Jane E. Dalton

Fas-Fas Ligand Interactions Are Essential for the Binding to and Killing of Activated Macrophages by γδ T Cells

γδ T cells have a direct role in resolving the host immune response to infection by eliminating populations of activated macrophages. Macrophage reactivity resides within the Vγ1/Vδ6.3 subset of γδ T cells, which have the ability to kill activated macrophages following infection with Listeria monocytogenes (Lm). However, it is not known how γδ T cell macrophage cytocidal activity is regulated, or what effector mechanisms γδ T cells use to kill activated macrophages. Using a macrophage-T cell coculture system in which peritoneal macrophages from naive or Lm-infected TCRδ−/− mice were incubated with splenocytes from wild-type and Fas ligand (FasL)-deficient mice (gld), the ability of Vγ1 T cells to bind macrophages was shown to be dependent upon Fas-FasL interactions. Combinations of anti-TCR and FasL Abs completely abolished binding to and killing of activated macrophages by Vγ1 T cells. In addition, confocal microscopy showed that Fas and the TCR colocalized on Vγ1 T cells at points of contact with macrophages. Collectively, these studies identify an accessory or coreceptor-like function for Fas-FasL that is essential for the interaction of Vγ1 T cells with activated macrophages and their elimination during the resolution stage of pathogen-induced immune responses.

Delineation of the function of a major gamma delta T cell subset during infection.

γδ T cells play important but poorly defined roles in pathogen-induced immune responses and in preventing chronic inflammation and pathology. A major obstacle to defining their function is establishing the degree of functional redundancy and heterogeneity among γδ T cells. Using mice deficient in Vγ1+ T cells which are a major component of the γδ T cell response to microbial infection, a specific immunoregulatory role for Vγ1+ T cells in macrophage and γδ T cell homeostasis during infection has been established. By contrast, Vγ1+ T cells play no significant role in pathogen containment or eradication and cannot protect mice from immune-mediated pathology. Pathogen-elicited Vγ1+ T cells also display different functional characteristics at different stages of the host response to infection that involves unique and different populations of Vγ1+ T cells. These findings, therefore, identify distinct and nonoverlapping roles for γδ T cell subsets in infection and establish the complexity and adaptability of a single population of γδ T cells in the host response to infection that is not predetermined, but is, instead, shaped by environmental factors.

The Interaction of γδ T Cells with Activated Macrophages Is a Property of the Vγ1 Subset

Immunoregulation is an emerging paradigm of γδ T cell function. The mechanisms by which γδ T cells mediate this function, however, are not clear. Studies have identified a direct role for γδ T cells in resolving the host immune response to infection, by eliminating populations of activated macrophages. The aim of this study was to identify macrophage-reactive γδ T cells and establish the requirements/outcomes of macrophage-γδ T cell interactions during the immune response to the intracellular bacterium, Listeria monocytogenes (Lm). Using a macrophage-T cell coculture system in which peritoneal macrophages from naive or Lm-infected TCRδ−/− mice were incubated with splenocytes from naive and Lm-infected αβ/γδ T cell-deficient and wild-type mice, the ability to bind macrophages was shown to be restricted to γδ T cells and the GV5S1 (Vγ1) subset of γδ T cells. Macrophage adherence resulted in a 4- to 10-fold enrichment of Vγ1+ T cells. Enrichment of Vγ1 T cells was dependent upon the activation status of macrophages, but independent of the activation status of γδ T cells. Vγ1 T cells were cytotoxic for activated macrophages with both the binding to and killing of macrophages being TCR dependent because anti-TCRγδ Abs inhibited both Vγ1 binding and killing activities. These studies establish the identity of macrophage cytotoxic γδ T cells, the conditions under which this interaction occurs, and the outcome of this interaction. These findings are concordant with the involvement of Vγ1 T cells in macrophage homeostasis during the resolution of pathogen-mediated immune responses.

Post kala-azar dermal leishmaniasis: an unresolved mystery

Highlights • Sodium antimony gluconate contributes towards the pathogenesis of PKDL.• UV light plays a pivotal role in the development of PKDL.• Development of PKDL can be viewed as a reinfection or activation of latent Leishmania parasites.• PKDL can be resolved by mounting an effective tissue-specific memory T cell response.• Host genetic factors play a contributory role.

IL-10-producing Th1 cells and disease progression are regulated by distinct CD11c⁺ cell populations during visceral leishmaniasis.

IL-10 is a critical regulatory cytokine involved in the pathogenesis of visceral leishmaniasis caused by Leishmania donovani and clinical and experimental data indicate that disease progression is associated with expanded numbers of CD4+ IFNγ+ T cells committed to IL-10 production. Here, combining conditional cell-specific depletion with adoptive transfer, we demonstrate that only conventional CD11chi DCs that produce both IL-10 and IL-27 are capable of inducing IL-10-producing Th1 cells in vivo. In contrast, CD11chi as well as CD11cint/lo cells isolated from infected mice were capable of reversing the host protective effect of diphtheria toxin-mediated CD11c+ cell depletion. This was reflected by increased splenomegaly, inhibition of NO production and increased parasite burden. Thus during chronic infection, multiple CD11c+ cell populations can actively suppress host resistance and enhance immunopathology, through mechanisms that do not necessarily involve IL-10-producing Th1 cells.

Bone marrow-derived and resident liver macrophages display unique transcriptomic signatures but similar biological functions

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Inhibition of receptor tyrosine kinases restores immunocompetence and improves immune-dependent chemotherapy against experimental leishmaniasis in mice

Receptor tyrosine kinases are involved in multiple cellular processes, and drugs that inhibit their action are used in the clinic to treat several types of cancer. However, the value of receptor tyrosine kinase inhibitors (RTKIs) for treating infectious disease has yet to be explored. Here, we have shown in mice that administration of the broad-spectrum RTKI sunitinib maleate (Sm) blocked the vascular remodeling and progressive splenomegaly associated with experimental visceral leishmaniasis. Furthermore, Sm treatment restored the integrity of the splenic microarchitecture. Although restoration of splenic architecture was accompanied by an increase in the frequency of IFN-gamma+CD4+ T cells, Sm treatment alone was insufficient to cause a reduction in tissue parasite burden. However, preconditioning by short-term Sm treatment proved to be successful as an adjunct therapy, increasing the frequency of IFN-gamma+ and IFN-gamma+TNF+CD4+ T cells, enhancing NO production by splenic macrophages, and providing dose-sparing effects when combined with a first-line immune-dependent anti-leishmanial drug. We propose, therefore, that RTKIs may prove clinically useful as agents to restore immune competence before the administration of chemo- or immunotherapeutic drugs in the treatment of visceral leishmaniasis or other diseases involving lymphoid tissue remodeling, including cancer.

A Requirement for the Vγ1+ Subset of Peripheral γδ T Cells in the Control of the Systemic Growth of Toxoplasma gondii and Infection-Induced Pathology

γδ T cells are a diverse population of T cells that are widely distributed and are a common feature of pathogen-induced immune responses. It is not clear, however, whether different populations of γδ T cells have specific functions, and what factors determine the functional properties of individual populations. A murine model of peroral Toxoplasma gondii infection was used to determine the contribution Vγ1+ intestinal intraepithelial lymphocytes (IELs) vs systemic Vγ1+ T cells make to the acute and chronic stages of the host immune response, and whether the macrophage cytocidal activity of Vγ1+ T cells described in bacterial infections is seen in other, unrelated infectious disease models. In response to oral infection with virulent type 1 or avirulent type II strains of T. gondii, TCR-δ−/− mice rapidly developed severe ileitis. In contrast, in mice deficient in Vγ1+ T cells and IELs and wild-type mice, inflammation was delayed in onset and less severe. The protective effect of (Vγ1−) IELs to Toxoplasma infection was unrelated to their cytolytic and cytokine (Th1)-producing capabilities. Systemic Vγ1+ T cells were shown to play an essential role in limiting parasite growth and inflammation in peripheral tissues and, in particular, in the CNS, that was associated with their ability to efficiently kill parasite-elicited and infected macrophages. These findings suggest that macrophage cytocidal activity of Vγ1+ T cells may be a universal feature of pathogen-induced immune responses and that microenvironmental factors influence the involvement and function of γδ T cells in the host response to infection.

Compartment-Specific Remodeling of Splenic Micro-Architecture during Experimental Visceral Leishmaniasis

Progressive splenomegaly is a hallmark of visceral leishmaniasis in humans, canids, and rodents. In experimental murine visceral leishmaniasis, splenomegaly is accompanied by pronounced changes in microarchitecture, including expansion of the red pulp vascular system, neovascularization of the white pulp, and remodeling of the stromal cell populations that define the B-cell and T-cell compartments. Here, we show that Ly6C/G+ (Gr-1+) cells, including neutrophils and inflammatory monocytes, accumulate in the splenic red pulp during infection. Cell depletion using monoclonal antibody against either Ly6C/G+ (Gr-1; RB6) or Ly6G+ (1A8) cells increased parasite burden. In contrast, depletion of Ly6C/G+ cells, but not Ly6G+ cells, halted the progressive remodeling of Meca-32+ and CD31+ red pulp vasculature. Strikingly, neither treatment affected white pulp neovascularization or the remodeling of the fibroblastic reticular cell and follicular dendritic cell networks. These findings demonstrate a previously unrecognized compartment-dependent selectivity to the process of splenic vascular remodeling during experimental murine visceral leishmaniasis, attributable to Ly6C+ inflammatory monocytes.

Immunomodulators: use in combined therapy against leishmaniasis

Host immune responses have a direct impact on the efficacy of chemotherapy against leishmaniasis, one of the most important of the neglected tropical diseases. Although there have been significant advances in recent years to minimize drug toxicity, cost and treatment duration, notably with the develop ment of short-course combined therapy for visceral leishmaniasis, the potential for drug resistance and a paucity of new drugs in development remain major challenges for the future. The use of immunomodulators in combination with conventional chemotherapy to enhance host immune responses may have several advantages as a means to improving current therapeutic regimens.