On T cell memory: arguments for antigen dependence

Approach to the Adult Acute Lymphoblastic Leukemia Patient

During recent decades, understanding of the molecular mechanisms of acute lymphoblastic leukemia (ALL) has improved considerably, resulting in better risk stratification of patients and increased survival rates. Age, white blood cell count (WBC), and specific genetic abnormalities are the most important factors that define risk groups for ALL. State-of-the-art diagnosis of ALL requires cytological and cytogenetical analyses, as well as flow cytometry and high-throughput sequencing assays. An important aspect in the diagnostic characterization of patients with ALL is the identification of the Philadelphia (Ph) chromosome, which warrants the addition of tyrosine kinase inhibitors (TKI) to the chemotherapy backbone. Data that support the benefit of hematopoietic stem cell transplantation (HSCT) in high risk patient subsets or in late relapse patients are still questioned and have yet to be determined conclusive. This article presents the newly published data in ALL workup and treatment, putting it into perspective for the attending physician in hematology and oncology.

Have Cells Harboring the HIV Reservoir Been Immunoedited?

Immunoediting is an important concept in oncology, delineating the mechanisms through which tumors are selected for resistance to immune-mediated elimination. The recent emergence of immunotherapies, such as checkpoint inhibitors, as pillars of cancer therapy has intensified interest in immunoediting as a constraint limiting the efficacy of these approaches. Immunoediting manifests at a number of levels for different cancers, for example through the establishment of immunosuppressive microenvironments within solid tumors. Of particular interest to the current review, selection also occurs at the cellular level; and recent studies have revealed novel mechanisms by which tumor cells acquire intrinsic resistance to immune recognition and elimination. While the selection of escape mutations in viral epitopes by HIV-specific T cells, which is a hallmark of chronic HIV infection, can be considered a form of immunoediting, few studies have considered the possibility that HIV-infected cells themselves may parallel tumors in having differential intrinsic susceptibilities to immune-mediated elimination. Such selection, on the level of an infected cell, may not play a significant role in untreated HIV, where infection is propagated by high levels of cell-free virus produced by cells that quickly succumb to viral cytopathicity. However, it may play an unappreciated role in individuals treated with effective antiretroviral therapy where viral replication is abrogated. In this context, an "HIV reservoir" persists, comprising long-lived infected cells which undergo extensive and dynamic clonal expansion. The ability of these cells to persist in infected individuals has generally been attributed to viral latency, thought to render them invisible to immune recognition, and/or to their compartmentalization in anatomical sites that are poorly accessible to immune effectors. Recent data from ex vivo studies have led us to propose that reservoir-harboring cells may additionally have been selected for intrinsic resistance to CD8+ T cells, limiting their elimination even in the context of antigen expression. Here, we draw on knowledge from tumor immunoediting to discuss potential mechanisms by which clones of HIV reservoir-harboring cells may resist elimination by CD8+ T cells. The establishment of such parallels may provide a premise for testing therapeutics designed to sensitize tumor cells to immune-mediated elimination as novel approaches aimed at curing HIV infection.

Analysis of adenovirus-induced immunity to infection with Listeria monocytogenes: Fading protection coincides with declining CD8 T cell numbers and phenotypic changes

Defining correlates of T cell mediated protection is important in order to accelerate the development of efficient T cell based vaccines conferring long-term immunity. Extensive studies have provided important insight regarding the characteristics and functional properties of the effector and memory CD8 T cells induced by viral vector based vaccines. However, long-term protection has been difficult to achieve with T cell inducing vaccines, and the determinants underlying this loss in protection over time are still not fully defined. In this study we analyzed different parameters of the CD8 T cell response as a function of time after vaccination with a human serotype 5 adenovector expressing the glycoprotein (GP) of LCMV tethered to the MHC class II-associated invariant chain. Using this vector we have previously found that CD8 T cells mediate protection from challenge with GP-expressing Listeria monocytogenes at 60 days post vaccination, but only little protection after further 60 days, and we now confirm this observation. A comparison of vaccine-primed CD8 T cells early and late after vaccination revealed a minor decline in the overall numbers of antigen specific memory CD8 T cells during this interval. More importantly, we also observed phenotypic changes over time with a distinct decline in the frequency and number of KLRG1⁺ CD8 T cells, and, notably, adoptive transfer studies confirmed that memory CD8 T cells expressing KLRG1 are central to protection from systemic L. monocytogenes infection. Together these findings imply that multiple factors including changes in memory T cell numbers and phenotypic composition over time influence the longevity of CD8 T-cell mediated protection.

Combined local and systemic immunization is essential for durable T-cell mediated heterosubtypic immunity against influenza A virus

The threat from unpredictable influenza virus pandemics necessitates the development of a new type of influenza vaccine. Since the internal proteins are highly conserved, induction of T cells targeting these antigens may provide the solution. Indeed, adenoviral (Ad) vectors expressing flu nucleoprotein have previously been found to induce short-term protection in mice. In this study we confirm that systemic (subcutaneous (s.c.) immunization rapidly induced heterosubtypic protection predominantly mediated by CD8 T cells, but within three months clinical protection completely disappeared. Local (intranasal (i.n.)) immunization elicited delayed, but more lasting protection despite relatively inefficient immunization. However, by far, the most robust protection was induced by simultaneous, combined (i.n. + s.c.) vaccination, and, notably, in this case clinical protection lasted at least 8 months without showing any evidence of fading. Interestingly, the superior ability of the latter group to resist reinfection correlated with a higher number of antigen-specific CD8 T cells in the spleen. Thus, detailed analysis of the underlying CD8 T cell responses highlights the importance of T cells already positioned in the lungs prior to challenge, but at the same time underscores an important back-up role for circulating antigen-specific cells with the capacity to expand and infiltrate the infected lungs.

Generation of Adaptive Immune Responses Following Influenza Virus Challenge is Not Compromised by Pre-Treatment with the TLR-2 Agonist Pam2Cys

Immunostimulatory agents provide a new category of anti-microbial agents that activate the host’s innate immune system allowing control of viral and/or bacterial infections. The TLR-2 agonist PEG-Pam₂Cys has been shown to mediate potent anti-viral activity against influenza viruses when administered prophylactically (1). Here, we demonstrate that the treatment of mice with PEG-Pam₂Cys does not compromise their ability to generate adaptive immune responses following subsequent challenge with influenza virus. The antibody induced in mice pre-treated with Pam₂Cys possessed hemagglutination-inhibiting activities and the CD8⁺ T-cell responses that were elicited provided protection against heterologous viral challenge. In the absence of an effective influenza vaccine, an agent that provides immediate protection against the virus and does not compromise the induction of influenza-specific immunity on exposure to infectious virus provides an opportunity for population immunity to be achieved through natural exposure to virus.

Gene expression profiles of T cells from hepatitis E virus infected patients in acute and resolving phase

BACKGROUND AND AIMS

Approximately 50% of acute viral hepatitis in young adults and in pregnant women is due to hepatitis E virus (HEV) infection in developing countries. T cell-mediated immune injury probably plays a key role in the pathogenesis of acute hepatitis illness. However, there is a paucity of data on the global gene expression programs activated on T cells, which are subsequently responsible for T cell recruitment to the liver and triggering of immune injury.

PATIENTS AND METHODS

We performed a flow cytometric analysis of T cells in individuals with acute hepatitis E (AVH-E; n=10), resolving phase of HEV (n=9), and ten healthy controls (HC). Further transcriptional profiling analysis was performed using Affymetrix GeneChip DNA microarrays to identify the genes that were differentially expressed in AVH-E and HC.

RESULTS

Patients with AVH-E showed higher frequencies of CD8+ (27 ± 4%; P=0.02) and activated CD38+ CD69+ T cells (25% ± 3%; P=0.04) than in resolving phase patients (20 ± 2% and 9.1 ± 4%, respectively), who in turn exhibited higher CCR9 expression than cells from patients in active phase. The naïve T cell population (CD3+ CD45RA+) was decreased upon HEV infection (29 ± 4% in AVH-E vs. 53.1 ± 3.2% in HC; P=0.05); however, the CD11a high subpopulation within CD4+ CD45RA+ cells was increased in both AVH-E (6.1%) and resolving phase (7.7%) patients. Gene ontology analysis suggested that during AVH-E infection, there is in CD4+ T cells an activation of genes involved in pro-inflammatory responses. Additional RT-PCR analysis confirmed that in cells from AVH-E patients, there is an increased expression of CCR5, CCR9, CXCR3, CXCR4, STAT1, IRF-9, IFN-α, and TNF-α, together with a down-regulation of IL-2, SOCS3, and IL-10, with respect to cells from resolving phase patients.

CONCLUSIONS

Our findings suggest the involvement of a circulating CD45RA+ CD11a high population with CCR5 expression in the pathogenesis processes of AVH-E. The obtained results help to understand the underlying inflammatory process occurring in HEV infection, which can lead to either resolution or immunopathology.

Principles of memory CD8 T-cells generation in relation to protective immunity

Memory T-cell responses are of vital importance in understanding the host's response against pathogens and cancer cells and to begin establishing the correlation of protection against disease. In this review, we discuss our own data in the general context of current knowledge to sketch tentative working principles for the induction of protective T-cell responses by vaccination. We draw attention to quantitative and qualitative aspects of the initial contact with antigen, as well as to the kinetics of events leading to the generation of memory T cells thereafter. Our arguments are based on the current distinction of memory T cells into two lineages: effector memory T cells (T(EM)) and central memory T cells (T(CM)). Our provisional conclusion is that protective T-cell responses correlate positively with the T cells of the central memory phenotype. In proposing a set of working principles to enable protective memory T cells by vaccination we address vaccination both in the context of the immunologically-inexperienced and immunologically-experienced individual, respectively. Finally, we draw attention to the interplay between systemic and local immunity as important factors in determining the success of memory T-cell responses in protecting the individual. We believe that considerations on the immunodynamics of memory induction and maintenance, memory lineage differentiation and their relation to protection may help design strategies to control disease caused by pathogens and cancer.

Evaluation of albumin microspheres as oral delivery system for Mycobacterium tuberculosis vaccines

Mucosal immunization has been suggested to be the best option for preventing Mycobacterium tuberculosis infection. The purpose of this study was to develop albumin microspheres containing Mycobacterium tuberculosis antigens and to determine if oral administration of the microspheres can induce antigen-specific mucosal and systemic immune responses. Albumin microspheres containing Mycobacterium tuberculosis dead cells and cell lysate were prepared. The physico-chemical characteristics of the formulations were determined and the microspheres were administered to animal models to evaluate the induction of immune responses to the antigens. The results showed that the particle sizes, zeta potential and dissolution pattern of the microspheres were ideal for oral delivery of vaccines. In vivo studies showed high production of antigen-specific antibody production in serum, nasal, salivary and faecal samples. From the results of the study, it can be concluded that oral administration of Mycobacterium tuberculosis microspheres was successful in inducing antigen-specific systemic and mucosal immune responses.

A simple model of pathogen-immune dynamics including specific and non-specific immunity

We present and analyze a model for the dynamics of the interactions between a pathogen and its host's immune response. The model consists of two differential equations, one for pathogen load, the other one for an index of specific immunity. Differently from other simple models in the literature, this model exhibits, according to the hosts' or pathogen's parameter values, or to the initial infection size, a rich repertoire of behaviours: immediate clearing of the pathogen through aspecific immune response; or acute infection followed by clearing of the pathogen through specific immune response; or uncontrolled infections; or acute infection followed by convergence to a stable state of chronic infection; or periodic solutions with intermittent acute infections. The model can also mimic some features of immune response after vaccination. This model could be a basis on which to build epidemic models including immunological features.

Antigen and memory CD8 T cells: were they both right?

Picture yourself as a researcher in immunology. To begin your project, you ask a question: Do CD8 T cells require antigen to maintain a memory response? This question is of prime importance to numerous medical fields. In chronologic order, you digest the literature, but unfortunately, you hit a major stumbling block in the 1990s. The crux of the problem is that which so often happens in science: two well-recognized, capable groups emerge with diametrically opposed conclusions, leaving you pondering which set of wellcontrolled data to believe. Fortunately, years later, a surprising group of articles sheds light on this mystery and subtly reconciles these two positions.

Lymphoid reservoirs of antigen-specific memory T helper cells

How vaccines control the development of antigen-specific effector and memory T helper cells is central to protective immunity but remains poorly understood. Here we found that protein vaccination selected high-affinity, CXCR5+ICOS(hi) follicular B-helper T cells (T(FH) cells) that developed in draining lymphoid tissue to regulate B cell responses. In the memory phase, reservoirs of antigen-specific CXCR5+ICOS(lo) T(FH) cells persisted with less effector activity but accelerated antigen-recall ability. This new compartment of memory T(FH) cells was retained in draining lymphoid sites with antigen-specific memory B cells, persistent complexes of peptide and major histocompatibility complex class II and continued expression of CD69. Thus, protein vaccination promotes B cell immunity by selecting high-affinity effector T(FH) cells and creating lymphoid reservoirs of antigen-specific memory T(FH) cells in vivo.

Differential susceptibility of leukocyte subsets to cytotoxic T cell killing: Implications for HIV immunopathogenesis

BACKGROUND

Cytotoxic T lymphocytes (CTL) are crucial for the host defense against viral infection. In many cases, this anti-viral immune response contributes to host pathogenesis, through inflammation and tissue destruction. Few studies have explored the relative susceptibility of infected cells to CTL killing, and the range of cell types that may be effectively killed by CTLs in vivo, both of which are key to understanding both immune control of infection and immune-related pathogenesis.

METHODS

We developed and optimized a highly sensitive method to quantify the relative susceptibility of leukocyte subsets to CTL-mediated killing. Maximal sensitivity was achieved by uniquely measuring cell death occurring during the assay culture.

RESULTS

We found that leukocyte subsets have a wide range of susceptibility to antigen-specific CTL-mediated lysis. Generally, T cells were more susceptible than B or NK cells, with CD4 T cells being more susceptible than CD8 T cells. In all lymphocyte lineages, susceptibility was greater for more differentiated subsets compared with their naïve counterparts; however, for dendritic cells, immature cells are more susceptible than mature cells. We focused on the susceptibility of T cell subsets, and found that naïve cells are far more resistant than memory cells, and in particular, CCR5+ or HLA-DR+ memory cells are highly susceptible to CTL-mediated killing.

CONCLUSIONS

These results provide an explanation for the observation that certain subsets of CD4 T cells are ablated during chronic HIV infection, and indicate which subsets are most likely to contain the persistent viral reservoir.

Heat denaturation, a simple method to improve the immunotherapeutic potential of allergens

Allergen-specific immunotherapy (SIT) leads to a long-term amelioration of IgE- and Th2-mediated allergic diseases. However, SIT efficiency is low, with years of treatment along with frequent allergic side effects. The goal of this study was to reduce the side effects by destroying IgE-binding epitopes, i.e. by heat-denaturation, while preserving the therapeutic effect. Mice were immunised with bee venom, birch pollen, grass pollen or cat hair allergens, or with ovalbumin. Heat-denatured allergens bound less IgE but enhanced Th1-dependent IgG2a production as measured by ELISA. The strong IgG2a antibody responses also prevented allergic anaphylaxis in mice, as measured by body temperature drop after a challenge with a high allergen dose. We found that optimal heat-denaturation of allergens left a small proportion in the native conformation to sufficiently stimulate B cells, while non-B cell-mediated effects were probably amplified. The enhanced immunogenicity of heat-denatured allergens is likely explained by enhanced antigen presentation to T cells due to the particulate nature of heat-denatured proteins. This enables Th1 skewing of the immune response with strong production of IgG2a in mice. Therefore, heat-denaturation represents probably the simplest way to enhance the efficiency of SIT while reducing its side effects.

Potent protective cellular immune responses generated by a DNA vaccine encoding HSV-2 ICP27 and the E. coli heat labile enterotoxin

A mouse model was employed to evaluate protective cellular immune responses induced by an immediate early antigen of HSV-2. Particle-mediated DNA vaccination of mice with a DNA plasmid-encoding ICP27 resulted in the induction of ICP27-specific IFN-gamma and TNF-alpha production in Balb/c mice, but little protection to intranasal challenge with wild type HSV-2. However, when the DNA vaccine was supplemented with as little as 50ng of a vector encoding the A and B subunits of the Escherichia coli heat labile enterotoxin (LT), animals were profoundly protected from morbidity and mortality. The ICP27+LT-mediated protection was correlated with a large increase in ICP27-specific IFN-gamma and TNF-alpha production but cytokine-specific monoclonal antibody treatment at the time of challenge showed that protection was mediated predominantly by IFN-gamma. Furthermore, depletion of T cell subsets prior to infectious challenge demonstrated that removal of either CD8+ or CD4+ T cells impaired protection with CD8+ T cells appearing to play a direct effector role. These data demonstrate that augmented cellular immune responses resulting from LT vector plus antigen vector administration to the skin are biologically significant, leading to enhanced protection against mucosal pathogenic challenge.

Toll-like receptor ligands as adjuvants in allergen-specific immunotherapy

BACKGROUND

Allergen-specific immunotherapy (SIT) leads to long-term amelioration of T-helper type 2 (Th2)-mediated allergic symptoms and is therefore recommended as a first line therapy for allergies. The major disadvantage of SIT is its low efficiency, requiring treatment over years.

OBJECTIVE

In this study, we evaluated the potential of Toll-like receptor (TLR) ligands to facilitate Th1-type immune responses.

METHODS

The immunogenicity and therapeutic potential of the major bee venom allergen phospholipase A2 (PLA2) combined with various TLR ligands were tested in mice and compared with immune responses induced by conventional aluminium-based preparations.

RESULTS

Regarding total IgG against PLA2, TLR2/4-binding lipopolysaccharide and TLR3-binding polyriboinosinic polyribocytidylic (PolyI:C) were the superior adjuvants for prophylactic vaccination. However, TLR9-binding phosphorothioate-modified cytosine-guanosine-rich oligonucleotide (CpG), TLR-3-binding PolyI:C, and TLR2/6-binding peptidoglycan skewed the immune responses more towards IgG2a isotype and Th1 cytokines. Furthermore, in a therapeutic approach, CpG, PolyI:C and TLR7/8-binding 3M003 had immune modulating properties as they suppressed established IgE titres.

CONCLUSION

The potential of TLR ligands to adjuvate the immunogenicity of bee venom PLA2 and to skew the Th1-Th2 balance proved very heterogeneous. With respect to SIT, CpG, PolyI:C, and 3M003 were very promising. Hence, TLR ligands should be considered as adjuvants or immune modulators in SIT in human as to improve its efficiency regarding the Th1-Th2 balance of the immune response with a likely effect on therapy duration.

A DNA vaccine targeting Fos-related antigen 1 enhanced by IL-18 induces long-lived T-cell memory against tumor recurrence

A novel vaccination strategy induced specific CD8(+) T cell-mediated immunity that eradicated spontaneous and experimental pulmonary cancer metastases in syngeneic mice and was also effective in a therapeutic setting of established breast cancer metastases. This was achieved by targeting transcription factor Fos-related antigen 1(Fra-1), overexpressed by many tumor cells, with an ubiquitinated DNA vaccine against Fra-1, coexpressing secretory IL-18. Insight into the immunologic mechanisms involved was provided by adoptive transfer of T lymphocytes from successfully immunized BALB/c mice to syngeneic severe combined immunodeficient (SCID) mice. Specifically, long-lived T memory cells were maintained dormant in nonlymphoid tissues by IL-18 in the absence of tumor antigen. Importantly, a second tumor cell challenge of these SCID mice restored both, robust tumor-specific cytotoxicity and long-lived T-cell memory, capable of eradicating established pulmonary cancer metastases, suggesting that this vaccine could be effective against tumor recurrence.

Does programmed CTL proliferation optimize virus control?

CD8 T-cell or cytotoxic T-lymphocyte responses develop through an antigen-independent proliferation and differentiation program. This is in contrast to the previous thinking, which was that continuous antigenic stimulation was required. This Opinion discusses why nature has chosen the proliferation program and how it compares to continuous stimulation. Although the two mechanisms should not lead to significantly different dynamics during chronic infection, they do make a difference in acute infection. We argue that programmed proliferation is better at clearance, whereas continuous stimulation is better at limiting acute symptoms. The 7-10 programmed cell divisions observed in vivo might be an optimization of this trade-off. We also discuss the conditions under which the program does or does not require CD4 T-cell help for clearance.

Augmentation of post transplant immunity: antigen encounter at the time of hematopoietic stem cell transplantation enhances antigen-specific donor T-cell responses in the post transplant repertoire

After transplant, the immune system is reconstituted by cells derived from both hematopoietic stem cells and peripheral expansion from differentiated donor T cells. After transplant, immune function is poor despite transplantation of mature lymphocytes from immune-competent donors. We tested the hypothesis that early antigen encounter at the time of cell transplant would improve the desired donor T-cell responses. Two independent models of peptide-specific T-cell responses were studied. The model for CD4 cells employed T cells from transgenic (Tg) DO11.11 mice that constitutively express the T-cell receptor for the class II-restricted ovalbumin peptide 323-339. The model for CD8 cells employed non-Tg H2-Db-restricted T-cell responses to the influenza nucleoprotein peptide 366-374. As measured both functionally and by direct imaging of T cells using clonotypic reagents, encounter with specific antigen at the time of T-cell transplantation led to clonal expansion of donor T cells and preservation of donor T-cell function in the post transplant immune environment. Antigen-specific donor T-cell function was poor if antigen encounter was delayed or omitted. Severe parent>F1 graft-versus-host reactions blocked the effect of early antigen exposure. Vaccination of transplant recipients against microbial or leukemia antigens may be worthy of study.

Anti-mycobacterial immunity induced by a single injection of M. leprae Hsp65-encoding plasmid DNA in biodegradable microparticles

A single sub-cutaneous injection of a plasmid DNA encoding a mycobacterial heat shock protein 65 (Hsp65) entrapped in biodegradable poly(lactic-co-glycolic acid) microspheres produced high titers of antibodies, measured 5 months after the injection in BALB/c mice. Splenocytes secreted IFN-gamma and exerted an anti-bacterial effect on macrophages infected in vitro with Mycobacterium tuberculosis. The results are encouraging with regard to obtaining good compliance and vaccination coverage with candidate plasmid DNA vaccines, especially in developing countries.

Evolution of Immunological Memory and the Regulation of Competition between Pathogens

Memory is a central characteristic of immune responses. It is defined as an elevated number of specific immune cells that remain after resolution of infection and can protect the host against reinfection. The evolution of immunological memory is subject to debate. The advantages of memory discussed so far include protection from reinfection, control of chronic infection, and the transfer of immune function to the next generation. Mathematical models are used to identify a new force that can drive the evolution of immunological memory: the duration of memory can regulate the degree of competition between different pathogens. While a long duration of memory provides lasting protection against reinfection, it may also allow an inferior pathogen species to persist. This can be detrimental for the host if the inferior pathogen is more virulent. On the other hand, a shorter duration of memory ensures that an inferior pathogen species is excluded. This can be beneficial for the host if the inferior pathogen is more virulent. Thus, while in the absence of pathogen diversity memory is always expected to evolve to a long duration, under specific circumstances, memory can evolve toward shorter durations in the presence of pathogen diversity.

A transgenic mouse model genetically tags all activated CD8 T cells

Identifying and characterizing Ag-specific CD8+ T cells are central to the study of immunological memory. Although powerful strategies such as MHC tetramers and peptide-induced cytokine production assays exist for identifying Ag-specific CD8+ T cells, alternate strategies that are not dependent upon a priori knowledge of the immunodominant and subdominant antigenic epitopes, as well as the MHC background of the animal are of obvious utility. In this study, we present a transgenic mouse model that uses Cre-loxP recombination to permanently mark all activated CD8+ T cells with beta-galactosidase. We used the lymphocytic choriomeningitis virus infection model to track the dynamics of the antiviral CD8+ T cell responses. We show that in this transgenic mouse model system, all of the antiviral effector and memory CD8+ T cells are contained within the beta-gal-marked CD8+ T cell population.

Remembrance of antigens past: new insights into memory T cells

Memory immune responses against foreign antigens protect the host from pathogens previously encountered via illness or vaccination, yet can also contribute to the pathology of autoimmune disease when generated against self-antigens. Memory immune responses are classically attributed to the reactivation of long-lived, antigen-specific T lymphocytes that arise directly from differentiated effector T cells and persist in a uniformly quiescent state. Recent findings in both humans and mice, using new biochemical, molecular and cellular approaches, have identified novel features of memory T cells providing new insight into models for memory cell development and differentiation. Biochemical and molecular studies on memory T cells have identified novel markers for memory T cells that may play integral roles in their generation and maintenance. Recent cellular immunological studies have uncovered remarkable heterogeneity amongst antigen-specific memory T cells. Memory cell heterogeneity in the expression of homing and chemokine receptor delineates functional subsets of memory T cells that differ in their proliferative capacity, differentiation potential, homing properties and protective abilities. These findings suggest that memory T cells with diverse properties residing in both lymphoid and nonlymphoid tissues may be necessary to elicit a rapid and effective protective recall immune response involving both cellular and humoral immunity.

Phase I study of intranodal delivery of a plasmid DNA vaccine for patients with Stage IV melanoma

BACKGROUND

Based on the likelihood of transfecting large numbers of local antigen-presenting cells, a Phase I study in patients with Stage IV melanoma was conducted to determine the practicality, toxicity of, and immune responses to repeated infusions into a groin lymph node of escalating doses of a DNA plasmid encoding tyrosinase epitopes.

METHODS

Cohorts of 8 patients each received 200 microg, 400 microg, or 800 microg of DNA intranodally by pump over 96 hours every 14 days for 4 cycles. Blood was collected for immunologic assays and to measure plasmid in serum prior to treatment, 4 weeks later, and 8 weeks later. Scans and X-rays were performed at baseline and after 8 weeks.

RESULTS

Treatment was tolerated well, with only five patients demonstrating Grade 1-2 toxicity. Vaccine delivery by 96-hour infusions of plasmid into a groin lymph node resulted in only 1 episode of catheter leakage in 107 cannulations. Detection of plasmid in serum was rare and transient in two patients. Immune responses by peptide-tetramer assay to tyrosinase 207-216 were detected in 11 of 26 patients. No clinical responses were seen. Survival of the heavily pretreated patients on this trial was unexpectedly long, with 16 of 26 patients alive at a median follow-up of 12 months.

CONCLUSIONS

Infusion of a DNA plasmid vaccine into a groin lymph node was practical and well tolerated. Immune responses to a novel tyrosinase epitope were noted. Overall survival in this trial of heavily pretreated patients was unexpectedly long, with 16 of 26 patients alive after a follow-up of 12 months, favoring immune responders.

Acute shock induced by antigen vaccination in NOD mice

Type 1 diabetes in NOD mice can be prevented through autoantigen vaccination by shifting lymphocyte differentiation toward a T-helper 2 (Th(2)) response. However, in other models of autoimmunity, this approach may be accompanied by unexpected triggering of Th(2)-dependent anaphylactic shock. To test the safety of vaccination therapy in the NOD mouse model, we evaluated the effects of immunization with a wide battery of antigens in NOD, BALB/c, and C57BL/6 mice. Surprisingly, a nondiabetogenic antigen, hen egg white lysozyme, induced severe shock exclusively in NOD mice (shock in 11 of 11 mice, lethal in 3 mice). Shock severity was further increased by a more pronounced Th(2) setting generated by 1alpha,25(OH)(2)D(3) administration (17 of 17 mice, lethal in 14 mice, P < 0.0001). Pretreatment with dexamethasone resulted in full rescue, indicating an immune-mediated mechanism. Serum IgE levels and Th(1)/Th(2) cytokine profile analysis showed that the shock phenomenon was paralleled by a Th(2) response. mRNA expression of platelet-activating factor receptor (PAF-R) was significantly higher in NOD mice (P < 0.01) and was further increased by 1alpha,25(OH)(2)D(3). Pretreatment with WEB2086 (PAF-R antagonist) again protected all mice from lethal shock, indicating PAF as an anaphylaxis effector. In conclusion, in NOD mice, vaccination leading to a Th(2) immune shift can result in a lethal anaphylactic reaction.

Dendritic cell amplification of HIV type 1-specific CD8+ T cell responses in exposed, seronegative heterosexual women

In the Heterosexual AIDS Transmission Study (HATS), the frequency of high-risk sexual activity and viral load in the seropositive partner were shown to correlate with HIV-1 transmission. However, these parameters could not account for the status of some exposed, seronegative (ESN) individuals who remained uninfected despite years of exposure. To test the hypothesis that antiviral immune responses are a correlate of nontransmission in this cohort, we developed two sensitive methods for assessing HIV-1-specific humoral and cell-mediated responses. To quantify T cell responses, autologous mature dendritic cells (DCs) were used as antigen-presenting cells to elicit HIV-1-specific IFN-gamma production by ELISPOT. Antibody responses to HIV-1 gp120 were assessed by combination immunoprecipitation-Western blot (IP-WB). Previous studies of this cohort, using limiting dilution analysis, did not reveal HIV-1-specific cytotoxic T lymphocyte activity. However, when autologous DCs were used to present HIV-1 antigens, T cells from three of eight ESN women (38%) responded by producing IFN-gamma. T cells from three of four seropositive partners responded to HIV-1 antigens, whereas five negative controls did not. The use of DCs as antigen-presenting cells increased sensitivity by 2- to 30-fold relative to standard ELISPOT. Using IP-WB, low levels of gp120-reactive antibodies were detected in plasma from 1 of 14 ESN women. These results support the hypothesis that HIV-1-specific T cell responses play a role in immune surveillance in this cohort of North American serodiscordant couples. This report also demonstrates the ability of dendritic cells to reveal T cell responses that might be overlooked by other methods.

Uncertainties - discrepancies in immunology

The many immunological observations and results from in-vitro or in-vivo experiments vary, and their interpretations differ enormously. A major problem is that within a normal distribution of biological phenomena, which are measurable with many methods, virtually anything is possible. Within a coevolutionary context, the definition of biologically relevant thresholds is an important key to improve our understanding of weaknesses and strengths of the immune system. This review is a personal view, comparing textbook rules and experiments using model antigens with observations on immunity against infections or tumors to critically evaluate our perception and understanding of specificity, affinity maturation, antigen presentation, selection of the class of the immune response, immunological memory and protective immunity, positive selection of T cells and self/nonself discrimination.

Parasitological and immunological follow-up of American tegumentary leishmaniasis patients

A long-term evaluation of human American tegumentary leishmaniasis patients was conducted to detect immunological and/or parasitological indicators associated with cure or protection against leishmaniasis. Cutaneous leishmaniasis (CL) and mucosal leishmaniasis (ML) patients from endemic areas of Leishmania braziliensis infection in Brazil were studied during the active disease, at the end of therapy, and up to 10 years after the end of therapy. For immunological studies, lymphocyte proliferative responses, phenotypic characterization of CD4+ and CD8+ T cells reactive to L. braziliensis and cytokine production in vitro were assayed. In CL, with its tendency for healing lesions, at or shortly after the completion of therapy the ratio of CD4+ to CD8+ T cells was approximately one and production of interferon gamma (IFN gamma) remained roughly constant. In ML, these apparently beneficial CD4+/CD8+ ratios and cytokine patterns appeared later. The long-term memory T cell responses were associated with preferential induction of CD4+ subpopulations and IFN gamma production that probably led to protection against relapses or reinfection. Deoxyribonucleic acid (DNA) was isolated from peripheral blood and oligonucleotides that amplify the conserved region of the minicircle molecules of Leishmania were used in a 'hot-start' polymerase chain reaction (PCR). Leishmania DNA was found in about one-quarter of the patients with active disease as well as in individuals who had received chemotherapy. The PCR was also positive in one-third of the individuals with a positive skin test but no past or present history of leishmaniasis. The well-modulated T cell response leading to long-term protection observed in CL patients could result from a favourable host genetic background and/or a particular parasite genotype, leading to a beneficial T cell immune response even in the presence of parasite antigens. The possibility of parasite persistence after clinical cure suggests that the immune response can control, but not fully eliminate, the infection. It could prevent the parasite from causing disease, maintaining a leishmanial antigen-specific response and hampering reinfection.

Antiinfection immunity and autoimmunity

This review summarizes findings that indicate that immune reactions of T and B cells depend on thresholds of the binding avidity of the receptor, on antigen amounts, on the time period during which antigen is available in secondary lymphatic tissues, as well as on the detection methods used. Usually immunologically ignored, strictly extralymphatic host cells or their antigens may be released by infection or immunopathology to reach lymphatic organs and induce autoimmune diseases. If we know the infection, we call the disease immunopathologically mediated; if we do not recognize or know it, we call the disease autoimmune.

Impairment of immunological memory in the absence of MHC despite survival of memory T cells

The mechanisms by which immunological memory is maintained after infection or vaccination are still a matter of debate. Long-term survival of memory T cells does not require major histocompatibility complex (MHC) contact. We show here that compared with memory CD4+ T cells that maintain contact with MHC class II, memory CD4+ T cells deprived of MHC class II contact show distinct functional defects upon antigen re-encounter. Thus, in contrast to their survival, maintenance of the typical quality of memory T cells crucially depends on MHC-derived signals.

Expression of the serpin serine protease inhibitor 6 protects dendritic cells from cytotoxic T lymphocyte-induced apoptosis: differential modulation by T helper type 1 and type 2 cells

Dendritic cells (DCs) play a central role in the immune system as they drive activation of T lymphocytes by cognate interactions. However, as DCs express high levels of major histocompatibility complex class I, this intimate contact may also result in elimination of DCs by activated cytotoxic T lymphocytes (CTLs) and thereby limit induction of immunity. We show here that immature DCs are indeed susceptible to CTL-induced killing, but become resistant upon maturation with anti-CD40 or lipopolysaccharide. Protection is achieved by expression of serine protease inhibitor (SPI)-6, a member of the serpin family that specifically inactivates granzyme B and thereby blocks CTL-induced apoptosis. Anti-CD40 and LPS-induced SPI-6 expression is sustained for long periods of time, suggesting a role for SPI-6 in the longevity of DCs. Importantly, T helper 1 cells, which mature DCs and boost CTL immunity, induce SPI-6 expression and subsequent DC resistance. In contrast, T helper 2 cells neither induce SPI-6 nor convey protection, despite the fact that they trigger DC maturation with comparable efficiency. Our data identify SPI-6 as a novel marker for DC function, which protects DCs against CTL-induced apoptosis.

Low level viral persistence after infection with LCMV: a quantitative insight through numerical bifurcation analysis

Many important viruses persist at very low levels in the body in the face of host immunity, and may influence the maintenance of this state of 'infection immunity'. To analyse low level viral persistence in quantitative terms, we use a mathematical model of antiviral cytotoxic T lymphocyte (CTL) response to lymphocytic choriomeningitis virus (LCMV). This model, described by a non-linear system of delay differential equations (DDEs), is studied using numerical bifurcation analysis techniques for DDEs. Domains where low level LCMV coexistence with CTL memory is possible, either as an equilibrium state or an oscillatory pattern, are identified in spaces of the model parameters characterising the interaction between virus and CTL populations. Our analysis suggests that the coexistence of replication competent virus below the conventional detection limit (of about 100 pfu per spleen) in the immune host as an equilibrium state requires the per day relative growth rate of the virus population to decrease at least 5-fold compared to the acute phase of infection. Oscillatory patterns in the dynamics of persisting LCMV and CTL memory, with virus population varying between 1 and 100 pfu per spleen, are possible within quite narrow intervals of the rates of virus growth and precursor CTL population death. Whereas the virus replication rate appears to determine the stability of the low level virus persistence, it does not affect the steady-state level of the viral population, except for very low values.

Memory T cells constitute a subset of the human CD8+CD45RA+ pool with distinct phenotypic and migratory characteristics

Using HLA class I-viral epitope tetramers to monitor herpes virus-specific CD8(+) T cell responses in humans, we have shown that a significant fraction of responding cells revert from a CD45RO(+) to a CD45RA(+) state after priming. All tetramer-binding CD45RA(+) cells, regardless of epitope specificity, expressed a phenotype LFA-1(high)CCR7(low) that was stable for at least 10 years in infectious mononucleosis patients and indefinitely in asymptomatic carriers. CD8(+)CD45RA(+)LFA-1(high) cells were not present in cord blood but in adults account for up to 50% of CD8(+)CD45RA(+) cells. These CD45RA(+)LFA-1(high) cells have significantly shorter telomeres than CD45RA(+)LFA-1(low) cells, suggesting that the latter represent a naive population, while the former are memory cells. CD45RA(+) memory cells are a stable population of noncycling cells, but on stimulation they are potent producers of IFN-gamma, while naive CD8(+) cells produce only IL-2. The chemokine receptor profile and migratory potential of CD45RA(+) memory cells is very similar to CD45RO(+) cells but different to naive CD8 cells. In accord with this, CD45RA(+) memory cells were significantly underrepresented in lymph nodes, but account for virtually all CD8(+)CD45RA(+) T cells in peripheral tissues of the same individuals.

Involvement of inhibitory NKRs in the survival of a subset of memory-phenotype CD8+ T cells

Inhibitory natural killer receptors (NKRs) such as killer cell immunoglobulin-like receptors (KIRs) in humans and Ly49 molecules in mice are expressed on NK cells and recognize multiple major histocompatibility (MHC) class I proteins. In humans and mice, a subset of CD8+ T cells also expresses NKRs and harbors a memory phenotype. Using mice that are transgenic for KIR2DL3 and its cognate HLA-Cw3 ligand, we show that engagement of inhibitory NKRs selectively drives the in vivo accumulation of a subset of memory-phenotype CD8+ T cells that express the beta chain of the interleukin 2 receptor. In vitro, recognition of MHC class I molecules by inhibitory NKRs on T cells down-regulated activation-induced cell death. These results unveil an MHC class I-dependent pathway that promotes the survival of a subset of memory-phenotype CD8+ T cells and also reveal an unexpected biological function for inhibitory NKRs on T cells.

Preferential localization of effector memory cells in nonlymphoid tissue

Many intracellular pathogens infect a broad range of host tissues, but the importance of T cells for immunity in these sites is unclear because most of our understanding of antimicrobial T cell responses comes from analyses of lymphoid tissue. Here, we show that in response to viral or bacterial infection, antigen-specific CD8 T cells migrated to nonlymphoid tissues and were present as long-lived memory cells. Strikingly, CD8 memory T cells isolated from nonlymphoid tissues exhibited effector levels of lytic activity directly ex vivo, in contrast to their splenic counterparts. These results point to the existence of a population of extralymphoid effector memory T cells poised for immediate response to infection.

Cytotoxic T-lymphocyte memory, virus clearance and antigenic heterogeneity

Cytotoxic T-lymphocyte (CTL) memory to viruses has traditionally been studied in an isolated setting. However, recent experiments have indicated that the presence of antigenically heterologous challenges can result in the attrition of CTL memory. Here we use mathematical models in order to explore the consequence of these dynamics for the ability of the immune system in controlling multiple infections. Mathematical models suggest that antigen-independent persistence of CTL memory is required in order to resolve and clear an infection. This ensures strong immunological pressure at low loads when the virus population declines towards extinction. If the number of antigenic stimuli exposed to the immune system crosses a threshold, we find that immunological pressure is significantly reduced at low loads and this can prevent virus clearance and reduces overall control of viral replication. Hence, exposure to many heterologous challenges reduces the ability of CTL memory to contribute to virus control. The higher the number of infections present in the host, the higher the overall virus load and the higher the total number of memory CTLs. Beyond a given threshold, addition of new viruses to the system results in accelerated loss of virus control which eventually leads to a reduction in the overall memory CTL population. These dynamics might contribute to the progressively weaker immunity observed as a result of ageing. In this context, antigenically variable pathogens expose the immune system to many heterologous challenges within a short period of time and this could result in accelerated ageing of the immune system. These results have important implications for vaccination and treatment strategies directed against viral infections.

Direct analysis of the dynamics of the intestinal mucosa CD8 T cell response to systemic virus infection

The CD8 T cell response to vesicular stomatitis virus infection was characterized in the spleen and intestinal mucosa using MHC tetramers. Surprisingly, the primary response persisted in the lamina propria long after the splenic response had declined. Furthermore, the response was characterized by a protracted effector phase in which cytolytic activity in the lamina propria, but not in the spleen, was maintained. The appearance of Ag-specific cells in the intestinal mucosa was largely, though not exclusively, a result of beta(7) integrin-mediated migration. Infection with Listeria monocytogenes or with vaccinia virus also led to sustained mucosal responses. After reinfection of vesicular stomatitis virus-primed mice with a serotypically distinct virus, a sustained recall response was detected in all tissues. In CD40(-/-) mice, the mucosal, but not the splenic, response was compromised, resulting in diminished mucosal memory. The recall response was CD40 independent and correlated with memory levels, indicating that the mucosal and systemic responses operated independently. These findings illustrated the integrated yet distinct nature of systemic vs mucosal immune responses.

Predicting the dynamics of antiviral cytotoxic T-cell memory in response to different stimuli: cell population structure and protective function

This paper examines the numerical and functional consequences of various stimuli on antiviral CD8+ T-cell memory using a mathematical model. The model is based upon biological evidence from the murine model of infection with lymphocytic choriomeningitis virus (LCMV) that the phenotype of immunological memory represents low-level responses driven by various stimuli, and the memory CTL population is partitioned between resting, cycling and effector cells. These subpopulations differ in their lifespan, their potential to mediate antiviral protection and in the stimuli needed for their maintenance. Three types of maintenance stimuli are examined: non-antigen-specific (bystander) stimulation, persisting antigen stimulation and reinfection-mediated stimulation. The modelling predicts that: (i) stable persistence of CTL memory requires the presence of either bystander or antigen-specific stimulation above a certain threshold depending on the sensitivity of memory CTL to stimulation and their life-span; (ii) a relatively low level of stimuli (approximately 10(4) fold less on a per CTL basis compared to acute infection) is needed to stabilize the expanded memory CTL population; (iii) the presence of CTL subsets in the memory pool of different activation states and lifespans ensures the robustness of memory persistence in the face of temporal variation in the low-level stimuli and; (iv) an 'optimal' population structure of the memory CTL pool, in terms of immediate protection, requires the presence of both activated cycling and effector CTL. For this, persisting antigen alone or synergistically with bystander signals provide the appropriate stimulation, so that the stimuli equivalent to approximately 30 p.f.u. of LCMV in the spleen are sufficient to maintain approximately 10(5)-10(6) specific CTL in the memory pool. These observations are relevant both to our understanding of natural protective immunity and to vaccine design.

Enrichment of immediate-early 1 (m123/pp89) peptide-specific CD8 T cells in a pulmonary CD62L(lo) memory-effector cell pool during latent murine cytomegalovirus infection of the lungs

Interstitial cytomegalovirus (CMV) pneumonia is a clinically relevant complication in recipients of bone marrow transplantation (BMT). Recent data for a model of experimental syngeneic BMT and concomitant infection of BALB/c mice with murine CMV (mCMV) have documented the persistence of tissue-resident CD8 T cells after clearance of productive infection of the lungs (J. Podlech, R. Holtappels, M.-F. Pahl-Seibert, H.-P. Steffens, and M. J. Reddehase, J. Virol. 74:7496-7507, 2000). It was proposed that these cells represent antiviral "standby" memory cells whose functional role might be to help prevent reactivation of latent virus. The pool of pulmonary CD8 T cells was composed of two subsets defined by the T-cell activation marker L-selectin (CD62L): a CD62L(hi) subset of quiescent memory cells, and a CD62L(lo) subset of recently resensitized memory-effector cells. In this study, we have continued this line of investigation by quantitating CD8 T cells specific for the three currently published antigenic peptides of mCMV: peptide YPHFMPTNL processed from the immediate-early protein IE1 (pp89), and peptides YGPSLYRRF and AYAGLFTPL, derived from the early proteins m04 (gp34) and M84 (p65), respectively. IE1-specific CD8 T cells dominated in acute-phase pulmonary infiltrates and were selectively enriched in latently infected lungs. Notably, most IE1-specific CD8 T cells were found to belong to the CD62L(lo) subset representing memory-effector cells. This finding is in accordance with the interpretation that IE1-specific CD8 T cells are frequently resensitized during latent infection of the lungs and may thus be involved in the maintenance of mCMV latency.

Protective long-term antibody memory by antigen-driven and T help-dependent differentiation of long-lived memory B cells to short-lived plasma cells independent of secondary lymphoid organs

Memory is a hallmark of immunity. Memory carried by antibodies is largely responsible for protection against reinfection with most known acutely lethal infectious agents and is the basis for most clinically successful vaccines. However, the nature of long-term B cell and antibody memory is still unclear. B cell memory was studied here after infection of mice with the rabies-like cytopathic vesicular stomatitis virus, the noncytopathic lymphocytic choriomeningitis virus (Armstrong and WE), and after immunization with various inert viral antigens inducing naive B cells to differentiate either to plasma cells or memory B cells in germinal centers of secondary lymphoid organs. The results show that in contrast to very low background levels against internal viral antigens, no significant neutralizing antibody memory was observed in the absence of antigen and suggest that memory B cells (i) are long-lived in the absence of antigen, nondividing, and relatively resistant to irradiation, and (ii) must be stimulated by antigen to differentiate to short-lived antibody-secreting plasma cells, a process that is also efficient in the bone marrow and always depends on radiosensitive, specific T help. Therefore, for vaccines to induce long-term protective antibody titers, they need to repeatedly provide, or continuously maintain, antigen in minimal quantities over a prolonged time period in secondary lymphoid organs or the bone marrow for sufficient numbers of long-lived memory B cells to mature to short-lived plasma cells.

Selective bystander proliferation of memory CD4+ and CD8+ T cells upon NK T or T cell activation

Ag-experienced or memory T cells have increased reactivity to recall Ag, and can be distinguished from naive T cells by altered expression of surface markers such as CD44. Memory T cells have a high turnover rate, and CD8(+) memory T cells proliferate upon viral infection, in the presence of IFN-alphabeta and/or IL-15. In this study, we extend these findings by showing that activated NKT cells and superantigen-activated T cells induce extensive bystander proliferation of both CD8(+) and CD4(+) memory T cells. Moreover, proliferation of memory T cells can be induced by an IFN-alphabeta-independent, but IFN-gamma- or IL-12-dependent pathway. In these conditions of bystander activation, proliferating memory (CD44(high)) T cells do not derive from activation of naive (CD44(low)) T cells, but rather from bona fide memory CD44(high) T cells. Together, these data demonstrate that distinct pathways can induce bystander proliferation of memory T cells.