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.

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.

Macrophages are key players of the innate immunity and versatile cells that can adapt to microenvironmental signals resulting in distinct polarization states with altered phenotype and function. The dynamic ratio of M1 and M2 polarized macrophages is critical for the appropriate development and resolution of inflammation. In chronic diseases, this normal progression is arrested in the M1 dominant phase. Here we show the selective elimination of M1 macrophages by CD64-targeted immunotoxins both in vitro and in vivo in a transgenic mouse model of chronic cutaneous inflammation. We corroborated this using a skin biopsy from a patient with atopic dermatitis. The reduction of M1 cells was also accompanied by an altered cytokine profile from inflammatory IL-6(high)/IL10(low) to anti-inflammatory IL-6(low)/IL-10(high), highlighting the importance of M1 macrophages in maintaining chronic inflammation. In addition to elimination, we also show that both populations display phenotypic plasticity and that after conversion into either type, still only M1 polarization results in sensitivity towards the CD64-directed immunotoxin. Reduced endosomal protease activity in M1 versus M2 macrophages was found to be the critical factor for M1-selectivity. Our results provide novel insights into the biology of polarized macrophages and suggest that targeting CD64 expressed by M1 macrophages, can be instrumental in developing novel intervention strategies for chronic inflammatory diseases.