Diseases caused by chronic inflammation (e.g., arthritis, multiple sclerosis and diabetic ulcers) are multicausal, thus making treatment difficult and inefficient. Due to the age-associated nature of most of these disorders and the demographic transition towards an overall older population, efficient therapeutic intervention strategies will need to be developed in the near future. Over the past decades, elimination of activated macrophages using CD64-targeting immunotoxins has proven to be a promising way of resolving inflammation in animal models. More recent data have shown that the M1-polarized population of activated macrophages in particular is critically involved in the chronic phase. We recapitulate the latest progress in the development of IT. These have advanced from full-length antibodies, chemically coupled to bacterial toxins, into single chain variants of antibodies, genetically fused with fully human enzymes. These improvements have increased the range of possible target diseases, which now include chronic inflammatory diseases. At present there are no therapeutic strategies focusing on macrophages to treat chronic disorders. In this review, we focus on the role of different polarized macrophages and the potential of CD64-based IT to intervene in the process of chronic inflammation.

Human cytolytic fusion proteins (hCFPs) are comprised of a specific cell-surface-binding moiety and an effector molecule of human origin. In contrast to common immunotoxins, including bacterial or plant toxins, they are considered not to be immunogenic. Two examples for human pro-apoptotic effector proteins are the serine protease Granzyme B and the RNase Angiogenin. Pre-clinical testing of functionality in in vitro and in vivo studies is essential for therapeutics. Establishing relevant animal models that have predictive value for therapeutic success is a great challenge in biomedical research. In this study, we investigated the species-dependent cytotoxic activity of two hCFPs prior to their application in a murine inflammation model. We found that in vitro and ex vivo either hCFP was able to kill human cells only, leaving murine cells unaffected. In contrast, no species-dependency was found for the bacterial Pseudomonas exotoxin A based immunotoxin H22(scFv)-ETA’. This species-dependent functioning has to be carefully considered when performing pre-clinical studies in animal models.

Some of the most promising small molecule toxins used to generate antibody drug conjugates (ADCs) include anti-mitotic agents (e.g., auristatin and its derivatives) which are designed to attack cancerous cells at their most vulnerable state during mitosis. We were interested in identifying a human cystostatic protein eventually showing comparable activities and allowing the generation of corresponding targeted fully human cytolytic fusion proteins. Recently, we identified the human microtubule associated protein tau (MAP tau), which binds specifically to tubulin and modulates the stability of microtubules, thereby blocking mitosis and presumably vesicular transport. By binding and stabilizing polymerized microtubule filaments, MAP tau-based fusion proteins skew microtubule dynamics towards cell cycle arrest and apoptosis. This biological activity makes rapidly proliferating cells (e.g., cancer and inflammatory cells) an excellent target for MAP tau-based targeted treatments. Their superior selectivity for proliferating cells confers additional selectivity towards upregulated tumor-associated antigens at their surface, thereby preventing off-target related toxicity against normal cells bearing tumor-associated antigens at physiologically normal to low levels. In this review, we highlight recent findings on MAP tau-based targeted cytolytic fusion proteins reported in preclinical immunotherapeutic studies.