It is now broadly accepted that myeloid cells constitute an important immunosuppressive population in the tumor microenvironment (TME), and thus represent cellular targets of interest in immuno-oncology. In order to reliably test myeloid-targeting compounds, a relevant preclinical model presenting functional myeloid cells that express druggable targets is of utmost importance. As previously discussed, myeloid cells are present and functional in reconstituted BRGSF-HIS mice, and their numbers can be boosted upon hFlt3-L treatment. Notably, BRGSF-HIS myeloid cells also express various druggable targets of interest.

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Regulators of tumor immunosuppression, ILT2 and ILT4, are expressed and functional in BRGSF-HIS mice

The inhibitory leukocyte Ig-like receptors ILT2 and ILT4, also referred as LILRB1 and LILRB2 respectively, are known to regulate a broad range of cellular functions involved in immuno-tolerance, autoimmune diseases, and immune evasion of tumors.^1,2 Interestingly, there are no mouse orthologs for these inhibitory receptors, rendering ILT2/ILT4-targeting therapies’ assessment more challenging in preclinical settings. The most relevant preclinical model in this case is reconstituted mice with a human immune system. To assess the BRGSF-HIS model’s relevance for the assessment of ILT2/ILT4-targeting compounds, receptors’ expression was assessed by flow cytometry on splenocytes from 23-week-old hFlt3-L-boosted BRGSF-HIS mice. Results showed that a vast majority of dendritic cells (DC) and monocytes express hILT2 in the spleen, blood, and bone marrow (not shown), whereas ILT4 expression was confined to DC and monocytes. Additional experiments (not shown) demonstrated that ILT4 is functional in BRGSF-HIS mice, indicating that BRGSF-HIS mice represent a relevant model for ILT2/ILT4-targeting therapies assessment.

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Myeloid cells in BRGSF-HIS mice express immune checkpoint VISTA, a potent inhibitor of T-cell function

V-domain Ig suppressor of T-cell activation (VISTA) is a B7 family member mainly expressed on myeloid cells.³ It has been shown to reduce antitumor immune response, and thereby represents a promising target in cancer immunotherapy. BRGSF-HIS mice pertinence as a preclinical model for VISTA-targeting therapies was tested by assessing VISTA expression on myeloid cells in this model. Splenocytes from 25-week-old hFlt3-L-boosted BRGSF-HIS mice were collected and analyzed by flow cytometry. Results showed that human VISTA is preferentially expressed on monocytes and, to a lesser extent, on DCs and basophils. A similar expression pattern was observed in the blood (not shown), and additional experiments showed that VISTA is functional in BRGSF-HIS mice, suggesting that BRGSF-HIS mice can reliably be used to test VISTA-targeting compounds.

Interestingly, various other druggable targets of interest were tested and shown to be expressed on myeloid cells in BRGSF-HIS mice; for example, LAIR1, VSIG4, TLR2, TLR4, TLR8, TREM1, TREM2, and STING. Importantly, these mice were shown to be permissive to human tumor development,⁴ and the TME in BRGSF-HIS mice was shown to include a significant number of myeloid cells.

‍Taken together, these data demonstrate that BRGSF-HIS mice represent a relevant model to assess myeloid-targeting compounds in oncology, but also in auto-immunity, inflammation, and many more applications.

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References:

1. Liu L, Wang L, Zhao L, He C, Wang G. The Role of HLA-G in Tumor Escape: Manipulating the Phenotype and Function of Immune Cells. Front Oncol. 2020 Dec 23;10:597468.
2. Chen HM, van der Touw W, Wang YS, Kang K, Mai S, Zhang J, Alsina-Beauchamp D, Duty JA, Mungamuri SK, Zhang B, Moran T, Flavell R, Aaronson S, Hu HM, Arase H, Ramanathan S, Flores R, Pan PY, Chen SH. Blocking immunoinhibitory receptor LILRB2 reprograms tumor-associated myeloid cells and promotes antitumor immunity. J Clin Invest. 2018 Dec 3;128(12):5647-5662.
3. Yuan L, Tatineni J, Mahoney KM, Freeman GJ. VISTA: A Mediator of Quiescence and a Promising Target in Cancer Immunotherapy. Trends Immunol. 2021 Mar;42(3):209-227.
4. Marín-Jiménez JA, Capasso A, Lewis MS, Bagby SM, Hartman SJ, Shulman J, Navarro NM, Yu H, Rivard CJ, Wang X, Barkow JC, Geng D, Kar A, Yingst A, Tufa DM, Dolan JT, Blatchford PJ, Freed BM, Torres RM, Davila E, Slansky JE, Pelanda R, Eckhardt SG, Messersmith WA, Diamond JR, Lieu CH, Verneris MR, Wang JH, Kiseljak-Vassiliades K, Pitts TM, Lang J. Testing Cancer Immunotherapy in a Human Immune System Mouse Model: Correlating Treatment Responses to Human Chimerism, Therapeutic Variables and Immune Cell Phenotypes. Front Immunol. 2021 Mar 29;12:607282.