Increased CD8+T Cell Memory to Concurrent Infection at the Expense of Increased Erosion of Pre-existing Memory: The Paradoxical Role of IL-15

Increased Levels of Serum IL-15 and TNF-β Indicate the Progression of Human Intracranial Aneurysm

Objective

Existing evidence suggests that chronic inflammation promotes the progression of human intracranial aneurysm (IA) and many cytokines have been detected to participate in the process of inflammation. However, rare cytokines in plasma have been used as proxies for progression of IA. This study aimed to identify novel cytokines as biomarkers to predict the development of IA.

Methods

Patients with unruptured intracranial aneurysms (UIAs) undergoing microsurgical clipping were prospectively recruited from January 2017 to June 2020 and were separated into two groups based on their ELAPSS score (low risk group < 10, intermediate-high risk group ≥ 10). Propensity score matching (PSM) was used to reduce imbalances in the baseline characteristics between groups. All blood samples were collected before surgery. A human serum 48-cytokines examination was performed to analyze the concentrations of serological cytokines. Clinical data and cytokines were compared between groups.

Results

A total of 184 patients were enrolled in this study. The low risk group contained 77 patients and 107 patients were included in the intermediate-high risk group. Finally, there were 69 patients in each group after PSM with a matching rate of 1:1. The concentrations of 3 serum cytokines were significantly increased in intermediate-high risk patients, namely, interleukin-15 (IL-15), monocyte chemoattractant protein-1 (MCP-1), and tumor necrosis factor-β (TNF-β) (P < 0.05, |log₂ fold change| > 2). The result of receiver operator characteristic (ROC)curve revealed that TNF-β had the highest predictive accuracy, with an area under the curve (AUC) value of 0.725 [95% confidence interval (CI) 0.639–0.811, P < 0.001] followed by IL-15 (AUC = 0.691, 95% CI 0.602–0.781, P < 0.001) and MCP-1 (AUC = 0.661, 95% CI 0.569–0.753, P = 0.001). Multivariate logistic analysis demonstrated high IL-15 [odds ratio (OR), 3.23; 95% CI, 1.47–7.12; P = 0.004] and high TNF-β (OR, 8.30; 95% CI, 3.25–21.25; P < 0.001) as the risk factors that correlated with intermediate-high risk of IA progression.

Conclusion

UIA patients with intermediate-high growth risk exhibited increased serum levels of IL-15, MCP-1, and TNF-β. Serum IL-15, and TNF-β could serve as biomarkers to predict the progression of UIAs.

Recombinant BCG to Enhance Its Immunomodulatory Activities

The bacillus Calmette-Guérin (BCG) is an attenuated Mycobacterium bovis derivative that has been widely used as a live vaccine against tuberculosis for a century. In addition to its use as a tuberculosis vaccine, BCG has also been found to have utility in the prevention or treatment of unrelated diseases, including cancer. However, the protective and therapeutic efficacy of BCG against tuberculosis and other diseases is not perfect. For three decades, it has been possible to genetically modify BCG in an attempt to improve its efficacy. Various immune-modulatory molecules have been produced in recombinant BCG strains and tested for protection against tuberculosis or treatment of several cancers or inflammatory diseases. These molecules include cytokines, bacterial toxins or toxin fragments, as well as other protein and non-protein immune-modulatory molecules. The deletion of genes responsible for the immune-suppressive properties of BCG has also been explored for their effect on BCG-induced innate and adaptive immune responses. Most studies limited their investigations to the description of T cell immune responses that were modified by the genetic modifications of BCG. Some studies also reported improved protection by recombinant BCG against tuberculosis or enhanced therapeutic efficacy against various cancer forms or allergies. However, so far, these investigations have been limited to mouse models, and the prophylactic or therapeutic potential of recombinant BCG strains has not yet been illustrated in other species, including humans, with the exception of a genetically modified BCG strain that is now in late-stage clinical development as a vaccine against tuberculosis. In this review, we provide an overview of the different molecular engineering strategies adopted over the last three decades in order to enhance the immune-modulatory potential of BCG.

Altered plasma levels of βC and γC chain cytokines and post-treatment modulation in tuberculous lymphadenitis

BACKGROUND

Alterations in β common (βC) and γ common (γC) chain cytokines have been described in pulmonary tuberculosis. However, their role in tuberculous lymphadenitis (TBL) disease has not been assessed.

METHODS

Thus, in the present study, we have examined the systemic levels of βC and γC chain cytokines in TBL, latent tuberculosis (LTB) and healthy control (HC) individuals. We have examined the discriminatory potential of both family of cytokines using ROC analysis. Finally, we measured the pre and post-treatment responses of these cytokines after anti-tuberculosis treatment.

RESULTS

TBL individuals exhibit significantly increased (IL-3) and diminished systemic levels of (IL-5, GM-CSF) βC cytokines compared to LTB and HC individuals. TBL individuals also exhibit significantly diminished (IL-2, IL-7) and elevated (IL-4, IL-9) levels of γC cytokines compared to LTB and/or HC. ROC analysis shows a clear discriminatory capacity of both βC (IL-5) and γC (IL-2) chain cytokines to distinguish TBL from LTB and HCs. The systemic levels of βC chain cytokines were not significantly altered, but in contrast γC (IL-2 and IL-7) cytokines were significantly modulated after treatment. Finally, no significant correlation was observed for βC and γC chain cytokines with their respective lymphocyte count of TBL individuals.

CONCLUSIONS

Hence, we conclude that altered plasma levels of βC and γC cytokines are the characteristics of immune alteration in TBL disease and certain cytokines were modulated after treatment.

Mathematical models of memory CD8+ T-cell repertoire dynamics in response to viral infections

Immunity to diseases is conferred by pathogen-specific memory cells that prevent disease reoccurrences. A broad repertoire of memory T-cells must be developed and maintained to effectively protect against viral invasions; yet, the total number of memory T-cells is constrained between infections. Thus, creating memory to new infections can require attrition of some existing memory cells. Furthermore, some viruses induce memory T-cell death early in an infection, after which surviving cells proliferate to refill the memory compartment.We develop mathematical models of cellular attrition and proliferation in order to examine how new viral infections impact existing immunity. With these probabilistic models, we qualitatively and quantitatively predict how the composition and diversity of the memory repertoire changes as a result of viral infections. In addition, we calculate how often immunity to prior diseases is lost due to new infections. Comparing our results across multiple general infection types allows us to draw conclusions about, which types of viral effects most drastically alter existing immunity. We find that early memory attrition does not permanently alter the repertoire composition, while infections that spark substantial new memory generation drastically shift the repertoire and hasten the decline of existing immunity.

The role of interleukin-15 in inflammation and immune responses to infection: implications for its therapeutic use

Interleukin-15 (IL-15) is a pleiotropic cytokine with a broad range of biological functions in many diverse cell types. It plays a major role in the development of inflammatory and protective immune responses to microbial invaders and parasites by modulating immune cells of both the innate and adaptive immune systems. This review provides an overview of the mechanisms by which IL-15 modulates the host response to infectious agents and its utility as a cytokine adjuvant in vaccines against infectious pathogens.

Immunotherapy for malignant melanoma--tracing Ariadne's thread through the labyrinth

A working group (FEBIM) within the European Organisation for Research and Treatment of Cancer undertook extensive studies on the possible association of infectious diseases and the risk of malignant melanoma. These studies provided evidence that several infectious diseases and also some vaccines including the anti-tuberculosis vaccine, BCG, derived from Mycobacterium bovis, confer a significant level of protection against this form of cancer. In recent years, the importance of immunoregulatory networks in the establishment of tolerance to tumour antigens and the key role of the innate immune system in the development of such networks have been recognised. The molecular patterns of micro-organisms activate pattern recognition receptors on antigen presenting cells and determine the qualitative nature of the ensuing immune response. Bacteria in the actinomycetales family, notably members of the genus Mycobacterium, exhibit particularly powerful adjuvant activity and profoundly affect underlying patterns of immune reactivity. In particular, there is growing evidence that a heat-killed preparation of a strain of Mycobacterium vaccae is able to down-regulate patterns of immune reactivity that favour the tumour and to induce those that lead to anti-cancer immune responses. The results of preliminary clinical observations with melanoma patients, and published studies on other cancers, point to the need for more formal clinical trials.

Respiratory infections: do we ever recover?

Although the outcome of respiratory infection alters with age, nutritional status, and immunologic competence, there is a growing body of evidence that we all develop a unique but subtle inflammatory profile. This uniqueness is determined by the sequence of infections or antigenic insults encountered that permanently mold our lungs through experience. This experience and learning process forms the basis of immunologic memory that is attributed to the acquired immune system. But what happens if the pathogen is not homologous to any preceding it? In the absence of cross-specific acquired immunity, one would expect a response similar to that of a subject who had never been infected with anything before. It is now clear that this is not the case. Prior inflammation in the respiratory tract alters immunity and pathology to subsequent infections even when they are antigenically distinct. Furthermore, the influence of the first infection is long lasting, not dependent on the presence of T and B cells, and effective against disparate pathogen combinations. We have used the term "innate imprinting" to explain this phenomenon, although innate education may be a closer description. This educational process, by sequential waves of infection, may be beneficial, as shown for successive viral infections, or significantly worse, as illustrated by the increased susceptibly to life-threatening bacterial pneumonia in patients infected with seasonal and pandemic influenza. We now examine what these long-term changes involve, the likely cell populations affected, and what this means to those studying inflammatory disorders in the lung.

Comparative ability of plasmid IL-12 and IL-15 to enhance cellular and humoral immune responses elicited by a SIVgag plasmid DNA vaccine and alter disease progression following SHIV(89.6P) challenge in rhesus macaques

Plasmid-based IL-12 has been demonstrated to successfully enhance the immunogenicity of DNA vaccines, thus enabling a reduction of the amount of DNA required for immunization. IL-15 is thought to affect the maintenance and enhance effector function of CD8(+) memory T cells. Since the ability to elicit a long-term memory response is a desirable attribute of a prophylactic vaccine, we sought to evaluate the ability of these plasmid-based cytokines to serve as vaccine adjuvants in rhesus macaques. Macaques were immunized with plasmid DNA encoding SIVgag in combination with plasmid IL-12, IL-15, or a combination of IL-12 and IL-15. The plasmid-based cytokines were monitored for their ability to augment SIVgag-specific cellular and humoral immune responses and to alter the clinical outcome following pathogenic SHIV(89.6P) challenge. Macaques receiving SIVgag pDNA in combination with plasmid IL-12 alone, or in combination with plasmid IL-12 and IL-15, demonstrated significantly elevated cell-mediated and humoral immune responses resulting in an improved clinical outcome following virus challenge compared to macaques receiving SIVgag pDNA alone. Macaques receiving SIVgag pDNA in combination with plasmid IL-15 alone demonstrated minor increases in cell-mediated and humoral immune responses, however, the clinical outcome following virus challenge was not improved. These results have important implications for the continued development of plasmid DNA vaccines for the prevention of HIV-1 infection.

Interleukin-15 mediates protection against experimental tuberculosis: a role for NKG2D-dependent effector mechanisms of CD8+ T cells

CD8+ T cells are involved in protection against Mycobacterium tuberculosis infection and represent a promising target for new vaccine strategies. Because IL-15 is important for the homeostasis of CD8+ T cells, we studied the immune response in IL-15-deficient mice during tuberculosis. In the absence of IL-15, CD8+ T cells failed to efficiently accumulate in draining lymph nodes and at the site of infection. The expression of antigen-specific effector functions, such as the production of interferon-gamma and cytotoxicity, were impaired in CD8+ T cells, but not CD4+ T cells, from IL-15-deficient mice. This defect was associated with an increased mortality of IL-15-deficient mice during the chronic phase of infection. The lectin-like stimulatory receptor natural killer group 2D (NKG2D) was up-regulated on CD8+ T cells only from wild-type mice, but not from IL-15-deficient mice. Mechanistically, blocking NKG2D function with an mAb inhibited M. tuberculosis-directed CD8+ T cell responses in vitro. We conclude that in addition to regulating the expansion of CD8+ T cells, IL-15 is also necessary for inducing effector mechanisms in CD8+ T cells that depend on NKG2D expression. Hence, our results implicate IL-15 and NKG2D as promising targets for modulating CD8+ T cell-mediated protection against tuberculosis.

Progress in tuberculosis vaccine development

The first tuberculosis vaccine candidates have reached clinical testing. Novel subunit vaccine candidates aimed at boosting previous BCG-prime vaccination and novel viable attenuated vaccine candidates aimed at substituting BCG have both completed the preclinical stage. Despite these achievements, rational vaccine design against tuberculosis has not come to an end. Novel findings in basic immunology and microbiology will advance further improvements in vaccine development. These include the potential role of crosspriming to induce more potent T-cell responses, the role of memory T cells and regulatory T cells in sustaining or curtailing optimal immune responses, respectively, as well as the involvement of cytokines in T-cell migration to nonimmunologic tissue sites and in the generation of memory. Knowledge about basic mechanisms underlying optimum protection will not only have a direct impact on future vaccine design against tuberculosis but also help in the formulation of a set of biomarkers with predictive value for vaccine efficacy assessment.

Chronic Leishmania donovani infection promotes bystander CD8+-T-cell expansion and heterologous immunity

It has been proposed that long-lived memory T cells generated by vaccination or infection reside within a memory compartment that has a finite size. Consequently, in a variety of acute infection models interclonal competition has been shown to lead to attrition of preexisting memory CD8+ T cells. Contrary to expectations, therefore, we found that chronic Leishmania donovani infection of Listeria-immune mice results in heightened protection against subsequent Listeria challenge. This protection was associated with bystander expansion of Listeria-specific CD8+ T cells and a bias in these cells toward a central memory T-cell phenotype with an enhanced capacity for gamma interferon production. We propose that splenomegaly, which is characteristic of visceral leishmaniasis and other tropical infections, may help promote heterologous immunity by resetting the size of the memory compartment during chronic infection.

CD8 memory T cells: cross-reactivity and heterologous immunity

Virus-specific memory T cell populations demonstrate plasticity in antigen recognition and in their ability to accommodate new memory T cell populations. The degeneracy of T cell antigen recognition and the flexibility of diverse antigen-specific repertoires allow the host to respond to a multitude of pathogens while accommodating these numerous large memory pools in a finite immune system. These cross-reactive memory T cells can be employed in immune responses and mediate protective immunity, but they can also induce life-threatening immunopathology or impede transplantation tolerance and graft survival. Here we discuss examples of altered viral pathogenesis occurring as a consequence of heterologous T cell immunity and propose models for the maintenance of a dynamic pool of memory cells.

Reducing the Stimulation of CD8+T Cells during Infection with Intracellular Bacteria Promotes Differentiation Primarily into a Central (CD62LhighCD44high) Subset

During infection with lymphocytic choriomeningitis virus, CD8(+) T cells differentiate rapidly into effectors (CD62L(low)CD44(high)) that differentiate further into the central memory phenotype (CD62L(high)CD44(high)) gradually. To evaluate whether this CD8(+) T cell differentiation program operates in all infection models, we evaluated CD8(+) T cell differentiation during infection of mice with recombinant intracellular bacteria, Listeria monocytogenes (LM) and Mycobacterium bovis (BCG), expressing OVA. We report that CD8(+) T cells primed during infection with the attenuated pathogen BCG-OVA differentiated primarily into the central subset that correlated to reduced attrition of the primed cells subsequently. CD8(+) T cells induced by LM-OVA also differentiated into central phenotype cells first, but the cells rapidly converted into effectors in contrast to BCG-OVA. Memory CD8(+) T cells induced by both LM-OVA as well as BCG-OVA were functional in that they produced cytokines and proliferated extensively in response to antigenic stimulation after adoptive transfer. During LM-OVA infection, if CD8(+) T cells were guided to compete for access to APCs, then they received reduced stimulation that was associated with increased differentiation into the central subset and reduced attrition subsequently. Similar effect was observed when CD8(+) T cells encountered APCs selectively during the waning phase of LM-OVA infection. Taken together, our results indicate that the potency of the pathogen can influence the differentiation and fate of CD8(+) T cells enormously, and the extent of attrition of primed CD8(+) T cells correlates inversely to the early differentiation of CD8(+) T cells primarily into the central CD8(+) T cell subset.

IL-15-independent antiviral function of primary and memory CD8+ T cells

Memory CD8⁺ T cells are comprised of CD122^hi IL-15-dependent and CD122^lo IL-15-independent subsets. Induction and retention of IL-15-independent memory CD8⁺ T cells was assessed in IL-15^−/− and wild-type (wt) mice immunized with recombinant vaccinia virus (rVV) or Sindbis virus (rSIN) vectors expressing the identical foreign epitope. Both vectors induced epitope-specific CD8⁺ T cell expansion and function, independent of IL-15. Similar kinetics of rVV clearance confirmed effective CD8⁺ T cell function in IL-15^−/− mice. CD44^hi CD122^hi CD8⁺ T cells, mainly of the CD62L^−/lo phenotype, increased more dramatically and declined more rapidly in IL-15^−/− mice, independent of the vector. Rapid IL-15-independent memory CD8⁺ T cell expansion following challenge of immune mice compensated for the limited memory CD8⁺ populations in IL-15^−/− mice. However, despite expansion and expression of potent effector function, viral clearance was delayed in the absence of IL-15, coinciding with a rapid loss in cytolytic function.

Apoptosis and loss of virus-specific CD8+ T-cell memory

CD8⁺ T-cell memory to viruses is stable in the absence but volatile in the presence of other infections. Apoptotic events that occur early in acute infections delete pre-existing memory T cells, leaving the host with reduced memory (except for cross-reactive responses) to previously encountered viruses. Apoptotic events also silence the acute immune response, leaving the host with a residual population of memory T cells. Persistent infections can induce apoptotic deletions of memory T cells that are specific to the persisting virus and to previously encountered pathogens.

Molecular adjuvants for mucosal immunity

Mucosal surfaces represent the entry route of a multitude of viral pathogens. For many of these viruses, such as the herpes simplex viruses and human immunodeficiency virus, no effective vaccine exists. Hence, it is important that prospective vaccines engender maximal immunity at these susceptible sites. Genetic vaccines encoding adjuvant molecules represent one approach to optimize mucosal as well as systemic immunity. Promising candidates include various inflammatory cytokines and chemokines that might be used to enhance the primary response to a level sufficient for protection. Encouraging studies involving cytokines such as granulocyte/macrophage colony-stimulating factor, interleukin-2 (IL-2), IL-12, IL-18, and many others are examined. Notable chemokines that may offer hope in such efforts include IL-8, RANTES, CCL19, CCL21, and a few others. Combinatorial approaches utilizing several cytokines and chemokines will most likely yield the greatest success. In addition, as more is discovered regarding the requirements for memory development of T cells, boosters involving key cytokines such as IL-15 and IL-23 may prove beneficial to long-term maintenance of the memory pool. This review summarizes the progress in the use of genetic vaccines to achieve mucosal immunity and discusses the needed strategies to maximize long-term prospective immunity at this vulnerable entry site.

Proliferation and Differentiation of CD8+ T Cells in the Absence of IL-2/15 Receptor β-Chain Expression or STAT5 Activation

Major gains in the efficacy of T cell-based therapies for cancer and infectious diseases could be realized through improved understanding of the signals that control expansion and differentiation of CD8(+) cytolytic T cells. IL-2, IL-15, and the downstream transcription factor STAT5 have all been implicated as important regulators of these processes, yet there are conflicting data regarding their contribution to in vivo T cell responses. We used a murine adoptive T cell transfer model to examine the contribution of IL-2 and IL-15 signaling to the proliferation and differentiation of naive, CD8(+) T cells bearing an OVA-specific TCR transgene (OT-I). OT-I T cells failed to express the high affinity IL-2R (CD25) while proliferating in vivo, irrespective of the mode of Ag delivery. Moreover, OT-I T cells rendered genetically deficient in the shared IL-2/IL-15Rbeta subunit (IL-2Rbeta) demonstrated normal Ag-induced proliferation and cytolytic activity in vivo. Accordingly, activation of STAT5 was not detected in proliferating IL-2Rbeta-deficient OT-I T cells, thus implicating a STAT5-independent cytokine or costimulatory pathway in this process. Even though IL-2 and IL-15 were dispensable for CD8(+) T cell proliferation, systemic infusion of IL-2 nevertheless promoted the expansion of OT-I T cells in vivo. Thus, IL-2 and IL-15 signals are not essential for CD8(+) T cell proliferation or differentiation, but IL-2 can promote supraphysiological expansion when supplied exogenously. These findings challenge current models that place CD8(+) T cell proliferation under the control of STAT5-dependent cytokines and suggest new approaches to the therapeutic manipulation of T cell numbers in vivo.

Immunological memory in humans

Specificity and memory are the defining characteristics of adaptive immune responses. Vaccines are predicated on the existence of immune memory, and the robustness of immune memory is a primary determinant of vaccine efficacy. How is immune memory maintained? Much progress has been made in this area over the past several years, and new human studies have added key insights into the longevity of B and T cell immune memory in the absence of antigen.