Multicellular Control of T cell Responses

Introduction/Rationale: T cells must maintain a precise boundary between self and non-self to effectively clear pathogens while minimizing collateral damage. Traditionally, maintenance of this boundary has been ascribed to mechanisms operating within individual cells. However, self-activated T cells are common even at homeostasis, implying the need for additional controls. We hypothesized that higher order, multicellular mechanisms, enabled by spatial inhomogeneity within lymphoid tissues, provides this regulation, and that the local density of activated T cells (“activation density”) is the key parameter distinguishing self from non-self responses.

Methods: We immunized mice with protein antigens containing peptides of varying diversity to modulate activation density during the initial priming phase. Using an in vivo dye-labeling approach, we identified endogenous antigen-presenting cells (APCs) engaged in T cell priming. We applied highly multiplexed confocal microscopy to draining lymph nodes to visualize how activation density shapes T cell activation states. We complemented these experiments with a computational model to explore physiological parameters influencing activation density.

Results: We found that antigen peptide diversity tuned activation density, giving rise to activation dense multicellular domains around antigen-bearing APCs. Elevated activation density was associated with reduced regulatory T cell control, enhanced cytokine sensitivity, and increased proliferative potential. Importantly, spatial coordination among activated T cells was required for achieving high activation density.

Conclusion: Together, these findings reveal that T cell activation outcomes are not solely determined by cell-intrinsic signaling, but also by collective organization within tissue microenvironments. Such multicellular control mechanisms provide a new framework for understanding how immune responses distinguish self from non-self and may explain how subtle changes in spatial organization give rise to pathological states.