Interactions between dendritic cells and CD4+ T cells during Plasmodium infection

Modulation of Signal Regulatory Protein α (SIRPα) by Plasmodium Antigenic Extract: A Preliminary In Vitro Study on Peripheral Blood Mononuclear Cells

Signal regulatory protein α (SIRPα) is an immunoreceptor expressed in myeloid innate immune cells that signals for inhibition of both phagocytosis and inflammatory response. Malaria parasites have evolutionarily selected multiple mechanisms that allow them to evade host immune defenses, including the modulation of cells belonging to innate immunity. Notwithstanding, little attention has been given to SIRPα in the context of immunosuppressive states induced by malaria. The present study attempted to investigate if malaria parasites are endowed with the capacity of modulating the expression of SIRPα on cells of innate immune system. Human peripheral blood mononuclear cells (PBMC) from healthy individuals were incubated in the presence of lipopolysaccharide (LPS) or crude extracts of P. falciparum or P. vivax and then, the expression of SIRPα was evaluated by flow cytometry. As expected, LPS showed an inhibitory effect on the expression of SIRPα in the population of monocytes, characterized by cell morphology in flow cytometry analysis, while Plasmodium extracts induced a significant positive modulation. Additional phenotyping of cells revealed that the modulatory potential of Plasmodium antigens on SIRPα expression was restricted to the population of monocytes (CD14⁺CD11c⁺), as no effect on myeloid dendritic cells (CD14⁻CD11c⁺) was observed. We hypothesize that malaria parasites explore inhibitory signaling of SIRPα to suppress antiparasitic immune responses contributing to the establishment of infection. Nevertheless, further studies are still required to better understand the role of SIRPα modulation in malaria immunity and pathogenesis.

The protective effect of CD40 ligand-CD40 signalling is limited during the early phase of Plasmodium infection

γδ T cells are essential for eliminating Plasmodium berghei XAT. Because administration of the agonistic anti-CD40 antibody can induce elimination of P. berghei XAT parasites in γδ T cell-deficient mice, we considered that γδ T cells might activate dendritic cells via CD40 signalling during infection. Here we report that administration of the anti-CD40 antibody to γδ T cell-deficient mice 3-10 days post-P. berghei XAT infection could eliminate the parasites. Our data suggest that dendritic cell activation via γδ T cells expressing CD40 ligand is critical during the early phase of infection.

Imaging of the spleen in malaria

Splenomegaly, albeit variably, is a hallmark of malaria; yet, the role of the spleen in Plasmodium infections remains vastly unknown. The implementation of imaging to study the spleen is rapidly advancing our knowledge of this so-called "blackbox" of the abdominal cavity. Not only has ex vivo imaging revealed the complex functional compartmentalization of the organ and immune effector cells, but it has also allowed the observation of major structural remodeling during infections. In vivo imaging, on the other hand, has allowed quantitative measurements of the dynamic passage of the parasite at spatial and temporal resolution. Here, we review imaging techniques used for studying the malarious spleen, from optical microscopy to in vivo imaging, and discuss the bright perspectives of evolving technologies in our present understanding of the role of this organ in infections caused by Plasmodium.

Immune activation and regulation in simian immunodeficiency virus-Plasmodium fragile-coinfected rhesus macaques

Human immunodeficiency virus (HIV) is characterized by immune activation, while chronic malaria is associated with elevated interleukin-10 (IL-10) levels. How these apparently antagonizing forces interact in the coinfected host is poorly understood. Using a rhesus macaque model of simian immunodeficiency virus (SIV)-Plasmodium fragile coinfection, we evaluated how innate immune effector cells affect the balance between immune activation and regulation. In vitro Toll-like receptor (TLR) responses of peripheral blood myeloid dendritic cells (mDC) and monocytes were temporarily associated with acute parasitemic episodes and elevated plasma IL-10 levels. Prolonged infection resulted in a decline of mDC function. Monocytes maintained TLR responsiveness but, in addition to IL-12 and tumor necrosis factor alpha, also produced IL-10. Consistent with the role of spleen in the clearance of parasite-infected red blood cells, coinfected animals also had increased splenic IL-10 mRNA levels. The main cellular source of IL-10 in the spleens of coinfected animals, however, was not splenic macrophages but T cells, suggesting an impairment of adaptive immunity. In contrast to those in spleen, IL-10-positive cells in axillary lymph nodes of coinfected animals were predominantly mDC, reminiscent of the immunosuppressive phenotype of peripheral blood mDC. Concurrent with IL-10 induction, however, SIV infection promoted elevated systemic IL-12 levels. The continuously increasing ratio of plasma IL-12 to IL-10 suggested that the overall host response in SIV-P. fragile-coinfected animals was shifted toward immune activation versus immune regulation. Therefore, SIV-P. fragile coinfection might be characterized by earlier manifestation of immune dysfunction and exhaustion than that of single-pathogen infections. This could translate into increased morbidity in HIV-malaria-coinfected individuals.

Plasmodium berghei sporozoite challenge of vaccinated BALB/c mice leads to the induction of humoral immunity and improved function of CD8(+) memory T cells

Protection against malaria can be achieved by induction of a strong CD8(+) T-cell response against the Plasmodium circumsporozoite protein (CSP), but most subunit vaccines suffer from insufficient memory responses. In the present study, we analyzed the impact of postimmunization sporozoite challenge on the development of long-lasting immunity. BALB/c mice were immunized by a heterologous prime/boost regimen against Plasmodium berghei CSP that induces a strong CD8(+) T-cell response and sterile protection, which is short-lived. Here, we show that protective immunity is prolonged by a sporozoite challenge after immunization. Repeated challenges induced sporozoite-specific antibodies that showed protective capacity. The numbers of CSP-specific CD8(+) T cells were not substantially enhanced by sporozoite infections; however, CSP-specific memory CD8(+) T cells of challenged mice displayed a higher cytotoxic activity than memory T cells of immunized-only mice. CD4(+) T cells contributed to protection as well; but CD8(+) memory T cells were found to be the central mediator of sterile protection. Based on these data, we suggest that prolonged protective immunity observed after immunization and infection is composed of different antiparasitic mechanisms including CD8(+) effector-memory T cells with increased cytotoxic activity as well as CD4(+) memory T cells and neutralizing antibodies.

The role of the spleen in malaria

The spleen is a complex organ that is perfectly adapted to selectively filtering and destroying senescent red blood cells (RBCs), infectious microorganisms and Plasmodium-parasitized RBCs. Infection by malaria is the most common cause of spleen rupture and splenomegaly, albeit variably, a landmark of malaria infection. Here, the role of the spleen in malaria is reviewed with special emphasis in lessons learned from human infections and mouse models.

Blood-stage Plasmodium berghei infection leads to short-lived parasite-associated antigen presentation by dendritic cells

Despite extensive evidence that Plasmodium species are capable of stimulating the immune system, the association of malaria with a higher incidence of other infectious diseases and reduced responses to vaccination against unrelated pathogens suggests the existence of immune suppression. Recently, we provided evidence that blood-stage Plasmodium berghei infection leads to suppression of MHC class I-restricted immunity to third party (non-malarial) antigens as a consequence of systemic DC activation. This earlier study did not, however, determine whether reactivity was also impaired to MHC class II-restricted third party antigens or to Plasmodium antigens themselves. Here, we show that while P. berghei-expressed antigens were presented early in infection, there was a rapid decline in presentation within 4 days, paralleling impairment in MHC class I- and II-restricted presentation of third party antigens. This provides important evidence that P. berghei not only causes immunosuppression to subsequently encountered third party antigens, but also rapidly limits the capacity to generate effective parasite-specific immunity.

Efficient phagosomal maturation and degradation of Plasmodium-infected erythrocytes by dendritic cells and macrophages

Dendritic cells (DC) and macrophages phagocytose pathogens and degrade them in their phagosomes to allow for proper presentation of foreign antigens to other cells of the immune system. The Plasmodium parasite, causative agent of malaria, infects RBC that are phagocytosed by DC and macrophages during the course of infection. Under specific conditions, the functionality of these cells can be affected by phagocytosis of Plasmodium-infected RBC. We investigated whether phagosomal maturation and degradation of Plasmodium yoelii-infected RBC in phagosomes is affected in DC and macrophages. We show that recruitment of the phagolysosomal marker Lamp-1 and of MHC-II, as well as acidification of phagosomes, was achieved in a timely manner. Using P. yoelii-infected RBC labelled with a fluorescent dye or transgenic green fluorescent protein (GFP)-expressing parasites, we found a gradual, rapid decrease in the phagosome fluorescence signal, indicating that P. yoelii-infected RBC are efficiently degraded in macrophages and DC. We also observed that pre-incubation of DC with infected RBC did not affect phagosomal maturation of newly internalized P. yoelii-infected RBC. In conclusion, after phagocytosis, Plasmodium-infected RBC are degraded by DC and macrophages, suggesting that the process of phagosomal maturation is effectively completed in malaria.

Modulation of dendritic cell responses by parasites: a common strategy to survive

Parasitic infections are one of the most important causes of morbidity and mortality in our planet and the immune responses triggered by these organisms are critical to determine their outcome. Dendritic cells are key elements for the development of immunity against parasites; they control the responses required to eliminate these pathogens while maintaining host homeostasis. However, there is evidence showing that parasites can influence and regulate dendritic cell function in order to promote a more permissive environment for their survival. In this review we will focus on the strategies protozoan and helminth parasites have developed to interfere with dendritic cell activities as well as in the possible mechanisms involved.