Chemokine Fusion Proteins
“Osprey’s therapeutic approach to inflammation and autoimmunity is to neutralize specific pathological leukocytes using chemokine fusion proteins,” explains Hongsheng Su, Ph.D., director of process development at Osprey Pharmaceuticals.
Dr. Su’s research group is investigating a class of novel fusion proteins called leukocyte population modulators (LPM). The lead therapeutic candidate, CCL2-LPM, is a recombinant fusion protein composed of a member of the chemokine superfamily, CCL2 (also known as macrophage chemoattractant protein-1, MCP-1) fused to a bacterial and eukaryotic protein synthesis inhibitory enzyme (RIP, ribosomal inactivating protein), in this case a truncated Shiga toxin A1 subunit. CCL2-LPM is currently in clinical trials.
Chemokines, a group of small protein molecules, are powerful chemoattractants that participate in leukocyte trafficking, extravasation (exudation of lymphatic fluid into the tissues), and recruitment to specific sites.
The CCL2 chemokine and its receptor, CCR2, play a decisive role in a number of inflammatory diseases, including those of the kidney and some cancers. By recruiting active pathological leukocytes, a cascade is initiated, which facilitates tissue damage, autoimmunity, fibrosis, and metastasis. For this reason, Dr. Su and his colleagues propose that engineering a fusion protein in which the chemokine CCL2 is fused to a protein-synthesis inhibitor would bring this destructive chain of events to a halt.
However, production of a protein that inhibits protein synthesis presents the investigator with a quandary, since it is not immediately obvious how one might prevent the newly synthesized inhibitor from shutting down its own continuing synthesis.
In order to deal with this challenge, Dr. Su and his collaborators developed a protocol using the compound 4-aminopyrazolo(3,4-d)-pyrimidine (4APP), which blocks the action of the Shiga A1. 4APP binds to the SA1, protecting cells from the toxic actions of the SA1 protein.
By employing this strategy the team was able to grow transformed cells containing the fusion protein gene to high densities, and isolate gram/L quantities of the product. The expressed and highly purified protein was effective in neutralizing cells of the monocyte lineage, which possess the CCL2 chemokine receptor. According to Dr. Su, “early clinical trials in IgA nephropathy patients are currently ongoing.”