Comparative antigenicity of recombinant wild-type staphylokinase (SakSTAR) and a selected mutant (SakSTAR.M38) in a baboon thrombolysis model

Facile generation and use of immunogenic polypeptide fusions to a sparingly soluble non-antigenic protein carrier

Many researchers attempt to prepare antipeptide antibodies by immunizing animals with preparations of fusion proteins or conjugates between the target peptide and a larger protein (such as GST or KLH). Often, the immune response to the larger protein dominates. We have engineered a protein to be sparingly soluble in aqueous solution and nonantigenic, and show that fusions of this sparingly soluble non-antigenic protein (SSNAP) to target peptide sequences can be purified easily to a point suitable for immunizations. When animals are immunized with such fusion proteins, the majority of the immune response is to the target peptide. In all three cases tested, the peptide-specific immune response generated using the SSNAP carrier was greater than that obtained with peptides chemically linked to BSA or KLH, or expressed as fusion proteins to GST. The SSNAP carrier induced a very early IgG response with all classes of IgG well represented in the specific antibody response. All of the SSNAP fusion peptide-derived antibodies were capable of recognizing the full-length target protein in both ELISA and Western analysis. Based on the superior performance of the SSNAP antigens, these studies suggest this novel strategy will have broad utility for the generation of peptide antibodies.

Recombinant staphylokinase variants with altered immunoreactivity. III: Species variability of antibody binding patterns

BACKGROUND

The "charged cluster-to-alanine" substitution variants SakSTAR(K35A,E38A,K74A,E75A,R77A) and SakSTAR(K74A,E75A,R77A,E80A,D82A), previously identified as SakSTAR.M38 and SakSTAR.M89, respectively, induce less antibody formation in patients than wild-type recombinant staphylokinase (SakSTAR), but their specific activities are reduced by 50%. Therefore, the effect of the reversal of one or more of these substituted amino acids on the ratio of activity to antigenicity was studied.

METHODS AND RESULTS

Fourteen mutants with one to four "alanine-to-wild-type" reversals were expressed in Escherichia coli and highly purified (> 95%). In rabbits immunized with wild-type SakSTAR, the combined K35,E38,K74,E75,R77 or K74,E75,R77,E80,E82 epitope accounted for only 30% of antibody absorption from plasma, and no clear immunodominant residue could be identified. In baboons immunized with SakSTAR, the K35,E38 and K74,E75,R77 epitopes or the K74,E75,R77 and E80,D82 epitopes contributed equally to account for 50% of total antibody binding, but no immunodominant residues were apparent. In pooled plasma from patients with peripheral arterial occlusion treated with wild-type SakSTAR, about 40% of the antibodies depended on K74 of epitope K74,E75,R77 for binding, whereas epitopes K35,E38 and E80,D82 had a negligible contribution toward antibody recognition.

CONCLUSIONS

The recognition pattern by SakSTAR variants of antibodies induced with wild-type SakSTAR differs markedly among species. This implies that a systematic evaluation of reduced antigen recognition and antibody induction in humans will require the development of human or humanized systems. Surprisingly, SakSTAR(K74), with a single substitution of Lys74 with Ala, had an intact specific activity but did not absorb 40% of the antibodies induced in patients by treatment with wild-type SakSTAR.

Recombinant staphylokinase variants with altered immunoreactivity. IV: Identification of variants with reduced antibody induction but intact potency

BACKGROUND

The thrombolytic potency and antibody induction of selected variants of recombinant staphylokinase (SakSTAR), including SakSTAR(K74) with Lys74, SakSTAR(E75) with Glu75-, SakSTAR(EER) with Glu38, Glu75, and Arg77, and SakSTAR(K74ER) with Lys74, Glu75, and Arg77 replaced by Ala, were studied.

METHODS AND RESULTS

In rabbits, SakSTAR(74) and SakSTAR(EER) elicited significantly less circulating neutralizing activity than SakSTAR and SakSTAR(E75) (P = .005 and P = .0002 versus SakSTAR, respectively). In baboons, SakSTAR(K74) induced significantly fewer antibodies than wild-type SakSTAR (P < .05). Intra-arterial administration in patients with peripheral arterial occlusion of SakSTAR(K74) (n = 11) or SakSTAR(K74ER) (n = 6) induced significantly fewer circulating neutralizing antibodies [median values and interquartile ranges, 20 (3.8 to 26) and 2.4 (0.2 to 59) micrograms compound neutralized/mL plasma after 3 to 4 weeks (P = .01 and P = .035 versus SakSTAR, respectively)] than SakSTAR (n = 9) [median value and interquartile range, 93 (24 to 110) micrograms compound neutralized/mL plasma]. Overt neutralizing antibody induction (> 10 micrograms compound neutralized/mL plasma) occurred in all 9 patients given wild-type SakSTAR, in 6 of the 11 SakSTAR(K74ER) patients (P = .038 versus SakSTAR), and in 2 of the 6 SakSTAR(K74ER) patients (P = .011 versus SakSTAR).

CONCLUSIONS

SakSTAR(K74), a variant of recombinant staphylokinase with a single substitution of Lys74 with Ala, and SakSTAR(K74), with Lys74, Glu75, and Arg77 substituted with Ala, have intact thrombolytic potencies but induce significantly less antibody formation in patients.