. To compare the fates of alloreactive naive (T(N)) or memory (T(M)) T cells, we developed a model of GVHD in which donor T cells express a transgene-encoded TCR specific for an antigenic peptide that is ubiquitously expressed in the recipient. Small numbers of naive TCR transgenic (Tg) T cells induced a robust syndrome of GVHD in transplanted recipients. We then used an established method to convert TCR Tg cells to T(M) and tested these for GVHD induction. This allowed us to control for the potentially different frequencies of alloreactive T cells among T(N) and T(M), and to track fates of alloreactive T cells after transplantation. T(EM) caused minimal, transient GVHD whereas central memory T cells (T(CM)) caused potent GVHD. Surprisingly, T(EM) were not inert: they, engrafted, homed to target tissues, and proliferated extensively, but they produced less IFN-γ and their expansion in target tissues was limited at later time points, and local proliferation was reduced. Thus, cell-intrinsic properties independent of repertoire explain the impairment of T(EM), which can initiate but cannot sustain expansion and tissue damage.

Transcriptional control of rapid recall by memory CD4 T cells

Memory T cells are distinguished from naive T cells by their rapid production of effector cytokines, although mechanisms for this recall response remain undefined. In this study, we investigated transcriptional mechanisms for rapid IFN-γ production by Ag-specific memory CD4 T cells. In naive CD4 T cells, IFN-γ production only occurred after sustained Ag activation and was associated with high expression of the T-bet transcription factor required for Th1 differentiation and with T-bet binding to the IFN-γ promoter as assessed by chromatin immunoprecipitation analysis. By contrast, immediate IFN-γ production by Ag-stimulated memory CD4 T cells occurred in the absence of significant nuclear T-bet expression or T-bet engagement on the IFN-γ promoter. We identified rapid induction of NF-κB transcriptional activity and increased engagement of NF-κB on the IFN-γ promoter at rapid times after TCR stimulation of memory compared with naive CD4 T cells. Moreover, pharmacologic inhibition of NF-κB activity or peptide-mediated inhibition of NF-κB p50 translocation abrogated early memory T cell signaling and TCR-mediated effector function. Our results reveal a molecular mechanism for memory T cell recall through enhanced NF-κB p50 activation and promoter engagement, with important implications for memory T cell modulation in vaccines, autoimmunity, and transplantation.

Bystander stimulation of activated CD4+ T cells of unrelated specificity following a booster vaccination with tetanus toxoid

Antigen-specific CD4(+) T cells are central to natural and vaccine-induced immunity. An ongoing antigen-specific T-cell response can, however, influence surrounding T cells with unrelated antigen specificities. We previously observed this bystander effect in healthy human subjects following recall vaccination with tetanus toxoid (TT). Since this interplay could be important for maintenance of memory, we have moved to a mouse model for further analysis. We investigated whether boosting memory CD4(+) T cells against TT in vivo would influence injected CD4(+) TCR transgenic T cells (OT-II) specific for an unrelated OVA peptide. If OT-II cells were pre-activated with OVA peptide in vitro, these cells showed a bystander proliferative response during the ongoing parallel TT-specific response. Bystander proliferation was dependent on boosting of the TT-specific memory response in the recipients, with no effect in naive mice. Bystander stimulation was also proportional to the strength of the TT-specific memory T-cell response. T cells activated in vitro displayed functional receptors for IL-2 and IL-7, suggesting these as potential mediators. This crosstalk between a stimulated CD4(+) memory T-cell response and CD4(+) T cells activated by an unrelated antigen could be important in human subjects continually buffeted by environmental antigens.

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.

Ablation of SLP-76 signaling after T cell priming generates memory CD4 T cells impaired in steady-state and cytokine-driven homeostasis

The intracellular signaling mechanisms regulating the generation and long-term persistence of memory T cells in vivo remain unclear. In this study, we used mouse models with conditional deletion of the key T cell receptor (TCR)-coupled adaptor molecule SH2-domain-containing phosphoprotein of 76 kDa (SLP-76), to analyze signaling mechanisms for memory CD4 T cell generation, maintenance, and homeostasis. We found that ablation of SLP-76 expression after T cell priming did not inhibit generation of phenotypic effector or memory CD4 T cells; however, the resultant SLP-76-deficient memory CD4 T cells could not produce recall cytokines in response to TCR-mediated stimulation and showed decreased persistence in vivo. In addition, SLP-76-deficient memory CD4 T cells exhibited reduced steady-state homeostasis and were impaired in their ability to homeostatically expand in vivo in response to the gamma(c) cytokine IL-7, despite intact proximal signaling through the IL-7R-coupled JAK3/STAT5 pathway. Direct in vivo deletion of SLP-76 in polyclonal memory CD4 T cells likewise led to impaired steady-state homeostasis as well as impaired homeostatic responses to IL-7. Our findings demonstrate a dominant role for SLP-76-dependent TCR signals in regulating turnover and perpetuation of memory CD4 T cells and their responses to homeostatic cytokines, with implications for the selective survival of memory CD4 T cells following pathogen exposure, vaccination, and aging.

The magnitude of CD4+ T cell recall responses is controlled by the duration of the secondary stimulus

The parameters controlling the generation of robust CD4(+) T cell recall responses remain poorly defined. In this study, we compare recall responses by CD4(+) and CD8(+) memory T cells following rechallenge. Homologous rechallenge of mice immune to either lymphocytic choriomeningitis virus or Listeria monocytogenes results in robust CD8(+) T cell recall responses but poor boosting of CD4(+) T cell recall responses in the same host. In contrast, heterologous rechallenge with a pathogen sharing only a CD4(+) T cell epitope results in robust boosting of CD4(+) T cell recall responses. The disparity in CD4(+) and CD8(+) T cell recall responses cannot be attributed to competition for growth factors or APCs, as robust CD4(+) and CD8(+) T cell recall responses can be simultaneously induced following rechallenge with peptide-pulsed dendritic cells. Instead, CD4(+) T cell recall responses are dependent on the duration of the secondary challenge. Increasing the rechallenge dose results in more potent boosting of CD4(+) T cell recall responses and artificially limiting the duration of secondary infection following heterologous rechallenge adversely impacts the magnitude of CD4(+) T cell, but not CD8(+) T cell, recall responses. These findings suggest that rapid pathogen clearance by secondary CTL following homologous rechallenge prevents optimal boosting of CD4(+) T cell responses and therefore have important practical implications in the design of vaccination and boosting strategies aimed at promoting CD4(+) T cell-mediated protection.

The UDP-glucose receptor P2RY14 triggers innate mucosal immunity in the female reproductive tract by inducing IL-8

Innate mucosal immune responses, including recognition of pathogen-associated molecular patterns through Toll-like receptors, play an important role in preventing infection in the female reproductive tract (FRT). Damaged cells release nucleotides, including ATP and uridine 5'-diphosphoglucose (UDP-glucose), during inflammation and mechanical stress. We show in this report that P2RY14, a membrane receptor for UDP-glucose, is exclusively expressed in the epithelium, but not the stroma, of the FRT in humans and mice. P2RY14 and several proinflammatory cytokines, such as IL-8, are up-regulated in the endometria of patients with pelvic inflammatory disease. UDP-glucose stimulated IL-8 production via P2RY14 in human endometrial epithelial cells but not stromal cells. Furthermore, UDP-glucose enhanced neutrophil chemotaxis in the presence of a human endometrial epithelial cell line in an IL-8-dependent manner. Administration of UDP-glucose into the mouse uterus induced expression of macrophage inflammatory protein-2 and keratinocyte-derived cytokine, two murine chemokines that are functional homologues of IL-8, and augmented endometrial neutrophil recruitment. Reduced expression of P2RY14 by small interfering RNA gene silencing attenuated LPS- or UDP-glucose-induced leukocytosis in the mouse uterus. These results suggest that UDP-glucose and its receptor P2RY14 are key front line players able to trigger innate mucosal immune responses in the FRT bypassing the recognition of pathogen-associated molecular patterns. Our findings would significantly impact the strategic design of therapies to modulate mucosal immunity by targeting P2RY14.

Generation and maintenance of memory CD4(+) T Cells

In the course of an immune response to an infectious microbe, pathogen-specific naïve CD4(+) T cells proliferate extensively and differentiate into effector cells. Most of these cells die rapidly, but a small fraction of effector cells persist as memory cells to confer enhanced protection against the same pathogen. Recent advances indicate that strong TCR stimulation during the primary response is essential for the generation of long-lived memory CD4(+) T cells. Memory cells appear to be derived equally from all subsets of effector cells, and memory cells can also acquire additional functional capabilities during the secondary response. Resting memory CD4(+) cells are dependent on signals from contact with IL-7 and IL-15, but not MHC class II, for their survival and intermittent homeostatic proliferation.

CD4 memory T cells: what are they and what can they do?

Immunological memory provides the basis for successful vaccines. It is important to understand the properties of memory cells. There is much known about the phenotype and functions of memory CD8 T cells, less about memory B cells, while CD4 memory T cells have proved difficult to study. Differences in the types of memory CD4 cells studied and the difficulties of tracking the small number of cells have led to conflicting and unclear results. Here we discuss the different systems used to study CD4 memory cells and ask whether, and in what circumstances, memory CD4 cells could provide protection against infections.

Biochemical signaling pathways for memory T cell recall

Memory T cells exhibit low activation thresholds and rapid effector responses following antigen stimulation, contrasting naive T cells with high activation thresholds and no effector responses. Signaling mechanisms for the distinct properties of naive and memory T cells remain poorly understood. Here, I will discuss new results on signal transduction in naive and memory T cells that suggest proximal control of activation threshold and a distinct biochemical pathway to rapid recall. The signaling and transcriptional pathways controlling immediate effector function in memory T cells closely resemble pathways for rapid effector cytokine production in innate immune cells, suggesting memory T cells use innate pathways for efficacious responses.

Self-antigen prevents CD8 T cell effector differentiation by CD134 and CD137 dual costimulation

We compared how CD4 vs CD8 cells attain the capacity to express the effector cytokine IFN-gamma under both immunogenic and tolerogenic conditions. Although the Ifng gene locus was epigenetically repressed in naive Ag-inexperienced CD4 cells, it had already undergone partial remodeling toward a transcriptionally competent configuration in naive CD8 cells. After TCR stimulation, CD8 cells fully remodeled the Ifng locus and gained the capacity to express high levels of IFN-gamma more rapidly than CD4 cells. Enforced dual costimulation through OX40 and 4-1BB redirected CD8 cells encountering soluble exogenous peptide to expand and differentiate into IFN-gamma and TNF-alpha double-producing effectors rather than becoming tolerant. Despite this and the stronger tendency of CD8 compared with CD4 cells to differentiate into IFN-gamma-expressing effectors, when parenchymal self-Ag was the source of tolerizing Ag, enforced dual costimulation selectively boosted expansion but did not push effector differentiation in CD8 cells while both expansion and effector differentiation were dramatically boosted in CD4 cells. Notably, enforced dual costimulation was able to push effector differentiation in CD8 cells encountering cognate parenchymal self-Ag when CD4 cells were simultaneously engaged. Thus, the ability of enforced OX40 plus 4-1BB dual costimulation to redirect CD8 cells to undergo effector differentiation was unexpectedly influenced by the source of tolerizing Ag and help was selectively required to facilitate CD8 cell effector differentiation when the tolerizing Ag derived from self.

Anergy in memory CD4+ T cells is induced by B cells

Induction of tolerance in memory T cells has profound implications in the treatment of autoimmune diseases and transplant rejection. Previously, we reported that the presentation of low densities of agonist peptide/MHC class II complexes induced anergy in memory CD4(+) T cells. In the present study, we address the specific interaction of different types of APCs with memory CD4(+) T cells. A novel ex vivo anergy assay first suggested that B cells induce anergy in memory T cells, and an in vivo cell transfer assay further confirmed those observations. We demonstrated that B cells pulsed with defined doses of Ag anergize memory CD4 cells in vivo. We established that CD11c(+) dendritic cells do not contribute to anergy induction to CD4 memory T cells, because diphtheria toxin receptor-transgenic mice that were conditionally depleted of dendritic cells optimally induced anergy in memory CD4(+) T cells. Moreover, B cell-deficient muMT mice did not induce anergy in memory T cells. We showed that B2 follicular B cells are the specific subpopulation of B cells that render memory T cells anergic. Furthermore, we present data showing that anergy in this system is mediated by CTLA-4 up-regulation on T cells. This is the first study to demonstrate formally that B cells are the APCs that induce anergy in memory CD4(+) T cells.

Arsenic interferes with the signaling transduction pathway of T cell receptor activation by increasing basal and induced phosphorylation of Lck and Fyn in spleen cells

Arsenic is known to produce inhibition as well as induction of immune cells proliferative responses depending on the doses as one of its mechanisms of immunotoxicity. Here we evaluate the effect of arsenic exposure on the activation of splenic mononuclear cells (SMC) in male CD57BL6N mice. Intra-gastric exposure to arsenic (as sodium arsenite) for 30 days (1, 0.1, or 0.01 mg/kg/day), reduced the proportion of CD4+ cells and the CD4+/CD8+ ratio in the spleen, increasing the proportion of CD11b+ cells. Arsenic exposure did not modify the proportion of B cells. SMC showed an increased level of phosphorylation of lck and fyn kinases (first kinases associated to TCR complex when activated). Although normal levels of apoptosis were observed on freshly isolated SMC, an increase in apoptotic cells related with the increase in phosphorylation of lck and fyn was observed when SMC were activated with Concanavalin-A (Con-A). Arsenic exposure reduced the proliferative response of SMC to Con-A, and also reduced secretion of IL-2, IL-6, IL-12 and IFNgamma. No effect was observed on IL-4, and IL-10 secretion. The same effects were observed when SMC of exposed animals were activated with anti-CD3/CD28 antibodies for 24 h, but these effects were transitory since a recovery, up to control levels or even higher, were observed after 72 h of stimulation. This study demonstrates that repeated and prolonged exposure to arsenic alters cell populations and produces functional changes depending on the specific activation pathway, and could be related with the phosphorylation status of lck and fyn kinases.

Proximal signaling control of human effector CD4 T cell function

The functional coupling of T cell receptor (TCR)-mediated signaling events in primary human T cells remains undefined. We demonstrate here that alterations in the expression of proximal TCR-coupled signaling subunits are associated with distinct effector capacities in differentiated human CD4 T cells. Analysis of proximal signaling profiles using biochemical and single cell approaches reveals decreased CD3zeta and ZAP-70 expression correlating with functional anergy, with increased CD3zeta/ ZAP-70 expression and phosphorylation connoting acquisition of effector capacity. By contrast, the FcRgamma signaling subunit known to be expressed in human effector cells and in T cells from the autoimmune disease SLE is up-regulated upon activation, yet does not correlate with functional capacity in effector cells, and does not alter signaling or function in primary FcRgamma transfectants. Our results have implications for targeting signaling molecules in immunotherapy and evaluating the functional consequence of signaling alterations associated with autoimmunity and chronic diseases.