Owen Proudfoot

Dendritic Cells Induce Immunity and Long-Lasting Protection against Blood-Stage Malaria despite an In Vitro Parasite-Induced Maturation Defect

ABSTRACT Dendritic cells (DC) suffer a maturation defect following interaction with erythrocytes infected with malaria parasites and become unable to induce protective malaria liver-stage immunity. Here we show that, by contrast, maturation-arrested DC in vitro are capable of the successful induction of antigen-specific gamma interferon (IFN-γ) and interleukin 4 (IL-4) T-cell responses, antibody responses, and potent protection against lethal blood-stage malaria challenge in vivo. Similar results were found with DC pulsed with intact parasitized Plasmodium yoelii or Plasmodium chabaudi erythrocytes. Cross-strain protection was also induced. High levels of protection (80 to 100%) against lethal challenge were evident from 10 days after a single immunization and maintained up to 120 days. Interestingly, correlation studies versus blood-stage protection at different time points suggest that the immune effector mechanisms associated with protection could change over time. Antibody-independent, T-cell- and IL-12-associated protection was observed early after immunization, followed by antibody and IL-4-associated, IFN-γ-independent protection in long-term studies. These results indicate that DC, even when clearly susceptible to parasite-induced maturation defect effects in vitro, can be central to the induction of protection against blood-stage malaria in vivo.

Selectively Impaired CD8+ but Not CD4+ T Cell Cycle Arrest during Priming as a Consequence of Dendritic Cell Interaction with Plasmodium-Infected Red Cells

Individuals living in malaria-endemic areas show generally low T cell responses to malaria Ags. In this study, we show murine dendritic cell (DC) interaction with parasitized erythrocytes (pRBC) arrested their maturation, resulting in impaired ability to stimulate naive, but not recall T cell responses in vitro and in vivo. Moreover, within the naive T cell population, pRBC-treated DC were selectively deficient in priming CD8+ but not CD4+ T cells. Indeed, DC that had taken up pRBC were shown for the first time to efficiently prime CD4+ T cell responses to a known protective merozoite Ag, MSP4/5. In contrast, impaired priming resulted in decreases in both proliferation and cytokine production by CD8+ T cells. Deficient priming was observed to both a model and a Plasmodium berghei-specific CD8+ T cell epitope. The mechanisms underlying the inability of parasite-treated DC to prime CD8+ T cells were explored. pRBC treatment of DC from wild-type C57BL/6, but not from IL-10 knockout animals, suppressed DC-mediated T cell priming across a Transwell, suggesting active IL-10-dependent suppression. CD8+ T cells were arrested at the G0 stage of the cell cycle after two cell divisions post-Ag stimulation. The proliferation arrest was partially reversible by the addition of IL-2 or IL-7 to responder cultures. These results suggest that in malaria-endemic areas, priming of CD8+ T cell responses may be more difficult to induce via vaccination than the priming of CD4+ T cells. Moreover, pathogens may selectively target the CD8+ T cell arm of protective immunity for immune evasion.

Molecular basis of improved immunogenicity in DNA vaccination mediated by a mannan based carrier

Receptor mediated gene delivery is an attractive non-viral method for targeting genetic material to specific cell types. We have previously utilized oxidized (OMPLL) and reduced mannan poly-L-lysine (RMPLL) to target DNA vaccines to antigen presenting cells and demonstrated that it could induce far stronger immune responses in mice compared to naked DNA immunization. In this study, we describe the immune enhancing attributes of mannan-PLL mediated DNA vaccination at the molecular level. Several attributes observed in similar gene delivery conjugates, such as entry via the endocytic pathway, low toxicity, protection from nucleases and compaction of particle size, were also evident here. In addition, OMPLL and RMPLL conjugates had profound effects on the antigen presentation functions of dendritic cells and macrophages, through the stimulation of cytokine production and maturation of dendritic cells. Interestingly, we demonstrate that OMPLL-DNA and RMPLL-DNA are able to mediate dendritic cell activation via toll-like receptor 2 as opposed to mannan alone which mediates via toll-like receptor 4. Overall, this report leads to greater understanding of how oxidized and reduced mannan mediated gene delivery could augment immune responses to DNA vaccination and provide insights into ways of further improving its immunogenicity.

Dendritic cell vaccination

There has been a surge of interest in the use of dendritic cell (DC) vaccination as cellular immunotherapy for numerous cancers. Despite some encouraging results, this therapeutic modality is far from being considered as a therapy for cancer. This review will first discuss preclinical DC vaccination in murine models of cancer, with an emphasis on comparative studies investigating different methods of antigen priming. We will then comment on the various murine DC subsets and how these relate to human DC preparations used for clinical studies. Finally, the methodology used to generate human DCs and some recent clinical trials in several cancers are reviewed.

Immunogenetics and the design of Plasmodium falciparum vaccines for use in malaria-endemic populations

There are about 500,000,000 cases of clinical malaria of varying severity per year. Some 2 million deaths per year result, almost all of which occur in children living in sub-Saharan Africa and are due to Plasmodium falciparum infection (1). Optimism stemming from the development of vaccines that show some protection against malaria in animals and in malaria-naive humans challenged with laboratory strains of P. falciparum has been dampened by their failure to provide significant, long-lasting protection to individuals living in regions where the disease is endemic. One consequence of the great number of deaths in young children is that malaria has exerted an almost unparalleled selective pressure on humans, leading to the appearance of gene polymorphisms at high frequency, including some for lethal hemoglobinopathies (2). Polymorphic forms of a number of host genes involved in immunity have been associated with protection or susceptibility to malaria. These are likely to predispose populations to unique immune response patterns to vaccines, which may enhance or interfere with their efficacy. Here, we consider some recent findings on host diversity, particularly of gene products involved in immunity. We then discuss the results from recent human vaccine trials, as well as potential strategies to optimize vaccines for use in malaria-endemic areas. Immunity to malaria is quite complex and still not completely understood. The cellular arm of the immune system is considered more important in controlling liver-stage infections, although antibodies contribute to protection; humoral immune mechanisms may be more important in controlling the blood stages. The role of other immune system components is not as well defined, but an involvement in resistance to malaria is often inferred where a particular polymorphism is common in individuals living in malaria-endemic areas. Genes that have come under specific scrutiny include those for components of the innate and acquired immune systems: mannose-binding protein (MBP), inducible nitric oxide synthase (iNOS), Fc receptors, cytokines and cytokine receptors, and the class I and class II MHC molecules.