Project Summary/Abstract Cancer metastasis occurs when cancer cells spread to distal organs from their site of origin, and it is the leading cause of death in patients with solid cancers. When cancer spreads to the lung, various types of innate immune cells either help or inhibit this colonization process. Monocytes are one of the earliest cell types to flood the lung during the beginning of metastasis. These monocytes eventually become tumor-supporting metastasis-associated macrophages (MAMs), and many studies in the field have reported that monocytes and MAMs actively aid metastatic cancer to infiltrate the lung and take residence there. My project aims to understand how monocytes differentiate into MAMs; in particular, what signals and factors are critical for this transition to occur in the lung. Furthermore, from my preliminary data, I have identified a time window during which monocytes have undergone clear phenotypic changes but have not yet fully developed into MAMs. Interestingly, this transition period also coincides with the anti-tumor activity of other immune cell types, such as natural killer (NK) cells. Therefore, I aim to study the interactions between pro-tumoral monocytes and anti- tumoral NK cells during the first several hours of cancer metastasis to the lung. By understanding how the mechanisms that underlie these interactions help to increase the odds of metastatic establishment in the lung, I will be able to contribute valuable insights towards novel therapeutic efforts to prevent metastasis.