T cell clonal conditioning: a phase occurring early after antigen presentation but before clonal expansion is impacted by Toll-like receptor stimulation

Evaluating the glycolytic potential of mouse costimulated effector CD8+ T cells ex vivo

Studying the metabolic fitness of T cells is fundamental to understand how immune responses are regulated. Here, we describe a step-by-step protocol optimized to efficiently generate and isolate effector antigen-specific CD8+ T cells ex vivo using costimulation. We also detail steps to evaluate their metabolic activity using Seahorse technology. This protocol can be used to measure the glycolytic potential of effector murine T cells in response to different manipulations, such as infections, adjuvant studies, gene editing, or metabolite supplementation. For complete details on the use and execution of this protocol, please refer to Agliano et al. (2022).

Therapeutic blockade of CD54 attenuates pulmonary barrier damage in T cell-induced acute lung injury

Acute respiratory distress syndrome (ARDS) is a serious, often fatal condition without available pharmacotherapy. Although the role of innate cells in ARDS has been studied extensively, emerging evidence suggests that T cells may be involved in disease etiology. Staphylococcus aureus enterotoxins are potent T-cell mitogens capable of triggering life-threatening shock. We demonstrate that 2 days after inhalation of S. aureus enterotoxin A, mice developed T cell-mediated increases in vascular permeability, as well as expression of injury markers and caspases in the lung. Pulmonary endothelial cells underwent sequential phenotypic changes marked by rapid activation coinciding with inflammatory events secondary to T-cell priming, followed by reductions in endothelial cell number juxtaposing simultaneous T-cell expansion and cytotoxic differentiation. Although initial T-cell activation influenced the extent of lung injury, CD54 (ICAM-1) blocking antibody administered well after enterotoxin exposure substantially attenuated pulmonary barrier damage. Thus CD54-targeted therapy may be a promising candidate for further exploration into its potential utility in treating ARDS patients.

Optimal CD4 T cell priming after LPS-based adjuvanticity with CD134 costimulation relies on CXCL9 production

LPS is a powerful adjuvant, and although LPS-mediated TLR4 signaling has been exquisitely delineated, the in vivo mechanism of how TLR4 responses impact T cell priming is far less clear. Besides costimulation, TNF and type 1 IFN are dominant cytokines released after TLR4 activation and can shape T cell responses, but other downstream factors have not been examined extensively. Depending on context, we show that IFNαR1 blockade resulted in minor to major effects on specific CD4 T cell clonal expansion. To help explain these differences, it was hypothesized that IFNαR1 blockade would inhibit specific T cell migration by reducing chemokine receptor signaling, but specific CD4 T cells from IFNαR1-blocked mice were readily able to migrate in response to specific chemokines. Next, we examined downstream factors and found that type 1 IFN signaling was necessary for chemokine production, even when mice were immunized with specific Ag with LPS and CD134 costimulation. IFNαR1 signaling promoted CXCL9 and CXCL10 synthesis, suggesting that these chemokines might be involved in the LPS and CD134 costimulation response. After immunization, we show that CXCL9 blockade inhibited CD4 T cell accumulation in the liver but also in LNs, even in the presence of elevated serum IFN-β levels. Thus, whereas type 1 IFN might have direct effects on primed CD4 T cells, the downstream chemokines that play a role during migration also impact accumulation. In sum, CXCL9 production is a key benchmark for productive CD4 T cell vaccination strategies.

Imaging Asymmetric T Cell Division

Asymmetric cell division (ACD) controls cell fate decisions in model organisms such as Drosophila and C. elegans and has recently emerged as a mediator of T cell fate and hematopoiesis. The most appropriate methods for assessing ACD in T cells are still evolving. Here we describe the methods currently applied to monitor and measure ACD of developing and activated T cells. We provide an overview of approaches for capturing cells in the process of cytokinesis in vivo, ex vivo, or during in vitro culture. We provide methods for in vitro fixed immunofluorescent staining and for time-lapse analysis. We provide an overview of the different approaches for quantification of ACD of lymphocytes, discuss the pitfalls and concerns in interpretation of these analyses, and provide detailed methods for the quantification of ACD in our group.

A virtual lymph node model to dissect the requirements for T-cell activation by synapses and kinapses

The initiation of T-cell responses in lymph nodes requires T cells to integrate signals delivered by dendritic cells (DCs) during long-lasting contacts (synapses) or more transient interactions (kinapses). However, it remains extremely challenging to understand how a specific sequence of contacts established by T cells ultimately dictates T-cell fate. Here, we have coupled a computational model of T-cell migration and interactions with DCs with a real-time, flow cytometry-like representation of T-cell activation. In this model, low-affinity peptides trigger T-cell proliferation through kinapses but we show that this process is only effective under conditions of high DC densities and prolonged antigen availability. By contrast, high-affinity peptides favor synapse formation and a vigorous proliferation under a wide range of antigen presentation conditions. In line with the predictions, decreasing the DC density in vivo selectively abolished proliferation induced by the low-affinity peptide. Finally, our results suggest that T cells possess a biochemical memory of previous stimulations of at least 1–2 days. We propose that the stability of T-cell–DC interactions, apart from their signaling potency, profoundly influences the robustness of T-cell activation. By offering the ability to control parameters that are difficult to manipulate experimentally, the virtual lymph node model provides new possibilities to tackle the fundamental mechanisms that regulate T-cell responses elicited by infections or vaccines.

Polarity and asymmetric cell division in the control of lymphocyte fate decisions and function

Polarity is important in several lymphocyte processes including lymphocyte migration, formation of the immunological synapse, and asymmetric cell division (ACD). While lymphocyte migration and immunological synapse formation are relatively well understood, the role of lymphocyte ACD is less clear. Recent advances in measuring polarity enable more robust analyses of asymmetric cell division. Use of these new methods has produced crucial quantification of ACD at precise phases of lymphocyte development and activation. These developments are leading to a better understanding of the drivers of fate choice during lymphocyte activation and provide a context within which to explain the effects of ACD.

Responses to LPS boost effector CD8 T-cell accumulation outside of signals 1 and 2

Immunization with adjuvant plus antigen induces durable T-cell immunity and is a mainstay of vaccines. Here, the consequence of separating antigen stimulation of T cells from the adjuvant response was studied in a re-transfer model. Effector CD8 T cells in recipient mice were exposed to lipopolysaccharide (LPS), the Toll-like receptor 4 (TLR4) ligand, which significantly increased persistence. While accumulation in lymphoid and non-lymphoid organs was evident, this result depended upon the timing of LPS administration and the presence of the TLR4 adaptor TRIF in the recipient mice. Interestingly, there was very little impact of the LPS response on subset differentiation, which rather appeared to be programmed by antigen and costimulation. To discern factors that limit accumulation, interleukin 10 (IL-10) was targeted since it is a product of TLR4 triggering and mitigates inflammation. Blockade of IL-10 increased accumulation even though the effector CD8 T cells were well past the priming phase, but upon recall interferon-γ secretion was not affected as would be expected when IL-10 is present during priming. Thus, the adjuvant-altered microenvironment is effective not only in the presence of antigen but also during a window of effector CD8 T-cell stasis, suggesting that pathogen-associated molecular pattern molecules released during co-infection, or by vaccines, could alter the survival fate of specific effector T cells.Cellular & Molecular Immunology advance online publication, 20 July 2015; doi:10.1038/cmi.2015.69.

Regulatory T cell ablation causes acute T cell lymphopenia

Regulatory T (Treg) cells enforce T cell homeostasis and maintain peripheral T cell tolerance. Here we report a previously unappreciated phenomenon of acute T cell lymphopenia in secondary lymphoid organs and non-lymphoid tissues triggered by Treg cell depletion that precedes the expansion of self-reactive T cells. Lymphopenia affects both neonates and adults indicating a dominant role of Treg cells in maintaining peripheral T cell numbers regardless of the developmental stage. The lymphopenia was neither triggered by caspase-dependent apoptosis nor macrophage-mediated clearance of T cells, nor diminished survival of naïve or recently activated T cells due to paucity of IL-7. It is possible that transient lymphopenia associated with congenital or acute Treg cell deficiency may contribute to the development of T cell mediated autoimmune disorders.

Splenic priming of virus-specific CD8 T cells following influenza virus infection

In healthy individuals, influenza virus (IAV) infection generally remains localized to the epithelial cells of the respiratory tract. Previously, IAV-specific effector CD8 T cells found systemically during the course of IAV infection were thought to have been primed in lung-draining lymph nodes with subsequent migration to other tissues. However, little is known about whether other lymphoid sites participate in the generation of virus-specific CD8 T cells during localized IAV infection. Here, we present evidence of early CD8 T cell priming in the spleen following respiratory IAV infection independent of lung-draining lymph node priming of T cells. Although we found early indications of CD8 T cell activation in the lymph nodes draining the respiratory tract, we also saw evidence of virus-specific CD8 T cell activation in the spleen. Furthermore, CD8 T cells primed in the spleen differentiated into memory cells of equivalent longevity and with similar recall capacity as CD8 T cells primed in the draining lymph nodes. These data showed that the spleen contributes to the virus-specific effector and memory CD8 T cell populations that are generated in response to respiratory infection.

Guarding the perimeter: protection of the mucosa by tissue-resident memory T cells

Mucosal tissues are continually bombarded with infectious agents seeking to gain entry into the body. The absence of a tough physical exterior layer surrounding these tissues creates a unique challenge for the immune system, which manages to provide broad protection against a plethora of different organisms with the aid of special adaptations that augment immunity at these vulnerable sites. For example, specialized populations of memory T lymphocytes reside at initial sites of pathogen entry into the body, where they provide an important protective barrier. Similar anatomically-confined populations of pathogen-specific CD8 T cells can be found near the outer margins of the body following recovery from a variety of local infections, where they share very similar phenotypic characteristics. How these tissue-resident T cells are retained in a single anatomic location where they can promote immunity is beginning to be defined. Here, we will review current knowledge of the mechanisms that help establish and maintain these regional lymphocytes in the mucosal tissues and discuss relevant data that enhance our understanding of the contribution of these lymphocyte populations to protective immunity against infectious diseases.

Modeling of influenza-specific CD8+ T cells during the primary response indicates that the spleen is a major source of effectors

The biological parameters that determine the distribution of virus-specific CD8(+) T cells during influenza infection are not all directly measurable by experimental techniques but can be inferred through mathematical modeling. Mechanistic and semimechanistic ordinary differential equations were developed to describe the expansion, trafficking, and disappearance of activated virus-specific CD8(+) T cells in lymph nodes, spleens, and lungs of mice during primary influenza A infection. An intensive sampling of virus-specific CD8(+) T cells from these three compartments was used to inform the models. Rigorous statistical fitting of the models to the experimental data allowed estimation of important biological parameters. Although the draining lymph node is the first tissue in which Ag-specific CD8(+) T cells are detected, it was found that the spleen contributes the greatest number of effector CD8(+) T cells to the lung, with rates of expansion and migration that exceeded those of the draining lymph node. In addition, models that were based on the number and kinetics of professional APCs fit the data better than those based on viral load, suggesting that the immune response is limited by Ag presentation rather than the amount of virus. Modeling also suggests that loss of effector T cells from the lung is significant and time dependent, increasing toward the end of the acute response. Together, these efforts provide a better understanding of the primary CD8(+) T cell response to influenza infection, changing the view that the spleen plays a minor role in the primary immune response.

Attenuated T cell responses to a high-potency ligand in vivo

According to this study, the strongest T cell receptor ligands in vitro do not necessarily induce the strongest T cell responses in vivo, suggesting that vaccine designers may need to reconsider their strategies.

αβ T cell receptor (TCR) recognition of foreign peptides bound to major histocompatibility complex (pMHC) molecules on the surface of antigen presenting cells is a key event in the initiation of adaptive cellular immunity. In vitro, high-affinity binding and/or long-lived interactions between TCRs and pMHC correlate with high-potency T cell activation. However, less is known about the influence of TCR/pMHC interaction parameters on T cell responses in vivo. We studied the influence of TCR/pMHC binding characteristics on in vivo T cell immunity by tracking CD4⁺ T cell activation, effector, and memory responses to immunization with peptides exhibiting a range of TCR/pMHC half-lives and in vitro T cell activation potencies. Contrary to predictions from in vitro studies, we found that optimal in vivo T cell responses occur to ligands with intermediate TCR/pMHC half-lives. The diminished in vivo responses we observed to the ligand exhibiting the longest TCR/pMHC half-life were associated with attenuation of intracellular signaling, expansion, and function over a broad range of time points. Our results reveal a level of control over T cell activation in vivo not recapitulated in in vitro assays and highlight the importance of considering in vivo efficacy of TCR ligands as part of vaccine design.

As an important part of immune system, T cells fight infections by recognizing signs of foreign invaders. A molecule on the surface of these cells—called the T cell receptor—recognizes and binds to protein components (peptides) from bacteria, viruses, and other pathogens that are displayed on the surface of other cells. The T cells can then use this information to orchestrate the fight against infection. Vaccination involves injecting into the body foreign peptides that mimic a pathogen, therefore tricking it into raising a T cell response against that pathogen that will be protective in the event of a real infection. We studied T cell responses in mice injected with one of several peptides to which the T cell receptor binds more or less strongly. Contrary to expectations, we found that the peptide that interacted most strongly with the T cell receptor did not provoke the strongest T cell response. This may be nature's way of preventing harmful inflammatory damage due to excessively strong T cell activation. Our work shows that peptides that bind the T cell receptor with medium strength may be best to use for vaccines. Current vaccine strategies seeking to design peptides that bind with maximum strength to the T cell receptor may need to be reconsidered in light of our findings.

Antigen presenting and effector cell cluster formation in BALB/c mice during mousepox: model studies*

AIMS

The objective of this study was to access APC-effector cell cluster formation in genetically susceptible BALB/c (H-2(d) ) mice infected with highly virulent Moscow strain of ectromelia virus (ECTV-MOS) and estimate of lymphocyte activation based upon expression of CD62L and CD44 molecules.

METHODS AND RESULTS

APC-effector cell clusters were obtained by enzymatic digestion from draining lymph nodes (DLNs) and spleens of BALB/c mice. We found that APCs infected with ECTV-MOS form unstable clusters with effector cells, and thus may diminish T-cell activation at the early stage of mousepox. Different types of effector cells including T-cell subsets (CD4(+) and CD8(+) ), B cells and polymorphonuclear cells colocalize within individual clusters. Increase in CD19(+) B cells within APC-effector cell clusters during severe clinical mousepox may reflect B-cell activation.

CONCLUSIONS

Our studies indicated vigorous changes in APC-effector cell cluster formation in genetically susceptible BALB/c mice during mousepox (up to 2 weeks). ECTV-MOS can modulate APC interactions with effector cells and consequently may impair T-cell activation probably owing to unstable cluster formation and/or subsequent weak stimulation by infected APCs at the early stages of mousepox.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of APC-effector cell cluster formation in BALB/c mice during mousepox. It gives us a new light on the mutual cell-cell interactions and development of the immune response during ECTV-MOS infection.

T cell and APC dynamics in situ control the outcome of vaccination

The factors controlling the progression of an immune response to generation of protective memory are poorly understood. We compared the in situ and ex vivo characteristics of CD8 T cells responding to different forms of the same immunogen. Immunization with live Listeria monocytogenes, irradiated L. monocytogenes (IRL), or heat-killed L. monocytogenes (HKL) induced rapid activation of CD8 T cells. However, only IRL and live L. monocytogenes inoculation induced sustained proliferation and supported memory development. Gene and protein expression analysis revealed that the three forms of immunization led to three distinct transcriptional and translational programs. Prior to cell division, CD8 T cell-dendritic cell clusters formed in the spleen after live L. monocytogenes and IRL but not after HKL immunization. Furthermore, HKL immunization induced rapid remodeling of splenic architecture, including loss of marginal zone macrophages, which resulted in impaired bacterial clearance. These results identify initial characteristics of a protective T cell response that have implications for the development of more effective vaccination strategies.

Dendritic cells and Stat3 are essential for CD137-induced CD8 T cell activation-induced cell death

Agonistic anti-CD137 mAbs either positively or negatively regulate T cell function. When administered at the beginning of lymphocytic choriomeningitis virus Armstrong infection anti-CD137 induced immunosuppression and T cell deletion, and in the case of influenza infection led to increased mortality. In contrast, 72 h delay in anti-CD137 treatment led to an enhanced virus-specific CD8 T cell response and rapid viral clearance. Virus-specific CD8 T cells in anti-CD137-injected mice rapidly upregulate Fas expression, and although necessary, was insufficient to induce CD8 T cell deletion. Strikingly, CD137 signaling in T cells was found to be insufficient to induce suppression or deletion. Rather, immunosuppression and T cell deletion was only observed if CD137 signals were provided to T cells and dendritic cells (DCs). In vitro CD137 crosslinking in DCs led to phosphorylation of Stat3, and importantly, anti-CD137 treatment of lymphocytic choriomeningitis virus Armstrong infected Stat3 conditional knock-out mice induced neither immune suppression or T cell deletion. Taken together, these data suggest that CD137 signaling in DCs can regulate CD8 T cell survival through a Stat3 and Fas-mediated pathway.

Analysis of apoptosis of memory T cells and dendritic cells during the early stages of viral infection or exposure to toll-like receptor agonists

Profound type I interferon (IFN-I)-dependent attrition of memory CD8 and CD4 T cells occurs early during many infections. It is dramatic at 2 to 4 days following lymphocytic choriomeningitis virus (LCMV) infection of mice and can be elicited by the IFN-inducing Toll receptor agonist poly(I:C). We show that this attrition occurs in many organs, indicating that it is due to T cell loss rather than redistribution. This loss correlated with elevated intracellular staining of T cells ex vivo for activated caspases but with only low levels of ex vivo staining with annexin V, probably due to the rapid clearance of apoptotic cells in vivo. Instead, a high frequency of annexin V-reactive CD8alpha(+) dendritic cells (DCs), which are known to be highly phagocytic, accumulated in the spleen as the memory T cell populations disappeared. After short in vitro incubation, memory phenotype T cells isolated from LCMV-infected mice (day 3) or mice treated with poly(I:C) (12 h) displayed substantial DNA fragmentation, as detected by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) assay, compared to T cells isolated from uninfected mice, indicating a role for apoptosis in the memory T cell attrition. This apoptosis of memory CD8 T cells early during LCMV infection was reduced in mice lacking the proapoptotic molecule Bim. Evidence is presented showing that high levels of T cell attrition, as found in young mice, correlate with reduced immunodomination by cross-reactive memory cells.

TLR2 engagement on dendritic cells promotes high frequency effector and memory CD4 T cell responses

Ligation of TLR by distinct pathogen components provides essential signals for T cell priming, although how individual TLR engagement affects primary and memory T cell responses is not well defined. In this study, we demonstrate distinct effects of TLR2 vs TLR4 engagement on primary and memory CD4 T cell responses due to differential effects on APC. Priming of influenza hemagglutinin (HA)-specific naive CD4 T cells with HA peptide and the TLR2 agonist Pam3CysK in vivo resulted in a high frequency of activated HA-specific CD4 T cells that predominantly produced IL-2 and IL-17, whereas priming with HA peptide and the TLR4 agonist LPS yielded a lower frequency of HA-specific CD4 T cells and predominant IFN-gamma producers. TLR2 agonist priming depended on TLR2 expression by APC, as wild-type CD4 T cells did not expand in response to peptide and Pam3CysK in TLR2-deficient hosts. TLR2-mediated priming also led to an increased frequency of Ag-specific memory CD4 T cells compared with TLR4 priming and mediated enhanced secondary responses to influenza challenge. Our results show that TLR engagement on APC influences both primary and secondary CD4 T cell responses, and suggest that long-term functional capacities of T cells are set by innate signals during early phases of an infection.

T cells demonstrate a Th1-biased response to native beta2-glycoprotein I in a murine model of anti-phospholipid antibody induction

Anti-phospholipid syndrome (APS) is an autoimmune disorder characterized by the presence of autoantibody (AAb) to phospholipid (PL)-binding proteins, such as beta2-glycoprotein I (beta2GPI), and clinical manifestations including thrombosis and/or recurrent pregnancy loss. beta2GPI-reactive T cells are clearly implicated in the generation of these AAb, but the mechanism responsible for their activation remains unclear. We hypothesized that immunization of mice with human beta2GPI, in the context of a potent innate immune activator lipopolysaccharide (LPS), would generate not only high titers of anti-PL AAb, but also a strong beta2GPI-specific T cell response. Healthy, nonautoimmune C57BL/6 mice were immunized repeatedly with human beta2GPI in the presence of LPS. High titers of anti-PL to beta2GPI appeared after the second immunization, with T cell reactivity to beta2GPI detectable only after the fourth immunization. Splenic T cells from these mice proliferated in response to native beta2GPI, alone or bound to anionic PL. These T cells produced IL-2 and IFN-gamma, but not IL-4 or IL-10, indicating a Th1 bias of the beta2GPI-specific response. These findings suggest that T cells responsive to beta2GPI may become activated in APS patients by exposure to their cognate Ag in the context of innate immune activation and a pro-inflammatory environment.

Characterization of preexisting MAGE-A3-specific CD4+ T cells in cancer patients and healthy individuals and their activation by protein vaccination

Vaccination with cancer/testis Ag MAGE-A3 in the form of recombinant protein often induces specific humoral and cellular immune responses. Although Ag-specific CD4+ T cells following vaccination are detectable by cytokine production after a single in vitro stimulation, their detection before vaccination is difficult because of low frequency. In this study, we have applied a sensitive method using CD154 (CD40L) staining to detect MAGE-A3-specific CD4+ T cells. MAGE-A3-specific T cell responses were analyzed in four healthy donors, two lung cancer patients with spontaneous serum Abs to MAGE-A3, and two baseline seronegative lung cancer patients throughout vaccination with MAGE-A3 protein. MAGE-A3-specific CD4+ T cells were detected in all individuals tested, at low frequency in healthy donors and seronegative cancer patients and higher frequency in patients seropositive for MAGE-A3. Polyclonal expansion of CD154-expressing CD4+ T cells after cell sorting generated a large number of MAGE-A3-specific CD4+ T cell lines from all individuals tested, enabling full characterization of peptide specificity, HLA-restriction, and avidity. Application of this method to cancer patients vaccinated with MAGE-A3 protein with or without adjuvant revealed that protein vaccination induced oligoclonal activation of MAGE-A3-specific CD4+ T cells. It appeared that MAGE-A3 protein vaccination in the presence of adjuvant selectively expanded high avidity CD4+ T cells, whereas high avidity T cells disappeared after multiple vaccinations with MAGE-A3 protein alone.

Lipopolysaccharide potentiates effector T cell accumulation into nonlymphoid tissues through TRIF

LPS is a natural adjuvant that potentiates Ag-specific T cell survival and Th1 differentiation by stimulating MyD88 and Toll/IL-1R domain-containing adaptor-inducing IFN-beta (TRIF) signaling pathways. In this study, we reveal the TRIF pathway is critical for amplifying murine effector T cell accumulation into nonlymphoid tissues following immunization with Ag plus LPS. Although LPS increased the accumulation of splenic T cells in TRIF-deficient mice, markedly fewer T cells were recovered from liver and lung in comparison to wild type. Most of the T cells primed in TRIF-deficient mice failed to up-regulate CXCR3 and had an overall reduced capacity to produce IFN-gamma, demonstrating effector T cell differentiation was linked to their migration. To investigate the role of TRIF-dependent cytokines, neutralization studies were performed in wild type mice. Although TNF neutralization reduced T cell numbers, its coneutralization with IL-10 unexpectedly restored the T cells, suggesting the balance between pro- and anti-inflammatory cytokines influences T cell survival rather than their magnitude. To investigate a role for costimulatory molecules, we tested whether the T cell defect in TRIF-deficient mice could be corrected with enforced costimulation. Boosting with a CD40 agonist in addition to LPS restored the effector CD8 T cell response in livers of TRIF-deficient mice while only partially restoring CD4 T cells, suggesting that LPS primes CD8 and CD4 T cell immunity through different mechanisms. Overall, our data support targeting TRIF for vaccines aimed to direct immune responses to nonlymphoid tissues.

Understanding how lipopolysaccharide impacts CD4 T-cell immunity

Lipopolysaccharide (LPS) is a natural adjuvant synthesized by gram-negative bacteria that has profound effects on CD4 T-cell responses. LPS stimulates cells through the Toll-like receptor 4 (TLR4), causing the release of inflammatory cytokines and upregulation of costimulatory molecules on antigen-presenting cells (APCs). The combination of signals from antigens, costimulation, and cytokines allows CD4 T cells to overcome suppressive barriers and accumulate in large numbers. T cells that are primed in an LPS-stimulated environment are programmed for long-term survival following clonal expansion. LPS is well-known for generating Th1 responses. However, under appropriate conditions it can also support differentiation into other T-helper lineages, demonstrating its pleiotropic nature. Although molecular analyses have provided insights into how immune responses are controlled by LPS in vivo, its powerful adjuvant activity is also associated with toxicity. Research on partial TLR4 agonists such as monophosphoryl lipid A have demonstrated that toxicity and immunogenicity are not always linked, making them useful candidates for human vaccines. In this sense, many years of LPS research have ultimately contributed to vaccine design, and the next generation may involve studying how the balance between different CD4 T-cell subsets is controlled.

Endogenous naive CD8+ T cell precursor frequency regulates primary and memory responses to infection

Through genetic recombination, the adaptive immune system generates a diverse T cell repertoire allowing recognition of a vast spectrum of foreign antigens. Any given CD8+ T cell specificity is thought to be rare, but none have been directly quantified. Here, major histocompatibility complex tetramer and magnetic-bead technology were coupled to quantitate naive antigen-specific CD8+ T cells and the early response to infection. Among six specificities measured, the number of naive antigen-specific precursors ranged from approximately 80 to 1200 cells/mouse. After vesicular stomatitis virus infection, the antigen-specific CD8+ T cell response occurred in discrete phases: prolonged activation of a subset of cells over the first 72 hr followed by a rapid proliferative burst. Naive precursor frequency altered response kinetics and regulated immunodominance, as well as the time required for the responding population to shift toward CD62L(hi) memory cells. Thus, initial endogenous precursor frequencies were surprisingly diverse and not only regulated initial immune response characteristics but also controlled memory CD8+ T cell lineage decisions.

Tentative T cells: memory cells are quick to respond, but slow to divide

T cell memory is a cornerstone of protective immunity, and is the key element in successful vaccination. Upon encountering the relevant pathogen, memory T cells are thought to initiate cell division much more rapidly than their naïve counterparts, and this is thought to confer a significant biological advantage upon an immune host. Here, we use traceable TCR-transgenic T cells to evaluate this proposed characteristic in CD4+ and CD8+ memory T cells. We find that, even in the presence of abundant antigen that was sufficient to induce in vivo IFNgamma production by memory T cells, both memory and naïve T cells show an extended, and indistinguishable, delay in the onset of proliferation. Although memory cells can detect, and respond to, virus infection within a few hours, their proliferation did not begin until approximately 3 days after infection, and occurred simultaneously in all anatomical compartments. Thereafter, cell division was extraordinarily rapid for both naïve and memory cells, with the latter showing a somewhat accelerated accumulation. We propose that, by permitting memory T cells to rapidly exert their effector functions while delaying the onset of their proliferation, evolution has provided a safeguard that balances the risk of infection against the consequences of severe T cell-mediated immunopathology.

The lipopolysaccharide adjuvant effect on T cells relies on nonoverlapping contributions from the MyD88 pathway and CD11c+ cells

Bacterial LPS is a natural adjuvant that induces profound effects on T cell clonal expansion, effector differentiation, and long-term T cell survival. In this study, we delineate the in vivo mechanism of LPS action by pinpointing a role for MyD88 and CD11c(+) cells. LPS induced long-term survival of superantigen-stimulated CD4 and CD8 T cells in a MyD88-dependent manner. By tracing peptide-stimulated CD4 T cells after adoptive transfer, we showed that for LPS to mediate T cell survival, the recipient mice were required to express MyD88. Even when peptide-specific CD4 T cell clonal expansion was dramatically boosted by enforced OX40 costimulation, OX40 only synergized with LPS to induce survival when the recipient mice expressed MyD88. Nevertheless, these activated, but moribund, T cells in the MyD88(-/-) mice acquired effector properties, such as the ability to synthesize IFN-gamma, demonstrating that effector differentiation is not automatically coupled to a survival program. We confirmed this notion in reverse fashion by showing that effector differentiation was not required for the induction of T cell survival. Hence, depletion of CD11c(+) cells did not affect LPS-driven specific T cell survival, but CD11c(+) cells were paramount for optimal effector T cell differentiation as measured by IFN-gamma potential. Thus, LPS adjuvanticity is based on MyD88 promoting T cell survival, while CD11c(+) cells support effector T cell differentiation.

Kupffer cell heterogeneity: functional properties of bone marrow derived and sessile hepatic macrophages

Kupffer cells form a large intravascular macrophage bed in the liver sinusoids. The differentiation history and diversity of Kupffer cells is disputed; some studies argue that they are derived from blood monocytes, whereas others support a local origin from intrahepatic precursor cells. In the present study, we used both flow cytometry and immunohistochemistry to distinguish 2 subsets of Kupffer cells that were revealed in the context both of bone marrow transplantation and of orthotopic liver transplantation. One subset was radiosensitive and rapidly replaced from hematogenous precursors, whereas the other was relatively radioresistant and long-lived. Both were phagocytic but only the former population was recruited into inflammatory foci in response to CD8(+) T-cell activation. We propose the name "sessile" for the radioresistant Kupffer cells that do not participate in immunoinflammatory reactions. However, we found no evidence that these sessile Kupffer cells arise from immature intrahepatic precursors. Our conclusions resolve a long-standing controversy and explain how different experimental approaches may reveal one or both of these subsets.

Immunization with an apoptotic cell-binding protein recapitulates the nephritis and sequential autoantibody emergence of systemic lupus erythematosus

The initial events predisposing to loss of tolerance in patients with systemic lupus erythematosus (SLE) are largely unknown, as are the events that precipitate the transition from preclinical to overt disease. We hypothesized that induction of murine SLE would require tipping the balance between tolerance and immunity in two ways: 1) an immunogen that could take advantage of apoptotic cells as a scaffold for epitope spread, and 2) an immune activator that would generate a strong and persistent T cell response to the inciting immunogen. We show that immunization of C57BL/6 and BALB/c mice with human beta(2)-glycoprotein I, an apoptotic cell-binding protein, in the presence of LPS induces a long-lived, potent response to beta(2)-glycoprotein I that results in epitope spread to multiple SLE autoantigens. SLE-specific autoantibodies emerged in a sequential manner that recapitulated the order seen in human SLE. Moreover, immunized mice developed overt glomerulonephritis closely resembling human lupus nephritis.

Cutting edge: Paracrine, but not autocrine, IL-2 signaling is sustained during early antiviral CD4 T cell response

IL-2 is expressed predominantly by activated T cells, and regulates T cell function by activating, via its receptor, the latent transcription factor STAT5. This signaling can occur in either a paracrine (between cells) or an autocrine (same cell) manner, although the kinetics by which these two signaling modes operate during in vivo T cell responses are unknown. In the current study, IL-2 expression and signaling in a clonotypic population of antiviral CD4+ T cells was analyzed by flow cytometry during the initial 24 h of priming. IL-2 expression and STAT5 activation peaked in parallel, but surprisingly, were almost completely mutually exclusive. Thus, only paracrine IL-2 signaling could be observed. As an additional indication of the efficiency of paracrine IL-2 signaling, polyclonal CD4+CD25+Foxp3+ regulatory T cells displayed detectable STAT5 activation under steady-state conditions, which was strongly enhanced by neighboring IL-2-expressing antiviral CD4 cells.

A role for IFNgamma in differential superantigen stimulation of conventional versus plasmacytoid DCs

Superantigens (SAgs) are known to play a role in food poisoning, toxic shock syndrome and have been identified as a potential mediator of autoimmunity. Although much is known about the effects of SAgs on T cells, by comparison few studies have investigated how SAgs influence innate immune cells. In particular no study has examined how SAgs affect murine plasmacytoid dendritic cells (pDC). We report that in vivo administration of staphylococcal enterotoxin A (SEA) increased the number of pDCs in secondary lymphoid organs, and induced CD86 and CD40 expression. Similar to SEA activation of conventional DCs (cDCs), pDCs relied on T cells, but not on CD40. Nonetheless, pDCs strictly required IFNgamma for upregulation of CD86 and CD40, but cDCs did not depend upon IFNgamma for activation. Further, even though IFNgamma deficient pDCs were not activated by SEA, they were still capable of producing wild-type levels of IFNalpha in response to CpG oligodeoxynucleotide (ODN). The source of IFNgamma for pDC activation was not T cells, nor did pDCs themselves have to synthesize or bind IFNgamma, but the presence of IFNgamma was essential. After SEA stimulation, IFNgamma deficient mice fail to induce expression of the pDC dependent chemokines CXCL9, and demonstrated a defect in recruitment of pDCs to marginal zones of lymphoid organs. Thus, SEA exerts its combined effect on pDC activation, recruitment and chemokine induction through the action of IFNgamma. This fundamental dichotomy of the effects of SAgs on pDCs versus cDCs show how a non-PAMP from bacteria, can selectively and indirectly stimulate innate cell subpopulations much in the same way that differential TLR expression influences cells of the innate immune system.

T cell conditioning explains early disappearance of the memory CD8 T cell response to infection

Memory CD8 T cells respond more rapidly to acute intracellular infections than naive CD8 T cells. An understanding of the biological processes involved in memory CD8 T cell recognition of Ag and up-regulation of effector mechanism necessitates analyzing memory CD8 T cells at early time points after infection. In the current study, we show that memory CD8 T cells ostensibly disappear from the spleens, blood, and peripheral organs of mice early after infection with Listeria monocytogenes. This disappearance is critically dependent on Ag, and cell-associated Ag alone can mediate this phenomenon. Further investigations, however, suggest that this disappearance is secondary to T cell-APC interactions, also known as T cell conditioning, and disruption of these putative interactions during splenic processing improves recovery of Ag-specific memory CD8 T cell populations after immunization. Conventional analyses of memory CD8 T cell populations early after infection and possibly in the presence of low levels of Ag (as during chronic infections) may exclude significant numbers of the responding CD8 T cell population.

Effects of LPS-mediated bystander activation in the innate immune system

LPS induces dendritic cell (DC) activation, but the precise in vivo mechanism is unclear since DCs express low levels of TLR4. Here, it is shown that DCs can be activated in response to LPS through a bystander mechanism. This result was obtained using chimeric mice reconstituted with LPS-responsive and nonresponsive bone marrow cells. Thus, after indirect in vivo conditioning by LPS, bystander-activated DCs (LPS nonresponsive) up-regulated CD86. This up-regulation occurred even when LPS-responsive cells were MyD88 deficient. Functional analysis demonstrated that in vivo LPS conditioning endowed both the LPS-responsive and bystander cells with the ability to produce IFN-gamma in response to TLR9 stimulation in vitro. IFN-gamma production was also shown to be important for enhanced T-bet gene expression but not important for up-regulation of CD86. To investigate aspects of the mechanism, we used intracellular cytokine staining and found that NKDCs were responsible for at least some of the IFN-gamma production. Thus, our in vivo results demonstrated that bacterial LPS can bridge activation of various cellular populations of the innate immune system through a bystander mechanism.

Generation and maintenance of human memory cells during viral infection

Long-term maintenance of memory T cell response is the hallmark of immune protection and hence the holy grail of most vaccine development studies. Persistent memory cells, developed after either viral infection or vaccination, ensure the generation of an antiviral response upon reexposure to the pathogen. During acute viral infections, as in the case of measles and influenza viruses, strong T cell effector functions, which eradicate the virus and protect patients against reexposure, are achieved by the generation of persistent protective memory cells. However, in chronic infections, T cells drastically lose effector functions before acquiring a memory phenotype. Chronic infections can be categorized into infections where viremia is controlled and protective memory cells are maintained as in the case of EBV and CMV infections, or where the virus persists and memory cells are exhausted and disrupted as in the case of human immunodeficiency virus infection. In this review, we will discuss the different phenotypical and functional characteristics of memory cells subsets, the importance of the role they play during acute and chronic infections, and the mechanisms behind their effectiveness and persistence.

IL-18 bridges innate and adaptive immunity through IFN-gamma and the CD134 pathway

IL-18 induces inflammation resulting in either enhanced protection from pathogens or exacerbation of autoimmunity, and T cells are profoundly activated during these responses. How IL-18 influences T cell activation is unknown, but this study in mice shows that IL-18 boosted Ag-specific T cell clonal expansion of effector T cells and induced a subpopulation of IFN-gamma superproducing T cells. Commitment to IFN-gamma production through IL-18 was independent of NK cells and IL-12 but dependent on host-derived IFN-gamma. To determine how expansion of these effectors occurred, IL-18 was shown to induce OX40L on dendritic cells, whereas peptide stimulation induced CD134 (OX40) on specific T cells. CD134 blockade inhibited T cell effector expansion thereby reducing the number of IFN-gamma superproducers by 12-fold. Thus, independent of IL-12, IL-18 impacts T cell immunity throughout lymphoid and nonlymphoid tissue by bridging the innate and adaptive arms of the immune system through IFN-gamma and the CD134 costimulatory pathway.

Precursor frequency, nonlinear proliferation, and functional maturation of virus-specific CD4+ T cells

The early events regulating antiviral CD4 responses were tracked using an adoptive transfer model. CD4+ T cell expansion was nonlinear, with a lengthy lag phase followed by 2 days of explosive proliferation. A small number of naive Ag-specific CD4+ T cells were found in nonlymphoid tissues and, in the 8 days following infection, the number of activated cells increased in all tissues analyzed, and their effector functions matured. Finally, we show that a naive mouse contains approximately 100 naive CD4+ precursor cells specific for a single epitope, a precursor frequency of approximately 10(-5), similar to that of naive CD8+ T cells, indicating that the approximately 50-fold difference in size of the two responses to virus infection is determined by something other than the number of precursor cells.

Patients, Pathogens, and Protective Immunity: The Relevance of Virus-Induced Alloreactivity in Transplantation

Successful transplantation requires the establishment of an ongoing state in which there is simultaneous inhibition of the undesired T cell-dependent rejection response and yet retention of the ability to develop effective cell-mediated primary and memory responses to pathogens. The complexity of attaining such a precarious state is underscored by the growing body of evidence that alloreactivity can be profoundly influenced by infections that occur before, concurrent with, or subsequent to an organ transplant. In this review, we explore the growing list of mechanisms that have been identified by which pathogen-host interactions might influence rejection, including the degeneracy of TCR recognition leading to cross-reactive immune responses, the effects of pathogens on innate immune mechanisms, and the potential impact of virally induced lymphopenia.

Francisella tularensis induces aberrant activation of pulmonary dendritic cells

Francisella tularensis is an obligate intracellular bacterium that induces severe, acute, often fatal disease when acquired by the respiratory route. Despite the seriousness of this pathogen, very little is understood about its interaction with key target cells in the airways and lungs (alveolar macrophages and airway dendritic cells (DC)) after inhalation. In this study we demonstrate replication of F. tularensis in primary DC. Early after infection, F. tularensis induced increased expression of MHC class II and CD86 on DC, but not macrophages. This was followed by depletion of DC from the airways and lungs. Despite logarithmic replication and phenotypic maturation of DC, F. tularensis failed to induce production of several key proinflammatory cytokines, including TNF-alpha and IL-6, from DC. However, F. tularensis infection did elicit production of the potent immunosuppressive cytokine, TGF-beta. Furthermore, F. tularensis actively suppressed the ability of DC to secrete cytokines in response to specific TLR agonists. Finally, we also found that infection of DC and macrophages in the lungs appears to actually increase the severity of pulmonary infection with F. tularensis. For example, depletion of airway DC and alveolar macrophages before infection resulted in significantly prolonged survival times. Together, these data suggest F. tularensis is able to selectively uncouple Ag-presenting functions from proinflammatory cytokine secretion by critical APCs in the lungs, which may serve to create a relatively immunosuppressive environment favorable to replication and dissemination of the organism.

Early interaction of Yersinia pestis with APCs in the lung

Despite the importance of pneumonic plague, little is known of the early pulmonary immune responses that occur following inhalation of Yersinia pestis. Therefore, we conducted studies to identify the early target cells for uptake of Y. pestis in the lungs following intratracheal or i.v. inoculation. Following intratracheal inoculation, Y. pestis was rapidly internalized primarily by a distinctive population of CD11c+DEC-205+CD11b- cells in the airways, whereas i.v. inoculation resulted in uptake primarily by CD11b+CD11c- macrophages and granulocytes in lung tissues. The airway cells internalized and were infected by Y. pestis, but did not support active replication of the organism. Intratracheal inoculation of Y. pestis resulted in rapid activation of airway CD11c+ cells, followed within 24 h by the selective disappearance of these cells from the airways and lungs and the accumulation of apoptotic CD11c+ cells in draining lymph nodes. When CD11c+ cells in the airways were depleted using liposomal clodronate before infection, this resulted in a significantly increased replication of Y. pestis in the lungs and dissemination to the spleen and draining lymph nodes. These findings suggest that CD11c+ cells in the airways play an important role in suppressing the initial replication and dissemination of inhaled Y. pestis, although these results will also require confirmation using fully virulent strains of Y. pestis. Depletion of these airway cells by Y. pestis may therefore be one strategy the organism uses to overcome pulmonary defenses following inhalation of the organism.

Between adaptive and innate immunity: TLR4-mediated perforin production by CD28null T-helper cells in ankylosing spondylitis

CD3+CD4+CD28null and CD3+CD8+CD28null T cells are enriched in patients with immune-mediated diseases compared with healthy controls. This study shows that CD4+CD28null T cells express Toll-like receptors recognizing bacterial lipopolysaccharides in ankylosing spondylitis, psoriatic arthritis and rheumatoid arthritis. In ankylosing spondylitis, TLR4 (23.1 +/- 21.9%) and, to a smaller extent, TLR2 (4.1 +/- 5.8%) were expressed on CD4+CD28null T cells, whereas expression was negligible on CD4+CD28+ and CD8+ T cells. CD4+CD28null T cells produced perforin upon stimulation with lipopolysaccharide, and this effect was enhanced by autologous serum or recombinant soluble CD14. Perforin production could be prevented with blocking antibodies directed against CD14 or TLR4. Incubation of peripheral blood mononuclear cells with tumour necrosis factor alpha led to an upregulation of TLR4 and TLR2 on CD4+CD28null T cells in vitro, and treatment of patients with antibodies specifically directed against tumour necrosis factor alpha resulted in decreased expression of TLR4 and TLR2 on CD4+CD28null T cells in vivo. We describe here a new pathway for direct activation of cytotoxic CD4+ T cells by components of infectious pathogens. This finding supports the hypothesis that CD4+CD28null T cells represent an immunological link between the innate immune system and the adaptive immune system.

In situ characterization of CD4+ T cell behavior in mucosal and systemic lymphoid tissues during the induction of oral priming and tolerance

The behavior of antigen-specific CD4⁺ T lymphocytes during initial exposure to antigen probably influences their decision to become primed or tolerized, but this has not been examined directly in vivo. We have therefore tracked such cells in real time, in situ during the induction of oral priming versus oral tolerance. There were marked contrasts with respect to rate and type of movement and clustering between naive T cells and those exposed to antigen in immunogenic or tolerogenic forms. However, the major difference when comparing tolerized and primed T cells was that the latter formed larger and longer-lived clusters within mucosal and peripheral lymph nodes. This is the first comparison of the behavior of antigen-specific CD4⁺ T cells in situ in mucosal and systemic lymphoid tissues during the induction of priming versus tolerance in a physiologically relevant model in vivo.

Frontline: An in-depth evaluation of the production of IL-2 by antigen-specific CD8 T cells in vivo

IL-2 is an important cytokine that is capable of inducing both proliferation and apoptosis of activated T cells. CD4 T cells are thought to be the major producers of IL-2, but CD8 T cells also produce copious amounts of this cytokine. However, our current understanding regarding the kinetics of IL-2 production by antigen-specific CD8 T cells, and the proportion of these cells that produce IL-2 in vivo, is extremely limited. We now demonstrate that virus-specific CD8 T cells initiate IL-2 production by 6 h post-infection and prior to cell division in vivo. Interestingly, peak levels of IL-2 production were achieved very early during the response and prior to the proliferative peak. We also show -- using transgenic mice expressing herpes simplex virus-1 thymidine kinase under the control of the IL-2 promoter -- that, unlike what has been reported for antigen-specific CD4 T cells, the majority of antigen-specific CD8 T cells produce IL-2 during primary as well as secondary immune responses in vivo.