The contribution of the Fas/FasL apoptotic pathway in ulcer formation during Leishmania major-induced cutaneous Leishmaniasis

Immunological role of keratinocytes in leishmaniasis

Following inoculation of Leishmania, a protozoan parasite, into the skin of a mammal, the epidermal keratinocytes recognize the parasite and influence the local immune response that can give rise to different outcomes of leishmaniasis. The early keratinocyte-derived cytokines and keratinocytes-T cells interactions shape the anti-leishmanial immune responses that contribute to the resistance or susceptibility to leishmaniasis. The keratinocyte-derived cytokines can directly potentiate the leishmanicidal activity of monocytes and macrophages. As keratinocytes express MHC-II and enhance the expression of costimulatory molecules, these cells act as antigen-presenting cells (APCs) in cutaneous leishmaniasis (CL). Depending on the epidermal microenvironment, the keratinocytes induce various types of effector CD4⁺ T cells. Keratinocyte apoptosis and necrosis have been also implicated in ulceration in CL. Further, keratinocytes contribute to the healing of Leishmania-related cutaneous wounds. However, keratinocyte-derived IL-10 may play a key role in the development of post-kala-azar dermal leishmaniasis (PKDL). In this review, a comprehensive discussion regarding the multiple roles played by keratinocytes during leishmaniasis was provided, while highlighting novel insights concerning the immunological and pathological roles of these cells.

The Impact of Neutrophil Recruitment to the Skin on the Pathology Induced by Leishmania Infection

Leishmania (L.) are obligate intracellular protozoan parasites that cause the leishmaniases, a spectrum of neglected infectious vector-borne diseases with a broad range of clinical manifestations ranging from local cutaneous, to visceral forms of the diseases. The parasites are deposited in the mammalian skin during the blood meal of an infected female phlebotomine sand fly. The skin is a complex organ acting as the first line of physical and immune defense against pathogens. Insults to skin integrity, such as that occurring during insect feeding, induces the local secretion of pro-inflammatory molecules generating the rapid recruitment of neutrophils. At the site of infection, skin keratinocytes play a first role in host defense contributing to the recruitment of inflammatory cells to the infected dermis, of which neutrophils are the first recruited cells. Although neutrophils efficiently kill various pathogens including Leishmania, several Leishmania species have developed mechanisms to survive in these cells. In addition, through their rapid release of cytokines, neutrophils modulate the skin microenvironment at the site of infection, a process shaping the subsequent development of the adaptive immune response. Neutrophils may also be recruited later on in unhealing forms of cutaneous leishmaniasis and to the spleen and liver in visceral forms of the disease. Here, we will review the mechanisms involved in neutrophil recruitment to the skin following Leishmania infection focusing on the role of keratinocytes in this process. We will also discuss the distinct involvement of neutrophils in the outcome of leishmaniasis.

Immunopathology of Leishmaniasis: An Update

Leishmaniasis represents a severe, increasing, public health problem. The perspective of its control is highly dependent on research progress, on therapeutic manipulations of the immune system, and on vaccine development. There is a correlation between the clinical outcome of Leishmania infection and the cytokine response profile. While a protective immune response against Leishmania has been clearly identified to be related to the influence of a type-1 response and IFN-γ production, the precise role of T helper (TH) 2 cytokines in non-healing infections requires further exploration. IL-4 and IL-13 (TH2 cytokines) can promote disease progression in cutaneous leishmaniasis, whereas IL-4 would appear to enhance protective type-1 responses in visceral leishmaniasis. Thus, the TH1/TH2 paradigm of resistance/susceptibility to intracellular parasites is probably an oversimplification of a more complicated network of regulatory/counter regulatory interactions. Moreover, the presence of antigen specific regulatory T cell subsets may provide an environment that contributes to the balance between TH1 and TH2 cells. Finally, the involvement of CD8+ T cells has been described, but the modality of their function in this kind of infection has not been so far elucidated.

Inhibition of caspase-8 activity promotes protective Th1- and Th2-mediated immunity to Leishmania major infection

We investigated how apoptosis pathways mediated by death receptors and caspase-8 affect cytokine responses and immunity to Leishmania major parasites. Splenic CD4 T cells undergo activation-induced apoptosis, and blockade of FasL-Fas interaction increased IFN-γ and IL-4 cytokine responses to L. major antigens. To block death receptor-induced death, we used mice expressing a T cell-restricted transgene for vFLIP. Inhibition of caspase-8 activation in vFLIP mice enhanced Th1 and Th2 cytokine responses to L. major infection, even in the Th1-prone B6 background. We also observed increased NO production by splenocytes from vFLIP mice upon T cell activation. Despite an exacerbated Th2 response, vFLIP mice controlled better L. major infection, with reduced lesions and lower parasite loads compared with WT mice. Moreover, injection of anti-IL-4 mAb in infected vFLIP mice disrupted control of parasite infection. Therefore, blockade of caspase-8 activity in T cells improves immunity to L. major infection by promoting increased Th1 and Th2 responses.

FasL and TRAIL signaling in the skin during cutaneous leishmaniasis - implications for tissue immunopathology and infectious control

Cutaneous leishmaniasis (CL) is associated with chronic inflammation and ulceration of the skin. Tissue macrophages serve as host cells and immune activation is necessary for parasite clearance. The balance between immune-mediated tissue destruction and successful clearance of infection is delicate and ulceration has been proposed to be a result of infiltration of activated immune cells into the skin. FasL and TRAIL play a dual role in skin homeostasis through induction of apoptosis as well as proinflammatory signaling. During leishmaniasis, dysregulation of both FasL and TRAIL has been described by us and others but the resulting pathogenic effects in the skin during human leishmaniasis are not fully elucidated. Targeting disease specific immune deviations has proven to be a promising new approach for the therapy of autoimmune diseases. Potentially, targeting FasL or TRAIL in combination with microcidals could offer a future treatment strategy to reduce the disfiguring immunopathology associated with CL. In this mini review we will discuss how FasL and TRAIL-induced signaling may influence on the extent of tissue inflammation and the efficacy of parasite clearance in leishmaniasis.

Immunological perspectives of leishmaniasis

Leishmania parasites have been widely used in experimental models to understand generation, maintenance and failure of immune responses underlying resistance and susceptibility to infection. The clinical outcomes of Leishmania infection depend on the infecting species and the immune status of the host. Noticeably most people exposed Leishmania never develop overt disease. Understanding the immunological events that result in failure or successful control of the parasites is fundamental to both design and evaluation of vaccines and therapies against the leishmaniases. Recent studies visualizing immune response to Leishmania major in the skin have given new insights into the different immune cells acting as hosts the parasite during different stage of infection. Control of Leishmania infection and disease progression has been associated with generation of T-helper (Th) 1 and Th2 responses respectively. Though still valid in several aspects, the Th1/Th2 paradigm is an oversimplification in need of revision. Th2 polarization has never explained severity of human leishmanial disease and a number of other T-cell subsets, including regulatory T- and Th17- cells, have important roles in susceptibility and resistance of both experimental and human leishmanial disease. This review gives an updated overview of immunological response considered to be of importance in protection, susceptibility, disease progression and cure of leishmaniasis, with a special emphasis on human diseases.

Apoptosis induced by parasitic diseases

Fatalities caused by parasitic infections often occur as a result of tissue injury that results from a form of host-cell death known as apoptosis. However, instead of being pathogenic, parasite-induced apoptosis may facilitate host survival. Consequently, it is of utmost importance to decipher and understand the process and the role of apoptosis induced or controlled by parasites in humans. Despite this, few studies provide definitive knowledge of parasite-induced host-cell apoptosis. Here, the focus is on a consideration of host-cell apoptosis as either a pathogenic feature or as a factor enabling parasite survival and development.

Cell death by apoptotic-like mechanisms could be described as a ride to death with a return ticket, as initiation of the pathway may be reversed, with the potential that it could be manipulated for therapeutic purposes. The management of host-cell apoptosis could thus be an adjunctive factor for parasitic disease treatment. Evidence that the apoptotic process could be reversed by anti-apoptotic drugs has recently been obtained, leading to the possibility of host-cell rescue after injury. An important issue will be to predict the beneficial or deleterious effects of controlling human cell death by apoptotic-like mechanisms during parasitic diseases.

Targeting caspases in intracellular protozoan infections

Caspases are cysteine aspartases acting either as initiators (caspases 8, 9, and 10) or executioners (caspases 3, 6, and 7) to induce programmed cell death by apoptosis. Parasite infections by certain intracellular protozoans increase host cell life span by targeting caspase activation. Conversely, caspase activation, followed by apoptosis of lymphocytes and other cells, prevents effective immune responses to chronic parasite infection. Here we discuss how pharmacological inhibition of caspases might affect the immunity to protozoan infections, by either blocking or delaying apoptosis.

Systemic FasL and TRAIL neutralisation reduce leishmaniasis induced skin ulceration

Cutaneous leishmaniasis (CL) is caused by Leishmania infection of dermal macrophages and is associated with chronic inflammation of the skin. L. aethiopica infection displays two clinical manifestations, firstly ulcerative disease, correlated to a relatively low parasite load in the skin, and secondly non-ulcerative disease in which massive parasite infiltration of the dermis occurs in the absence of ulceration of epidermis. Skin ulceration is linked to a vigorous local inflammatory response within the skin towards infected macrophages. Fas ligand (FasL) and Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) expressing cells are present in dermis in ulcerative CL and both death ligands cause apoptosis of keratinocytes in the context of Leishmania infection. In the present report we show a differential expression of FasL and TRAIL in ulcerative and non-ulcerative disease caused by L. aethiopica. In vitro experiments confirmed direct FasL- and TRAIL-induced killing of human keratinocytes in the context of Leishmania-induced inflammatory microenvironment. Systemic neutralisation of FasL and TRAIL reduced ulceration in a model of murine Leishmania infection with no effect on parasitic loads or dissemination. Interestingly, FasL neutralisation reduced neutrophil infiltration into the skin during established infection, suggesting an additional proinflammatory role of FasL in addition to direct keratinocyte killing in the context of parasite-induced skin inflammation. FasL signalling resulting in recruitment of activated neutrophils into dermis may lead to destruction of the basal membrane and thus allow direct FasL mediated killing of exposed keratinocytes in vivo. Based on our results we suggest that therapeutic inhibition of FasL and TRAIL could limit skin pathology during CL.

Cutaneous leishmaniases are associated with parasite-induced inflammatory lesions of the skin. The degree of clinical pathology is not associated with parasitic burden; on the contrary, ulcerative lesions are associated with low infectious load, and non-ulcerative lesions are associated with an abundant parasite infiltration. Leishmania are intracellular parasites in mammalian hosts and reside in macrophages in the deep layers of the skin, the dermis. The exact mechanism of ulceration in CL is not known and Leishmania parasites do not directly induce destruction of keratinocytes in the most superficial layer of the skin, the epidermis. In this study we investigated if ulcerated lesions were associated with higher expression of FasL- and TRAIL-induced cell-death of keratinocytes. We found a higher expression of FasL and TRAIL in human skin samples from ulcerative as compared to non-ulcerative leishmaniasis. In a mouse model of ulcerative leishmaniasis neutralisation of FasL and TRAIL reduced ulceration. We suggest that FasL and TRAIL participate in the ulcer formation during leishmaniasis both as a chemoattractant of activated neutrophils leading to tissue destruction and through direct killing of keratinocytes. Possible approaches to use this concept in therapeutical interventions with the aim to reduce immunopathology associated with leishmaniasis are discussed.

Mucocutaneous leishmaniasis masquerading as Wegener granulomatosis

A 43-year-old Brazilian female presented in 2001 with nasal stuffiness and sinusitis. A biopsy was consistent with limited Wegener's granulomatosis although antineutrophil cytoplasmic antibodies were negative. Her nasal inflammation progressed despite trials of prednisone, methotrexate, and azathioprine. A septal perforation developed and a repeat biopsy showed granulomatous inflammation. In 2006 the patient was referred to Division of Rheumatology, University of California, Los Angeles. The nose was grossly erythematous and a magnetic resonance imaging revealed nasal destruction and sinusitis. Palatine biopsies showed chronic inflammation. Cyclophosphamide at 150 mg/d resulted in markedly improved mucocutaneous lesions. The patient developed a leg and arm rash in 2007. A skin biopsy was positive for Leishmania braziliensis. The cyclophosphamide was discontinued and amphotericin B was initiated with transient benefit. Remission was achieved with pentavalent antimony. Despite multiple nasopharyngeal biopsies, for a 6-year span, mucocutaneous leishmaniasis masqueraded as Wegener's granulomatosis. Cyclophosphamide not only resulted in clinical improvement, due to reduced inflammatory response, but also allowed widespread cutaneous dissemination.

T cells specific to leishmania and other nonrelated microbial antigens can migrate to human leishmaniasis skin lesions

Immunopathological studies have contributed to the characterization of in situ inflammatory infiltrates in cutaneous leishmaniasis (CL). However, little is known about the T-cell antigen reactivity of these lesions. Our objective was to analyze the responsiveness of lymphocytes from CL lesions to leishmanial and nonrelated antigens in terms of proliferation and the production of cytokines. Mononuclear cells were extracted from lesions, and blood from CL patients infected with Leishmania (Viannia) braziliensis. Activated cells accounted for 35-45% of lesions T-cell subsets. Elevated levels of C1.7/CD244(+)CD8(+) T cells suggest in situ cytotoxic effector function. Lymphocytes isolated from the leishmaniasis lesions proliferated and produced IFN-gamma in response to leishmanial antigens as well as to irrelevant antigens such as Toxoplasma gondii (Tg). Patients presenting with larger lesions had the highest lymphocyte proliferation indexes. A high frequency of Tg-specific cells was detected in the lesions by limiting dilution assay, similar to the frequency of Leishmania-specific cells. Importantly, Tg-reactive cells were not found in lesions of patients without a history of toxoplasmosis. The proportion of Leishmania-reactive CD4(+) and CD8(+) T cells in the lesions was quite variable. Overall, these data suggest that T cells reactive to nonrelevant antigens can migrate to leishmanial lesions and possibly influence the pathogenesis of the disease.

Caspase-8 deficiency in epidermal keratinocytes triggers an inflammatory skin disease

Expression of enzymatically inactive caspase-8, or deletion of caspase-8 from basal epidermal keratinocytes, triggers chronic skin inflammation in mice. Unlike similar inflammation resulting from arrest of nuclear factor kappaB activation in the epidermal cells, the effect induced by caspase-8 deficiency did not depend on TNF, IL-1, dermal macrophage function, or expression of the toll-like receptor adapter proteins MyD88 or TRIF. Both interferon regulatory factor (IRF) 3 and TANK-binding kinase were constitutively phosphorylated in the caspase-8-deficient epidermis, and knockdown of IRF3 in the epidermis-derived cells from these mice abolished the expression of up-regulated genes. Temporal and spatial analyses of the alterations in gene expression that result from caspase-8 deficiency reveal that the changes are initiated before birth, around the time that cornification develops, and occur mainly in the suprabasal layer. Finally, we found that caspase-8-deficient keratinocytes display an enhanced response to gene activation by transfected DNA. Our findings suggest that an enhanced response to endogenous activators of IRF3 in the epidermis, presumably generated in association with keratinocyte differentiation, contributes to the skin inflammatory process triggered by caspase-8 deficiency.

Tropical dermatology: Tropical diseases caused by protozoa

Protozoan infections are very common among tropical countries and have an important impact on public health. Leishmaniasis is the most widely disseminated protozoan infection in the world, while the trypanosomiases are widespread in both Africa and South America. Amebiasis, a less common protozoal infection, is a cause of significant morbidity in some regions. Toxoplasmosis and pneumocystosis (formerly thought to be caused by a protozoan) are worldwide parasitic infections with a very high incidence in immunocompromised patients but are not restricted to them. In the past, most protozoan infections were restricted to specific geographic areas and natural reservoirs. There are cases in which people from other regions may have come in contact with these pathogens. A common situation involves an accidental contamination of a traveler, tourist, soldier, or worker that has contact with a reservoir that contains the infection. Protozoan infections can be transmitted by arthropods, such as sandflies in the case of leishmaniasis or bugs in the case of trypanosomiases. Vertebrates also serve as vectors as in the case of toxoplasmosis and its transmission by domestic cats. The recognition of the clinical symptoms and the dermatologic findings of these diseases, and a knowledge of the geographic distribution of the pathogen, can be critical in making the diagnosis of a protozoan infection.

LEARNING OBJECTIVES

After completing this learning activity, participants should be able to recognize the significance of protozoan infections worldwide, identify the dermatologic manifestations of protozoan infections, and select the best treatment for the patient with a protozoan infection.

A new function of the Fas-FasL pathway in macrophage activation

Upon infection with the protozoan parasite Leishmania major, susceptible BALB/c mice develop unhealing lesions associated with the maturation of CD4(+)Th2 cells secreting IL-4. In contrast, resistant C57BL/6 mice heal their lesions, because of expansion and secretion of IFN-gamma of CD4(+) Th1 cells. The Fas-FasL pathway, although not involved in Th cell differentiation, was reported to be necessary for complete resolution of lesions. We investigate here the role of IFN-gamma and IL-4 on Fas-FasL nonapoptotic signaling events leading to the modulation of macrophage activation. We show that addition of FasL and IFN-gamma to BMMø led to their increased activation, as reflected by enhanced secretion of TNF, IL-6, NO, and the induction of their microbicidal activity, resulting in the killing of intracellular L. major. In contrast, the presence of IL-4 decreased the synergy of IFN-gamma/FasL significantly on macrophage activation and the killing of intracellular L. major. These results show that FasL synergizes with IFN-gamma to activate macrophages and that the tight regulation by IFN-gamma and/or IL-4 of the nonapoptotic signaling events triggered by the Fas-FasL pathway affects significantly the activation of macrophages to a microbicidal state and may thus contribute to the pathogenesis of L. major infection.

Tracing immunity to human leishmaniasis

People who have recovered from leishmaniasis are believed to have long-lasting protection against subsequent infection. Understanding the immunological changes that are associated with protection from cure of and susceptibility to the disease are fundamental to both designing and evaluating vaccine candidates against the leishmaniases. In the quest for a vaccine against leishmaniasis, appropriate surrogate markers of immunity would be valuable and cost effective. Biomarkers would ease screening and selection of potentially efficient vaccine candidates. Moreover, biomarkers of disease may be used to monitor disease and aid therapeutic prognosis. This would be useful in the evaluation of both existing and new drugs, making invasive post-treatment evaluation redundant. Biomarkers may also be indicative of the severity of the disease and may be able to predict the outcome of an infection and indicate whether the patient will spontaneously recover, exhibit mild symptoms or if the disease is disseminating and will be severe. In this article we discuss the immunological changes associated with different forms of human leishmaniasis and the value of appropriate immunological biomarkers in finding an effective vaccine and an evaluation of therapies against leishmanial disease will be given.

Recent developments leading toward a paradigm switch in the diagnostic and therapeutic approach to human leishmaniasis

PURPOSE OF REVIEW

To identify recent papers showing how human and parasite genetics influence leishmaniasis, and how understanding of the immunopathology may be utilized in immunotherapy for these diseases.

RECENT FINDINGS

Progress has been made in recent years showing the complexity within populations of Leishmania spp. and indicating that different strains lead to diverse clinical pictures and responses to treatment. Thus detection of parasite genetic tags for the precise identification of infecting strains, and for predictive diagnosis of clinical and therapeutic fates seems now possible. Host genetic loci involved in disease outcome have been detected, which may also be explored for better case management. These developments in diagnosis will demand expanding the therapeutic arsenal to take their expected effect. This is starting to be fulfilled by immunotherapies successfully employed to treat cases refractory to standard first line drugs, as the result of a more profound comprehension of the immunopathology of the leishmaniases.

SUMMARY

The knowledge mounting has already helped explain why different patients present different forms of leishmaniasis and respond differently to treatment, and may be on the verge of catalyzing a major change in the already over a century old paradigm of diagnosing and managing these patients.

Inhibition of caspase-8 activity reduces IFN-gamma expression by T cells from Leishmania major infection

Following infection with Leishmania major, T cell activation and apoptosis can be detected in draining lymph nodes of C57BL/6-infected mice. We investigated the mechanisms involved in apoptosis and cytokine expression following T cell activation. After two weeks of infection, apoptotic T cells were not detected in draining lymph nodes but activation with anti-CD3 induced apoptosis in both CD4 and CD8 T cells. Treatment with anti-Fas Ligand, caspase-8 or caspase- 9 inhibitors did not block activation-induced T-cell death. We also investigated whether the blockade of caspase-8 activity would affect the expression of type-1 or type-2 cytokines. At early stages of infection, both CD4 and CD8 T cells expressed IFN-gamma upon activation. Treatment with the caspase-8 inhibitor zIETD-fmk (benzyl-oxycarbonyl-Ile- Glu(OMe)-Thr-Asp(OMe)-fluoromethyl ketone) reduced the proportion of CD8 T cells and IFN-gamma expression in both CD4 and CD8 T cells. We conclude that a non apoptotic role of caspase-8 activity may be required for T cell-mediated type-1 responses during L. major infection.

Fas ligand-induced proinflammatory transcriptional responses in reconstructed human epidermis. Recruitment of the epidermal growth factor receptor and activation of MAP kinases

Fas ligand (FasL) exerts potent proapoptotic and proinflammatory actions on epidermal keratinocytes and has been implicated in the pathogenesis of eczema, toxic epidermal necrolysis, and drug-induced skin eruptions. We used reconstructed human epidermis to investigate the mechanisms of FasL-induced inflammatory responses and their relationships with FasL-triggered caspase activity. Caspase activity was a potent antagonist of the pro-inflammatory gene expression triggered by FasL prior to the onset of cell death. Furthermore, we found that FasL-stimulated autocrine production of epidermal growth factor receptor (EGFR) ligands, and the subsequent activation of EGFR and ERK1 and ERK2 mitogen-activated protein kinases, were obligatory extracellular steps for the FasL-induced expression of a subset of inflammatory mediators, including CXCL8/interleukin (IL)-8, ICAM-1, IL-1alpha, IL-1beta, CCL20/MIP-3alpha, and thymic stromal lymphopoietin. These results expand the known physiological role of EGFR and its ligands from promoting keratinocyte mitogenesis and survival to mediating FasL-induced epidermal inflammation.

Protozoan parasites: programmed cell death as a mechanism of parasitism

Programmed cell death (PCD) is a potent mechanism to remove parasitized cells, but it has also been shown that protozoan parasites can induce or inhibit apoptosis in host cells. In recent years, it has become clear that unicellular parasites can also undergo PCD, meaning that they commit suicide in response to various stimuli. This review focuses on the role of protozoan PCD and on the interaction between protozoan parasites and the host cell death machinery from the perspective of parasite survival strategies.

FasL and TRAIL induce epidermal apoptosis and skin ulceration upon exposure to Leishmania major

Receptor-mediated apoptosis is proposed as an important regulator of keratinocyte homeostasis in human epidermis. We have previously reported that Fas/FasL interactions in epidermis are altered during cutaneous leishmaniasis (CL) and that keratinocyte death through apoptosis may play a pathogenic role for skin ulceration. To further investigate the alterations of apoptosis during CL, a keratinocyte cell line (HaCaT) and primary human epidermal keratinocytes were incubated with supernatants from Leishmania major-infected peripheral blood mononuclear cells. An apoptosis-specific microarray was used to assess mRNA expression in HaCaT cells exposed to supernatants derived from L. major-infected cultures. Fas and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) mRNA and protein expression were significantly up-regulated, and apoptosis was detected in both HaCaT and human epidermal keratinocyte cells. The keratinocyte apoptosis was partly inhibited through blocking of Fas or FasL and even more efficiently through TRAIL neutralization. Up-regulation of Fas on keratinocytes in epidermis and the presence of FasL-expressing macrophages and T cells in dermis were previously reported by us. In this study, keratinocytes expressing TRAIL, as well as the proapoptotic receptor TRAIL-R2, were detected in skin biopsies from CL cases. We propose that activation of Fas and TRAIL apoptosis pathways, in the presence of inflammatory mediators at the site of infection, leads to tissue destruction and ulceration during CL.

Pro- and anti-apoptotic activities of protozoan parasites

During infection, programmed cell death, i.e. apoptosis, is an important effector mechanism of innate and adaptive host responses to parasites. In addition, it fulfils essential functions in regulating host immunity and tissue homeostasis. Not surprisingly, however, adaptation of parasitic protozoa to their hosts also involves modulation or even exploitation of cell death in order to facilitate parasite survival in a hostile environment. During recent years, considerable progress has been made in our understanding of apoptosis during parasitic infections and there is now convincing evidence that apoptosis and its modulation by protozoan parasites has a major impact on the parasite-host interaction and on the pathogenesis of disease. This review updates our current knowledge on the diverse functions apoptosis may fulfil during infections with diverse protozoan parasites including apicomplexans, kinetoplastids and amoebae. Furthermore, we also summarize common mechanistic themes of the pro- and anti-apoptotic activities of protozoan parasites. The diverse and complex effects which parasitic protozoa exert on apoptotic cell death within the host highlight fascinating interactions of parasites and their hosts. Importantly, they also stress the importance of further investigations before the modulation of host cell apoptosis can be exploited to combat parasitic infections.