Human CD4+ Memory T Cells Can Become CD4+IL-9+ T Cells

Background

IL-9 is a growth factor for T- and mast-cells that is secreted by human Th2 cells. We recently reported that IL-4+TGF-β directs mouse CD4⁺CD25⁻CD62L⁺ T cells to commit to inflammatory IL-9 producing CD4⁺ T cells.

Methodology/Principal Findings

Here we show that human inducible regulatory T cells (iTregs) also express IL-9. IL-4+TGF-β induced higher levels of IL-9 expression in plate bound-anti-CD3 mAb (pbCD3)/soluble-anti-CD28 mAb (sCD28) activated human resting memory CD4⁺CD25⁻CD45RO⁺ T cells as compared to naïve CD4⁺CD25⁻CD45RA⁺ T cells. In addition, as compared to pbCD3/sCD28 plus TGF-β stimulation, IL-4+TGF-β stimulated memory CD4⁺CD25⁻CD45RO⁺ T cells expressed reduced FOXP3 protein. As analyzed by pre-amplification boosted single-cell real-time PCR, human CD4⁺IL-9⁺ T cells expressed GATA3 and RORC, but not IL-10, IL-13, IFNγ or IL-17A/F. Attempts to optimize IL-9 production by pbCD3/sCD28 and IL-4+TGF-β stimulated resting memory CD4⁺ T cells demonstrated that the addition of IL-1β, IL-12, and IL-21 further enhance IL-9 production.

Conclusions/Significance

Taken together these data show both the differences and similarities between mouse and human CD4⁺IL9⁺ T cells and reaffirm the powerful influence of inflammatory cytokines to shape the response of activated CD4⁺ T cells to antigen.     

Role of TCR-induced extracellular signal-regulated kinase activation in the regulation of early IL-4 expression in naive CD4+ T cells

Although extracellular signal-regulated kinase (Erk) activation influences IL-4 production in various experimental systems, its role during Th differentiation is unclear. In this study, we show that Erk plays a critical role in IL-4 expression during TCR-induced Th differentiation of naive CD4(+) T cells. Stimulation of CD4(+) T cells with a high affinity peptide resulted in sustained Erk activation and Th1 differentiation. However, reduction of Erk activity led to a dramatic increase in IL-4 production and Th2 generation. Analysis of RNA and nuclear proteins of CD4(+) T cells 48 h after stimulation revealed that this was due to early IL-4 expression. Interestingly, transient Erk activation resulted in altered AP-1 DNA binding activity and the induction of an AP-1 complex that was devoid of Fos protein and consisted of Jun-Jun dimers. These data show that in the presence of a strong TCR signal, IL-4 expression can be induced in naive CD4(+) T cells by altering the strength of Erk activation. In addition, these data suggest that TCR-induced Erk activation is involved in the regulation of IL-4 expression by altering the composition of the AP-1 complex and its subsequent DNA binding activity.     

CEACAM1 dynamics during neisseria gonorrhoeae suppression of CD4+ T lymphocyte activation

Neisseria gonorrhoeae colony opacity-associated (Opa) proteins bind to human carcinoembryonic antigen cellular adhesion molecules (CEACAM) found on host cells including T lymphocytes. Opa binding to CEACAM1 suppresses the activation of CD4(+) T cells in response to a variety of stimuli. In this study, we use primary human CD4(+) T cells isolated from peripheral blood to define the molecular events occurring subsequent to Opa-CEACAM1 binding. We establish that, in contrast to other cell types, T cells do not engulf N. gonorrhoeae upon CEACAM1 binding. Instead, the bacteria recruit CEACAM1 from intracellular stores and maintain it on the T cell surface. Upon TCR ligation, the co-engaged CEACAM1 becomes phosphorylated on tyrosine residues within the ITIMs apparent in the cytoplasmic domain. This allows the recruitment and subsequent activation of the src homology domain 2-containing tyrosine phosphatases SHP-1 and SHP-2 at the site of bacterial attachment, which prevents the normal tyrosine phosphorylation of the CD3zeta-chain and ZAP-70 kinase in response to TCR engagement. Combined, this dynamic response allows the bacteria to effectively harness the coinhibitory function of CEACAM1 to suppress the adaptive immune response at its earliest step.     

Suppression of CD4+ T lymphocyte effector functions by CD4+CD25+ cells in vivo

CD4+CD25+ regulatory T cells have been extensively studied during the last decade, but how these cells exert their regulatory function on pathogenic effector T cells remains to be elucidated. Naive CD4+ T cells transferred into T cell-deficient mice strongly expand and rapidly induce inflammatory bowel disease (IBD). Onset of this inflammatory disorder depends on IFN-gamma production by expanding CD4+ T cells. Coinjection of CD4+CD25+ regulatory T cells protects recipient mice from IBD. In this study, we show that CD4+CD25+ regulatory T cells do not affect the initial activation/proliferation of injected naive T cells as well as their differentiation into Th1 effectors. Moreover, naive T cells injected together with CD4+CD25+ regulatory T cells into lymphopenic hosts are still able to respond to stimuli in vitro when regulatory T cells are removed. In these conditions, they produce as much IFN-gamma as before injection or when injected alone. Finally, when purified, they are able to induce IBD upon reinjection into lymphopenic hosts. Thus, prevention of IBD by CD4+CD25+ regulatory T cells is not due to deletion of pathogenic T cells, induction of a non reactive state (anergy) among pathogenic effector T cells, or preferential induction of Th2 effectors rather than Th1 effectors; rather, it results from suppression of T lymphocyte effector functions, leading to regulated responses to self.     

IL-4 modulation of CD4+CD25+ T regulatory cell-mediated suppression

Murine CD4(+)CD25(+) T regulatory (Treg) cells were cocultured with CD4(+)CD25(-) Th cells and APCs or purified B cells and stimulated by anti-CD3 mAb. Replacement of APCs by B cells did not significantly affect the suppression of CD4(+)CD25(-) Th cells. When IL-4 was added to separate cell populations, this cytokine promoted CD4(+)CD25(-) Th and CD4(+)CD25(+) Treg cell proliferation, whereas the suppressive competence of CD4(+)CD25(+) Treg cells was preserved. Conversely, IL-4 added to coculture of APCs, CD4(+)CD25(-) Th cells, and CD4(+)CD25(+) Treg cells inhibited the suppression of CD4(+)CD25(-) Th cells by favoring their survival through the induction of Bcl-2 expression. At variance, suppression was not affected by addition of IL-13, although this cytokine shares with IL-4 a receptor chain. When naive CD4(+)CD25(-) Th cells were replaced by Th1 and Th2 cells, cell proliferation of both subsets was equally suppressed, but suppression was less pronounced compared with that of CD4(+)CD25(-) Th cells. IL-4 production by Th2 cells was also inhibited. These results indicate that although CD4(+)CD25(+) Treg cells inhibit IL-4 production, the addition of IL-4 counteracts CD4(+)CD25(+) Treg cell-mediated suppression by promoting CD4(+)CD25(-) Th cell survival and proliferation.     

Innate immune activation of CD4 T cells in salmonella-infected mice is dependent on IL-18

Production of IFN-gamma by CD4 T cells is generally thought to be mediated by TCR triggering, however, Ag-nonspecific activation of effector CD8 T cells has been reported in infection models. In this study, we demonstrate that Ag-experienced CD4 T cells in the spleen of Salmonella-infected mice acquire the capacity to rapidly secrete IFN-gamma in response to stimulation with bacterial lysate or LPS. This innate responsiveness of T cells was transient and most apparent during, and immediately following, active Salmonella infection. Furthermore, innate T cell production of IFN-gamma in response to bacterial lysate or LPS was Ag independent and could be induced in Listeria-infected mice and in the absence of MHC class II expression. IL-18 was required for maximal innate responsiveness of CD4 T cells in Salmonella-infected mice and for optimal bacterial clearance in vivo. These data demonstrate that CD4 T cells acquire the capacity to respond to innate stimuli during active bacterial infection, a process that may contribute significantly to amplifying effector responses in vivo.     

Frequency analysis of CD4+CD8+ T cells cloned with IL-4

The coexpression of both CD4 and CD8 molecules on T cells occurs in the peripheral blood at a low frequency and can be generated transiently on CD4+ peripheral blood T cells by treatment with lectin which induces CD8 biosynthesis and cell surface expression. We have cloned T cells in a nonselective fashion from normal subjects in the presence of either IL-2, rIL-4 and IL-2, or rIL-4 and have examined the phenotypic expression of CD4 and CD8. The addition of excess rIL-4 increased the expression of CD8 on the surface of CD4+ T cell clones but did not increase CD4 expression on CD8+ T cell clones. There were three patterns of CD4 and CD8 expression observed: high density CD8 with no CD4 expression; high density CD4 with low CD8 expression; or high density CD4 with higher cell surface CD8 expression which was regulated by the presence of rIL-4. CD4+ T cell clones originally cultured in IL-2 and rIL-4 and subsequently grown in IL-2 alone exhibited decreased expression of the CD8 molecule. The increased expression of CD8 did not correlate with NK activity or lectin-dependent cytotoxicity in an antigen independent system. In addition, rIL-4 alone or in combination with IL-2 appeared to accelerate the growth curve of T cell clones as compared to IL-2 alone. These results show that IL-4 can upregulate CD8 expression on CD4+ T cell clones while not effecting CD4 expression on CD8+ T cell clones. As class I MHC is the ligand for the CD8 molecule, expression of CD8 induced by IL-4 on CD4+ T cells may allow for increased nonspecific cell to cell contact during the course of an inflammatory response.     

Effector mechanisms in cyclosporine A-induced syngeneic graft-versus-host disease. Role of CD4+ and CD8+ T lymphocyte subsets

Syngeneic graft-versus-host disease (SGVHD) is a T cell-mediated autoimmune disease occurring postsyngeneic bone marrow transplantation and the administration of the potent immunosuppressive agent, cyclosporine A. Paradoxically, cyclosporine A disrupts the immunologic homeostasis governing self-tolerance. Our studies using an adoptive transfer model attempted to identify effector mechanisms associated with the autoimmune disease. Both CD4+ and CD8+ splenic T cells isolated from autoimmune donors were required for the adoptive transfer of active disease into lethally irradiated secondary recipients reconstituted with normal bone marrow. Doses of more than 5 x 10(6) of nylon wool depleted splenocytes from autoimmune donors effectively transferred disease into lethally irradiated secondary recipients. Splenocytes that are T cell depleted or CD4(+)-enriched cells did not elicit disease upon adoptive transfer. Nylon wool fractionated CD8+ splenocytes also failed to adoptively transfer disease unless high doses (greater than or equal to 30 x 10(6)) were used. The disease transferred with the CD8+ subset presented as acute type SGVHD and was self-limiting. The recombination of the individually isolated T cell subsets not only restored but also enhanced immune reactivity upon adoptive transfer. Moreover, use of the recombined subsets resulted in progressive disease with the development of chronic type SGVHD. The titration of each subset to the other suggested that a minimal number of CD4+ T cells was required to potentiate the CD8+ autoreactive cells in vivo. Further analysis of the helper cell involved demonstrated that it had a CD4+ CD45r- phenotype, characteristic of an amplifying helper cell population. Administration of IL-2 did not substitute for CD4+ Th cells but yet amplified the activity of unfractionated cells or recombined subsets implicating the role of other factors in the pathogenesis of SGVHD. Delineation of the effector mechanisms involved in SGVHD is critical in determining the underlying events that trigger either the production of autoreactive cells or the perturbation of the regulation of these autoreactive cells, culminating in autoimmunity.     

CD28, IL-2-independent costimulatory pathways for CD8 T lymphocyte activation.

We investigate, here, the mechanism of the costimulatory signals for CD8 T cell activation and confirm that costimulation signals via CD28 do not appear to be required to initiate proliferation, but provide survival signals for CD8 T cells activated by TCR ligation. We show also that IL-6 and TNF-alpha can provide alternative costimulatory survival signals. IL-6 and TNF-alpha costimulate naive CD8 T cells cultured on plate-bound anti-CD3 in the absence of CD28 ligation. They act directly on sorted CD8-positive T cells. They also costimulate naive CD8 T cells from Rag-2-deficient mice, bearing transgenic TCRs for HY, which lack memory cells, a potential source of IL-2 secretion upon activation. IL-6 and TNF-alpha provide costimulation to naive CD8 T cells from CD28, IL-2, or IL-2Ralpha-deficient mice, and thus function in the absence of the B7-CD28 and IL-2 costimulatory pathways. The CD8 T cell generated via the anti-CD3 plus IL-6 and TNF-alpha pathway have effector function in that they express strong cytolytic activity on Ag-specific targets. They secrete only very small amounts of any of the cytokines tested upon restimulation with peptide-loaded APC. The ability of the naive CD8 T cells to respond to TCR ligation and costimulatory signals from IL-6 and TNF-alpha provides a novel pathway that can substitute for signals from CD4 helper cells or professional APC. This may be significant in the response to viral Ags, which can be potentially expressed on the surface of any class I MHC-expressing cell.     

CD8+ T cell-mediated airway hyperresponsiveness and inflammation is dependent on CD4+IL-4+ T cells

CD4+ T cells, particularly Th2 cells, play a pivotal role in allergic airway inflammation. However, the requirements for interactions between CD4+ and CD8+ T cells in airway allergic inflammation have not been delineated. Sensitized and challenged OT-1 mice in which CD8+ T cells expressing the transgene for the OVA(257-264) peptide (SIINFEKL) failed to develop airway hyperresponsiveness (AHR), airway eosinophilia, Th2 cytokine elevation, or goblet cell metaplasia. OT-1 mice that received naive CD4+IL-4+ T cells but not CD4+IL-4- T cells before sensitization developed all of these responses to the same degree as wild-type mice. Moreover, recipients of CD4+IL-4+ T cells developed significant increases in the number of CD8+IL-13+ T cells in the lung, whereas sensitized OT-1 mice that received primed CD4+ T cells just before challenge failed to develop these responses. Sensitized CD8-deficient mice that received CD8+ T cells from OT-1 mice that received naive CD4+ T cells before sensitization increased AHR and eosinophil numbers in bronchoalveolar lavage fluid when challenged with allergen. In contrast, sensitized CD8-deficient mice receiving CD8+ T cells from OT-1 mice without CD4+ T cells developed reduced AHR and eosinophil numbers in bronchoalveolar lavage fluid when challenged. These data suggest that interactions between CD4+ and CD8+ T cells, in part through IL-4 during the sensitization phase, are essential to the development of CD8+IL-13+ T cell-dependent AHR and airway allergic inflammation.     

Cutting edge: IL-12 induces CD4+CD25- T cell activation in the presence of T regulatory cells

IL-12p40 cytokines have been implicated in the development of organ-specific autoimmune diseases as well as pathogen-specific adaptive immunity. In addition to inducing IFN-gamma, IL-12 stimulates effector CD4(+) T cells to express adhesion molecules and homing receptors that facilitate their migration to sites of inflammation. In this study, we expand upon those observations by demonstrating an alternative pathway by which IL-12 could promote Th1 inflammatory responses in mice, namely, by restoring proliferation and cytokine expression by effector T cells in the presence of CD4(+)CD25(+) regulatory T cells (Treg). This effect of IL-12 was not replicated by IL-23 or IFN-gamma and was dependent on signaling through the IL-12R expressed on CD25(-) responder cells, but not on Treg. Our studies suggest that IL-12 could act in concert with other proinflammatory factors to stimulate CD4(+)CD25(-) T cell activation in the presence of Treg.     

Activation of virus-specific CD8+ T cells by lipopolysaccharide-induced IL-12 and IL-18

Virus-specific T cells represent a hallmark of Ag-specific, adaptive immunity. However, some T cells also demonstrate innate functions, including non-Ag-specific IFN-gamma production in response to microbial products such as LPS or exposure to IL-12 and/or IL-18. In these studies we examined LPS-induced cytokine responses of CD8(+) T cells directly ex vivo. Following acute viral infection, 70-80% of virus-specific T cells will produce IFN-gamma after exposure to LPS-induced cytokines, and neutralization experiments indicate that this is mediated almost entirely through production of IL-12 and IL-18. Different combinations of these cytokines revealed that IL-12 decreases the threshold of T cell activation by IL-18, presenting a new perspective on IL-12/IL-18 synergy. Moreover, memory T cells demonstrate high IL-18R expression and respond effectively to the combination of IL-12 and IL-18, but cannot respond to IL-18 alone, even at high cytokine concentrations. This demonstrates that the synergy between IL-12 and IL-18 in triggering IFN-gamma production by memory T cells is not simply due to up-regulation of the surface receptor for IL-18, as shown previously with naive T cells. Together, these studies indicate how virus-specific T cells are able to bridge the gap between innate and adaptive immunity during unrelated microbial infections, while attempting to protect the host from cytokine-induced immunopathology and endotoxic shock.     

Uncoupling of IL-2 signaling from cell cycle progression in naive CD4+ T cells by regulatory CD4+CD25+ T lymphocytes

Prior reports have shown that CD4(+)CD25(+) regulatory T cells suppress naive T cell responses by inhibiting IL-2 production. In this report, using an Ag-specific TCR transgenic system, we show that naive T cells stimulated with cognate Ag in the presence of preactivated CD4(+)CD25(+) T cells also become refractory to the mitogenic effects of IL-2. T cells stimulated in the presence of regulatory T cells up-regulated high affinity IL-2R, but failed to produce IL-2, express cyclins or c-Myc, or exit G(0)-G(1). Exogenous IL-2 failed to break the mitotic block, demonstrating that the IL-2 production failure was not wholly responsible for the proliferation defect. This IL-2 unresponsiveness did not require the continuous presence of CD4(+)CD25(+) regulatory T cells. The majority of responder T cells reisolated after coculture with regulatory cells failed to proliferate in response to IL-2, but were not anergic and proliferated in response to Ag. The mitotic block was also dissociated from the antiapoptotic effects of IL-2, because IL-2 still promoted the survival of T cells that had been cocultured with CD4(+)CD25(+) T cells. IL-2-induced STAT5 phosphorylation in the cocultured responder cells was intact, implying that the effects of the regulatory cells were downstream of receptor activation. Our results therefore show that T cell activation in the presence of CD4(+)CD25(+) regulatory T cells can induce an alternative stimulation program characterized by up-regulation of high affinity IL-2R, but a failure to produce IL-2, and uncoupling of the mitogenic and antiapoptotic effects of IL-2.     

Lectin-mediated induction of IL-4-producing CD4+ T cells.

Cultured murine CD4+ T cells have been shown to differentiate into IL-2 or IL-4-producing subsets. The factors responsible for the development of CD4+ T cells which produce IL-2 but not IL-4 and cells capable of producing IL-4 but not IL-2 are unknown. Here we describe a system that allows the controlled induction of IL-2- or IL-4-producing T cells after one single round of activation. Freshly isolated CD8-depleted T cells were activated with various polyclonal T cell activators for 48 h, washed, and then expanded under different conditions. IL-2 and IL-4 production were induced by restimulation of T cells and were measured with CTLL cells that respond to both cytokines and mAb to IL-2 and IL-4. T cells produced mainly IL-2 and small amounts of IL-4 when restimulated after expansion culture for 12 days with rIL-2 alone. However, after expansion for 12 days in the presence of rIL-2 plus Con A, we observed a 30- to 100-fold up-regulation of IL-4 activity and a 100-fold down-regulation of IL-2 when assessed by responses of CTLL cells incubated with the supernatant of restimulated T cells and by responses of CTLL cells cocultured with restimulated cells. An increase of IL-4 and decrease of IL-2 was also observed when the results were based on the cell numbers at the beginning of the expansion culture. The induction of IL-4 and the down-regulation of IL-2 1) were not reproduced with alpha-methyl-mannoside-treated supernatant of Con A-stimulated spleen cells, 2) were not dependent on the presence of large numbers of APC, 3) did not result from differential consumption of lymphokines after restimulation, 4) were not due to a difference in the time course of IL-2 or IL-4 release in either T cell population, and 5) were obtained regardless of the agents used to activate or to restimulate the T cells. Because Con A remained detectable on the T cell surface and because expansion of activated T cells with IL-2 plus Con A for several days was necessary, our results indicate that mainly IL-4-producing CD4+ T cells can be induced by prolonged engagement of T cell surface molecules.     

CD25+ CD4+ T cells regulate the expansion of peripheral CD4 T cells through the production of IL-10

The mechanisms by which the immune system achieves constant T cell numbers throughout life, thereby controlling autoaggressive cell expansions, are to date not completely understood. Here, we show that the CD25(+) subpopulation of naturally activated (CD45RB(low)) CD4 T cells, but not CD25(-) CD45RB(low) CD4 T cells, inhibits the accumulation of cotransferred CD45RB(high) CD4 T cells in lymphocyte-deficient mice. However, both CD25(+) and CD25(-) CD45RB(low) CD4 T cell subpopulations contain regulatory cells, since they can prevent naive CD4 T cell-induced wasting disease. In the absence of a correlation between disease and the number of recovered CD4(+) cells, we conclude that expansion control and disease prevention are largely independent processes. CD25(+) CD45RB(low) CD4 T cells from IL-10-deficient mice do not protect from disease. They accumulate to a higher cell number and cannot prevent the expansion of CD45RB(high) CD4 T cells upon transfer compared with their wild-type counterparts. Although CD25(+) CD45RB(low) CD4 T cells are capable of expanding when transferred in vivo, they reach a homeostatic equilibrium at lower cell numbers than CD25(-) CD45RB(low) or CD45RB(high) CD4 T cells. We conclude that CD25(+) CD45RB(low) CD4 T cells from nonmanipulated mice control the number of peripheral CD4 T cells through a mechanism involving the production of IL-10 by regulatory T cells.     

Compartmentalization of a CD4+ T lymphocyte subpopulation in tuberculous pleuritis

The study of T lymphocytes from pleural fluid and tissue of patients with tuberculous pleuritis provides an opportunity to evaluate the human immune response to infection at the site of disease activity. Therefore, we investigated the phenotype and function of CD4+ pleural fluid cells from patients with tuberculous pleuritis. Pleural fluid was enriched with CD4+CDw29+ T lymphocytes, which are thought to represent "memory" T cells. Immunoperoxidase staining of pleural tissue confirmed the predominance of CD4+CDw29+ T lymphocytes at the site of disease activity. CD4+ subpopulations were evaluated for their ability to contribute to a cell-mediated immune response against Mycobacterium tuberculosis by assaying immune function in vitro. Pleural fluid-derived CD4+CDw29+ cells, but not CD4+CDw29- lymphocytes, proliferated vigorously and produced high levels of IFN-gamma when stimulated with purified protein derivative of M. tuberculosis. CD4+CDw29+ clones produced IFN-gamma specifically in response to purified protein derivative of M. tuberculosis but not to an irrelevant Ag, tetanus toxoid. IFN-gamma levels were markedly elevated in pleural fluid, compared to peripheral blood, suggesting production of this lymphokine in vivo at the site of tissue inflammation. The sum of these data indicate that, in tuberculous pleuritis, CD4+CDw29+ cells are concentrated at the site of disease activity, produce IFN-gamma and are likely to play an important role in the local human cell-mediated immune response to M. tuberculosis.     

Distinct IL-2 receptor signaling pattern in CD4+CD25+ regulatory T cells

Despite expression of the high-affinity IL-2R, CD4(+)CD25(+) regulatory T cells (Tregs) are hypoproliferative upon IL-2R stimulation in vitro. However the mechanisms by which CD4(+)CD25(+) T cells respond to IL-2 signals are undefined. In this report, we examine the cellular and molecular responses of CD4(+)CD25(+) Tregs to IL-2. IL-2R stimulation results in a G(1) cell cycle arrest, cellular enlargement and increased cellular survival of CD4(+)CD25(+) T cells. We find a distinct pattern of IL-2R signaling in which the Janus kinase/STAT pathway remains intact, whereas IL-2 does not activate downstream targets of phosphatidylinositol 3-kinase. Negative regulation of phosphatidylinositol 3-kinase signaling and IL-2-mediated proliferation of CD4(+)CD25(+) T cells is inversely associated with expression of the phosphatase and tensin homologue deleted on chromosome 10, PTEN.     

Functional analysis of B and T lymphocyte attenuator engagement on CD4+ and CD8+ T cells

T cell activation can be profoundly altered by coinhibitory and costimulatory molecules. B and T lymphocyte attenuator (BTLA) is a recently identified inhibitory Ig superfamily cell surface protein found on lymphocytes and APC. In this study we analyze the effects of an agonistic anti-BTLA mAb, PK18, on TCR-mediated T cell activation. Unlike many other allele-specific anti-BTLA mAb we have generated, PK18 inhibits anti-CD3-mediated CD4+ T cell proliferation. This inhibition is not dependent on regulatory T cells, nor does the Ab induce apoptosis. Inhibition of T cell proliferation correlates with a profound reduction in IL-2 secretion, although this is not the sole cause of the block of cell proliferation. In contrast, PK18 has no effect on induction of the early activation marker CD69. PK18 also significantly inhibits, but does not ablate, IL-2 secretion in the presence of costimulation as well as reduces T cell proliferation under limiting conditions of activation in the presence of costimulation. Similarly, PK18 inhibits Ag-specific T cell responses in culture. Interestingly, PK18 is capable of delivering an inhibitory signal as late as 16 h after the initiation of T cell activation. CD8+ T cells are significantly less sensitive to the inhibitory effects of PK18. Overall, BTLA adds to the growing list of cell surface proteins that are potential targets to down-modulate T cell function.     

Autophagy and CD4+ T lymphocyte destruction by HIV-1

The first step of HIV-1 infection is mediated by the binding of envelope glycoproteins (Env) to CD4 and two major coreceptors, CCR5 or CXCR4. The HIV-1 strains that use CCR5 are involved in primo-infection whereas those HIV-1 strains that use CXCR4 play a major role in the demise of CD4+ T lymphocytes and a rapid progression toward AIDS. Notably, binding of X4 Env expressed on cells to CXCR4 triggers apoptosis of uninfected CD4+ T cells. We now have just demonstrated that, independently of HIV-1 replication, transfected or HIV-1-infected cells that express X4 Env induce autophagy and accumulation of Beclin 1 in uninfected CD4+ T lymphocytes via CXCR4. Moreover, autophagy is a prerequisite to Env-induced apoptosis in uninfected bystander T cells, and CD4+ T cells still undergo an Env-mediated cell death with autophagic features when apoptosis is inhibited. To the best of our knowledge, these findings represent the first example of autophagy triggered through binding of virus envelope proteins to a cellular receptor, without viral replication, leading to apoptosis. Here, we proposed hypotheses about the significance of Env-induced Beclin 1 accumulation in CD4+ T cell death and about the role of autophagy in HIV-1 infected cells depending on the coreceptor involved.     

Role of lymphokine-secreting CD8+ T cells in cytotoxic T lymphocyte responses against vaccinia virus

The present study has examined the relative role of CD4+ and CD8+ Th cells in the generation and reactivation of antivaccinia virus memory CTL responses. We show that mice primed in vivo to vaccinia virus generate in vitro antivaccinia virus memory CTL responses through both CD4+ and CD8+ Th cell pathways, with the CD4+ Th pathway being the more prominent of the two. In addition, we show that vaccinia virus-specific CD8+ Th cell function is mediated through production of lymphokines, including IL-2, and that the CD8+ Th cell component in the CTL response is labile, decreasing progressively with increasing time after in vivo priming. Thus, this study demonstrates the existence of two phenotypically distinct Th cell pathways in the generation of antivirus CTL responses.