Using humanised mice to understand Plasmodium falciparum growth and development

Abstract

Malaria caused ~250 million cases and ~600,000 deaths in 2022 alone. Although treatments and vaccines are currently available, resistance to frontline drugs and waning efficacy of the available vaccines dampen malaria eradication efforts. Therefore, novel and innovative approaches are needed to better understand the causative Plasmodium parasite and to develop more effective anti-malarial therapies. In-vitro culture has been used to study P. falciparum erythrocytic cycle since 1976, allowing the characterisation of genes and testing of therapies. However, it is known that P. falciparum undergoes genetic changes under continuous culture and there are limitations in the host-parasite interactions that can be explored in-vitro. Humanised mice (HuMice) are immunodeficient mice that can be engrafted with human cells and tissues. They are the only small animal model that can be used to study human restricted P. falciparum in-situ and may provide a unique opportunity to model human malaria infection in-vivo. Herein I describe work where I successfully establish P. falciparum infections in HuMice to characterise parasite changes in different HuMice models in-vivo in the presence or absence of a human immune system model. HuMice were engrafted with either human erythrocytes alone to maintain infection or with human immune cells and human erythrocytes to determine the effect of the human immune system on parasite growth in-vivo. I established two different human immune system HuMice models to characterise changes in parasite growth in-vivo. First, using stem cells to reconstitute the human immune system allowing the development of functional human immune system cells including B and T cells. Secondly, a new approach using PBMC allowing for the reconstitution of differentiated immune cells. The results showed that mice with immune cells had higher parasitaemia than mice engrafted with human erythrocytes alone. The human immune cells in the models were responding to infection, with T cells expressing pro-inflammatory TNFα and IFNγ and B cells producing IgG in response to P. falciparum antigens. RNA-Seq data revealed an upregulation of P. falciparum genes in parasites from humanised mice compared to in-vitro including genes involved in immune evasion. The results show the usefulness of HuMice in studying P. falciparum biology and outline considerations for the selection of models appropriate to research questions.