Structural basis for RIFIN-mediated activation of LILRB1 in malaria

Abstract

The Plasmodium species that cause malaria are obligate intracellular parasites, and disease symptoms occur as they replicate within human blood. Despite risking immune detection, the parasite delivers proteins that bind host receptors to infected erythrocyte surfaces. In the causative agent of the most deadly human malaria, Plasmodium falciparum, RIFINs form the largest erythrocyte surface protein family 1 . Some RIFINs can bind inhibitory immune receptors, acting as targets for unusual antibodies containing a LAIR1 ectodomain 2 - 4 , or as ligands for LILRB1 5 . RIFINs stimulate LILRB1 activation and signalling5, thereby potentially dampening human immune responses. To understand this process, we determined a structure of a RIFIN bound to LILRB1. We show that the RIFIN mimics the natural activating ligand of LILRB1, MHC class I, in its LILRB1-binding mode. A single RIFIN mutation disrupts the complex, blocks LILRB1 binding by all tested RIFINs and abolishes signalling in a reporter assay. In a supported lipid bilayer system, which mimics NK cell activation by antibody- dependent cell-mediated cytotoxicity, both RIFIN and MHC are recruited to the NK cell immunological synapse and reduce cell activation, as measured by perforin mobilisation. Therefore, LILRB1-binding RIFINs mimic the binding mode of the natural ligand of LILRB1 and suppress NK cell function.

Analysis of Pir Gene Expression Across the Plasmodium Life Cycle

“…Whether this relates to a function in sequestration, evasion of adaptive immunity or a completely different function is unclear. Molecular mimicry as a means of immune suppression has been described for P. falciparum RIFINs, another sequence-variable multigene family, some of which mimic the natural ligand for LILRB1 [69] and suppress the activity of NK cells. P. knowlesi pir sequences have also been shown to have a striking resemblance to 50% of CD99, a T-cell regulating protein, suggesting molecular mimicry and potential immune-modulating activity [8].…”

Structure of the Plasmodium -interspersed repeat proteins of the malaria parasite

“…The expression of different P. chabaudi pirs has recently been associated with acute or chronic P. chabaudi infection (28,29). The growing evidence that RIFINs of P. falciparum bind to inhibitory immune receptors, such as LILRB1 and LAIR1 (17), and dampen immune signaling (33), is thus tantalizing. Could PIR proteins also interact with inhibitory immune receptors, thereby promoting chronic disease?…”

“…Alternatively, subgroups of RIFINs have been shown to interact with human inhibitory immune receptors, such as LILRB1 and LAIR1 (17). The LILRB1-binding RIFINs mimic the natural ligand of LILRB1, MHC class I, allowing the RIFIN to inhibit markers of natural killer cell activation, most likely reducing parasite clearance (33). These studies imply wide-ranging roles for the small VSAs in both mammalian and insect hosts.…”

“…To explore this question, we determined the structure of the extracellular domain of a CIR protein from P. chabaudi. We compared this structure with that of the variable domain of a LILRB1-binding RIFIN (33) and used structure-guided sequence analysis to predict which small VSAs are part of the PIR superfamily. This provides a framework for understanding PIR protein function and evolution.…”

Combined Immunofluorescence (IFA) and Fluorescence In Situ Hybridization (FISH) Assays for Diagnosing Babesiosis in Patients from the USA, Europe and Australia

“…Chronic babesiosis can be symptomatic or asymptomatic and occur in immunocompromised patients, the elderly and some immunocompetent patients [ 24 ]. The biological basis for such persistence has not been well studied but may involve multiple immune evasion mechanisms employed by Babesia parasites analogous to the mechanisms that have been better studied in malaria parasites [ 17 ], including the recently identified roles for iRBC surface-expressed variant proteins of the rifin family in impairing protective immune responses in malaria [ 25 ]. Additionally, mutations in the cytb and rpl4 genes of B. microti governing atovaquone and azithromycin resistance, which is now commonly seen in clinical practice, can also contribute to chronic babesiosis [ 26 , 27 ].…”