Postdoctoral fellow Ragon Institute, Massachusetts, United States
Disclosure(s):
Tomer Milo, PhD: No financial relationships to disclose
Introduction/Rationale: Regulatory T cells (Tregs) respond more strongly to self antigens than conventional T cells (Tconvs), effectively preventing autoimmunity. However, Treg reactivities to foreign antigens, and their relative strength compared to Tconvs, remain less understood. Because Tconv self-reactivity promotes their sensitivity to foreign antigens, we hypothesized that Tregs—being even more self-reactive—would be the most sensitive to foreign stimuli, a prediction we experimentally validated. This finding raises a fundamental paradox: how can Tconvs overcome Treg suppression to mount effective immune responses against pathogens? To address this, we developed and experimentally tested a mathematical theory grounded in spatially localized T cell decision-making within lymph nodes - a framework we term spatial stochasticity.
Methods: We quantified self- and foreign-reactivity distributions of endogenous, polyclonal Tconvs and Tregs using reporter mice that measure TCR signaling strength and MHC-II tetramers. We then applied high-resolution, multiplexed 3D imaging of optically cleared lymph nodes to directly test predictions in situ.
Results: Remarkably, Tregs exhibited higher sensitivity not only to self but also to foreign antigens compared to Tconvs. In a well-mixed lymph node where Tconvs and Tregs compete globally, such an advantage would imply that Tregs always dominate, precluding productive pathogen responses. However, our data and modeling indicate that lymph nodes are not well-mixed but instead organized into numerous microdomains—where small numbers of T cells are stochastically sampled to compete locally—thereby breaking this global advantage and allowing rare, highly responsive Tconvs to initiate effector responses.
Conclusion: Spatial stochasticity redefines self/non-self discrimination as a spatially distributed, probabilistic, population-based decision process across lymph node microdomains. As a result, Tregs provide quality control during infections by selecting the most competitive T cell clones.
