Myeloid cells are attracting a lot of attention lately in immuno-oncology. Often in preclinical research, finding a relevant and physiological model can be a real challenge. Reconstituted mice, immunodeficient mice with a reconstituted human immune system, represent a good model as their immune system is entirely human. In the BRGSF-HIS model, the myeloid compartment can be efficiently boosted upon Flt3-L treatment. More importantly, these human immune cells must be functional for the model to be relevant, and they should be present and functional for a large window of time, to offer flexibility in studies. This might look like a long list of prerequisites, but BRGSF-HIS mice actually check all these items.
As a first indicator of monocyte activation in BRGSF-HIS, splenocytes and bone marrow from mice, with and without Flt3-L-boosts, were collected, and CD14+ cells were enriched and treated with different TLR agonists for 24 hours. Upon TLR4 and TLR7/8 agonist stimulation, a significant increase in the expression of the co-stimulatory molecule CD83 on CD14+monocytes was observed. This increase was associated with the production of cytokines such as TNF-α, and IL-1β, CCL2, IL-17, TGFβ, and IL-6, confirming that monocytes from BRGSF-HIS mice can be activated. It is interesting to note that the mice used in this study were 38–39 weeks old, confirming the long lifespan and large therapeutic window that this model offers.
To further investigate macrophage functionality in BRGSF-HIS mice, Flt3-L-boosted BRGSF-HIS mice were treated with thioglycolate, and four days later, peritoneal macrophages and bone marrow were collected and incubated with bispecific antibodies (CD47-TAA) and TAA-expressing cells for 2.5 hours. This experiment, performed in collaboration with Lightchain Bioscience, showed that peritoneal macrophages from BRGSF-HIS mice express FcɣRI, II, and III (not shown). Also, a phagocytic index was determined by fluorescence as the average number of target cells engulfed by 100 macrophages.
Interestingly, it showed that human macro phages from BRGSF-HIS mice phagocyte target cells when incubated with the relevant bispecific antibody, demonstrating that BRGSF-HIS macrophages can elicit antibody-dependent cellular phagocytosis (ADCP).
Additionally, anti-CD20 monoclonal antibody Rituximab (RTX) was administered to Flt3-L-boosted BRGSF-HIS mice to investigate its impact on circulating cells and splenocytes. RTX treatment induced a virtually complete depletion of circulating B cells at 48 hours, accompanied by a severe decrease in resident cells in the spleen (IgM+/IgD+ cells in CD19+splenocytes). As RTX has been shown to bind via FcɣR (mostly FcɣRIIa and FcɣRIIIa), and triggers complement-dependent cytotoxicity, antibody-dependent cellular cytotoxicity, and ADCP1, these data suggest an efficient FcɣR mediated monocyte and NK cell cytotoxicity in BRGSF-HIS mice.
Another type of myeloid cells has been linked to autoimmune and inflammatory diseases, as well as some cancers: pDCs. In 2018, Fournier et al.2 described a new antibody, targeting pDC marker CD303, able to induce strong activity against these cells. This antibody’s in vivo efficacy was demonstrated in BRGSF-HIS mice. Indeed, a single administration of anti-CD303 antibody efficiently depleted circulating pDCs (90%). This depletion was less drastic in the spleen, and absent in the bone marrow. Interestingly, the authors also showed that anti-CD303 antibody depletes pDCs in vitro through ADCC and ADCP, and efficiently inhibits pDCs’ ability to secrete IFNα upon TLR9 stimulation. Taken together, these data suggest that, in vivo, pDC depletion is mediated by ADCC and ADCP, indicating once more that the BRGSF-HIS myeloid compartment is functional.
All these data converge to prove the functionality of BRGSF-HIS myeloid compartment, rendering it a most relevant model to investigate and target myeloid cells in immunotherapy. As innate immunity has been shown to be involved in many pathologies, BRGSF-HIS mice could prove helpful and pertinent, not only in oncology but in auto-immunity, inflammation, and many more applications.
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