Calreticulin functions in antimicrobial immunity of obscure puffer Takifugu obscurus

Calreticulin (Crt) is a highly conserved and multi-functional protein with lectin-like properties and important immunological activities. In this study, a Crt homolog, namely, ToCrt, was cloned and characterized from the obscure puffer Takifugu obscurus with an open reading frame of 1278 bp encoding a putative protein of 425 amino acids. The deduced amino acid sequence of ToCrt consisted of three conserved structural domains: N-domain, P-domain, and C-terminal domain. In the phylogenetic tree, ToCrt formed a separate cluster with three Crts from other pufferfish species (Takifugu rubripes, Takifugu flavidus, and Takifugu bimaculatus). The mRNA transcript of ToCrt was ubiquitously expressed in all the examined tissues in a decreasing order: liver, spleen, kidney, gills, intestine, and heart. After Vibrio harveyi, Edwardsiella tarda, and Aeromonas hydrophila stimulations, the levels of ToCrt mRNA in the kidney and spleen were significantly upregulated compared with that in the control group. The recombinant calreticulin domain of ToCrt (rToCrt) could bind three Gram-negative bacteria (V. harveyi, E. tarda, and A. hydrophila) and polysaccharides from bacterial cell walls such as lipopolysaccharide and peptidoglycan. Meanwhile, rToCrt could agglutinate different kinds of microorganisms and exhibit antimicrobial activity. These results suggested that T. obscurus ToCrt could serve as an antimicrobial effector in the host immune response against invading microorganisms.

The measles virus hemagglutinin downregulates the cellular receptor SLAM (CD150)

Signaling lymphocyte activation molecule (SLAM), a cellular receptor for measles virus, was downregulated from the surface of cells infected with either the Edmonston or wild-type KA strain of measles virus. Transfection of the expression plasmid encoding the Edmonston or KA hemagglutinin, but not the fusion protein, induced downregulation of SLAM in not only cells expressing the envelope protein on the surface, but those not expressing it. After cocultivation with cells expressing the hemagglutinin, SLAM-expressing cells also exhibited downregulation of SLAM. Thus, the measles virus hemagglutinin can induce downregulation of SLAM in cells either expressing or coming in contact with it.