Abstract A31: Immune cell influx during postpartum mammary gland involution reveals immunosuppression and tumor promotion
Authors: Martinson Holly , Jindal Sonail , Borges Virginia , Schedin Pepper 
Published: August 2014

Young women with breast cancer diagnosed in the postpartum period, defined as up to 5 years from last child delivery, have a poorer prognosis compared to women who have never been pregnant. Our lab has proposed that the poor prognosis associated with postpartum breast cancers is due to gland remodeling following pregnancy, known as postpartum involution. Using the mouse mammary gland as a model, our understanding of postnatal involution has greatly improved, however our understanding of the immune systems role during involution is limited. Recently M2 macrophages, that have tumor promotional characteristics, have been identified as essential for epithelial cell death during normal postpartum involution, demonstrating involution requires assistance from the immune system. We hypothesize that immune cells within the normal involuting gland set up an immunosuppressive environment that favors the growth and metastasis of breast cancer cells. To characterize the role of immune cells during involution and in the promotion of postpartum breast cancer, we created an immunocompetent mouse model using the Balb/c mouse.

Using flow cytometry, we have determined that dendritic cells, GR1+ monocytic immature myeloid cells (iMCs), and T cells increase exponentially in the mammary gland during involution. Functional assays using myeloid cells isolated from the mammary gland indicate the iMCs isolated from involuting glands are capable of suppressing T cell activation ex vivo. The ability of iIMCs to suppress T cell activation indicates they are capable of immune suppression. To address whether iMCs are able to promote tumor progression in the postpartum breast cancer setting, D2A1 a triple negative mammary carcinoma cell line and Balb/c mouse were utilized. D2A1 cells injected into the involuting mammary fat pad have a significant growth advantage over D2A1 cells injected into the nulliparous gland, demonstrating that mammary gland involution promotes the growth of mammary carcinoma cells in this immune competent model. T cells identified by flow cytometry are significantly lower in number systemically and within the tumors of involution-injected mammary glands than T cells in nulliparous mice, indicating systemic immunosuppression. To demonstrate immunosuppressive function, iMCs were isolated from tumors and incubated with T cells ex vivo. iMCs from involution tumors were significantly more suppressive when compared to iMCs from nulliparous mice. These data suggest that transient exposure of latent cancer cells to the immune suppressed microenvironment of involution confers a prolonged poor prognostic signature.

Further, we report evidence that postpartum involution can persistently alter breast cancer in young women. CD45+ immune cell infiltrate was elevated in mammary lobules during postpartum breast involution and in breast cancers diagnosed within 2 years of a completed pregnancy, suggesting an immunosuppressed population of immune cells is enriched in women diagnosed with postpartum breast cancer. While it is unknown if the immune cell infiltrate in normal involuting breast tissue or in postpartum breast cancers has immunosuppressive function, this may be expected based on our rodent data that shows postpartum breast cancers are characterized by high monocyte population with immune suppressive function.

Our study demonstrates monocytes infiltrating the involuting mammary gland are capable of suppressing T cell function supporting the notion that immune suppression can promote a more aggressive tumor microenvironment, providing justification for targeting the immune-suppressive monocytes as a potential therapy for postpartum breast cancer patients.

Citation Format: Holly Martinson, Sonail Jindal, Virginia Borges, Pepper Schedin. Immune cell influx during postpartum mammary gland involution reveals immunosuppression and tumor promotion. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr A31.