Exoerythrocytic Plasmodium Parasites Secrete a Cysteine Protease Inhibitor Involved in Sporozoite Invasion and Capable of Blocking Cell Death of Host Hepatocytes

Abstract

Plasmodium parasites must control cysteine protease activity that is critical for hepatocyte invasion by sporozoites, liver stage development, host cell survival and merozoite liberation. Here we show that exoerythrocytic P. berghei parasites express a potent cysteine protease inhibitor (PbICP, P. berghei inhibitor of cysteine proteases). We provide evidence that it has an important function in sporozoite invasion and is capable of blocking hepatocyte cell death. Pre-incubation with specific anti-PbICP antiserum significantly decreased the ability of sporozoites to infect hepatocytes and expression of PbICP in mammalian cells protects them against peroxide- and camptothecin-induced cell death. PbICP is secreted by sporozoites prior to and after hepatocyte invasion, localizes to the parasitophorous vacuole as well as to the parasite cytoplasm in the schizont stage and is released into the host cell cytoplasm at the end of the liver stage. Like its homolog falstatin/PfICP in P. falciparum, PbICP consists of a classical N-terminal signal peptide, a long N-terminal extension region and a chagasin-like C-terminal domain. In exoerythrocytic parasites, PbICP is posttranslationally processed, leading to liberation of the C-terminal chagasin-like domain. Biochemical analysis has revealed that both full-length PbICP and the truncated C-terminal domain are very potent inhibitors of cathepsin L-like host and parasite cysteine proteases. The results presented in this study suggest that the inhibitor plays an important role in sporozoite invasion of host cells and in parasite survival during liver stage development by inhibiting host cell proteases involved in programmed cell death.

Cysteine Protease Inhibitor (AcStefin) Is Required for Complete Cyst Formation of Acanthamoeba

“…In protozoan parasites, endogenous protease inhibitors have been found to regulate protease activity by blocking deleterious effects of host proteases from Plasmodium falciparum (34), Trypanosoma cruzi (31), Leishmania mexicana (32), and Entamoeba (36). In Plasmodium, falstatin, a cysteine protease inhibitor of P. falciparum, is required to facilitate red cell invasion (34), and PbICP (Plasmodium berghei inhibitor of cysteine proteases) has been reported to play important roles in sporozoite invasion and host cell survival (35). In T. cruzi, the overexpression of chagasin, an inhibitor of cysteine proteases (including cruzain), decreased infectivity in cell culture (31), and in L. mexicana, virulence in mice was markedly abolished by disrupting the activities of cysteine protease inhibitors (32).…”

“…In protozoan parasites, chagasin, a specific parasite-derived inhibitor of clan CA (family C1 cysteine peptidases), was identified in Trypanosoma cruzi (31). Chagasin and chagasin-like inhibitors (ICP [inhibitor of cysteine peptidases]) lack significant identity with proteins of the cystatin I25 family and are classified as chagasins (clan XI, family I42), which have been suggested to regulate endogenous and/or host cell proteases (31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42). However, little is known regarding the presence of cystatins of the I25 family in protozoan parasites or of their involvements in differentiation.…”

Autophagy in protists

“…The inhibitor can potentially control parasites, as well as host cell-derived peptidases, which sheds some additional light on parasite-host cell interaction. 188 Since autophagy plays a role in antigen presentation (reviewed in ref. 189), the hypothesis that intracellular Theileria parasites modify the autophagic response of infected leukocytes was tested.…”

Looking under the skin: the first steps in malarial infection and immunity

“…Initially, host cell apoptosis is blocked 61 , possibly via inhibitor of cysteine proteases (ICP; called falstatin in P. falciparum) 62 , which has been suggested to inhibit cathepsin L-like host cell proteases but not cathepsin B-type parasite proteases. The P. yoelii liver stage has been shown to modify the levels of several hepatocyte proteins, including p53, which is suppressed to enhance survival of the infected cell 63 .…”