Guadecitabine treatment in vivo reduces tumor size and specifically targets the myeloid lineage with minimal effects on lymphoid populations
4T1 tumor-bearing WT Balb/cJ mice were treated daily on days 10, 11, 12, and 13 with 50 mg guadecitabine. By day 16, guadecitabine treatment had resulted in a significant reduction in tumor volume (Figure 1a). Histologic examination revealed that control tumors had thick outer capsules surrounding the majority of the tumor, while tumors from guadecitabine-treated mice had thinner capsules that were often disrupted or fragmented. Tumors from guadecitabine-treated mice also had increased TUNEL+ apoptotic cells (Figure 1b).
Progression of certain cancers can force the bone marrow and spleen into a phase of excessive myelopoiesis, whereby immature myeloid cells spill out into the circulation. This accumulation of myeloid cells is the underlying cause of the splenomegaly seen in the 4T1 model, and provides a reservoir of recirculating MDSCs(7,15,16). Indeed, within the spleens of control 4T1 tumor-bearing mice we saw a large increase in cellularity due to a massive expansion of total MDSCs (Figure 1c), with the granulocytic MDSCs accounting for 27.75% ± 1.627% of the total cell population (Supplemental Figure 1a). There was a similar increase in total cellularity and number of MDSCs found in the bone marrow and blood (Figure 1d,e), as has been previously reported(16–18). Representative flow images for each treatment group and tissue sample are shown in Supplemental Figure 1. With guadecitabine treatment, however, the excessive myeloid populations were largely absent in each tissue compartment. In the remaining splenic MDSCs, we saw a significant increase in the immune-stimulatory markers MHC II, CD80, and CD86 (Figure 1f). Together, these data suggest that guadecitabine depletes MDSCs by targeting excessive myelopoiesis. Additionally, guadecitabine appears to push remaining MDSCs toward a mature, antigen presenting cells (APC) phenotype.
To study the effects of guadecitabine on MDSCs further, we moved to an in vitro model. To do this we utilized MDSCs isolated from ADAM10Tg mice. These MDSCs result from a defect in hematopoiesis, do not overexpress ADAM10, are suppress T cells, express MDSC markers, are heterogeneous, have been extensively characterized, and are not tumor derived(19–22). ADAM10Tg mice do not have any mature B cells. This makes them easy to harvest and isolate for experiments. MDSCs were treated in vitro with increasing doses of guadecitabine for 24, 48, or 72 hours and then an MTT cytotoxicity assay was performed. Guadecitabine was only cytotoxic directly after extended periods of time (Supplemental Figure 2a). The remaining MDSCs after 24 hours of treatment had increased expression of MHC II, CD80, and CD86, similar to in vivo experiments, but only in the Ly6C+ population (Supplemental Figure 2b). Next, we examined the direct effect on 4T1 tumor cells in vitro, showing a direct cytotoxic effect only after a prolonged direct exposure to guadecitabine (Supplemental Figure 2c). After 24 hours of treatment, 4T1 tumor cells showed increased MHC I expression most dramatically in the presence of IFNγ (Supplemental Figure 2d). Using a T cell suppression assay, T cell proliferation was unaltered in the presence of MDSCs from ADAM10Tg mice treated with guadecitabine, as compared with MDSCs from vehicle treated mice, that inhibited T cell proliferation (Supplemental Figure 2e). This shows that MDSC survival and suppressive function are impaired by guadecitabine treatment.
While reducing suppressive myeloid cells can be beneficial, the lymphoid compartment is vital for anti-tumor immunity. Because of the robust MDSC expansion in the spleen, the percentage of B and T cells at day 16 was reduced in control tumor-bearing mice (Supplemental Figure 3a,b,c). The absolute number of B and T cells was increased, suggesting immune activation. In guadecitabine-treated mice, the B and T cells were present at normal, although not elevated above, naïve levels. Additionally, the highly ordered structure of the spleen is essential to ensure proper cellular interactions. H&E staining illustrates that tumor-bearing control spleens have an enlargement of the red pulp due to accumulated MDSCs (Supplemental Figure 3d). This expansion is absent with guadecitabine treatment, and importantly the spleens maintain appropriate separation of red and white pulp. Based on the cell numbers and intact architecture, guadecitabine does not appear to affect the splenic lymphoid populations. Due to the increase in BM immature myeloid cells that was reversed with guadecitabine treatment at day 16, the development of myeloid progenitors was examined in the BM. With guadecitabine treatment, the common myeloid progenitors (CMP) and megakaryocyte-erythrocyte progenitors (MEP) are significantly reduced, while the granulocyte-macrophage progenitors (GMP) are unaffected (Supplemental Figure 3e)
To investigate the temporal effects of guadecitabine, we next performed a time-course study. We observed an immediate slowing of tumor growth that reached significance by day 16 (Figure 2a). Figure 2b-d shows a steady increase in spleen, bone marrow, and blood cellularity in control tumor-bearing mice. MDSCs begin to accumulate in the bone marrow and blood around day 12, while the splenomegaly was slightly delayed until day 14. In each of these tissue compartments, however, guadecitabine instantly halted and reversed the accumulation of MDSCs. By day 16, the total MDSC populations were back to naïve levels.
Guadecitabine’s effect on tumor growth is T cell-dependent
Cytotoxic T lymphocytes (CTLs) are the main effector cells responsible for cell-mediated killing of tumors. During an adaptive immune response, antigen-specific T cells become activated and expand to boost their anti-tumor activity. MDSCs have been shown many times to diminish the cytotoxic ability of CTLs in tumor-bearing hosts(7,8,23–29). We therefore wanted to investigate whether the reduced tumor burden resulted from a direct effect of guadecitabine on the 4T1 tumor cells in vivo or was secondary to the immunomodulatory effect of the drug.
Athymic nude mice bearing 4T1 tumors were either untreated or treated with guadecitabine as above. The tumors grew at an equal pace with or without guadecitabine (Figure 3a,b). The treatments had the same effect of reversing the tumor-induced increase in cellularity and MDSC accumulation within the spleen, bone marrow, and circulation (Figure 3c-e). TUNEL staining of the tumor sections, however, indicated few obvious apoptotic cells in either group (Figure 3f), indicating that guadecitabine does not have a direct cytotoxic effect on 4T1 tumors in vivo. Together, these data suggest that the effect of guadecitabine on tumor growth in vivo is T cell-dependent.
In order to confirm the role of T cells, we performed a series of depletion experiments to target and remove T cells with depletion antibodies according to the schedule in Supplemental Figure 4a. We confirmed the specificity and completeness of the a-CD4 and a-CD8 depletions, showing that only the intended T cell populations were removed without affecting B cells and MDSCs (Supplemental Figure 5a-c). As expected, mice receiving the isotype control + guadecitabine had smaller tumors than the isotype alone (Supplemental Figure 4b). In mice which underwent CD8+ T cell depletion, however, we observed comparable tumor growth with or without guadecitabine treatment (Supplemental Figure 4c). When CD4+, in addition to CD8+, cells were depleted, there was no additional effect on the tumor, suggesting CD4+ T cells do not play a significant role (Supplemental Figure 4d). Together, these experiments confirm the role of T cells, but also indicate that CD8+ CTLs are the important population involved in the enhanced tumor immunity.
Guadecitabine diminishes the T cell-inhibitory environment of the spleen
The draining lymph nodes (dLNs) are a site of robust immune activity and often of great interest in tumor studies(30). We harvested dLN from WT tumor-bearing control or guadecitabine-treated mice on day 16 and restimulated the cells in vitro with ionomycin+PMA. Flow cytometry analysis showed no difference in the percent of CD8+ T cells producing IFNg from guadecitabine-treated mice versus tumor-bearing controls (Figure 4a). We did not observe MDSC infiltration into the dLN (Supplemental Figure 6a) of these mice, leading us to conclude that T cells are being affected elsewhere.
Several groups have published on the importance of the spleen as a priming zone for T cell activity(7,29). Others have previously reported on the requirement of direct contact between MDSCs and T cells in order for suppression to occur(27,29). We therefore hypothesized that the MDSC accumulation within the spleen interacts with and suppresses CTLs as they recirculate. When day 16 splenocytes were restimulated in culture, there was a significant increase in the percent of CD8+ cells from guadecitabine-treated mice that produced IFNg (Figure 4b). To further investigate the T cell activity, we calculated the total number of IFN-producing CD8+ T cells between the groups and found no difference (Figure 4c). This reveals that guadecitabine elicits a higher degree of activation from the same number of splenic CTLs. This enhanced activation is further evidenced by the greater proportion of IFNγ-producing cells within the spleen (9.663% ± 0.9034 ) compared to the highly active dLN (5.149% ± 0.6741). We also confirmed previous reports that MDSCs only affect CD8+ T cells(27), as we saw no effect on IFNγ production by CD4+ T cells (Supplemental Figure 6b). As the tumor site is additionally an important interaction site for CTLs and MDSCs, we assessed frozen tumor sections for CD8a and IFNγ by immunoflourescent staining. Guadecitabine treatment increased both CD8a and IFNγ at the tumor site (Figure 4d). This was confirmed with significantly elevated Ifng expression in guadecitabine treated tumor tissue (Figure 4e). Next, we assessed MDSC activity and presence in the tumor and spleen and found that guadecitabine reduced arginase1 staining, and resulted in almost complete absence of Gr1 staining (Supplemental Figure 7a,b).
Used in combination with AIT, guadecitabine further slows tumor growth and prolongs overall survival
We next tested the efficacy of guadecitabine administered in combination with the transfer of antigen-experienced lymphocytes. Lymphocytes from tumor-bearing donor mice were expanded ex vivo as previously described, resulting in 94.4% T cell purity (Supplemental Figure 8a)(10). Recipient animals were challenged with a 50,000-cell 4T1 flank tumor on day 0 then treated as shown in Figure 5a. Briefly, CYP and lymphocyte transfer coincided with the first treatments of guadecitabine on days 3 and 4. We observed a beneficial reduction in tumor size in mice that received guadecitabine or AIT alone. When combined, however, there was an impressive four-week delay in tumor growth, with complete regression in 2 of 5 mice (Figure 5b) and improved survival (Figure 5e). Statistical significance was determined up to day 17, when all treatment groups remained experimentally viable. By comparing the areas under the curves (AUC), each treatment group was significantly reduced compared to the control mice(31). Additionally, the tumor measurements at day 17 show that the combination therapy resulted in significantly reduced tumor areas beyond guadecitabine or AIT alone. This separation of tumor growth curves continued to increase as the experiment progressed.
We also tested a different schedule, in which the CYP/AIT was delayed until the last treatment of guadecitabine on day 6 (Supplemental Figure 8b); this allowed time for guadecitabine to take effect before the antigen-experienced lymphocytes were introduced and poses a greater challenge to the efficacy of AIT against larger tumors. In this case, the synergistic effect of guadecitabine and AIT persisted further out until day 40 (Supplemental Figure 8c,d). In addition, 4 of 5 mice were cured, and we observed a higher overall survival when AIT occurred after guadecitabine (Supplemental Figure 8f) with similar statistical significance (Supplemental Figure 8d,e).
Guadecitabine similarly reduces E0771 tumor burden.
Finally, we wanted to test the effectiveness of guadecitabine in another breast cancer model. WT C57Bl/6 mice were injected subcutaneously with 200,000 E0771 cells. The tumors were allowed to become established for 3 days before the mice were treated daily on days 3, 4, 5, and 6 with 50 mg guadecitabine. Similar to the 4T1 model, guadecitabine significantly reduced the growth of E0771 tumors (Figure 6a). Additionally, adding guadecitabine significantly improved the impact of AIT (Figure 6b,c).