Ablative Tumor Radiation Can Change the Tumor Immune Cell Microenvironment to Induce Durable Complete Remissions

Abstract

Purpose: The goals of the study were to elucidate the immune mechanisms that contribute to desirable complete remissions of murine colon tumors treated with single radiation dose of 30 Gy. This dose is at the upper end of the ablative range used clinically to treat advanced or metastatic colorectal, liver, and non-small cell lung tumors.

Experimental design: Changes in the tumor immune microenvironment of single tumor nodules exposed to radiation were studied using 21-day (>1 cm in diameter) CT26 and MC38 colon tumors. These are well-characterized weakly immunogenic tumors.

Results: We found that the high-dose radiation transformed the immunosuppressive tumor microenvironment resulting in an intense CD8(+) T-cell tumor infiltrate, and a loss of myeloid-derived suppressor cells (MDSC). The change was dependent on antigen cross-presenting CD8(+) dendritic cells, secretion of IFNγ, and CD4(+)T cells expressing CD40L. Antitumor CD8(+) T cells entered tumors shortly after radiotherapy, reversed MDSC infiltration, and mediated durable remissions in an IFNγ-dependent manner. Interestingly, extended fractionated radiation regimen did not result in robust CD8(+) T-cell infiltration.

Conclusions: For immunologically sensitive tumors, these results indicate that remissions induced by a short course of high-dose radiotherapy depend on the development of antitumor immunity that is reflected by the nature and kinetics of changes induced in the tumor cell microenvironment. These results suggest that systematic examination of the tumor immune microenvironment may help in optimizing the radiation regimen used to treat tumors by adding a robust immune response.

Conflict of interest statement

No potential conflicts of interest were disclosed

©2015 American Association for Cancer Research.

Figures

Figure 1. Immunosuppressive tumor microenvironment in CT 26 tumors established for 21 days

A, Mononuclear cells from day 21 subcutaneous CT26 tumors, spleens from day 21 tumor-bearing mice, and spleens from normal mice were analyzed for expression of CD25, PD-1 and Tim-3 on CD4+ and CD8+ T cells, and for MDSC phenotype cells (CD11b+ Gr1hi) and TAM phenotype cells (CD11b+ Gr1lo). Percentages of each subset in boxes on representative two color analysis panels are shown, and arrows identify gated subsets. Staining for CD11b, Gr-1, CD4 and CD8 used a live mononuclear cell gate. B, CD8+, CD4+, CD11b Gr-1hi (MDSCs) (n=6) and CD11b Gr-1lo (MΩ) (n=6) cells are shown as a mean percentage +/− SE among mononuclear cells in tumor and spleens at day 21 after tumor implantation, and mean percentage of CD4+CD25+ Foxp3+ cells shown among total CD4+ T cells (n=6). *-p<0.05, ** -p<0.01,***-p<0.001, NS p>0.05. C, Representative staining of cultures in which tumor-derived MDSCs were incubated with syngeneic splenic T cells loaded with CFSE and with CD3/CD28 beads in vitro for 5 days. MDSC: T cell ratio was 1:1. HTS Transwell-96 well plates with 0.4 µm membranes were used for transwell studies. iNOS (L-NMMA) and L-arginase (nor-NOHA) inhibitors were used at 2 different concentrations (0.5 µM and 1.5 µM). Percentage of gated CD8+ T cells that diluted CFSE is shown. D, CFSE dilution by CD8+ T cells is shown as a mean percentage +/− SE of triplicate wells. CFSE-labeled T cells were cultured with tumor-derived MDSCs in the presence of CD3/CD28 beads. MDSC:T cell ratios were 1:1, 1:5,1:10. 1:20 and 1:40 (n=7). E, CFSE dilution by CD8+ T cells is shown as a mean percentage +/− SE of triplicate wells. CFSE-labeled T cells were cultured with tumor-derived MDSCs in the presence of CD3/CD28 beads at the 1:1 ratio, and L-NMMA and nor-NOHA inhibitors were added in concentrations 0.5 µM or 1.5 µM. (n=7) F, Primary CT26 tumors were established at day 0. Tumor-bearing animals were challenged with 5×106 CT26 cells on the opposite flank at day 21. Growth curves of the second tumor and fraction of mice with progressive second tumor growth are shown (n=5).

Figure 2. Treatment of advanced CT26 tumors by single high dose radiation leads to complete remission, and development of systemic long-term immunity that can be adoptively transferred by T cells

A, Experimental scheme. CT26 colon tumors were established for 21 days subcutaneously and mice received a single dose of local tumor irradiation (LTI). Survival after single doses of irradiation 15 (n=8), 20 (n=5) and 30 Gy (n=15), or without radiation (n=9) is shown. There were significant differences in survival in groups with untreated tumors vs tumors treated with 15 Gy (p< 0.01) or in groups treated with 30 Gy vs 15 Gy (p< 0.05 ) or by Mantel-Cox test. B, Experimental scheme. Mice with complete remissions of 21 day tumors after 30 Gy of LTI (n=12) were selected for this study. As controls, a group of normal mice was vaccinated subcutaneously (s.c.) with 1×106 irradiated tumor cells (50Gy in vitro) and 30 µg CpG (n=10). Vaccinated (n=10) or irradiated (n=12) mice were challenged with 5×106 of CT26 cells subcutaneously 100-150 days after treatment. Tumor growth curves, fraction of protected mice and survival are shown. There were significant differences in survival of vaccinated or untreated vs irradiated mice (p<0.05). C, Experimental scheme. T cells (6×106) and T cell depleted (TCD) bone marrow cells (1×107) were harvested from mice that were in remissions after 30Gy for at least 100 days, and transferred into syngeneic tumor-bearing mice (7 day tumors) conditioned with 8 Gy of total body irradiation (TBI) (n=5). T cells and TCD bone marrow transfer from untreated mice served as controls (n=5). Survival for 100 days is shown. There was a significant difference in survival between groups without the transplant procedure (n=9) vs with transplants from LTI donors (p<0.001) (n=5), but not with transplants from naïve mice (p>0.1)(n=5). D. Primary CT26 tumors were established at day 0. 30 Gy LTI to primary tumor was given at day 21, and mice were challenged with 5×106 of CT26 cells on the opposite flank at days 21 (n=5) or 51 (n=5) after primary tumor implantation. Growth curves are for second tumors on the contralateral flank. There was a significant difference in the fraction without tumor growth in groups with LTI challenged at day 21 vs 51 (p<0.05 by Chi Square test).

Figure 3. Change of balance of tumor-infiltrating cells after 30Gy LTI

A, Mice with 21 day tumors received a single dose of LTI (30 Gy). Tumor infiltrating mononuclear cells were analyzed 14 days after LTI completion. Control tumor-bearing mice received no LTI and cells were analyzed at day 35. Representative stainings of CD4+ and CD8+ T cells, MDSCs and TAMs are shown as well as the expression of PD-1 and Tim-3. B, Immunohistochemistry of tumors at day 35 that were untreated or received 30 Gy LTI at day 21. Tumor tissue sections were stained with anti-CD3 and anti-CD11b antibodies using a two stage procedure. CD11b+-red, CD3+ is green and DAPI staining is blue. C, Kinetics of tumor-infiltrating cells after LTI was analyzed at days 1,2,3, 6 and 14 after LTI that are days 22, 23, 24, 27, and 35 after tumor induction, respectively. Cell subsets (CD11b Gr-1hi, CD11b Gr-1lo, CD11c, CD4+ and CD8+) (n=8) are shown as a mean percentage +/− SE among live mononuclear cells in tumor. D, Kinetics of tumor-infiltrating cells after LTI was analyzed at days 1,2,3, 6 and 14 after LTI that are days 22, 23, 24, 27, and 35 after tumor induction, respectively. Cell subsets (CD11b Gr-1hi, CD11b Gr-1lo, CD11c, CD4+ and CD8+) are shown as a mean absolute number +/− SE per mg of tumor (n=8).

Figure 4. Curative effect of radiation requires CD4 and CD8 T cells, as well as cross-presenting dendritic cells, and is mediated by IFN-γ

A, Survival of tumor-bearing animals after 30 Gy LTI depends on CD4+ and CD8+ T cells. Wild-type animals with 21 day CT26 tumors received 30Gy LTI (n=14) and anti-CD8 (n=8) or CD4 (n=5) depleting antibodies. B, Curative effect of LTI depends on CD8+ dendritic cells. Survival after single dose of 30 Gy LTI at day 21 in Batf3−/− mice with (n=9) and without (n=10) add back of purified CD8+ CD11c+ spleen cells is shown. The latter cells (5×106) were injected intratumorally within 1 day after irradiation. C, Cell subsets (CD11b+Gr-1hi, CD11b+Gr-1lo, CD11c, CD4+ and CD8+) are shown as a mean percentage +/− SE among mononuclear cells in tumor at day 14 after LTI in wild-type mice with or without CD4+ (n=5) or CD8+ T cells (n=5) depletion, in Batf3−/− mice with (n=5) and without (n=5) add back of CD8+ CD11c+ cells, and IFN-γ−/− mice (n=5). D, IFN-γ is necessary for curative effect of LTI. IFN-γ−/− (n=13), perforin−/− (n=5) and TNF-α −/− (n=5), and TLR4−/− (n=14) tumor-bearing mice were used. Survival after single dose of LTI at day 21 is shown.

Figure 5. Addition of conventional daily fractionated to single high dose radiation results in decreased survival of tumor-bearing mice and metastatic spread of the tumor

A, Balb/c mice with 21 d CT 26 tumors received a single dose 30 Gy LTI (n=15), fractionated LTI 3Gy daily for 10 days (n=8), and combined regimen single dose 30Gy LTI on day 21 and fractionated 3Gyx10 started on day 24 (n=8). Survival of tumor-bearing mice shown. There were significant differences in survival in groups with LTI 30Gy vs fractionated or combined regimens (p< 0.01) by Mantel-Cox test. B, Cell subsets (CD8+, CD4+, CD11b Gr-1hi and CD11b Gr-1lo) are shown as a mean percentage +/− SE among live mononuclear cells in tumor at day 35 after tumor implantation (day 14 after LTI started) (n=5). C, H&E staining of the lungs (day 14 after LTI) of tumor-bearing animals that received combined LTI regimen (30Gy+3Gx10). D, Tumor growth curves and fraction of mice that survived at least 100 days are shown after single dose of 30Gy or combined regimen of 30Gy+3Gyx10. E, Cell subsets (CD8+, CD4+, CD11b Gr-1hi and CD11b Gr-1lo) are shown (after a single dose of 30 Gy (n=5) or combined regimen of 30 Gy+3Gyx10) (n=5) as a mean number +/− SE per mg of tumor at day 35 after tumor implantation (day 14 after LTI started).

Figure 6. Studies with the MC38 colon tumor. Curative effect of single high dose radiation requires CD40L expression on CD4+ T cells

A. MC38 colon tumors were established for 21 days subcutaneously in C57/B6 mice or C57B6 RAG2−/− mice, and the mice received a single dose of LTI 30Gy. Survival after single doses of irradiation 30 Gy (n=17), or without radiation is shown (n=8). There were significant differences in survival in WT mice treated with 30 Gy vs RAG2−/− treated group (p< 0.05), or WT untreated group (p<0.01) by Mantel-Cox test. B, Cell subsets (CD8+, CD4+, CD11b+ Gr-1hi and CD11b+ Gr-1lo) are shown as a mean percentage +/− SE among mononuclear cells in tumor at day 35 after tumor implantation in untreated WT (n=5) vs mice treated with LTI 30Gy (n=5) at day 21. C, MC38 tumors were established for 21 day. Survival of TLR4−/− (n=7), FasL−/− (n=12), and CD40 L−/− (n=10) tumor-bearing mice that received 30Gy LTI at day mice is shown. Survival of unirradiated wild type and gene inactivated mice given MC38 tumors was not significantly different (data not shown). D, Scheme: 6×106 of CD4+ T cells from CD40L−/− or WT mice and 2×106 of CD8+ from wild-type mice were transferred into RAG2−/− mouse within 6 hours after they received 3 Gy total body irradiation. 6 weeks after TBI, MC38 tumors were established subcutaneously. MC38 tumors received 30 Gy LTI at day 21. E, Survival of MC38 tumor-bearing RAG2−/− mice or mice reconstituted by CD40L−/− CD4+ (n=10) or CD4 WT T cells (n=7) and CD8+ WT T cells. All mice received 30 Gy LTI at day 21. F, Cell subsets (CD8+, CD4+, CD11b+ Gr-1hi, CD11c+ and CD11b+ Gr-1lo) are shown as a mean percentage +/− SE live among mononuclear cells in tumors in RAG2−/− mice reconstituted with CD40L−/− CD4+ T cells and CD8+ WT T cells two weeks after 30 Gy LTI (n=5).