Date Published: December 1, 2016
Publisher: Public Library of Science
Author(s): Phillip A. Swanson, Geoffrey T. Hart, Matthew V. Russo, Debasis Nayak, Takele Yazew, Mirna Peña, Shahid M. Khan, Chris J. Janse, Susan K. Pierce, Dorian B. McGavern, Christopher A Hunter.
Cerebral malaria (CM) is a severe complication of Plasmodium falciparum infection that results in thousands of deaths each year, mostly in African children. The in vivo mechanisms underlying this fatal condition are not entirely understood. Using the animal model of experimental cerebral malaria (ECM), we sought mechanistic insights into the pathogenesis of CM. Fatal disease was associated with alterations in tight junction proteins, vascular breakdown in the meninges / parenchyma, edema, and ultimately neuronal cell death in the brainstem, which is consistent with cerebral herniation as a cause of death. At the peak of ECM, we revealed using intravital two-photon microscopy that myelomonocytic cells and parasite-specific CD8+ T cells associated primarily with the luminal surface of CNS blood vessels. Myelomonocytic cells participated in the removal of parasitized red blood cells (pRBCs) from cerebral blood vessels, but were not required for the disease. Interestingly, the majority of disease-inducing parasite-specific CD8+ T cells interacted with the lumen of brain vascular endothelial cells (ECs), where they were observed surveying, dividing, and arresting in a cognate peptide-MHC I dependent manner. These activities were critically dependent on IFN-γ, which was responsible for activating cerebrovascular ECs to upregulate adhesion and antigen-presenting molecules. Importantly, parasite-specific CD8+ T cell interactions with cerebral vessels were impaired in chimeric mice rendered unable to present EC antigens on MHC I, and these mice were in turn resistant to fatal brainstem pathology. Moreover, anti-adhesion molecule (LFA-1 / VLA-4) therapy prevented fatal disease by rapidly displacing luminal CD8+ T cells from cerebrovascular ECs without affecting extravascular T cells. These in vivo data demonstrate that parasite-specific CD8+ T cell-induced fatal vascular breakdown and subsequent neuronal death during ECM is associated with luminal, antigen-dependent interactions with cerebrovasculature.
Malaria, a disease caused by protozoan parasites of the genus Plasmodium, is a leading cause of morbidity and mortality in the developing world. Of the 627,000 annual deaths due to malaria, the vast majority are caused by Plasmodium falciparum infections . Human cerebral malaria (HCM) is one of several clinical manifestations of severe P. falciparum infection and is diagnosed by coma and parasitemia in the absence of meningitis, hyperglycemia, and postictal state . HCM is fatal in 15–30% of affected individuals [3, 4], while an additional 10% of survivors suffer long-term neurological sequelae such as ataxia, hemiplegia, and cognitive impairment . Yet, the underlying cause of HCM remains unknown.
The pathological mechanisms underlying HCM are not entirely understood. Because ECM shares many of the pathological features of HCM, we set out to uncover novel mechanistic insights into the immunopathogenesis of this disorder. We made several important observations that significantly advance our understanding of cerebral malaria. Studies have shown that CM in humans and rodents is associated with BBB breakdown, edema, and hemorrhaging. During the peak of ECM, we noted that the meninges in addition to the brain parenchyma show evidence of profound vascular pathology, and this was associated a reduction in tight junction protein expression. Importantly, death from ECM is linked to marked vascular leakage and neuronal cell death in the brainstem, which is consistent with the edema and subsequent cerebral herniation recently observed in children with HCM . Mechanistically, we uncovered that this fatal disease is caused by the activities of parasite-specific CD8+ T cells operating along cerebral blood vessels. As the disease developed, cerebrovascular ECs were highly activated by IFNγ, which promoted induction of cell adhesion and antigen presenting molecules. This in turn facilitated cognate peptide-MHC I dependent engagement by parasite-specific CD8+ T cells primarily on the luminal surfaces of cerebral blood vessels. The pathological significance of these interactions was demonstrated in mice rendered genetically deficient in their ability to present antigen in MHC I on ECs. These mice had reduced cerebrovascular engagement by CD8+ T cells and were resistant to fatal disease. Lastly, therapeutic administration of antibodies specific for VLA-4 and LFA-1 rapidly displaced CD8+ T cells from cerebral blood vessels and promoted survival, thus providing a simple yet effective means to treat this disease.