PROJECT SUMMARY A major component of many solid tumors, including lung cancers, are bone marrow (BM)-derived immune cells that migrate to tumors and aid in their continued growth. While the activity of these cells including tumor associated macrophages (TAMs) has been the subject of intense investigation, we recently identified that BM- derived hematopoietic stem and progenitor cells (HSPCs) are also present in growing tumors and can be functionally maintained intratumorally for long periods of time. Interestingly, the numbers of HSPCs present in tumors directly correlates to the eventual regrowth rates of tumors following radiation therapy (RT). The data suggests that HSPCs represent another important cell population involved in tumor biology, however; their mechanism of action is still unclear. Filling this gap in knowledge will add to the ever-changing understanding of tumor biology. The objective of this proposal is to determine how HSPCs are maintained in tumors and how HSPCs promote tumor regrowth post-RT. Our preliminary data support the idea that HSPCs are maintained through interactions of the integrin CD49f and laminins present within the tumor extracellular matrix. In Specific Aim 1, we will show that this interaction is indeed responsible for HSPC maintenance using in vitro and in vivo strategies that block or enhance this interaction followed by analysis of their effects on HSPC functionality. We will also define the intracellular signaling pathways involved in this process with initial studies focusing on focal adhesion kinase (FAK) signaling. These studies will characterize for the first time a tumor specific niche capable of maintaining HSPCs outside of the BM. In Specific Aim 2, we will demonstrate that tumor treatment with RT exacerbates HSPC migration to tumors and concomitantly disrupts the interaction between CD49f and laminin. We will also show that RT produces tumor microenvironments that favor the differentiation of these ‘released’ HSPCs into tumor supportive macrophages (specifically M2 polarized) to aid in tumor recovery. We will also test the effects of blocking the activity of HSPCs on tumor growth and regrowth post-RT. By completing the proposed studies, our long-term goal is to use the knowledge gained to make a significant contribution towards the development of more robust treatment strategies for patients suffering with solid tumor based cancers.