Malaria, and its causative agent, the parasite Plasmodium falciparum, is transmitted person-to-person through a bite from an Anopheles mosquito. After being bitten by an infected mosquito, P. falciparum parasites in the sporozoite stage (PfSpz) enter the body and travel to the liver where they infect liver cells—this stage of the disease is asymptomatic. In liver cells the sporozoites divide into merozoites (PfRBC) that then exit the liver cell and infect red blood cells. This infection of red blood cells causes the symptomatic stage of malaria.
In the field of public health, it is generally accepted that immunity from malaria infections is usually short-lived and weak at best. A study by Teirlinck et al published December 1 in PLoS Pathogens set out to test whether or not lasting immunity is actually induced by infection with P. falciparum. In order to direct their experiment, the researchers chose to investigate recent findings that cytokines, small signaling proteins, play a role in the cellular immune response to P. falciparum. To do this, the researchers infected two groups via mosquito bites; group A was treated with the prophylactic anti-malaria drug chloroquine while group B received no prophylaxis.
The researchers first had to prove that previously naïve (people who have never had malaria) could be infected and mount a response. To do this, a day after infecting groups A and B, the researchers tested the cellular response by measuring interferon γ (IFNγ) levels and found that group A had significantly high levels of IFNγ which persisted throughout the study. Group B also had measurable levels of cellular response, but the researchers found that there was a greater response to PfRBC than PfSpz. The researchers then continued to measure the cellular response up to 400 days after infection and found that cellular responses persisted—albeit with large differences between individuals. An increased level of IFNγ is concerning however, because it is one of the primary cytokines for a destructive, inflammatory immune response to other pathogens. There is a possibility then that these increased levels of IFNγ could lead to other immune issues.
Based on prior findings, Teirlinck et al set out to understand what immune cells were producing the IFNγ. These prior findings stated that αβ T-cells, γδ T-cells, and natural killer (NK) cells were able to respond to PfRBC infection. The researchers found that there was only a significant increase in the number of γδ T-cells in both groups after 35 days. γδ T-cells are known as a “bridge between innate and adaptive immunity” because they are able to mount a specified response to a pathogen. γδ T-cells restrict their TCR, a pathogen-presentation protein, resulting in its use as a pathogen patternrecognition receptor—a characteristic of innate immune cells. All immune cell types tested, NK, αβ T-cells, and γδ T-cells expressed higher levels of IFNγ post-exposure to PfRBCs, and interestingly γδ T-cells produced IFNγ prior to PfRBC infection. Post infection, an IFNγ response was measurable in four cell types: γδ T-cells, αβ T-cells, γδNKT cells, and NK cells, with γδ T-cells producing the most IFNγ, and NK cells producing the least. The researchers also found that CD4 T-cells had significantly higher IFNγ production than CD8T-cells, but the majority of IFNγ producing T-cells did not express CD4 or CD8. These data are a bit confusing as a PfRBC infection is intracellular, and should stimulate a CD8 response, but the CD8 T-cells are not active. The high activity of the γδ T-cells suggests that the immune system recognizes a non-protein signal from PfRBC. Interestingly, the greatest IFNγ producer in PfSpz infection was a CD4 T-cell—the type of cell one would expect.
The researchers then tested the groups 35 days after exposure. They found that 84% of the cells, in vitro, producing INFγ in response to PfRBC were some type of memory cell. The IFNγ output of these memory cells remained constant throughout the testing period, and 400 days after exposure 72% of cells still produced IFNγ. These results were similar for PfSpz. The establishment of memory cells is a key component to mounting an immune response after an initial infection. These memory cells allow for an almost immediate response by rapidly dividing upon interaction with their antigen. The establishment of memory cells is an essential part of developing a vaccine, and this greater understanding of the mechanisms by which they are produced for PfRBC and PfSpz is invaluable.
To better understand which cells mounted the most effective response, the researchers tested immune responses to PfRBC and PfSpz. To do this they tested memory cells in order to measure their IL-2 and IFNγ production. IL-2 is a cytokine that is important in the proliferation of T-cells. By measuring the IL-2 production of memory T-cells in conjunction with IFNγ, the researchers were able to establish that T-cells in response to PfSpz were not only more proliferative, but more active as well. At day 400 post-exposure, the researchers found that a memory T-cell response with IL-2 and IFNγ was made by approximately 19.2% of the cells in vitro.
This study provides a mountain of information to vaccine developers. By establishing the natural immune reaction to a PfRBC and PfSpz infection, the researchers have given the vaccine developers a map of how and where to point the vaccine. From the information, the most affective vaccines will be of the PfSpz stage, and attempt to induce a T-cell response from both αβ T-cells and γδ T-cells. This study however has one overarching weakness however: the majority of the studies were carried out in vitro. Without studying the responses in a living system, the researchers cannot make any certifiable claims about the immune system. This study shows much promise in the field of Plasmodium falciparum vaccine development.
Teirlinck AC , McCall MBB , Roestenberg M , Scholzen A , Woestenenk R , et al. 2011 Longevity and Composition of Cellular Immune Responses Following Experimental Plasmodium falciparum Malaria Infection in Humans. PLoS Pathog 7(12): e1002389. doi:10.1371/journal.ppat.1002389
Life cycle of the Malaria Parasite. Nation Institutes of Health. http://www.niaid.nih.gov/topics/malaria/pages/lifecycle.aspx