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: analysis of human V alpha 24+CD8+ NK T cells activated by alpha-galactosylceramide-pulsed monocyte-derived dendritic cells

Human Valpha24(+) NKT cells constitute a counterpart of mouse Valpha14(+) NKT cells, both of which use an invariant TCR-alpha chain. The human Valpha24(+) NKT cells as well as mouse Valpha14(+) NKT cells are activated by glycolipids in a CD1d-restricted manner and produce many immunomodulatory cytokines, possibly affecting the immune balance. In mice, it has been considered from extensive investigations that Valpha14(+)CD8(+) NKT cells that express invariant TCR do not exist. Here we introduce human Valpha24(+)CD8(+) NKT cells. These cells share important features of Valpha24(+) NKT cells in common, but in contrast to CD4(-)CD8(-) (double-negative) or CD4(+) Valpha24(+) NKT cells, they do not produce IL-4. Our discovery may extend and deepen the research field of Valpha24(+) NKT cells as well as help to understand the mechanism of the immune balance-related diseases.     

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: 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: a novel mechanism for rescue of B cells from CD95/Fas-mediated apoptosis.

CD95-induced apoptosis contributes to the maintenance of homeostasis in both B and T lymphocyte-mediated immunity. B cells increase CD95 expression in response to activation signals and become susceptible to CD95-induced apoptosis. Protection from CD95-mediated death signals can be induced in mature B cells by signals delivered through the B cell Ag receptor. In this paper we demonstrate for the first time that rescue from apoptosis can occur independently of de novo protein synthesis. This rescue from apoptosis prevents activation of caspase 8, the apical caspase in the CD95 death pathway, and CD95-FADD (Fas-associated death domain containing protein) association does not occur normally. Thus B cell activation signals can biochemically modify proximal elements of the CD95 death pathway and regulate the sensitivity of cells to apoptosis induction at an early stage in programmed cell death.     

Cutting edge: antibody production to pneumococcal polysaccharides requires CD1 molecules and CD8+ T cells

T cell involvement in Ab responses to thymus-independent type 2 Ags is an immunologic enigma. The identity of these cells and the mechanisms of their TCR engagement to carbohydrate molecules remain unknown. We measured IgG Ab production after immunization with pneumococcal polysaccharides in mice with disruptions in selected genes of the T cell pathway. Nonclassical MHC class I-like CD1 molecules and MHC class I-dependent CD8+ cells were found to be essential. Our findings set forth a new paradigm for humoral responses in which CD1 expression as well as a subset of CD8+ cells are required to provide helper function for Ab production against thymus-independent type 2 polysaccharides, similar to MHC class II-restricted CD4+ cells for protein Ags.     

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: 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: 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: 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.     

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: pulmonary immunopathology mediated by antigen-specific expression of TNF-alpha by antiviral CD8+ T cells

Respiratory virus infection results in considerable pulmonary immunopathology, a component of which results from the host immune responses. We have developed a murine model to specifically examine the lung injury due to CD8(+) T cell recognition of an influenza hemagglutinin (HA) transgene on lung epithelium in the absence of replicating virus, after adoptive transfer. Lung injury is largely mediated by chemokines expressed by the epithelial cells upon T cell recognition mediated by TNF-alpha. To determine the critical source of TNF-alpha, HA-specific TNF(-/-) CD8(+) T cells were transferred into HA transgenic animals, and lung injury was not observed, though these T cells exhibited no defect in antiviral activity in vivo. This indicates that the initiating event in the injury process is Ag-specific expression of TNF-alpha by antiviral CD8(+) T cells upon recognition of alveolar epithelial Ag, and that the effector activities responsible for viral clearance may be dissociable from those resulting in immunopathology.     

Cutting edge: soluble IL-6R is produced by IL-6R ectodomain shedding in activated CD4 T cells

IL-6 trans-signaling via the soluble IL-6R (sIL-6R) plays an important role in the progression of several autoimmune diseases and cancer by providing IL-6-responsiveness to cells lacking IL-6R. However, the potential sources of sIL-6R are less understood. In this study we show that sIL-6R is produced by both naive and memory CD4 T cells upon TCR activation. The production of sIL-6R by activated CD4 T cells is mediated by shedding of the membrane-bound IL-6R, and this process correlates with the expression of the metalloproteinase ADAM17 in these cells. In contrast to CD4 T cells, CD8 T cells do not express ADAM17 and their production of sIL-6R is negligible. Thus, during an immune response CD4 T cells are an important source of sIL-6R. Production of sIL-6R by autoreactive CD4 T cells may contribute to their role in the development of autoimmune disease by conferring IL-6-responsiveness to cells lacking IL-6R such as synoviocytes.     

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: 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: 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: direct action of thymic stromal lymphopoietin on activated human CD4+ T cells

Thymic stromal lymphopoietin (TSLP) is a cytokine that promotes CD4(+) T cell homeostasis and contributes to allergic and inflammatory responses. TSLP can act directly on mouse CD4(+) T cells, but in humans, the available data have indicated that TSLP receptors are not expressed on CD4(+) T cells and that TSLP instead activates dendritic cells, which in turn promote the proliferation and differentiation of CD4(+) T cells. We now unexpectedly demonstrate the presence of TSLP receptors on activated human CD4(+) T cells. Strikingly, whereas freshly isolated peripheral blood human T cells show little if any response to TSLP, TCR stimulation allows a potent response to this cytokine. Moreover, TSLP increases the sensitivity of human CD4(+) T cells to low doses of IL-2, augmenting responsiveness of these cells to TCR engagement. Our results establish that human CD4(+) T cells are direct targets for TSLP.     

Cutting edge: virus-specific CD4+ memory T cells in nonlymphoid tissues express a highly activated phenotype

Recent studies have shown that CD4(+) memory T cells persist in nonlymphoid organs following infections. However, the development and phenotype of these peripheral memory cells are poorly defined. In this study, multimerized MHC-Ig fusion proteins, with a covalently attached peptide sequence from the Sendai virus hemagglutinin/neuraminidase gene, have been used to identify virus-specific CD4(+) T cells during Sendai virus infection and the establishment of peripheral CD4(+) memory populations in the lungs. We show declining frequencies of virus-specific CD4(+) T cells in the lungs over the course of approximately 3 mo after infection. Like peripheral CD8(+) T cells, the CD4(+) have an acutely activated phenotype, suggesting that a high level of differentiation is required to reach the airways and persist as memory cells. Differences in CD25 and CD11a expression indicate that the CD4(+) cells from the lung airways and parenchyma are distinct memory populations.     

Cutting edge: neuronal recognition by CD8 T cells elicits central diabetes insipidus

An increasing number of neurologic diseases is associated with autoimmunity. The immune effectors contributing to the pathogenesis of such diseases are often unclear. To explore whether self-reactive CD8 T cells could attack CNS neurons in vivo, we generated a mouse model in which the influenza virus hemagglutinin (HA) is expressed specifically in CNS neurons. Transfer of cytotoxic anti-HA CD8 T cells induced an acute but reversible encephalomyelitis in HA-expressing recipient mice. Unexpectedly, diabetes insipidus developed in surviving animals. This robust phenotype was associated with preferential accumulation of cytotoxic CD8 T cells in the hypothalamus, upregulation of MHC class I molecules, and destruction of vasopressin-expressing neurons. IFN-γ production by the pathogenic CD8 T cells was necessary for MHC class I upregulation by hypothalamic neurons and their destruction. This novel mouse model, in combination with related human data, supports the concept that autoreactive CD8 T cells can trigger central diabetes insipidus.     

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.