Immunological differences between hosts, such as diverse T-cell receptor (TCR) repertoires, are widely credited for reducing the risk of pathogen spread and adaptation in a population. Within-host immunological diversity might likewise be important for robust pathogen control, but to what extent naïve TCR repertoires differ across different locations in the same host is unclear. T-cell zones (TCZs) in secondary lymphoid organs provide secluded micro-environmental niches. By harboring distinct

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... Rs, such niches could enhance within-host immunological diversity. On the other hand, rapid T cell migration is expected to dilute such diversity. Here, we combined tissue micro-dissection and deep sequencing of the TCR beta chain to examine the extent to which TCR repertoires differ between TCZs in murine spleens. In the absence of antigen, we found little evidence for differences between different TCZs of the same spleen. Yet, three days after immunization with sheep red blood cells, we observed a >10-fold rise in the number of clones that appeared to localize to individual zones. Remarkably, these differences largely disappeared at 4 days after immunization, when hallmarks of an ongoing immune response were still observed. These data suggest that in the absence of antigen, any repertoire differences observed between TCZs of the same host can largely be attributed to random clone distribution. Upon antigen challenge, segregated TCR compartments appear and disappear within days. Such "transient mosaic" dynamics could be an important barrier for pathogen adaptation and spread during an immune response.