Marcela F. Lopes

Macrophage Interactions with Neutrophils Regulate Leishmania major Infection

Macrophages are host cells for the pathogenic parasite Leishmania major. Neutrophils die and are ingested by macrophages in the tissues. We investigated the role of macrophage interactions with inflammatory neutrophils in control of L. major infection. Coculture of dead exudate neutrophils exacerbated parasite growth in infected macrophages from susceptible BALB, but killed intracellular L. major in resistant B6 mice. Coinjection of dead neutrophils amplified L. major replication in vivo in BALB, but prevented parasite growth in B6 mice. Neutrophil depletion reduced parasite load in infected BALB, but exacerbated infection in B6 mice. Exacerbated growth of L. major required PGE2 and TGF-β production by macrophages, while parasite killing depended on neutrophil elastase and TNF-α production. These results indicate that macrophage interactions with dead neutrophils play a previously unrecognized role in host responses to L. major infection.

Intracellular protozoan parasites and apoptosis: diverse strategies to modulate parasite-host interactions.

Programmed cell death (apoptosis) is an important regulator of the hosts response during infection with a variety of intracellular protozoan parasites. Parasitic pathogens have evolved diverse strategies to induce or inhibit host-cell apoptosis, thereby modulating the hosts immune response, aiding dissemination within the host or facilitating intracellular survival. Here, we review the molecular and cell-biological mechanisms of the pathogen-induced modulation of host-cell apoptosis and its effects on the parasite-host interaction and the pathogenesis of parasitic diseases. We also discuss the previously unrecognized phenomenon of apoptotic cell death in (unicellular) protozoan parasites and its potential implications.

Neutrophils Activate Macrophages for Intracellular Killing of Leishmania major through Recruitment of TLR4 by Neutrophil Elastase

We investigated the role of neutrophil elastase (NE) in interactions between murine inflammatory neutrophils and macrophages infected with the parasite Leishmania major. A blocker peptide specific for NE prevented the neutrophils from inducing microbicidal activity in macrophages. Inflammatory neutrophils from mutant pallid mice were defective in the spontaneous release of NE, failed to induce microbicidal activity in wild-type macrophages, and failed to reduce parasite loads upon transfer in vivo. Conversely, purified NE activated macrophages and induced microbicidal activity dependent on secretion of TNF-α. Induction of macrophage microbicidal activity by either neutrophils or purified NE required TLR4 expression by macrophages. Injection of purified NE shortly after infection in vivo reduced the burden of L. major in draining lymph nodes of TLR4-sufficient, but not TLR4-deficient mice. These results indicate that NE plays a previously unrecognized protective role in host responses to L. major infection.

INCREASED SUSCEPTIBILITY OF FAS LIGAND-DEFICIENT GLD MICE TO TRYPANOSOMA CRUZI INFECTION DUE TO A TH2-BIASED HOST IMMUNE RESPONSE

Infection of BALB / c mice with Trypanosoma cruzi resulted in up‐regulated expression of Fas and Fas ligand (FasL) mRNA by splenic CD4+ T cells, activation‐induced CD4+ T cell death (AICD), and in Fas  :  FasL‐mediated cytotoxicity. When CD4+ T cells from infected mice were co‐cultured with T. cruzi‐infected macrophages, onset of AICD exacerbated parasite replication. CD4+ T cells from T. cruzi‐infected FasL‐deficient BALB gld / gld mice had no detectable AICD in vitro and their activation with anti‐TCR did not exacerbate T. cruzi replication in macrophages. However, infection of BALB gld / gld mice with T. cruzi resulted in higher and more prolonged parasitemia, compared to wild‐type mice. Secretion of Th2 cytokines IL‐10 and IL‐4 by CD4+ T cells from infected gld mice was markedly increased, compared to controls. In addition, in vivo injection of anti‐IL‐4 mAb, but not of an isotype control mAb, reduced parasitemia in both gld and wild‐type mice. These results indicate that, besides controlling CD4+ T cell AICD and parasite replication in vitro, an intact Fas  :  FasL pathway also controls the host cytokine response to T. cruzi infection in vivo, being required to prevent an exacerbated Th2‐type immune response to the parasite.

Costimulation of Host T Lymphocytes by a Trypanosomal trans-Sialidase: Involvement of CD43 Signaling

Trans-sialidase is a membrane-bound and shed sialidase from Trypanosoma cruzi, the protozoan parasite responsible for Chagas disease. We investigated the role of soluble trans-sialidase on host CD4+ T cell activation. Trans-sialidase activated naive CD4+ T cells in vivo. Both enzymatically active and inactive recombinant trans-sialidases costimulated CD4+ T cell activation in vitro. Costimulation resulted in increased mitogen-activated protein kinase activation, proliferation, and cytokine synthesis. Furthermore, active and inactive trans-sialidases blocked activation-induced cell death in CD4+ T cells from T. cruzi-infected mice. By flow cytometry, inactive trans-sialidase bound the highly sialylated surface Ag CD43 on host CD4+ T cells. Both costimulatory and antiapoptotic effects of trans-sialidases required CD43 signaling. These results suggest that trans-sialidase family proteins are involved in exacerbated host T lymphocyte responses observed in T. cruzi infection.

The macrophage haunted by cell ghosts: a pathogen grows

Abstract Apoptosis is induced in the course of immune responses to infectious agents. The last step of apoptosis is recognition and ingestion of the dying cells by phagocytes. Here, Marcela F. Lopes and colleagues discuss recent studies and argue that phagocytosis of apoptotic cells plays a previously unrecognized role in regulating the nature of immune responses against pathogens.

Pathogen-induced proapoptotic phenotype and high CD95 (Fas) expression accompany a suboptimal CD8+ T-cell response: reversal by adenoviral vaccine.

MHC class Ia-restricted CD8+ T cells are important mediators of the adaptive immune response against infections caused by intracellular microorganisms. Whereas antigen-specific effector CD8+ T cells can clear infection caused by intracellular pathogens, in some circumstances, the immune response is suboptimal and the microorganisms survive, causing host death or chronic infection. Here, we explored the cellular and molecular mechanisms that could explain why CD8+ T cell-mediated immunity during infection with the human protozoan parasite Trypanosoma cruzi is not optimal. For that purpose, we compared the CD8+ T-cell mediated immune responses in mice infected with T. cruzi or vaccinated with a recombinant adenovirus expressing an immunodominant parasite antigen. Several functional and phenotypic characteristics of specific CD8+ T cells overlapped. Among few exceptions was an accelerated expansion of the immune response in adenoviral vaccinated mice when compared to infected ones. Also, there was an upregulated expression of the apoptotic-signaling receptor CD95 on the surface of specific T cells from infected mice, which was not observed in the case of adenoviral-vaccinated mice. Most importantly, adenoviral vaccine provided at the time of infection significantly reduced the upregulation of CD95 expression and the proapoptotic phenotype of pathogen-specific CD8+ cells expanded during infection. In parallel, infected adenovirus-vaccinated mice had a stronger CD8 T-cell mediated immune response and survived an otherwise lethal infection. We concluded that a suboptimal CD8+ T-cell response is associated with an upregulation of CD95 expression and a proapoptotic phenotype. Both can be blocked by adenoviral vaccination.

Viral FLIP Impairs Survival of Activated T Cells and Generation of CD8+ T Cell Memory

Viral FLIPs (vFLIPs) interfere with apoptosis signaling by death-domain-containing receptors in the TNFR superfamily (death receptors). In this study, we show that T cell-specific transgenic expression of MC159-vFLIP from the human Molluscum contagiosum virus blocks CD95-induced apoptosis in thymocytes and peripheral T cells, but also impairs postactivation survival of in vitro activated primary T cells despite normal early activation parameters. MC159 vFLIP impairs T cell development to a lesser extent than does Fas-associated death domain protein deficiency or another viral FLIP, E8. In the periphery, vFLIP expression leads to a specific deficit of functional memory CD8+ T cells. After immunization with a protein Ag, Ag-specific CD8+ T cells initially proliferate, but quickly disappear and fail to produce Ag-specific memory CD8+ T cells. Viral FLIP transgenic mice exhibit impaired CD8+ T cell responses to lymphocytic choriomeningitis virus and Trypanosoma cruzi infections, and a specific defect in CD8+ T cell recall responses to influenza virus was seen. These results suggest that vFLIP expression in T cells blocks signals necessary for the sustained survival of CD8+ T cells and the generation of CD8+ T cell memory. Through this mechanism, vFLIP proteins expressed by T cell tropic viruses may impair the CD8+ T cell immune responses directed against them.

Programmed T-cell death in experimental chagas disease

In mature T cells, programmed cell death is thought to serve a regulatory function by limiting both the duration and amplitude of immune responses. Programmed cell death might also be involved in immuno-pathogenesis of certain infectious diseases: recent evidence suggests that programmed T-cell death plays an important role in immune suppression during viral infections. In this article, George DosReis, Maria Evangelina Fonseca and Marcela Lopes review their findings on programmed T-cell death in experimental infection induced by the protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease. They also discuss the differential behavior of CD4(+) and CD8(+) T-cell subsets regarding programmed cell death, and same possible pathogenic aspects of host-parasite interaction, where abnormal or exaggerated programmed T-cell death could be involved.

The Fas death pathway controls coordinated expansions of Type 1 CD8 and Type 2 CD4 T cells in Trypanosoma cruzi infection

We investigated the role of the Fas ligand (FasL)/Fas death pathway on apoptosis and cytokine production by T cells in Trypanosoma cruzi infection. Anti‐FasL, but not anti‐TNF‐α or anti‐TRAIL, blocked activation‐induced cell death of CD8 T cells and increased secretion of IL‐10 and IL‐4 by CD4 T cells from T. cruzi‐infected mice. CD4 and CD8 T cells up‐regulated Fas/FasL expression during T. cruzi infection. However, Fas expression increased earlier in CD8 T cells, and a higher proportion of CD8 T cells was activated and expressed IFN‐γ compared with CD4 T cells. Injection of anti‐FasL in infected mice reduced parasitemia and CD8 T cell apoptosis and increased the ratio of CD8:CD4 T cells recovered from spleen and peritoneum. FasL blockade increased the number of activated T cells, enhanced NO production, and reduced parasite loads in peritoneal macrophages. Injection of anti‐FasL increased IFN‐γ secretion by splenocytes responding to T. cruzi antigens but also exacerbated production of type 2 cytokines IL‐10 and IL‐4 at a late stage of acute infection. These results indicate that the FasL/Fas death pathway regulates apoptosis and coordinated cytokine responses by type 1 CD8 and type 2 CD4 T cells in T. cruzi infection.