Involvement of a Host Erythrocyte Sialic Acid Content in Babesia bovis Infection

Abstract

ABSTRACT. Host sialic acid (SA) has recently been suggested to play an important role in erythrocyte (RBC) infection by Babesia spp. The present study attempted to further determine the specific type of SAs important in the RBC invasion. Bovine RBC was found to bear abundant α2-3-linked SA residues but not α2-6-linked SA in nature, confirmed by flow cytometric analysis of the neuraminidase (Nm)-treated RBCs. Lectin-blot analyses revealed the removal of α2-3-linked SAs from the 97-, 33-, and 31-kDa bands by the Nm treatment. Addition of the Nm-treated RBCs into an in vitro culture of B. bovis resulted in a decreased population of the parasitized RBCs. The thin smear samples from the cultures were then observed under a confocal laser scanning microscope after staining with the α2-3-linked SA-specific lectin: a selective invasion of B. bovis was found only in the intact RBCs bearing the SAs, but not in the desialylated RBCs. Furthermore, a significant reduction of the parasitized RBCs was also observed in the culture supplemented with exogenous 3'-sialyllactose containing the α2-3-linked SAs. However, the complete inhibition of parasite proliferation was not achieved in the culture. These findings indicate that while the α2-3-linked SA-dependent pathway is needed for highly efficient invasion of host RBCs by B. bovis, there might also be other potential alternative pathways. KEY WORDS: Babesia bovis, erythrocyte, invasion, sialic acid.J. Vet. Med. Sci. 69(10): 999-1004, 2007 Babesia bovis, a hemoprotozoan parasite invading and replicating within bovine erythrocytes (red blood cells; RBCs), causes a serious disease characterized by fever, anemia, hypotensive shock syndrome, and, in severe cases, a fetal cerebral disease with a high mortality rate [9,28]. Therefore, the spread of B. bovis infection results in great economic losses all over the world [2]. Much effort has so far been directed to the development of effective control strategies, including vaccination and chemotherapy against the bovine babesiosis [3, 26]. However, since detailed information about the molecular interactions between Babesia parasites and the host RBCs is limited, the control of babesiosis is still insufficient.Recently, sialic acids (SAs), which are the terminal or subterminal non-reducing units attached to the underlying galactose residues of many sialoglycoproteins on the host cell membrane [24], have been shown to play an important role in RBC invasion by Babesia parasites [7,12,33]. Gaffar et al. [7] reported that the SA residues, regardless of the type of linkages (α2-3-and α2-6-linkages), are important in the RBC invasion by B. bovis and that B. bovis can invade not only bovine RBCs but also human and other animal RBCs in an SA-dependent manner. Interestingly, our recent study suggested that the host SAs are important not only for RBC invasion by B. caballi but also for the intraerythrocytic maturation of B. equi [16]. However, the knowledge of molecular interaction between Babesia parasites and host SAs is still limited. The present...

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“…Thus, several parasite surface molecules that are apparently involved in erythrocyte invasion have been discovered (9,10). In addition, sialic acid present on erythrocyte surface proteins and cholesterol abundance in the erythrocyte membrane, as well as extracellular Ca 2+ concentrations also play a critical role in parasite invasion (9,11,12). Furthermore, circumstantial evidence has been presented that Babesia-invasion pathways may be functionally redundant as has been demonstrated for Plasmodium (12,13,14).…”

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