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Granzyme B (also termed fragmentin 2) is a prototypic member of a subfamily of serine proteases expressed in the cytoplasmic granules of cytotoxic T lymphocytes and natural killer cells, and has been implicated in the destruction of targeted cells. Studies on the role of all granzymes in the cytolytic response would be greatly facilitated by the availability of specific anti-granzyme antisera. Three synthetic peptides corresponding to amino acid residues 1-17, 92-109 and 139-157 of human granzyme B were predicted to be immunogenic in the mouse, based on their hydrophilicity, accessibility to solvent, polymorphism with respect to mouse granzyme B and by comparison with X-ray crystallographic models of the rat mast cell protease II. Each peptide was conjugated to keyhole limpet hemocyanin and used to produce monoclonal antibodies in BALB/c mice. The monoclonal antibodies produced generally exhibited strong and specific reactivity with the respective immunizing peptide. However, only those antibodies detecting the peptide corresponding to residues 139-157 were able to detect native or denatured granzyme B, in direct binding studies with purified granzyme B or by immunoblotting. As an alternative approach for antiserum production, mice were immunized with whole, proteolytically active granzyme B isolated by immuno-affinity purification from NK tumour cell lysates, using one of the monoclonal antibodies generated. Despite the overall structural similarities between the various human granzymes, these mouse antisera surprisingly reacted only with granzyme B. Indeed, the reactivity of these polyclonal antisera was specifically abrogated by preincubation with the peptide corresponding to amino acid residues 139-157. This peptide stretch therefore represents an immunodominant portion of the granzyme B molecule in the mouse. Given the analogous structures of serine protease families expressed in leukocytes, these findings have implications for the production of monospecific antisera to granzymes and related proteases.