The High Affinity CXCR4 Inhibitor, BL-8040, Impairs the Infiltration, Migration, Viability, and Differentiation of Regulatory T Cells
Authors: Abraham Michal , Mishalian Inbal , Vainstein Abi , Shemesh-Darvish Liron , Sorani Ella , Eizenberg Orly , Peled Amnon 
Published: November 2021

Abstract
Introduction: Regulatory T (Treg) cells, an immunosuppressive subset of CD4+ T cells characterized by the expression of the master transcription factor forkhead box protein P3 (FOXP3), are a component of the immune system with essential roles in maintaining self-tolerance. Treg cells which are indispensable for preventing autoimmunity, also suppress effective tumor immunity (Togashi et al. Nat Rev Clin Oncol 2019)
Treg cells abundantly infiltrate into tumor tissues, present in the tumor microenvironment where they promote tumor development and progression by dampening antitumor immune responses. The abundantly infiltrate of Treg cells into tumor tissues is often associated with poor prognosis in cancer patients (Tanaka et al Eur. J. Immunol. 2019).
The chemokine receptor CXCR4 and its ligand, stromal cell-derived factor-1 (SDF-1/CXCL12) are critically involved in immune cell trafficking. CXCR4 overexpression, which has been identified in multiple cancer types, also supports cancer metastasis, recurrence and therapeutic resistance. More importantly, CXCR4 was shown to enhance tumor immune evasion by recruiting Treg (Santagata et al. Oncotarget. 2017)
Objective: To study the effect of the high affinity CXCR4 antagonist, BL-8040, on the biology of Treg cells. To study how BL-8040 affects the ability of these cells to penetrate into the tumors, their migratory ability, their survival and also the differentiation of naive T cells towards Treg.
Methods:C57BL/6 mice bearing LivMet pancreatic tumors and control mice were used for in-vivo study. In-vitro study was done with CD4 +CD25 hiFOXP3 + (Treg) cells which were isolated from fresh whole blood. CD4 +CD25 - cells were served as T conventional cells (Tconv). Differentiation of Treg cells was done from Naïve CD4+ T cells which were isolated from cord blood and stimulated with anti-CD3/CD28, TGF-b, IL-2 with or without BL-8040 for 6 days.
Results: When mice bearing pancreatic cancer were treated with BL-8040, we found a significant reduction in the number of infiltrating Treg into the tumor. Following treatment with BL-8040 there was no alteration in the number of Treg in the blood neither in control mice nor in mice bearing tumors. To further understand the mechanism by which BL-8040 effected Treg cells we isolated Treg and Tconv cells and found that Treg cells express lower level of CXCR4, as compared to Tconv (Figure1). Further to, when we compared their motility, we found that Treg migration less efficiently towards CXCL12. Despite this, BL8040 more efficiently suppressed CXCL12 induced migration of Treg compared to Tconv. 100 nM of BL8040 was found to inhibits the migration of 82 % from the Treg compared to only 56.6% of Tconv cells (Figure 2). CXCR4 involves classical pathways of cell survival. In order to study the role of CXCR4 in the viability of Treg, we incubated Treg and Tconv cells in the presence of BL-8040 for 24 hr. We found that Treg cells are more sensitive to BL-8040 treatment with 19.2% of cell death compared to only 3.5% of Tconv cell death (Figure 3). Treg are one of the lineages of T helper (Th) cells which differentiated from naïve CD4 T cells. We found that BL-8040 inhibits the differentiation of naive CD4 T cells toward Treg. 10uM of BL-8040 shows a 5-fold inhibition in Treg differentiation from naïve CD4 T cells (Figure 4).
Conclusions: In this work we show that the CXCR4 antagonist, BL-8040, can act as an immunomodulator by affecting the biology of regulatory T cells. BL8040 reduce the amount of infiltrating Treg into the tumors, impaired the migratory capacity of Treg toward CXCL12 and induces their cell death. Furthermore, BL-8040 was found to inhibit the differentiation of naïve CD4 T cells toward Treg. Taking all these together, BL-8040 may therefore improve the anticancer immune response, without impairing the activity of Tconv and thus can potentially serve as an effective immunomodulatory agent for cancer.
Figure 1 Figure 1.

Disclosures
Abraham: Biokine Therapeutics: Current Employment. Vainstein: BioLineRx LTD: Current Employment. Shemesh-Darvish: BioLineRx LTD: Current Employment. Sorani: BioLineRx LTD: Current Employment. Eizenberg: Biokine Therapeutics Ltd: Current Employment. Peled: Biokine Therapeutics Ltd: Current Employment; Gamida Cell: Research Funding.