The inability to slow or revert progression to posttraumatic osteoarthritis (PTOA) after injury has devastating consequences for a young population, whose only therapeutic option remains knee replacement at a young age. Despite extensive research, PTOA pathophysiology continues to be poorly understood which translates in suboptimal diagnosis tools and limited treatment options. Although there is consensus that cell death (apoptosis) and low-grade inflammation play a major role in PTOA onset, we lack understanding on how these factors tip the healing response to injury into a vicious cycle of perpetuating joint damage. Unfortunately, blocking inflammation or apoptosis alone has proven to be insufficient to prevent progression to PTOA. In our preliminary studies we found a synergistic interaction between apoptotic articular chondrocytes and the inflammatory response of synovial cells that exacerbates apoptosis and inflammation creating a devastating production of pro- inflammatory and catabolic signaling. Thus, we hypothesize that a combinatorial treatment targeting apoptosis and inflammation is needed to reinstate joint homeostasis after injury. Here we propose a comprehensive approach to characterize the crosstalk between inflammation and apoptosis in vitro and in vivo. We will identify in vitro the crosstalk mechanisms and the molecular interplay between apoptosis and inflammation by analyzing the transcriptome of articular chondrocytes and synovial cells exposed to one or both stimuli and by characterizing the paracrine signaling. In vitro studies will allow us to study single interactions between cells and stimuli. We will then assess inflammation and apoptosis in vivo. Studying the dynamic crosstalk between events in knee joints represents a major challenge since we need to capture two processes in the same animal in vivo overtime. Molecular imaging represents a natural approach that can capture both inflammation and apoptosis in the same animal overtime and make possible the study of the dynamic synergistic interaction between these two processes. We will combine our novel MRI contrast agent that detects pro-inflammatory signaling in articular cartilage, with optical imaging agents targeting apoptosis and a positron emission tomography (PET) tracer targeting the folate receptor expressed in activated macrophages. This multimodal imaging approach will allow longitudinal assessment of the activity and timing of inflammation and apoptosis during the progression to PTOA. We will investigate in vivo the effect of blocking apoptosis has in inflammation and vice versa. Finally, a combinatorial treatment targeting apoptosis and inflammation will be explored to reinstate joint homeostasis after injury. These studies will lay the groundwork for the development of therapeutic strategies targeting the crosstalk between apoptosis and inflammation but also contribute to a fundamental understanding of how the interplay of crucial pathological mechanism can...