Tumor immunity: a balancing act between T cell activation, macrophage activation and tumor-induced immune suppression

Abstract

The mouse 4T1 mammary carcinoma is a BALB/c-derived tumor that spontaneously metastasizes and induces immune suppression. Although >95% of wild type BALB/c mice die from metastatic 4T1 tumor even if the primary mammary tumor is surgically removed, >65% of BALB/c mice with a deleted Signal Transducer Activator of Transcription 6 (STAT6) gene survive post-surgery. STAT6-deficiency also confers enhanced immunity against spontaneously developing breast cancer since NeuT+/- mice that are STAT6-deficient develop mammary tumors later and survive longer than NeuT+/- mice that are STAT6-competent. Rejection of metastastic disease and survival of STAT6-deficient mice after removal of primary tumor involve three mechanisms: (1) The generation of M1 type macrophages that produce nitric oxide and are tumoricidal; (2) A decrease to normal in the elevated levels of myeloid suppressor cells that accumulate during primary tumor growth; and (3) CD8+ tumor-specific T lymphocytes. STAT6-deficient, but not wild type BALB/c, mice generate nitric oxide producing macrophages because they lack the STAT6 transcription factor which is necessary for signaling through the type 2 IL-4Ralpha complex, and which induces the production of arginase instead of nitric oxide.

Fig. 1

Deletion of the STAT6 gene delays tumor progression and extends survival time of mice that spontaneously develop mammary carcinoma. NeuT^+/− mice, which spontaneously develop multifocal breast cancer, were crossed and backcrossed to STAT6^−/− mice to obtain STAT6^−/− neuT^+/− mice. The STAT6^−/− neuT^+/− and neuT^+/− mice were observed weekly for A the number of primary mammary tumors per mouse, the mean tumor diameter (TD) of individual tumors, and the sum of the diameters of all tumors per mouse; and B survival time

Fig. 2

Resistance to metastatic mammary carcinoma requires M1 macrophages and CD8⁺ T cells and is counter-acted by myeloid suppressor cells (MSC). A Mice are inoculated in the mammary gland on day 0 with 7000 4T1 mammary carcinoma cells; primary tumors are surgically removed on day 21–28; and mice are either followed for survival or sacrificed 10 days after surgery and their spleens analyzed for MSC or their bone-marrow-derived macrophages assayed for arginase and iNOS activity. B BALB/c, STAT6^−/−, and STAT6^−/− IFNγ^−/− mice were treated according to the schedule shown in Part A, and their splenocytes were analyzed by flow cytometry for Gr1⁺ CD11b⁺ MSC. Data are shown as percentage of mice that have normal levels of MSC (<8% of splenocytes are Gr1⁺ CD11b⁺). C STAT6^−/− mice were treated according to the schedule in Part A and concomitantly depleted for CD4⁺ or CD8⁺ T cells (left hand panel), or depleted for phagocytic cells/macrophages (right-hand panel)

Fig. 3

Proposed pathways for immunological resistance to metastatic mammary carcinoma in post-surgery mice. Resistance requires three mechanisms: (1) Reduction in tumor-induced myeloid suppressor cells (MSC); (2) Activation of tumor-specific CD8⁺ T lymphocytes; and (3) Activation of tumoricidal M1 macrophages. See text for detailed description