UC Irvine

Macrophages play a key role in the innate immune system, and their activation is tightly regulated to avoid excess and harmful inflammation. Studies have revealed the roles of soluble and adhesive cues in regulating macrophage polarization. The importance of intercellular communication within a macrophage population has also been demonstrated. However, as macrophages become crowded, the increasing cell density could alter the surrounding soluble and adhesive environments, as well as the coordination within a population. How these changes in the multicellular microenvironment affect macrophage functions is not well understood.

In my dissertation, I investigated the effects of the multicellular environment and its alteration on macrophage functions, characterizing how these factors affect paracrine signaling at macroscopic and microscopic levels, as well as how it could affect the physical interaction of cells. We characterized the regulatory effects of density-dependent intercellular communication on macrophage populations by stimulating cells in bulk, in small groups, and in isolation. We also assessed how cell crowding affects the function of a mechanosensitive ion channel, Piezo1, as this channel is important for the physical sensing of macrophages. We found that macrophage activation depends on the communication among the cell population, and both seeding density and absolute cell number contribute to the resultant responses. In addition, Piezo1 is found to contribute to the crowding-dependent regulation of macrophage activation. These works demonstrated that altering cell density could have broad-spectrum effects on macrophage functions. As a result, the impact of seeding density and its impacts on the multicellular environment should not be overlooked when designing an experiment that uses macrophages.

Overall, this study expanded our knowledge on how the macrophage population could regulate cellular functions by altering their surrounding soluble and adhesive environment in a density-dependent manner. This work could help explain the discrepancy in experimental results that can occur between different studies, as well as among different culture systems. Furthermore, this density-dependent regulation of macrophage activation may represent a novel regulatory mechanism with meaningful impacts toward cellular functions in vivo. Future work would focus on understanding how cell crowding could affect other aspects of cellular processes, and how these effects could further modulate macrophage functions in both in vitro and in vivo settings.