CD8+ T cell responses to human immunodeficiency viruses type 2 (HIV-2) and type 1 (HIV-1) gag proteins are distinguishable by magnitude and breadth but not cellular phenotype

The mechanisms underlying the relatively slow progression of human immunodeficiency virus type 2 (HIV-2) compared with HIV-1 infection are undefined and could be a result of more effective immune responses. We used HIV-2 and HIV-1 IFN-gamma enzyme-linked immunospot assays to evaluate CD8(+) T cell responses in antiretroviral-naive HIV-2- ('HIV-2(+)') and HIV-1-infected ('HIV-1(+)') individuals. Gag-specific responses were detected in the majority of HIV-2(+) and HIV-1(+) subjects. Overlapping gag peptide analysis indicated a significantly greater magnitude and breadth of responses in the HIV-1(+) cohort, and this difference was attributable to low responses in HIV-2(+) subjects with undetectable viral load (medians 2107 and 512 spot-forming units per 10(6) PBMC, respectively, p=0.007). We investigated the phenotype of viral epitope-specific CD8(+) T cells identified with HLA-B53- and HLA-B58-peptide tetramers (8 HIV-2(+), 11 HIV-1(+) subjects). HIV-2-specific CD8(+) T cells were predominantly CD27(+) CD45RA(-), and only a minority expressed perforin. The limited breadth and low frequency of CD8(+) T cell responses to HIV-2 gag in aviremic HIV-2(+) subjects suggests that these responses reflect antigen load in plasma, as is the case in HIV-1 infection. Immune control of HIV-2 does not appear to be related to the frequency of perforin-expressing virus-specific CD8(+) T cells.     

Unidirectional development of CD8+ central memory T cells into protective Listeria-specific effector memory T cells

Three distinct subsets of antigen-experienced CD8(+) T cells have been identified so far: short-living effector T cells (T(EC)) and two long-living subsets, described as central (T(CM)) and effector memory (T(EM)) T cells. The lineage relationships of these subpopulations as well as their involvement in protection have not yet been conclusively determined. We recently described a novel marker combination (CD127 and CD62L) to identify all three major CD8(+) T cell subsets in mice infected with Listeria monocytogenes (L.m.). Extensive lineage relationship analyses on highly purified subpopulations after in vitro and in vivo stimulation demonstrated that T(CM) can develop into T(EM) or T(EC), whereas T(EM) can only progress to T(EC) cells. Short-living T(EC) never regained a T(EM) or T(CM) phenotype. These data strongly suggest a hierarchical and unidirectional order of developmental stages. In vivo priming protocols that preferentially induced one of the different CD8(+) T cell subsets demonstrated that predominance of T(EM) (CD40 stimulation) correlated best with effective protection against L.m., whereas generation of neither T(CM) (by immunization with heat-killed L.m.) nor T(EC) (by systemic co-administration of CpG during primary infection) conferred substantial long-term protective immunity. These findings have important implications for the design of more effective T cell-based vaccines.     

Tolerogenic dendritic cells impede priming of naïve CD8+ T cells and deplete memory CD8+ T cells

Type 1 diabetes is a T‐cell‐mediated autoimmune disease in which autoreactive CD8⁺ T cells destroy the insulin‐producing pancreatic beta cells. Vitamin D3 and dexamethasone‐modulated dendritic cells (Combi‐DCs) loaded with islet antigens inducing islet‐specific regulatory CD4⁺ T cells may offer a tissue‐specific intervention therapy. The effect of Combi‐DCs on CD8⁺ T cells, however, remains unknown. To investigate the interaction of CD8⁺ T cells with Combi‐DCs presenting epitopes on HLA class I, naive, and memory CD8⁺ T cells were co‐cultured with DCs and proliferation and function of peptide‐specific T cells were analyzed. Antigen‐loaded Combi‐DCs were unable to prime naïve CD8⁺ T cells to proliferate, although a proportion of T cells converted to a memory phenotype. Moreover, expansion of CD8⁺ T cells that had been primed by mature monocyte‐derived DCs (moDCs) was curtailed by Combi‐DCs in co‐cultures. Combi‐DCs expanded memory T cells once, but CD8⁺ T‐cell numbers collapsed by subsequent re‐stimulation with Combi‐DCs. Our data point that (re)activation of CD8⁺ T cells by antigen‐pulsed Combi‐DCs does not promote, but rather deteriorates, CD8⁺ T‐cell immunity. Yet, Combi‐DCs pulsed with CD8⁺ T‐cell epitopes also act as targets of cytotoxicity, which is undesirable for survival of Combi‐DCs infused into patients in therapeutic immune intervention strategies.     

Down-regulation of CXCR4 expression on human CD8+ T cells during peripheral differentiation

Multi-color flow cytometric analysis on human CD8(+) T cell subsets revealed that CXCR4 is predominantly expressed on CD8(+) T cells with the naive CD27(+)CD28(+)CD45RA(+) phenotype, and is down-regulated during differentiation into those with an effector phenotype. The down-regulation of CXCR4 expression during peripheral differentiation was supported by the fact that the expression of CXCR4 on CD8(+) T cells was negatively correlated with that of perforin. The analysis of CCR5, CCR7, and CXCR4 co-expression further showed that CD8(+) T cells expressing a high level of CXCR4 are CCR7(+)CCR5(-) naive or central memory subsets, and those expressing a low level of CXCR4 were included in the CCR7(-)CCR5(+/-) memory/effector and effector subsets. Epstein Barr virus-specific CD8(+) T cells, which mostly express the memory phenotype, expressed CXCR4, while human cytomegalovirus-specific CD8(+) T cells, which mostly express the effector phenotype, partially expressed this receptor, showing that the expression of CXCR4 is also down-regulated during differentiation of viral antigen-specific CD8(+) T cells. The classification of human CD8(+) T cells based on the expression of these chemokine receptors should prove useful for studies that clarify the differentiation of human CD8(+) T cells.     

Immunisation with recombinant modified vaccinia virus Ankara expressing HIV-1 gag in HIV-1-infected subjects stimulates broad functional CD4+ T cell responses

Virus-specific CD4+ T cells with IL-2-secreting and/or proliferative capacity are detected readily in HIV-1-infected long-term nonprogressors and rarely in persons with untreated progressive infection. The contribution of these cells to viraemia control is uncertain, but this question might be addressed in clinical therapeutic vaccination studies. However, the quality of T helper responses induced by currently available HIV-1 vaccine candidates has not been explored in depth. We determined the effect of vaccination with modified vaccinia virus Ankara (MVA) expressing HIV-1 gag p24/p17 (MVA.HIVA) on HIV-1-specific CD4+ T cell responses in 16 chronically infected, highly active antiretroviral therapy (HAART)-treated subjects using CD8-depleted IFN-gamma ELISPOT assays, intracellular cytokine staining assays for IL-2 and IFN-gamma, and a CFSE-based proliferation assay. Gag-specific CD4+ T cell responses were significantly increased in magnitude and breadth after vaccination and targeted both known and new epitopes, several of which were also recognised by healthy HIV-uninfected volunteers immunised with the same vaccines. The frequencies of CD4+ T cells expressing IL-2 or IFN-gamma, alone or simultaneously, were also augmented. These findings indicate that functional virus-specific T helper cells can be boosted by vaccination in chronic HIV-1 infection. Further evaluation of their role in viraemia control is warranted.     

Incomplete effector/memory differentiation of antigen-primed CD8+ T cells in gene gun DNA-vaccinated mice

DNA vaccination is an efficient way to induce CD8+ T cell memory, but it is still unclear to what extent such memory responses afford protection in vivo. To study this, we induced CD8+ memory responses directed towards defined viral epitopes, using DNA vaccines encoding immunodominant MHC class I-restricted epitopes of lymphocytic choriomeningitis virus covalently linked to beta2-microglobulin. This vaccine construct primed for a stronger recall response than did a more conventional minigene construct. Despite this, vaccinated mice were only protected against systemic infection whereas protection against the consequences of peripheral challenge was limited. Phenotypic analysis revealed that DNA vaccine-primed CD8+ T cells in uninfected mice differed from virus-primed CD8+ T cells particularly regarding expression of very-late antigen (VLA)-4, an adhesion molecule important for targeting T cells to inflammatory sites. Thus, our DNA vaccine induces a long-lived memory CD8+ T cell population that provides efficient protection against high-dose systemic infection. However, viral replication in solid non-lymphoid organs is not curtailed sufficiently fast to prevent significant virus-induced inflammation. Our results suggest that this is due to qualitative limitations of the primed CD8+ T cells.     

Switching from a restricted to an effective CD4 T cell response by activating CD8+ murine dendritic cells with a Toll-like receptor 9 ligand

Freshly isolated quiescent splenic dendritic cell (DC) subtypes differ in their capacity to activate naive CD4 T cells in culture. The CD8+ DC showed a reduced capacity to stimulate T cell proliferation compared to either of the CD8- DC subsets, regardless of antigen and DC dose. In contrast to CD8- DC, the quiescent CD8+ DC did not induce IFN-gamma production from CD4 T cells. The difference between the DC subtypes appeared to be at the level of initial surface molecule interactions, but could not be attributed to differences in expression of MHC class II or B7 family molecules, or to the expression of Fas ligand on DC. However, when activated by inclusion of the Toll-like receptor 9 ligand CpG in culture, CD8+ DC became potent stimulators of both CD4 T cell proliferation and IFN-gamma production. In contrast, similar activation of CD8- DC produced a more modest increase in capacity to stimulate CD4 T cell proliferation and no increase in capacity to stimulate IFN-gamma production. The difference between a quiescent and an activated state is therefore more extreme for CD8+ than for CD8- DC. The especially tight regulation of the activity of CD8+ DC may be essential for the maintenance of self tolerance.     

Reduced antigen concentration and costimulatory blockade increase IFN-gamma secretion in naive CD8+ T cells

CD8+ T cells are killer cells but also major producers of IFN-gamma. We have investigated the effects of peptide antigen titration and costimulatory blockade on IFN-gamma production and proliferation by naive CD8+ T cells. Mature dendritic cells (DC) pulsed with high amounts of agonist peptide triggered proliferation but little IFN-gamma secretion in individual T cells. In contrast, immature DC pulsed with similar amounts of peptide induced IFN-gamma secretion in a larger fraction of T cells but triggered less proliferation. Blocking B7.2 or lowering the amount of peptide on mature DC led to a response similar to that induced by immature DC, suggesting that differences in stimulatory strength were responsible for the different responses. Using splenic antigen-presenting cells (APC) we demonstrate that reducing the amount of peptide in combination with B7 blockage enhanced IFN-gamma secretion and decreased proliferation in naive CD8+ T cells in an additive way. Our data suggest that IFN-gamma secretion and proliferation are independently and inversely controlled by stimulatory strength in naive CD8+ T cells. This may enable CD8+ T cells to respond with IFN-gamma secretion to immature APC with few peptide ligands consistent with an early immunoregulatory role of CD8+ T cells.     

Virtual memory CD8+ T cells restrain the viral reservoir in HIV-1-infected patients with antiretroviral therapy through derepressing KIR-mediated inhibition

The viral reservoir is the major hurdle in developing and establishing an HIV cure. Understanding factors affecting the size and decay of this reservoir is crucial for the development of therapeutic strategies. Recent work highlighted that CD8+ T cells are involved in the control of viral replication in ART-treated HIV-1-infected individuals, but how CD8+ T cells sense and restrict the HIV reservoir are not fully understood. Here, we demonstrate that a population of unconventional CD45RA+, PanKIR+, and/or NKG2A+ virtual memory CD8+ T cells (T_VM cells), which confer rapid and robust protective immunity against pathogens, plays an important role in restraining the HIV DNA reservoir in HIV-1-infected patients with effective ART. In patients undergoing ART, T_VM cells negatively correlate with HIV DNA and positively correlate with circulating IFN-α2 and IL-15. Moreover, T_VM cells constitutively express high levels of cytotoxic granule components, including granzyme B, perforin and granulysin, and demonstrate the capability to control HIV replication through both cytolytic and noncytolytic mechanisms. Furthermore, by using an ex vivo system, we showed that HIV reactivation is effectively suppressed by T_VM cells through KIR-mediated recognition. This study suggests that T_VM cells are a promising target to predict posttreatment virological control and to design immune-based interventions to reduce the reservoir size in ART-treated HIV-1-infected individuals.     

Autoreactive human peripheral blood CD8+ T cells with a regulatory phenotype and function

Despite substantial advances in our understanding of CD4+ CD25+ regulatory T cells, a possible equivalent regulatory subset within the CD8+ T cell population has received less attention. We now describe novel human CD8+/TCR alphabeta+ T cells that have a regulatory phenotype and function. We expanded and cloned these cells using autologous LPS-activated dendritic cells. The clones were not cytolytic, but responded in an autoreactive HLA class I-restricted fashion, by proliferation and production of IL-4, IL-5, IL-13 and TGFbeta1, but not IFN-gamma. They constitutively expressed CD69 and CD25 as well as molecules associated with CD4+ CD25+ regulatory T cells, including cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and Foxp3. They suppressed IFN-gamma production and proliferation by CD4+ T cells in vitro in a cell contact-dependent manner, which could be blocked using a CTLA-4-specific mAb. They were more readily isolated from patients with ankylosing spondylitis and may therefore be up-regulated in response to inflammation. We suggest that they are the CD8+ counterparts of CD4+ CD25+ regulatory T cells. They resemble recently described CD8+ regulatory cells in the rat that were able to abrogate graft-versus-host disease. Likewise, human HLA-restricted CD8+ regulatory T cells that can be cloned and expanded in vitro may have therapeutic applications.     

Origins and functional basis of regulatory CD11c+CD8+ T cells

Previously, we showed that CD11c defines a novel subset of CD8⁺ T cells whose in vivo activity is therapeutic for arthritis; however, the mechanisms directing their development, identity of their precursors, and basis of their effector function remain unknown. Here, we show that the novel subset develops from CD11c^surface?CD8⁺ T cells and undergoes robust expansion in an antigen‐ and 4‐1BB (CD137)‐dependent manner. CD11c⁺CD8⁺ T cells exist in naïve mice (<3%) with limited suppressive activity. Once activated, they suppress CD4⁺ T cells in vivo and in vitro. Suppression of CD4⁺ by CD11c⁺CD8⁺ T cells is related to an increase in IDO activity induced in competent cells via the general control non‐derepressible‐2 pathway. CD11c⁺CD8⁺ T cells are refractory to the effect of IDO but constrict in a novel 1‐methyl D ,L ‐tryptophan‐dependent mechanism resulting in reversal of their suppressive effects. Thus, our data uncover, for the first time, the origin, development, and basis of the suppressive function of this novel CD11c⁺CD8⁺ T‐cell subpopulation that has many signature features of Treg.     

CD4 T cell activation by myelin oligodendrocyte glycoprotein is suppressed by adult but not cord blood CD25+ T cells

Regulatory T cells expressing CD25 have been shown to protect rodents from organ-specific autoimmune diseases. Similar CD25+ cells with a memory phenotype exerting suppressive function after polyclonal or allogeneic stimulation are also present in adult human blood. We demonstrate that adult human CD25+ cells regulate the response to myelin oligodendrocyte glycoprotein (MOG), as depletion of CD25(+) cells increases responses of PBMC and the addition of purified CD25+ cells suppresses MOG-specific proliferation and IFN-gamma production of CD4(+)CD25(-) T cells. In contrast, cord blood CD25+ cells do not inhibit responses to self antigens, and only a small subpopulation of cord CD25+ cells expresses the typical phenotype of adult regulatory T cells (CD45RA(-) and GITR(+)) enabling suppression of polyclonal responses. We conclude that activation of self-reactive T cells in normal healthy individuals is prevented by the presence of self-antigen-specific CD25+ regulatory T cells and that the majority of these cells mature after birth.     

IL-18, but not IL-12, is required for optimal cytokine production by influenza virus-specific CD8+ T cells

The potent innate cytokines IL-12 and IL-18 are considered to be important antigen-independent mediators of IFN-gamma production by NK cells and T lymphocytes. The present analysis addresses the physiological role of IL-12 and IL-18 in the generation of virus-specific CD8+ T cells. Both wt C57BL/6J (B6) mice and mice with disrupted IL-12p40 (IL-12p40(-/-)) or IL-18 (IL-18(-/-)) genes were infected with an influenza A virus and the characteristics of the resultant epitope-specific CD8+ T cell responses were compared. While IL-12 appeared to have no notable effect on either virus growth or on CD8+ T cell response profiles, the absence of IL-18 was associated with delayed virus clearance from the lung and, despite normal numbers, a significantly reduced production of IFN-gamma, TNF-alpha, and IL-2 by epitope-specific CD8+ T cells. While this cytokine phenotype was broadly maintained in IL-12p40/IL-18 double-knockout mice, no evidence was seen for any additive effect. Together, our results suggest that IL-18, but not IL-12, induces optimal, antigen-specific production of key cytokines by CD8+ T cells for the efficient clearance of influenza virus from the lungs of infected mice.     

B cells can prime naive CD4+ T cells in vivo in the absence of other professional antigen-presenting cells in a CD154-CD40-dependent manner

The role of B cells as APC is well established. However, their ability to prime naive T cells in vivo has been difficult to examine because of the presence of dendritic cells. The current studies were undertaken to examine this issue in a model of adoptive transfer of antigen-specific B cells and T cells into histoincompatible Rag2(-/-) mice. By means of this system, we were able to demonstrate that antigen-specific B cells are competent APC for naive CD4(+) T cells specific for the same antigen. In vivo antigen presentation resulted in expansion of both CD4(+) T cells and B cells. The antigen-presenting function of the transferred B cells was dependent on the CD154-CD40 interaction, as transfer of CD154-deficient antigen-specific CD4(+) T cells or CD40-deficient B cells failed to induce T and B cell expansion in response to immunization. These results indicate that antigen-specific B cells have the capacity to induce primary T cell responses in the absence of other competent APC.     

The functional avidity of virus-specific CD8+ T cells is down-modulated in Borna disease virus-induced immunopathology of the central nervous system

Borna disease virus (BDV) infection of the central nervous system (CNS) leads to severe neurological symptoms in susceptible MRL mice. The disease is mainly mediated by CD8+ T cells specific for the immunodominant epitope TELEISSI in the BDV nucleoprotein. In this study, TELEISSI/MHC class I tetramers were used to directly visualize antigen-specific CD8+ T cells. We found that on average approximately 30% of the ex vivo analyzed CD8+ T cells in the CNS of diseased mice were specific for TELEISSI. Unexpectedly, the frequency of tetramer-reactive brain-derived CD8+ T cells doubled following overnight culture in the absence of antigen. The majority of CD8+ T cells showed enhanced tetramer binding without up-regulation of T cell receptor surface expression. The frequency of IFN-gamma-secreting CD8+ T cells after antigen-specific stimulation was higher in overnight cultures than in freshly isolated BDV-specific brain lymphocytes, and enhanced tetramer binding correlated with elevated sensitivity to lower levels of peptide antigen in cytotoxicity assays. These results indicate that the functional avidity of virus-specific CD8+ T cells was down-modulated in vivo. Thus, quantification of tissue-infiltrating CD8+ T cells by the tetramer technique must be interpreted with caution as it may underestimate the real frequency of antigen-specific CD8+ T cells.     

Differences between CD8+ T cells in lupus-prone (NZB x NZW) F1 mice and healthy (BALB/c x NZW) F1 mice may influence autoimmunity in the lupus model

Immunization with portions of a murine antibody to DNA induced Ig peptide-reactive peripheral CD8+ inhibitory T (Ti) cells in non-autoimmune (BALB/c x NZW) F1 (CWF1) mice. Those Ti suppressed in vitro production of IgG anti-DNA by lymphocytes from MHC-matched, lupus-prone (NZB x NZW) F1 (BWF1) mice, primarily via secretion of transforming growth factor-beta (TGF-beta). However, splenic CD8+ cells from immunized BWF1 mice failed to suppress anti-DNA. Therefore, BWF1 mice were studied for defects in peripheral CD8+ T cells. The potential to suppress autoimmunity mediated by activated CD4+ helper T and B cells in BWF1 mice was assessed. As BWF1 mice aged, peripheral CD8+ T cells expanded little; fewer than 10% displayed surface markers of activation and memory. In contrast, quantities of splenic CD4+ T and B cells increased; high proportions displayed activation/memory markers. In old compared to young BWF1 mice, splenic cell secretion of two cytokines required for generation of CD8+ T effectors, IL-2 and TGF-beta, was decreased. Immunizing BWF1 mice activated peptide-reactive CD8+ T cells, but their number was decreased compared to young BWF1 or old normal mice. While peptide-reactive splenic CD8+ T cells from immunized BWF1 mice did not survive in short-term cultures, similar CD8+ T cell lines from immunized CWF1 mice expanded and on transfer into BWF1 mice delayed autoimmunity and prolonged survival. Therefore, CD8+ T cells in old BWF1 mice are impaired in expansion, acquisition of memory, secretion of cytokine, and suppression of autoimmunity. Understanding these defects might identify targets for therapy in systemic lupus erythematosus.     

The ester-bonded palmitoyl side chains of Pam3CysSerLys4 lipopeptide account for its powerful adjuvanticity to HLA class I-restricted CD8+ T lymphocytes

Molecularly defined adjuvants are urgently required to implement immunization protocols by which CD8+ T cells induction is envisaged. We show here that the synthetic lipopeptide Pam3CysSerLys4 (P3CSK4) strongly enhances the expansion of antigen-specific IFN-gamma+CD8+ cells in vitro. These effects critically depend on the presence of two ester-bonded palmitoylated side chains. In fact, T cell expansion is impaired in the presence of derivatives bearing two non-palmitoylated fatty acid chains, while derivatives with only one amide-bonded palmitoylated residue are completely inactive and behave like the non-lipidated peptide backbone. P3CSK4 is not mitogenic for T lymphocytes and can modulate DC immune biological properties. Indeed, doses as low as 100 ng/ml increase CD86, CD83 and CD40 surface expression on DC, fail to induce CCR7, and trigger a defined pattern of soluble factors associated to immune effector functions. In particular, substantial amounts of TNF-alpha, IL-6, CCL2 and CXCL10, in the absence of IFN-alpha, IFN-gamma, IL-15, IL-12p70 and CX3CL1, can be measured. Accordingly, antigen-specific CD8+ T cells expanded in vitro express CCR2 and CXCR3 chemokine receptors. Altogether our data suggest that human DC are able to respond to chemically different synthetic lipopeptide analogs and that optimal adjuvanticity to CD8+ T cell induction is achieved by the palmitoylated structures.     

IFN-alpha-conditioned dendritic cells are highly efficient in inducing cross-priming CD8(+) T cells against exogenous viral antigens

Dendritic cells (DC) generated after a short-term exposure of monocytes to IFN-alpha and GM-CSF (IFN-DC) are highly effective in inducing cross-priming of CD8(+ )T cells against viral antigens. We have investigated the mechanisms responsible for the special attitude of these DC and compared their activity with that of reference DC. Antigen uptake and endosomal processing capabilities were similar for IFN-DC and IL-4-derived DC. Both DC types efficiently cross-presented soluble HCV NS3 protein to the specific CD8(+) T cell clone, even though IFN-DC were superior in cross-presenting low amounts of viral antigens. Moreover, when DC were pulsed with inactivated HIV-1 and injected into hu-PBL-SCID mice, the generation of virus-specific CD8(+ )T cells was markedly higher in animals immunized with IFN-DC than in mice immunized with CD40L-matured IL-4-DC. Of interest, in experiments with purified CD8(+ )T cells, IFN-DC were superior with respect to CD40L-matured IL-4-DC in inducing in vitro cross-priming of HIV-specific CD8(+ )T cells. This property correlated with enhanced potential to express the specific subunits of the IL-23 and IL-27 cytokines. These results suggest that IFN-DC are directly licensed for an efficient CD8(+) T cell priming by mechanisms likely involving enhanced antigen presentation and special attitude to produce IL-12 family cytokines.     

Low numbers of CD8+ T lymphocytes in hereditary haemochromatosis are explained by a decrease of the most mature CD8+ effector memory T cells

Low CD8⁺ T lymphocyte numbers have long been described in hereditary haemochromatosis (HH). Recently, two conserved haplotypes localized near the microsatellite D6S105 at the major histocompatibility complex (MHC) class I region were described predicting the clinical expression of HH and the CD8⁺ T lymphocyte numbers. The A‐A‐T haplotype was associated with a severe clinical expression of HH and low CD8⁺ T lymphocyte numbers, while the G‐G‐G haplotype was associated with a milder clinical expression of HH and high CD8⁺ T lymphocyte numbers. As CD8⁺ T lymphocytes are a very heterogeneous population, in this study we analysed the CD8⁺ subpopulations of naive, central memory (T_CM) and effector memory (T_EM), and further subsets of CD8⁺ T_EM cells in 47 HH patients and 68 controls. In addition, association studies were conducted between the conserved haplotypes and the CD8⁺ T cell subpopulations in HH. Variations of the numbers of naive and central memory cells with age were similar between HH patients and controls. For T_EM cells and the T_EM CD27^‐CD28^‐ subset no effect of age was observed in HH [R² = 0·001, not significant (n.s.) and R² = 0·01, n.s., respectively] contrasting with the increasing of these subpopulations with age in controls (R² = 0·09, P = 0·017 and R² = 0·22, P = 0·0005, respectively). Interestingly, patients homozygous for the A‐A‐T haplotype have lower numbers of CD8⁺ T_EM cells due especially to lower numbers of T_EM CD27^‐CD28^‐ (0·206 ± 0·119 and 0·066 ± 0·067 × 10⁶ cells/ml, respectively) than patients carrying the G‐G‐G haplotype (0·358 ± 0·195 and 0·246 ± 0·202 × 10⁶ cells/ml, respectively). This may suggest an inability of HH patients to differentiate the CD8⁺ T cells into the most mature phenotype.     

CD28 days later: Resurrecting costimulation for CD8+ memory T cells

Rapid activation and proliferative expansion of specific CD8⁺ memory T (CD8⁺T_M) cells upon antigen re‐encounter is a critical component of the adaptive immune response that confers enhanced immune protection. In this context, however, the requirements for costimulation in general, and CD28 signaling in particular, remain incompletely defined. In the current issue of the European Journal of Immunology, Fröhlich et al. [Eur. J. Immunol. 2016. 46: 1644‐1655] provide definitive evidence that optimal elaboration of CD8⁺T_M‐cell recall responses is indeed contingent on CD28 expressed by these cells. Here, we discuss the “CD28 costimulation paradigm” in its historical context and highlight some of the unresolved complexities pertaining to CD28‐dependent interactions that shape CD8⁺ T‐cell phenotypes, functionalities, and recall reactivity.