Targeting Immunosuppressive Macrophages in the Cutaneous T-Cell Lymphoma Microenvironment

Abstract

Introduction: Immunosuppressive M2-like macrophages contribute to malignancy progression in cutaneous T-cell lymphoma (CTCL). Hence targeting M2-like macrophage differentiation and function may have potential therapeutic utility in CTCL. Targetable signaling pathways in M2 macrophages include the mannose receptor CD206 and the pattern recognition receptor Toll-like Receptor 4 (TLR4). Objective: To evaluate TLR4 and CD206 function in CTCL-associated macrophages and their therapeutic potential as targets of immunomodulation. Methods: We assessed TLR4 and CD206 expression in human CTCL lesional skin biopsies and in transwell co-cultures containing macrophages and CTCL cells. Using an established murine model of CTCL, we quantified tumor burden of mouse MBL2 cells after transplantation into immunocompetent mice with and without manipulating TLR4 or CD206 function. To abrogate TLR4 function we used TLR4 deficient mice and to stimulate CD206 signaling we used a novel CD206 binding peptide, RP-832c. Results: Human CTCL lesional skin biopsies and MBL2 tumors displayed co-localized expression of CD206 and TLR4 on macrophages. CTCL-associated macrophages in co-cultures exhibited an M2-like polarized phenotype as well as increased TLR4 expression. In contrast, when these co-cultures were treated with RP-832c, macrophages displayed the M1-like phenotype. In mice lacking TLR4, CTCL tumor growth was reduced (Figure 1) and M1 to M2 macrophage ratios were significantly higher compared to that of wild type. Similarly, CTCL tumors in mice treated with RP-832c displayed a shift from M2 macrophage phenotype to M1 and exhibited attenuated tumor growth (Figure 2). Conclusion: Our data suggest that both TLR4 inhibition and CD206 activation in the CTCL tumor microenvironment inhibit tumor growth by inducing a phenotypic shift from immunosuppressive M2-like to anti-tumor M1-like macrophages. Therefore, topical or systemic formulations of CD206 binding peptide, RP-832c, and small molecule TLR4 inhibitors present promising immunomodulatory therapeutic options in CTCL. Figure 1: MBL2 cells were injected s.c. into flanks of C57BL/6 and TLR4-/- recipients (5 mice /grp.) and tumor volumes monitored overtime. Shown are the mean and standard error for each group. Figure 2: MBL2 cells were injected s.c. into flanks of C57BL/6 recipients (5 mice /grp.) and mice were treated daily with i.p. injections of RP832c or saline starting 10 days post inoculation. Tumor volumes were monitored overtime. Shown are the mean and standard error for each group. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal