Cytokines are pivotal in the generation and resolution of the inflammatory response. The midkine/pleiotrophin (MK/PTN) family of cytokines, composed of just two members, was discovered as heparin-binding neurite outgrowth-promoting factors. Since their discovery, expression of this cytokine family has been reported in a wide array of inflammatory diseases and cancer. In this minireview, we will discuss the emerging appreciation of the functions of the MK/PTN family in the immune system, which include promoting lymphocyte survival, sculpting myeloid cell phenotype, driving immune cell chemotaxis, and maintaining hematopoiesis.

Immune profiling of tissue through multiplex immunohistochemistry is important for the investigation of immune cell dynamics and it can contribute to disease prognosis and evaluation of treatment response in cancer patients. However, protocols for mouse formalin-fixed paraffin-embedded (FFPE) tissue have been less successful. Given that FFPE remains the most common method to fix and analyze mouse tissue, this has limited the options to study the immune system and the impact of novel therapeutics in preclinical models. In an attempt to address this, we developed an improved immunohistochemistry protocol with a more effective antigen retrieval buffer. We also validated 22 antibodies specific for mouse immune cell markers to distinguish B cells, T cells, NK cells, macrophages, dendritic cells, and neutrophils. In addition, we designed and tested novel strategies to identify immune cells for which unique antibodies are currently not available. Lastly, in the 4T1 model of breast cancer, we demonstrate the utility of our protocol and antibody panels in the quantitation and spatial distribution of immune cells.

Metastatic cancer cells adapt to thrive in secondary organs. To investigate metastatic adaptation, we performed transcriptomic analysis of metastatic and non-metastatic murine breast cancer cells. We found that pleiotrophin (PTN), a neurotrophic cytokine, is a metastasis-associated factor that is expressed highly by aggressive breast cancers. Moreover, elevated PTN in plasma correlated significantly with metastasis and reduced survival of breast cancer patients. Mechanistically, we find that PTN activates NF-κB in cancer cells leading to altered cytokine production, subsequent neutrophil recruitment, and an immune suppressive microenvironment. Consequently, inhibition of PTN, pharmacologically or genetically, reduces the accumulation of tumor-associated neutrophils and reverts local immune suppression, resulting in increased T cell activation and attenuated metastasis. Furthermore, inhibition of PTN significantly enhanced the efficacy of immune checkpoint blockade and chemotherapy in reducing metastatic burden in mice. These findings establish PTN as a previously unrecognized driver of a prometastatic immune niche and thus represents a promising therapeutic target for the treatment of metastatic breast cancer.

No abstract