Renewal of peripheral CD8+ memory T cells during secondary viral infection of antibody-sufficient mice

The survival of memory CD8 T cells that is mediated by IL-15 correlates with sustained protection against malaria

Ag-specific memory T cell responses elicited by infections or vaccinations are inextricably linked to long-lasting protective immunity. Studies of protective immunity among residents of malaria endemic areas indicate that memory responses to Plasmodium Ags are not adequately developed or maintained, as people who survive episodes of childhood malaria are still vulnerable to either persistent or intermittent malaria infections. In contrast, multiple exposures to radiation-attenuated Plasmodium berghei sporozoites (Pb γ-spz) induce long-lasting protective immunity to experimental sporozoite challenge. We previously demonstrated that sterile protection induced by Pb γ-spz is MHC class I-dependent and CD8 T cells are the key effectors. IFN-γ(+) CD8 T cells that arise in Pb γ-spz-immunized B6 mice are found predominantly in the liver and are sensitive to levels of liver-stage Ag depot and they express CD44(hi)CD62L(lo) markers indicative of effector/effector memory phenotype. The developmentally related central memory CD8 T (TCM) cells express elevated levels of CD122 (IL-15Rβ), which suggests that CD8 TCM cells depend on IL-15 for maintenance. Using IL-15-deficient mice, we demonstrate in this study that although protective immunity is inducible in these mice, protection is short-lived, mainly owing to the inability of CD8 TCM cells to survive in the IL-15-deficient milieu. We present a hypothesis consistent with a model whereby intrahepatic CD8 TCM cells, being maintained by IL-15-mediated survival and basal proliferation, are conscripted into the CD8 effector/effector memory T cell pool during subsequent infections.

Phenotyping of circulating CD8⁺ T cell subsets in human cutaneous leishmaniasis

Recovery from CL is usually accompanied with long-lasting protection and induction of strong immune response. The phenotypes, generation and maintenance of central (=T(CM)) and effector (=T(EM)) memory T cell subsets in human leishmaniasis are not well known. Profile of T cell subsets were analyzed on peripheral CD8⁺ T cells from volunteers with history of cutaneous leishmaniasis (HCL). In HCL and control groups, mean frequencies of CCR7⁺CD45RA⁺CD8⁺ naïve and CCR7⁻CD45RA⁻CD8⁺ T(EM) cells were higher than other subsets before culture, but after stimulation with soluble Leishmania antigen, the frequency of naïve T cells was significantly decreased and the frequency of T(EM) cells was significantly increased. T(EM) phenotype composed the highest portion of proliferating Carboxy Fluorescein diacetate Succinimidyl Ester (CFSE)-dim population which was significantly higher in HCL volunteers than in control group. Stimulation of isolated CD8⁺ memory T cells, but not naïve T cells, from HCL volunteers induced a significantly higher IFN-γ production compared with that of healthy controls. Intracellular IFN-γ staining provided the same result. Memory population is shown to be responsible for Leishmania-induced IFN-γ production. Leishmania-reactive proliferating T(EM) cells were identified as the most frequent subset which may play a role in recall immune response and protection against Leishmania infection.

B cells modulate T cells so as to favour T helper type 1 and CD8+ T-cell responses in the acute phase of Trypanosoma cruzi infection

In this study, we have evaluated the production of pro- and anti-inflammatory cytokines and the formation of central and effector memory T cells in mice lacking mature B cells (mu MT KO). The results show that Trypanosoma cruzi infection in C57Bl/6m mu MT KO mice is intensified in relation to control mice and this exacerbation is related to low levels of inflammatory cytokines produced during the acute infection and the lower numbers of central and effector memory CD4(+) and CD8(+) T cells generated during the acute phase of the infection. In addition, a marked reduction in the CD8(+) T-cell subpopulation was observed in mu MT KO infected mice. In agreement to this, the degree of tissue parasitism was increased in mu MT mice and the tissue inflammatory response was much less intense in the acute phase of the infection, consistent with a deficit in the generation of effector T cells. Flow cytometry analysis of the skeletal muscle inflammatory infiltrate showed a predominance of CD8(+) CD45Rb low in B-cell-sufficient C57Bl/6 mice, whereas the preponderant cell type in mu MT KO skeletal muscle inflammatory infiltrate was CD4(+) T cells. In addition, CD8(+) T cells found in skeletal muscle from mu MT KO infected mice were less activated than in control B-cell sufficient infected mice. These results suggest that B cells may participate in the generation of effector/memory T cells. In addition and more importantly, B cells were crucial in the maintenance of central and effector memory CD8(+) T cell, as well as the determination of the T cell cytokine functional pattern, and they may therefore account for critical aspects of the resistance to intracellular pathogens, such as T. cruzi.

Activation phenotype, rather than central- or effector-memory phenotype, predicts the recall efficacy of memory CD8+ T cells

The contributions of different subsets of memory CD8⁺ T cells to recall responses at mucosal sites of infection are poorly understood. Here, we analyzed the CD8⁺ T cell recall responses to respiratory virus infection in mice and demonstrate that activation markers, such as CD27 and CD43, define three distinct subpopulations of memory CD8⁺ T cells that differ in their capacities to mount recall responses. These subpopulations are distinct from effector– and central–memory subsets, coordinately express other markers associated with activation status, including CXCR3, CD127, and killer cell lectin-like receptor G1, and are superior to CD62L in predicting the capacity of memory T cells to mediate recall responses. Furthermore, the capacity of vaccines to elicit these memory T cell subpopulations predicted the efficacy of the recall response. These findings extend our understanding of how recall responses are generated and suggest that activation and migration markers define distinct, and unrelated, characteristics of memory T cells.

Characterization of primary and memory CD8 T-cell responses against ranavirus (FV3) in Xenopus laevis

In mammals, resistance to primary and secondary viral infections critically involves major histocompatibility complex class I-restricted cytotoxic CD8+ T lymphocytes (CTLs). Although many gene homologues involved in CTL function have been identified in all vertebrate classes, antiviral CTL responses have been poorly characterized for ectothermic vertebrates. Because of the threat of emerging wildlife viral diseases to global biodiversity, fundamental research on comparative viral immunity has become crucial. Ranaviruses (family Iridoviridae) are double-stranded DNA viruses possibly implicated in the worldwide decline of amphibian populations. We used the frog Xenopus laevis as a model to evaluate adaptive immune responses to the ranavirus frog virus 3 (FV3). FV3 infects the kidneys of adults but is cleared within 4 weeks, with faster clearance upon secondary infections. In vivo depletion of CD8+ T cells markedly decreases the survival of adults after viral infection. To further investigate the involvement of anti-FV3 CD8+ T-cell effectors in host resistance in vivo, we determined the proliferation kinetics of CD8+ T cells in the spleen by bromodeoxyuridine incorporation and their infiltration of kidneys by immunohistology. Upon primary infection, CD8+ T cells significantly proliferate in the spleen and accumulate in infected kidneys from day 6 onward, in parallel with virus clearance. Earlier proliferation and infiltration associated with faster viral clearance were observed during a secondary infection. These results provide in vivo evidence of protective antigen-dependent CD8+ T-cell proliferation, recognition, and memory in fighting a natural pathogen in Xenopus.

Aging and CD8+ T cell immunity to respiratory virus infections

The capacity of the immune system to mediate effective immune responses to pathogens declines with age. In the case of immune responses to newly encountered antigens, several studies have demonstrated that this decline reflects both a loss of naïve T cells and changes in the repertoire and function of these cells over time. However, comparatively little is known about the impact of age on established memory T cells pools. Here we discuss age-related changes in memory CD8(+) T cell pools elicited by influenza and parainfluenza viruses and the impact of these changes on immunity in general.

T‐cell memory and recall responses to respiratory virus infections

The respiratory tract is characterized by its large surface area and the close association of an extensive vasculature with the external environment. As such, the respiratory tract is a major portal of entry for many pathogens. The immune system is able to effectively control most pulmonary pathogens and establish immunological memory that is capable of mediating an accelerated and enhanced recall response to secondary pathogen challenge. A key component of the recall response in the lung involves the rapid response of antigen-specific memory CD8+ T cells. Recent studies have shown that memory CD8+ T cells are extremely heterogeneous in terms of phenotype, function, anatomical distribution, and longevity. However, we have little understanding of how the different subsets of memory cells actually contribute to the recall response, especially with respect to peripheral or mucosal sites, such as the lung. Since immunological memory is the cornerstone of vaccination, it is essential that we understand how different memory CD8+ T-cell subsets are initially generated, maintained over time, and contribute to recall responses. This review focuses on memory T cells that mediate recall responses to influenza and parainfluenza virus infections in the lung.

Kupffer cell-dependent hepatitis occurs during influenza infection

Respiratory infections, including influenza in humans, are often accompanied by a hepatitis that is usually mild and self-limiting. The mechanism of this kind of liver damage is not well understood. In the present study, we show that influenza-associated hepatitis occurs due to the formation of inflammatory foci that include apoptotic hepatocytes, antigen-specific CD8(+) T cells, and Kupffer cells. Serum aminotransaminase levels were elevated, and both the histological and serum enzyme markers of hepatitis were increased in secondary influenza infection, consistent with a primary role for antigen-specific T cells in the pathogenesis. No virus could be detected in the liver, making this a pure example of "collateral damage" of the liver. Notably, removal of the Kupffer cells prevented the hepatitis. Such hepatic collateral damage may be a general consequence of expanding CD8(+) T-cell populations during many extrahepatic viral infections, yielding important implications for liver pathobiology.

T cell memory in the lung airways

The respiratory tract poses a substantial challenge to the immune system due to its large surface area, an extensive vasculature that is in very close proximity to the external environment, and repeated exposure to potentially pathogenic organisms in the air. Yet many lung pathogens are controlled by appropriate immune responses. The underlying mechanisms of the adaptive cellular immune response in protecting the respiratory tract are poorly understood. Recently, it has emerged that memory CD4(+) and CD8(+) T cells are present in the lung airways, and evidence is mounting that these cells play a key role in pulmonary immunity to pathogen challenge by immediately engaging the pathogen at the site of infection when pathogen loads are low. For example, in the case of respiratory virus infections, there is evidence that both CD4(+) and CD8(+) memory cells in the lung airways mediate substantial control of a secondary respiratory virus infection in the lungs. Here we address recent developments in our understanding of lung airway memory T cells and their role in infectious disease.

Kupffer cell-dependent hepatitis occurs during influenza infection

Respiratory infections, including influenza in humans, are often accompanied by a hepatitis that is usually mild and self-limiting. The mechanism of this kind of liver damage is not well understood. In the present study, we show that influenza-associated hepatitis occurs due to the formation of inflammatory foci that include apoptotic hepatocytes, antigen-specific CD8(+) T cells, and Kupffer cells. Serum aminotransaminase levels were elevated, and both the histological and serum enzyme markers of hepatitis were increased in secondary influenza infection, consistent with a primary role for antigen-specific T cells in the pathogenesis. No virus could be detected in the liver, making this a pure example of "collateral damage" of the liver. Notably, removal of the Kupffer cells prevented the hepatitis. Such hepatic collateral damage may be a general consequence of expanding CD8(+) T-cell populations during many extrahepatic viral infections, yielding important implications for liver pathobiology.

Modulation of Memory CD4 T Cell Function and Survival Potential by Altering the Strength of the Recall Stimulus

Optimization of long term immunity depends on the functional persistence of memory T cells; however, there are no defined strategies for promoting memory T cell function and survival. In this study, we hypothesized that varying the strength of the recall stimulus could modulate the function and survival potential of memory CD4 T cells. We tested the ability of peptide variants of influenza hemagglutinin (HA) exhibiting strong and weak avidity for an HA-specific TCR, to modulate HA-specific memory CD4 T cells in vitro and in vivo. In vitro stimulation with a weak avidity peptide (L115) uncoupled memory CD4 T proliferation from effector cytokine production with low apoptosis, whereas stimulation with a strong avidity peptide (Y117) fully recalled memory T cell functions but triggered increased apoptosis. To determine how differential recall would affect memory T cells in vivo, we boosted BALB/c hosts of transferred, CFSE-labeled HA-specific memory CD4 T cells with native HA, Y117, and L115 variant peptides and found differences in early Ag-driven memory T cell proliferation and IL-7R expression, with subsequent changes in memory T cell yield. High avidity boosting resulted in rapid proliferation, extensive IL-7R down-regulation, and the lowest yield of HA-specific memory cells, whereas low avidity boosting triggered low in vivo proliferation, maintenance of IL-7R expression, and the highest memory T cell yield. Our results indicate that memory CD4 T cell function and survival can be modulated at the recall level, and can be optimized by low level stimulation that minimizes apoptosis and enhances responses to survival factors.

Anatomical features of anti-viral immunity in the respiratory tract

The mucosal surfaces of the lungs are a major portal of entry for virus infections and there are urgent needs for new vaccines that promote effective pulmonary immunity. However, we have only a rudimentary understanding of the requirements for effective cellular immunity in the respiratory tract. Recent studies have revealed that specialized cellular immune responses and lymphoid tissues are involved in the protection of distinct anatomical microenvironments of the respiratory tract, such as the large airways of the nose and the alveolar airspaces. This review discusses some of the anatomical features of anti-viral immunity in the respiratory tract including the role of local lymphoid tissues and the relationship between effector and memory T cells in the airways, the lung parenchyma, and lymphoid organs.

Jump-starting the immune system: prime-boosting comes of age

A major challenge for immunologists has been the development of vaccines designed to emphasize cellular immune responses. One particularly promising approach is the prime-boost strategy, which has been shown to generate high levels of T-cell memory in animal models. Recently, several papers have highlighted the power of prime-boost strategies in eliciting protective cellular immunity to a variety of pathogens and have demonstrated efficacy in humans. Coupled with recent advances in our understanding of the mechanisms underlying the generation, maintenance and recall of T-cell memory, the field is poised to make tremendous progress. This Review discusses the impact of these recent developments on the future of prime-boost vaccine strategies.