Significance: IFN-γ is an example of a pleiotropic cytokine that plays critical roles in promoting both protective immune responses and immunopathological processes. In response to this cytokine, cells activate a well-identified and conserved JAK/STAT signaling pathway that can still elicit distinct responses, even in the same cell type. How this outcome specificity is achieved remains largely unknown. We have found that IFN-γ regulates CD8 T cell differentiation through noncanonical pathways that are enabled by integrin costimulation of the STAT1 pathway. This leads to cooperative effects between activating cell types that is dictated by proximity. This study demonstrates the importance of cofactors and microenvironment in eliciting specific cytokine functions.

Effective responses to intracellular pathogens are characterized by T cell clones with a broad affinity range for their cognate peptide and diverse functional phenotypes. How T cell clones are selected throughout the response to retain a breadth of avidities remains unclear. Here, we demonstrate that direct sensing of the cytokine IFN-γ by CD8⁺ T cells coordinates avidity and differentiation during infection. IFN-γ promotes the expansion of low-avidity T cells, allowing them to overcome the selective advantage of high-avidity T cells, whilst reinforcing high-avidity T cell entry into the memory pool, thus reducing the average avidity of the primary response and increasing that of the memory response. IFN-γ in this context is mainly provided by virtual memory T cells, an antigen-inexperienced subset with memory features. Overall, we propose that IFN-γ and virtual memory T cells fulfil a critical immunoregulatory role by enabling the coordination of T cell avidity and fate.

T cells are essential mediators of the adaptive immune system, which constantly patrol the body in search for invading pathogens. During an infection, T cells that recognise the pathogen are recruited, expand and differentiate into subtypes tailored to the infection. In addition, they differentiate into subsets required for short and long-term control of the pathogen, i.e., effector or memory. T cells have a remarkable degree of plasticity and heterogeneity in their response, however, their overall response to a given infection is consistent and robust. Much research has focused on how individual T cells are activated and programmed. However, in order to achieve a critical level of population-wide reproducibility and robustness, neighbouring cells and surrounding tissues have to provide or amplify relevant signals to tune the overall response accordingly. The characteristics of the immune response—stochastic on the individual cell level, robust on the global level—necessitate coordinated responses on a system-wide level, which facilitates the control of pathogens, while maintaining self-tolerance. This global coordination can only be achieved by constant cellular communication between responding cells, and faults in this intercellular crosstalk can potentially lead to immunopathology or autoimmunity. In this review, we will discuss how T cells mount a global, collective response, by describing the modes of T cell-T cell (T-T) communication they use and highlighting their physiological relevance in programming and controlling the T cell response.

T cells use the T cell antigen receptor (TCR) to discriminate between higher-affinity foreign and lower-affinity self peptide-MHC (pMHC) antigens. The OT-I mouse TCR is widely used to study anti-gen discrimination utilising many pMHCs, including foreign and self antigens. Previous studies sug-gested that OT-I T cells achieve near-perfect discrimination between higher and lower affinity antigens. Moreover, these 3D affinities measured in solution did not correlate with the 2D membrane affinities, suggesting a complex relationship between 3D and 2D affinities. In contrast, other TCRs have shown imperfect antigen discrimination and strong correlations between 3D and 2D affinities. To resolve these discrepancies, we extended a protocol for measuring ultra-low TCR/pMHC affinities to accurately de-termine the 3D affinities of the OT-I TCR binding 19 pMHC complexes. These revised 3D affinities now strongly correlate with the 2D affinities, and accurately predict functional responses. Importantly, we now find that the OT-I TCR exhibits enhanced yet imperfect antigen discrimination, similar to other TCRs, allowing it to detect abnormally high levels of low-affinity self-antigens. Finally, we show that discrimination is highest with low-affinity pMHC ligands, a finding predicted by the kinetic-proofreading model of antigen discrimination. This work underscores the ability of T cells to effectively gauge prox-ies for 3D affinity within the 2D cell-cell interface, with significant implications for the mechanisms underlying antigen discrimination. Lay T cells protect the body from infection by distinguishing between foreign and self molecules. They do this using a specialized receptor called the T cell receptor (TCR), which senses differences in how strongly it binds to foreign versus self molecules. Scientists have used a mouse TCR called OT-I to understand this process, concluding that OT-I T cells exhibit near perfect discrimination between foreign and self antigens based on a sharp affinity threshold. However, conflicting results from experiments on other TCRs hinted at a more complicated picture. In this study, we used an improved method to accurately measure OT-I TCR binding affinity to 19 peptides, including foreign and self antigens. In contrast to previous results and like other TCRs, we found that OT-I T cells display enhanced but im-perfect discrimination, enabling them to be activated by high levels of self antigen. The new data also revealed that the ability of T cells to discriminate antigens is particularly high at low affinities. These findings reconcile apparent discrepancies between the OT-I TCR and other TCRs, and have implications for understanding diseases where T cells often respond to self antigen, such as autoimmunity and cancer. Graphical