CD8‐β ADP‐ribosylation affects CD8+ T‐cell function

The CD8αβ coreceptor is crucial for effective peptide: MHC‐I recognition by the TCR of CD8⁺ T cells. Adenosine diphosphate ribosyl transferase 2.2 (ART2.2) utilizes extracellular NAD⁺ to transfer ADP‐ribose to arginine residues of extracellular domains of surface proteins. Here, we show that in the presence of extracellular NAD⁺, ART2.2 caused ADP‐ribosylation of CD8‐β on murine CD8⁺ T cells in vitro and in vivo. Treatment with NAD⁺ prevented binding of anti‐CD8‐β mAb YTS156.7.7 but not of mAb H35–17.2, indicating that NAD⁺ caused modification of certain epitopes and not a general loss of CD8‐β. Loss of antibody binding was strictly dependent on ART2.2, because it was not observed on ART2‐deficient T cells or in the presence of inhibitory anti‐ART2.2 single‐domain antibodies. ADP‐ribosylation of CD8‐β occurred during cell isolation, particularly when cells were isolated from CD38‐deficient mice. Incubation of ART2‐expressing, but not of ART2‐deficient, OVA‐specific CD8⁺ T cells with NAD⁺ interfered with binding of OVA_257–264:MHC‐I tetramers. In line with this result, treatment of WT mice with NAD⁺ resulted in reduced CD8⁺ T‐cell mediated cytotoxicity in vivo. We propose that ADP‐ribosylation of CD8‐β can regulate the coreceptor function of CD8 in the presence of elevated levels of extracellular NAD⁺.     

Distinct involvement of CD45 in antigen receptor signalling in CD4+ and CD8+ primary T cells

We demonstrate that pretreatment of primary CD4⁺, but not CD8⁺ T cells with anti-CD45 inhibits activation signals induced through the T cell receptor for antigen (TCRαβ). Specifically, anti-TCRαβ-mediated tyrosine phosphorylation of phospholipase C-γ1 is inhibited, and this in turn correlates with the inhibition of subsequent Ca²⁺ mobilization and DNA synthesis. In marked contrast, none of these activation parameters are affected by anti-CD45 in CD8⁺ T cells. Perturbation of TCRαβ signalling in CD4⁺ cells is observed in conditions which do not detectably affect the level of CD45 expression, or its membrane distribution. Further, changes in the intrinsic phosphatase activity of CD45 are not detectable. While anti-CD45 ablates TCRαβ signalling, anti-CD3?-mediated activation is unaffected. This suggests that elements of the antigen receptor complex can be functionally uncoupled, and indicates that the requirements for CD45 in signalling through these two elements are different. The results demonstrate that the involvement of CD45 in coupling TCRαβ to second messenger-generating pathways is under distinct physical and/or functional constraints in primary CD4⁺ and CD8⁺ T cells.     

Unlike CD4+ T-cell help, CD28 costimulation is necessary for effective primary CD8+ T-cell influenza-specific immunity

The importance of costimulation on CD4⁺ T cells has been well documented. However, primary CTLs against many infections including influenza can be generated in the absence of CD4⁺ T‐cell help. The role of costimulation under such “helpless” circumstances is not fully elucidated. Here, we investigated such a role for CD28 using CTLA4Ig transgenic (Tg) mice. To ensure valid comparison across the genotypes, we showed that all mice had similar naïve precursor frequencies and similar peak viral loads. In the absence of help, viral clearance was significantly reduced in CTLA4Ig Tg mice compared with WT mice. CD44⁺BrdU⁺influenza‐specific CD8⁺ T cells were diminished in CTLA4Ig Tg mice at days 5 and 8 postinfection. Adoptive transfer of ovalbumin‐specific transgenic CD8⁺ T cells (OT‐I)‐I cells into WT or CTLA4Ig Tg mice revealed that loss of CD28 costimulation resulted in impairment in OT‐I cell division. As shown previously, neither viral clearance nor the generation of influenza‐specific CD8⁺ T cells was affected by the absence of CD4⁺ T cells alone. In contrast, both were markedly impaired by CD28 blockade of “helpless” CD8⁺ T cells. We suggest that direct CD28 costimulation of CD8⁺ T cells is more critical in their priming during primary influenza infection than previously appreciated.     

The expansion and selection of T cell receptor αβ intestinal intraepithelial T cell clones

In conventional mice, the T cell receptor (TCR)αβ⁺ CD8αα⁺ and CD8αβ⁺ subsets of the intestinal intraepithelial lymphocytes (IEL) constitute two subpopulations. Each comprise a few hundred clones expressing apparently random receptor repertoires which are different in individual genetically identical mice (Regnault, A., Cumano, A., Vassalli, P., Guy-Grand, D. and Kourilsky, P., J. Exp. Med. 1994. 180: 1345). We analyzed the repertoire diversity of sorted CD8αα and CD8αβ⁺ IEL populations from the small intestine of individual germ-free mice that contain ten times less TCRαβ⁺ T cells than conventional mice. The TCRβ repertoire of the CD8αα and the CD8αβ IEL populations of germ-free adult mice shows the same degree of oligoclonality as that of conventional mice. These results show that the intestinal microflora is not responsible for the repertoire oligoclonality of TCRαβ⁺ IEL. The presence of the microflora leads to an expansion of clones which arise independently of bacteria. To evaluate the degree of expansion of IEL clones in conventional mice, we went on to measure their clone sizes in vivo by quantitative PCR in the total and in adjacent sections of the small intestine of adult animals. We found that both the CD8αα and the CD8αβ TCRαβ IEL clones have a heterogeneous size pattern, with clones containing from 3 × 10³ cells up to 1.2 × 10⁶ cells, the clones being qualitatively and quantitatively different in individual mice. Cells from a given IEL clone are not evenly distributed throughout the length of the small intestine. The observation that the TCRαβ IEL populations comprise a few hundred clones of very heterogeneous size and distribution suggests that they arise from a limited number of precursors, which may be slowly but continuously renewed, and undergo extensive clonal expansion in the epithelium.     

CD5− CD8αβ intestinal intraepithelial lymphocytes (IEL) are induced to express CD5 upon antigen-specific activation: CD5− and CD5+ CD8αβ IEL do not represent separate T cell lineages

We followed αβ T cell receptor (TCR) usage in subsets of gut intraepithelial lymphocytes (IEL) in major histocompatibility complex class I-restricted αβ TCR-transgenic (tg) mice. The proportion of tg αβ TCR⁺ CD8αβ IEL is reduced compared with CD8⁺ splenocytes of the same animal, particularly under conventional conditions of maintenance. Further fractionation of CD8αβ IEL according to the expression level of surface CD5 revealed that in conventionally housed animals tg TCR⁺ CD5^? CD8αβ IEL are as frequent as in specific pathogen-free (SPF) mice, whereas tg TCR⁺ CD5^int or, even more pronounced, tg TCR⁺ CD5^hi CD8αβ IEL are greatly diminished when compared with mice kept under SPF conditions. Upon antigen-specific stimulation of CD5^? CD8αβ IEL in vitro, CD5 surface expression is up-regulated on a large fraction of cells within 48 h. Up-regulation of CD5 surface expression is further enhanced by the presence of the anti-αIEL monoclonal antibody 2E7. This clearly demonstrates that CD5^?, and CD5⁺ CD8αβ IEL cannot be considered as separate T cell lineages.     

Positive selection of self‐antigen‐specific CD8+ T cells by hematopoietic cells

In contrast to thymic epithelial cells, which induce the positive selection of conventional CD8⁺ T cells, hematopoietic cells (HCs) select innate CD8⁺ T cells whose Ag specificity is not fully understood. Here we show that CD8⁺ T cells expressing an H‐Y Ag‐specific Tg TCR were able to develop in mice in which only HCs expressed MHC class I, when HCs also expressed the H‐Y Ag. These HC‐selected self‐specific CD8⁺ T cells resemble innate CD8⁺ T cells in WT mice in terms of the expression of memory markers and effector functions, but are phenotypically distinct from the thymus‐independent CD8⁺ T‐cell population. The peripheral maintenance of H‐Y‐specific CD8⁺ T cells required presentation of the self‐Ag and IL‐15 on HCs. HC‐selected CD8⁺ T cells in mice lacking the Tg TCR also showed these features. Furthermore, by using MHC class I tetramers with a male Ag peptide, we found that self‐Ag‐specific CD8⁺ T cells in TCR non‐Tg mice could develop via HC‐induced positive selection, supporting results obtained from H‐Y TCR Tg mice. These findings indicate the presence of self‐specific CD8⁺ T cells that are positively selected by HCs in the peripheral T‐cell repertoire.     

Central tolerance spares the private high‐avidity CD4+ T‐cell repertoire specific for an islet antigen in NOD mice

Although central tolerance induces the deletion of most autoreactive T cells, some autoreactive T cells escape thymic censorship. Whether potentially harmful autoreactive T cells present distinct TCRαβ features remains unclear. Here, we analyzed the TCRαβ repertoire of CD4⁺ T cells specific for the S100β protein, an islet antigen associated with type 1 diabetes. We found that diabetes‐resistant NOD mice deficient for thymus specific serine protease (TSSP), a protease that impairs class II antigen presentation by thymic stromal cells, were hyporesponsive to the immunodominant S100β_1‐15 epitope, as compared to wild‐type NOD mice, due to intrathymic negative selection. In both TSSP‐deficient and wild‐type NOD mice, the TCRαβ repertoire of S100β‐specific CD4⁺ T cells though diverse showed a specific bias for dominant TCRα rearrangements with limited CDR3α diversity. These dominant TCRα chains were public since they were found in all mice. They were of intermediate‐ to low‐avidity. In contrast, high‐avidity T cells expressed unique TCRs specific to each individual (private TCRs) and were only found in wild‐type NOD mice. Hence, in NOD mice, the autoreactive CD4⁺ T‐cell compartment has two major components, a dominant and public low‐avidity TCRα repertoire and a private high‐avidity CD4⁺ T‐cell repertoire; the latter is deleted by re‐enforced negative selection.     

Distinct roles for CD4 and CD8 as co-receptors in T cell receptor signalling

We demonstrate that CD4 and CD8 modify signals induced through the T cell receptor for antigen (TCRαβ) in distinct fashions. Pretreatment of CD4⁺ lymph node T cells with CD4-specific monoclonal antibody results in a tenfold inhibition of DNA synthesis induced by anti-TCRαβ. In contrast, pretreatment of CD8⁺ T cells with CD8-specific mAb has no effect on DNA synthesis subsequently induced through TCRαβ. While inhibiting late activation signals, pretreatment with anti-CD4 does not detectably alter the pattern of anti-TCRαβ-induced tyrosine phosphorylation of cellular proteins, nor subsequent Ca²⁺ mobilization. The distinct biological consequences of anti-CD4 and anti-CD8 pretreatment correlate with the differential association of their respective ligands with the cellular protein tyrosine kinase, p56^lck. While both T cell lineages contain similar levels of cellular p56^lck, tenfold more is associated with CD4 than with CD8. This difference is associated with the differential effects of pretreatment with anti-CD4 and anti-CD8 on the distribution and activity of p56^lck. Further, antibody-mediated aggregation of TCRαβ on CD4⁺ T cells induces the appearance of a p56^lck species with decreased mobility in sodium dodecylsulfate-polyacrylamide gel electrophoresis. This effect is observed in CD4⁺ T cells exclusively and involves the fraction of p56^lck which is not associated with CD4. The results presented here demonstrate that the signalling elements which couple the antigen receptor to second messenger-generating systems are under distinct physical and/or functional constraints in the two T cell lineages.     

Effector and memory CD8+ T cells as seen in immunity to malaria

Summary: Transgenic (Tg) mice carrying a T‐cell receptor (TCR) specific for a CD8⁺ T‐cell epitope expressed in pre‐erythrocytic stages of Plasmodium yoelii has proven to be a valuable tool to advance our understanding of this anti‐parasite T‐cell response, as it occurs in vivo. The visualization of CD8⁺ T cells in vivo and ex vivo greatly facilitated research aimed at characterizing basic features of this T‐cell response such as the kinetics of differentiation and proliferation and the in vivo antigen presentation. Importantly, this research unveiled the existence of early self‐regulatory mechanisms controlling the magnitude of the CD8⁺ T‐cell response and also identified CD4⁺ T cells as critical elements in the development of memory populations. This review discusses our recent research using Tg mice and highlights our progress in understanding the CD8⁺ T‐cell‐mediated immunity against malaria liver stages.     

The multiple roles of the CD8 coreceptor in T cell biology: opportunities for the selective modulation of self-reactive cytotoxic T cells

Short peptide fragments generated by intracellular protein cleavage are presented on the surface of most nucleated cells bound to highly polymorphic MHCI molecules. These pMHCI complexes constitute an interface that allows the immune system to identify and eradicate anomalous cells, such as those that harbor infectious agents, through the activation of CTLs. Molecular recognition of pMHCI complexes is mediated primarily by clonally distributed TCRs expressed on the surface of CTLs. The coreceptor CD8 contributes to this antigen-recognition process by binding to a largely invariant region of the MHCI molecule and by promoting intracellular signaling, the effects of which serve to enhance TCR stimuli triggered by cognate ligands. Recent investigations have shed light on the role of CD8 in the activation of MHCI-restricted, antigen-experienced T cells and in the processes of T cell selection and lineage commitment in the thymus. Here, we review these data and discuss their implications for the development of potential therapeutic strategies that selectively target pathogenic CTL responses erroneously directed against self-derived antigens.     

Topical treatment of all‐trans retinoic acid inhibits murine melanoma partly by promoting CD8+ T‐cell immunity

All‐trans retinoic acid (atRA), the main biologically active metabolite of vitamin A, has been implicated in immunoregulation and anti‐cancer. A recent finding that vitamin A could decrease the risk of melanoma in humans indicates the beneficial role of atRA in melanoma. However, it remains unknown whether topical application of atRA could inhibit melanoma growth by influencing tumour immunity. We demonstrate topical application of tretinoin ointment (atRA as the active ingredient) effectively inhibited B16F10 melanoma growth. This is accompanied by markedly enhanced CD8⁺ T‐cell responses, as evidenced by significantly increased proportions of effector CD8⁺ T cells expressing granzyme B, tumour necrosis factor‐α, or interferon‐γ, and Ki67⁺ proliferating CD8⁺ T cells in atRA‐treated tumours compared with vaseline controls. Furthermore, topical atRA treatment promoted the differentiation of effector CD8⁺ T cells in draining lymph nodes (DLN) of tumour‐bearing mice. Interestingly, atRA did not affect tumoral CD4⁺ T‐cell response, and even inhibited the differentiation of interferon‐γ‐expressing T helper type 1 cells in DLN. Importantly, we demonstrated that the tumour‐inhibitory effect of atRA was partly dependent on CD8⁺ T cells, as CD8⁺ T‐cell depletion restored tumour volumes in atRA‐treated mice, which, however, was still significantly smaller than those in vaseline‐treated mice. Finally, we demonstrated that atRA up‐regulated MHCI expression in B16F10 cells, and DLN cells from tumour‐bearing mice had a significantly higher killing rate when culturing with atRA‐treated B16F10 cells. Hence, our study demonstrates that topical atRA treatment effectively inhibits melanoma growth partly by promoting the differentiation and the cytotoxic function of effector CD8⁺ T cells.     

IL‐15‐dependent CD8+CD122+ T cells ameliorate experimental autoimmune encephalomyelitis by modulating IL‐17 production by CD4+ T cells

Interleukin‐15 (IL‐15) is an inflammatory cytokine whose role in autoimmune diseases has not been fully elucidated. Th17 cells have been shown to play critical roles in experimental autoimmune encephalomyelitis (EAE) models. In this study, we demonstrate that blockade of IL‐15 signaling by TMβ‐1 mAb treatment aggravated EAE severity. The key mechanism was not NK‐cell depletion but depletion of CD8⁺CD122⁺ T cells. Adoptive transfer of exogenous CD8⁺CD122⁺ T cells to TMβ‐1‐treated mice rescued animals from severe disease. Moreover, transfer of preactivated CD8⁺CD122⁺ T cells prevented EAE development and significantly reduced IL‐17 secretion. Naïve effector CD4⁺CD25^? T cells cultured with either CD8⁺CD122⁺ T cells from wild‐type mice or IL‐15 transgenic mice displayed lower frequencies of IL‐17A production with lower amounts of IL‐17 in the supernatants when compared with production by effector CD4⁺CD25^? T cells cultured alone. Addition of a neutralizing antibody to IL‐10 led to recovery of IL‐17A production in Th17 cultures. Furthermore, coculture of CD8⁺CD122⁺ T cells with effector CD4⁺ T cells inhibited their proliferation significantly, suggesting a regulatory function for IL‐15 dependent CD8⁺CD122⁺ T cells. Taken together, these observations suggest that IL‐15, acting through CD8⁺CD122⁺ T cells, has a negative regulatory role in reducing IL‐17 production and Th17‐mediated EAE inflammation.     

Interleukin (IL)-21-independent pathogen-specific CD8+ T-cell expansion, and IL-21-dependent suppression of CD4+ T-cell IL-17 production

Although interleukin‐21 (IL‐21) potently activates and controls the differentiation of immune cells after stimulation in vitro, the role for this pleiotropic cytokine during in vivo infection remains poorly defined. Herein, the requirement for IL‐21 in innate and adaptive host defence after Listeria monocytogenes infection was examined. In the innate phase, IL‐21 deficiency did not cause significant defects in infection susceptibility, or in the early activation of natural killer and T cells. In the adaptive phase, L. monocytogenes‐specific CD8⁺ T cells expand to a similar magnitude in IL‐21‐deficient mice compared with control mice. Interestingly, the IL‐21‐independent expansion of L. monocytogenes‐specific CD8⁺ T cells was maintained even in the combined absence of IL‐12 and type I interferon (IFN) receptor. Similarly, L. monocytogenes‐specific CD4⁺ T cells expanded and produced similar levels of IFN‐γ regardless of IL‐21 deficiency. Unexpectedly however, IL‐21 deficiency caused significantly increased CD4⁺ T‐cell IL‐17 production, and this effect became even more pronounced after L. monocytogenes infection in mice with combined defects in both IL‐12 and type I IFN receptor that develop a T helper type 17‐dominated CD4⁺ T‐cell response. Despite increased CD4⁺ T‐cell IL‐17 production, L. monocytogenes‐specific T cells re‐expanded and conferred protection against secondary challenge with virulent L. monocytogenes regardless of IL‐21 deficiency, or combined defects in IL‐21, IL‐12, and type I IFN receptor. Together, these results demonstrate non‐essential individual and combined roles for IL‐21, IL‐12 and type I IFNs in priming pathogen‐specific CD8⁺ T cells, and reveal IL‐21‐dependent suppression of IL‐17 production by CD4⁺ T cells during in vivo infection.     

MHC I tetramer staining tends to overestimate the number of functionally relevant self-reactive CD8 T cells in the preimmune repertoire

Previous studies that used peptide-MHC (pMHC) tetramers (tet) to identify self-specific T cells have questioned the effectiveness of thymic-negative selection. Here, we used pMHCI tet to enumerate CD8 T cells specific for the immunodominant gp33 epitope of lymphocytic choriomeningitis virus glycoprotein (GP) in mice transgenically engineered to express high levels of GP as a self-antigen in the thymus. In GP-transgenic mice (GP⁺), monoclonal P14 TCR⁺ CD8 T cells that express a GP-specific TCR could not be detected by gp33/D^b-tet staining, indicative of their complete intrathymic deletion. By contrast, in the same GP⁺ mice, substantial numbers of polyclonal CD8 T cells identifiable by gp33/D^b-tet were present. The gp33-tet staining profiles of polyclonal T cells from GP⁺ and GP-negative (GP⁻) mice were overlapping, but mean fluorescence intensities were ∼15% lower in cells from GP⁺ mice. Remarkably, the gp33-tet⁺ T cells in GP⁺ mice failed to clonally expand after lymphocytic choriomeningitis virus infection, whereas those of GP⁻ mice did so. In Nur77^GFP-reporter mice, dose-dependent responses to gp33 peptide-induced TCR stimulation revealed that gp33-tet⁺ T cells with high ligand sensitivity are lacking in GP⁺ mice. Hence, pMHCI tet staining identifies self-specific CD8 T cells but tends to overestimate the number of truly self-reactive cells.     

TLR2 engagement on CD4+ T cells enhances effector functions and protective responses to Mycobacterium tuberculosis

We have previously demonstrated that mycobacterial lipoproteins engage TLR2 on human CD4⁺ T cells and upregulate TCR‐triggered IFN‐γ secretion and cell proliferation in vitro. Here we examined the role of CD4⁺ T‐cell‐expressed TLR2 in Mycobacterium tuberculosis (MTB) Ag‐specific T‐cell priming and in protection against MTB infection in vivo. Like their human counterparts, mouse CD4⁺ T cells express TLR2 and respond to TLR2 costimulation in vitro. This Th1‐like response was observed in the context of both polyclonal and Ag‐specific TCR stimulation. To evaluate the role of T‐cell TLR2 in priming of CD4⁺ T cells in vivo, naive MTB Ag85B‐specific TCR transgenic CD4⁺ T cells (P25 TCR‐Tg) were adoptively transferred into Tlr2^?/? recipient C57BL/6 mice that were then immunized with Ag85B and with or without TLR2 ligand Pam₃Cys‐SKKKK. TLR2 engagement during priming resulted in increased numbers of IFN‐γ‐secreting P25 TCR‐Tg T cells 1 week after immunization. P25 TCR‐Tg T cells stimulated in vitro via TCR and TLR2 conferred more protection than T cells stimulated via TCR alone when adoptively transferred before MTB infection. Our findings indicate that TLR2 engagement on CD4⁺ T cells increases MTB Ag‐specific responses and may contribute to protection against MTB infection.     

IL‐15 is critical for the maintenance and innate functions of self‐specific CD8+ T cells

IL‐15 is a pleiotropic cytokine involved in host defense as well as autoimmunity. IL‐15‐deficient mice show a decrease of memory phenotype (MP) CD8⁺ T cells, which develop naturally in naïve mice and whose origin is unclear. It has been shown that self‐specific CD8⁺ T cells developed in male H‐Y antigen‐specific TCR transgenic mice share many similarities with naturally occurring MP CD8⁺ T cells in normal mice. In this study, we found that H‐Y antigen‐specific CD8⁺ T cells in male but not female mice decreased when they were crossed with IL‐15‐deficient mice, mainly due to impaired peripheral maintenance. The self‐specific TCR transgenic CD8⁺ T cells developed in IL‐15‐deficient mice showed altered surface phenotypes and reduced effector functions ex vivo. Bystander activation of the self‐specific CD8⁺ T cells was induced in vivo during infection with Listeria monocytogenes, in which proliferation but not IFN‐γ production was IL‐15‐dependent. These results indicated important roles for IL‐15 in the maintenance and functions of self‐specific CD8⁺ T cells, which may be included in the naturally occurring MP CD8⁺ T‐cell population in naïve normal mice and participate in innate host defense responses.     

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.     

Epitope specificity of memory CD8+ T cells dictates vaccination-induced mortality in LCMV-infected perforin-deficient mice

Perforin‐deficient (PKO) mice serve as models for familial hemophagocytic lympho‐histiocytosis, a uniformly fatal disease associated with viral infection of perforin‐deficient humans. Naïve perforin‐deficient BALB/c mice survive while vaccinated PKO mice containing virus‐specific memory CD8 ⁺ T cells rapidly succumb to lymphocytic choriomeningitis virus (LCMV) infection. Thus, vaccination converts a nonlethal persistent infection into a fatal disease mediated by virus‐specific memory CD8 ⁺ T cells. Here, we determine the extent to which vaccination‐induced mortality in PKO mice following LCMV challenge is due to differences in vaccine modalities, the quantity or epitope specificity of memory CD8⁺ T cells. We show that LCMV‐induced mortality in immune PKO mice is independent of vaccine modalities and that the starting number of memory CD8 ⁺ T cells specific to the immunodominant epitope NP_118‐126 dictates the magnitude of secondary CD8 ⁺ T‐cell expansion, the inability to regulate production of CD8⁺ T‐cell‐derived IFN‐γ, and mortality in the vaccinated PKO mice. Importantly, mortality is determined by the epitope specificity of memory CD8 ⁺ T cells and the associated degree of functional exhaustion and cytokine dysregulation but not the absolute magnitude of CD8 ⁺ T‐cell expansion. These data suggest that deeper understanding of the parameters that influence the outcome of vaccine‐induced diseases would aid rational vaccine design to minimize adverse outcomes after infection.     

Inhibition of Alloantigen‐Primed Memory CD4+ and CD8+ T Cells by Hematopoietic Chimerism in Mice

Donor‐reactive memory T cells present a special hurdle in transplantation. Although hematopoietic chimerism is effective for inducing donor‐specific tolerance, the effects on memory T cells are unclear. Here, we induced stable chimerism and tolerance in mice (Tolerance group, n = 6) by donor‐specific transfusion (DST) plus anti‐CD154 monoclonal antibody (mAb), avoiding the toxic myeloablative conditioning treatment to assist bone marrow transplantation (DST/aCD154&BMTx). We then transferred memory CD4⁺ or CD8⁺ T cells from donor antigen primed mice to the tolerance‐induced recipients 4 days after heart transplantation (Tol/CD4⁺ Tm group and Tol/CD8⁺ Tm group, n = 6, respectively), but neither of these memory T‐cell subsets had an effect on the permanent graft survival (median survival time > 100 days). The unaltered rate of memory T cells in spleen and anergy to donor antigen in vitro demonstrated that these memory T cells were well controlled. The chimerism‐promoting protocol DST/aCD154&BMTx produced an immune environment that included high levels of regulatory T cells (Tregs), microchimerism and TGF‐β, all of which may act in suppressing the donor‐reactive memory CD4⁺ or CD8⁺ T cells. These findings have potentially important implications for designing approaches to suppressing memory T cells for success of transplantation.