Cutting edge: increased expression of Bcl-2 in antigen-specific memory CD8+ T cells

Bcl-2 plays a critical role in regulating cell survival and apoptosis. We examined Bcl-2 expression in virus-specific CD8 T cells during the expansion, death, and memory phases of the T cell response following infection of mice with lymphocytic choriomeningitis virus (LCMV). Naive CD8 T cells expressed a basal level of Bcl-2 that was down-regulated in effector CD8 T cells just before the death phase. Bcl-2 levels remained low during the death phase but surviving memory CD8 T cells expressed higher levels of Bcl-2 than naive cells. These changes were shown to occur in LCMV TCR transgenic cells as well as virus-specific CD8 T cells in C57BL/6 and BALB/c mice identified by MHC class I tetramers. In all instances, memory CD8 T cells expressed higher levels of Bcl-2, suggesting that increased Bcl-2 expression plays a role in the long-term maintenance of memory CD8 T cells in vivo.     

Cutting edge: OFF cycling of TNF production by antigen-specific CD8+ T cells is antigen independent

Although they are known for their capacity to kill infected cells, Ag-specific CD8(+) T cells elaborate other effector mechanisms, including TNF and IFN-gamma, that contribute to defense against infection. Ag-specific CD8(+) T cells rapidly turn ON and turn OFF IFN-gamma production in direct response to Ag contact, presumably to minimize the potential immunopathology that could result from inappropriate secretion of this inflammatory mediator. In this study, we show, using in vitro propagated and directly ex vivo-analyzed Ag-specific CD8(+) T cells, that in contrast to Ag-dependent ON/OFF cycling of IFN-gamma production, the cessation of TNF production by the same IFN-gamma producing cells is rapid and Ag independent.     

Cutting edge: cognate CD4 help promotes recruitment of antigen-specific CD8 T cells around dendritic cells

The cellular orchestration underlying help provided by CD4 T lymphocytes to CD8 T cell responses is not fully understood. We documented that the formation of three-cell clusters occurred as soon as day 1 and relied on long-lasting CD4 and CD8 T cell interactions with dendritic cells (DCs). The influence of CD4 help on CD8 T cell differentiation could be observed as early as the second round of cell division. Importantly, our results identify a new facet to the phenomenon of CD4 help in which DCs, upon cognate interactions with CD4 T cells, increase their ability to attract/retain Ag-specific CD8 T cells. Our results support a model in which CD4 help operates rapidly, in part by favoring CD8 T cells recruitment around those DCs that are the most competent for priming.     

Cutting edge: cross-presented intracranial antigen primes CD8+ T cells

The CNS is considered immune privileged due to the blood-brain barrier and the absence of conventional lymphatics. Nonetheless, T cell immune responses specific for CNS Ag have been documented. Where these events are initiated and what cellular mechanisms are involved remain unknown. In this study, we established an experimental mouse model to evaluate the requirements for priming CD8+ T cells following the cross-presentation of intracranial Ag. Surprisingly, we find that even with a damaged blood-brain barrier, Ag presentation occurs in regional lymph nodes and not within the CNS itself. Only once the responding cells have expanded can they traffic to the site of CNS injury. Cross-presentation of intracranial Ag is efficient and the subsequent priming of CD8+ T cells is dependent on CD4+ T cell help and CD40 signaling in host APCs. Our findings have important implications for the initiation of T cell immune responses toward CNS Ags.     

Cutting edge: control of CD8+ T cell activation by CD4+CD25+ immunoregulatory cells

CD4(+)CD25(+) regulatory T cells inhibit organ-specific autoimmune diseases induced by CD4(+)CD25(-) T cells and are potent suppressors of CD4(+)CD25(-) T cell activation in vitro. We demonstrate that CD4(+)CD25(+) T cells also suppress both proliferation and IFN-gamma production by CD8(+) T cells induced either by polyclonal or Ag-specific stimuli. CD4(+)CD25(+) T cells inhibit the activation of CD8(+) responders by inhibiting both IL-2 production and up-regulation of IL-2Ralpha-chain (CD25) expression. Suppression is mediated via a T-T interaction as activated CD4(+)CD25(+) T cells suppress the responses of TCR-transgenic CD8(+) T cells stimulated with soluble peptide-MHC class I tetramers in the complete absence of APC. These results broaden the immunoregulatory role played by CD4(+)CD25(+) T cells in the prevention of autoimmune diseases, but also raise the possibility that they may hinder the induction of effector CD8(+) T cells to tumor or foreign Ags.     

Cutting edge: engagement of NKG2A on CD8+ effector T cells limits immunopathology in influenza pneumonia

Influenza pneumonia results in considerable lung injury, a significant component of which is mediated by CD8+ T cell Ag recognition in the distal airways and alveoli. TNF-alpha produced by Ag-specific CD8+ T cells appears primarily responsible for this immunopathology, and we have examined the negative regulation of CD8+ TNF production by CD94/NKG2A engagement with its receptor, Qa-1b. TNF production by antiviral CD8+ T cells was significantly enhanced by NKG2A blockade in vitro, and mice deficient in the NKG2A ligand, Qa-1b, manifested significantly greater pulmonary pathology upon CD8+ T cell-mediated clearance in influenza pneumonia. Furthermore, blockade of NKG2A ligation resulted in the enhancement of lung injury induced by CD8+ effector cell recognition of alveolar Ag in vivo in the absence of infectious virus. These data demonstrate that CD94/NKG2A transduces a biologically important signal in vivo to activated CD8+ T cells that limits immunopathology in severe influenza infection.     

Cutting edge: expression of chemokine receptor CXCR1 on human effector CD8+ T cells

IL-8 is a potent inflammatory cytokine that induces chemotaxis of neutrophils expressing CXCR1 and CXCR2, thus indicating its involvement in the migration of these cells to inflammatory sites where bacteria proliferate. Presently, we showed that CXCR1(+) cells were predominantly found among CD8(+) T cells having effector phenotype, and that the expression of CXCR1 was positively correlated with that of perforin, suggesting that CXCR1 is expressed on effector CD8(+) T cells. Indeed, human CMV-specific CD8(+) T cells from healthy individuals, which mostly express the effector phenotype and have cytolytic function, expressed CXCR1, whereas EBV-specific CD8(+) T cells, which mostly express the memory phenotype and have no cytolytic function, did not express this receptor. The results of a chemotaxis assay showed that the migration of CXCR1(+)CD8(+) T cells was induced by IL-8. These results suggest that the IL-8-CXCR1 pathway plays an important role in the homing of effector CD8(+) T cells.     

Cutting edge: CD8+CD122+ regulatory T cells produce IL-10 to suppress IFN-gamma production and proliferation of CD8+ T cells

We recently identified CD8+CD122+ regulatory T cells that directly control CD8+ and CD4+ cells without intervention of APCs. In this study, we investigated the effector mechanism of CD8+CD122+ regulatory T cells by using an in vitro regulation system. The profile of cytokine expression revealed that IL-10 was predominantly produced by CD8+CD122+ cells, whereas other cytokines were similarly expressed in CD8+CD122+ cells and CD8+CD122- cells. Suppression of both proliferation and IFN-gamma production by CD8+CD122- cells by CD8+CD122+ cells was blocked by adding anti-IL-10 Ab to the culture but not by adding anti-TGF-beta Ab. When IL-10 was removed from the conditioned medium from CD8+CD122+ cells, the conditioned medium no longer showed regulatory activity. Finally, CD8+CD122+ cells from IL-10-deficient mice had no regulatory activity in vitro and reduced regulatory activity in vivo. Our results clearly indicate that IL-10 is produced by CD8+CD122+ cells and mediates the regulatory activity of these cells.     

Peripheral deletion of antigen-specific T cells leads to long-term tolerance mediated by CD8+ cytotoxic cells

Peripheral deletion is one mechanism by which potentially self-reactive clones are removed whether they escape thymic deletion. We have examined the consequences of deleting Ag-specific T cells by i.v. injection of soluble Ag. Deletion of DO11.10 T cells by peptide was mediated predominately via a Fas/FasL mechanism. Animals that underwent deletion were tolerant to subsequent immunization with Ag, even when tolerant mice were given fresh Ag-specific DO11.10 T cells before immunization. Tolerance was mediated by CD8(+) T cells that killed the DO11.10-transgenic T cells in vivo. These data demonstrate that the programmed cell death of large numbers of T cells leads to peripheral tolerance mediated by CD8(+) CTLs.     

Gene expression in antigen-specific CD8+ T cells during viral infection

Following infection with intracellular pathogens, Ag-specific CD8(+) T cells become activated and begin to proliferate. As these cells become activated, they elaborate effector functions including cytokine production and cytolysis. After the infection has been cleared, the immune system returns to homeostasis through apoptosis of the majority of the Ag-specific effector cells. The surviving memory cells can persist for extended periods and provide protection against reinfection. Little is known about the changes in gene expression as Ag-specific cells progress through these stages of development, i.e., naive to effector to memory. Using recombinant MHC class I tetramers, we isolated Ag-specific CD8(+) T cells from mice infected with lymphocytic choriomeningitis virus at various time points and performed semiquantitative RT-PCR. We examined expression of: 1) genes involved in cell cycle control, 2) effector and regulatory functions, and 3) susceptibility to apoptosis. We found that Ag-specific CD8(+) memory T cells contain high steady-state levels of Bcl-2, BAX:, IFN-gamma, and lung Kruppel-like factor (LKLF), and decreased levels of p21 and p27 mRNA. Moreover, the pattern of gene expression between naive and memory cells is distinct and suggests that these two cell types control susceptibility to apoptosis through different mechanisms.     

Cutting edge: cure of colitis by CD4+CD25+ regulatory T cells

CD4(+)CD25(+) regulatory T cells have been shown to prevent T cell-mediated immune pathology; however, their ability to ameliorate established inflammation has not been tested. Using the CD4(+)CD45RB(high) T cell transfer model of inflammatory bowel disease, we show that CD4(+)CD25(+) but not CD4(+)CD25(-)CD45RB(low) T cells are able to cure intestinal inflammation. Transfer of CD4(+)CD25(+) T cells into mice with colitis led to resolution of the lamina propria infiltrate in the intestine and reappearance of normal intestinal architecture. CD4(+)CD25(+) T cells were found to proliferate in the mesenteric lymph nodes and inflamed colon. They were located between clusters of CD11c(+) cells and pathogenic T cells and found to be in contact with both cell types. These studies suggest that manipulation of CD4(+)CD25(+) T cells may be beneficial in the treatment of chronic inflammatory diseases.     

Cutting edge: virus selectively primes human langerhans cells for CD70 expression promoting CD8+ T cell responses

The two outermost compartments of skin are populated by different Ag-presenting dendritic cell types. Epidermal Langerhans cells (LCs) are evolutionarily adapted to the continuous presence of harmless skin commensals by the selective lack of cell surface TLRs that sense bacteria. In this article, we analyze the ability of LCs and dermal dendritic cells (DDCs) to respond to virus infection. Live virus and intracellular TLR3-agonist dsRNA commit LCs more effectively than DDCs to stimulate naive CD8(+) T cell expansion and their differentiation into effector cells. This potent CD8(+) T cell-promoting capacity of LCs is causally related to high levels of virus-induced CD70 expression but not to IL-12 production. These data suggest a remarkable specialization of LCs in the induction of pathogen class-specific adaptive immunity. Whereas LCs ignore bacteria, they are superior to DDCs to initiate effective CD70-mediated CD8(+) T cells in response to virus stimulation.     

Cutting edge: intravenous soluble antigen is presented to CD4 T cells by CD8- dendritic cells, but cross-presented to CD8 T cells by CD8+ dendritic cells

Mouse spleen contains three distinct mature dendritic cell (DC) populations (CD4(+)8(-), CD4(-)8(-), and CD4(-)8(+)) which retain a capacity to take up particulate and soluble AGS: Although the three splenic DC subtypes showed similar uptake of injected soluble OVA, they differed markedly in their capacity to present this Ag and activate proliferation in OVA-specific CD4 or CD8 T cells. For class II MHC-restricted presentation to CD4 T cells, the CD8(-) DC subtypes were more efficient, but for class I MHC-restricted presentation to CD8 T cells, the CD8(+) DC subtype was far more effective. This differential persisted when the DC were activated with LPS. The CD8(+) DC are therefore specialized for in vivo cross-presentation of exogenous soluble Ags into the class I MHC presentation pathway.     

Cutting edge: sustained expansion of CD8+ T cells requires CD154 expression by Th cells in acute graft versus host disease

Brief treatment with alphaCD154 Ab has been shown to prevent acute graft versus host disease (aGvHD). We extend these data to show that in the absence of CD154 function, donor T cells are unable to expand or generate high level anti-host CTL activity. Using transgenic (Tg) alloreactive CD8+ T cells adoptively transferred into allogeneic recipients, we show that short-term expansion of the CD8+ Tg T cells occurred in the absence of Th cells, and this short-term expansion could be facilitated with an agonistic alphaCD40. While CD40 agonism could enhance short-term expansion, sustained expansion of CD8+ Tg T cells required bona fide CD154-expressing CD4+ alloreactive Th cells. While CD154 was necessary for CD8+ Tg T cell sustained expansion, IL-2 was also implicated as essential. These observations suggest alphaCD154 therapy in GvHD is effective because the treatment causes an abortive CD8 alloresponse leading to the exhaustion or deletion of alloreactive CD8+ clones preventing the development of disease.     

Cutting edge: direct suppression of B cells by CD4+ CD25+ regulatory T cells

Regulatory T cells (Tregs) can potentially migrate to the B cell areas of secondary lymphoid tissues and suppress T cell-dependent B cell Ig response. T cell-dependent Ig response requires B cell stimulation by Th cells. It has been unknown whether Tregs can directly suppress B cells or whether they must suppress Th cells to suppress B cell response. We report here that Foxp3+ Tregs are found in T-B area borders and within germinal centers of human lymphoid tissues and can directly suppress B cell Ig response. Although Tregs can effectively suppress T cells, they can also directly suppress B cell response without the need to first suppress Th cells. The direct suppression of B cell Ig production by Tregs is accompanied by inhibition of Ig class switch recombination.     

Cutting edge: rapid in vivo killing by memory CD8 T cells

In this study, we examined the cytotoxic activity of effector and memory CD8 T cells in vivo. At the peak of the CTL response following an acute lymphocytic choriomeningitis virus infection, effector CD8 T cells exhibited extremely rapid killing and started to eliminate adoptively transferred target cells within 15 min by a perforin-dependent mechanism. Although resting memory CD8 T cells are poorly cytolytic by in vitro (51)Cr release assays, there was rapid elimination (within 1-4 h) of target cells after transfer into immune mice, and both CD62L(high) and CD62L(low) memory CD8 T cells were able to kill rapidly in vivo. Strikingly, when directly compared on a per cell basis, memory CD8 T cells were only slightly slower than effector cells in eliminating target cells. These data indicate that virus specific memory CD8 T cells can rapidly acquire cytotoxic function upon re-exposure to Ag and are much more efficient killers in vivo than previously appreciated.     

Cutting edge: cytolytic effector function in human circulating CD8+ T cells closely correlates with CD56 surface expression

Recent data suggest that human effector CD8+ T cells express a distinct CD27-CD45RAhigh (CD57+CD28-CD11ahigh) phenotype. Here, we propose that CTL effector function correlates better with CD56 (neuronal cell adhesion molecule (NCAM)) surface expression. CD56 was absent on cord blood CD8+ T cells, but was expressed by 4-30% of freshly isolated circulating CD8+ T cells from 15 adults. Dramatic oligoclonal expansions in 3/3 individuals were confined to the CD56+ subset of CD8+ T cells. The CD56+ subset generally contained high amounts of intracellular perforin and granzyme B. Finally, direct cytolytic capacity was closely restricted to the CD56+(CD45RAhigh) cells, better than to CD27-CD45RAhigh cells in 5/5 individuals analyzed. Thus, the phenotype corresponding to the circulating effector CD8+ T cell pool may be simplified and more precisely defined by the use of just two surface markers: CD8 and CD56.     

Cutting edge: requirement for IL-15 in the generation of primary and memory antigen-specific CD8 T cells

IL-15 and IL-15Ralpha are required for generation of memory-phenotype CD8 T cells in unimmunized mice. However, the role of IL-15 in primary expansion and generation of Ag-specific memory CD8 T cells in vivo has not been investigated. We characterized the CD8 T cell response against vesicular stomatitis virus (VSV) in IL-15(-/-) and IL-15Ralpha(-/-) mice. Surprisingly, IL-15 was required for primary expansion of VSV-specific CD8 T cells. The generation of VSV-specific memory CD8 T cells was also impaired without IL-15 signaling, and this defect correlated with a decrease in memory CD8 T cell turnover. Despite minimal proliferation without IL-15, a subset of memory cells survived long-term. IL-15Ralpha expression was low on naive CD8 T cells, up-regulated on Ag-specific effector cells, and sustained on memory cells. Thus, IL-15 was important for the generation and the subsequent maintenance of antiviral memory CD8 T cells.     

Cutting edge: detection of antigen-specific CD4+ T cells by HLA-DR1 oligomers is dependent on the T cell activation state

Class I MHC tetramers have proven to be invaluable tools for following and deciphering the CD8(+) T cell response, but the development of similar reagents for detection of CD4(+) T cells based on class II MHC proteins has been more difficult. We evaluated fluorescent streptavidin-based oligomers of HLA-DR1 for use as reagents to analyze Ag-specific human CD4(+) T cells. Staining was blocked at low temperatures and by drugs that disrupt microfilament formation and endocytosis. Cell-associated MHC oligomers were resistant to a surface stripping protocol and were observed by microscopy in intracellular compartments. This behavior indicates that detection of CD4(+) T cells using class II MHC oligomers can depend on an active cellular process in which T cells cluster and/or endocytose their Ag receptors. T cells of identical specificity but in different activation states varied greatly in their ability to be detected by class II MHC oligomers.     

Cutting edge: blockade of the CD28/B7 costimulatory pathway inhibits intestinal allograft rejection mediated by CD4+ but not CD8+ T cells.

The effect of blocking the CD28/B7 costimulatory pathway on intestinal allograft rejection was examined in mice. Murine CTLA4Ig failed to prevent the rejection of allografts transplanted into wild-type or CD4 knockout (KO) mice but did inhibit allograft rejection by CD8 KO recipients. This effect was associated with decreased intragraft mRNA for IFN-gamma and TNF-alpha and increased mRNA for IL-4 and IL-5. This altered pattern of cytokine production was not observed in allografts from murine CTLA4Ig-treated CD4 KO mice. These data demonstrate that blockade of the CD28/B7 pathway has different effects on intestinal allograft rejection mediated by CD4+ and CD8+ T cells and suggest that these T cell subsets have different costimulatory requirements in vivo. The results also suggest that the inhibition of CD4+ T cell-mediated allograft rejection by CTLA4Ig may be related to down-regulation of Th1 cytokines and/or up-regulation of Th2 cytokines.