Interleukin-4 up-regulates mouse mammary tumor virus expression yet is not required for in vivo virus spread

Adaptive Immune Regulation of Mammary Postnatal Organogenesis

Postnatal organogenesis occurs in an immune competent environment and is tightly controlled by interplay between positive and negative regulators. Innate immune cells have beneficial roles in postnatal tissue remodeling, but roles for the adaptive immune system are currently unexplored. Here we show that adaptive immune responses participate in the normal postnatal development of a non-lymphoid epithelial tissue. Since the mammary gland (MG) is the only organ developing predominantly after birth, we utilized it as a powerful system to study adaptive immune regulation of organogenesis. We found that antigen-mediated interactions between mammary antigen-presenting cells and interferon-γ (IFNγ)-producing CD4+ T helper 1 cells participate in MG postnatal organogenesis as negative regulators, locally orchestrating epithelial rearrangement. IFNγ then affects luminal lineage differentiation. This function of adaptive immune responses, regulating normal development, changes the paradigm for studying players of postnatal organogenesis and provides insights into immune surveillance and cancer transformation.

MMTV infectious cycle and the contribution of virus-encoded proteins to transformation of mammary tissue

Mouse mammary tumor virus has served as a major model for the study of breast cancer since its discovery 1920's as a milk-transmitted agent. Much is known about in vivo infection by this virus, which initially occurs in lymphocytes that then carry virus to mammary tissue. In addition to the virion proteins, MMTV encodes a number of accessory proteins that facilitate high level in vivo infection. High level infection of lymphoid and mammary epithelial cells ensures efficient passage of virus to the next generation. Since MMTV causes mammary tumors by insertional activation of oncogenes, which is thought to be a stochastic process, mammary epithelial cell transformation is a by-product of the infectious cycle. The envelope protein may also participate in transformation. Although there have been several reports of a similar virus in human breast cancer, the existence of a human MTV has not been definitely established.

The IL-4/IL-13/Stat6 signalling pathway promotes luminal mammary epithelial cell development

Naïve T helper cells differentiate into Th1 and Th2 subsets, which have unique cytokine signatures, activators and transcriptional targets. The Th1/Th2 cytokine milieu is a key paradigm in lineage commitment, and IL-4 (Il4), IL-13 (Il13) and Stat6 are important mediators of Th2 development. We show here, for the first time, that this paradigm applies also to mammary epithelial cells, which undergo a switch from Th1 to Th2 cytokine production upon the induction of differentiation. Thus, the Th1 cytokines IL-12 (Il12), interferon gamma (INFgamma; also known as Ifng) and Tnfalpha are downregulated concomitantly with the upregulation of the Th2 cytokines IL-4, IL-13 and IL-5 (Il5) as epithelial cells commit to the luminal lineage. Moreover, we show that Th2 cytokines play a crucial role in mammary gland development in vivo, because differentiation and alveolar morphogenesis are reduced in both Stat6 and IL-4/IL-13 doubly deficient mice during pregnancy. This unexpected discovery demonstrates a role for immune cell cytokines in epithelial cell fate and function, and adds an unexpected tier of complexity to the previously held paradigm that steroid and peptide hormones are the primary regulators of mammary gland development.

Regression of a mammary adenocarcinoma in STAT6-/- mice is dependent on the presence of STAT6-reactive T cells

Polarization of the immune response toward a type 1 cytokine profile has been posited to be associated with a therapeutic antitumor immune response. STAT6-/- mice are unable to generate a type 2 immune response, and instead mount an enhanced type 1 response. STAT6-/- mice are significantly more resistant to 4T1, a mammary adenocarcinoma cell line, resisting a 10-fold higher tumor dose compared with wild-type (wt) BALB/c mice. An analysis of the T cells from tumor-bearing STAT6-/- mice revealed that they contained a population primed by a peptide (STAT6(531-539)) of the STAT6 protein expressed in 4T1. The adoptive transfer of T cells from STAT6(531-539)-vaccinated STAT6-/- mice significantly reduced the number of 4T1 pulmonary metastases in recipient mice. Additionally, the role of these STAT6(531-539)-reactive T cells against s.c. 4T1 tumor challenge was determined by tumor-challenging wt BALB/c mice reconstituted with STAT6-/- bone marrow, thereby assessing whether a polarized type 1 immune response in the absence of STAT6-reactive T cells was sufficient to reject a 4T1 tumor challenge. T cells from the STAT6-/- bone marrow chimeras failed to recognize the STAT6(531-539), and these mice proved to be as susceptible as wt BALB/c mice to 4T1 challenge. This demonstrated that the absence of STAT6(531-539)-reactive T cells correlated with the inability to reject 4T1 challenge. Additionally, these data emphasize that the enhanced ability to mount a type 1-polarized immune response is inconsequential if a sufficient antitumor immune response is not primed by the tumor.