This study aims to illuminate how professional interactions among teachers form different types of teacher leadership in schools, which, in turn, shape different levels of teachers' involvement in activities related to their professional learning community (PLC). To investigate certain patterns of teachers' interactions that contribute to the formation of teacher leadership and thereby the development of PLC, two international schools that offer a full continuum of three International Baccalaureate (IB) programs were purposively chosen. The schools have similar numbers of students and are located in two different countries in East Asia. Using a mixed-methods research approach, the patterns of teacher professional interactions were first examined using social network analysis. Next, the degree to which teachers in the case study schools enact practices related to PLCs was assessed and compared through a multivariate analysis of variance (MANOVA) test. Further, based on findings from social network analysis, in-depth semistructured interviews were conducted with selected key participants to understand the linkage between teacher leadership and teachers' involvement in their PLC in each case school. From the findings, we propose four conceptual types of teacher leadership that explain how teacher leadership shapes different levels of teachers' engagement in PLC in the two case schools.