Induction of CD8+ regulatory T cells in the intestine by Heligmosomoides polygyrus infection

This study determined whether Heligmosomoides polygyrus induces intestinal regulatory T cells. Splenic T cells proliferate strongly when cultured with anti-CD3 and antigen-presenting cells (APC). Lamina propria T cells from mice with H. polygyrus mixed with normal splenic T cells from uninfected mice inhibited proliferation over 90%. Lamina propria T cells from mice without H. polygyrus only modestly affected T cell proliferation. The worm-induced regulatory T cell was CD8+ and required splenic T cell contact to inhibit proliferation. The regulation also was IL-10 independent, but TAP-dependent, suggesting that it requires major histocompatibility complex (MHC) class I interaction. Additional studies employed mice with transgenic T cells that did not express functional TGF-beta receptors. The lamina propria T regulator inhibited proliferation of these transgenic T cells nearly 100%, suggesting that TGF-beta signaling via the T cell was not required. CD8+ T cells were needed for worms to reverse piroxicam-induced colitis in Rag mice (T and B cell deficient) reconstituted with IL-10-/- T cells. Thus H. polygyrus induces a regulatory CD8+ lamina propria T cell that inhibits T cell proliferation and that appears to have a role in control of colitis.     

CD8+ T cell protective immunity against Chlamydia pneumoniae includes an H2-M3-restricted response that is largely CD4+ T cell-independent

CD8+ T cells are important for immunity to the intracellular bacterial pathogen Chlamydia pneumoniae (Cpn). Recently, we reported that type 1 CD8+ (Tc1) from Cpn-infected B6 mice recognize peptides from multiple Cpn Ags in a classical MHC class Ia-restricted fashion. In this study, we show that Cpn infection also induces nonclassical MHC class Ib-(H2-M3)-restricted CD8+ T cell responses. H2-M3-binding peptides representing the N-terminal formylated sequences from five Cpn Ags sensitized target cells for lysis by cytolytic effectors from the spleens of infected B6 mice. Of these, only peptides fMFFAPL (P1) and fMLYWFL (P4) stimulated IFN-gamma production by infection-primed splenic and pulmonary CD8+ T cells. Studies with Cpn-infected Kb-/-/Db-/- mice confirmed the Tc1 cytokine profile of P1- and P4-specific CD8+ T cells and revealed the capacity of these effectors to exert in vitro H2-M3-restricted lysis of Cpn-infected macrophages and in vivo pulmonary killing of P1- and P4-coated splenocytes. Furthermore, adoptive transfer of P1- and P4-specific CD8+ T cells into naive Kb-/-/Db-/- mice reduced lung Cpn loads following challenge. Finally, we show that in the absence of MHC class Ia-restricted CD8+ T cell responses, CD4+ T cells are largely expendable for the control of Cpn growth, and for the generation, memory maintenance, and secondary expansion of P1- and P4-specific CD8+ T cells. These results suggest that H2-M3-restricted CD8+ T cells contribute to protective immunity against Cpn, and that chlamydial Ags presented by MHC class Ib molecules may represent novel targets for inclusion in anti-Cpn vaccines.     

Fingerprints of anergic T cells

Peripheral T cell tolerance may result from activation-induced cell death [1], anergy [1], and/or immune response modulation by regulatory T cells [2]. In mice that express a transgenic receptor specific for peptide 111-119 of influenza hemagglutinin presented by E(d) class II MHC molecules as well as hemagglutinin under control of the immunoglobulin-kappa promoter, we have found that anergic T cells [3] can also have immunoregulatory function and secrete IL-10 [4]. In order to obtain information on molecular mechanisms involved in anergy and immunoregulation, we have compared expression levels of 1176 genes in anergic, naive, and recently activated CD4+ T cells of the same specificity by gene array analysis. The results provide a plausible explanation for the anergic phenotype in terms of proliferation, provide new information on the surface phenotype of in vivo-generated anergic CD4+ T cells, and yield clues with regard to new candidate genes that may be responsible for the restricted cytokine production of in vivo-anergized CD4+ T cells. The molecular fingerprints of such T cells should enable the tracking of this small population in the normal organism and the study of their role in immunoregulation.     

The role of B7 costimulation in CD4/CD8 T cell homeostasis

The effect of B7-mediated costimulation on T cell homeostasis was examined in studies of B7-1 (CD80) and B7-2 (CD86) transgenic as well as B7-deficient mice. B7 overexpression in transgenic mice resulted in marked polyclonal peripheral T cell hyperplasia accompanied by skewing toward an increased proportion of CD8 single-positive cells and a decreased proportion of CD4 single-positive cells in thymus and more markedly in peripheral T cells. B7-induced T cell expansion was dependent on both CD28 and TCR expression. Transgenic overexpression of B7-1 or B7-2 resulted in down-regulation of cell surface CD28 on thymocytes and peripheral T cells through a mechanism mediated by intercellular interaction. Mice deficient in B7-1 and B7-2 exhibited changes that were the reciprocal of those observed in B7-overexpressing transgenics: a marked increase in the CD4/CD8 ratio in peripheral T cells and an increase in cell surface CD28 in thymus and peripheral T cells. These reciprocal effects of genetically engineered increase or decrease in B7 expression indicate that B7 costimulation plays a physiological role in the regulation of CD4+ and CD8+ T cell homeostasis.     

Ontogeny and expression of non-MHC-restricted T cell antigen-binding molecules by thymocytes and peripheral T cell subsets

The ontogeny of T cells which express major histocompatibility complex (MHC)-unrestricted T cell antigen-binding molecules (TABM) on the cell membrane was investigated. We used a rabbit anti-mouse TABM antiserum to investigate the expression of TABM by subsets of adult thymocytes, peripheral T cells, and thymocytes during gestation. TABM are expressed by CD4+, CD8-, CD4+, CD8+ thymocytes and single-positive thymocytes. During gestation, TABM are expressed as early as Day 16, and at birth the expression of TABM on thymocytes has reached adult levels. Data are also presented which suggest that the expression of membrane TABM (mTABM) on peripheral T cells can be upregulated during T cell activation. The results suggest that TABM are expressed by different T cell subsets and that TABM+ cells may utilize the same intrathymic developmental pathway as that of T cells which express the alpha/beta T cell receptor.     

Cutting edge: B cells promote CD8+ T cell activation in MRL-Fas(lpr) mice independently of MHC class I antigen presentation

Spontaneous CD8+ T cell activation in MRL-Faslpr mice is B cell dependent. It is unclear whether this B-dependent activation is mediated by direct Ag presentation via MHC class I proteins (i.e., cross-presentation) or whether activation occurs by an indirect mechanism, e.g., via effects on CD4+ cells. To determine how CD8+ T cell activation is promoted by B cells, we created mixed bone marrow chimeras where direct MHC class I Ag presentation by B cells was abrogated while other leukocyte compartments could express MHC class I. Surprisingly, despite the absence of B cell class I-restricted Ag presentation, CD8+ T cell activation was intact in the chimeric mice. Therefore, the spontaneous B cell-dependent CD8+ T cell activation that occurs in systemic autoimmunity is not due to direct presentation by B cells to CD8+ T cells.     

Generation of T cell help through a MHC class I-restricted TCR

CD4+ T cells that are activated by a MHC class II/peptide encounter can induce maturation of APCs and promote cytotoxic CD8+ T cell responses. Unfortunately, the number of well-defined tumor-specific CD4+ T cell epitopes that can be exploited for adoptive immunotherapy is limited. To determine whether Th cell responses can be generated by redirecting CD4+ T cells to MHC class I ligands, we have introduced MHC class I-restricted TCRs into postthymic murine CD4+ T cells and examined CD4+ T cell activation and helper function in vitro and in vivo. These experiments indicate that Ag-specific CD4+ T cell help can be induced by the engagement of MHC class I-restricted TCRs in peripheral CD4+ T cells but that it is highly dependent on the coreceptor function of the CD8beta-chain. The ability to generate Th cell immunity by infusion of MHC class I-restricted Th cells may prove useful for the induction of tumor-specific T cell immunity in cases where MHC class II-associated epitopes are lacking.     

An MHC class Ib-restricted CD8+ T cell response to lymphocytic choriomeningitis virus

Conventional MHC class Ia-restricted CD8(+) T cells play a dominant role in the host response to virus infections, but recent studies indicate that T cells with specificity for nonclassical MHC class Ib molecules may also participate in host defense. To investigate the potential role of class Ib molecules in anti-viral immune responses, K(b-/-)D(b-/-)CIITA(-/-) mice lacking expression of MHC class Ia and class II molecules were infected with lymphocytic choriomeningitis virus (LCMV). These animals have a large class Ib-selected CD8(+) T cell population and they were observed to mediate partial (but incomplete) virus clearance during acute LCMV infection as compared with K(b-/-)D(b-/-)β(2)-microglobulin(-/-) mice that lack expression of both MHC class Ia and class Ib molecules. Infection was associated with expansion of splenic CD8(+) T cells and induction of granzyme B and IFN-γ effector molecules in CD8(+) T cells. Partial virus clearance was dependent on CD8(+) cells. In vitro T cell restimulation assays demonstrated induction of a population of β(2)-microglobulin-dependent, MHC class Ib-restricted CD8(+) T cells with specificity for viral Ags and yet to be defined nonclassical MHC molecules. MHC class Ib-restricted CD8(+) T cell responses were also observed after infection of K(b-/-)D(b-/-)mice despite the low number of CD8(+) T cells in these animals. Long-term infection studies demonstrated chronic infection and gradual depletion of CD8(+) T cells in K(b-/-)D(b-/-)CIITA(-/-) mice, demonstrating that class Ia molecules are required for viral clearance. These findings demonstrate that class Ib-restricted CD8(+) T cells have the potential to participate in the host immune response to LCMV.     

Role of CD4 T cell help and costimulation in CD8 T cell responses during Listeria monocytogenes infection

CD4 T cells are known to assist the CD8 T cell response by activating APC via CD40-CD40 ligand (L) interactions. However, recent data have shown that bacterial products can directly activate APC through Toll-like receptors, resulting in up-regulation of costimulatory molecules necessary for the efficient priming of naive T cells. It remains unclear what role CD4 T cell help and various costimulation pathways play in the development of CD8 T cell responses during bacterial infection. In this study, we examined these questions using an intracellular bacterium, Listeria monocytogenes, as a model of infection. In CD4 T cell-depleted, CD4(-/-), and MHC class II(-/-) mice, L. monocytogenes infection induced CD8 T cell activation and primed epitope-specific CD8 T cells to levels commensurate with those in normal C57BL/6 mice. Furthermore, these epitope-specific CD8 T cells established long-term memory in CD4(-/-) mice that was capable of mounting a protective recall response. In vitro analysis showed that L. monocytogenes directly stimulated the activation and maturation of murine dendritic cells. The CD8 T cell response to L. monocytogenes was normal in CD40L(-/-) mice but defective in CD28(-/-) and CD137L(-/-) mice. These data show that in situations where infectious agents or immunogens can directly activate APC, CD8 T cell responses are less dependent on CD4 T cell help via the CD40-CD40L pathway but involve costimulation through CD137-CD137L and B7-CD28 interactions.     

Small intestine CD11c+ CD8+ T cells suppress CD4+ T cell-induced immune colitis

The large (LI) and small intestine (SI) differ in patterns of susceptibility to chronic mucosal inflammation. In this study, we evaluated whether this might, in part, reflect differences in resident mucosal CD11c(+) T cells. These cells comprised 39-48% (SI) and 12-17% (LI) of the intraepithelial compartment, most of which were T-cell receptor-αβ(+). In the SI, the majority of these cells were CD103(+) CD8(+) NK1.1(-), whereas the opposite phenotype prevailed in the LI. In transfer models of CD4(+) T cell-induced colitis, small numbers (2.5 × 10(5)) of SI CD11c(+) CD8(+) T cells suppressed proinflammatory cytokine-producing CD4(+) T cells in mesenteric lymph nodes and mucosa-associated lymphoid compartments (SI and LI) and protected mice from chronic inflammation. On a per-cell basis, the regulatory function of SI CD11c(+) T cells in CD4(+) T cell colitis was potent compared with other reported regulatory CD4(+) or CD8(+) T cells. In contrast, neither LI CD11c(+) T cells nor SI CD11c(-) T cells were effective in such immunoregulation. SI CD11c(+) CD8(+) T cells were similarly effective in suppressing CD4(+)CD45RB(hi) T cell colitis, as evidenced by inhibition of intracellular proinflammatory cytokine expression and histological inflammation. These findings indicate that SI CD11c(+) CD8(+) T cells are a distinct intestinal T cell population that plays an immunoregulatory role in control of proinflammatory CD4(+) T cells and maintenance of intestinal mucosal homeostasis.     

Dual function of the NK cell receptor 2B4 (CD244) in the regulation of HCV-specific CD8+ T cells

The outcome of viral infections is dependent on the function of CD8+ T cells which are tightly regulated by costimulatory molecules. The NK cell receptor 2B4 (CD244) is a transmembrane protein belonging to the Ig superfamily which can also be expressed by CD8+ T cells. The aim of this study was to analyze the role of 2B4 as an additional costimulatory receptor regulating CD8+ T cell function and in particular to investigate its implication for exhaustion of hepatitis C virus (HCV)-specific CD8+ T cells during persistent infection. We demonstrate that (i) 2B4 is expressed on virus-specific CD8+ T cells during acute and chronic hepatitis C, (ii) that 2B4 cross-linking can lead to both inhibition and activation of HCV-specific CD8+ T cell function, depending on expression levels of 2B4 and the intracellular adaptor molecule SAP and (iii) that 2B4 stimulation may counteract enhanced proliferation of HCV-specific CD8+ T cells induced by PD1 blockade. We suggest that 2B4 is another important molecule within the network of costimulatory/inhibitory receptors regulating CD8+ T cell function in acute and chronic hepatitis C and that 2B4 expression levels could also be a marker of CD8+ T cell dysfunction. Understanding in more detail how 2B4 exerts its differential effects could have implications for the development of novel immunotherapies of HCV infection aiming to achieve immune control.     

CD18 is required for optimal development and function of CD4+CD25+ T regulatory cells

CD4+CD25+ T regulatory (Treg) cells inhibit immunopathology and autoimmune disease in vivo. CD4+CD25+ Treg cells' capacity to inhibit conventional T cells in vitro is dependent upon cell-cell contact; however, the cell surface molecules mediating this cell:cell contact have not yet been identified. LFA-1 (CD11a/CD18) is an adhesion molecule that plays an established role in T cell-mediated cell contact and in T cell activation. Although expressed at high levels on murine CD4+CD25+ Treg cells, the role of LFA-1 in these cells has not been defined previously. We hypothesized that LFA-1 may play a role in murine CD4+CD25+ Treg function. To evaluate this, we analyzed LFA-1-deficient (CD18-/-) CD4+CD25+ T cells. We show that CD18-/- mice demonstrate a propensity to autoimmunity. Absence of CD18 led to diminished CD4+CD25+ T cell numbers and affected both thymic and peripheral development of these cells. LFA-1-deficient CD4+CD25+ T cells were deficient in mediating suppression in vitro and in mediating protection from colitis induced by the transfer of CD4+CD25- T cells into lymphopenic hosts. Therefore, we define a crucial role for CD18 in optimal CD4+CD25+ Treg development and function.     

Requirement for CD4+ T cells in V beta 4+CD8+ T cell activation associated with latent murine gammaherpesvirus infection.

A CD8+ T cell lymphocytosis in the peripheral blood is associated with the establishment of latency following intranasal infection with murine gammaherpesvirus-68. Remarkably, a large percentage of the activated CD8+ T cells of mice expressing different MHC haplotypes express V beta 4+ TCR. Identification of the ligand driving the V beta 4+CD8+ T cell activation remains elusive, but there is a general correlation between V beta 4+CD8+ T cell stimulatory activity and establishment of latency in the spleen. In the current study, the role of CD4+ T cells in the V beta 4+CD8+ T cell expansion has been addressed. The results show that CD4+ T cells are essential for expansion of the V beta 4+CD8+ subset, but not other V beta subsets, in the peripheral blood. CD4+ T cells are required relatively late in the antiviral response, between 7 and 11 days after infection, and mediate their effect independently of IFN-gamma. Assessment of V beta 4+CD8+ T cell stimulatory activity using murine gammaherpesvirus-68-specific T cell hybridomas generated from latently infected mice supports the idea that CD4+ T cells control levels of the stimulatory ligand that drives the V beta 4+CD8+ T cells. As V beta 4+CD8+ T cell expansion also correlates with levels of activated B cells, these data raise the possibility that CD4+ T cell-mediated B cell activation is required for optimal expression of the stimulatory ligand. In addition, in cases of low ligand expression, there may also be a direct role for CD4+ T cell-mediated help for V beta 4+CD8+ T cells.     

Function of the CD4 and CD8 molecules on human cytotoxic T lymphocytes: regulation of T cell triggering

The function of the T cell differentiation antigens CD4 (Leu-3/T4) and CD8 (Leu-2/T8) on human cytotoxic T lymphocytes (CTL) is presently seen only in conjugate formation between CTL and target cell via class II or class I MHC antigens rather than in the later killing steps. In this study, human CD4+ and CD8+ CTL clones were used to investigate the effects of monoclonal antibodies against these differentiation antigens on nonspecific triggering of cytotoxicity. Cytotoxicity was induced either by antibodies against the CD3 (T3) antigen or by the lectins Con A and PHA. Anti-CD4 or anti-CD8 antibodies specifically inhibited all types of cytotoxicity of CD4+ or CD8+ CTL, respectively, regardless of the specificity of the CTL for class I or class II HLA antigens and regardless of whether target cells expressed class I or class II antigens. These results are incompatible with an exclusive role of the CD4 and CD8 molecules in MHC class recognition and are discussed with respect to a function as negative signal receptors for these molecules on CTL.     

CD69 regulates type I IFN-induced tolerogenic signals to mucosal CD4 T cells that attenuate their colitogenic potential

CD69 is highly expressed by lymphocytes at mucosal surfaces. We aimed to investigate the role of CD69 in mucosal immune responses. The expression of CD69 by CD4 T cells isolated from the spleen, mesenteric lymph nodes, small intestinal lamina propria, and colonic lamina propria was determined in specific pathogen-free B6 and TCR transgenic animals, as well as in germ-free B6 mice. Transfer colitis was induced by transplanting RAG(-/-) mice with B6 or CD69(-/-)CD45RB(high) CD4 T cells. CD69 expression by CD4 T cells is induced by the intestinal microflora, oral delivery of specific Ag, and type I IFN (IFN-I) signals. CD4 T cells from CD69(-/-) animals produce higher amounts of the proinflammatory cytokines IFN-γ, TNF-α, and IL-21, whereas the production of TGF-β1 is decreased. CD69-deficient CD4 T cells showed reduced potential to differentiate into Foxp3(+) regulatory T cells in vivo and in vitro. The transfer of CD69(-/-)CD45RB(high) CD4 T cells into RAG(-/-) hosts induced an accelerated colitis. Oral tolerance was impaired in CD69(-/-) and IFN-I receptor 1-deficient mice when compared with B6 and OT-II × RAG(-/-) animals. Polyinosinic-polycytidylic acid treatment of RAG(-/-) mice transplanted with B6 but not CD69(-/-) or IFN-I receptor 1-deficient CD45RB(high) CD4 T cells attenuated transfer colitis. CD69 deficiency led to the increased production of proinflammatory cytokines, reduced Foxp3(+) regulatory T cell induction, impaired oral tolerance, and more severe colitis. Hence, the activation Ag CD69 plays an important role in regulating mucosal immune responses.     

H2-M3-restricted T cells participate in the priming of antigen-specific CD4+ T cells

H2-M3-restricted CD8+ T cells provide early protection against bacterial infections. In this study, we demonstrate that activated H2-M3-restricted T cells provide early signals for efficient CD4+ T cell priming. C57BL/6 mice immunized with dendritic cells coated with the MHC class II-restricted listeriolysin O peptide LLO(190-201) (LLO) generated CD4+ T cells capable of responding to Listeria monocytogenes (LM) infection. Inclusion of a H2-M3-restricted formylated peptide fMIGWII (fMIG), but not MHC class Ia-restricted peptides, during immunization with LLO significantly increased IFN-gamma-producing CD4+ T cell numbers, which was associated with increased protection against LM infection. Studies with a CD4+ T cell-depleting mAb indicate that the reduction in bacterial load in fMIG plus LLO immunized mice is likely due to augmented numbers of LLO-specific CD4+ T cells, generated with the help of H2-M3-restricted CD8+ T cells. We also found that augmentation of LLO-specific CD4+ T lymphocytes with H2-M3-restricted T cells requires presentation of LLO and fMIG by the same dendritic cells. Interestingly, the augmented CD4+ T cell response generated with fMIG also increased primary LM-specific responses by MHC class Ia-restricted CD8 T cells. Coimmunization with LLO and fMIG also increases the number of memory Ag-specific CD4+ T cells. We also demonstrate that CD8 T cells restricted to another MHC class Ib molecule, Qa-1, whose human equivalent is HLA-E, are also able to enhance Ag-specific CD4+ T cell responses. These results reveal a novel function for H2-M3- and Qa-1-restricted T cells; provision of help to CD4+ Th cells during the primary response.     

Paradoxical effect of reduced costimulation in T cell-mediated colitis

B7-1 and B7-2 play different roles in the pathogenesis of autoimmunity, but this is controversial. We analyzed colitis induced by transfer of CD45RB(high)CD4(+) T cells to RAG(-/-) recipients lacking B7-1 and/or B7-2. Surprisingly, disease was greatly accelerated in RAG(-/-) recipients deficient for either B7-1 or B7-2, especially in the B7-2(-/-) recipients. This accelerated colitis induction correlated with increased T cell division in vivo and production of Th1 cytokines. Although colitis pathogenesis following T cell transfer was inhibited in the absence of CD40L expression, CD40-CD40L interactions were not required in the B7-2(-/-) RAG(-/-) recipients. In vitro priming by APCs lacking either B7-1 or B7-2 caused decreased IL-2 production, which led to decreased CTLA-4 expression, although T cells primed in this way could respond vigorously upon restimulation by producing increased IL-2 and proinflammatory cytokines. Consistent with this mechanism, we demonstrate that blocking IL-2 early after T cell transfer accelerated colitis. Our data therefore outline a mechanism whereby synergistic costimulation by B7-1 and B7-2 molecules during priming is required for optimal IL-2 production. The consequent inhibitory effect of full CTLA-4 expression, induced by IL-2, may slow colitis, even in the absence of regulatory T cells.     

TGF-beta 1 plays an important role in the mechanism of CD4+CD25+ regulatory T cell activity in both humans and mice

In previous studies, we have shown that murine CD4+CD25+ regulatory T cells produce high levels of TGF-beta1 in a cell surface and/or secreted form, and blockade of such TGF-beta1 by anti-TGF-beta curtails the ability of these cells to suppress CD25- T cell proliferation and B cell Ig production in in vitro suppressor assays. In further support for the role of TGF-beta1 in suppression by CD4+CD25+ T cells, we show in this study that another TGF-beta1-blocking molecule, recombinant latency-associated peptide of TGF-beta1 (rLAP), also reverses suppression by mouse CD4+CD25+ T cells as well as their human counterparts, CD4+CD25(high) T cells. In addition, we show that CD25- T cells exposed to CD4+CD25+ T cells in vitro manifest activation of Smad-2 and induction of CD103, the latter a TGF-beta-inducible surface integrin. In further studies, we show that while CD4+CD25+ T cells from TGF-beta1-deficient mice can suppress CD25- T cell proliferation in vitro, these cells do not protect recipient mice from colitis in the SCID transfer model in vivo, and, in addition, CD4+LAP+, but not CD4+LAP- T cells from normal mice protect recipient mice from colitis in this model. Together, these studies demonstrate that TGF-beta1 produced by CD4+CD25+ T cells is involved in the suppressor activity of these cells, particularly in their ability to regulate intestinal inflammation.