It is with deep sadness that we mourn the passing of Ulrich (Uli) Theopold, of Stockholm University, who passed away on September 23 after a long battle with cancer. We remember and honor his life and his work. Uli was a distinguished scholar, mentor, researcher, and a dedicated member of the editorial board of the Journal of Innate Immunity. He was a luminary in the world of innate immunity, leaving an indelible mark on the field and touching the lives of countless colleagues, students, and friends (Fig. 1).

Uli Theopold, 1957–2023 (photo courtesy of Eva Dalin)

Uli Theopold, 1957–2023 (photo courtesy of Eva Dalin)

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Uli studied genetics at Albert-Ludwigs-Universität in Freiburg, Germany, and continued with his PhD studies at the same institute under the supervision of the Nobel Laureate Georges Köhler. In 1989, he defended his thesis about induced immune tolerance in transgenic mice. During a period in Otto Schmidt’s laboratory in Freiburg, Uli became involved with the sophisticated molecular interactions between parasitoid wasps and their caterpillar hosts, raising a long-lasting interest in insect immunity. This led him to contact Dan Hultmark, at Stockholm University, where the pioneering work led by the late Hans G Boman had expanded into a dynamic research environment with several groups studying insect immunity from different angles. In 1991, Uli joined the Hultmark laboratory as a postdoc, backed up with a generous Human Frontier Fellowship, to work with the model organism Drosophila melanogaster. He soon set out to follow his own original ideas, and during three very productive years, Uli cloned a whole suite of genes, expressed in Drosophila blood cells. Then, in 1994, Uli and his wife Gaby moved to Adelaide, Australia, where Uli reconnected with Otto Schmidt and started as an independent group leader. There he collaborated with Otto Schmidt on the wasp-caterpillar system, and in parallel, he followed up on his own findings from Stockholm. In 2000, Uli returned to Stockholm University, where he was recruited to an associate professorship. Finally, this gave him the opportunity to carve out a research field of his own: insect blood clotting. With his background in biochemistry and insect physiology, he was the first to characterize clotting in Drosophila and reveal its importance in wound healing and immunity. He also worked on the molecular mechanisms underlying the rupture of crystal cells, specialized hemocytes involved in the melanization reaction. His laboratory showed that these cells undergo a special form of programmed cell death similar to the pyroptosis that affects mammalian inflammatory cells. In 2008, Uli started a long-lasting collaboration with the laboratory of Pavel Hyršl, Brno, Czech Republic. From that time, the research incorporated the use of entomopathogenic nematodes and their symbiotic bacteria as a novel tool to study Drosophila immunity. This led to identification of new molecular components found to be crucial during early stages of nemato-bacterial infections. More recently, Uli’s laboratory analyzed the interactions of the immune system with a new model of salivary gland tumors. This led to important insights into complex interplays between tissues undergoing hypertrophic growth on the one hand, and immune and stress responses on the other hand, with wider implications for therapeutic treatment of tumors in humans.

Uli’s works are remarkable for several reasons and are illustrative of his scientific ethos. He tackled neglected fields of research that consistently required methodological innovation. His work is descriptive and follows a stepwise process to fully characterize all elements of the topic at hand. He combined biochemical and genetic approaches and used his deep knowledge of other non-model insects to push Drosophila research. Uli was motivated by strong fundamental curiosity to understand the fly immune system without being drawn to passing fashions in the field or the bias to focus only on mechanisms that are conserved with mammals. Consequently, Uli’s scientific contributions are solid and serve as references that endure the test of time.

Uli Theopold was not only dedicated to the fields of insect and invertebrate immunology. He served for many years as section editor for invertebrate immunology in this journal (JIN). He was also a member of the editorial boards for Developmental & Comparative Immunology and for Frontiers in Insect Physiology. He was one of the organizers for the Jacques Monod Conferences in Insect Immunity, 2006 and 2009, in France.

In addition to his passion for research, Uli was also a committed university teacher. He enjoyed interacting with undergraduate students and conveying his enthusiasm for life and nature, from molecular biology and immunology to physiology, inter-organismal relationships, and host-parasite interactions. As a director of studies in molecular biosciences, Uli was deeply involved in developing educational programs and was always considering how to engage students in their studies. Uli was also someone who we, as colleagues, could always count on; he would often volunteer to take yet another lecture or step in as course leader when needed. Even during the rough periods of cancer treatment, he would come in and take his part of the teaching curriculum, up to the very last month of his life.

The impact Uli had as a mentor and for the next generation of scientists can hardly be overestimated. Since his passing, it is touching how many young scientists have expressed how much he meant to them. He gave his research group members – from interns to postdocs – intellectual freedom and encouraged creative thinking, and everyone’s ideas were valued in frequent and multifaceted scientific discussions. In combination with respectful critique, his style of communication and mentoring was ahead of its time, and his mentees matured into competent scientists.

Uli was not just a great scientist. He was modest and kind, and stands out as one of the nicest academics you could hope to meet. Discussing science was never as fun and exciting as when discussing with him. All his colleagues will remember his smile and kindness toward his community. We will sadly miss him and send our support to his wife and three daughters that surrounded Uli with affection.