Immunological signatures can predict whether malaria-infected children will develop fever or other symptoms, suggests a study publishing September 3 in the journal Immunity.
Surprisingly, activation of the well-known tumor-suppressor protein p53 is associated with enhanced protection against malaria fever – and increasing p53 in human immune cells and in mice results in a decrease in malaria-induced inflammation.
The authors say the findings could lead to new strategies for dampening the harmful inflammatory responses associated with some infections and identifying individuals who might be at risk for such responses.
Surprisingly, activation of the well-known tumor-suppressor protein p53 is associated with enhanced protection against malaria fever – and increasing p53 in human immune cells and in mice results in a decrease in malaria-induced inflammation.
The authors say the findings could lead to new strategies for dampening the harmful inflammatory responses associated with some infections and identifying individuals who might be at risk for such responses.
“Malaria is caused by the Plasmodium falciparum parasite and remains a major killer of children in Africa,” says senior study author Peter Crompton of the National Institute of Allergy and Infectious Diseases.
“Our limited understanding of how the human immune system controls malaria-induced inflammation and parasite growth impedes the development of vaccines and adjunctive therapies for this devastating disease.”
Malaria is caused by parasites that are transmitted to people through the bites of infected mosquitoes.
In 2017, there were approximately 219 million malaria cases worldwide and 435,000 malaria deaths. P. falciparum is the most prevalent malaria parasite in Africa and is responsible for most malaria deaths globally.
In areas of intense transmission, children who survive the first five years of life have typically acquired immunity to severe malaria. But in non-immune individuals, P. falciparum malaria can cause fever and rapidly progress to severe illness and death if not treated early.
The development of a safe and effective vaccine could play a critical role in malaria elimination efforts.
Although progress is being made, a malaria vaccine that reliably induces long-term protection remains elusive.
The complexity of the Plasmodium parasite and the incomplete understanding of critical processes, such as host immune protection and disease pathogenesis, have hampered efforts to develop a vaccine.
Antimalarial drugs, in combination with mosquito control programs, have played a key role in controlling malaria in endemic areas, resulting in significant reduction of the geographic range of malarial disease worldwide.
But the emergence and spread of drug-resistant parasites and insecticide-resistant mosquitos have contributed to a re-emergence of malaria, turning back the clock on control efforts.
The need for new strategies to prevent malaria infection and disease has become a critical priority on the global malaria research agenda.
To gain insights into host factors that might protect against malaria disease, Crompton and first author Tuan Tran of Indiana University School of Medicine applied a systems biology approach to study children who differed in their ability to control parasite growth and fever following P. falciparum infection.
They collected and analyzed blood samples from healthy, uninfected Malian children at enrollment before the malaria season, during bi-weekly scheduled visits, and at their first malaria episode of the ensuing season.
Specifically, the researchers integrated whole-blood transcriptomics with flow-cytometric analysis of blood cells and cytokine and antibody profiles.
They focused on children aged 6-11 years, the age during which malaria immunity begins to be acquired in this region.
During the first malaria season, the researchers identified three distinct outcomes of P. falciparum infection.
Twenty children were immune and showed no symptoms, 26 children showed early fever at the time of infection, and 34 children experienced delayed fever two days to two weeks later.
Protection from malaria symptoms was associated with a pre-infection signature of B cell enrichment, platelet and monocyte activity, T helper cell responses, including interferon-driven pro-inflammatory responses, and p53 activation.
In addition, control of parasite growth was associated with increased immunoglobulin G and Fc receptor activation prior to infection.
After this analysis, the researchers next set out to specifically investigate the role of p53 in malaria.
Using multiple approaches in the laboratory, they found that increased p53 attenuates malaria-induced inflammation in human monocytes and in a mouse model of malaria, providing evidence that p53 activation contributes to the control of malaria fever.
However, this study does not prove that p53 is directly involved in controlling the inflammatory response to malaria in humans.
“There has been extensive research on p53 in the context of cancer, but much less is known about its role in the immune response to infections, particularly in humans,” Crompton says.
“It may be that low expression of p53 in blood could serve as a marker for individuals or populations at greater risk of harmful inflammation when infected by malaria or other pathogens.
Or perhaps increasing p53 pathways relevant to controlling inflammation could potentially reduce the severity of late-stage malaria.”
Crompton notes that it will also be important to investigate whether the findings are generalizable to other human populations and other infectious diseases, and potentially to autoimmune diseases.
“It will also be interesting to investigate the nexus between pathogen-induced modulation of p53 expression, particularly in the context of chronic or repeated infections, and cancer risk,” he says.
Plasmodium gametocytes develop from their asexual progenitors and are the only malaria parasite life-stage infective to mosquitoes. In preparation for their development in the mosquito, gametocytes cease to express many proteins involved in the parasites cycle of asexual replication, and upregulate others that are involved in sexual development1,2. Some of these have essential roles in mosquito-stage development3,4, which culminates in the insect becoming infectious to humans.
In surveys where mosquitoes were fed directly on the skin or on a blood sample, it was noted that some gametocytaemic individuals including those with high gametocyte densities were not infectious5,6. This may be associated with naturally acquired antibodies that interfere with parasite development within mosquitoes to reduce or prevent mosquito infection7,8. The role of antibodies in determining transmission efficiency can be tested in mosquito-feeding assays, in which purified antibodies are added to a mosquito blood meal that contains gametocytes9,10. Evidence for naturally acquired transmission-reducing activity (TRA) is provided if test antibodies in endemic serum cause a reduction in the number of developing Plasmodium oocysts relative to mosquitoes fed the same infectious blood meal without test antibodies. TRA can be experimentally induced after immunisation of animals with inactivated Plasmodium gametocytes or gametes8,11. The gametocyte and gamete proteins P48/45 and P230 have been identified as targets of transmission-blocking monoclonal antibodies (mAb), which act by inhibiting the protein’s role in gamete fertilisation in the mosquito gut3,4,12. Monoclonal antibodies specific to the zygote and ookinete proteins P25 and P28 were also shown to block transmission, by inhibiting ookinete invasion of the midgut epithelium13. All four have been produced as recombinant proteins that can induce antibody-mediated TRA in animal models13–15.
Because P25 and P28 mRNA is translationally repressed until zygote formation16, antibodies specific to these proteins appear absent in endemic populations17,18. In contrast, responses to P48/45 and P230 are commonly observed in naturally infected individuals10,19,20. The presence of antibodies to Plasmodium falciparum Pfs48/45 and Pfs230 in endemic sera has been associated with TRA in several, but not all sero-epidemiological surveys10,21–28. Importantly, many individuals with functional TRA do not have measurable antibodies against these two proteins10,23,25,28–31. Despite these observations, there has been no attempt to demonstrate the contribution of naturally acquired α-Pfs48/45 and α-Pfs230 antibodies to transmission inhibition, and little investigation of alternative targets of natural TRA. The gametocyte proteome has now been described in detail32,33. Utilising the protein microarray platform, genome scale data sets have been combined with functional and immunological data to provide valuable insight into mechanisms and markers of malaria humoral immunity34,35.
Here, we aim to investigate the immune signature of naturally occurring antibody-mediated TRA, to expand and prioritise antigenic targets for functional characterisation as biomarkers, or transmission-blocking vaccine candidates. To this purpose, we utilise a protein microarray comprising an inclusive selection of proteins expressed during gametocyte development. We assess antibody responses to these proteins and to correctly folded Pfs48/4536 and Pfs23015 in 648 malaria-exposed individuals from Burkina Faso, the Gambia, Cameroon, and from migrants from the Netherlands. Using purified total IgG, we assess the functional TRA of antibodies from the same individuals by determining their effect on mosquito infection density in standard membrane-feeding assays (SMFA) with cultured P. falciparum gametocytes and Anopheles stephensi mosquitoes. Our analysis reveals significant associations between high-level TRA and responses to Pfs48/45, Pfs230 and 43 novel gametocyte proteins. For a subset of 366 gametocyte-positive donors who had provided blood samples to Anopheles gambiae s.s. or An. coluzzii mosquitoes in field-based membrane-feeding assays, we determine the association of these antibody responses with mosquito infection rates during natural infections. For Pfs48/45 and Pfs230, we provide functional data that demonstrate their role in natural TRA. For one of the newly identified proteins (PfGEST), we provide experimental data that does not support its functional role in natural TRA.
More information:Immunity, Tran et al.: “A molecular signature in blood reveals a role for p53 in regulating malaria-induced inflammation” https://www.cell.com/immunity/fulltext/S1074-7613(19)30335-8 , DOI: 10.1016/j.immuni.2019.08.009
Journal information: Immunity
Provided by Cell Press
