A rapid crosstalk of human gammadelta T cells and monocytes drives the acute inflammation in bacterial infections

Human cytomegalovirus elicits fetal gammadelta T cell responses in utero

The fetus and infant are highly susceptible to viral infections. Several viruses, including human cytomegalovirus (CMV), cause more severe disease in early life compared with later life. It is generally accepted that this is a result of the immaturity of the immune system. gammadelta T cells are unconventional T cells that can react rapidly upon activation and show major histocompatibility complex-unrestricted activity. We show that upon CMV infection in utero, fetal gammadelta T cells expand and become differentiated. The expansion was restricted to Vgamma9-negative gammadelta T cells, irrespective of their Vdelta chain expression. Differentiated gammadelta T cells expressed high levels of IFN-gamma, transcription factors T-bet and eomes, natural killer receptors, and cytotoxic mediators. CMV infection induced a striking enrichment of a public Vgamma8Vdelta1-TCR, containing the germline-encoded complementary-determining-region-3 (CDR3) delta1-CALGELGDDKLIF/CDR3gamma8-CATWDTTGWFKIF. Public Vgamma8Vdelta1-TCR-expressing cell clones produced IFN-gamma upon coincubation with CMV-infected target cells in a TCR/CD3-dependent manner and showed antiviral activity. Differentiated gammadelta T cells and public Vgamma8Vdelta1-TCR were detected as early as after 21 wk of gestation. Our results indicate that functional fetal gammadelta T cell responses can be generated during development in utero and suggest that this T cell subset could participate in antiviral defense in early life.

Loss of CD4+ T cell IL-6R expression during inflammation underlines a role for IL-6 trans signaling in the local maintenance of Th17 cells

IL-6 responses are classically orchestrated via a membrane-bound IL-6R (CD126) alpha subunit (classical IL-6R signaling) or through a soluble form of this cognate receptor (IL-6 trans signaling). Appraisal of IL-6R expression on human and mouse T cells emphasized that IL-6R expression is closely linked with that of CCR7 and CD62L. In this regard, infiltrating effector T cells from clinical and experimental peritonitis episodes lose IL-6R expression, and anti-CD3/CD28 Ab costimulation of peripheral T cells in vitro leads to a downregulation in IL-6R expression. Consequently, IL-6 signaling through membrane-bound IL-6R seems to be limited to naive or central memory T cell populations. Loss of IL-6R expression by activated T cells further suggests that these effector cells might still retain IL-6 responsiveness via IL-6 trans signaling. Using IL-6R-deficient mice and recombinant tools that modulate the capacity of IL-6 to signal via its soluble receptor, we report that local control of IL-6 trans signaling regulates the effector characteristics of the T cell infiltrate and promotes the maintenance of IL-17A-secreting CD4(+) T cells. Therefore, we concluded that classical IL-6R signaling in naive or central memory CD4(+) T cells is required to steer their effector characteristics, whereas local regulation of soluble IL-6R activity might serve to maintain the cytokine profile of the Th cell infiltrate. Therefore, the activation status of a T cell population is linked with an alteration in IL-6 responsiveness.

Monocytes and gammadelta T cells: close encounters in microbial infection

gammadelta T cells comprise an evolutionarily conserved yet poorly understood subset of T cells. Numerous features place these unconventional lymphocytes at the branching point between antigen-presenting cells and natural killer cells of the innate immune system and major-histocompatibility-complex-restricted alphabeta T cells of the adaptive immune system. We propose a role for human Vgamma9/Vdelta2 T cells in the generation of monocyte-derived inflammatory dendritic cells during infection. Our model incorporates the peculiar innate-like specificity of Vgamma9/Vdelta2 T cells for the microbial metabolite (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), co-recruitment of monocytes and Vgamma9/Vdelta2 T cells to sites of infection, and their crosstalk, with profound consequences for the initiation of antigen-specific alphabeta T-cell responses. Vgamma9/Vdelta2 T cells act thus as a cellular switch between innate and adaptive defence mechanisms.

Linking innate and adaptive immunity: human Vgamma9Vdelta2 T cells enhance CD40 expression and HMGB-1 secretion

γδ T cells play an important role in regulating the immune response to stress stimuli; however, the mean by which these innate lymphocytes fulfill this function remains poorly defined. The main subset of human peripheral blood γδ T cells responds to nonpeptidic antigens, such as isopentylpyrophosphate (IPP), a metabolite in the mevalonate pathway for both eukaryote and prokaryote cells. IPP-primed γδ T cells significantly augment the inflammatory response mediated by monocytes and αβ T cells to TSST-1, the staphylococcal superantigen that is the major causative agent of toxic shock syndrome. Here we show that the small pool of activated peripheral γδ T cells induces an early upregulation of CD40 on monocytes and the local release of High Mobility Group Box-1 (HMGB-1), the molecule designated as the late mediator of systemic inflammation. This finding provides a new basis for how γδ T cells may serve as influential modulators of both endogenous and exogenous stress stimuli.

Highly active microbial phosphoantigen induces rapid yet sustained MEK/Erk- and PI-3K/Akt-mediated signal transduction in anti-tumor human gammadelta T-cells

Background

The unique responsiveness of Vγ9Vδ2 T-cells, the major γδ subset of human peripheral blood, to non-peptidic prenyl pyrophosphate antigens constitutes the basis of current γδ T-cell-based cancer immunotherapy strategies. However, the molecular mechanisms responsible for phosphoantigen-mediated activation of human γδ T-cells remain unclear. In particular, previous reports have described a very slow kinetics of activation of T-cell receptor (TCR)-associated signal transduction pathways by isopentenyl pyrophosphate and bromohydrin pyrophosphate, seemingly incompatible with direct binding of these antigens to the Vγ9Vδ2 TCR. Here we have studied the most potent natural phosphoantigen yet identified, (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), produced by Eubacteria and Protozoa, and examined its γδ T-cell activation and anti-tumor properties.

Methodology/Principal Findings

We have performed a comparative study between HMB-PP and the anti-CD3ε monoclonal antibody OKT3, used as a reference inducer of bona fide TCR signaling, and followed multiple cellular and molecular γδ T-cell activation events. We show that HMB-PP activates MEK/Erk and PI-3K/Akt pathways as rapidly as OKT3, and induces an almost identical transcriptional profile in Vγ9⁺ T-cells. Moreover, MEK/Erk and PI-3K/Akt activities are indispensable for the cellular effects of HMB-PP, including γδ T-cell activation, proliferation and anti-tumor cytotoxicity, which are also abolished upon antibody blockade of the Vγ9⁺ TCR Surprisingly, HMB-PP treatment does not induce down-modulation of surface TCR levels, and thereby sustains γδ T-cell activation upon re-stimulation. This ultimately translates in potent human γδ T-cell anti-tumor function both in vitro and in vivo upon transplantation of human leukemia cells into lymphopenic mice,

Conclusions/Significance

The development of efficient cancer immunotherapy strategies critically depends on our capacity to maximize anti-tumor effector T-cell responses. By characterizing the intracellular mechanisms of HMB-PP-mediated activation of the highly cytotoxic Vγ9⁺ T-cell subset, our data strongly support the usage of this microbial antigen in novel cancer clinical trials.