Cerebral Malaria Is Regulated by Host-Mediated Changes in Plasmodium Gene Expression

Cerebral malaria (CM), the deadliest complication of Plasmodium infection, is a complex and unpredictable disease. However, our understanding of the host and parasite factors that cause CM is limited. Using a mouse model of CM, experimental CM (ECM), we performed a three-way comparison between ECM-susceptible C57BL/6 mice infected with ECM-causing Plasmodium ANKA parasites [ANKA(C57BL/6)], ECM-resistant BALB/c mice infected with Plasmodium ANKA [ANKA(BALB/c)], and C57BL/6 mice infected with Plasmodium NK65 that does not cause ECM [NK65(C57BL/6)]. All ANKA(C57BL/6) mice developed CM. In contrast, in ANKA(BALB/c) and NK65(C57BL/6), infections do not result in CM and proceed similarly in terms of parasite growth, disease course, and host immune response. However, parasite gene expression in ANKA(BALB/c) was remarkably different than that in ANKA(C57BL/6) but similar to the gene expression in NK65(C57BL/6). Thus, Plasmodium ANKA has an ECM-specific gene expression profile that is activated only in susceptible hosts, providing evidence that the host has a critical influence on the outcome of infection. IMPORTANCE Hundreds of thousands of lives are lost each year due to the brain damage caused by malaria disease. The overwhelming majority of these deaths occur in young children living in sub-Saharan Africa. Thus far, there are no vaccines against this deadly disease, and we still do not know why fatal brain damage occurs in some children while others have milder, self-limiting disease progression. Our research provides an important clue to this problem. Here, we showed that the genetic background of the host has an important role in determining the course and the outcome of the disease. Our research also identified parasite molecules that can potentially be targeted in vaccination and therapy approaches.

Testing the impact of a single nucleotide polymorphism in a Plasmodium berghei ApiAP2 transcription factor on experimental cerebral malaria in mice

Cerebral malaria (CM) is the deadliest form of severe Plasmodium infections. Currently, we have limited understanding of the mechanisms by which Plasmodium parasites induce CM. The mouse model of CM, experimental CM (ECM), induced by infection with the rodent parasite, Plasmodium berghei ANKA (PbANKA) has been extensively used to study the pathophysiology of CM. Recent genomic analyses revealed that the coding regions of PbANKA and the closely related Plasmodium berghei NK65 (PbNK65), that does not cause ECM, differ in only 21 single nucleotide polymorphysims (SNPs). Thus, the SNP-containing genes might contribute to the pathogenesis of ECM. Although the majority of these SNPs are located in genes of unknown function, one SNP is located in the DNA binding site of a member of the Plasmodium ApiAP2 transcription factor family, that we recently showed functions as a virulence factor alternating the host’s immune response to the parasite. Here, we investigated the impact of this SNP on the development of ECM. Our results using CRISPR-Cas9 engineered parasites indicate that despite its immune modulatory function, the SNP is neither necessary nor sufficient to induce ECM and thus cannot account for parasite strain-specific differences in ECM phenotypes.

Fluorescence-Detected Conformational Changes in Duplex DNA in Open Complex Formation by Escherichia coli RNA Polymerase: Upstream Wrapping and Downstream Bending Precede Clamp Opening and Insertion of the Downstream Duplex

FRET (fluorescence resonance energy transfer) between far-upstream (-100) and downstream (+14) cyanine dyes (Cy3, Cy5) showed extensive bending and wrapping of λP_R promoter DNA on Escherichia coli RNA polymerase (RNAP) in closed and open complexes (CC and OC, respectively). Here we determine the kinetics and mechanism of DNA bending and wrapping by FRET and of formation of RNAP contacts with -100 and +14 DNA by single-dye protein-induced fluorescence enhancement (PIFE). FRET and PIFE kinetics exhibit two phases: rapidly reversible steps forming a CC ensemble ({CC}) of four intermediates [initial (RP_C), early (I_1E), mid (I_1M), and late (I_1L)], followed by conversion of {CC} to OC via I_1L. FRET and PIFE are first observed for I_1E, not RP_c. FRET and PIFE together reveal large-scale bending and wrapping of upstream and downstream DNA as RP_C advances to I_1E, decreasing the Cy3-Cy5 distance to ∼75 Å and making RNAP-DNA contacts at -100 and +14. We propose that far-upstream DNA wraps on the upper β'-clamp while downstream DNA contacts the top of the β-pincer in I_1E. Converting I_1E to I_1M (∼1 s time scale) reduces FRET efficiency with little change in -100 or +14 PIFE, interpreted as clamp opening that moves far-upstream DNA (on β') away from downstream DNA (on β) to increase the Cy3-Cy5 distance by ∼14 Å. FRET increases greatly in converting I_1M to I_1L, indicating bending of downstream duplex DNA into the clamp and clamp closing to reduce the Cy3-Cy5 distance by ∼21 Å. In the subsequent rate-determining DNA-opening step, in which the clamp may also open, I_1L is converted to the initial unstable OC (I₂). Implications for facilitation of CC-to-OC isomerization by upstream DNA and upstream binding, DNA-bending transcription activators are discussed.

A single-nucleotide polymorphism in a Plasmodium berghei ApiAP2 transcription factor alters the development of host immunity

The acquisition of malaria immunity is both remarkably slow and unpredictable. At present, we know little about the malaria parasite genes that influence the host's ability to mount a protective immune response. Here, we show that a single-nucleotide polymorphism (SNP) resulting in a single amino acid change (S to F) in an ApiAP2 transcription factor in the rodent malaria parasite Plasmodium berghei (Pb) NK65 allowed infected mice to mount a T helper cell 1 (T_H1)-type immune response that controlled subsequent infections. As compared to PbNK65^S, PbNK65^F parasites differentially expressed 46 genes, most of which are predicted to play roles in immune evasion. PbNK65^F infections resulted in an early interferon-γ response and a later expansion of germinal centers, resulting in high levels of infected red blood cell-specific T_H1-type immunoglobulin G2b (IgG2b) and IgG2c antibodies. Thus, the Pb ApiAP2 transcription factor functions as a critical parasite virulence factor in malaria infections.

The role of transcription factor T-bet in B cell mediated responses to Plasmodium infection

Malaria is a global health concern which affects over 200 million individuals worldwide. Although the immune system rapidly responds to Plasmodium infection with specific antibodies, natural antibodies fail to establish long term protection, often leading to repetitive infections and chronicity. Recent studies showed that in a chronic malaria setting both B cells and T cells induce expression of T-bet transcription factor. It is not completely understood whether this is a part of the specific host defense or a result of disease pathogenesis. Here using transgenic mouse models, we show that knockout of T-bet or IFN-g similarly reduces survival of mice compared to WT upon infection with Plasmodium chabaudi, therefore suggesting a protective role for T-bet. To determine the specific role T-bet plays in B cell responses to Plasmodium, we first identified that T-bet expression in mouse B cells are triggered by dual stimulation of IFN-g and B cell receptor. Similarly using Tbet-zsgreen transgenic mice we showed that conditions specific to Plasmodium chabaudi infection drive T-bet expression in multiple B cell subsets in a time dependent fashion. However, no such induction is shown when mice are immunized with T-dependent protein antigens adjuvanted with Alum. On the other hand, bone marrow chimeric animals reconstituted with equal amounts of congenically labeled WT and T-bet KO bone marrow cells showed that for most B cell subsets the lack of T-bet does not change the ratio of WT and KO cells upon Plasmodium infection indicating that T-bet expression may not be equally critical for all B cell responses to infection. These findings suggest a complex role of T-bet in the development of humoral responses.

Elucidating synergistic and antagonistic effects of B cell receptor and toll-like receptors 3, 4, and 9 in B cell activation

B cells are key components of the humoral immune response. Two forms of B cell activation come from engagement of the B cell receptor (BCR) with foreign antigens and from pathogen-associated molecular patters (PAMPs) binding to toll-like receptors (TLRs). For TLRs, MyD88 and TRIF are the major adaptors that lead to NFkB and IRF3 activation respectively. Among various TLRs that are present in B cells, TLR9 is only able to use MyD88 and TLR3 only signals through TRIF, while TLR4 uses both pathways. Therefore, these TLRs provide excellent tools to investigate the roles of TRIF and MyD88 pathways in B cell function and the interplay between them. Here, using stimulation through these three different TLRs either individually or in combination with each other and with/or without anti-IgM, we observed that there is synergy and antagonism between these pathways in terms of activation induced metabolic boost, cytokine production, IgM secretion, expression of various cell surface markers, and antigen presentation. We identified the TRIF pathway as an inducer of Type-I interferon secretion by B cells, which eventually favors the expression of costimulatory molecules such as CD86. On the other hand, MyD88 stimulation antagonized the expression of costimulatory molecules and dramatically induced rapid proliferation and production of IgM as well as proinflammatory cytokines such as IL-6 and TNF. Using B cells obtained from MyD88 or TRIF KO mice, we showed that the interplay between pathways is of limited scale; however, for TLR4 signaling MyD88 has a largely permissive effect while TRIF determines the magnitude of response. These results further our understanding of the complex TLR signaling machinery in B lymphocytes.