Cytokine expression in malaria-infected non-human primate placentas

Malaria parasites are known to mediate the induction of inflammatory immune responses at the maternal-foetal interface during placental malaria (PM) leading to adverse consequences like pre-term deliveries and abortions. Immunological events that take place within the malaria-infected placental micro-environment leading to retarded foetal growth and disruption of pregnancies are among the critical parameters that are still in need of further elucidation. The establishment of more animal models for studying placental malaria can provide novel ways of circumventing problems experienced during placental malaria research in humans such as inaccurate estimation of gestational ages. Using the newly established olive baboon (Papio anubis)-Plasmodium knowlesi (P. knowlesi) H strain model of placental malaria, experiments were carried out to determine placental cytokine profiles underlying the immunopathogenesis of placental malaria. Four pregnant olive baboons were infected with blood stage P. knowlesi H strain parasites on the one fiftieth day of gestation while four other uninfected pregnant olive baboons were maintained as uninfected controls. After nine days of infection, placentas were extracted from all the eight baboons through cesarean surgery and used for the processing of placental plasma and sera samples for cytokine sandwich enzyme linked immunosorbent assays (ELISA). Results indicated that the occurrence of placental malaria was associated with elevated concentrations of tumour necrosis factor alpha (TNF-{\alpha}) and interleukin 12 (IL-12). Increased levels of IL-4, IL-6 and IL-10 and interferon gamma (IFN-{\gamma}) levels were detected in uninfected placentas. These findings match previous reports regarding immunity during PM thereby demonstrating the reliability of the olive baboon-P. knowlesi model for use in further studies.

💡 Research Summary

Placental malaria (PM) remains a major cause of adverse pregnancy outcomes, including pre‑term delivery, abortion, and low birth weight, largely because the immunological events occurring at the maternal‑fetal interface are not fully understood. Human studies are hampered by difficulties in accurately determining gestational age, ethical constraints, and limited access to placental tissue. To address these challenges, the authors established a novel non‑human primate model using olive baboons (Papio anubis) infected with the Plasmodium knowlesi H strain. Four pregnant baboons were inoculated with blood‑stage parasites on gestational day 50, while four uninfected pregnant baboons served as controls. Nine days post‑infection, all animals underwent cesarean delivery, and placental plasma and serum were harvested for cytokine analysis using sandwich ELISA.

The key findings were a marked elevation of the pro‑inflammatory cytokines tumor necrosis factor‑alpha (TNF‑α) and interleukin‑12 (IL‑12) in the infected placentas, whereas the uninfected placentas displayed higher concentrations of the anti‑inflammatory/immune‑regulatory cytokines interleukin‑4 (IL‑4), interleukin‑6 (IL‑6), interleukin‑10 (IL‑10), and the Th1 cytokine interferon‑gamma (IFN‑γ). This cytokine profile mirrors observations in human PM, where heightened TNF‑α and IL‑12 are associated with placental pathology, and reduced IL‑10/IL‑4 levels correlate with poor pregnancy outcomes.

The study’s strengths lie in the precise control over infection timing and gestational stage, which eliminates the temporal uncertainty common in human investigations. The baboon’s placental architecture and gestational length closely resemble those of humans, making the model physiologically relevant. Moreover, the use of P. knowlesi—a parasite capable of infecting both non‑human primates and humans—enhances the translational value of the findings.

Nevertheless, the work has limitations. The sample size (n = 4 per group) is modest, reducing statistical power. Only a single P. knowlesi strain was examined, precluding assessment of strain‑specific immune responses. The study focused exclusively on cytokine concentrations without integrating histopathological evaluation, fetal growth metrics, or birth outcomes, which would have provided a more comprehensive picture of the functional consequences of the observed immune shifts.

In conclusion, the olive baboon–P. knowlesi model successfully recapitulates the cytokine milieu characteristic of human placental malaria, validating its utility for mechanistic studies and pre‑clinical testing of vaccines or therapeutics aimed at mitigating PM‑related morbidity. Future investigations should expand the cohort size, incorporate multiple parasite strains, and link immunological data with placental pathology and fetal development outcomes to fully exploit this promising platform for translational malaria research.