Humanized Mice as a Model for Reprogramming Tumor-Associated Macrophages

Numerous drugs in preclinical development focus on repolarizing M2-like macrophages toward the inflammatory M1 phenotype. These reprogrammed macrophages secrete various chemokines and cytokines that engage additional immune effector cells to convert immunologically "cold" tumors into "hot" tumors.

Targeting Pattern Recognition Receptors in the TME

Bolt Biotherapeutics was spun out of Dr. Edgar Engleman's lab at Stanford University to commercialize the research team's pioneering work in myeloid biology. Recently, the poster's lead author, Justin Kenkel, PhD, a principal scientist at Bolt Biotherapeutics, and Shelley Ackerman, PhD, Director, met with Taconic Biosciences to discuss the findings and significance of the poster presentations. Their research represents a translational extension of a project originally pursued by Dr. Kenkel while he was a postdoctoral fellow in the Engleman Lab⁴.

The therapeutic described in the posters, BDC-3042, is an agonistic antibody for the C-type lectin receptor Dectin 2. This is a pattern recognition receptor (PRR) that senses glycan structures found within fungi and other microorganisms⁵. Macrophages and other innate immune cells are activated by characteristic molecular motifs from microbial and viral pathogens via an assortment of PRRs that trigger acute microbicidal and inflammatory responses required to contain and eliminate infection. Upon ligation, Dectin 2 mediates phagocytosis, antigen processing and presentation, and secretion of pro-inflammatory cytokines and chemokines—inducing a classically activated M1 functional profile within the triggered macrophages. In the lead up to in vivo studies, Bolt Biotherapeutics demonstrated that BDC-3042 can activate human macrophages in a series of in vitro and ex vivo assays, eliciting secretion of an array of pro-inflammatory cytokines and chemokines.

Researchers at Bolt Biotherapeutics engrafted huNOG-EXL mice with a triple-negative breast cancer cell line that they found supports macrophage infiltration into the TME. The tumors subsequently grew in vivo and were infiltrated by human immune cells, including Dectin-2-expressing TAMs. Mice were then treated with BDC-3042 and benchmarked against a standard-of-care drug, pembrolizumab, which targets an immune checkpoint pathway in T cells. Treatment with BDC-3042 was shown to inhibit tumor growth to a significantly greater extent than pembrolizumab when comparing tumor volumes from huNOG-EXL cohorts humanized with HSCs across nine different donors. This crucial proof of concept work supports further clinical evaluation of the drug candidate.

What attracted Bolt scientists to Dectin-2, instead of more well-known PRR family members? Dr. Kenkel explained that Dectin 2 expression is largely restricted to monocytes and macrophages within the myeloid compartment, as compared to other PRRs that are often more broadly expressed. This selectivity may mean treatment is accompanied by a lower risk of triggering systemic inflammatory responses.

Identification of Dectin-2 expression within multiple tumor types affirms it is a broadly applicable target preferentially localized within the TME⁶. Furthermore, its expression and druggability can be modeled within the huNOG-EXL mouse. Data presented at the meetings last year demonstrated that Dectin-2 is most highly expressed by macrophages residing in the TME in the humanized mouse model (Figure 3). Correspondingly, BDC-3042 bound preferentially to macrophages found within the TME while showing minimal binding to macrophages in other tissues (Figure 4).

Additional Considerations

The Bolt poster presentations offer insight into other important attributes of the model. A key consideration for using HIS mice is that they are most typically engrafted with immune cells from a healthy, but otherwise random, donor. Among lots of animals, the researcher should be prepared to encounter a degree of variability in immune composition and responses. This is evident in the poster's depiction of the immune richness within the TME of mice grouped by donor. Some are richly infiltrated; others are sparsely infiltrated. The ratios of macrophages to cytotoxic T cells to regulatory T cells further define the relative "hotness or coldness" of the tumor.

At Bolt Biotherapeutics, this is viewed as an informative representation of heterogeneity rather than a hurdle of the model. Dr. Ackerman explains that patients in the clinic are heterogeneous in terms of TME immune composition and responsiveness to immune-targeting agents. Therefore, when good study design comprises a representative sampling of HIS donors—three or more, as a best practice for Bolt Biotherapeutics—clinical heterogeneity may be better predicted. Many published studies do not address this variability.