PD-1 deficiency promotes TFH cells expansion in ITV-immunized mice by upregulating cytokines secretion

Background

T follicular helper (TFH) cells are fundamental for the development of humoral immunity. In our previous study, we found that PD-1 deficiency substantially promoted the expansion of Plasmodium-specific TFH cells and enhanced the humoral immunity of ITV (infection treatment vaccine)-immunized mice. However, the underlying mechanism by which PD-1 signaling modulates TFH cells activation remains unclear.

Methods

Mice were immunized with the ITV following the standard procedures. The activation phenotype of CD11c⁺CXCR5⁺ dendritic cells (DCs), the frequency and number of splenic follicular regulatory T cells (TFR cells), Plasmodium-specific TFH cells and germinal center (GC) B cells were analyzed by FACS. The levels of serum cytokines were quantified using the cytometric bead array (CBA) and in vivo cytokine neutralization was carried out according to a previously described protocol and verified by serum cytokine detection.

Results

We found that PD-1^-/- naïve and immunized mice had more TFR cells in the spleen than WT and WT immunized mice. Additionally, CXCR5⁺ DC, which prime TFH cells, were activated at similar levels in ITV-immunized WT and PD-1^-/- mice. However, the serum levels of IL-10, IFN-γ and MCP-1 were significantly increased in ITV-immunized PD-1^-/- mice, and treatment with an anti-IL-10, anti-IFN-γ or anti-MCP-1 neutralizing antibody in vivo markedly impaired the development of TFH cells and GC B cells.

Conclusions

Our findings demonstrate that the modulation of TFH cells by PD-1 signaling is dependent on the cytokines IL-10, IFN-γ and MCP-1 in ITV-immunized mice. These results could facilitate the design of an effective malaria vaccine with the aim of inducing humoral immune responses.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2984-4) contains supplementary material, which is available to authorized users.

Molecular and immunological tools for the evaluation of the cellular immune response in the neotropical monkey Saimiri sciureus, a non-human primate model for malaria research

Background

The neotropical, non-human primates (NHP) of the genus Saimiri and Aotus are recommended by the World Health Organization as experimental models for the study of human malaria because these animals can be infected with the same Plasmodium that cause malaria in humans. However, one limitation is the lack of immunological tools to assess the immune response in these models. The present study focuses on the development and comparative use of molecular and immunological methods to evaluate the cellular immune response in Saimiri sciureus.

Methods

Blood samples were obtained from nineteen uninfected Saimiri. Peripheral blood mononuclear cells (PBMC) from these animals and splenocytes from one splenectomized animal were cultured for 6, 12, 18, 24, 48, 72 and 96 hrs in the presence of phorbol-12-myristate-13-acetate and ionomycin. The cytokine levels in the supernatant were detected using human and NHP cytometric bead array Th1/Th2 cytokine kits, the Bio-Plex Pro Human Cytokine Th1/Th2 Assay, enzyme-linked immunosorbent assay, enzyme-linked immunospot assays and intracellular cytokine secretion assays. Cytokine gene expression was examined through TaqMan® Gene Expression Real-Time PCR using predesigned human gene-specific primers and probes or primers and probes designed based on published S. sciureus cytokine sequences.

Results

The use of five assays based on monoclonal antibodies specific for human cytokines facilitated the detection of IL-2, IL-4 and/or IFN-γ. TaqMan array plates facilitated the detection of 12 of the 28 cytokines assayed. However, only seven cytokines (IL-1A, IL-2, IL-10, IL-12B, IL-17, IFN-β, and TNF) presented relative expression levels of at least 70% of the gene expression observed in human PBMC. The use of primers and probes specific for S. sciureus cytokines facilitated the detection of transcripts that showed relative expression below the threshold of 70%. The most efficient evaluation of cytokine gene expression, in PBMC and splenocytes, was observed after 6–12 hrs of culture, except for LTA in PBMC, whose expression was best analysed after 24 hrs of culture.

Conclusions

Real-time PCR facilitates the analysis of a large number of cytokines altered during malaria infection, and this technique is considered the best tool for the evaluation of the cellular immune response in S. sciureus.

Malaria vaccine development and how external forces shape it: an overview

The aim of this paper is to analyse the current status and scientific value of malaria vaccine approaches and to provide a realistic prognosis for future developments. We systematically review previous approaches to malaria vaccination, address how vaccine efforts have developed, how this issue may be fixed, and how external forces shape vaccine development. Our analysis provides significant information on the various aspects and on the external factors that shape malaria vaccine development and reveal the importance of vaccine development in our society.

Evaluation of haematological changes in Plasmodium-berghei-infected mice administered with aqueous extract of Phyllantus amarus

This study was designed to evaluate the changes in some hematological parameters of P-berghei-infected mice treated with aqueous extract of Phyllantus amarus, a plant that is used traditionally to treat malaria patients in some Nigerian communities. The aqueous extract of the leaves at 200, 400 and 600 mg kg(-1) body weight/day dose levels were used to treat the test groups immediately after infection for the suppressive test and 72 hours post infection for the curative test while a standard antimalarial drug, Artesunate, at a dose of 50 mg kg(-1) body weight was administered on the positive control group. The negative control group was left untreated. The level of parasitemia, variation in weight, Percentage Packed Cell Volume (% PCV), erythrocytes (RBC) and leukocytes (WBC) counts in the different groups were monitored throughout the period of study. The crude extract was screened for its phytochemical composition. The crude extract at 200, 400 and 600 mg kg(-1) body weight/day suppressed parasitemia by 54.67, 61.25 and 61.24% after treating for four days in the suppressive test as against 72.32% for the standard drug while the level of parasitemia was reduced by 64.35, 66.71 and 67.13%, respectively after treating for five days in the curative test as against 71.87% for the standard drug. The variations in the values of Percentage Packed Cell Volume (% PCV), weight, leukocyte and erythrocyte counts for treated groups before and after treatment was not significant (p < 0.05). Alkaloids, flavonoids, tannins, glycosides, saponin, carbohydrate and phenols were found to be present in the crude extract. The findings of this study show that the use of Phyllantus amarus as antimalaria regimen by local medical practitioners does not adversely affect the weight and the haematological parameters determined.

Experimental models in vaccine research: malaria and leishmaniasis

Animal models have a long history of being useful tools, not only to test and select vaccines, but also to help understand the elaborate details of the immune response that follows infection. Different models have been extensively used to investigate putative immunological correlates of protection against parasitic diseases that are important to reach a successful vaccine. The greatest challenge has been the improvement and adaptation of these models to reflect the reality of human disease and the screening of vaccine candidates capable of overcoming the challenge of natural transmission. This review will discuss the advantages and challenges of using experimental animal models for vaccine development and how the knowledge achieved can be extrapolated to human disease by looking into two important parasitic diseases: malaria and leishmaniasis.

Immunological consequences of arthropod vector-derived salivary factors

Diseases, such as malaria, dengue, leishmaniasis and tick-borne encephalitis, affect a substantial percentage of the world's population and continue to result in significant morbidity and mortality. One common aspect of these diseases is that the pathogens that cause them are transmitted by the bite of an infected arthropod (e.g. mosquito, sand fly, tick). The pathogens are delivered into the skin of the mammalian host along with arthropod saliva, which contains a wide variety of bioactive molecules. These saliva components are capable of altering hemostasis and immune responses and may contribute to the ability of the pathogen to establish an infection. The biological and immunological events that occur during pathogen transmission are poorly understood but may hold the key to novel approaches to prevent transmission and/or infection. In May 2011, the National Institute of Allergy and Infectious Diseases (NIAID) of the US National Institutes of Health (NIH) in the Department of Health and Human Services hosted a workshop entitled Immunological Consequences of Vector-Derived Factors which brought together experts in skin immunology, parasitology and vector biology to outline the gaps in our understanding of the process of pathogen transmission, to explore new approaches to control pathogen transmission, and to initiate and foster multidisciplinary collaborations among these investigators.

Protection from experimental cerebral malaria with a single dose of radiation-attenuated, blood-stage Plasmodium berghei parasites

Background

Whole malaria parasites are highly effective in inducing immunity against malaria. Due to the limited success of subunit based vaccines in clinical studies, there has been a renewed interest in whole parasite-based malaria vaccines. Apart from attenuated sporozoites, there have also been efforts to use live asexual stage parasites as vaccine immunogens.

Methodology and Results

We used radiation exposure to attenuate the highly virulent asexual blood stages of the murine malaria parasite P. berghei to a non-replicable, avirulent form. We tested the ability of the attenuated blood stage parasites to induce immunity to parasitemia and the symptoms of severe malaria disease. Depending on the mouse genetic background, a single high dose immunization without adjuvant protected mice from parasitemia and severe disease (CD1 mice) or from experimental cerebral malaria (ECM) (C57BL/6 mice). A low dose immunization did not protect against parasitemia or severe disease in either model after one or two immunizations. The protection from ECM was associated with a parasite specific antibody response and also with a lower level of splenic parasite-specific IFN-γ production, which is a mediator of ECM pathology in C57BL/6 mice. Surprisingly, there was no difference in the sequestration of CD8+ T cells and CD45+ CD11b+ macrophages in the brains of immunized, ECM-protected mice.

Conclusions

This report further demonstrates the effectiveness of a whole parasite blood-stage vaccine in inducing immunity to malaria and explicitly demonstrates its effectiveness against ECM, the most pathogenic consequence of malaria infection. This experimental model will be important to explore the formulation of whole parasite blood-stage vaccines against malaria and to investigate the immune mechanisms that mediate protection against parasitemia and cerebral malaria.

TLR9 and endogenous adjuvants of the whole blood-stage malaria vaccine

Vaccination has been a successful tool in the protection against many infectious diseases, and recent advances in biotechnology have created new techniques and strategies to produce safe and efficacious vaccines for human use. However, developing a protective vaccine against malaria has been a challenge. In this article, we focus on an old approach with some new modifications, the so-called whole-parasite vaccination strategy against blood-stage Plasmodium falciparum, the deadliest human malarial agent. In addition, we discuss recent developments in our understanding of how the endogenous adjuvant activity in the parasites, which functions via Toll-like receptor 9, acts as a double-edged sword between protective vaccination and pathological responses against malaria infection.

Immune-mediated mechanisms of parasite tissue sequestration during experimental cerebral malaria

Cerebral malaria is a severe complication of malaria. Sequestration of parasitized RBCs in brain microvasculature is associated with disease pathogenesis, but our understanding of this process is incomplete. In this study, we examined parasite tissue sequestration in an experimental model of cerebral malaria (ECM). We show that a rapid increase in parasite biomass is strongly associated with the induction of ECM, mediated by IFN-gamma and lymphotoxin alpha, whereas TNF and IL-10 limit this process. Crucially, we discovered that host CD4(+) and CD8(+) T cells promote parasite accumulation in vital organs, including the brain. Modulation of CD4(+) T cell responses by helminth coinfection amplified CD4(+) T cell-mediated parasite sequestration, whereas vaccination could generate CD4(+) T cells that reduced parasite biomass and prevented ECM. These findings provide novel insights into immune-mediated mechanisms of ECM pathogenesis and highlight the potential of T cells to both prevent and promote infectious diseases.

Comparison of Plasmodium berghei challenge models for the evaluation of pre-erythrocytic malaria vaccines and their effect on perceived vaccine efficacy

Background

The immunological mechanisms responsible for protection against malaria infection vary among Plasmodium species, host species and the developmental stage of parasite, and are poorly understood. A challenge with live parasites is the most relevant approach to testing the efficacy of experimental malaria vaccines. Nevertheless, in the mouse models of Plasmodium berghei and Plasmodium yoelii, parasites are usually delivered by intravenous injection. This route is highly artificial and particularly in the P. berghei model produces inconsistent challenge results. The initial objective of this study was to compare an optimized intravenous (IV) delivery challenge model with an optimized single infectious mosquito bite challenge model. Finding shortcomings of both approaches, an alternative approach was explored, i.e., the subcutaneous challenge.

Methods

Mice were infected with P. berghei sporozoites by intravenous (tail vein) injection, single mosquito bite, or subcutaneous injection of isolated parasites into the subcutaneous pouch at the base of the hind leg. Infection was determined in blood smears 7 and 14 days later. To determine the usefulness of challenge models for vaccine testing, mice were immunized with circumsporozoite-based DNA vaccines by gene gun.

Results

Despite modifications that allowed infection with a much smaller than reported number of parasites, the IV challenge remained insufficiently reliable and reproducible. Variations in the virulence of the inoculum, if not properly monitored by the rigorous inclusion of sporozoite titration curves in each experiment, can lead to unacceptable variations in reported vaccine efficacies. In contrast, mice with different genetic backgrounds were consistently infected by a single mosquito bite, without overwhelming vaccine-induced protective immune responses. Because of the logistical challenges associated with the mosquito bite model, the subcutaneous challenge route was optimized. This approach, too, yields reliable challenge results, albeit requiring a relatively large inoculum.

Conclusions

Although a single bite by P. berghei infected Anopheles mosquitoes was superior to the IV challenge route, it is laborious. However, any conclusive evaluation of a pre-erythrocytic malaria vaccine candidate should require challenge through the natural anatomic target site of the parasite, the skin. The subcutaneous injection of isolated parasites represents an attractive compromise. Similar to the mosquito bite model, it allows vaccine-induced antibodies to exert their effect and is, therefore not as prone to the artifacts of the IV challenge.

Anopheles gambiae innate immunity

The successful development of Plasmodium in Anopheles mosquitoes is governed by complex molecular and cellular interactions that we are just beginning to understand. Anopheles immune system has received particular attention as genetic evidence points clearly to its critical role in eliminating the majority of parasites invading the midgut epithelium. Several factors regulating Plasmodium development have been identified and tentatively assigned to the individual steps leading to mosquito immune reactions; non-self-recognition, signal modulation, signal transduction and effector mechanisms. Detailed knowledge of these steps and their underlying molecular mechanisms may offer novel perspectives to abort Plasmodium development in the vector. Here, we summarize our current knowledge of mosquito innate immunity highlighting both, recent advances and areas where additional research is required.

Malaria vaccine research: lessons from 2008/9

Vaccination remains a crucial component of any initiative to control or eradicate malaria. With increasing reports of insecticide resistance in mosquitoes, and malaria parasite resistance to first-line drugs, it is clear that the development of an effective malaria vaccine is an urgent requirement for the improvement of global human health. This article highlights malaria vaccine research-related discoveries from 2008/9 to suggest that the time is now ripe for researchers to develop malaria vaccines that target many antigens from multiple stages of the parasite’s lifecycle. We also call for greater bidirectional information transfer between preclinical and clinical trials, to facilitate more efficient improvement of malaria vaccine candidates.